METALLURGICAL AND MATERIALS TRANSACTIONS A
ABSTRACTS
Volume 29A, No. 1, January 1998

This Month Featuring: Symposium on Fundamentals of Gamma Titanium Aluminides: Part I; Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Environment; Welding & Joining; Surface Treatment; Solidification; Materials Processing. View January 1998 Contents.

SYMPOSIUM ON FUNDAMENTALS OF GAMMA TITANIUM ALUMINIDES: PART I

Phase Transformation Behavior of Gamma Titanium Aluminide Alloys during Supertransus Heat Treatment
S.L. SEMIATIN, V. SEETHARAMAN, D.M. DIMIDUK, and K.H.G. ASHBEE
Microstructure evolution in a wrought near-gamma titanium alloy, Ti-45Al-2Cr-2Nb, was investigated by a series of heat treatments comprised of initial heating high in the alpha-plus-gamma phase field followed by short-time heating in the single-phase alpha field. The initial heating step led to a dispersion of gamma particles which pinned the alpha grain boundaries. The kinetics of the gamma grain dissolution during subsequent heating in the single-phase field were interpreted in terms of models for both interface reaction-controlled and diffusion-controlled processes. The model for diffusion-controlled dissolution yielded predictions comparable to the observed times, whereas the model for interface reaction-controlled behavior predicted dissolution kinetics over an order of magnitude slower than observed. The growth of the alpha grains, both before and after the dissolution of the gamma phase, was also modeled. Section size limitations to the ability to use supertransus heating to obtain uniform and moderately fine alpha grain sizes were examined using the transformation models and a simple heat transfer analysis approach. The results were validated through the heat treatment of subscale and full-scale forgings.

Microstructure Evolution through the Phase Transformation in a Ti-48 At. Pct Al Alloy
TATSUO KUMAGAI, EIJI ABE, and MORIHIKO NAKAMURA
The phase transformation during rapid quenching and subsequent isothermal aging has been investigated in a Ti-48 at pct Al alloy. The microstructure changes from a completely massively transformed -grain structure to a mixed microstructure of the massively transformed grains and the untransformed (meaning massively untransformed) fine 2/ lamellae with an increase in the cooling rate from the high-temperature phase field. Fine grains are generated from these fine 2/ lamellae by subsequent aging at 1323 K. The fine grains contain many defects, such as dislocations, microtwins (or stacking faults), domain boundaries, and variants, which are frequently observed in the massive grains. This result suggests that the formation mechanism of the fine grains during aging is similar to that of the massive grains. When the fine / lamellar sample, which is formed by preliminary aging at a lower temperature (1173 K), is aged at a higher temperature (1323 K), apparent changes in microstructure could not be recognized. This result indicates that the fine -grain formation is closely related to the 2 phase transformation in the fine 2/ lamellae.

Supertransus Processing of TiAl-Based Alloys
G.E. FUCHS
Fine grained lamellar microstructures would be expected to exhibit high strength, creep resistance, fracture toughness, and moderate ductility. High-temperature extrusion was used to produce fine-grained lamellar microstructures in both ingot metallurgy (I/M) and powder metallurgy (P/M) Ti-48Al-2Nb-2Cr alloys. The effect of processing parameters, such as extrusion temperature and cooling rate, on the microstructure and properties was determined. In addition, the thermal stability of the microstructure was evaluated by subsequent heat treatments. Although fine-grained lamellar microstructures were generated in both ingot and powder metallurgy materials, processing had a significant effect on the microstructure and properties of the resultant materials.

The Role of Grain Size and Selected Microstructural Parameters in Strengthening Fully Lamellar TiAl Alloys
DENNIS M. DIMIDUK, PETER M. HAZZLEDINE, TRIPLICANE A. PARTHASARATHY, SRIRAM SESHAGIRI, and MADAN G. MENDIRATTA
More than 5 years ago, wrought processing was first used to produce fully lamellar (FL) microstructures in TiAl alloys having grain sizes less than 400 µm. These alloys exhibit an improvement in overall balance of properties, especially at high temperatures. More recently, such microstructural forms led to exceptional yield strengths (500 to 1000 MPa at low temperatures) while maintaining attractive high-temperature properties. The improvements appeared to be related to an unusually high apparent sensitivity of strength to grain size. Studies reported an apparent value for the slope of the Hall-Petch (HP) plot approaching 5 MPa for FL gamma alloys, while that for single-phase or duplex microstructures is near unity. The present investigations examine the slope of the HP plot for FL microstructures, paying particular attention to the lamellar microstructural variables. Results show that the 2 lamellar thickness and spacing and the lamellar thickness can vary over more than two orders of magnitude with typical process methods. These spacings influence the value of k in the HP (grain size) relationship. Since they often change concomitantly with grain size in processing, they can give rise to a large scatter in the HP plot. The investigations also examine the flow behavior, glide barriers, and slip multiplicity for polysynthetically twinned (PST) crystals (the single-grain analogue of FL material), and then map this behavior into an explanation of the yield behavior of high-strength FL gamma alloys.

Physical Constants, Deformation Twinning, and Microcracking of Titanium Aluminides
M.H. YOO and C.L. FU
Physical properties that are relevant to mechanical behavior of single-phase TiAl and Ti3Al and two- phase TiAl/Ti3Al alloys are summarized. By using planar-fault energies and temperature-dependent elastic constants, dislocation dissociation reactions applicable to twin formation in TiAl are analyzed, and a pole mechanism based on a jogged [110]/2 ordinary dislocation is proposed to explain the available experimental data on deformation twinning in -TiAl single crystals. The strong plastic anisotropy reported in TiAl polysynthetically twinned (PST) crystals is attributed in part to the localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. The experimental findings reported on cleavage habit planes of PST crystals are discussed in terms of the calculated ideal work of adhesion and possible extrinsic factors.

