METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 27A, No. 11, November 1996 This Month Featuring: Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Physical Chemistry; Environment; Welding & Joining; Surface Treatment; Materials Processing; Composite Materials. View November 1996 Table of Contents.

ALLOY PHASES

TRANSFORMATIONS

High-Resolution Transmission Electron Microscopy Investigation of the Face-Centered Cubic/Hexagonal Close-Packed Martensite Transformation in Co-31.8 Wt Pct Ni Alloy: Part 2. Plate Intersections, Extended Defects, and Nucleation Mechanisms
D.W. BRAY and J.M. HOWE
The face-centered cubic/hexagonal close-packed (fcc/hcp) martensite phase transformation in a Co-31.8 wt pct Ni alloy was studied by high-resolution transmission electron microscopy (HRTEM). The HRTEM was used to study the structure and properties of intersections between martensite plates and other defects observed in the alloy such as stacking fault tetrahedra (SFT) and Z-type defects. The HRTEM was also used to attempt to determine if various proposed mechanisms for the fcc/hcp martensite transformation were operating. There is evidence to suggest that the reflection mechanism proposed by Bollmann and the dipole mechanism proposed by Hirth are active in the fcc/hcp martensitic transformation, although the evidence is not completely certain in either case. Growth of the hcp phase by a four- or six-plane mechanism as proposed by Mahajan et al. is possible in theory but was not observed in this study. Transformation by previously proposed pole mechanisms was also not observed in this study, although evidence for a new type of pole mechanism was found. The formation of SFT along the fcc/hcp martensite interface was observed to occur by the cross-slip of Shockley partial dislocations out of the fcc/hcp interface onto conjugate fcc matrix planes, followed by further cross-slip to form the SFT, as previously observed for grain boundaries in fcc alloys.

A Model Describing Neutron Irradiation-Induced Segregation to Grain Boundaries in Dilute Alloys
R.G. FAULKNER, SHENHUA SONG, and P.E.J. FLEWITT
A model describing neutron irradiation-induced grain boundary segregation at a given temperature is established for dilute alloys based on a complex diffusion mechanism and combined with McLean's equilibrium segregation model. In the model, irradiation-enhanced solute diffusion is taken into consideration. The diffusion equations are more rigorously solved than in earlier models, so that an accurate definition of the grain boundary solute concentration is given as a function of time. The effect of the temperature dependence of dislocation density is accommodated and the estimation method for complex diffusion is reappraised. Theoretical predictions are made for segregation of phosphorus in neutron-irradiated -Fe. There exists a transition temperature below which combined irradiation-induced nonequilibrium and irradiation-enhanced equilibrium segregation is dominant and above which thermal equilibrium segregation is dominant. The peaks in the temperature dependence of segregation shift to lower temperatures with decreasing neutron dose rate and/or increasing neutron dose. The combined radiation-induced nonequilibrium and radiation-enhanced equilibrium peak segregation temperature is about 150°C for P grain boundary segregation in neutron-irradiated -Fe at dose rate = 10^-6 dpa/s and dose = 1 dpa. The thermal equilibrium segregation peak is around 550°C for the same conditions. Comparison of some experimental and predicted results shows that the predictions are generally consistent with the observations.

Preferential Coarsening of " Precipitates in INCONEL 718 During Creep
MING GAO, SHUCHUN CHEN, D. GARY HARLOW, and ROBERT P. WEI
Preferential coarsening of the " (Ni₃Nb) precipitates was observed in polycrystalline INCONEL 718 during creep which involved principally the growth of one of three variants at the expense of the others. The tendency and extent of preferential coarsening are analyzed in terms of the interaction between the applied stress and the transformation strains and probabilistically in terms of the orientation of the " precipitates with respect to the tensile stress axis.

Austenite Grain Growth Kinetics in Al-Killed Plain Carbon Steels
MATTHIAS MILITZER, ALAN GIUMELLI, E. BRUCE HAWBOLT, and T. RAY MEADOWCROFT
Austenite grain growth kinetics have been investigated in three Al-killed plain carbon steels. Experimental results have been validated using the statistical grain growth model by Abbruzzese and Lücke, which takes pinning by second-phase particles into account. It is shown that the pinning force is a function of the pre-heat-treatment schedule. Extrapolation to the conditions of a hot-strip mill indicates that grain growth occurs without pinning during conventional processing. Analytical relations are proposed to simulate austenite grain growth for Al-killed plain carbon steels for any thermal path in a hot-strip mill.

