METALLURGICAL AND MATERIALS TRANSACTIONS A
ABSTRACTS
Volume 28A, No. 12, December 1997

This Month Featuring: Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Environment; Welding & Joining; Surface Treatment; Solidification; Materials Processing; Composite Materials. View December 1997 Contents.

ALLOY PHASES

Discontinuous Cellular Precipitation in a High-Refractory Nickel-Base Superalloy
J.D. NYSTROM, T.M. POLLOCK, W.H. MURPHY, and A. GARG
A discontinuous precipitation reaction has been investigated in a high-refractory content nickel-base alloy. The reaction transforms the two-phase -' parent microstructure into a three-phase cellular structure with a ' matrix containing Re-rich P-phase and agglomerated lamellae. The reaction has been studied in polycrystalline material and in bicrystals with varying degrees of boundary misori entation at temperatures in the range of T/Tm = 0.78 to 0.85. The early stages of the reaction are characterized by heterogeneous nucleation of P-phase precipitates and migration of the grain bound ary. At low-angle, near-tilt boundaries misoriented by less than 10 deg, nucleation of P-phase particles was observed, but the cellular reaction did not occur, due to limited boundary mobility and diffusivity. The high degree of supersaturation of Re and W in the initial -' alloy appears to be the primary driving force for the reaction. Small amounts of creep deformation did not significantly influence the extent of the transformation. The diffusivity of Re associated with the moving boundary was calculated to be 5 x 10-8 cm2 s-1 at 1093°C, which is approximately four orders of magnitude greater than the bulk lattice diffusivity of tungsten.

C-Ni and Al-C-Ni Phase Diagrams and Thermodynamic Properties of C in the Alloys from 1550°C to 2300°C
L.L. ODEN and N.A. GOKCEN
The C-Ni and Al-C-Ni phase diagrams were determined by chemical analysis of alloys saturated with carbon within sealed graphite crucibles. The solubility of carbon in nickel over the temperature range 1550°C to 2300°C is given by log (at. pct C) = 2.0376 - 1874.68/T, where T is temperature in kelvin. Isothermal sections for the ternary system were determined at intervals of 150°C over the range of temperatures investigated. The univariant points on the 1700°C, 1850°C, and 2000°C isotherms were determined by metallographic examination of rapidly cooled alloys to be about 67.2Al-1.1C-31.7Ni, 70.3Al-2.3C-27.4Ni, and 82.5Al-7.0C-10.5Ni, respectively, where all concen trations are in atomic percent. Graphite, Al4C3 (decomposition temperature 2156°C), and AlNi (decomposition temperature 1638°C) were the only solid phases observed within the temperature range investigated. The excess partial Gibbs energy for dissolved C, , in liquid Al-C-Ni solutions in equilibrium with C, as calculated from the experimental solubilities and thermodynamic data on Al-Ni, is

= -RT ln y = 136,290 - 57.98T + 3.0T ln T + x2(-63,170 + 14.4T)

+ z 2 (-104,716 + 31.76T) + xz [-23,326 + 20.82T + x(-37,040

+ 17.22T) + xz(747,390 - 202.02T)]

where R is the gas constant and x, y, and z are the atomic fractions of Al, C, and Ni, respectively. The equations for for C in equilibrium with Al4C3 have also been obtained for the binary and ternary alloys.

TRANSFORMATIONS

Texture Development in Dual-Phase Cold-Rolled 18 pct Ni Maraging Steel
Z. AHMAD, M. FAROOQUE, A. UL HAQ, and A.Q. KHAN
Austenite and martensite textures were studied in 18 pct Ni 350-maraging steel as a function of various degrees of cold rolling. The austenite phase in the samples was produced by repeated thermal cycling between ambient and 800°C. The austenite phase thus formed was mechanically unstable and transformed to the martensite phase after 30 pct cold rolling. The texture developed as a result of cold rolling, and its effect upon microstructure and hardness has been studied.

Building a Continuous Cooling Transformation Diagram of - CEZ Alloy by Metallography and Electrical Resistivity Measurements
C. ANGELIER, S. BEIN, and J. BÉCHET
The phase transformations of Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe (-CEZ) have been studied during con tinuous cooling after -solution treatment. For this purpose, electrical resistivity measurements and metallographical examinations have been carried out, and the continuous cooling transformation (CCT) diagram of -CEZ alloy has been plotted. The different kinds of -phase decomposition schemes in -CEZ alloy during continuous cooling have been investigated in detail. Two main morphological features of the / structure are involved, depending on the cooling rate: the basket [chweave and the colony structures are observed for high and low cooling rates, respectively. For the intermediate cooling rates, the two morphologies coexist. Finally, a generalized scheme of the + transformation sequences during continuous cooling is presented.

