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Volume 29A, No. 7, July 1998 This Month Featuring: Symposium on Structure and Properties of Bulk Amorphous Alloys; Alloy Phases; Transformations; Mechanical Behavior; Physical Chemistry; Welding & Joining; Solidification; Materials Processing; Composite Materials. View July 1998 Contents.
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Tm(= Tm - Tx) of less than 100 K and, hence, their large glass-forming ability is due to the steep increase in viscosity in the supercooled liquid state. The high Tx/Tm enables the development of a fully relaxed, clustered amorphous structure including Nd-Nd and Nd-Fe atomic pairs. It is, therefore, presumed that the hard magnetic properties are due to the development of Nd-Nd and Nd-Fe atomic pairs with large random magnetic anisotropy. The Nd- and Pr-based bulk amorphous alloys can be regarded as a new type of clustered amorphous material, and the control of the clustered amorphous structure is expected to enable the appearance of novel functional properties which cannot be obtained for an ordinary amorphous structure.
Bulk Amorphous Metallic Alloys: Synthesis by Fluxing Techniques and Properties
YI HE. TONGDE SHEN, and R.B. SCHWARZ
Bulk amorphous alloys having dimensions of at least 1 cm in diameter have been prepared in the Pd-Ni-P, Pd-Cu-P, Pd-Cu-Ni-P, and Pd-Ni-Fe-P systems using a fluxing and water-quenching technique. The compositions for bulk glass formation have been determined in these systems. For these bulk metallic glasses, the difference between the crystallization temperature (Tx) and the glass transition temperature (Tg, T = Tx - Tg) ranges from 60 to 110 K. These large values of T open the possibility for the fabrication of amorphous near-net-shaped components using techniques such as injection molding. The thermal, elastic, and magnetic properties of these alloys have been studied, and we have found that bulk amorphous Pd40Ni22.5Fe17.5P20 has spin glass behavior for temperatures below 30 K.
Structure of Bulk Amorphous Pd-Ni-P Alloys Determined by Synchrotron Radiation
T. EGAMI, W. DMOWSKI, YI HE and R.B. SCHWARZ
The atomic structure of Pd-Ni-P bulk amorphous alloys was studied by the anomalous (resonance) X-ray scattering technique using synchrotron radiation tuned near the Pd K-edge. Bulk samples of Pd40Ni40P20, Pd30Ni50P20, and Pd50Ni34P16 amorphous alloys were prepared by the flux method in the form of rods. The results show that the structure of these alloys can be described basically by the dense random packed structure with small chemical short-range order. It is suggested that the exceptional stability of these glasses originates mainly from the instability of the competing crystalline phases rather than the atomic ordering in these glasses.
Test Environments and Mechanical Properties of Zr-Base Bulk Amorphous Alloys
C.T. LIU, L. HEATHERLY, D.S. EASTON, C.A. CARMICHAEL, J.H. SCHNEIBEL, C.H. CHEN, J.L. WRIGHT, M.H. YOO, J.A. HORTON and A. INOUE
The mechanical properties of two Zr-base bulk amorphous alloys (BAA), Zr-10Al-30Cu-5Ni (BAA-10) and Zr-10Al-5Ti-17.9Cu-14.6Ni (BAA-11), were studied by both tensile and compressive tests at room temperature in various test environments. The BAA ingots up to 7 mm in diameter were successfully produced by both arc melting and drop casting and induction melting and injection casting. The BAA specimens deformed mainly elastically, followed by catastrophic failure along shear bands. Examination of the fracture region revealed ductile fracture features resulting from a substantial increase in temperature, which was attributable to the conversion of the stored elastic strain energy to heat. Surprisingly, "liquid droplets" located at major shear-band cracks adjacent to the fracture section were observed, indicating the occurrence of local melting during fracture. The angle orientation of shear bands, shear-band cracks, and fracture surfaces relative to the stress axis is quite different for BAA specimens tested in tension and compression. This suggests that both shear stress and normal stress may play a role in developing shear bands during plastic deformation. The tensile properties of BAAs were found to be insensitive to the test environment at room temperature. However, the reaction of BAAs with distilled water and heavy water was detected by laser desorption mass spectrometry (LDMS). These results suggest that moisture-induced hydrogen embrittlement in BAAs may be masked by catastrophic fracture following shear bands.
Bulk Titanium-Rich Alloys Containing Nanoscale Disordered Regions
DONGJIAN LI, K.J. DOHERTY, S.J. POON, and G.J. SHIFLET
Nanoscale disordered regions were obtained in titanium-rich alloys via annealing of a metastable crystalline phase. This metastable bcc solid solution (
) was formed in the alloy Ti65Cr13Cu16Mn4Fe2 following arc melting and cooling or simply by air cooling a solutionized ingot. Since only air cooling was required to retain the phase, large ingots can be produced. During subsequent annealing, partial amorphization occurred in conjunction with a phase transformation from to '. The premise for this amorphization is the reduction of the free energy difference between an amorphous phase and a metastable crystalline phase with proper alloy additions. Comparisons are made with amorphized powders at the same composition produced by mechanical milling.
