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Volume 26A, No. 6, June 1995 This Month Featuring: 1993 Edward DeMille Campbell Memorial Lecture; 1993 Distinguished Lecture in Materials and Society; Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Physical Chemistry; Welding and Joining; Surface Treatment; Electronic, Magnetic, and Optical Material; Solidification. View June 1995 Table of Contents.
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)m,(010)L-J//(
)m and (001)L-J//(211)m. The rotation axis and rotation angle between two variants of the L-J phase are [021] and 90 deg. The L-J phase has bever been observed in various Cu-Al, Cu-Mn, and Cu-Al-Mn alloy systems before.
Phase in Some Rapidly Quenched Titanium-Eutectoid Alloys The decomposition of the phase in rapidly quenched Ti-2.8 at. pct Co, Ti-5.4 at. pct Ni, Ti-4.5 at. pct, and 5.5 at. pct Cu alloys has been investigated by electron microscopy. During rapid quenching, two competitive phase transformations, namely martensitic and eutectoid transformation, have occurred, and the region of eutectoid transformation is extended due to the high cooling rates involved. The phase decomposed into nonlamellar eutectoid product (bainite) having a globular morphology in Ti-2.8 pct Co and Ti-4.5 pct Cu (hypoeutectoid) alloys. In the near-eutectoid Ti-5.5 pct Cu alloy, the decomposition occurred by a lamellar (pearlite) type, whereas in Ti-5.4 pct Ni (hypereutectoid), both morphologies were observed. The interfaces between the proeutectoid 
and the intermetallic compound in the nonlamellar type as well as between the proeutectoid and the pearlite were often found to be partially coherent. These findings are in agreement with the Lee and Aaronson model proposed recently for the evolution of bainite and pearlite structures during the solid-state transformations of some titanium-eutectoid alloys. The evolution of the Ti2Cu phase during rapid quenching involved the formation of a metastable phase closely related to an "
-type" phase before the equilibrium phase formed. Further, the lamellar intermetallic compound Ti2Cu was found to evolve by a sympathetic nucleation process. Evidence is established for the sympathetic nucleation of the proeutectoid crystals formed during rapid quenching.
Structure of Mechanically Alloyed Ti-Al-Nb Powders
GUO-HAO CHEN, C. SURYANARAYANA, and F.H. (SAM) FROES
Ti-Al-Nb ternary powder mixtures containing 24Al-11Nb, 25Al-25Nb, 37.5Al-12.5Nb, and 28.5Al-23.9Nb (at. pct) were mechanically alloyed in a SPEX 8000 mixer mill using a ball-to-powder weight ratio of 10:1. The structural evolution in these alloys was investigated by X-ray diffraction and transmission electron microscopy techniques. A solid solution of Al and Nb in Ti was formed at an early stage of milling, followed by the B2/body-centered cubic (bcc) and amorphous phases at longer milling times. The stability of these phases and their transformation to other phases have been investigated by heat treating these powders at different temperatures. The B2/bcc phase transformed into an orthorhombic (O-Ti2AlNb) or a mixture of the orthorhombic (O) and hexagonal close-packed (2-Ti3Al) phases, the proportion of phases being dependent on the powder composition. Milling beyond the amorphous phase formation resulted in the formation of an fcc phase in all the powders, which appears to be TiN, formed as a result of contamination of the powder.
The Liquid Film Migration in a Sintered Fe-Cr-C Base Alloy
HYEON-SANG SHIN, JEONG-SEOG, KIM DUK-YONG YOON, and YOUNG-DUH SONG
In an Fe-5Cr-1Mo-2Cu-0.5P-3C (wt pct) alloy prepared by liquid-phase sintering at 1120°C, the intergranular liquid films migrate during cooling and isothermal heat treatment at temperatures where the solid and liquid phases coexist. The liquid film migration (LFM) occurs extensively even during rapid cooling, apparently because of a large driving force. The migrating liquid films solidify to form network carbides. In the regions swept by the migrating liquid films, the concentrations of the substitutional solute atoms are slightly different from those in the original grains. When cooled rapidly, martensite is abundant in these regions. It is possible that the driving force for LFM in this alloy stems in part from the C concentration change associated with the concentration changes of the slowly diffusing substitutional solute atoms.