Tension and Compression Testing of Single-Crystalline Gamma Ti-55.5 Pct Al
MARC ZUPAN and K.J. HEMKER
High-quality single crystals 6 to 10 mm in diameter of -Ti 55.5 pct Al have been grown using the optical float zone furnace technique. These crystals have been oriented and cut into microsample tension and compression specimens with a gage area of 250 x 250 µm and an effective gage length of 300 µm. These specimens have been deformed using a microsample testing machine which applies loads on the order of 50 N and measures strain using an interferometric strain/displacement gage. Stress-strain curves have been obtained for four different orientations and two temperatures and as a function of the sense of the applied load. Of special interest is the availability of tensile data for the resolved shear stress. Preliminary comparison of tension and compression microsample tests indicates that the tension-compression asymmetry is negligible at 500 K.

Fundamental Aspects of Fatigue and Fracture in a TiAl Sheet Alloy
K.S. CHAN and D.S. SHIH
The fatigue mechanisms in a TiAl sheet alloy, heat treated to the lamellar and equiaxed microstructures, were studied to determine the effects of microstructure on the initiation of microcracks and their subsequent growth into large cracks. The nucleation and growth history of individual microcracks were followed. For comparison, fatigue crack growth and fracture toughness were also characterized using specimens containing a machined notch with a fatigue precrack. The results indicated that microcracks initiated at grain/colony boundaries and at slip bands. Most microcracks were arrested after nucleation, but a few grew at stress intensity ranges below the large crack threshold. The populations of nonpropagating and propagating cracks varied with life fractions. Ligaments in the wake of a fatigue crack were more severely strained than the crack-tip region of the main crack, and, as a result, they were more prone to fatigue failure. The destruction of the crack-wake ligaments is expected to result in lower fracture resistance in materials under cyclic loading than those under monotonic loading.

Changes in Microstructure during Primary Creep of a Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si Alloy
D.Y. SEO, T.R. BIELER, S.U. AN, and D.E. LARSEN
Cast gamma titanium aluminides are gaining acceptance as potential replacements for superalloy and steel components in many applications. One particular alloy with W, Mo, and Si additions has shown exceptional primary creep resistance. Quantitative microscopic comparisons were made between microstructures in undeformed and deformed regions in creep specimens deformed to strains between 0.1 and 1.5 pct strain, using optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM) techniques. As-hot isostatically pressed ("hipped") and heat-treated (1010°C for 50 hours) conditions were compared. The as-hipped specimen had a higher lamellar volume fraction, and it crept more than 100 times faster. The lamellar spacing in the lamellar grains systematically decreased by 15 to 35 pct, with increasing stress, during the first 0.1 to 2 pct strain. Precipitates containing W, Mo, and/or Si were observed in the deformed gage and undeformed grip sections of the heat-treated specimens. Precipitation is nucleated by heat treatment, but, during creep deformation, a more homogeneous and faster growth process occurs in the gage section than in the aged but undeformed grip section. The gage section had a 35 pct higher precipitate volume fraction, but their average size was smaller. A lower volume fraction of lamellar grains and the presence of precipitates account for the excellent creep resistance in the heat-treated alloy.

Microstructural Evolution during Creep of Single-Phase Gamma TiAl
MIN LU and K.J. HEMKER
Mechanical experiments and transmission electron microscope (TEM) observations indicate that single-phase -TiAl does exhibit primary, secondary, and inverse creep, but not steady-state creep. Constant stress creep tests of -Ti-51Al-2Mn conducted at 550°C, 597°C, and 703°C have been interrupted at different stages in the creep process. The TEM observations of these specimens were used to document the microstructural evolution that occurs during creep. Superdislocation motion was activated and subsequently exhausted during primary creep. Ordinary dislocations were observed to be pinned during primary creep, but with time, these dislocations began to bow past their pinning points. The extended region of inverse creep has been related to the bowing and multiplication of these ordinary dislocations. Quantitative measurements of dislocation density were performed, and while the density of superdislocations remained constant, the density of ordinary dislocations increased by an order of magnitude during the life of a creep test. The acceleration in the creep rate has been related to this increase in the density of ordinary dislocations, but the change in dislocation density was not high enough to account for the increase in the creep rate. This suggests that both the mobility and density of ordinary dislocations increase as creep progresses.

