METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 27A, No. 10, October 1996 This Month Featuring: Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Physical Chemistry; Environment; Electronic, Magnetic, & Optical Material; Solidification; Materials Processing; Composite Materials. View October 1996 Table of Contents.

ALLOY PHASES

Influence of Thermal Aging on the Intergranular Corrosion Resistance of types 304LN and 316LN Stainless Steels
U. KAMACHI MUDALI, R.K. DAYAL, J.B. GNANAMOORTHY, and P. RODRIGUEZ
Intergranular corrosion (IGC) resistance of types 304LN and 316LN stainless steels (SS) thermally aged at 823, 873, and 923 K for various durations was assessed by ASTM A262 practice A test (electrolytic etch test) and electrochemical potentiodynamic reactivation (EPR) test. The results indicated that the type 316LN SS has significantly improved IGC resistance compared to 304LN SS. Based on the results of these tests, time-temperature-sensitization (TTS) diagrams were developed for both alloys. The secondary precipitates formed during thermal aging treatments were electrochemically extracted and analyzed by X-ray diffraction (XRD) to determine the types of precipitates formed during the aging treatments. The results indicated that the precipitates were mostly of M₂₃C₆ carbides.

Lattice Misfits in Four Binary Ni-Base /' Alloys at Ambient and Elevated Temperatures
A.B. KAMARA, A.J. ARDELL, and C.N.J. WAGNER
High-temperature X-ray diffractometry was used to determine the in situlattice parameters, a and a',and lattice misfits, = (a_' -a)/a, of the matrix () and dispersed'-type (Ni₃X) phases in polycrystalline binary Ni-Al, Ni-Ga, Ni-Ge, and Ni-Si alloys as functions of temperature, up to about 680°C. Concentrated alloys containing large volume fractions of the ' phase (~0.40 to 0.50) were aged at 700°C to produce large, elastically unconstrained precipitates. The room-temperature misfits are 0.00474 (Ni-Al), 0.01005 (Ni-Ga), 0.00626 (Ni-Ge), and -0.00226 (Ni-Si), with an estimated error of ±4 pct. The absolute values of the lattice constants of the and ' phases, at compositions corresponding to thermodynamic equilibrium at about 700°C, are in excellent agreement with data from the literature, with the exception of Ni₃Ga, the lattice constant of which is much larger than expected. In Ni-Ge alloys, decreases to 0.00612 at 679°C, and in Ni-Ga alloys, the decrease is to 0.0097. In Ni-Si and Ni-Al alloys, exhibits a stronger temperature dependence, changing to -0.00285 at 683°C (Ni-Si) and to 0.00424 at 680°C (Ni-Al). Since the times required to complete the high-temperature X-ray diffraction (XRD) scans were relatively short (2.5 hours at most), we believe that the changes in observed are attributable to differences between the thermal expansion coefficients of the and ' phases, because the compositions of the phases in question reflect the equilibrium compositions at 700°C. Empirical equations are presented that accurately describe the temperature dependences of a, a_', and over the range of temperatures of this investigation.

A Thermodynamic Evaluation of the Nickel-Silicon System
MIKAEL LINDHOLM and BO SUNDMAN
The nickel-silicon system has been the subject of some interest in recent years. The present work has the aim of describing this binary system thermodynamically using the most recent experimental data and computational facilities. This has been done by the CALPHAD method, and two different models for the liquid were tested. The order-disorder transformation of Ni₃Si has been modeled using a sublattice model.

Kinetics of Phase Evolution of Zn-Fe Intermetallics
Z.T. LIU, M. BOISSON, and O.N.C. UWAKWEH
The intermetallic phases, (Fe₃Zn₁₀), ₁ (Fe₅Zn₂₁), (FeZn₇), and (FeZn₁₃), are mechanically alloyed through ball milling of pure elemental Fe and Zn powders under a controlled atmosphere of argon gas.The state of the as-ball-milled materials was crystalline, except for the phase, which was amorphous. Phase-evolution kinetics through differential scanning calorimeter (DSC) measurements of the as-ball-milled powders show three characteristic transition temperatures for the ₁ and phases, two for the phase, and only one for the phase. The activation energies for the evolution of the milled powders to their equilibrium crystalline phases are 170±1, 251±2, 176±1, and 737±4 kJ/mol for the , ₁, , and phases, respectively. These values show that the mechanisms for the metastable-to-stable phase transition in these intermetallics are different, whereas diffusion over short distances may be part of the entire process in all cases.

High-Resolution Electron Microscopy Analysis of Structural Defects in a (2/1, 5/3)-Type Approximant of a Decagonal Quasicrystal of an Al-Pd-Mn alloy
D.P. YU, G. REN, and Z. ZHANG
Structural defects were analyzed by means of high-resolution electronmicroscopy (HREM) in a crystalline (2/1, 5/3)-type Fibonacci approximant of an Al-Pd-Mn alloy system. A kind of stacking fault is observed with a projected displacement vector R parallel to the [-3 0 29] direction; its amplitude |R| = 2a sin 18deg=1.19 nm, and its habit plane lies in the (1 0 1) plane. Two kinds of domain boundaries have been found and the domains are related by a 180 deg rotation around thec-axis plus a displacement along the [3 0 -29] or the [-3 0-29] direction in a plane perpendicular to the b-axis. The domain boundary planes are the {1 0 1} planes.