Modeling Recovery and Recrystallization Kinetics in Cold-Rolled Ti-Nb Stabilized Interstitial-Free Steel
K. MUKUNTHAN and E.B. HAWBOLT
The recovery and recrystallization kinetics in an 80 pct cold-rolled Ti-Nb stabilized interstitial-free (IF) steel have been characterized for isothermal (500 to 760°C) and continuous heating (0.025°C s^-1 to 20.2°C s^-1) annealing. Isothermal recovery kinetics, as monitored by {220} X-ray peak resolution measurements, were described using a semiempirical logarithmic equation. The IF steel recovered relatively easily, with approximately 45 to 60 pct of the total peak resolution occurring prior to the onset of recrystallization. An iterative procedure was adopted to separate the diffraction effects associated with the concurrent recovery and recrystallization processes. Microstructural observations indicated that the recrystallization event was heterogeneous, with preferential nucleation and early site saturation at grain boundaries in the cold-rolled material. Isothermal recrystallization kinetics, determined by quantitative metallography, were described using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Speich-Fisher (SF) relationships. An alternative description of the isothermal recrystallization kinetics was provided by the experimentally determined microstructural path function, independent of the thermal path, and an empirical kinetic function describing the interface averaged growth rate. The kinetic analysis yielded an apparent recrystallization activation energy of 501.7 kJ/mole, indicating severe retardation of recrystallization in IF steels. Recovery and recrystallization kinetics during continuous heating have been modeled using the isothermal kinetic parameters, assuming the validity of the principle of additivity. The results were validated by experimental measurements obtained at heating rates simulating both batch and continuous annealing. Although the Scheil additivity equation overestimated the recrystallization start time for continuous heating conditions, the associated higher temperature and more rapid initial recrystallization resulted in similar overall kinetics.

The Effect of Metallic Elements on the Crystallization Behavior of Amorphous Fe-Si-B Alloys
T. NAOHARA
The crystallization behavior of amorphous Fe_84-XSi₆B₁₀M_X (M = Nb, Zr, V, or Cu) alloys was examined using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) with the aim of clarifying the effect of additional M elements. The compositional dependence of the first crystallization temperature T_X1 increased in the order of Zr > Nb > V; however, the addition of 1 at. pct Cu caused a decrease in T_X1. Such an effect of the M elements on the thermal stability of an amorphous phase was interpreted in terms of the difference in the atomic size. These alloys were composed of a mixed structure of -Fe and amorphous phases after aging for 3.6 ks in the first exothermic temperature range. The addition of more than 3 at. pct Nb or Zr significantly affected the morphology and grain size of the -Fe phase. However, their particles possessed dendritic morphology with a grain size of 0.1 to 0.3 µm, when the Nb or Zr content was less than 2 at. pct. Further addition of these elements brought about the formation of spherical -Fe particles. The average grain size, for instance, was as small as 20 nm in the aged alloy containing 6 at. pct Nb, which shows that a remarkable grain refinement occurs with increasing Nb content.

On the Effect of Stress on Nucleation and Growth of Precipitates in an Al-Cu-Mg-Ag Alloy
B. SKROTZKI, G.J. SHIFLET, and E.A. STARKE, JR.
A study has been made of the effect of an externally applied tensile stress on and ' precipitate nucleation and growth in an Al-Cu-Mg-Ag alloy and a binary Al-Cu alloy which was used as a model system. Both solutionized and solutionized and aged conditions were studied. The mechanical properties have been measured and the microstructures have been characterized by transmission electron microscopy (TEM). The volume fraction and number density, as well as the precipitate size, have been experimentally determined. It was found that for as-solutionized samples aged under stress, precipitation occurs preferentially parallel to the stress axis. A threshold stress has to be exceeded before this effect can be observed. The critical stress for influencing the precipitate habit plane is between 120 and 140 MPa for and between 16 and 19 MPa for ' for the aging temperature of 160°C. The major effect of the applied stress is on the nucleation process. The results are discussed in terms of the role of the lattice misfit between the matrix and the precipitate nucleus.