Recrystallization and Grain Growth Phenomena in a Particle-Reinforced Aluminum Composite
D C. VAN AKEN P.E. KRAJEWSKI, G.M VYLETEL, J.E. ALLISON, and J.W JONES
Recrystallization and grain growth in a 2219/TiC/15p composite were investigated as functions of the amount of deformation and deformation temperature Both cold and hot deformed samples were annealed at the normal solution treatment temperature of 535°C. It was shown that large recrystallized grain diameters, relative to the interparticle spacing, could be produced in a narrow range of deformation for samples cold-worked, and those hot-worked below 450°C. For cold-worked samples between 4 to 6 pct deformation, the recrystallized grain diameters varied from 530 to 66 µm as the amount of deformation increased. Subsequent grain growth was not observed in these recrystallized materials and noncompact grain shapes were observed. For deformations greater than 15 pct, recrystallized grain diameters less than the interparticle spacing were observed and subsequent grain growth produced a pinned grain diameter of 27 µm The pinned grain diameter agreed well with an empirical model based on three dimensional (3-D) Monte Carlo simulations of grain growth and particle pinning in a two-phase material. Tensile properties were determined as a function of grain size, and it was shown that grain size had a weak influence on yield strength. A maximum in the yield strength was observed at a grain size larger than the normal grain growth and particle-pinned diameter
Communication: Grain-Boundary Precipitation and Fracture Behavior of an Al-Cu-Li-Mg-Ag Alloy
YIWEN MOU, J.M. HOWE, and E.A. STARKE, JR.
Local Fatigue Damage Accumulation around Notch Attending Crack Initiation
Y. IINO
The subsequent recrystallization technique was used to study the process of local damage accumulation around a notch under conditions of low-cycle fatigue. A 0.8-in. compact tension specimen of 304 stainless steel with a notch radius of 1 mm. The accumulated plastic zone around the notch increases with the number of cycles N. The accumulated plastic strain within the zone also increases with N, producing the strain gradient (damage gradient). A fatigue crack initiates when the accumulated plastic strain at the notch root reaches a critical value equal to the fracture strain of the material; that is, when the accumulated plastic work at the crack initiation site becomes critical. The fatigue crack emanating from a notch root grows through the pre-existing damaged zone. It is shown that this local damage accumulation approach can explain the fast growth of a short crack from a notch.
Thermal Fracture Endurance of Cast Irons with Application Study of Pig Iron Ingot Molds
JYE-LONG LEE and SHEN-CHIH LEE
Pig iron ingot molds manufactured with flake, compacted graphite cast iron, and spheroidal graphite cast iron were installed on a pig iron casting machine and subjected to thermal cycling for studying thermal fracture endurance of the three cast irons. The effects of graphite morphology on the fracture mechanism were analyzed by examining the fracture patterns, microstructures, and microcracks in the failed molds. The determining factors of thermal fracture endurance were elucidated with thermal fracture resistance indices. Compacted graphite cast iron exhibited better thermal fracture endurance than flake and spheroidal graphite cast irons because of its higher strength-to-thermal stress ratio.
Defect Structures and Nonbasal Slip of C36 Laves Phase MgNi2 in a Two-Phase Alloy
YAPING LIU, JAMES D. LIVINGSTON, and SAMUEL M. ALLEN
The microstructure of the C36 Laves phase MgNi2 in a two-phase Ni-15 at. pct Mg alloy was studied using electron microscopy. Both intrinsic and extrinsic stacking faults were found on the basal plane. The displacement vectors of the faults and Burgers vectors of partial dislocations either lying on the basal faults or bounding each side of the faults were analyzed using high-resolution transmission electron microscopy (TEM). Nonbasal faults were found on the pyramidal planes {
} and {
} and the prismatic plane {
}, all connected to basal faults. The displacement vector of the pyramidal faults was determined to be 1/12 <
>. The microstructure was also studied after room-temperature compression, and nonbasal slip was found to be the major deformation mode.