Development of Ultrafine Lamellar Structures in Two-Phase -TiAl Alloys
P.J. MAZIASZ and C.T. LIU
Processing of two-phase -TiAl alloys (Ti-47Al-2Cr-2Nb, or minor modifications thereof) above the -transus temperature (T) produced unique refined-colony/ultrafine lamellar structures in both powder- and ingot-metallurgy (PM and IM, respectively) alloys. These ultrafine lamellar structures consist of fine laths of the and 2 phases, with average interlamellar spacings (L) of 100 to 200 nm and 2-2 spacings () of 200 to 500 nm, and are dominated by /2 interfaces. This characteristic microstructure forms by extruding PM Ti-47Al-2Cr-2Nb alloys at 1400°C and also forms with finer colony size but slightly coarser, fully lamellar structures by hot-extruding similar IM alloys. Alloying additions of B and W refine L and in both IM Ti-47Al (cast and heat treated at 1400°C) and IM Ti-47Al-2Cr-2Nb alloys (extruded at 1400°C). The ultrafine lamellar structure in the PM alloy remains stable during heat treatment at 900°C for 2 hours but becomes unstable after 4 hours at 982°C; the ultrafine lamellar structure remains relatively stable after aging for >5000 hours at 800°C. Additions of B + W dramatically improve the coarsening resistance of L and in the IM Ti-47Al alloys aged for 168 hours at 1000°C. In both the PM and IM Ti-47Al-2Cr-2Nb alloys, these refined-colony/ultrafine lamellar structures correlate with high strength and good ductility at room temperature, and very good strength at high temperatures. While refining the colony size improves the room-temperature ductility, alloys with finer L are stronger at both room and high temperatures. Additions of B + W produce finer as-processed L and in IM TiAl alloys and stabilize such structures during heat treatment or aging.

ALLOY PHASES

Ternary Alloying Study of MoSi2
DANQING YI, ZONGHE LAI, CHANGHAI LI, O.M. AKSELSEN, and J.H. ULVENSOEN
Ternary alloying of MoSi2 with adding a series of transition elements was investigated by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Iron, Co, Ni, Cr, V, Ti, and Nb were chosen as alloying elements according to the AB2 structure map or the atomic size factor. The studied MoSi2 base alloys were prepared by the arc melting process from high-purity metals. The EDS analysis showed that Fe, Co, and Ni have no solid solubility in as-cast MoSi2, while Cr, V, Ti, and Nb exhibit limited solid solubilities, which were determined to be 1.4 ± 0.7, 1.4 ± 0.4, 0.4 ± 0.1, and 0.8 ± 0.1. Microstructural characterization indicated that Mo-Si-MVIII (MVIII = Fe, Co, Ni) and Mo-Si-Cr alloys have a two-phase as-cast microstructure, i.e., MoSi2 matrix and the second-phase FeSi2, CoSi, NiSi2, and CrSi2, respectively. In as-cast Mo-Si-V, Mo-Si-Ti, and Mo-Si-Nb alloys, besides MoSi2 and C40 phases, the third phases were observed, which have been identified to be (Mo, V)5Si3, TiSi2, and (Mo, Nb)5Si3.

TRANSFORMATIONS

Growth of Semicoherent TiFe Nanoparticles in -Ti Matrix in the Ti73Fe27 Rapidly Quenched Ribbon
C.-L. LEE, B. RADOJEVIC, F.-R. CHEN, and T.-P. PERNG
A thin Ti73Fe27 ribbon was prepared by rapid quenching from the melt. The as-quenched ribbon was in a metastable condition with a small amount of nanoparticles of TiFe, of a size of 138 ± 31 nm, embedded in the -Ti matrix. The -Ti matrix was supersaturated with Fe, and the fraction of the matrix was higher than that in the equilibrium state. High-resolution imaging of the interface of the TiFe/-Ti showed that a periodic array of dislocations were present in the interface to accommodate the lattice mismatch. The spacing between the dislocations in the as-quenched specimen was 5.0 ± 0.5 nm. When the ribbon was heated to 700°C, growth of the TiFe nanoparticles to a size of 228 ± 37 nm took place in the -Ti matrix. The amount of -Ti was reduced, as well as the Fe content in -Ti. The interface between the TiFe and -Ti remained semicoherent, except that the spacing between the interfacial dislocations was reduced to 3.5 ± 0.6 nm. The persistence of the semicoherent interface was ascribed to the same crystal structure and close lattice parameters shared by TiFe and -Ti. The growth kinetics of the TiFe nanoparticles during heating was examined based on the modified theory of isothermal heating. It can be considered to be controlled by the diffusion of Fe atoms in the -Ti matrix to the TiFe phase. Prolonged heating of the ribbon below the eutectoid temperature led to partial transformation of -Ti to -Ti.

Stress-Assisted Transformation in Ti-60 Wt Pct Ta Alloys
R.W. MARGEVICIUS and J.D. COTTON
An investigation of the influence of processing variables on mechanical properties and phase development for a Ti-60 wt pct Ta (Ti-28.5 at. pct Ta) alloy was conducted. The alloy was hot-rolled, subjected to heat-treatment temperatures above the (bcc) transus (1 hour at 700°C, 800°C, or 900°C), and water quenched. All heat treatments produced a combination of metastable (bcc) and metastable " (orthorhombic martensite), with the amount of retained essentially independent of heat treatment, ranging from 20 to 33 vol pct. Deformation of as-rolled and heat-treated tension specimens showed an anomalous leveling of the stress-strain curve in the stress-strain curves at low strains. X-ray diffraction (both simple 2 diffractometry and texture analysis) on both deformed and undeformed material determined that the leveling of the stress-strain curve was a result of the " martensitic transformation. The stress required to initiate the transformation increased with prequench temperature. This was determined to be due to the presence of athermal . Grain growth kinetics have been determined in the course of this work.

A Correlation Method for Determination of Crystallization Mechanism and Activation Energy of Amorphous Alloy
DEJIU SHEN, YULIN WANG, LIYANG LI, GUANZHONG XING, and ZHONGYI SHEN
A correlation method is advanced for determining the crystallization mechanism and activation energy of amorphous alloys by investigating the crystallization rate of amorphous Zr70Cu30 alloy under high pressure. The results show that this method is much closer to phase transformation reality than the Kissinger method. In principle, this method also should be applicable to kinetics of common solid-state phase transformation.