An Analysis of the Formation of Bulk Amorphous Silicon from the Melt
YAN SHAO, FRANS SPAEPEN, and DAVID TURNBULL
The possibility of forming bulk amorphous silicon from the undercooled liquid is analyzed by calculations of the rates for homogeneous nucleation of the amorphous and crystalline phases, the rate of heterogeneous nucleation of the crystalline phase on the liquid-amorphous interface, and the possibility of crystallization of the amorphous phase in the solid state. It appears that bulk formation of the amorphous phase may be possible in appropriately seeded millimeter-sized samples. A new lower limit for the amorphous-liquid interfacial tension is presented as well.
Metastability and Thermophysical Properties of Metallic Bulk Glass Forming Alloys
R.K. WUNDERLICH and H.J. FECHT
The absence of crystallization over a wide time/temperature window can be used to produce bulk metallic glass by relatively slow cooling of the melt. For a number of alloys, including several multicomponent Zr-based alloys, the relevant thermodynamic and thermomechanical properties of the metastable glassy and undercooled liquid states have been measured below and above the glass transition temperature. These measurements include specific heat, viscosity, volume, and elastic properties as a function of temperature. As a result, it becomes obvious that the maximum undercooling for these alloys is given by an isentropic condition before an enthalpic or isochoric instability is reached. Alternatively, these glasses can also be produced by mechanical alloying, thus replacing the thermal disorder by static disorder and resulting in the same thermodynamic glass state. During heating through the undercooled liquid, a nanoscale phase separation occurs for most glasses as a precursor of crystallization.
Thermodynamic and Kinetic Properties of Amorphous and Liquid States
A.V. GRANATO
The magnitude and temperature dependence of the liquid state shear modulus G, specific heat Cp, diffusivity D, and viscosity 
should all be closely related, according to the interstitialcy model, if a recent proposal by Dyre et al. is generally true. They suppose that the viscosity is given by = 0 exp (F/kT), where 0 is a reference viscosity and F is given by the work required to shove aside neighboring particle in a diffusion process, where F = GVc and Vc is a characteristic volume. In the interstitialcy model the high frequency thermodynamic liquid state shear modulus is given by G(T) = G0 exp ;ob-
(T/T0 - 1);cb, where G0 is the shear modulus at a reference temperature T0, which can be taken as the glass temperature. The resulting non-Arrhenius behavior of the viscosity is compared with experimental data. A critical quantitative analysis for a Zr41.2Ti13.8Cu12.5 Ni10Be225 alloy does not support the shoving model, but the thermodynamic properties can be understood in terms of mixed interstitials composed of metal-beryllium complexes.
Ab Initio Studies of the Electronic Structure and Energetics of Bulk Amorphous Metals
D.M.C. NICHOLSON, G.M. STOCKS, W.A. SHELTON, YANG WANG, and J.C. SWIHART
Bulk amorphous metals (BAMs) are an interesting class of new materials possessing unique properties that offer exciting possibilities for applications to a broad range of technologies. In contrast to the previous generation of amorphous metals, BAMs can be produced in bulk form at cooling rates as low as ;sl1 K/s. The understanding of the structure, properties, and required cooling rates for BAM formation is hindered by the large number of constituents in typical alloys. In this article, we present the results of first principles local density approximation studies of the electronic structure and energetics of model Ni-Pd-P, Zr-Ni-Cu, and Zr-Ni-Al amorphous alloys that relate to two of the simplest BAMs, namely, Ni0.4Pd0.4P0.2 and Zr0.6Al0.15Ni0.25. The calculations are based on large unit cell (;sl300-atom) structural models for which the electronic structure is calculated using the first principles order-N locally self-consistent multiple scattering method.
Electronic Structure and Related Properties of Metallic Glasses: Linear Muffin-Tin Orbital Approach
S.K. BOSE
We describe various aspects of electronic structure calculation for amorphous metals using the linear muffin-tin orbital (LMTO) scheme. We discuss calculations of the magnetic properties within the framework of multiple scattering formalism assuming collinear magnetic structure and using the linear muffin-tin orbital Green's function method. Results for the effective exchange coupling parameters and local magnetic moments for amorphous Fe and Co are discussed with special emphasis on the dependence of these quantities on the local and global features of the structure. A method of calculating the electronic transport properties in transition metal glasses, based on the LMTO-recursion method and the Kubo-Greenwood formula, is presented. The practical limitations of the method and the ways to overcome them are briefly discussed.
