| METALLURGICAL AND MATERIALS TRANSACTIONS A | |
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Volume 26A, No. 7, July 1995 This Month Featuring: Alloy Phases; Transformations; Mechanical Behavior; Physical Chemistry; Environment; Welding and Joining; Electronic, Magnetic, & Optical Material; Solidification; Materials Processing; Composite Materials. View July 1995 Table of Contents.
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Alloy Design and Coarsening Phenomenon of L12 Precipitates in High-Temperature Al-2 At. Pct (Ti,V,Zr) Systems
SEUNG ZEON HAN, SEOG CHUL CHUNG, and HYUCK MO LEE
Aging works of two melt-spun Al-2 at pct (Ti,V,Zr) alloys showed that metastable L12Al3(Ti,V,Zr) precipitates were dominant and did not transform to stable D023 ones: the average radius was 3 to 4 nm and the interparticle spacing was 10 to 30 nm at 698 K up to 400 hours. Coarsening kinetics was found to be very sluggish and was coincident with the low lattice mismatch. Due to the low coarsening rate and the high thermal stability of the precipitated phase, rapidly solidified Al-Ti-V-Zr systems show promise as base of high-strength Al alloys for high-temperature applications.
Effects of Cold Work on Precipitation in Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) Alloys
S.P. RINGER B.C. MUDDLE and I.J. POLMEAR
A study has been made of the effects of cold work prior to aging on precipitation hardening in selected Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys. General aging characteristics have been determined by differential scanning calorimetry and response to hardening has been correlated with microstructure using transmission electron microscopy (TEM) selected area electron diffraction (SAED) and quantitative stereology. Particular attention has been given to the phases 
and T1 that form on the {111}
planes although information on the precipitates 
', S' (or S) and 
' is also reported. Although and T1 have similar morphologies and habit planes their response to cold work prior to aging is different. Deformation promotes T1 formation at the expense of the ' phase in Al-Cu-Li alloys and at the expense of ', 
, and S' in Al-Cu-Li-Mg-Ag alloys. On the other hand in Al-Cu-Mg-Ag alloys deformation assists precipitation of ' at the expense of phase and some decrease is recorded in the hardening response. Prior cold work is also found to reduce the response during natural aging in most alloys. These results are discussed in terms of the role of particular alloying additions.
High-Temperature Metallurgy of Advanced Borated Stainless Steels
C.V. ROBINO and M.J. CIESLAK
The high-temperature metallurgy of advanced borated stainless steels has been evaluated through differential thermal analysis, aging studies, quantitative metallography, and impact toughness measurements. Differential thermal analysis (DTA) was conducted on alloys containing a range of boron concentrations and was used to determine the temperatures associated with melting/solidification reactions. Aging studies, conducted at temperatures near the solidus, were used to determine the effects of elevated temperature exposures on impact properties and microstructure. Differential thermal analysis quantified the solidus and liquidus temperatures as a function of boron concentration. Impact testing of samples aged at temperatures near the solidus indicated only moderate reductions in toughness. Particle shape measurements indicated that the boride particles were initially nearly spherical and remained so during elevated temperature exposure, although some faceting and agglomeration of the borides did occur. Measurements of boride particle size distributions were used to define the time, temperature, and composition dependence of the boride coarsening. Coarsening data were analyzed in terms of current coarsening models. These analyses indicated that the boride particle coarsening followed the theoretically predicted tl/3 time dependence and that the coarsening rate increased with increasing volume fraction of the boride phase. Analysis of the particle size data for aging at various temperatures indicated that the boride coarsening was consistent with the activation energy for Cr diffusion in austenite. Scanning electron microscopy of the impact fracture surfaces showed that the failure mode in isothermally heat-treated samples was ductile and essentially identical to the failure mode for as-received material.
On Grain and Subgrain Rotations in Two Dimensions
T.O. SAETRE and N. RYUM
Computer simulations of subgrain growth by coalescence in two dimensions have been carried out. The model governing the dynamics allows the subgrains to rotate in order to reduce the sub-boundary energies. The purpose of the model is to study this mechanism separately; thus the sub-boundaries are not allowed to migrate out of their initial positions. Hence a coarsening of the subgrain structure occurs due to coalescence only. Results from several simulations are discussed. It was found that the mean subgrain size increased as an exponential function of time. The effect of the initial distribution of orientations and angles of misorientation has also been studied. It was found that the width of the distribution of orientations is important for the evolution of the mean subgrain size. The model and consequently the simulations concern subgrain rotations leading to coalescence. Based on these results the general case of grain rotations in two dimensions has been discussed. It has been suggested that grain rotations depend on the grain boundary energy as a function of the misorientation.