Simulation of the Precipitation of Sigma Phase in Duplex Stainless Steels
M.B. CORTIE and E.M.L.E.M. JACKSON
The precipitation of sigma phase within the ferrite component of a duplex stainless steel has been simulated using a two-dimensional computer model which takes into account the partitioning of alloy elements between ferrite and austenite. The model is based on a cellular automaton and, despite having a rather simple set of transition rules, is able to simulate changes in the volume fractions of the austenite, ferrite, and sigma phases. The microstructures produced are similar in appearance to those in the real system. Comparison of the model and the real system may assist in the assessment of the various phenomena occurring. Use is made of the model to examine many of the factors that might conceivably be harnessed to retard precipitation of the sigma phase in duplex stainless steels.

Interphase Boundary Precipitation in a Ti-1.7 At. Pct Er Alloy
M.V. KRAL, W.H. HOFMEISTER, and J.E. WITTIG
Microstructures in a Ti-1.7 at. pct Er alloy were studied in the arc-cast, rapidly solidified, and annealed conditions. Transmission electron microscopy (TEM) of the rapidly solidified materials revealed 3- to 20-nm-diameter precipitates that were distributed in regularly spaced, approximately planar sheets throughout equiaxed Ti grains. The precipitate sheet morphology is similar to the interphase boundary carbide sheets that have been documented in many alloy steels. In addition, precipitate fibers with cross sections of approximately 5 nm and up to 500 nm in length were often found adjacent to particle sheets. Electron diffraction experiments showed that the structure and lattice spacings of the sheet and fibrous particles are consistent with elemental erbium. Subsequent annealing treatments resulted in the formation of a face-centered cubic allotrope of Er2O3. The present work describes the precipitate morphologies and crystallography and discusses the applicability of current ledge growth models of interphase boundary precipitation to titanium-erbium alloys.

A New Model for the Volume Fraction of Martensitic Transformations
H.Y. YU
A model using an energy balance is proposed to describe the volume fraction of multiple-interface martensitic transformations. For martensitic transformations without external stresses at quenching temperature T < Ms, the volume fraction of martensite () is proportional to the undercooling (Ms - T) and inversely proportional to a linear function of the quenching temperature (T); thus, = (Ms - T)/[Ms - Mf - (1 - )T], where is a material constant. For stress-induced martensitic transformations under stress aik with temperature T > Ms, the relationship is = 0[1 - ik (aikMsik]-1, where 0 is the initial detectable amount of martensite formed at martensitic starting stress Msik and ik is a material constant. It is found that the results obtained from this model are in good agreement with experimental results.

Communication: Discussion of "An Analysis of Static Recrystallization during Continuous, Rapid Heat Treatment"
V. ERUKHIMOVITCH AND J. BARAM

Authors' Reply
S.L. SEMIATIN, I.M. SUKONNIK, AND V. SEETHARAMAN

Communication: Reduction and Removal of Martensite Stabilization in Cu-Zn-Al-Mn-Zr Shape Memory Alloys
C.W.H. LAM, C.Y. CHUNG, W.H. ZOU, AND J.K.L. LAI

TRANSPORT PHENOMENA

Discontinuity in Normal Spectral Emissivity of Solid and Liquid Copper at the Melting Point
H. WATANABE, M. SUSA, and K. NAGATA
Normal spectral emissivities for solid (,solid) and liquid (,liquid) copper at the melting point have been determined, and the discontinuity in ,liquid and ,solid at the melting point has been discussed. The spectral emissivity was determined by comparing the spectral radiation intensity of the sample to that of the fixed-point blackbody over the wavelength range 550 to 750 nm. The samples and fixed-point blackbody were heated in the continuous casting type cold crucible. Values of ,liquid and ,solid at 650 nm were 0.167 and 0.108, respectively. The emissivity ratio of ,liquid/,solid increased from approximately 1.2 to 1.8 as the wavelength increased in the range 550 to 750 nm. The discontinuity in ,liquid and ,solid at the melting point has been qualitatively explained by the optical ab sorption mechanism due to the intraband transition.

MECHANICAL BEHAVIOR

Numerical Simulation of the X-Ray Stress Analysis Technique in Polycrystalline Materials under Elastic Loading
D. CHIDAMBARRAO, Y.C. SONG, and I.C. NOYAN In this article, the correlation between the local stress distributions and the average stresses obtained by X-ray measurement is studied through numerical modeling. The finite element method (FEM) is used to simulate the local stress distributions within a three-dimensional random aggregate of ani sotropic and isotropic single crystals under uniaxial elastic tension. The local stresses are calculated for polycrystalline W, Al, or Cu slabs with the same geometry and crystallite distribution. The stress/strain values in the crystallites that satisfy the diffraction condition are then used to simulate the X-ray data that would be obtained if the diffraction peaks from such aggregates were actually measured. In particular, the 200, 222, and 420 reflections are analyzed. The correlations between the far-field stress, the local stresses, and the average stresses determined through X-ray analysis are presented.