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[001]z1The effect of precipitate formation on the shape-memory characteristics of the Cu-Zn-Al-Mn-Zr alloy is discussed and compared to some other Cu-based SMAs.
Evolution and Thermal Stability of Ni3V and Ni2V Phases in a Ni-29 At. Pct V Alloy
J.B. SINGH, M. SUNDARARAMAN, and P. MUKHOPADHYAY
Eutectoid decomposition of the disordered fcc Ni-V solid solution in the composition range of 25 to 33.3 at. pct V gives rise to a mixture of the ordered Ni3V and Ni2V phases. In the present work, the evolution and thermal stability of these phases were studied in a Ni-29 at. pct V alloy. Solution-treated and water-quenched specimens, when aged at 850°C, were found to exhibit two types of microstructure. In the first, the Ni2V phase precipitated in a lamellar Ni3V matrix where a pair of conjugate lamellae corresponded to two variants of the Ni3V phase. In the second morphology, the Ni2V phase precipitated within a Ni3V matrix comprising a single variant of the Ni3V phase. The Ni2V phase was observed to precipitate in a plate-shaped morphology, exhibiting {120}fcc-type, habit planes. The precipitation of the Ni2V plates in the Ni3V lamellae resulted in zigzag interfaces between adjacent Ni3V domains. Both the microstructures were found to be thermally quite stable and did not coarsen appreciably on prolonged aging. However, the prolonged aging caused the renucleation of the Ni3V and the Ni2V phases in the vicinity of the grain boundaries in a manner similar to "recrystallization." The stability of the aged microstructure could be attributed to the nature of the interfaces between different domains of the Ni3V and Ni2V phases.
0.1) and in the recrystallized state (n 0.4). Inclusion dimensions and distributions were found to be identical in these two states. The parameter 
of the RT model is found to depend on n, whereas the parameter q of the Gurson-type models does not vary with n. Numerical modeling of porosity variations in notched, round copper bars shows that both the parameter and the parameter q in the GT model depend on the stress triaxiality in the recrystallized material, whereas the parameter q remains a constant in the GLP model. Accounting for the ellipsoidal void shapes and for the presence of the inclusion significantly affects the prediction of porosity variations.
Effect of Ni on Vacancy Concentrations and Hardness in FeAl Alloys
L.M. PIKE, C.T. LIU, and Y.A. CHANG
The effects of nickel additions to iron aluminides are investigated with respect to vacancy concentrations and solid-solution hardening. Lattice parameters, densities, vacancy concentrations, and hardnesses were measured for FeAl alloys (40, 45, 48, 50, and 51 at. pct Al) with up to 12 at. pct Ni additions, as quenched at both 700°C and 1000°C. It is found that Ni does not prevent the elimination of thermal vacancies, as has been suggested in previous studies. However, the equilibrium vacancy concentrations are affected by the Ni additions. No clear evidence of solid-solution hardening by the Ni additions themselves was found. Instead, it appears that the role of Ni in hardening FeAl is through the effect the Ni has on the vacancy concentration. It was found that the hardness of most of the Ni-containing FeAl alloys could be directly correlated to the square root of the vacancy concentration.
The Relationship between Microstructure and the J-R Curve
ASHOK SAXENA and LAURENT CRETEGNY
A model is presented that establishes a quantitative relationship between the JIC and the J-R curve and the microstructural parameters, such as inclusion size and spacing, and the plastic deformation properties of ductile materials, such as yield strength and strain hardening exponents. The model assumes that ductile crack growth occurs by void nucleation, growth, and coalescence. Each of these processes is modeled in the crack tip environment to complete the model. The proposed model is evaluated using tests performed on 303 stainless steel (SS) at room temperature and on 1Cr-Mo-0.25V steel at 565°C. Satisfactory agreements are obtained between the experimentally obtained J-R curves and those predicted from the model. Limitations of the model are also discussed.
Visual Simulation of Fatigue Crack Growth
SHUANZHU WANG, HAROLD MARGOLIN, and FENG-BAO LIN
An attempt has been made to visually simulate fatigue crack propagation from a precrack. An integrated program was developed for this purpose. The crack-tip shape was determined at four load positions in the first load cycle. The final shape was a blunt front with an ``ear'' profile at the precrack tip. A more general model, schematically illustrating the mechanism of fatigue crack growth and striation formation in a ductile material, was proposed based on this simulation. According to the present model, fatigue crack growth is an intermittent process; cyclic plastic shear strain is the driving force applied to both state I and II crack growth. No fracture mode transition occurs between the two stages in the present study. The crack growth direction alternates, moving up and down successively, producing fatigue striations. A brief examination has been made of the crack growth path in a ductile two-phase material.