Modeling Al Enrichment in Galvanized Coatings
NAI-YONG TANG
Aluminum enrichment in galvanized coatings was shown to be due to the formation of an inhibition layer consisting of Fe2AI5 at the substrate/coating interface. The formation of the inhibition layer is a two-stage process. The first stage associated with a high rate of Al uptake is nucleation controlled and the successive stage is diffusional growth controlled. The critical nucleus size is approximately one molecule of the compound Fe2Al5 and the energy barrier for the heterogeneous nucleation is 0.94 eV. Aluminum uptake increases with increasing strip-entry temperature and thickness because both work to increase the effective temperature for the nucleation and growth processes. A model was proposed in which Al enrichment was shown as a function of bath Al content bath temperature strip-entry temperature strip thickness immersion time and coating weight. The model is in good agreement with experimental results available in the open literature.
Precipitation Hardening of Mg-Zn and Mg-Zn-RE Alloys
L.Y. WEI, G.L. DUNLOP, and H. WESTENGEN
The age-hardening response of two cast Mg-Zn-RE alloys has been investigated and compared with that of a binary Mg-Zn alloy. The microstructures of the aged specimens were examined by analytical electron mieroseopy. Formation of a fine dispersion of rodlike 
'1 precipitates is the main cause for age hardening while extensive precipitation of disc-shaped '2 coincides with the onset of overaging. Rare earth additions retard the formation of '2 precipitates and thus postpone overaging. Four different orientation relationships between '2 precipitates and matrix were found and explained in terms of the near-CSL model.
Recrystallization Kinetics in Copper: Comparison between Techniques
E. WOLDT and D. JUUL JENSEN
Six different experimental techniques (electron backscattering (Kikuchi) patterns, calorimetry, microand macrohardness, orientation contrast scanning electron microscopy, and neutron diffraction) have been used to determine the volume fraction of recrystallized material X(t) in a series of partly recrystallized copper samples. Before recrystallization, the copper samples were cold-rolled to 92 pct reduction in thickness. The results obtained with the different experimental techniques are compared, and the applicability and accuracy of the six techniques are considered. While five of the techniques reveal the same kinetics, although with different degrees of accuracy, the texture measurements by neutron diffraction seem to indicate a faster recrystallization process. Finally, the recrystallization kinetics and stored energy aspects of the copper material are discussed with respect to the JohnsonMehl-Avrami-Kolmogorov model.
Anelastic Behavior of Barium-Titanate-Based Ceramic Materials
W. DUFFY, Jr., B.L. CHENG, M. GABBAY, and G. FANTOZZI
The internal friction (Q-1) and Young's modulus (E) of BaTiO3-based ceramics were measured vs temperature from -100°C to 150°C. Rectangular bars of high-density (96 to 99 pct) materials were driven electrostatically in flexural vibration at a resonance frequency of about 3 kHz, at maximum strain levels of about 10-6. The curves of Q-1(T) and E(T) allow the study of the following three phase transformations: tetragonal to cubic (about 130°C in pure material), orthorhombic to tetragonal (about 0°C in pure material), and rhombohedral to orthorhombic (about -80°C in pure material). Internal friction and modulus data were obtained on pure material and on materials doped with niebium and cobalt to give semiconducting and insulating X7R behavior. Permittivity, dielectric loss, and microstructure data are given and used to aid interpretation of the mechanical measurement data.
Can the Average Particle Section Size in a Metallographic Plane Be Larger Than the True Average Particle Size In a Three-Dimensional Microstructure?
PASCAL LOUIS and ARUN M. GOKHALE
It is shown that the average particle section size observed in a me/allographic plane can be larger or smaller than the true average particle size in the three-dimensional (3-D) microstructure. The ratio of the average particle section size in a representative me/allographic plane to the true average particle size in the 3-D microstructure is linearly related to the square of the coefficient of variation (CV) of the size distribution function in the 3-D microstructure. For most of the size distributions encountered in material microstructures the average particle section size in a me/allographic plane is expected to be larger than the true average particle size in the 3-D microstructure. Experimental data on hollow spherical carbon particles in a polymer matrix composite are presented to illustrate the theoretical results.
Compatibility of Deformation in Two-Phase Ti-AI Alloys: Dependence on
Microstructure and Orientation Relationships
J. LUSTER and M.A. MORRIS
A two-phase alloy of composition Ti-47.5AI-2.5Cr has been studied under two heat-treated conditions in order to obtain different microstructures. These consisted of lamellar and equiaxed distributions of 
grains in which the 2 phase was distributed as long lamellae or smaller globules, respectively. The specific rotation relationships between / and /2 grams have been measured, and these have been used to understand their effect on the compatibility of deformation across adjacent grains. For this, detailed analysis of active slip systems has been carried out by transmission electron microscopy (TEM) observations of deformed samples. A theoretical calculation of a geometric compatibility factor characterizing the best slip transfer across adjacent grains has been used in such a way that it has been possible to deduce the role played by the type of orientation relationship between grains in producing active deformation systems that allow the maximum compatibility of deformation.