Application of Image Analysis for Characterization I of Spatial Arrangements of Features in Microstructure
PASCAL LOUIS and ARUN M. GOKHAL
A number of microstructural processes are sensitive to the spatial arrangements of features in microstructure. However, very little attention has been given in the past to the experimental measurements of the descriptors of microstructural distance distributions due to the lack of practically feasible methods. We present a digital image analysis procedure to estimate the microstructural distance distributions. The application of the technique is demonstrated via estimation of K function, radial distribution function, and nearest-neighbor distribution function of hollow spherical carbon particulates in a polymer matrix composite, observed in a metallographic section.
Internal Friction in AlCu-Al2O3 Metal-Matrix Composites
L. PARRINI and R. SCHALLER
In metal-matrix composites (MMCs) the metal matrix is exposed to plastic deformation and damage accumulation in the region close to the reinforcements, following mechanical or thermal stress. In this connection Al-4 wt pct Cu-based MMCs reinforced with 20 vol pct Al2O3 fibers were characterized by internal friction (IF) measurements. The IF measurements as a function of the vibration amplitude present a solid friction peak connected with the loosening of metal-fiber interfaces, while plastic deformation was associated with a high amplitude IF background. On this basis, IF measurements allowed us to identify the distribution of internal stresses and damage accumulation at matrix-fiber interfaces or plastic flow in the matrix in different thermo-mechanical conditions. Furthermore IF measurements allowed damage accumulation consequent to mechanical fatigue to be followed.
Nanometer-Scale Crack Initiation and Propagation Behavior of Fe3AI-Based Intermetallic Alloy
L.J. QIAO, X. MAO, and C.Z. CHEN
The initiation and propagation of nanometer-scale cracks have been investigated in detail by in situ transmission electron microscope (TEM) observations for the intermetallic compound Fe3Al under mode I loading. No dislocation was detected and no dislocation emission was found when cracks propagated directly from the thin edge of a double-jet hole where the thickness of the foil was below a critical thinness. Thinning took place in the thicker region of the foils because a great number of dislocations were emitted from the crack tip, and then an electron semitransparent region was formed in front of the crack tip. Following this process, a dislocation-free zone (DFZ) was formed. The maximum normal stress occurs in the zone. Nanometer-scale cracks initiated discontinuously ahead of the main crack tip in the highly stressed zone. The size of the smallest nanocrack observed was about 3 nm and the tip radius of the nanocracks was less than 1 nm when the applied loading was low. The radius of the main crack tip was about 2.5 nm. The distances between discontinuous nanocracks and the main crack tip were about 5 to 60 nm, depending on the applied tensile loading. A relationship was found between the tensile loading and the nanocrack distance from the crack tip. The distance increases with the tensile loading, which is consistent with an "elastic-plastic" theoretical model
Elevated Temperature Creep and Fracture of Properties of the 62Cu-35Au-3Ni Braze Alloy
J.J. STEPHENS and F.A. GREULICH
The Cu-Au-Ni braze alloys are used for metal/ceramic brazes in electronic assemblies because of their good wetting characteristics and low vapor pressure. We have studied the tensile creep properties of annealed 62Cu-35Au-3Ni alloy over the temperature range 250°C to 750°C. Two power-law equations have been developed for the minimum creep rate as a function of true stress and temperature. At the highest temperatures studied (650°C and 750°C). The minimum creep rate is well described with a stress exponent of 3.0 which can be rationalized in the context of Class I solid solution strengthening. The inverted shape of the creep curves observed at these temperatures is also consistent with Class I alloy behavior. At lower temperatures, power-law creep is well described with a stress exponent of 7.5, and normal three-stage creep curves are observed. Intergranular creep damage, along with minimum values of strain to fracture, is most apparent at 450°C and 550°C. The lower stress exponent in the Class I alloy regime helps to increase the strain to fracture at higher temperatures (650°C and 750°C). The minimum creep rate behavior of the 62Cu-35Au-3Ni alloy is also compared with those of the 74.2Cu-25.8Au alloy and pure Cu. This comparison indicates that the 62Cu-35Au-3Ni has considerably higher creep strength than pure Cu. This fact suggests that the 62Cu-35Au-3Ni braze alloy can be used in low mismatch metal-to-ceramic braze joints such as Mo to metallized alumina ceramic with few problems. However, careful joint design may be essential for the use of this alloy in high thermal mismatch metal-to-ceramic braze joints.