Crystallographic Preferred Orientation Induced by Cyclic Rolling Contact Loading
A.P. VOSKAMP and E.J. MITTEMEIJER
Fine focus X-ray diffraction methods have been applied to analyze the texture development of the ferrite phase during rolling contact fatigue of 6309 type deep groove ball bearing inner rings prepared from hardened and tempered SAE 52100 steel. Textures of the ferrite matrix as {100}<110> and {111}<211> (where {hkl} denotes the crystallographic plane that is preferably parallel with the contact surface and <uvw> denotes the crystallographic direction that is preferably parallel with and in the direction of over-rolling) have been identified in a small region below the rolling contact surface. These textures develop gradually with an increasing number of stress cycles and become noticeable in conjunction with changes in residual stress, microstrain, and volume fraction of retained austenite in the same region. Upon rolling contact loading, both textures can become very pronounced, while the shape of the subsurface volume, where plastic deformation takes place in particular, remains unchanged: material displacement in the subsurface volume is less than 5 µm in the three principal directions. Crack propagation in association with spalling fatigue failure has been shown to be related to the type of texture developed.

M₂C Precipitates in Isothermal Tempering of High Co-Ni Secondary Hardening Steel
CHOONG HWA YOO, HYUCK MO LEE, JIN W. CHAN, and JOHN W. MORRIS, Jr.
The effects of isothermal tempering on the coarsening behavior of hexagonal M₂C precipitates and the secondary hardening reaction in ultrahigh-strength AerMet 100 steel were investigated. The tempering temperatures were 468°C, 482°C, and 510°C, and the tempering time spanned the range from 1 to 400 hours. Experimental studies of the coarsening behavior of the carbides were made by utilizing transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffractometry (XRD). The hardness at the secondary hardening peak was about HRc 55. The average length and diameter of M₂C carbides were 4 to 8 nm and 1.5 to 2.5 nm, respectively, at all three tempering temperatures; hence, the aspect ratio was almost 3, an equilibrium value in this case. The size of the M₂C carbides increased monotonically with time, but the growth kinetics did not exactly follow the classical coarsening behavior. The amount of precipitated austenite increased with tempering time and temperature. M₂C precipitates were still relatively fine even after 200 hours of tempering. This feature seemed to be closely related to the high hardness maintained after prolonged tempering.

TRANSPORT PHENOMENA

MECHANICAL BEHAVIOR

Plastic Zones and Fatigue-Crack Closure Under Plane-Strain Double Slip
KEN GALL, HUSEYIN SEHITOGLU, and YAVUZ KADIOGLU
The results of a systematic investigation involving forward and reversed plastic zones for a growing fatigue crack under plane-strain double-slip conditions are presented. The study focuses on plastic-deformation fields outside the small-scale yielding regime. The size of the macroscopic forward plastic zone is found to be nearly proportional to the square of the applied-stress intensity over the critical resolved shear stress. The size of the forward plastic zone is also found to depend on the angles of the two microscopic slip directions with respect to the crack line. When the microscopic slip directions are kept symmetric about the crack-line normal, and the angle between them is varied, the forward plastic zone sizes hardly vary. However, when the angle between the slip lines is kept constant, and both planes are simultaneously rotated, the forward plastic zone sizes vary by a factor of three. The ratio of the reversed plastic zone size to the forward plastic zone size is also found to be dependent on the orientation of the microscopic slip planes. The ratio varies when the angle between the microscopic slip planes is changed, or when the orientations of both planes are rotated simultaneously. Stationary cracks are generally found to have larger reversed plastic zones than fatigue cracks, and the difference is attributed to crack closure.

The Plastic Anisotropy of an Al-Li-Cu-Zr Alloy Extrusion in Unidirectional Deformation
M.T. LYTTLE and J.A. WERT
The plastic anisotropy resulting from the initial deformation microstructure and various aging treatments applied to several regions of an AA2090 near-net-shape extrusion has been investigated. Yield behavior was measured by uniaxial compression in multiple orientations of each region. Two models of the plastic anisotropy were generated: the Taylor/Bishop-Hill model, based on crystallographic texture, and the plastic inclusion model, developed by Hosford and Zeisloft,^[5] which incorporates anisotropic-precipitate effects. In overaged conditions, the Taylor/Bishop-Hill model adequately describes the observed plastic anisotropy. As the strengthening increment due to second-phase particles increases, there is a concurrent increase in the magnitude of the precipitate contribution to anisotropy. This anisotropy can not be accurately predicted solely by crystallographic texture. By incorporation of terms describing the precipitate anisotropy, the plastic inclusion model correctly predicts the yield strength variation in all regions tested. Examination of the fundamental interaction between matrix and precipitation strengthening reveals that there is a stronger basis for taking the critical resolved shear stress (CRSS) of the precipitates as a constant, rather than their effective yield strength. This consideration provides a more consistent and accurate form of the plastic inclusion model.