Energy Dissipation Efficiency in Aluminum Dependent on Monotonic Flow Curves and Dynamic Recovery
H.J. McQUEEN E. EVANGELISTA N. JIN and M.E. KASSNER
In the hot working of Al the flow curves are usually monotonic reaching saturation at a lower strain 
s and stress 
s as temperature rises and strain rate declines. Microstructural examination confirms that the dislocation density rises to a steady-state level through formation of an equiaxed subgrain substructure with constant dimension that is larger for lower stress. The energy dissipation efficiency estimated by dynamic materials modeling for flow curves of the above type is the result of dynamic recovery not of dynamic recrystallization which is characterized by flow curves with a peak and marked softening to a steady-state regime.
Unconstrained and Constrained Tensile Flow and Fracture Behavior of an Nb-1.24 At. Pct Si Alloy
M.G. MENDIRATTA, R. GOETZ, D.M. DIMIDUK, and J.J. LEWANDOWSKI
The unnotched and notched tensile behavior of the -phase constituent (Nb with Si in solid solution) of the (Nb)/Nb5Si3 composite has been investigated at room temperature and -196°C. At room temperature, the unnotched tensile behavior comprises signifcant strengthening due to Si, low straio-rate sensitivity, low strain hardening, extensive ductility, and ductile microvoid coalescence fracture, even at strain rates as high as 1.1 s-1. At -196°C, the unnotched alloy exhibited much higher strength, good ductility, and cleavage fracture. At room temperature, the notched specimens exhibited cleavagelike fracture with significant plasticity, and at -196°C, they exhibited cleavagelike fracture with much lower plasticity at the notch. A finite-element analysis (FEA) of stress and strain fields in the vicinity of the notch root, together with unnotched tensile behavior, indicates that plasticity plays an important role in nucleating cracks,while the high-axial tensile stress component governs crack propagation. These results are used to rationalize toe observed toughening and fracture behavior of a (Nb)/Nb5Si3 composite.
The Role of Hydrogen in Stress-Corrosion Cracking of Austenitic Stainless Steel in Hot MgCI2 Solution
L. QIAO, X. MAO, and W. CHU
The role of hydrogen in stress-corrosion cracking (SCC) of austenitic stainless steel was investigated in boiling chloride solution. The tests in the mixed melted salt verified that hydrogen-induced cracking (HIC) could occur at 160°C if aufficient hydrogen could be supplied continuously. It was found that the threshold SCC intensity factors of both 321 and 310 steels were lower than those of HIC during dynamic charging at high fugacity at 40°C and 160°C. In addition, anodic polarization decreased hydrogen concentration and promoted SCC in hot LiCI solution, while cathodic polarization increased hydrogen concentration and restrained SCC. Hydrogen could be introduced into the specimen and be concentrated at the crack tip during SCC. It could promote anodic dissolution and SCC remarkably, although it was not enough to produce cracking.
The Influence of Heat Treatment on the High-Stress Abrasion Resistance and Fracture Toughness of Alloy White Cast Irons
I R SARE and B K ARNOLD
The influence of a range of austenitizing and subcritical (tempering) heat treatments on the high-stress abrasion resistance and fracture toughness of four commercially significant grades of alloy white cast iron was investigated Complementing an earlier study[1] on the influence of a more limited range of heat treatments on the gouging abrasion performance of the same alloys, the results showed that the effect of austenitizing temperature on high-stress abrasion pin test weight loss differed for each alloy With increasing austenitizing temperature, these results ranged from a substantial improvement in wear performance and retention of hardness through to virtually no change in wear performance and substantial falls in hardness Fracture toughness, however, increased markedly in all alloys with increasing austenitizing temperature Tempering treatments in the range 400°C to 600°C, following hardening at the austenitizing temperature used commonly in industrial practice for each alloy, produced significant changes in both hardness and wear performance, but negligible changes in fracture toughness Most importantly, the data showed that selection of the correct temperature for subcritical heat treatment to reduce the retained austenite content for applications involving repeated impact loading is critical if abrasion resistance is not to suffer
Modeling Microstructural Development during the Forging of Waspaloy
GANGSHU SHEN, S.L. SEMIATIN, and RAJIV SHIVPURI
A model for predicting the evolution of microstructure in Waspaloy during thermomechanical processing was developed in terms of dynamic recrystallization (DRX), metadynamic recrystallization, and grain growth phenomena. Three sets of experiments were conducted to develop the model: (1) preheating tests to model grain growth prior to hot deformation; (2) compression tests in a Gleeble testing machine with different deformation and cooling conditions to model DRX, metadynamic recrystallization, and short time grain growth during the post deformation dwell period and cooling; and (3) pancake and closed die forging tests conducted in a manufacturing environment to verify and refine the model. The microstructural model was combined with finite element modeling (FEM) to predict microstructure development during forging of Waspaloy. Model predictions showed good agreement with microstructures obtained in actual isothermal and hammer forgings carried out at a forging shop.