A Model for Roughness-Induced Fatigue Crack Closure
SHENG-HUI WANG, CLEMENS M;auULLER, and HANS ECKART EXNER
A model for predicting the crack closing stress intensity factor for roughness-induced closure of fatigue cracks is developed based on a two-dimensional approach considering crack opening and closure of an idealized crack path. The model highlights the contribution of irreversible cyclic planar slip at the crack tip, and is extended to real cases describing roughness-induced crack closure as a function of fracture surface roughness parameters at low K levels where planar slip prevails. The resulting equation indicates that roughness-induced crack closure depends on the maximum stress intensity factor, the standard deviation of heights as well as the standard deviation of angles of the crack profile elements, and the yield stress of the material. Comparison between the prediction of the model and experimental data of Kcl for lamellar microstructures of Ti-2.5Cu as well as TIMETAL 1100 shows good agreement.
Properties of Friction-Stir-Welded 7075 T651 Aluminum
M.W. MAHONEY, C.G. RHODES, J.G. FLINTOFF, R.A. SPURLING, and W.H. BINGEL
Friction stir welding (FSW), a new welding technique invented at TWI, was used to weld 7075 T651 aluminum, an alloy considered essentially unweldable by fusion processes. This weld process exposed the alloy to a short time, high-temperature spike, while introducing extensive localized deformation. Studies were performed on these solid-state welds to determine mechanical properties both in the longitudinal direction, i.e., within the weld nugget, and, more conventionally, transverse to the weld direction. Because of the unique weld procedure, a fully recrystallized fine grain weld nugget was developed. In addition, proximate to the nugget, both a thermomechanically affected zone (TMAZ) and heat affected zone (HAZ) were created. During welding, temperatures remained below the melting point and, as such, no cast or resolidification microstructure was developed. However, within the weld nugget, a banded microstructure that influences room-temperature fracture behavior was created. In the as-welded condition, weld nugget strength decreased, while ductility remained high. A low-temperature aging treatment failed to fully restore T651 strength and significantly reduced tensile ductility. Samples tested transverse to the weld direction failed in the HAZ, where coarsened precipitates caused localized softening. Subsequent low-temperature aging further reduced average strain to failure without affecting strength. Although reductions in strength and ductility were observed, in comparison to other weld processes, FSW offers considerable potential for welding 7075 T651 aluminum.
Creep Rupture Life Prediction of Short Fiber-Reinforced Metal Matrix Composites
H.W. NAM and K.S. HAN
The creep rupture life of an Al/Al2O3 composite and its creep behavior were studied. The metal matrix composite was produced by using a squeeze casting technique. High-temperature tensile tests and creep experiments were conducted on a 15 vol pct alumina fiber-reinforced AC2B Al alloy metal matrix composite (MMC). The high-temperature tensile strength of Al/Al2O3 composite is 14 pct higher than that of an AC2B Al alloy. The steady-state creep rate and the creep life were measured. The stress exponent in Norton's equation and the activation energy were computed. The stress exponents of the AC2B and Al/Al2O3 composites were found to be 4 and 12.3, respectively. The activation energy of the AC2B and Al/Al2O3 composites was found to be 242.74 and 465.35 kJ/mol, respectively. A new equation for predicting creep life was established, which was based on the conservation of the creep strain energy. The theoretical predictions were compared with those of the experiment results, and a good agreement was obtained. It was found that the creep life is inversely proportional to the (n + 1)th power of the applied stress and strain failure energy of creep is conserved. The creep fracture surface, examined by scanning electron microscopy (SEM), showed that the MMC specimen failed in a brittle manner.
Intergranular Fracture of Gamma Titanium Aluminides under Hot Working Conditions
V. SEETHARAMAN and S.L. SEMIATIN
A comparative study of the hot workability of a near gamma titanium aluminide alloy Ti-49.5Al-2.5Nb-1.1Mn in the cast and wrought conditions was performed. Tension tests conducted on coarse grain, cast material, and fine grain wrought material revealed a pronounced variation in both fracture/peak stress and ductility with temperature and strain rate. Brittle, intergranular fracture occurring at high strain rates was found to be controlled by wedge crack nucleation, whereas the ductile fracture observed at low strain rates was controlled by the growth of wedge cracks and cavities. Dynamic recrystallization was shown to be the main restorative mechanism to accommodate grain boundary sliding and thereby control the crack growth rates. The ductile-to-brittle (DB) transition was found to be determined by the critical values of a grain size-based stress intensity factor given by the product of the peak/fracture stress and the square root of grain size. A processing map for the near gamma titanium aluminides was constructed based on the comparative analysis of the hot tension and compression test results.
Communication: Optimizing the Rotation Conditions for Grain Refinement in Equal-Channel Angular Pressing
KEIICHIRO OH-ISHI, ZENJI HORITA, MINORU FURUKAWA, MINORU NEMOTO, AND TERENCE G. LANGDON
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