TRANSPORT PHENOMENA

In-Situ Nuclear Magnetic Resonance Investigation of Strain, Temperature, and Strain-Rate Variations of Deformation-Induced Vacancy Concentration in Aluminum
K. LINGA MURTY, K. DETEMPLE, O. KANERT, and J.Th.M. DEHOSSON
Critical strain to serrated flow in solid solution alloys exhibiting dynamic strain aging (DSA) or Portevin-LeChatelier effect is due to the strain-induced vacancy production. Nuclear magnetic resonance (NMR) techniques can be used to monitor in situ the dynamical behavior of point and line defects in materials during deformation, and these techniques are nondestructive and noninvasive. The new CUT-sequence pulse method allowed an accurate evaluation of the strain-enhanced vacancy diffusion and, thus, the excess vacancy concentration during deformation as a function of strain, strain rate, and temperature. Due to skin effect problems in metals at high frequencies, thin foils of Al were used and experimental results correlated with models based on vacancy production through mechanical work (vs thermal jogs), while in situ annealing of excess vacancies is noted at high temperatures. These correlations made it feasible to obtain explicit dependencies of the strain-induced vacancy concentration on test variables such as the strain, strain rate, and temperature. These studies clearly reveal the power and utility of these NMR techniques in the determination of deformation-induced vacancies in situ in a noninvasive fashion.

MECHANICAL BEHAVIOR

New Grain Formation during Warm Deformation of Ferritic Stainless Steel
ANDREY BELYAKOV, RUSTAM KAIBYSHEV, and TAKU SAKAI
Microstructural evolution accompanied by localization of plastic flow was studied in compression of a ferritic stainless steel with high stacking fault energy (SFE) at 873 K (0.5 Tm). The structure evolution is characterized by the formation of dense dislocation walls at low strains and subsequently of microbands and their clusters at moderate strains, followed by the evolution of fragmented structure inside the clusters of microbands at high strains. The misorientations of the fragmented boundaries and the fraction of high-angle grain boundaries increase substantially with increasing strain. Finally, further straining leads to the formation of new fine grains with high-angle boundaries, which become more equiaxed than the previous fragmented structure. The mechanisms operating during such structure changes are discussed in detail.

Age Hardening and the Potential for Superplasticity in a Fine-Grained Al-Mg-Li-Zr Alloy
MINORU FURUKAWA, PATRICK B. BERBON, ZENJI HORITA, MINORU NEMOTO, NIKOLAI K. TSENEV, RUSLAN Z. VALIEV, and TERENCE G. LANGDON
Experiments were conducted to determine the age-hardening characteristics and the mechanical properties of an Al-5.5 pct Mg-2.2 pct Li-0.12 pct Zr alloy processed by equal-channel angular (ECA) pressing to give a very fine grain size of ~1.2 µm. The results show that peak aging occurs more rapidly when the grain size is very fine, and this effect is interpreted in terms of the higher volume of precipitate-free zones in the fine-grained material. Mechanical testing demonstrates that the ECA-pressed material exhibits high strength and good ductility at room temperature compared to conventional Al alloys containing Li. Elongations of up to ~550 pct may be achieved at an elevated temperature of 603 K in the ECA-pressed condition, thereby confirming that, in this condition, the alloy may be a suitable candidate material for use in superplastic forming operations.

Plastic Instability during Creep Deformation of a NiAl-Hf Single-Crystal Alloy--A Case Study
A. GARG, S.V. RAJ, R.D. NOEBE, M.V. NATHAL, and R. DAROLIA
Tensile samples from NiAl-Hf single crystals, having the same nominal composition and heat treated and creep tested under identical conditions at 1144 K, were found to exhibit very different rupture lives and creep ductilities. A case study was conducted on two samples with creep rupture lives of 343.6 and 37.0 hours (with corresponding creep ductilities of 12.3 and 39.9 pct, respectively) in order to find the causes of such a large variation in creep properties. Detailed microstructural analyses using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) indicated that the sample with higher rupture life and lower ductility had deformed uniformly along the length of the gage section, whereas the sample with lower rupture life and higher ductility (sample L) deformed by localized plastic deformation resulting in shear failure. This shear failure was due to a plastic instability in sample L which was caused by the presence of a high density of large Hf-rich interdendritic particles that were formed during casting of the single-crystal ingot but did not go into solution during the homogenization heat treatment. The role of these particles in causing nonuniform deformation, which led to strain localization and a premature failure in sample L, has been described in detail.

A Model for Microstructure Evolution in Adiabatic Shear Bands
JOY A. HINES, KENNETH S. VECCHIO, and SAID AHZI
A mechanical subgrain rotation model is proposed to account for the recrystallized grains which have been observed to form in adiabatic shear bands in a number of materials. The model is based on a "bicrystal" approach using crystal plasticity theory to predict the evolution of subgrain misorientations. These mechanically induced rotations are shown to occur at the high strain rate associated with adiabatic shear band formation. Recrystallized grain formation is proposed to occur by the formation and mechanical rotation of subgrains during deformation, coupled with boundary refinement via diffusion during shear band cooling. This model is referred to as progressive subgrain misorientation recrystallization and appears to account for shear band microstructures in a variety of metals.