Effects of Morphology and Volume Fraction of 2 Phase on the Fatigue Crack Propagation of a Ti-24Al-11Nb Alloy
HYUN HO CHUNG, CHONG SOO LEE, and NACK JOON KIM
The main objective of the present investigation is to study the individual effect of 2 morphology and volume fraction on the fatigue crack propagation behavior of a two-phase (2 + ) Ti-24Al- 11Nb alloy. With systematic heat treatments, three distinct morphologies have been obtained: fine colony, fine equiaxed, and fine basket-weave structures. Volume fraction of 2 has been varied from 40 to 68 pct by changing the two-phase annealing temperature. It has been shown that the volume fraction and morphology of 2 phase have a large influence on the fatigue crack propagation behavior. Fine colony structure shows no variation in the fatigue crack propagation rate with the 2 volume fraction. On the other hand, equiaxed and basket-weave structures show the increased resistance to fatigue crack propagation as 2 volume fraction decreases. Such increased resistance to crack growth is attributed primarily to tortuous crack paths, which result in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanism. Quantitative metallography and experimental crack closure measurements are presented to substantiate such interpretations.

Experimental Evaluation of a Polycrystal Deformation Modeling Scheme Using Neutron Diffraction Measurements
BJØRN CLAUSEN and TORBEN LORENTZEN
The uniaxial behavior of aluminum polycrystals is simulated using a rate-independent incremental self-consistent elastic-plastic polycrystal deformation model, and the results are evaluated by neutron diffraction measurements. The elastic strains deduced from the model show good agreement with the experimental results for the 111 and 220 reflections, whereas the predicted elastic strain level for the 200 reflection is, in general, approximately 10 pct too low in the plastic regime.

The Effect of Processing on the Hot Workability of Ti-48Al-2Nb-2Cr Alloys
G.E. FUCHS
The hot compression behavior and microstructure evolution of ingot metallurgy (I/M) and powder metallurgy (P/M) processed samples of the near- Ti-aluminide alloy, Ti-48Al-2Nb-2Cr (at. pct), were determined. Three I/M conditions and two P/M conditions were examined in this study. Hot compression tests were performed in the temperature range of 1100°C to 1300°C at strain rates ranging from 1.67 x 10-1/s to 1.67 x 10-4/s. The P/M materials consolidated by either hot isostatic pressing ("hipping") or extrusion exhibited the best hot workability in most cases. The P/M materials possessed finer, more homogeneous microstructures than the I/M materials. It was also noted that improved workability was observed in materials with equiaxed microstructures without any lamellar constituents.

Ductile-Phase Toughening in V-V;i3Si In Situ Composites
G.A. HENSHALL, M.J. STRUM, B.P. BEWLAY, and J.A. SUTLIFF
This article describes the room-temperature fracture behavior of ductile-phase-toughened V-V3Si in situ composites that were produced by arc melting (AM), cold-crucible induction melting (IM), and cold-crucible directional solidification (DS). Composites were produced containing a wide range of microstructures, interstitial impurity contents, and volume fractions of the ductile V-Si solid solution phase, denoted (V). The fracture toughness of these composites generally increases as the volume fraction of (V) increases, but is strongly influenced by the microstructure, the mechanical properties of the component phases, and the crystallographic orientation of the (V) phase with respect to the maximum principal stress direction. For eutectic composites that have a (V) volume fraction of about 50 pct, the fracture toughness increases with decreasing "effective" interstitial impurity concentra tion, [I] = [N] + 1.33 [O] + 9 [H]. As [I] decreases from 1400 ppm (AM) to 400 ppm (IM), the fracture toughness of the eutectic composites increases from 10 to 20 MPa . Further, the fracture toughness of the DS eutectic composites is greater when the crack propagation direction is perpendicular, rather than parallel, to the composite growth direction. These results are discussed in light of conventional ductile-phase bridging theories, which alone cannot fully explain the fracture tough ness of V-Si in situ composites.