COMPOSITE MATERIALS

Low Quench Sensitivity of Superplastic 8090 Al-Li Thin Sheets
TSUNG-RONG CHEN, GUAN-JYE PENG, and J.C. HUANG
Quench sensitivity of the superplastic 8090 Al-Li thin sheets was investigated from the strengthening point of view for the various precipitates formed in the alloy. Specimens subjected to different cooling rates (water quenching, silicon oil cooling, and air cooling) from solution treatment (or superplastic forming) temperature were examined by tensile tests, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Experimental results show that ' is the predominant phase in the alloy under all cooling conditions. Because ' could be formed in a similar amount and size after the T6 treatment, and because the amount of S' precipitates and T₂ particles were extremely low, the superplastic 8090 Al-Li alloy would, thus, always exhibit low quench sensitivity. Based on DSC and high resolution electron microscopy (HREM) studies, new interpretations of the first two exotherms of the DSC traces were made. Strong evidence shows that the first exothermic peak is attributed to the further precipitation of ' phase, and the second one is contributed from the Al-Cu-Mg GPB zone formation.

Incipient Chemical Instabilities of Nanophase Fe-Cu Alloys Prepared by Mechanical Alloying
B. FULTZ, C.C. AHN, S. SPOONER, L.B. HONG, J. ECKERT, and W.L. JOHNSON
We used Mössbauer spectrometry, X-ray diffractometry, a novel imaging method of electron energy loss spectrometry, and small-angle neutron scattering (SANS) to study early stage thermal instabilities of nanophase Fe-Cu alloys prepared by mechanical attrition. Mössbauer spectrometry confirmed previous reports of an extended Cu solubility in the body-centered cubic (bcc) phase of the as-milled material. Mössbauer spectrometry also provided evidence that in the compositional range of bcc-face-centered cubic (fcc) two-phase coexistence,the bcc phase had a Cu concentration nearly the same as the overall composition of the alloy. After the as-milled powders were annealed at temperatures as low as 200°C, however, Mössbauer spectrometry showed significant chemical unmixing of the Cu and Fe atoms. In annealed bcc Fe-20 pct Cu alloys, SANS measurements indicated that Cu segregated to grain boundaries. This segregation of Cu atoms to bcc grain boundaries did not alter significantly the tendency for grain growth, however. X-ray diffractometry showed that grain growth during thermal annealing was similar for all alloys, although grain growth was small at temperatures below 300°C. The two-phase (bcc plus fcc) alloy of Fe-30 pct Cu was more unstable against chemical segregation than were the single-phase (bcc or fcc) alloys. Energy-filtered imaging indicated that the Cu atoms segregated to regions around the bcc grains, perhaps to the adjacent fcc crystallites.

Effects of Alloy Modification and Thermomechanical Processing on Recrystallization of Al-Mg-Mn Alloys
K. KANNAN, J.S. VETRANO, and C.H. HAMILTON
The 5083 Al alloy (Al-4.75Mg-0.8Mn) holds potential for superplastic forming (SPF), but slow rates of forming limit its use for many applications. Higher strain rates are believed possible through the development of finer grained microstructures or stabilized subgrain structures. Grain sizes after recrystallization and recrystallization characteristics are known to be dependent on the amount and distribution of second-phase particles in the matrix. In this study, the concentration and sizes of such particles were varied by additions of particle-forming elements of Mn and Zr and by modifications of the rolling and aging schedules (thermomechanical processing (TMP)). The investigation involved studying recrystallization kinetics at different temperatures and correlating the grain sizes with particle sizes and volume fractions. The addition of Mn and Zr, for the composition ranges and TMP methods studied, resulted in a substantial reduction of the recrystallization kinetics, but complete suppression of static recrystallization (or subgrain stabilization) was not observed. However, statically recrystallized grain sizes as small as 6 µm were achieved.

Alloy Phase Analysis from Measurements of Bulk Magnetic Properties
K.A. LINDAHL, D.L. OLSON and J.U. TREFNY
Measurements of bulk magnetic properties were investigated to evaluate whether they can be used to reveal the microstructure and phase stability of alloys. Specifically, phase transformations in aluminum-copper alloys were followed with magnetic susceptibility measurements. The results suggest that bulk magnetic measurements can be used to predict microstructure and, thus, properties of alloys. The ability to characterize alloy properties and phase stability through correlation with electromagnetic measurements may allow significant improvements in the nondestructive evaluation of advanced alloy properties and the prediction of service life.

A High-Resolution Transmission Electron Microscopy Study of the Precipitation Process in a Dilute Ti-N Alloy
D. SUNDARARAMAN, S. RANGANATHAN, and V.S. RAGHUNATHAN
The precipitation processes in dilute nitrogen alloys of titanium have been examined in detail by conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The alloy Ti-2 at. pct N on quenching from its high-temperature phase field has been found to undergo early stages of decomposition. The supersaturated solid solution (''-hcp)on decomposition gives rise to an intimately mixed, irresolvable product microstructure. The associated strong tweed contrast presents difficulties in understanding the characteristic features of the process. Therefore, HREM has been carried out with a view to getting a clear picture of the decomposition process. Studies on the quenched samples of the alloy suggest the formation of solute-rich zones of a few atom layers thick, randomly distributed throughout the matrix. On aging, these zones grow to a size beyond which the precipitate/matrix interfaces appear to become incoherent and the '(tetragonal) product phase is seen distinctly. The structural details, the crystallography of the precipitation process, and the sequence of precipitation reaction in the system are illustrated.