A Comparison of Shear Localization Susceptibility in U-0.75 Wt Pct Ti and W-Ni-Fe during High Strain Rate Deformation
ANNA K. ZUREK and PAUL S. FOLLANSBEE
A comparison of shear instability in U-Ti vs W alloy during high strain rate deformation is presented. Experimental quasi-static and dynamic deformation data are used to formulate the constitutive description based on the mechanical threshold stress model (MTS). The MTS model is used to predict the deformation behavior of U-Ti and W-alloy beyond the conventionally achievable experimental capabilities. We suggest that uranium alloys are more prone to catastrophic-localized deformation (adiabatic shearing) due to the existence of a soft high-temperature phase, which is reached by uranium alloys undergoing large strains at high strain rate of deformation.
Communication: Development of High-Strength Aluminum Alloys by Mesoscopic Structure Control
KOZO OSAMURA, OSAMU KUBOTA, PARSONS PROMSTIT, HIROSHI OKUDA, SHOJIRO OCHIAI, KAZUO FUJII, JUN KUSUI, TAKAMASA YOKOTE, and KOHEI KUBO
Communication: Discussion of "Effect of Fe on the SuperplastiC Deformation of AN-22 Pct Al"
NAI-YONG TANG
Communication: Author's Reply
FARGHALLI A. MOHAMED and PRABIR K. CHAUDHURY
hexagonal close-packed (hcp) martensite (35 vol pct), producing up to a 130 pct increase of surface hardness. The surface hardness increase was 40 to 60 pct the shot-peened specimen and about 60 pct for the cold-rolled specimen. The LSP strengthening effect on Hadfield steel was attributed to the combined effects of the partial dislocation/stacking fault arrays and the grain refinement due to the presence of the -hcp martensite For the cold-rolled and shot-peened specimens, the strengthening was a result of -hcp martensite and twins with dislocation effects, respectively. Shot peening resulted in a relatively higher compressive residual stress throughout the specimen than LSP.
Microstructural Study of the Interface in Laser-Clad Ni-AI Bronze on Al Alloy AA333 and Its Relation to Cracking
Y. LIU, J. MAZUMDER , and K. SHIBATA
The interface toughness between a laser clad and the substrate determines whether the cladding is useful for engineering application. The objective of this investigation is to correlate the interface properties of laser-clad Ni-Al bronze on Al alloy AA333 with the microstructure and crystal structure of the interface. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX) are used to examine the interface. In a good clad track, the interface is an irregular curved zone with a varying width (occasionally keyholing structure) from 30 to 150 µm. A compositional transition from the Cu-rich clad (83 wt pct Cu) to the Al-rich substrate (3.2 wt pct Cu) occurs across this interface. Three phases in the interface are identified in TEM: Al solid solution, 
phase, and y1 phase, as described in the Cu-Al binary phase diagram. In a good clad track, the and y1 phases are distributed in the Al solid solution. In a clad track with cracks, the interface structure spreads to a much larger scale from 300 µm to the whole clad region. Large areas of and y1 phases are observed. The mechanism of cracking at the interface is related to the formation of a two-phase region of and y1 phases. To understand the microstructure, a nonequilibrium quasi-binary Cu-Al phase diagram is proposed and compared with the equilibrium binary Cu-Al phase diagram. It is found that the occurrence of many phases such as 
1, 2, 
1, 2, 1, 2, 
0, 0, and , as described in the equilibrium binary Cu-Al phase diagram is suppressed by either the cladding process or by the alloying elements. The three identified phases (Al solid solution, phase, and 1 phase) showed significant extension of solubility.