Neutron Diffraction Measurements of Intergranular Strains in MONEL-400
T.M. HOLDEN, A.P. CLARKE, and R.A. HOLT
Measurements have been made of the strain response parallel and perpendicular to an applied stress in uniaxial tensile specimens of MONEL-400. The diffraction elastic constants are in close agreement with the Kröner model. Beyond the yield point, different crystallographic orientations respond quite differently to the applied stress and, upon unloading, residual intergranular strains are generated which are tensile for some orientations and compressive for others. A simple theory based on the Taylor-Bishop-Hill (TBH) model gives a good qualitative description of the effects.

A New Miniature Mechanical Testing Procedure: Application to Intermetallics
KAZUHIKO ISSHIKI, ZENJI HORITA, TAKAYOSHI FUJINAMI, TAKESHI SANO, MINORU NEMOTO, YAN MA, and TERENCE G. LANGDON
A new miniature double-shear testing procedure is introduced for the testing of semibrittle materials such as intermetallics. Detailed experiments on samples of pure Al and an Al-5 pct Mg solid solution alloy confirm that the miniature configuration provides consistent and reliable results when using either a circular or a square cross section. In addition, it is demonstrated that the data obtained using miniature specimens are consistent with results from conventional large double-shear specimens and from specimens tested under tensile conditions. The miniature testing technique is used to provide preliminary mechanical data on a Ni-45 pct Al-5 pct Fe intermetallic.

An Investigation of the Effects of Interfacial Microstructure on the Fatigue Behavior of a Four-Ply [75]4 Continuous Silicon Carbide (SCS-6) Fiber-Reinforced Titanium Matrix Composite
O. JIN, Y. LI, B.M. RABEEH, and W.O. SOBOYEJO
The effects of interfacial microstructure/thickness on the strength and fatigue behavior of a model four-ply [75]4 Ti-15V-3Al-3Cr-3Sn/SiC (SCS-6) composite are examined in this article. Interfacial microstructure was controlled by annealing at 815°C for 10, 50, or 100 hours. The reaction layer and coating thickness were observed to increase with increasing annealing duration. Damage initi ation/propagation mechanisms were examined in as-received material and composites annealed at 815°C for 10 and 100 hours. Fatigue behavior was observed to be dependent upon the stress amplitude. At high stress amplitudes, the failure was dominated by overload phenomena. However, at all stress levels, fatigue crack initiation occurred by early debonding and matrix deformation by stress-induced precipitation. This was followed by matrix crack growth and fiber fracture prior to the onset of catastrophic failure. Matrix shear failure modes were also observed on the fracture surfaces in addition to fatigue striations in the matrix. Correlations were also established between the observed damage modes and acoustic emission signals that were detected under monotonic and cyclic loading conditions.

Correlation of Microstructure and Thermal Fatigue Property of Three Work Rolls
SUNGHAK LEE, DO HYUNG KIM, JAE HWA RYU, and KEESAM SHIN
This is a study of the thermal fatigue property in three centrifugally cast work rolls, i.e., a nickel- grain cast-iron roll (Ni-grain roll), a high-chromium cast-iron roll (Hi-Cr roll), and a high-speed steel roll (HSS roll). The thermal fatigue mechanism was investigated with a focus on the roll microstruc ture and the increase in tensile stress which led the specimen to fracture when it reached the tensile strength. The thermal fatigue test results indicated that the thermal fatigue property was best in the HSS roll, followed by the Hi-Cr roll and the Ni-grain roll, respectively, and that the thermal fatigue life of each roll decreased with the increase of the mean temperature or of the temperature range of the thermal fatigue cycle. The results were then interpreted based on the amount of primary carbides and the cyclic softening phenomenon associated with the exposed time to elevated temperatures. The coarse primary carbides on the specimen surface acted as fatigue crack initiation sites, as they cleaved at a low stress level to form cracks. The HSS roll, having the highest tensile strength and the smallest amount of primary carbides, thus showed better thermal fatigue property than the other rolls. For the improvement of the thermal fatigue property of the rolls, this study suggests a homogeneous distribution of primary carbides by reducing the carbide segregation formed along the solidification cell boundary and by optimizing of the roll-casting process.

An Overview of the Principles of Modeling Charpy Impact Energy Data Using Statistical Analyses
R. MOSKOVIC and P.E.J. FLEWITT
Integrity assessments of Magnox nuclear reactors with steel pressure vessels quantify the temperature margins between the operating temperature of the plant, at any given location, and the onset of upper-shelf temperature. The onset of upper-shelf temperature can be estimated from the fracture toughness properties of each material used in the construction of the pressure vessels. Although start- of-life fracture toughness properties of the materials have been measured, such properties are not available for the neutron-irradiated and thermally aged condition. One of the main effects of neutron irradiation and temperature experienced during service is to increase the ductile-to-brittle transition temperature (DBTT), which can be represented in terms of temperature shifts. In the irradiation surveillance schemes for the Magnox reactors, these temperature shifts can be inferred from Charpy impact energy data which have been measured regularly during the service life. Since Charpy impact energy data are inherently scattered, it is necessary to optimize the interpretation of the data by statistical processing. A recent analysis undertaken by Moskovic et al. concluded that Bayesian analyses are best suited to address the problem. In this overview, we consider the requirements of such analyses and the various options available. We then consider the method proposed by Moskovic et al. with respect to the requirements of the inputs to the integrity assessment and the validity of this approach. In this method of analysis, the distribution of all possible values of model coefficients is established by judging the various possible combinations of these model coefficients in relation to the likelihood of the observed data. Analysis of artificially generated data has been used to compare the effectiveness of Bayesian analyses with those used traditionally.