Communication: Discussion of "Effects of Tensile Stress on Microstructural Change of Eutectoid Zn-Al Alloy"
ELIZABETH MARTINEZ, J. MONTEMAYOR-ALDRETE, D. MUÑOZ-ANDRADE, and G. TORRES-VILLASEÑOR

Communication: Author's Reply
Y.H. ZHU and E. OROZCO

TRANSPORT PHENOMENA

MECHANICAL BEHAVIOR

Constitutive Behavior of Tantalum and Tantalum-Tungsten Alloys
SHUH RONG CHEN and GEORGE T. GRAY III
The effects of strain rate, temperature, and tungsten alloying on the yield stress and the strain-hardening behavior of tantalum were investigated. The yield and flow stresses of unalloyed Ta and tantalum-tungsten alloys were found to exhibit very high rate sensitivities, while the hardening rates in Ta and Ta-W alloys were found to be insensitive to strain rate and temperature at lower temperatures or at higher strain rates. This behavior is consistent with the observation that overcoming the intrinsic Peierls stress is shown to be the rate-controlling mechanism in these materials at low temperatures. The dependence of yield stress on temperature and strain rate was found to decrease, while the strain-hardening rate increased with tungsten alloying content. The mechanical threshold stress (MTS) model was adopted to model the stress-strain behavior of unalloyed Ta and the Ta-W alloys. Parameters for the constitutive relations for Ta and the Ta-W alloys were derived for the MTS model, the Johnson-Cook (JC), and the Zerilli-Armstrong (ZA) models. The results of this study substantiate the applicability of these models for describing the high strain-rate deformation of Ta and Ta-W alloys. The JC and ZA models, however, due to their use of a power strain-hardening law, were found to yield constitutive relations for Ta and Ta-W alloys that are strongly dependent on the range of strains for which the models were optimized.

The Effects on Fracture Toughness of Ductile-Phase Composition and Morphology in Nb-Cr-Ti and Nb-Si In Situ Composites
D.L. DAVIDSON, K.S. CHAN, and D.L. ANTON
Niobium-chromium alloys, both single and two phase, were alloyed with titanium in order to enhance fracture toughness and fatigue crack growth resistance. The selection of titanium as an alloying element and the relationship of electronic bonding to toughness are examined. The results indicated that toughness increased with a decreasing number of D + s electrons. Titanium was found to increase the toughness of solid-solution Nb-Cr alloys from to 87MPa, while for the two-phase "in situ composites,'' toughness was increased from to 20 MPa, although this is less than expected. Fracture toughness of the composites correlated nonlinearly with the volume fraction of the phases. The evidence suggests that the toughness of the composites is decreased due to fracture of the intermetallic particles and constraint on matrix deformation imposed by the intermetallic.Fracture characteristics of the Nb-Cr-Ti materials are compared to those of Nb-Cr and Nb-Si materials.

Cleavage Initiation in the Intercritically Reheated Coarse-Grained Heat Affected Zone: Part II. Failure Criteria and Statistical Effects
C.L. DAVIS and J.E. KING
In part I of this article, cleavage initiation in the intercritically reheated coarse-grained heat affected zone (IC CG HAZ) of high-strength low-alloy (HSLA) steels was determined to occur between two closely spaced blocky MA particles. Blunt notch, crack tip opening displacement (CTOD), and precracked Charpy testing were used in this investigation to determine the failure criteria required for cleavage initiation to occur by this mechanism in the IC CG HAZ. It was found that the attainment of a critical level of strain was required in addition to a critical level of stress. This does not occur in the case of high strain rate testing, for example, during precracked Charpy testing. A different cleavage initiation mechanism is then found to operate. The precise fracture criteria and microstructural requirements (described in part I of this article) result in competition between potential cleavage initiation mechanisms in the IC CG HAZ.

Control of Superplastic Deformation Rate During Uniaxial Tensile Tests
P.A. FRIEDMAN and A.K. GHOSH
Precise determination of superplastic flow behavior involves imposing known and controlled strain rate during deformation of these alloys. Examination of tensile specimens after superplastic deformation has revealed variations in strain and strain rate occurring as a function of position and the difficulty of maintaining a constant strain rate during testing. To quantify these strain and strain-rate gradients within the specimens, interrupted tensile tests and tests on gridded tensile specimens were performed. It was observed that more uniform strain and strain rates could be achieved with longer gauge length specimens. While longer gauge lengths make it possible to have better control over the imposed strain rate by minimizing the effects of material flow from the specimen grip regions, it has been realized that for smaller specimen gauge lengths, typically used in most laboratories, a more complex control of crosshead speed (CHS) during a test is essential to characterize superplastic behavior. A mathematical model has been developed in order to gain better insight into this material flow and to provide an improved crosshead control schedule for constant strain-rate testing. The results of this analysis have been validated on a superplastic aluminum-magnesium alloy (5083 Al).

The Influence of Stress Triaxiality on the Damage Mechanisms in an Equiaxed / Ti-6Al-4V Alloy
A.L. HELBERT, X. FEAUGAS, and M. CLAVEL
The influence of the stress triaxiality on void formation, void growth, and fracture was investigated for an equiaxed Ti-6Al-4V alloy. Void nucleation in the phase was found to occur for a critical value of macroscopic plastic strain, whereas void nucleation at the / interface also depends on triaxiality. Under low triaxiality and important plastic strain, voids appear and grow in the area where the microshear bands develop, with an angle close to 45 deg to the stress axis in the particles. In contrast, with high triaxiality, voids nucleate preferably at the / interfaces and grow perpendicular to the stress axis by a cleavage mechanism. In a middle range of triaxiality and plastic strain, voids nucleate in because of the sufficient plastic strain and also at the / interfaces because of the sufficient triaxiality (). Void growth occurs with an angle of 60 deg to the stress axis, since is not high enough to create cleavage and _p is high enough to provide a ductile growth. Two types of fracture were identified and reported on a fracture map: under low triaxiality, failure appears by plastic instability, whereas for high triaxiality, the instability is induced by avoid-growth process discussed with the help of Rice and Tracey's approach.