Cutting Performance and Microstructure of High Speed Steels: Contributions of Matrix Strengthening and Undissolved Carbides
S. KARAGÖZ and H.F. FISCHMEISTER
While it is accepted that both the hot strength of the matrix and the amount of undissolved carbides are important for the cutting performance of high speed steels, the relative weights of their contributions are unknown. In this work, they are separately identified and a model is presented that provides a quantitative prediction of tool life (solely in uninterrupted cutting) on the basis of microstructural and compositional data over a wide range of alloy compositions and cutting speeds. The model seems to describe the individual contributions to tool life well enough to serve as a guide in alloy development. The model has been developed using 13 different steels, spanning the entire range of customary compositions. It is based on the following parameters: volume fractions and compositions of undissolved carbides; precipitates formed during tempering (secondary hardening) and during operation (tertiary precipitates); and, finally, residual solute in the matrix. Tool life is modeled as a linear combination of contributions from the undissolved carbides and from the precipitate population, including a contribution due to the action of Co, and with an additional term due to solute strengthening of the matrix. The weight factors are determined by multiple linear regression analysis. They reflect the relative importance of each contributing factor, and their dependence on cutting speed can be interpreted in terms of the change in operative wear mechanism with tool temperature.

Quantitative Analysis on Boundary Sliding and Its Accommodation Mode during Superplastic Deformation of Two-Phase Ti-6Al-4V Alloy
JI SIK KIM, YOUNG WON CHANG, and CHONG SOO LEE
A study has been made to investigate boundary sliding and its accommodation mode with respect to the variation of grain size and / volume fraction during superplastic deformation of a two-phase Ti-6Al-4V alloy. A load relaxation test has been performed at 600°C and 800°C to obtain the flow stress curves and to analyze the deformation characteristics by the theory of inelastic deformation. The results show that grain matrix deformation (GMD) is found to be dominant at 600°C and is well described by the plastic state equation. Whereas, at 800°C, phase/grain boundary sliding (P/GBS) becomes dominant and is fitted well with the viscous flow equation. The accommodation mode for fine-grained microstructures (3 µm) well agrees with the isostress model, while that for large-grained structures (11 µm) is a mixed mode of the isostress and isostrain-rate models. The sliding resistance analyzed for the different boundaries is lowest in the / boundary, and increases on the order of / << / /, which plays an important role in controlling the superplasticity of the alloys with various / phase ratios.

Correlation of Dynamic Torsional Properties with Adiabatic Shear Banding Behavior in Ballistically Impacted Aluminum-Lithium Alloys
CHANG GIL LEE and SUNGHAK LEE
The present study is concerned with a correlation between dynamic deformation properties obtained from the dynamic Kolsky bar test with the adiabatic shear banding behavior developed in Al-Li alloys upon ballistic impact, and then with the ballistic performance. The selected materials were a 2090 Al-Li alloy, a WELDALITE 049 alloy, and a 7039 Al alloy, to allow a comparative study of different strengths and microstructures. After the ballistic impact testing, the amount and the distribution of adiabatic shear bands were examined using optical and scanning electron microscopes. In the front side of the impacted area, many thin delaminated sheets and a large amount of fragmentation were observed in the 2090 alloy and the WELDALITE alloy, respectively. Near the impacted region, a large amount of plastic flow also existed, and adiabatic shear bands were hardly observed in the 2090 and the WELDALITE alloys, whereas they easily formed in the 7039 alloy. Since adiabatic shear bands usually deteriorate the impact resistance of target materials, the ballistic performance of each alloy was discussed by comparing the adiabatic shear banding behavior with microstructure, strength level, and dynamic torsional properties.

Grain Size Estimation in Anisotropic Materials
BILLY RAY MORRIS, ARUN M. GOKHALE, and GEORGE F. VANDER VOORT
The need for an efficient sampling technique for estimation of grain size in anisotropic materials is addressed through statistical analysis of quantitative metallographic data on a series of iron specimens cold rolled to different extents. The data are utilized to arrive at the most efficient procedure for measuring grain size in anisotropic materials. The analysis reveals that, for cold-rolled materials, design-based intersection counting on a single longitudinal metallographic plane is sufficient to reliably and efficiently estimate the average grain size.

Effect of Phase Morphology on Fatigue Crack Growth Behavior of - Titanium Alloy--A Crack Closure Rationale
VIKAS KUMAR SAXENA and V.M. RADHAKRISHNAN
Effect of phase morphology on fatigue crack growth (FCG) resistance has been investigated in the case of an - titanium alloy. Fatigue crack growth tests with on-line crack closure measurements are performed in the microstructures varying in primary (elongated/equiaxed/Widmanstätten) and matrix (transformed/metastable) phase morphologies. The microstructures comprising metastable matrix are observed to yield higher FCG resistance than those for transformed matrix, irrespective of primary phase morphology (equiaxed or elongated). But, the effect of primary phase morphology is dictated by the type of phase (transformed or metastable) matrix. It is observed that in the microstructures with metastable matrix, the equiaxed primary as second phase possesses higher FCG resistance as compared to that of elongated morphology. The trend is reversed if the metastable matrix is replaced by transformed phase. The fatigue crack path profiles are observed to be highly faceted. The detailed fractographic investigations revealed that tortuosity is introduced as a result of cleavage in or or in both the phases, depending upon the microstructure. The crack closure concept has been invoked to rationalize the phase morphology effects on fatigue crack growth behavior. The roughness-induced and plasticity-induced crack closure appear to be the main mechanisms governing crack growth behavior in - titanium alloy.