The Effect of Consolidation Temperature on Microstructure and Mechanical Properties in Powder Metallurgy-Processed 2XXX Aluminum Alloy Composites Reinforced with SiC Particulates
KEESAM SHIN, DONGSUP CHUNG, and SUNGHAK LEE
The effects of consolidation temperature on the development of microstructure and resulting mechanical properties of 2XXX aluminum composites were studied in an effort to fabricate composites with enhanced properties. Type 2009 and 2124 aluminum composites reinforced with 15 pct SiC particulates were produced at four different consolidation temperatures, i.e., 560°C, 580°C, 600°C, and 620°C, followed by extrusion at 450°C. The 2124 Al-SiCp composites consolidated at 560°C showed the most homogeneous and the finest microstructures with the best mechanical properties, which were even better than the whisker-reinforced counterparts. All the results of the tensile tests, hardness tests, in situ scanning electron microscope (SEM) observations of the fracture process, and the apparent fracture toughness indicated that the prominent mechanical property improvement ob served in the 2124 Al-SiCp was associated largely with the reduction of volume fraction of the detrimental coarse and brittle manganese-containing particles, as well as grain refinement. The detrimental manganese-containing particles that were routinely observed in the 2124 Al-SiC composites were very effectively refined by the reduction of consolidation temperature, and they rather contributed to the overall mechanical properties of the composites through Orowan-type strengthening and grain growth inhibition.

Cyclic Deformation Behavior of a Transformation-Induced Plasticity-Aided Dual-Phase Steel
KOH-ICHI SUGIMOTO, MITSUYUKI KOBAYASHI, and SHIN-ICHI YASUKI
Cyclic hardening-softening behavior of a TRIP-aided dual-phase (TDP) steel composed of a ferrite matrix and retained austenite plus bainite second phase was examined at temperatures ranging from 20°C to 200°C. An increment of the cyclic hardening was related to (1) a long-range internal stress due to the second phase and (2) the strain-induced transformation (SIT) behavior of the retained austenite, as follows. Large cyclic hardening, similar to a conventional ferrite-martensite dual-phase steel, appeared in the TDP steel deformed at 20°C, where the SIT of the retained austenite occurred at an early stage. This was mainly caused by a large increase in strain-induced martensite content or strain-induced martensite hardening, with a small contribution of the internal stress. In this case, shear and expansion strains on the SIT considerably decreased the internal stress in the matrix. With increasing deformation temperature or retained austenite stability, the amount of cyclic hardening decreased with a significant decrease in plastic strain amplitude. This interesting cyclic behavior was principally ascribed to the internal stress, which was enhanced by stable and strain-hardened retained austenite particles.

Communication: Analysis and Prevention of Cracking Phenomena Occurring during Softfacing of Brass on AlSl 4140 Steel Substrate
DONG-KUK KIM AND SUNGHAK LEE

Communication: Fracture Behavior of Squeeze-Cast Aluminum-Nickel Composites for Diesel Engine Piston Rings
SUNGHAK LEE, SEONG-HUN CHOO, AND MEUNGHO RHEE

ENVIRONMENT

Understanding the Potential and pH Dependency of High-Strength -Titanium Alloy Environmental Crack Initiation
D.G. KOLMAN and J.R. SCULLY
An explanation for the strong dependency of crack initiation of precracked high-strength -titanium alloys in room-temperature 0.6 M NaCl on applied potential and bulk-solution pH is presented. It is proposed that environment-assisted cracking (EAC) susceptibility in neutral aqueous NaCl results from (1) film rupture due to plastic deformation at actively deformed crack tips, (2) accelerated dissolution of titanium, (3) crack tip acidification by hydrolysis of titanium ions, (4) crack tip po tential excursions toward bare metal open-circuit potentials (OCPs) during film rupture due to large ohmic voltages in the crack solution, (5) accelerated crack tip proton or water reduction concurrent with titanium dissolution, (6) bare surface-dominated hydrogen ingress into a fracture process zone, and (7) crack initiation by hydrogen embrittlement. Evidence for each of the above stages of the crack initiation scenario is presented, with emphasis on crack tip electrode kinetics and ohmic voltage calculations which govern process zone-controlled hydrogen uptake. The seven stages are consistent with the strong dependencies of crack initiation and growth in precracked high-strength -titanium alloys on (1) solution pH, (2) applied potential, and (3) strain rate, and they explain the "apparent" EAC resistance of smooth- and blunt-notch specimens. The latter lack both occluded crack tip geometries to promote acidification and ohmic voltage drops below reversible hydrogen, as well as localization of dynamic plastic strain. Hydrogen uptake is, subsequently, limited.

