METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 28A, No. 1, January 1997 This Month Featuring: Transformations; Mechanical Behavior; Physical Chemistry; Environment; Welding & Joining; Solidification; Materials Processing; Composite Materials. View January 1997 Contents.

TRANSFORMATIONS

Simulation of Recrystallization Microstructures and Textures: Effects of Preferential Growth
D. JUUL JENSEN
Three-dimensional (3-D) simulations of recrystallization were carried out using an approach where grains of different orientations are characterized by different nucleation and growth parameters. The aim woriginal fatigue precrack and involves linear elastic fracture mechanics principles. In this case, the change from hydrogen-assisted microdamage (TTS) to cleavagelike topography takes place when a critical stress intensity factor (K_H) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the fatigue precrack plus the TTS area). It is shown that the value K_H depends on experimental variables-mainly on the fatigue precracking regime-and its value maions relate to common experimental observations. It was found that the nucleation and growth assumptions have very significant influences on the recrystallization characteristics. The preferential growth of one type of grain in particular affects the width of the recrystallized grain size distribution and the texture development. Furthermore, it was found that many different nucleation and growth assumptions can result in identical recrystallization microstructures and textures. The results of the simulations are discussed with reference to typical experimental findings.

Effect of Elastic Stress on Two-Phase Binary Diffusion Couples
WILLIAM C. JOHNSON
A front-tracking, finite difference approach has been used to examine the influence of misfit strain and applied stress on interdiffusion in binary, coherent, two-phase planar diffusion couples assuming local thermodynamic equilibrium at the interface. The phases are cubic; possess different lattice parameters, elastic constants, and diffusivities; and can be oriented in either the [001] or [111] direction. Interface compositions, which are time indenotch roots. These results, together on reply (adrParams) with webServerScripts.bbs local (basemsg, mailaddress) getpathargs (adrparams, @basemsg, @mailaddress) return (buildPage (bbs.data.readerTitle, bbs.getReplyPage (basemsg, mailaddress))) ÄÄÄÄÄ
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Diffusion Fields Associated with Size and Shape Coarsening of Oblate Spheroids
YIWEN MOU and J.M. HOWE
The diffusion field or solute concentration distributed around an oblate spheroidal particle simulating a disc-shaped precipitate has been solved for varying particle aspect ratios and varying concentrations along the precipitate surface because of the curvature effect. With oblate spheroidal coordinates, the principal curvatures of the oblate spheroidal surface are derived as functions of the angular variable, and the Laplace field equation is separated into two Legendre equations on the angular variable and on the radial variable. The analytical solution to the Laplace equation, fitting the present boundary conditions, is secured as the sum of a Legendre function and a Legendre series composed of Legendre functions of the second kind with imaginary arguments. The Legendre function gives the concentration distribution with an ignored curvature effect, whereas the series shows the contribution from the curvature effect. Numerical results of normalized concentrations are presented as functions of the radial and angular variables for selected aspect ratios. The concentration distributions around both oblate and prolate spheroidal particles are shown to reduce to the concentration distributed around a spherical particle when the aspect ratio of the spheroids approaches unity.

Study of the - Phase Transformations of a Ti-64 Sheet Induced from a High-Temperature State and a High-Temperature + State
H. MOUSTAHFID, N. GEY, M. HUMBERT, and M.J. PHILIPPE
The room-temperature textures of Ti-64 sheets, inherited from the - phase transformation of high-temperature textures of the material in the and + fields, respectively, have been studied. The corresponding high-temperature textures were also determined by a method developed in our laboratory. The knowledge of the high-temperature textures allowed us to discuss the variant selections through transformation modeling. As a result, a variant selection occurs in the presence of the stable grains of the + field.

Formation of Iron Nitride in FeN Martensite: Nitrogen Vacancies, Iron-Atom Displacements, and Misfit-Strain Energy
M.J. VAN GENDEREN, A. BÖTTGER, and E.J. MITTEMEIJER
The precipitation of " iron nitride in FeN martensite (containing about 5.9 N/100 Fe) is studied as a function of tempering time successively at 333 and 373 K by means of X-ray diffractometry (XRD). On the basis of peak (=lattice parameter) shifts and intensity changes of main and superstructure reflections, it is concluded that, during tempering, the structure of the formed " precipitates changes. At 333 K, initially stoichiometric Fe₁₆N₂ is formed and, with further tempering, structural vacancies are introduced, i.e., Fe₁₆N_2-x is formed. At 373 K, the vacancies are partially filled. The change of the " structure at 333 K is caused by the change of the matrix from martensite to ferrite, i.e., the misfit between the " and the matrix is initially relatively small and then increases. By allowing nitrogen-deficient ", the increase of the misfit can be diminished at the expense of some increase of the volume of ". Gibbs free energy calculations (at 333 K), in which differences in the misfit energy are decisive, indeed show that, in a martensite matrix, Fe₁₆N₂ is favored, whereas, in a ferrite matrix, Fe₁₆N_2-x is favored. Further, in this article, methods are presented to calculate structure factors for nitrogen-deficient " and to obtain, from measured (=strained) lattice parameters, strain-free " lattice parameters from which the composition can be deduced.