Characterization of the Wear Response of a Modified Zinc-Based Alloy vis-à-vis a Conventional Zinc-Based Alloy and a Bearing Bronze at a High Sliding Speed
B.K. PRASAD, A.K. PATWARDHAN, and A.H. YEGNESWARAN
In this investigation, an attempt has been made to examine the wear response of a modified zinc-based alloy at a high speed (4.60 m/s) of sliding over a range of applied pressures. A conventional zinc-based alloy and a bearing bronze have also been subjected to identical tests with a view to assess the working capability of the modified alloy with respect to the existing ones. The wear characteristics of the alloys have been correlated with their microstructural features, while operating wear mechanisms have been studied through analyses of wear surfaces, subsurfaces, and debris particles. The conventional zinc-based alloy attained most inferior wear behavior when compared with that of the modified (zinc-based) alloy and the bronze. Interestingly, the modified alloy exhibited its wear response to be much better than that of the conventional zinc-based alloy due to the presence of nickel/silicon containing (hard and thermally stable) microconstituents. Moreover, the modified alloy also seized at a pressure similar to that of the bronze, although its wear rate prior to seizure was more than that of the latter. The study clearly indicates that it is possible to develop modified versions of zinc-based alloys having much improved wear characteristics over the conventional variety; the information gains special attention in view of the high speed of sliding selected in this study.

Influence of Interstitials on the Mechanical Properties of Metallic Materials
R.E. REED-HILL, C.V. ISWARAN, and M.J. KAUFMAN
The role of interstitials on the mechanical properties of metals has become more clearly understood as the result of investigations of systems whose interstitial solute concentrations in solid solution are effectively constant over a wide temperature range (from 0 K to well into the plateau of the flow stress vs temperature diagram). These studies strongly imply that both solid solution hardening and dynamic strain aging are due to pinning of dislocations by interstitial solute atoms. Furthermore, the strain rate is controlled by a mechanism whose activation enthalpy is a function of the effective stress, as suggested by Yokobori.

Effects of Alkali-Metal Impurities on Fracture Toughness of 2090 Al-Li-Cu Extrusions
E.D. SWEET, S.P. LYNCH, C.G. BENNETT, R.B. NETHERCOTT, and I. MUSULIN
The effects of alkali-metal impurity (AMI) content, temperature, and crack-mouth-opening displacement (CMOD) rate on the fracture toughness of 2090-T8 Al-Li-Cu alloy extrusions were studied, particularly for short-transverse (S-L) orientations. Decreasing AMI content resulted in increasing room-temperature fracture toughness, especially for underaged S-L and T-L specimens. Unlike most Al-Li based alloys, material with very low (< 2 wt. ppm) AMIs produced by vacuum refining had a high S-L fracture toughness (up to MPa for proof strengths ~440 MPa) as well as high toughness in other orientations. The increase in room-temperature fracture toughness with decreasing AMI content was associated with a decrease in the proportion of brittle intergranular and cleavage-like islands, and a corresponding increase in the proportion of high energy dimpled fracture modes, on fracture surfaces. Both the present and previous studies indicate that the brittle islands result from liquid-metal embrittlement due to the presence of discrete sodium-potassium rich liquid phases. For medium to high AMI contents (5 to 37 wt ppm), S-L fracture toughness increased with decreasing temperature due to solidification of these phases and a consequent decrease in the mobility of embrittling atoms. The ability of embrittling atoms to keep up with crack tips also depended on crack velocity so that CMOD rate influenced fracture toughness. The grain structure (degree of recrystallization) appeared to be another important parameter affecting fracture toughness.