Design of Multiscalar Metallic Multilayer Composites for High Strength, High Toughness, and Low CTE Mismatch
D.J. SROLOVITZ, S.M. YALISOVE, and J.C. BILELLO
We propose a new class of multilayer composites that consists of alternating tough and strong layers. Both the tough and the strong layers are metallic, effectively reducing the coefficient of thermal expansion (CTE) mismatch problem that often plagues metal-ceramic composites. The high-strength layers are themselves very fine-scale metallic multilayer composites. The high strengths result from Orowan strengthening of these very fine-scale layers. We present detailed analyses of the flow stress, toughness, and thermal stability of these multiscalar metallic multilayer composites (M3C) as a guide for microstructural optimization. The dominant term in the flow stress is proportional to the volume fraction of the strong layers and scales inversely with thickness of the very fine-scale layers that make up the strong layer. The toughness is dominated by the plastic flow of the tough layers and is proportional to the volume fraction and flow stress of the tough layers, as mod)fied by plastic constraint. The thermal stability of M3Cs is discussed in the context of solubility, length scales, and interdiffusivity of the two metals. Preliminary results suggest that M3Cs do exhibit an unusual combination of high toughness and strength.
The Creep-Damage Constitutive and Life Predictive Model for Nickel-Base Single-Crystal Superalloys
Z.F. YUE, Z.Z. LU, and C.Q. ZHENG
A two-state-variable creep-damage constitutive and life predictive model that has been built is discussed in this article. The cavitation-controlled damage mechanism and microstructural degradation, i.e., material damage mechanism, are considered. The latter is derived mainly from the rafting and derafting of the precipitate '. The model has been verified by the creep experiments of nickel-base single-crystal DD3 at 760°C and 850°C. The steady creep and tertiary creep can be predicted satisfactorily. The active slip systems are confirmed as octahedral <112> {111} based on the lattice rotation. The parameter C reflecung material damage mechanism depends on the crystallographic orientation and can be assigned to the value C
<011> along (001) crystallographic orientation and C<011> along <011> orientation partially. The life in different crystallographic orientations can be predicted satisfactorily.
Communication: In Situ Method of Following the Reactivity between SiC and Liquid Aluminum
S. GOWRY and M. BOUCHARD
Effect of Ferrite Transformation on the Tensile and Stress Corrosion Properties of Type 316 L Stainless Steel Weld Metal Thermally Aged at 873 K
H. SHAIKH, H.S. KHATAK, S.K. SESHADRI, J.B. GNANAMOORTHY, and P. RODRIGUEZ
This article deals with the effect of the microstructural changes, due to transformation of delta ferrite, on the associated variations that take place in the tensile and stress corrosion properties of type 316 L stainless steel weld deposits when subjected to postweld heat treatment at 873 K for prolonged periods (up to 2000 hours). On aging for short durations (up to 20 hours), carbide/carbonitride was the dominant transformation product, whereas sigma phase was dominant at longer aging times. The changes in the tensile and stress corrosion behavior of the aged weld metal have been attributed to the two competitive processes of matrix softening and hardening. Yield strength (YS) was found to depend predominantly on matrix softening only, while significant changes in the ultimate tensile strength (UTS) and the work-hardening exponent, n, occurred due to matrix hardening. Ductility and stress corrosion properties were considerably affected by both factors. Fractographic observations on the weld metal tested for stress-corrosion cracking (SCC) indicated a combination of transgranular cracking of the austenite and interface, cracking.
[Ni(Al,Ti)]-,'[Ni2AlTi]-
'[Ni3(Al,Ti)] Metal-Metal Composites, ', and ') in the Ni-Ti-AI system was fabricated by extruding a mechanical mixture of the compounds. In this way, the microstructure could be engineered to disperse the most ductile intermetallic in between the more brittle components. An additional feature was to choose the , ', and ' powders with compositions which allowed them to coexist in equilibrium when coextruded to form the composite. It is found that this approach confers elevated temperature microstluctural stability, a crack arresting ability, reasonable ductility, and considerable strength even at high temperatures.
Communication: Effect of Process Parameter Variability on HIP Consolidation of Continuous-Fiber, Metal-Matrix Composites Made from Foil/Fiber/Foil Layups
P.D. NICOLAOU, S.L. SEMIATIN, AND W.H. ZIMMER
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