MECHANICAL BEHAVIOR

Ultrasonic Backscattering in Duplex Microstructures: Theory and Application to Titanium Alloys
Y.K. HAN and R.B. THOMPSON
A theory is presented for the ultrasonic backscattering in duplex microstructures. Assuming single scattering described by the Born approximation, we consider a microstructure consisting of macrograins containing colonies with crystallographically related orientations. General results are presented for the backscattering coefficient, assuming that all variants occur with equal probability. These are then applied to the particular case of titanium alloys, in which the macrograins are taken to be prior beta grains and the colonies are assumed to be alpha phase produced by a martensitic transformation. Numerical results illustrate the effects of ultrasonic frequency, colony size and ellipticity, and macrograin size and ellipticity on the backscattering.

The Effect of Carbon on the Loss of Room-Temperature Damping Capacity in Copper-Manganese Alloys
SHASHI LADDHA, DAVID C. VAN AKEN, and HUH-TSWEN LIN
A high damping Cu-Mn alloy with a nominal composition of 48Cu-48Mn-1.5Al-0.27Si-0.072Sn-0.028C-0.05Er (all compositions in wt pct) was studied to determine the mechanism of the loss of damping capacity during room-temperature storage. In this study, it was found that an Er-modified alloy sample that was artificially aged for 16 hours at 400°C was stable even after 68 weeks of room-temperature storage. However, a loss of damping capacity was exhibited in the same material when aged to produce an underaged or peakaged condition. The decrease in damping capacity was found to be thermally activated with at least three relaxation processes. Each of the three relaxation processes appear to be related to the diffusion of carbon within the Mn-rich regions and a single activation energy of 0.970 ± 0.05 eV was used to model these processes. Rapid loss of damping capacity was observed in the same alloy when doped with excess carbon. After 3 weeks of storage at room temperature, the damping of the carbon-doped material, artificially aged at 400°C for 4 hours, was reduced to one-third of its initial damping capacity.

Modeling the Mechanical Behavior of Tantalum
BING-JEAN LEE, KENNETH S. VECCHIO, SAID AHZI, and SCOTT SCHOENFELD
A crystal plasticity model is proposed to simulate the large plastic deformation and texture evolution in tantalum over a wide range of strain rates. In the model, a modification of the viscoplastic power law for slip and a Taylor interaction law for polycrystals are employed, which account for the effects of strain hardening, strain-rate hardening, and thermal softening. A series of uniaxial compression tests in tantalum at strain rates ranging from 10^-3 to 10⁴ s^-1 were conducted and used to verify the model's simulated stress-strain response. Initial and evolved deformation textures were also measured for comparison with predicted textures from the model. Applications of this crystal plasticity model are made to examine the effect of different initial crystallographic textures in tantalum subjected to uniaxial compression deformation or biaxial tensile deformation.

Correlation of Microstructure and Fracture Toughness in Three High-Speed Steel Rolls
SUNGHAK LEE, KEE-SUN SOHN, CHANG GIL LEE, and BYUNG IL JUNG
The objective of this study is to clarify the fracture characteristics of high-speed steel (HSS) rolls in terms of microstructural factors such as matrix phase and primary carbide particles. Three HSS rolls with different chromium contents were fabricated by centrifugal casting, and the effect of the chromium addition was investigated through microstructural analysis, fracture-mechanism study, and toughness measurement. The hard and brittle primary carbides, as well as the eutectic carbides (ledeburites), were segregated in the intercellular regions and dominated overall properties. Observation of the fracture process revealed that these primary carbides cleaved first to form microcracks at low stress-intensity factor levels and that the microcracks then readily propagated along the intercellular networks. The addition of chromium to a certain level yielded microstructural modification, including the homogeneous distribution of primary carbides, thereby leading to enhancement of fracture toughness of the HSS rolls.