WELDING & JOINING

CO2 Laser Beam Welding of 6061-T6 Aluminum Alloy Thin Plate
AKIO HIROSE, HIROTAKA TODAKA, and KOJIRO F. KOBAYASHI
Laser beam welding is an attractive welding process for age-hardened aluminum alloys, because its low heat input minimizes the width of weld fusion and heat-affected zones (HAZs). In the present work, 1-mm-thick age-hardened Al-Mg-Si alloy, 6061-T6, plates were welded with full penetration using a 2.5-kW CO2 laser. Fractions of porosity in the fusion zones were less than 0.05 pct in bead-on-plate welding and less than 0.2 pct in butt welding with polishing the groove surface before welding. The width of a softened region in the-laser beam welds was less than 1/4 times that of a tungsten inert gas (TIG) weld. The softened region is caused by reversion of strengthening '' (Mg2Si) precipitates due to weld heat input. The hardness values of the softened region in the laser beam welds were almost fully recovered to that of the base metal after an artificial aging treatment at 448 K for 28.8 ks without solution annealing, whereas those in the TIG weld were not recovered in a partly reverted region. Both the bead-on-plate weld and the butt weld after the postweld artificial aging treatment had almost equivalent tensile strengths to that of the base plate.

A Microstructural Study of Dislocation Substructures Formed in Metal Foil Substrates during Ultrasonic Wire Bonding
NIKHIL MURDESHWAR and JAMES E. KRZANOWSKI
A study has been conducted on the deformation mechanisms in metal substrates subject to aluminum ultrasonic wire bonding (UWB). Aluminum wires were bonded to copper, nickel, stainless steel, and aluminum bronze foil substrates and then removed in aqueous sodium hydroxide to permit thin sections of bonded areas to be examined in the transmission electron microscope (TEM). The results showed a variety of dislocation substructures formed during bonding, including dislocation cells, subgrains, and planar arrays. Aluminum and copper showed evidence of thermal effects on micro structural evolution during bonding, such as dislocation annihilation at cell walls in copper and complete recrystallization in aluminum. In the nickel and stainless steel substrates, which have higher recrystallization temperatures, thermal effects on microstructure were not observed. In addition, it was found that low stacking-fault energy (SFE) materials, such as aluminum bronze, were less likely to undergo cell formation, and only planar dislocation arrays formed. In general, it is clear that the process of UWB induces cyclic stresses in the substrates, which exceed the yield strength of the metals examined. In addition, there is some heat generated during the bonding process, which can influence the resultant deformation microstructure.

Interfacial Characteristics for Brazing of Aluminum Matrix Composites with Al-12Si Filler Metals
W.P. WENG and T.H. CHUANG
Discussions concerning the interfacial reactions and characterizations in brazing aluminum matrix composites are documented in this study. Joints of alumina particulate reinforced 6061 aluminum matrix composites were made using an Al-12 (wt pct) Si filler metal by vacuum brazing. The resulted maximum bonding strengths were 75.4, 81.5, and 71.8 MPa for 10, 15, and 20 vol pct alumina reinforcement, respectively. The microstructural examinations revealed that the bonding strength was strictly related to the reinforced alumina particles and the reaction products presented at the joint interfaces. During brazing, Mg segregated at the joining interface and alumina/6061 Al interface. Further, reactions between alumina and 6061 Al matrix resulted in the formation of Mg-rich phases, such as MgAl2O4 and MgO, near the joining interface and the alumina reinforcement. The Si in the filler material penetrated into the metal matrix composites (MMCs) matrix and segregated at the alumina/6061 Al interfaces. This phenomenon can be confirmed by a joint between two alumina bulk specimens.