Manifestations of Dynamic Strain Aging in Soft-Oriented NiAl Single Crystals
M.L. WEAVER, M.J. KAUFMAN, and R.D. NOEBE
The tensile and compressive properties of six NiAl-base single-crystal alloys have been investigated at temperatures between 77 and 1200 K. The normalized critical resolved shear stresses (CRSS/E) and work-hardening rates (/E) for these alloys generally decreased with increasing temperature. However, anomalous peaks or plateaus for these properties were observed in conventional purity (CPNiAl), Si-doped (NiAl-Si), C-doped low Si (UF-NiAl1), and Mo-doped (NiAl-Mo) alloys at intermediate temperatures (600 to 1000 K). This anomalous behavior was not observed in high-purity, low interstitial material (HP-NiAl). Low or negative strain-rate sensitivities (SRS) also were observed in all six alloys in this intermediate temperature range. Coincident with the occurrence of negative strain-rate sensitivities was the observation of serrated stress-strain curves in the CPNiAl and NiAl-Si alloys. These phenomena have been attributed to dynamic strain aging (DSA). Chemical analysis of the alloys used in this study suggests that the main specie responsible for strain aging in NiAl is C but indicate that residual Si impurities can enhance the strain aging effects. The corresponding dislocation microstructures at low temperatures (300 to 600 K) were composed of well-defined cells. At intermediate temperatures (600 to 900 K), either poorly defined cells or coarse bands of localized slip, reminiscent of the vein structures observed in low-cycle fatigue specimens deformed in the DSA regime, were observed in conventional purity, Si-doped, and in Mo-doped alloys. In contrast, a well-defined cell structure persisted in the low interstitial, high-purity alloy. At elevated temperatures (1000 K), more uniformly distributed dislocations and sub-boundaries were observed in all alloys. These observations are consistent with the occurrence of DSA in NiAl single-crystal alloys at intermediate temperatures.

The Cracking Mechanism of Silicon Particles in an A357 Aluminum Alloy
JIEN-WEI YEH and WEN-PIN LIU
The cracking of Si particles in an A357 Al alloy has been investigated over a spectrum of stress and strain by varying aging strength and applying different tensile strains. The variation of the fraction of broken Si particles with stress, strain, and cleavage plane orientation has been obtained. The features of cracking reveal that cracking of Si particles is a very localized event. A dislocation pileup mechanism is the most probable one among all crack-initiation theories for explaining the behavior. Based on this mechanism, further deduction has been made to obtain the relationship between the fraction of broken particles and metallurgical factors. The present data, along with Gurland's and that of Low et al., have been found to verify this relationship for the effect of stress, strain, and cleavage plane orientation.

Communication: The Behavior of Internal Markers in Ti-6Al-4V Deformed in Superplastic Tension
P.L. BLACKWELL and P.S. BATE

PHYSICAL CHEMISTRY

Thermochemistry of the Ni-Hf System--Intermetallic Phases
LÁSZLÓ BENCZE and KLAUS HILPERT
The vaporization of alloys of the Ni-Hf system was investigated in the temperature range between 1200 and 1650 K by Knudsen effusion mass spectrometry. The different compositions of the 16 alloy samples investigated covered the complete homogeneity range of the Ni-Hf system. The partial pressure of Ni was determined over all samples. The thermodynamic activities of Ni and Hf in the alloys were evaluated from these pressures and by Gibbs-Duhem integration. In addition, Gibbs energies of formation, enthalpies of formation, and entropies of formation resulted for the nine intermetallic phases of the Ni-Hf system. Beside similar thermodynamic data for the evaporation reactions were studied. The data obtained are discussed and a method for distinguishing the congruent melting compounds from the peritectic ones by defining stability factors calculated from the Gibbs energies of formation is suggested.

Thermodynamic Assessment of the Nb-N System
WEIMING HUANG
The phase equilibrium and thermodynamic information of the Nb-N system was reviewed and assessed by using thermodynamic models for the Gibbs energy of individual phases. Although there was a large amount of experimental information of the system, heat capacity data of the Nb₂N and NbN were not available either in low or high temperatures. In the present study, low-temperature heat capacity and the °S₂₉₈ values were estimated using estimated entropy Debye temperatures. Only the Nb₂N (hcp) and NbN (fcc) nitrides were considered to be the true binary phases and were included in the present evaluation in addition to the N₂ gas, liquid, and -solid solution (bcc). Three thermodynamic models were used: a two-sublattice model for the solid solution phases, a substitutional model for the liquid phase, and an ideal-gas model for the N₂ gas. The model parameters were evaluated by fitting to the selected data by means of a computer program. A consistent set of parameters was obtained which satisfactorily described most of the experimental and estimated data.