Room-Temperature Deformation Behavior of a Directionally Solidified (B2)-(Ni-Fe-Al) Intermetallic Alloy
A. MISRA, J.T. KIM, and R. GIBALA
The room-temperature mechanical behavior of a directionally solidified columnar-grained, single-phase (B2)-(Ni-20 at. pct Fe-30 at. pct Al) intermetallic alloy deformed along the ''hard'' <001> direction has been characterized. The 0.2 pct offset compressive yield stress was found to be comparable to that of <001> single crystals of stoichiometric NiAl. The dislocation substructure consisted of a preponderance of long, straight a<111> screw dislocations on {112} planes, with cross-slip on {123} and {110} planes. The superpartials were not resolved by weak-beam imaging conditions, indicating that the antiphase boundary (APB) energy of NiAl is not reduced significantly by the Fe addition. The dislocation substructure was analyzed as a function of strain and compared to the dislocation substructure in <001> NiAl and body-centered cubic (bcc) metals deformed at low homologous temperatures. The debris left behind by a<111> screw dislocations consisted of prismatic edge dipole loops 5 to 25 nm in diameter.

Effects of Crack Aspect Ratio on the Behavior of Small Surface Cracks in Fatigue: Part I. Simulation
K.S. RAVICHANDRAN
A simple simulation of alternate growth of a small surface crack in the surface and depth directions was performed to illustrate the changes in crack aspect ratio, induced by grain boundaries, as a function of crack size. It is shown that at small crack sizes, large variations in aspect ratio, a/c (a is the crack depth and c is the half-surface length), occur, due to local crack front perturbations induced by grains that are oriented for crack growth. At these crack sizes, the assumption of a semicircular crack shape (a/c = 1.0) was found to cause errors in stress intensity range (K) calculations. This, in turn, led to significant scatter or ''anomaly'' in small crack growth rates relative to large cracks. At large crack sizes, the effects of local crack front perturbations on crack aspect ratio and K were found to be insignificant. As a result, the scatter in crack growth data was found to decrease to a negligible level at large crack sizes. It is suggested that the limiting crack size above which the small crack behaves as a large crack, l₂ = 10d (d = grain size), proposed by Taylor and Knott, is related to the crack size above which the effects due to aspect ratio variations are small.

Effects of Crack Aspect Ratio on the Behavior of Small Surface Cracks in Fatigue: Part II. Experiments on a Titanium (Ti-8Al) Alloy
K.S. RAVICHANDRAN and J.M. LARSEN
The continuous variations in crack shape or aspect ratio, a/c (a is the crack depth and c is the half-surface length), of small surface cracks, induced by grain boundaries, have been investigated during the fatigue crack growth of small cracks in a titanium (Ti-8Al) alloy. The significance of the aspect ratio variations in explaining the ''anomalous'' small-crack behavior was evaluated. The aspect ratio data were determined from the measurements of crack compliance, made using a laser interferometric system, and the measurements of surface crack length (2c), made using a photomicroscopic system. The variations in aspect ratio were found to be large at small crack sizes of the order of a few grain diameters. The experimental a/c data were compared with the patterns of crack aspect ratio variation, obtained from theoretical simulations. The simulated data were generated by assuming alternate crack propagation at the surface and at the depth, the details of which are presented in Part I of the study accompanying this article. A good agreement was found between the simulated and the experimentally observed variations. After incorporating the a/c variations in K calculations, the scatter in the growth data of small cracks was significantly reduced and was found to be of the same order as in large cracks. Additionally, it is shown in this study that the conventional methods of analysis of small-crack data, performed with an assumption of a/c = 1, can result in significant errors in K calculation and an increased level of scatter in small-crack growth data. Small cracks also were found to exhibit low closure levels relative to large cracks. The results of the study strongly indicate that characteristics of small cracks, often referred to as anomalous, are due to the assumption of a/c = 1 in situations of large variations in aspect ratio, the use of conventional methods of data analysis, and the lower levels of crack closure found naturally in small cracks.

Tensile Flow and Work-Hardening Behavior of a Ti-Modified Austenitic Stainless Steel
P.V. SIVAPRASAD, S. VENUGOPAL, and S. VENKADESAN
The flow-stress data of a 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel in the temperature range 300 to 1023 K was analyzed in terms of Ludwigson and Voce equations. The parameters of these equations were critically examined with respect to the effect of Ti/C ratio and test temperature. It was found that the Ludwigson equation described the flow behavior adequately up to the test temperature of 923 K, whereas the Voce equation could be employed in the full temperature range. The peaks/plateaus observed in the variation of these parameters as a function of temperature in the intermediate temperature range have been identified as one of the manifestations of dynamic strain aging (DSA). Also, the variation of these parameters with temperature clearly could bring out the different domains of DSA observed in this alloy. The work-hardening analysis of the flow-stress data revealed that in the DSA regime, the onset of stage III hardening is athermal.