Tensile Properties and Fracture Toughness of a Ti-45Al-1.6Mn Alloy at Loading Velocities of up to 12 m/s
Z.M. SUN, T. KOBAYASHI, H. FUKUMASU, I. YAMAMOTO, and K. SHIBUE
A -base TiAl alloy with duplex microstructure of lamellar colonies and equiaxed grains was prepared with a reactive sintering method. Tensile tests and fracture toughness tests at loading velocities up to 12 m/s (strain rate for tensile tests up to 3.2 x 102/s) were carried out. The microstructure of the alloy before and after tensile deformation was carefully examined with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The fractography of the tensile specimens and fracture toughness specimens was studied. The experimental results demonstrated that the ultimate tensile strength (UTS) and yield strength (YS) increase with increasing strain rate up to 10/s and subsequently level off. The UTS and YS exhibited similar strain rate sensitivity. The strain rate sensitivity exponent at strain rates lower than 10/s is about 1.5 x 10-2 and at higher strain rates is almost zero. In this study, fracture toughness was found to be less sensitive to the loading velocity, having values of around 25 MPa , which is believed to be attributed to the high strain rate experienced at the crack tip. The predominant deformation mechanism for the strain rates used in this study was found to be twinning. However, in the low strain rate range, the dislocation motion mechanism was operative at the initial deformation stage and twinning dominated the later stage of the deformation process. In the high strain rate range, the entire deformation process was dominated by twinning. The interaction between deformation twinning and grain boundaries resulted in intergranular fracture in the grains and delamination of 2/ interfaces in the lamellar colonies.

Microstructures Controlling the Ductile Crack Growth Resistance of Low Carbon Steels
HIROSHI YOSHIDA and MICHIHIKO NAGUMO
Microstructures controlling the ductile crack growth resistance in the ductile-brittle fracture transition region have been investigated with three low carbon low alloy steels, which showed characteristic differences in the R curves. The crack growth resistance is related to both the primary dimple morphology and the total length of local shear zones appearing on the fracture surface; the latter contribution predominates over the former. The heterogeneity of the microstructures, which constrains slip propagation at the grain boundaries, supplies sites for easy void nucleation and induces local shear and the resulting surface roughness.

Effect of Matrix Constitutive Behavior and Inclusions on Forming Limits of Fe-42 Pct Ni Alloy Sheet
NORIO YUKI, ROBERT P. FOLEY, and GEORGE KRAUSS
The effects of matrix constitutive behavior and nonmetallic inclusions on forming limit strains have been examined with five laboratory heats of Fe-42 pct Ni alloy. The inclusion volume fraction was varied between approximately 0.01 and 1.59 pct by proper selection of Mn, S, and O contents. Each ingot was processed into 0.38-mm-thick sheets and heat treated to the recovered or the recrystallized condition. Forming limit strains were obtained from uniaxial tensile, plane-strain tensile, and hydraulic bulge tests by circle grid analysis. In any strain path, the limit strain increased with increasing the strain hardening exponent, n. The forming limit strains on the right-hand side of the forming limit diagram (FLD) decreased with inclusion volume fraction, while no effects of the inclusion volume fraction were observed on the left-hand side. A decrease in the slope of the FLD on the right-hand side due to an increase in the inclusion volume fraction qualitatively agrees with the theoretical calculation by Graf and Hosford, which was conducted on the basis of the Marciniak-Kuczynski (M-K) theory.

Creep and Rupture Properties of an Austenitic Fe-30Mn-9Al-1C Alloy
S.M. ZHU and S.C. TJONG
The creep deformation behavior and rupture properties of as-quenched austenitic Fe-30Mn-9Al-1C alloy have been studied at 923, 948, and 973 K under applied stresses ranging from 50 to 350 MPa. The creep curves of the alloy exhibited an extended tertiary stage prior to failure. The stress and temperature dependencies of the minimum creep rate indicated two regimes of creep deformation as well as a transition from creep to power-law breakdown. These two regimes of creep deformation were identified as a low-stress creep regime having an activation energy of 140 kJ/mol and a stress exponent of about 1, and a power-law creep regime having an activation energy of 350 kJ/mol and a stress exponent of about 6. Transmission electron microscope (TEM) observations of the deformed specimens revealed that a low density of dislocations, coarse dislocation networks, and profuse slip bands were developed in the low stress, power law, and power-law breakdown regimes, respectively. Optical microscope and scanning electron microscope (SEM) observations of the ruptured specimens showed that creep cavitation shifted from round-type in the low-stress creep regime to wedge-type in the power-law breakdown regime. The observed creep and rupture characteristics of the alloy are interpreted in terms of creep mechanisms, which involve the Coble creep and dislocation climb creep.