ENVIRONMENT

WELDING & JOINING

Transient Liquid-Phase Bonding in the NiAl/Cu/Ni System--A Microstructural Investigation
W.F. GALE and Y. GUAN
A transmission electron microscopy based investigation of microstructural development in NiAl-Ni transient liquid bonding, using commercial purity copper interlayers, is presented in this article. The article considers the mechanisms of isothermal solidification in NiAl/Cu/Ni joints and the influence of copper diffusion from the joint centerline on the microstructures of the adjacent NiAl and Ni substrates. Changes in the microstructure of the bond centerline due to entry of aluminum (from the NiAl substrate) and Ni (from both the NiAl and the Ni substrates) are discussed. Transfer of aluminum from the NiAl substrate to the Ni substrate is also examined. The precipitation of both L1₂ type ' and B2 type phases at the joint centerline is investigated. Precipitation of ' within both the NiAl and Ni substrates is considered. The formation of A1 type phases in the NiAl substrate is also examined.

A Model for Coupled Growth of Reaction Layers in Reactive Brazing of ZrO₂-Toughened Al₂O₃
T. TORVUND, Ø. GRONG, O.M. AKSELSEN, and J.H. ULVENSØEN
In the present investigation, process modeling techniques have been applied to describe coupled reaction layer growth in reactive brazing of ZrO₂-toughened Al₂O₃ with Ag-Ti filler metals. The model takes into account both the successive evolution of the titanium oxide layer at the ceramic/braze metal interface at elevated temperatures and the subsequent decomposition of the reaction products during cooling. The results are presented in the form of novel process diagrams which illustrate in a quantitative manner the microstructural connections throughout the various stages of the process. The diagrams can, in turn, be used to calculate the individual reaction layer thicknesses at room temperature and relate these directly to the content of reacting element in the braze alloy.

The Wear Behavior Between Hardfacing Materials
WEITE WU and LUNG-TIEN WU
Hardface weld cladding in industry most commonly uses cobalt-based STELLITE nos. 6 and 12 and nickel-based Colmonoy nos. 56, 83, and 88 for plasma transferred arc (PTA) welding of 4140 steel. Frictional and abrasion wear of weld layers are compared with that of the widely used nitridized, low-level SKD61, SACM1 steel alloys and with centrifugal-cast nickel-based Colmonoy No. 68 bimetal. Experimental results show that cobalt alloys are not suitable for low-alloy steel frictional wear. However, nickel alloys are quite compatible. Resistance to abrasive wear increased in the experimental materials according to the level of hardness. Wear resistance was compromised in experimental materials when the hard phase was too dispersed.

SURFACE TREATMENT

Influence of Plastic Deformation upon the Half-Width of Engineering Metallic Materials in Hard State
J.B. LI, H.B. XU, R. CHEN, and Z.G. WANG
The half-width values of the X-ray diffraction profiles are frequently used to characterize the static strength of a strengthened surface, or the depth distribution of this mechanical parameter, in a strengthened surface layer, especially in a shot-peening affected layer. However, for the unpeened surface and the base material of the shot-peened specimen of an alloy steel treated in hard state, the experimental results shown in this article indicate that uniaxial tensile or compressive plastic deformation increases the yield strengths while it decreases the half-width values. The half-width values of the shot-peened surface and surface layer greatly decrease, whereas the yield strength of this surface remarkably increases. Accordingly, in the authors' opinion, the half-width values could not correctly describe the static strengths of hard metallic materials, and, contrary to the viewpoint put forward by a lot of researchers, the shot-peened surfaces of such materials are work hardened instead of work softened. A model demonstrating that plastic deformation reduces the half-width values by decreasing the second kind internal stresses is developed.

MATERIALS PROCESSING

High-Temperature Deformation and Failure of an Orthorhombic Titanium Aluminide Sheet Material
P.D. NICOLAOU and S.L. SEMIATIN
The high-temperature deformation and failure behavior of an orthorhombic titanium aluminide sheet alloy (fabricated by diffusion bonding of six thin foils) was established by conducting uniaxial tension and plane-strain compression tests at 980°C and strain rates between 10^-4 and 10^-2 s^-1. The stress-strain response was characterized by a peak stress at low strains followed by moderate flow softening. Values of the strain-rate sensitivity index (m) were between 0.10 and 0.32, and the plastic anisotropy parameter (R) was of the order of 0.6 to 1.0. Cavity nucleation and growth were observed during tensile deformation at strain rates of 10^-3 s^-1 and higher. However, the combined effects of low m, low cavity growth rate , and flow softening were deduced to be the source of failure controlled by necking and flow localization rather than cavitation-induced fracture prior to necking.