SURFACE TREATMENT

Effect of Substrate Grain Size on Iron-Zinc Reaction Kinetics during Hot-Dip Galvanizing
C.E. JORDAN and A.R. MARDER
In the galvanizing process, it has been proposed that the grain size of the substrate steel influences the Fe-Zn alloy phase reaction kinetics and growth rate during immersion in the liquid Zn bath. Two grain sizes (nominally 15 and 85 µm) were developed in a decarburized low-carbon (0.005) steel and hot-dipped galvanized in 0.00 wt pct Al-Zn and 0.20 wt pct Al-Zn baths to study the effect of substrate grain size on Fe-Zn phase formation. Uniform attack of the substrate steel occurred in the 0.00 wt pct Al-Zn bath, since an Fe2Al5 inhibition layer did not form. No barrier to nucleation of the Fe-Zn phases exists in this Zn bath, and therefore, the substrate steel grain size had no significant effect on the kinetics of phase growth for the gamma, delta, and zeta phase layers. In the 0.20 wt pct Al-Zn bath, discontinuous Fe-Zn phase growth (outburst formation) occurred due to the initial formation of the Fe-Al inhibition layer. The nucleation of the Fe-Zn phases was significantly retarded in this bath for the large (85 µm) substrate grain size. Whereas outbursts were found in the 15-µm grain size substrate after 10 seconds of immersion time, it required 1200 seconds to nucleate just a few outbursts in the 85-µm substrate. These results support the mechanism that Fe-Al inhibition layer breakdown occurs along fast diffusion paths for Zn in the inhibition layer that correspond to the location of substrate steel grain boundaries where reaction with Fe can occur.

Effect of Phosphorous Surface Segregation on Iron-Zinc Reaction Kinetics during Hot-Dip Galvanizing
C.E. JORDAN, R. ZUHR, and A.R. MARDER
Phosphorous was ion implanted on one surface of a large grain (10 to 20 mm) low-carbon steel sheet in order to study the effect of surface segregation on the formation of Fe-Zn phases during galvanizing. Both an Al-free and a 0.20 wt pct Al-Zn bath at 450°C were used in this investigation. It was found that P surface segregation did not affect the kinetics of Fe-Zn phase growth for the total alloy layer or the individual Fe-Zn gamma, delta, and zeta phase alloy layers in the 0.00 wt pct Al-Zn baths. In the 0.20 wt pct Al-Zn bath, the Fe2Al5 inhibition layer formed with kinetics, showing linear growth on both the P-ion implanted and non-P-ion implanted surfaces. Fe-Zn phase growth only occurred after extended reaction times on both surfaces and was found to directly correspond to the location of substrate grain boundary sites. These results indicate that P surface segregation does not affect the growth of Fe-Zn phases or the Fe2Al5 inhibition layer. It was shown that in the 0.20 wt pct Al-Zn bath, substrate grain boundaries are the dominant steel substrate structural feature that controls the kinetics of Fe-Zn alloy phase growth.

SOLIDIFICATION

Eutectic Reaction and Nonconstant Material Parameters in Micro-Macrosegregation Modeling
HERVÉ COMBEAU and ASBJØRN MO
Recently, the authors presented a general framework for the calculation of the local solidification path in micro-macrosegregation computations. For a volume element whose overall solute content is not necessarily constant, the problems of solute diffusion in the primary solid phase and of remelting were addressed. In handling the eutectic reaction, the mass fraction and the solute profile in the primary phase at the beginning of the eutectic reaction were assumed to be "frozen." In the present study, this restriction has been relaxed, as the solute diffusion in the primary phase occurring during the eutectic reaction is taken into account as well as that taking place for temperatures between the liquidus and the eutectic. It is shown that there is a potential for dissolving the secondary phase, which is controlled by diffusion in the primary phase. Other extensions of the former modeling concept made in the present study are to allow different densities of the liquid and solid phases, nonlinear phase diagram characteristics, and a temperature and/or solute concentration-dependent solute diffusivity in the primary phase. Assuming that the variations of enthalpy and of average solute concentration are known from the solution of the macroscopic continuity equations, modeling examples focusing on the extensions of the former model have been carried out.

MATERIALS PROCESSING

Processing of AA3004 Alloy Can Stock for Optimum Strength and Formability
SHIXI DING and JAMES G. MORRIS
By making effective use of precipitation hardening of AA3004 alloys, a superstrengthening effect was achieved in the manufacturing of AA3004 can stock materials through modified processing, which combines a regular work hardening process with a further precipitation hardening process. The annealing operation in the conventional processing was replaced by a high-temperature solution treatment to regain a high supersaturation of solutes in solid solution. The combined operations of cold rolling and low-temperature aging were able to yield varied combinations of high mechanical strength and ductility. In this work, the effects of processing sequences on the mechanical property improvement and the texture control of the final gage sheets were studied. The mechanical properties of the materials were evaluated based on the tensile results. Texture analyses were performed by means of orientation distribution function (ODF) descriptions. Earing tests were carried out in order to determine the mechanical anisotropy of the final sheets.