Communication: Effects of Co and Ni on Secondary Hardening and Fracture Behavior of Martensitic Steels Bearing W and Cr
H. KWON, C.M. KIM, K.B. LEE, H.R. YANG, and J.H. LEE

Communication: Effect of Small Loads on Crack Growth Rate and Crack Tip Deformation in the Fatigue Process of A537 Steel
XUEJUN WEI, JIN LI, JINGWEI CHEN, and WEI KE

ENVIRONMENT

Hydrogen Uptake in Titanium Aluminides Covered with Oxide Layers
AKITO TAKASAKI, YOSHIO FURUYA, and YOUJI TANEDA
Two-phase (Ti3Al and TiAl) Ti-45Al and Ti-50Al (at. pct) alloys, which were preoxidized in static air at 1098 K (825°C) for times between 900 seconds (0.25 hours) and 14.4 ks (4 hours), were charged thermally at 773 K (500°C) with flowing hydrogen gas at a pressure of 0.1 MPa for 72 ks (20 hours), and the effect of oxide layers on hydrogen penetration (or occlusion) in the alloys was investigated by thermal desorption spectroscopy (TDS). The TDS main peak (accelerated hydrogen evolution) temperature increased with an increasing thickness of oxide layers for both alloys, which results in a diffusion of hydrogen through oxide layers. The onset temperature of hydrogen evolution showed the highest values for the alloys with thinner oxide layers and then decreased with increasing thickness of oxide layers, due to hydrogen trapping at the oxide surface. Total hydrogen uptake was the lowest for both of the alloys with the thinnest oxide layers and then increased with an increasing thickness of oxide layers. The thinnest oxide layer on the Ti-45Al alloy (about 600 nm) avoided about 97 pct of the hydrogen occluded in the alloy without an oxide layer, whereas that on the Ti-50Al alloy (about 300 nm) avoided about 83 pct. Titanium oxide (TiO2) was unstable and might be reduced to -titanium during heating (TDS analysis) at a vacuum level of 10-6 Pa, whereas aluminum oxide (Al2O3) did not change its chemical form.

WELDING & JOINING

Isothermal Solidification Kinetics of Diffusion Brazing
W.D. MacDONALD and T.W. EAGAR
Diffusion brazing (DB) is a process that produces interface-free joints that approach the bulk properties of the material that is to be joined. Solid-state diffusion of the melting point depressant (MPD) element into the joint metal causes the solid/liquid (S/L) interface to advance until the joint is solidified. The time required to complete this isothermal solidification stage was modeled using a moving boundary analysis. Precision measurements of the interlayer width as a function of time were made on the copper-silver system. The bonding apparatus consisted of a suspended load feedback system that prevented leakage of joint metal due to extrusion or surface wetting, thus preserving mass balance. The interlayer width vs bonding time measurements obtained were found to be within a factor of 4 of the theoretical model. The difference was ascribed to the difficulty in accurately measuring the actual liquid width and loss of silver due to vaporization. Large spherical protrusions grow during bonding, which further roughens the interface. However, experimental and predicted concentration profiles of silver were found to be in complete agreement. As the liquid remaining in the grain boundary grooves is diffused away, porosity can develop due to volume changes. By holding the joint at temperature for extended periods, complete solidification followed by homogenization will occur. Selecting the appropriate bonding temperature to achieve a specified maximum concentration of braze metal at the joint is dependent on both the isothermal solidification and homogenization kinetics.

SURFACE TREATMENT

Titanium Preconditioning of Al2O3 for Liquid-State Processing of Al-Al2O3 Composite Materials
D.A. JAVERNICK, P.R. CHIDAMBARAM, and G.R. EDWARDS
Chemical vapor deposition (CVD) of titanium onto the surface of alumina substrates was used to improve the wetting by molten aluminum prior to infiltration. The CVD titanium coatings on alumina substrates were characterized by X-ray diffraction and energy-dispersive X-ray spectroscopy (EDS) as dual-phase, elemental titanium and titanium aluminide, Ti3Al. The kinetics of the coating process were quantified and analyzed using established reaction kinetics models. An overall activation energy of 219 kJ/mole was calculated for the formation of the titanium coatings on planar alumina substrates, for the temperature range of 610°C to 870°C. The CVD-coated planar and porous alumina substrates were immersed in molten aluminum, and wetting of planar substrates and infiltration of porous bodies were documented. Formation of both aluminum-rich and titanium-rich reaction products was observed through EDS cross-sectional analysis.

SOLIDIFICATION

Prevention of Macrosegregation in Squeeze Casting of an Al-4.5 Wt Pct Cu Alloy
C.P. HONG, H.F. SHEN, and I.S. CHO
The squeeze casting of an Al-4.5 wt pct Cu alloy was carried out to investigate the conditions for the formation and the prevention of macrosegregation. The effects of the process parameters, applied pressure, die temperature, pouring temperature, delay time, and humidity on the formation of macrosegregation were investigated in correlation with the evolution of macrostructure and shrinkage defects. Two critical applied pressures were defined: one is the critical applied pressure, PSC, under which shrinkage defects form, and the other is the critical applied pressure, PMS, above which macrosegregates form in the squeeze castings. A quantitative diagram describing the optimum process conditions was proposed for obtaining sound squeeze castings. It was found that the pouring temperature, the die temperature, the delay time, and the humidity are closely related to the two critical applied pressures PSC and PMS, in different manners. It was concluded that sound castings without macrosegregation and shrinkage defects can only be obtained when the applied pressure is in the range of PSC < P < PMS.

An Analytical Solution of the Critical Interface Velocity for the Encapturing of Insoluble Particles by a Moving Solid/Liquid Interface
J.K. KIM and P.K. ROHATGI
An analytical model for the particle pushing phenomenon that occurs between spherical particles and advancing curved solid/liquid interfaces during solidification of pure melts is presented. An expression for the critical interface velocity for encapturing particles by moving solid/liquid interfaces has been developed for the steady-state condition. As a first step, the actual shape of the interface behind the particle is computed in terms of the thermal conductivity ratio of the particle to that of the melt and the temperature gradient ahead of the interface; based on assumed subject, the critical interface velocity is calculated using the force balance between the attractive forces and repulsive forces acting on the particle. The critical interface velocity under steady-state conditions in aluminum containing SiC particle (10 µm) comes out to be 5800 µm/s according to the present model; this calculated velocity is much closer to the experimental observations of Wu et al., as compared to the predictions of the models proposed by earlier workers.