Nonequilibrium Grain-Boundary Segregation and Ductile-Brittle-Ductile Transition in Fe-Mn-Ni-Ti Age-Hardening Alloy
N.H. HEO
Nonequilibrium segregation kinetics of alloying elements and a ductile-brittle-ductile transition behavior have been investigated in an Fe-8.4Mn-7.4Ni-1.7Ti alloy. The alloy experienced a ductile-brittle-ductile (DBD) transition during isothermal aging. In the brittle region, the alloy showed a decrease in intergranular fracture strength and a subsequent increase with aging time. This is due to the segregation of titanium to the grain boundaries and its desegregation into the matrix. The intergranular fracture strength was higher on the zero tensile elongation finish curve than on the start curve. This is because the grain-boundary segregation level of titanium is relatively lower on the finish curve. The lowest intergranular fracture strength increased with increasing aging temperature, which was attributed to a lower grain-boundary segregation level of titanium at higher temperature. Manganese caused an overall reduction in intergranular fracture strength and, as a result, the delayed occurrence of the zero tensile elongation (ZTE) finish curve in a temperature and log-time plot.

Effect of Stress State on the Stress-Induced Martensitic Transformation in Polycrystalline Ni-Ti Alloy
KURT JACOBUS, HUSEYIN SEHITOGLU, and MARK BALZER
The effect of stress state on the character and extent of the stress-induced martensitic transformation in polycrystalline Ni-Ti shape memory alloy has been investigated. Utilizing unique experimental equipment, uniaxial and triaxial stress states have been imposed on Ni-Ti specimens and the pseudoelastic transformation strains have been monitored. Comparisons between tests of differing stress states have been performed using effective stress and effective strain quantities; a strain offset method has been utilized to determine the effective stress required for transformation under a given stress state. Results of the tests under different stress states indicate that (1) despite the negative volumetric strain associated with the austenite-to-martensite transformation in Ni-Ti, effective stress for the onset of transformation decreases with increasing hydrostatic stress; (2) effective stress vs effective strain behavior differs greatly under different applied stress states; and (3) austenite in Ni-Ti is fully stable under large values of compressive hydrostatic stress.

Prediction of Creep-Rupture Life of Unidirectional Titanium Matrix Composites Subjected to Transverse Loading
REJI JOHN, M. KHOBAIB, and PAUL R. SMITH
Titanium matrix composites (TMCs) incorporating unidirectional fiber reinforcement are considered as enabling materials technology for advanced engines which require high specific strength and elevated temperature capability. The resistance of unidirectional TMCs to deformation under longitudinally applied sustained loading at elevated temperatures has been well documented. Many investigators have shown that the primary weakness ofthe unidirectional TMC is its susceptibility to failure under very low transverse loads, especially under sustained loading. Hence, a reliable modelis required to predict the creep-rupture life of TMCs subjected to different transverse stress levels over a wide range of temperatures. In this article, we propose a model to predict the creep-rupture life of unidirectional TMC subjected to transverse loading based on the creep-rupture life of unidirectional TMC subjected to transverse loading based on the creep-rupturebehavior of the corresponding fiberless matrix. The model assumes that during transverse loading, the effective load-carrying matrix ligament along a row of fibers controls the creep-rupture strength and the fibers do not contribute tothe creep resistance of the composite. The proposed model was verified using data obtained from different TMC fabricated using three matrix compositions, which exhibited distinctly different types of creep behavior. The results show that the creep-rupture life of the transverse TMC decreases linearly with increasing ratio of the fiber diameter to the ply thickness. The creep-rupturelife is also predicted to be independent of fiber spacing along the length ofthe specimen.

Intergranular Fracture in Some Precipitation-Hardened Aluminum Alloys at Low Temperatures
S. KURAMOTO, G. ITOH, and M. KANNO
Intergranular fracture at low temperatures from room temperature down to 4.2 K has been studied in some precipitation-hardened aluminum alloys. Microscopic appearance of intergranular facets is revealed to be greatly affected by the microstructure adjacent to the grain boundaries (GBs). When large precipitates on GBs and wide precipitation-free zones (PFZs) are present, coalescence of microvoids initiated at the GB precipitates causes the intergranular fracturewith dimples. This fracture process is found to be unaffected by deformation temperature. On the other hand, in the presence of fine precipitates on GBs and narrow PFZs, matrix slip localization exerts significant influence on thefracture behavior. At low temperatures, large stress concentration at GBs leadsto intergranular fracture, forming sharp ledges on the fracture surfaces, while at room temperature, the dynamic recovery process is thought to relax such stress concentration, resulting in a transgranular "ductile'' rupture.

Tension Characteristics of Notched Specimens for Al-Li-Cu-Zr Alloy Sheets with Various Cerium Contents
L. MENG and X.L. ZHENG
In the present article, the high strength Al-Li-Cu-Zr alloy sheets modified by a rare earth element, Ce, are considered for possible application in practical aircraft products containing structural notches or stress concentrations; accordingly, a study has been made on the effects of stress concentration levels and Ce contents on the tension strength of notched specimens for the alloy sheets. Moreover, a discussion has been set off on the theoretical predictability on the basis of a theoretical expression for the notch strength by means of the mechanical properties of the smooth specimens. The test results show that when the stress concentration level increases, the notch strength linearly decreases in the double logarithmic coordinate; by comparison with the Ce-free alloy, the Ce-containing alloy sheets exhibit an insignificantly varying notch strength when the Ce content changes from 0.13 to 0.31 wt pct in the transverse orientation specimens or is 0.21 wt pct in the longitudinal orientation specimens even though their ductility for the smooth specimens can be improved to a certain degree by the Ce modification. The test data of notched specimens under the theoretical stress concentration factor (K_t) from 2.0 to 8.0 agree better with the predicted values of notch strength. Therefore, in accordance with some engineering properties such as the ultimate tensile strength (UTS), percentage elongation (EL), and Young's modulus (E) of the smooth specimens, the notch strength ofthe alloy sheets under plane strain state can be easily estimated in a certain range of stress concentration levels.