Direct Consolidation of -TiAl-Mn-Mo from Elemental Powder Mixtures and Control of Porosity through a Basic Study of Powder Reactions
T.K. LEE, J.H. KIM, and S.K. HWANG
A gamma titanium aluminide was made by elemental powder metallurgy. For consolidation of the alloy from powder blending, either hot extrusion or hot forging was used. A good combination of tensile yield strength and ductility was obtained by hot extrusion that produced a grain size of 50 µm. Consolidation by forging, however, resulted in a porous microstructure. On the basis of an investigation of the cause of the porosity by an Al/Ti diffusion couple experiment and by characterization of the temperature peaks due to an exothermic reaction among elemental powder particles, it was concluded that a transient phase such as TiAl3 was the culprit. Being the source of Al diffusion, the transient phase leaves behind Kirkendall voids when it forms prior to the major exothermic reaction among elemental powder particles. From this study, two processing techniques to circumvent the porosity were proposed and verified: a fast heating to the consolidation temperature or sufficient soaking above the reaction temperature prior to consolidation. A sound, fully lamellar, -phase controlled microstructure was obtained by these methods.

COMPOSITE MATERIALS

Role of Cladding in the Notched Tensile Properties of a Titanium Matrix Composite
U. RAMAMURTY, F.W. ZOK, and F.A. LECKIE
The utility of Ti cladding in alleviating the notch sensitivity of both the monotonic and cyclic tensile properties of Ti matrix composites (TMC) has been examined. Experiments have been conducted on panels with two different clad thicknesses as well as on the TMC alone. Crack bridging models have been used to describe the composite behavior, incorporating explicitly the effects of the cladding. It is demonstrated that the notched strength can be raised up to the level corresponding to the un-notched TMC alone, with a critical clad thickness that depends on the fracture properties of the TMC and the notch size. The fatigue threshold can be elevated also, though it cannot reach the threshold of the un-notched TMC. The bridging models have been used to calculate the trends in the strength and the fatigue threshold with the clad thickness.

Phase-Stress Partition during Uniaxial Tensile Loading of a TiC-Particulate-Reinforced Al Composite
N. SHI, M.A.M. BOURKE, J.A. ROBERTS, and J.E. ALLISON
Using neutron diffraction, we measured during in situ loading the lattice elastic mean phase (LEMP) strains in the matrix and reinforcement of a 15 vol pct TiC-particulate-reinforced 2219 Al composite. From the strain components longitudinal to and transverse to loading, the in situ normal phase stresses (average normal stresses in the constituent phases) were obtained through Hooke's law. The internal stress partition between the matrix and reinforcement, i.e., load sharing, can then be inferred. Internal stress development was also modeled using the finite-element method (FEM), showing good agree ment with the experimental results. Both indicate that the relationship between the LEMP strains/phase stresses and the applied load noticeably deviates from linearity during composite mi croyielding, long before the nominal 0.2 pct proof stress is reached. The nonlinearity arises (despite the linear elastic relationship between phase stresses and LEMP strains) because the applied traction is not synonymous with the phase stresses, and the ratio of phase stresses may vary during loading. Notably, the morphology of the LEMP strain development with applied load differs in the directions parallel to or perpendicular to the load. The differences are explained by considering the evolution of local matrix plasticity. Thermal residual stresses and inelastic stress relaxation, driven by interfacial diffusion, are also discussed.

Dislocations in Continuous Filament Reinforced W/NiAl and Al2O3/NiAl Composites
L. WANG, K. XU, R.R. BOWMAN, and R.J. ARSENAULT
Continuous filament reinforced W/NiAl and Al2O3/NiAl composites (as-processed, annealed, and thermally cycled) have much higher dislocation densities than that of monolithic NiAl. These higher dislocation densities resulted from the relaxation of thermal residual stress, which developed during the cooling of the sample from elevated temperatures and was caused by the difference in the coefficients of thermal expansion between the matrix and the reinforcement. The dislocation density in the region adjacent to the matrix-filament interface was high and decreased only slightly with distance from the interface in the 30 vol pct composites. The as-processed and annealed composites exhibited a rather homogeneous dislocation density in the matrix. After thermal cycling, these com posites showed no large difference in the dislocation density and morphology. However, there were local regions of lower dislocation densities. This difference was examined in relationship to filament fracture, surface matrix cracking, and degree of bonding.

Communication: On the Creep Strengthening of SiC Particulates in SiC-Al Composites
FARGHALLI A. MOHAMED

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