Communication: The Effect of Phosphorus in Base Steel on the Forming Characteristics of Galvannealed Sheet Steels: New Average Iron Content Range of Galvannealed IFP Steel
C.S. LIN and M. MESHII
characterizing the <100> direction of the grains that is closest to the temperature gradient were then deduced from the EBSPs measurement. It was found that, near the surface of the chill, the distribution was close to the theoretical curve calculated for randomly oriented grains. As the distance from the chill increased, the measured distribution became narrower and was displaced toward smaller values. At 2 mm from the chill, the most probable orientation of the grains was found to be about 0.21 rad (12 deg). The information obtained with the EBSPs was then compared with the results of a three-dimensional stochastic model (3D SM) describing the formation of grain structure during solidification. The model accounts for the random location and orientation of the nuclei, for the growth kinetics and preferential <100> growth directions of the dendrites. Although this model assumes a uniform temperature within the specimen, the simulation results were found to be in good agreement with the EBSPs measurement.
Effect of Silicon Particles on the Fatigue Crack Growth Characteristics of A1-12 Wt Pct Si-0.35 Wt Pct Mg-(O to 0.02) Wt Pct Sr Casting Alloys
F.T. LEE, J.F. MAJOR ,and F.H. SAMUEL
Fatigue crack growth (FCG) characteristics and mechanisms in Al-Si-Mg eutectic casting alloys containing 0.35 wt pct Mg and 0 to 0.02 wt pct Sr were investigated as a function of stress ratio R stress-intensity-factor range 
K, and silicon (Si) particle size. The fatigue crack propagation behavior was compared with that observed in commercial casting alloy A356. At the same applied K level, the crack growth rate was found to increase with increasing stress ratio and Si particle size. Modified (fine Si morphology) and A356 alloys showed better FCG resistance than the unmodified (coarse Si morphology) ones, for a constant applied K due to increased closure. The effects of roughness-induced and plasticity-induced crack closures crack branching and crack meandering on the fatigue crack propagation observed in these alloys have been discussed. The fatigue crack propagation path is found to be dependent on the Si particle characteristics. The mechanisms of silicon particle decohesion and cracking are also discussed.
The Characterization of the Al-12 Wt Pct Si Flake Powder Produced by a Double-Disk Process
KUANG-YUAN SHUE and JIEN-WEI YEH
A double-disk (DD) process was developed in this study for producing highly elongated flakes which are very promising for application as a filler for conductive composites with a plastic matrix. The Al-12Si alloy was selected as the experimental alloy in view of its high stiffness, strength, and integrity which are beneficial for retaining its high aspect ratio during the fabrication of the composites. Various operational parameters, i.e., pouring temperature and the rotational speeds of upper disk and quenching disk, were monitored in the powder production to reveal their effects on the size distribution and morphology of flakes. The flake formation mechanism was also investigated. Experimental results indicate that the broad size distribution of the flakes produced by the double-disk process rises from the splat-quenching effect and the formation of interlinked flakes and whisker-shaped flakes. The mean particle size (or mean flake width) increases with a decreasing rotational speed of ccntrifugal disk or quenching disk speed, but is little influenced by pouring temperature. The length-to-width ratio of flakes varies with the rotational speed of quenching disk but inversely with pouring temperature, centrifugal disk speed, and flake width. The thickness varies inversely with all of these variables. The aspect ratio varies with pouring temperature, rotational speed of quenching disk, and flake width, but varies inversely with centrifugal disk speed. All of these variations might be accounted for by considering the solidification rate and dragging rate. Based on these two rates, theoretical relationships for length-to-width ratio, thickness, and aspect ratio as a function of various metallurgical factors are derived and correlate sufficiently with the trends of experimental data.
Theory of Layered-Structure Formation in Peritectic Systems
R. TRIVEDI
Two-phase growth in a peritectic system has been examined and a theoretical model is developed for the formation of a layered structure in which two phases form alternate layers that are oriented parallel to the interface. The width of each layer and the spacing between layers have been shown to be inversely proportional to velocity and the proportionality constants depend upon the nucleation temperatures for the two phases. The nucleation temperatures for the two phases are shown to play a critical role in the formation and stability of layered structures. The range of compositions over which layered structures can form is identified and it is shown that convection effects in the liquid will destabilize the layered structure. Several key experimental studies have been identified to evaluate minimum undercoolings required for the nucleation of each of the two phases. The morphology of the transition interface is shown to depend upon the relative effects of nucleation and growth of the two phases.
Correction
Contents, Metallurgical and Meterials Transactions B, Volume 26B, June 1995
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