Failure Characteristics of 6061/Al₂O₃/15p and 2014/Al₂O₃/15p Composites as a Function of Loading Rate
B.Y. LOU and J.C. HUANG
The effects of loading rate on the toughness and fracture mechanisms of twocast 6061/Al₂O₃/15p and 2014/Al₂O₃/15p composites under the as-worked (AW) and AW + T6 conditions have been examined. The quasistatic bending and high-rate impact tests were conducted over strain rates from 5 x 10^-4 to 1 x 10³ s^-1 using screw-driven or servohydraulic high-rate systems. The results showed that the peak loadP_MAX, specimen deflection d, specimen lateral expansion fraction w, crack initiation energy E_i, propagation energy E_p, total fracture energy E_t, and deformation zone all tended to increase with increasing strain rate. Under quasistatic loading, the composites failed predominantly by matrix/reinforcement interface decohesion. As the loading rate increased, reinforcement failure became the major failure mechanism. Differences in the effect of matrix microstructure and stress state on the fracture properties also are discussed. In comparing the fracture modes in the AW and AW + T6 specimens, the latter showed a higher tendency toward particle cracking. Based on mechanical data, the degree of specimen deflection and expansion and fracture modes, the AW composites exhibited a higher strain-rate dependence. The T6 specimens, due to their intrinsicly more brittle nature, appeared to be less influenced by loading rate over the strain-rate range examined.

Influence of Training Time and Temperature on Shape Memory Effect in Cu-Zn-Al Alloys
NANJU GU, HUIFEN PENG, and RUIXIANG WANG
The influence of training temperature on the shape memory effect (SME) for the CuZnAl shape memory alloy (SMA) has been studied. It is found that there exists an optimum upper training temperature resulting in the best SME which is about 353 K for the Cu-26Zn-4Al alloy. The preferential oriented martensitic variants will be formed during cooling, since there are aligned dislocations (resulting in the best two-way memory effect (TWME)) in the parent phase after training between 293 and 353 K. However, the TWME drops gradually due to the generation of dislocation tangles in the parent phase as the training temperature increases to 373 K. Further work on the effect of training time onthe SME shows that the shift of As temperature is little when alloys studied are trained at the optimum temperature for 15 to 40 seconds and there exists a certain training time without any shift of A_S temperature after 1000 thermal cyclings.

Simulation of the Hot-Tension Test Under Cavitating Conditions
P.D. NICOLAOU, S.L. SEMIATIN, and C.M. LOMBARD
A theoretical analysis of the isothermal hot-tension test under cavitating conditions for sheet samples was performed using a "direct-equilibrium'' approach. The effects of cavity growth rate , initial cavity volume fractionC_v0, strain-rate sensitivity exponent m, and specimen taper on engineering stress-strain curves, strain profiles, and failure modes were established. For a given value of m, it was predicted that the engineering stress-strain curves of cavitating and noncavitating samples are almost coincident except near failure. In fact,during quasistable deformation, the required load for a cavitating material is slightly higher than that for a noncavitating material because of strain rate and effective area effects. Model results also delineated the competition between failure controlled by localized necking vs fracture, the latter being defined by a critical-volume fraction of cavities. Specifically, at low strain-rate sensitivities m and cavity growth rates , failure was predicted to be controlled by necking. By contrast, at high values of m and , fracture prior to localized necking was predicted to predominate; in these cases, the cross-sectional area at the failure site was appreciable, thus resembling a form of brittle fracture. The validity of the modeling approach was confirmed through the analysis of data in the literature. However, model results did suggest that caution should be taken in the interpretation of experimental data because various combinations of C_v0 and can result in the same total elongation.

The Effect of Mo Addition on the Liquid-Phase Sintering of W Heavy Alloy
HEE-DONG PARK, WOON-HYUNG BAIK, SUK-JOONG L. KANG, and DUK-YONG YOON
The morphological and compositional changes of grains have been investigated in the initial stage of liquid-phase sintering of W-Mo-Ni-Fe powder compacts. Both large (5.4-µm) and small (1.3-µm) W powders have been used to vary their time of dissolution in the liquid matrix. When 80W-10Mo-7Ni-3Fe (wt pct) compacts of fine (about 1- to 2-µm) Mo, Ni, and Fe and coarse (5.4-µm) W powders are liquid-phase sintered at 1500°C, the Mo powder and a fraction ofthe W powder rapidly dissolve in the Ni-Fe liquid matrix. The W-Mo grains (containing small amounts of Ni and Fe) nucleate in the matrix and grow while the W particles slowly dissolve. In this transient initial stage of the liquid-phase sintering, duplex structures of coarse W-Mo grains and fine W particles are obtained. As the W particles dissolve in the liquid matrix during the sintering, the W content in the precipitated solid phase also increases.The dissolution of the small W particles is assessed to be driven partially by the coherency strain produced by Mo diffusion at the surface. During sintering, the W particles continuously dissolve while the W-Mo grains grow. When the compacts are prepared from a fine (1.3-µm) W powder, the W grains dissolve more rapidly, in about 1 hour, and only W-Mo grains remain. These observations show that the morphological evolution of grains during liquid-phase sintering can be strongly influenced by the chemical equilibrium process.