The Influence of Temperature Gradient Zone Melting on Microsegregation
T. KRAFT, O. POMPE, and H.E. EXNER
Adding an algorithm for considering temperature gradient zone melting (TGZM) to an existing numerical model for predicting microstructure and microsegregation allows the prediction of migration distances of dendrite arms and asymmetric concentration distributions in the arms. Provided that detailed information on the time dependence of the temperature gradient as well as the cooling rate is available from heat flow calculations, accurate predictions of the type and amount of secondary phases or dendrite arm spacings are possible for cooling conditions at which TGZM is active. Parameter studies are performed to investigate the influence of TGZM for typical temperature gradients (0.01 to 10 K/mm). Sawtoothlike concentration distributions are predicted for high-temperature gradients. A binary Al-6.8 wt pct Cu alloy is solidified unidirectionally and asymmetrical concentration profiles are measured. Considering TGZM in the simulation results in good agreement of model predictions with experimental measurements in the position of the minimum concentration and the asymmetric shape of the concentration profile as well as dendrite arm spacings and amount of second phase.

Microstructural Investigation of a Rapidly Solidified 12Cr-Mo-V Steel
N.H. PRYDS, E. JOHNSON, S. LINDEROTH, and A.S. PEDERSEN
Rapidly solidified martensitic stainless steel (11.59Cr-0.98Mo-0.28V (in wt pct)) ribbons have been produced by the melt-spinning process. The microstructure of the ribbons showed three distinct zones: a columnar, a cellular, and a cellular-dendritic zone. The height of the columnar grain zone is independent of the process parameters such as the wheel material or the wheel velocity. Due to a high level of undercooling and a high growth velocity of the solid/liquid interface, the rapid solidification process is found to suppress the formation of -ferrite and enhance the formation of austenite. The austenite is transformed into martensite upon cooling. In comparison with conventional solidification, a reduction in the initial austenite grain size has been found to result in a very fine lath martensite (M) structure. Investigations of the texture within the ribbons along the growth direction show a weak fiber texture. Transmission electron microscopy (TEM) has revealed a [111]M1 || [001]M2 and (011)M1 || (110)M2 orientation relationship between two neighboring martensite laths. The observed orientation relationship is a result of a superposition of both the Kurdjumov-Sachs (K-S) and Nishiyama-Wasserman (N-W) orientation relations.

Communication: Structural Transition and Macrosegregation of Al-Cu Eutectic Alloy Solidified in the Electromagnetic Centrifugal Casting Process
W.Q. ZHANG, Y.S. YANG, Y.F. ZHU, Q.M.LIU, and Z.Q.HU

MATERIALS PROCESSING

Dynamic Analysis of Unidirectional Pressure Infiltration of Porous Preforms by Pure Metals
DHIMAN K. BISWAS, JORGE E. GATICA, and SURENDRA N. TEWARI
Unidirectional pressure infiltration of porous preforms by molten metals is investigated numerically. A phenomenological model to describe fluid flow and transport phenomena during infiltration of fibrous preforms by a metal is formulated. The model describes the dynamics of the infiltration process, the temperature distribution, and solid fraction distribution. The numerical results are compared against classical asymptotic analyses and experimental results. This comparison shows that end effects may become important and render asymptotic results unreliable for realistic samples. Fiber volume fraction and initial temperature appear as the factors most strongly influencing infiltration. Metal superheating affects not only the length of the two-phase zone but also the solid fraction distribution in the two-phase zone. The effect of constant applied pressure, although significant on the infiltration velocity, is almost negligible on the two-phase zone length and on solid fraction distribution. When the initial preform temperature is below the metal melting point, and constant pressure is applied under adiabatic conditions, the flow ceases when sufficient solidification occurs to obstruct it. A comparison with literature experiments proves the model to be an efficient predictive tool in the analysis of infiltration processes for different preform/melt systems.

Mitigating Intergranular Attack and Growth in Lead-Acid Battery Electrodes for Extended Cycle and Operating Life
E.M. LEHOCKEY, G. PALUMBO, P. LIN, and A. BRENNENSTUHL
Deterioration in the performance of lead acid batteries is primarily governed by weight loss and growth of the positive electrodes, arising from creep and intergranular corrosion/cracking. The present investigation examines the impact of increasing the frequency of grain boundaries having low- misorientations ( 29), described by the Coincident Site Lattice (CSL) model, which are known to be resistant to these intergranular degradation phenomena. Electrode microstructures of various PbCaSn alloys processed to contain frequencies of special boundaries (in excess of 50 pct) exhibited reductions in weight loss of between 26 and 46 pct accompanied by declines in grid growth of between 41 and 72 pct. Moreover, the distribution of intergranular attack/cracking in the microstructure of these alloys can be predicted on the basis of the frequency of low- special boundaries and grain size. In general, improvements in corrosion and creep/cracking occur without compromising tensile properties such as yield strength, ultimate tensile strength (UTS), and ductility. Modifying the crystallographic structure of grain boundaries in Pb alloy battery electrodes, thus, provides an opportunity for minimizing grid thicknesses (weight) and, hence, material costs in battery production, or for maximizing energy densities (Wh/kg) and cycle life performance.

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