The Effect of Hydrostatic Pressurization on the Microhardness and Compressive Behavior of the Porous Mn-Modified Ll₂ Titanium Trialuminide
Z. WITCZAK and R.A. VARIN
Microhardness and compressive mechanical properties of the Mn-modified Ll₂ titanium trialuminide containing porosities from 0.07 to 0.14 and a small volume fraction of the Al₃Ti particles were studied after hydrostatic pressurization at 2 GPa. It is found that the reduction in porosity after hydrostatic pressurization (densification) increases approximately linearly with increasing initial porosity. Compressive yield strength, strength to fracture, and permanent deformation to fracture of both unpressurized and pressurized material decrease linearly with increasing actual volume fraction of porosity. At the same level of actual porosity, the compressive yield strength of the hydrostatically pressurized Ll₂ titanium trialuminide is always higher and its permanent deformation to fracture is always lower than that of the unpressurized titanium trialuminide. Such a behavior indicates that the pressure-induced dislocations generated at elastic discontinuities such as pores and the Al₃Ti particles become immobilized. This, in turn, leads to a quick work hardening. The behavior of compressive yield strength of the hydrostatically pressurized Ll₂ titanium trialuminide is opposite to that observed for a B2 NiAl intermetallic hydrostatically pressurized to 1.4 GPa and subsequently compressive and tensile tested by Margevicius and Lewandowski. This difference is discussed in terms of different crystallographic structure of titanium trialuminides (Ll₂) and NiAl (B2).

PHYSICAL CHEMISTRY

ENVIRONMENT

WELDING & JOINING

SOLIDIFICATION

MATERIALS PROCESSING

Effect of Cu Addition on Consolidating Ti₅Si₃ by the Elemental Powder-Metallurgical Method
K.J. PARK, J.K. HONG, and S.K. HWANG
Elemental powder metallurgy (EPM) was employed to synthesize intermetallic compounds based on Ti₅Si₃. Copper was chosen as a sintering-aid element on the basis of the ''figure of merit'' theory and its effect on improving the density was verified experimentally. Reactive sintering as well as pseudo-hot isostatic pressing (PHIP) were utilized to increase the integrity of the compound. Decreasing the particle size of elemental powders or prolonging the reactive sintering heat treatment time decreased porosity. A high relative density of more than 99 pct was obtained under a condition of 6 wt pct Cu addition, fine Ti powder, and 7 h holding time at sintering temperature (1450°C). The pseudo-hot isostatic pressing enhanced the density of reactive sintered compound further to approximately 99 pct, which is explained in terms of particle rearrangement.

Analysis of Orientation Clustering in a Directionally Solidified Nickel-Based Ingot
DAVID A. WEST and BRENT L. ADAMS
Orientation imaging microscopy was used to study the field of lattice orientations in a directionally solidified ingot of nickel-based alloy in order to understand the evolution of grain structure and microtexture as a function of distance from the chilled surface. The lattice field is organized into equiaxed and randomly oriented grains at the chill plane. However, with increasing distance away from the chill plane, the microstructure organizes into increasingly coarse and ramified grain clusters in the region of columnar growth. The clusters comprise sets of grains connected by small-angle grain boundaries. Thus, the clustering of neighboring dendrites reflects a sharing of low-angle misorientations owing to the development of a strong <100> fiber texture and its concomitant, a profound change in misorientation distribution. The absence of any preferred orientation within a single cluster suggests that the clusters consist of separate grains. The radius of gyration and the fractal dimension have been used to characterize clustering in the microstructure. Clustering was found to increase with distance from the chill plane as the strength of the fiber texture increases. A critical misorientation angle was found at which the largest cluster percolates the microstructure. This critical angle decreases with increasing strength of the fiber texture.

COMPOSITE MATERIALS

Friction and Abrasion Resistance of Cast Aluminum Alloy-Fly Ash Composites
P.K. ROHATGI, R.Q. GUO, P. HUANG and S. RAY
The abrasive wear properties of stir-cast A356 aluminum alloy-5 vol pct fly ash composite were tested against hard SiC_p abrasive paper and compared to those of the A356 base alloy. The results indicate that the abrasive wear resistance of aluminum-fly ash composite is similar to that of aluminum-alumina fiber composite and is superior to that of the matrix alloy for low loads up to 8 N (transition load) on a pin. At loads greater than 8 N, the wear resistance of aluminum-fly ash composite is reduced by debonding and fracture of fly ash particles. Microscopic examination of the worn surfaces, wear debris, and subsurface shows that the base alloy wears primarily by microcutting, but the composite wears by microcutting and delamination caused by crack propagation below the rubbing surface through interfaces between fly ash and silicon particles and the matrix. The decreasing specific wear rates and friction during abrasion wear with increasing load have been attributed to the accumulation of wear debris in the spaces between the abrading particles, resulting in reduced effective depth of penetration and eventually changing the mechanism from two-body to three-body wear, which is further indicated by the magnitude of wear coefficient.