Microstructure and Tensile Properties of Compacted, Mechanically Alloyed, Nanocrystalline Fe-Al
J. RAWERS, G. SLAVENS, D. GOVIER, C. DOGAN, and R. DOAN
Data on mechanical properties of nanocrystalline materials have been limited, due in part to the difficulty in producing consolidated nanocrystalline materials of sufficient quantity for characterization and evaluation. A second problem is consolidation and retention of the nanostructure. A vacuum hot-pressing consolidation program has been developed to produce full-dense compacts from attrition milled, mechanically alloyed, nanograin micron-size particles of Fe-2 wt pct Al powder. The resulting compacts were of sufficient size to allow evaluation of microstructure, density, hardness, and tensile properties. The compacted microstructure was a composite of pure iron submicrograins and Fe-Al nanograin clusters. Tensile strength was found to be proportional to the sample's density squared. For full-dense compacts, tensile strength of nanocrystalline compacts approaching 1 GPa was obtained.

High-Temperature Wear and Deformation Processes in Metal Matrix Composites
J. SINGH and A.T. ALPAS
Dry-sliding wear behaviors of a particulate-reinforced aluminum matrix composite 6061 Al-20 pct Al₂O₃ and an unreinforced 6061 Al alloy were investigated in the temperature range 25°C to 500°C against a SAE 52100 bearing steel counterface. Experiments were carried out at a constant sliding speed of 0.2 m·s^-1 at different test loads. The deformation behavior of the materials was studied by performing uniaxial compression tests in the same temperature range as the wear tests. Both alloys showed a mild-to-severe wear transition above a certain test temperature. In the mild wear regime, the wear rate and the coefficient of friction of the unreinforced 6061 Al decreased slightly with temperature, but the temperature had almost no effect on the wear rate and the coefficient of friction of the 6061 Al-20 pct Al₂O₃ in the same regime. Particulate reinforcement led to an increase in the transition temperature and a 50 to 70 pct improvement in the wear resistance in the severe wear regime. This was attributed to the formation of tribological layers consisting of comminuted Al₂O₃ particles at the contact surface. High-temperature compression tests showed that the flow strength of 6061 Al-20 pct Al₂O₃ and 6061 Al decreased monotonically with temperature and both alloys exhibited a work-softening behavior at temperatures higher than the inflection point on the flow stress vs temperature curves. The logarithmic maximum stress vs reciprocal temperature relationship was not linear, indicating that the deformation processes were too complicated to be characterized by a single activation energy over the whole temperature range. For the range of 250°C to 450°C, the activation energy for deformation was estimated to be 311 kJ·mol^-1 for both the matrix alloy and the composite. Severe wear proceeded by thermally activated deformation processes involving dynamic recrystallization along a subsurface strain gradient. A power-Arrhenius type relationship was found to describe well the observed dependence of severe wear rates on the applied load and temperature. This relationship was used to calculate an apparent activation energy for wear of 87 kJ·mol^-1 for the particulate-reinforced composite and 33kJ·mol^-1 for the matrix alloy. The wear regimes at elevated temperatures are represented in a deformation mechanism map and the relationship between high-strain deformation processes and severe wear are discussed.

The Effect of High-Energy Electron-Beam Irradiationon Microstructural Modification of a High-Speed Steel Roll
DONGWOO SUH, SUNGHAK LEE, YANGMO KOO, and HUI CHOON LEE
The purpose of this study is to investigate the microstructural modification in a high-speed steel (HSS) roll irradiated with an accelerated high-energy electron beam. The HSS roll samples were irradiated at the beam travel speeds of 2.5 to 25 mm/s using an electron accelerator (1.4 MeV). The microstructure was examined with a scanning electron microscope (SEM) capable of in situ fracture testing and simultaneous measurement of the apparent fracture toughness. Irradiation changed the matrix phase from tempered martensite to a mixture of retained austenite and martensite. Coarse primary carbides were partially or completely dissolved, depending on the heat input. Irradiation greatly improved the fracture properties because of the presence of retained austenite, which could retard crack propagation, although hardness was decreased. Occasional interior quench cracks were found in the heat-affected region. Appropriate processing methods, such as pre- or postirradiation, were suggested. A heat transfer analysis of the irradiated surface layer was also carried out to elucidate the influence of the irradiation parameters on the microstructure.

Effect of Postweld Treatment on the Fatigue Crack Growth Rate of Electron-Beam-Welded AISI 4130 Steel
CHIEN-CHUN WANG and YIH CHANG
This article studies the effect of in-chamber electron beam and ex-chamber furnace postweld treatments on the fatigue crack grow thrate of electron-beam-welded AISI 4130 steel. Mechanical properties of the weldment are evaluated by tensile testing, while the fatigue properties are investigated by a fatigue crack propagation method. Microstructural examination shows that both postweld treatments temper the weldment by the appropriate control of beam pattern width, input beam energy, and furnace temperature. In addition, the ductility, strength, and microhardness of the weldment also reflect this tempering effect. The fatigue crack growth rate is decreased after both postweld treatments. This is mainly caused by the existence of a toughened microstructure and relief of the residual stress due to the fact that (1) the residual stress becomes more compressive as more beam energy isdelivered into the samples and (2) postweld furnace tempering effectively releases the tensile stress into a compressive stress state.

Elevated Temperature Compressive Properties of N-Doped NiAl
J. DANIEL WHITTENBERGER, R.D. NOEBE, and A. GARG
The elevated temperature properties of NiAl slightly enriched with ~900 appm nitrogen by atomizing the aluminide to powder under a nitrogen atmosphere have been determined. Compression samples were machined from hot extruded material and tested in air between 1100 and 1400 K under both constant velocity and constant load conditions. It appears that N in solid solution contributes little to the creep strength of B2 nickel aluminide. Excess nitrogen leading to the formation of AlN and Al(O,N) particles, however, can have a pronounced effect on creep behavior. These fine second-phase particles stabilize a small grain structure which, in turn, can improve or reduce the mechanical strength, depending on the deformation conditions. Under certain test conditions, high-angle grain boundaries can break away from the particles and overall grain growth occurs, leaving behind a network of AlN and Al(O,N) particles. This network of particles is very effective in anchoring a small, stable subgrain structure that provides elevated temperature strength without being subject to undesirable, weakening grain-boundary deformation mechanisms. Overall, the results provide further evidence that creep in NiAl is dislocation-climb controlled which involves subgrain formation and that stabilization of subgrains will improve mechanical strength in the manner proposed by Sherby et al.