Determination of Hydrogen in Titanium Alloys by Cold Neutron Prompt Gamma Activation Analysis
RICK L. PAUL, HUGH M. PRIVETT III, RICHARD M. LINDSTROM, WADE J. RICHARDS, and ROBERT R. GREENBERG
Cold neutron prompt gamma-ray activation analysis (CNPGAA) has proven useful for the analysis of hydrogen in titanium alloys. The analysis is nondestructive, measures the entire sample, and the results are independent of the chemical form of hydrogen present. We have used the technique to measure H mass fractions as low as 50 mg/kg in titanium-alloy jet-engine compressor blades and to measure hydrogen in standards for neutron tomography.

Synthesis of RuAl by Reactive Powder Processing
I.M. WOLFF
The unusual combination of high-temperature strength and room-temperature ductility makes compounds based on RuAl attractive for structural applications. Difficulties inherent in the manufacture of RuAl by melt processing can be circumvented by powder metallurgical (PM) techniques. The present work shows that reactive hot isostatic pressing (RHIPing) based on the self-propagating exothermic reaction of the constituent powders allows homogeneous, high-density material to be made. Controlled process parameters include green density, prior degassing, the powder size distribution, heating rate, pressure, and homogenizing schedule. The nature of the reaction products was found to be determined primarily by the applied pressure during combustion. While concurrent pressure is required to ensure densification, pressurization leads to the formation of a host of nonequilibrium phases that necessitate extended homogenizing practices to drive the reaction to completion. The readily assimilable process parameters allow the production of structural components based on RuAl to be envisaged.

COMPOSITE MATERIALS

Microstructural Changes in a Mechanically Alloyed Al-6.2Zn-2.5Mg-1.7Cu Alloy (7010) With and Without Particulate SiC Reinforcement
A. BHADURI, V. GOPINATHAN, P. RAMAKRISHNAN, and A.P. MIODOWNIK
Elemental powders of Al, Zn, Mg, and Cu (corresponding to the composition of 7010 aluminium alloy) were milled in a high-energy attritor with and without additions of SiC particulates. The microstructural changes taking place in the milled powders (which eventually lead to mechanical alloying) are found to be retarded by SiC additions. High-resolution techniques such as electron probe microanalysis (EPMA) and transmission electron microscopy/energy-dispersive X-ray analysis (TEM/EDX) revealed the presence of localized solute-rich regions long after the diffraction line from these solutes had ceased to appear in the X-ray diffractograms. Zinc appears to be more difficult to be mechanically alloyed into aluminum than either Cu or Mg in spite of its comparatively larger diffusivity in aluminum.

Wear and Friction Behavior of Metal Impregnated Microporous Carbon Composites
GULTEKIN GOLLER, D.P. KOTY, S.N. TEWARI, M. SINGH, and A. TEKIN
Metal-matrix composites have been prepared by pressure-infiltration casting of copper-base alloy melts into microporous carbon preforms. The carbon preforms contained varying proportions of amorphous carbon and graphite. Load dependence of the wear and friction behavior of the composite pins has been examined under ambient conditions against cast-iron plates, using a pin-on-plate reciprocating wear tester. The wear resistance of the composite is significantly improved, as compared with the base alloy. Contrary to the normally expected behavior, the addition of graphite to the amorphous carbon does not reduce the friction coefficient, especially at high loads. The wear and friction behavior of the composites is very sensitive to the size and distribution of the microstructural constituents.

Reinforcement Shape Effects on the Fracture Behavior and Ductility of Particulate-Reinforced 6061-Al Matrix Composites
S.G. SONG, N. SHI, G.T. GRAY III, and J.A. ROBERTS
Particle shape effects on the fracture and ductility of a spherical and an angular particulate-reinforced 6061-Al composite containing 20 pct vol Al₂O₃ were studied using scanning electron microscopy (SEM) fractography and modeled using the finite element method (FEM). The spherical particulate composite exhibited a slightly lower yield strength and work hardening rate but a considerably higher ductility than the angular counterpart. The SEM fractographic examination showed that during tensile deformation, the spherical composite failed through void nucleation and linking in the matrix near the reinforcement/matrix interface, whereas the angular composite failed through particle fracture and matrix ligament rupture. The FEM results indicate that the distinction between the failure modes for these two composites can be attributed to the differences in the development of internal stresses and strains within the composites due to particle shape.