Communication: Ballistic Impact Behavior of Multilayered Armor Plates Processed by Hardfacing
R. KISHORE and T.K. SINHA

Communication: Effect of Alloying Additions on Fracture Behavior of Mo-Containing Secondary Hardenming Steels
H. KWON, C.M. KIM, K.B. LEE, H.R. YANG, and J.H. LEE

PHYSICAL CHEMISTRY

Influence of Gold Content on Copper Oxidation from Silver-Gold-Copper Alloys
D.R. SWINBOURNE, G.G. BARBANTE, and A. STRAHAN
In the final stages of the smelting of copper anode slimes, a silver alloy, known as "doré,'' is produced. Oxidation refining is used to remove copper since this element interferes with subsequent electroparting of the small amounts of gold and platinum group metals in the doré. The gold content of doré can be greatly increased by gold scrap additions and this may affectthe minimum achievable copper content of doré. In this work, silver-gold-copper alloys were oxidized by injecting pure oxygen at 1100°C in the absence of any slag cover. For the gold contents expected in practice, the equilibrium copper content of the doré did not increase significantly as the gold content increased. However, at the other extreme of composition, the equilibrium copper content was a very strong function of the silver content ofthe gold bullion. The activity coefficient of copper in silver-gold alloys was calculated and compared to those predicted from a ternary subregular solution model of the system Ag-Au-Cu. Satisfactory agreement was found.

ENVIRONMENT

ELECTRONIC, MAGNETIC, & OPTICAL MATERIAL

SOLIDIFICATION

Solidification of Undercooled Fe-Cr-Ni Alloys: Part II. Microstructural Evolution
T. KOSEKI and M.C. FLEMINGS
Results are reported on microstructures of Fe-Cr-Ni alloys, solidified over arange of undercoolings and quenched during or after recalescence. Alloys studied contained 70 wt pct Fe and with Cr varying from approximately 15 to 20 wt pct. The three lower Cr alloys were hypoeutectic (with fcc as primary phase in equilibrium solidification); the two higher Cr alloys were hypereutectic (with bcc as primary phase in equilibrium solidification). Results obtained are in agreement with predictions based on thermal analyses previously presented; they confirm and extend the understanding gained in that work. The primary phase to solidify in the hypoeutectic alloys is bcc when undercooling is greater than an amount which decreases with increasing Cr content. At the lower Cr contents, the stable fcc phase then forms by solid-state transformation of the metastable phase and its subsequent engulfment by additional fcc. At the higher Cr content, transformation is by a peritectic-like reaction in the semisolid state, except near the surface at higher undercoolings where the transformation is massive. In the hypereutectic alloys, primary solidification at all undercoolings is the stable bcc phase. Partial transformation to fcc occurs in the semisolid or solid state, depending on composition and undercooling.

MATERIALS PROCESSING

The Use of Microstructural Gradients in Hot Gas-Pressure Forming of Zn-Al Sheet
J.-Q. JIANG and P.S. BATE
The gas-pressure forming of Zn-22 pct Al sheet containing regions of different initial microstructure, introduced by selected-area heat treatment, at elevated temperature and low strain rates has been evaluated. The heat treatments used converted a fine, spheroidal, phase mixture into a lamellar one, and the fraction of transformed material could be controlled reasonably well. The mechanical response of the material, determined by tensile testing, was correlated with the microstructure and its evolution during deformation. The material behavior was formalized as a simple constitutive equation which was used in finite-element modeling of the forming process. It was possible to influence the thickness distribution of a formed shape in a controlled manner using this approach.

Communication: Temperature Dependence of the Rate Sensitivity and Its Effect on the Activation Energy for High-Temperature Flow
F. Montheillet and J.J. Jonas

COMPOSITE MATERIALS

Thermally Assisted and Mechanically Driven Solid-State Reactions for Formation of Amorphous Al₃₃Ta₆₇ Alloy Powders
M. SHERIF EL-ESKANDARANY
The rod milling technique using the mechanical alloying (MA) process has been employed for preparing amorphous Al₃₃Ta₆₇ alloy starting from elemental Al and Ta powders. X-ray diffraction (XRD), differential thermal analysis (DTA), differential scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are utilized to follow the progress of amorphization. The results show that during the first few kiloseconds of MA time, layered composite particles of Al and Ta are intermixed and form an amorphous phase upon heating to 685 K by DTA. This process is called thermally assisted solid-state amorphization (TASSA). During the early stage of milling, the number of layers of the composite particles increases. This leads to an increase in the heat formation of amorphous Al₃₃Ta₆₇ alloy via the TASSA process, H_a^TASSA. After 360 ks (100 h) of the MA time, all Al atoms emigrate to Ta lattices to form a solid solution phase and the powder particles have no more layered structure. At this stage of milling, the value of H_a^TASSA becomes zero. This solid solution phase is not stable against the shear forces that are generated by the rods and transforms completely to an amorphous phase upon milling for 720 ks (200 hours). This phase transformation is attributed to the accumulation of several lattice imperfections, such as point and lattice defects, which raise the free energy from the more stable phase (solid solution) to a less stable phase (amorphous). After 1440 ks (400 hours) of MA time, a homogeneous amorphous phase is formed. The amorphization process in this case is attributed to a mechanical driven solid-state amorphization (MDSSA). The heat of formation of the amorphous phase formed via the MDSSA process, H_a^MDSSA, has been calculated. Moreover, the crystallization characteristics indexed by the crystallization temperature, and the enthalphy of crystallization, of the amorphous phases formed by TASSA and MDSSA processes are investigated as a function of MA time. The role of amorphization via each process has been discussed.

Kinetics of Cyclic Oxidation and Cracking and Finite Element Analysis of MA956 and Sapphire/MA956 Composite System
KANG N. LEE, VINOD K. ARYA, GARY R. HALFORD, and CHARLES A. BARRETT
Sapphire fiber-reinforced MA956 composites hold promise for significant weight savings and increased high-temperature structural capability, as compared to unreinforced MA956. As part of an overall assessment of the high-temperature characteristics of this material system, cyclic oxidation behavior was studied at 1093°C and 1204°. Initially, both sets of coupons exhibited parabolic oxidation kinetics. Later, monolithic MA956 exhibited spallation and a linear weight loss, whereas the composite showed a linear weight gain without spallation. Weight loss of the monolithic MA956 resulted from the linking of a multiplicity of randomly oriented and closely spaced surface cracks that facilitated ready spallation. By contrast, cracking of the composite's oxide layer was nonintersecting and aligned nominally parallel with the orientation of the subsurface reinforcing fibers. Oxidative lifetime of monolithic MA956 was projected from the observed oxidation kinetics. Linear elastic, finite element continuum, and micromechanics analyses were performed on coupons of the monolithic and composite materials. Results of the analyses qualitatively agreed well with the observed oxide cracking and spallation behavior of both the MA956 and the Sapphire/MA956 composite coupons.

Loading Rate and Test Temperature Effects on Fracture of In Situ Niobium Silicide-Niobium Composites
JOSEPH D. RIGNEY and JOHN J. LEWANDOWSKI
Arc cast, extruded, and heat-treated in situ composites of niobium silicide (Nb₅Si₃) intermetallic with niobium phases (primary-Nb_p and secondary-Nb_s) exhibited high fracture resistance in comparison to monolithic Nb₅Si₃. In toughness tests conducted at 298 K and slow applied loading rates, the fracture process proceeded by the microcracking of the Nb₅Si₃ and plastic deformation of the Nb_p and Nb_s phases, producing resistance-curve behavior and toughnesses of 28 MPa with damage zone lengths less than 500 µm. The effects of changes in the Nb_p yield strength and fracture behavior on the measured toughnesses were investigated by varying the loading rates during fracture tests at both 77 and 298 K. Quantitative fractography was utilized to completely characterize each fracture surface created at 298 K in order to determine the type of fracture mode (i.e., dimpled, cleavage) exhibited by the Nb_p. Specimens tested at either higher loading rates or lower test temperatures consistently exhibited a greater amount of cleavage fracture in the Nb_p, while the Nb_s always remained ductile. However, the fracture toughness values determined from experiments spanning six orders of magnitude in loading rate at 298 and 77 K exhibited little variation, even under conditions when the majority of Nb_p phases failed by cleavage at 77 K. The changes in fracture mode with increasing loading rate and/or decreasing test temperature and their effects on fracture toughness are rationalized by comparison to existing theoretical models.

Influence of Particle Size and Volume Percent of Flaky Mo Particles on the Mechanical Properties of Al₂O₃/Mo Composites
Y. WAKU, M. SUZUKI, Y. ODA, and Y. KOHTOKU
The influence of particle size and volume percent of Mo particles on flake-forming behavior of Mo powders during a ball milling process and three-point flexural strength and fracture toughness of Al₂O₃ composites reinforced with flaky Mo particles have been investigated. The flake-forming behavior of Mo powders mixed with Al₂O₃ powders becomes prominent with increasing Mo particle size, while remaining almost independent of Mo volume percent. The microstructure of the composites reinforced with flaky Mo particles is anisotropic, depending on the arrangement of these Mo particles in the Al₂O₃ matrix. The microdispersion of flaky Mo particles contributes remarkably to an increase in both flexural strength and fracture toughness. The flexural strength increases with decreasing Mo particle size, while the fracture toughness increases with increasing Mo particle size, which corresponds to an increase of the flake-forming tendency of these particles. Furthermore, the flexural strength and fracture toughness can be simultaneously improved by increasing the volume fraction of flaky Mo particles. The microstructural observations indicate that the improvement in strength may be attributed to a grain-refining effect due to inhibition of grain growth of the matrix by the presence of Mo particles. In addition, the improvement in fracture toughness may be due to plastic deformation of Mo particles at a crack tip, which is accelerated more by the flaky rather than the small spherical shape.

Structure of Phases in the -Al₂O₃ Fiber Studied by Convergent Beam Electron Diffraction
SHUNCAI WANG and H.J. DUDEK
The phases in the -Al₂O₃ fibers were investigated using the methods of transmission electron microscopy (TEM): convergent beam electron diffraction (CBED) and high-resolution electron microscopy (HREM). A phase '-Al₂O₃ discovered previously by Vewerly in oxide layers with an fcc structure was found and new atomic positions are proposed. A new structure of -Al₂O₃ was also observed. It has a Pmma space group and lattice parameters of a = 2a, b = 1.5a, and c a'. The correlation of the observed Al₂O₃ lattices to the spinel lattice is discussed and translation of atom positions during the ' transformation is studied. All anions must change their positions by a small amount; one-third of the cation positions in ' and more than 90 pct of cation positions in experience a large translation during that transformation. This implies that for the ' transformation, the positions of cations in both lattices are important. The results are discussed in relation to the fiber-matrix interaction under spinel formation during thermal loading of -Al₂O₃-fiber-reinforced aluminum piston alloys.