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Volume 28A, No. 4, April 1997 This Month Featuring: Alloy Phases, Transformations, Mechanical Behavior, Solidification, Materials Processing, Composite Materials. View April 1997 Contents.
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| METALLURGICAL AND MATERIALS TRANSACTIONS A | |
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Volume 28A, No. 4, April 1997 This Month Featuring: Alloy Phases, Transformations, Mechanical Behavior, Solidification, Materials Processing, Composite Materials. View April 1997 Contents.
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-Co Solid Solution
-Co solid solution. The composition dependence of the activities within the -Co solid solution region was used to calculate the self-interaction coefficients of silicon in -Co: In 
Si
= -10.4 ± 0.2 and 
Si(Si)= 18.6 ± 0.8. The regular solution model was shown to be a fairly good description of the -Co solid solution with an energy parameter Z
CoSi = -120 ± 5 kJ·mol-1. These results compare well with literature data on similar systems (Fe-Si and Ni-Si).
Microstructure and Mechanical Properties of Sputter-Deposited Cu1-xTax Alloys
HONG WANG, M.J. ZALUZEC, and J.M. RIGSBEE
The microstructures and mechanical properties of a family of sputter-deposited Cu1-xTax (0 < x < 0.18) alloys have been investigated. The as-deposited microstructures for all film compositions consisted of a polycrystalline, face-centered-cubic (fcc) Cu matrix, with varying levels of Ta in solid solution, plus a very high density of discrete, 1 to 3 nm, fcc Ta particles. Decreased deposition temperature (-120°C vs 100°C) increased the level of Ta in solid solution. After annealing (900°C for 1 hour) the as-deposited 6 at. pct Ta films, the Cu matrix grains remained submicron and the Ta particles remained fcc with no apparent particle coarsening. Additionally, the fcc Ta particles were found before and after annealing to be oriented identically with the Cu matrix and aligned on {111} and {100} habit planes. Annealing 17 at. pct Ta films at 900°C for 1 hour resulted in the formation of body-centered-cubic (bcc) Ta particles (>50-nm diameter) in addition to the much smaller fcc Ta particles. Annealing the low and high Ta composition films at 900°C for as long as 100 hours produced no observed change in either the Cu matrix grain size or the size and distribution of the fcc and bcc Ta particles. Microhardness and nanoindentation mechanical property evaluations of bulk hot-pressed materials indicated that the high strengths of the composites were unchanged, even after annealing for 100 hours at 900°C.
-Al3(Ti,V)/
-(Ti,V) Alloys
-Al3(Ti, V) + 
-Al8V5, not previously reported in the Al-Ti-V system, has been observed in both alloys solidified at sufficient levels of undercooling. The phase is replaced by metallic 
-(Ti, V) phase during subsequent heat treatment in the range 1073 to 1273 K, and differential thermal analysis (DTA) of samples preannealed at 1173 K revealed an endothermic peak at ~1560 K, consistent with equilibrium eutectic melting of the form ( + ) L. Although the chill-cast alloys retained metastable intermediate high-temperature phases, duplex metallic-intermetallic microstructures, containing uniform fine-scale distributions of metallic -(Ti, V) solid solution in a -Al3(Ti, V) intermetallic matrix, have been produced in both alloys during isothermal heat treatments at temperatures in the range 1073 to 1273 K. For both alloys, the bulk Vickers hardness of such microstructures remained in excess of that of binary Al3Ti, while in the Al62Ti10V28 alloy, where the increased volume fraction of phase took the form of a near-continuous network within matrix, there was evidence arising from indentation tests of a substantial improvement in the cracking resistance compared to both chill-cast ternary alloy and binary Al3Ti.
Modeling Time Dependence of the Average Interface Migration Rate in Site-Saturated Recrystallization
P.R. RIOS
A model is proposed to describe the time dependence of the average interface migration rate observed during recrystallization. The main assumption is that the growth rate of the extended grains can be written as a function of the recrystallized fraction. from this assumption, it could be demonstrated that the average interface migration rate can be written as a product of two factors. One is a time-independent but temperature-dependent interface migration rate. The other factor can be make a function of normalized time only and therefore is independent of temperature. the model shows good agreement with the results of Speich and Fisher and Vandermeer and Rath. It is concluded that the present model offers a plausible description for the time dependence of the average interface migration rate in recrystallization.
Analysis of Grain Growth in a Two-Phase Gamma Titanium Aluminide Alloy
V. SEETHARAMAN and S.L. SEMIATIN
Microstructure evolution during annealing of a wrought near-gamma titanium aluminide alloy, Ti- 45.5Al-2Nb-2Cr (at. pct), in the temperature range 1200°C to 1320°C was investigated. The mean grain size of the alpha phase as well as the volume fraction and size of the gamma particles were evaluated as a function of annealing temperature and time. Isothermal annealing at temperatures above the alpha transus, T = 1300°C, led to rapid grain growth of the alpha phase, the kinetics of which could be described by a simple power-law type expression with a grain growth exponent p = 2.3. Alpha grain growth was significantly retarded during annealing at subtransus temperatures (1200°C 
T 1300°C) by the pinning influence of gamma-phase particles. Limiting grain size values predicted by computer simulation models applicable for high-volume fractions of precipitates/particles were in good agreement with experimental findings. The kinetics of alpha grain growth in the presence of gamma particles were analyzed, and the results showed that a grain growth exponent of p 
2.6 could satisfactorily account for the experimental results.
Aging Effects in a Cu-12Al-5Ni-2Mn-1Ti Shape Memory Alloy
Z.G. WEI, H.Y. PENG, W.H. ZOU, and D.Z. YANG
The isothermal aging effects in an as-quenched Cu-11.88Al-5.06Ni-1.65Mn-0.96Ti (wt pct) shape memory alloy at temperatures in the range 250°C to 400°C were investigated. The changes in the state of atomic order and microstructural evolutions were traced by means of in situ X-ray diffraction and electrical resistivity measurements, as well as transmission electron microscopy (TEM) and optical observations. The kinetics of the aging process, i.e., the temperature and time dependence of the properties including hardness, resistivity, martensitic transformation temperatures, and shape memory capacity were characterized, and at least three temperature-dependent aging stages were distinguished: (1) D03 or L21 atomic reordering, which causes the martensitic transformation temperatures to shift upward and leads the M18R martensite to tend to be a N18R type structure; (2) formation of solute-depleted bainite which results in a drastic depression in martensitic transformation temperatures and loss of the shape memory capacity, accompanied by the atomic disordering in both the remaining parent phase and bainite; and (3) precipitation of the equilibrium and 2 phases and destruction of the shape memory capacity.
Diffusional Breakdown of Nickel Protective Coatings on Copper Substrate in Silver-Copper Eutectic Melts
W.D. ZHUANG and T.W. EAGAR
Diffusion couples with electrolessly plated nickel diffusion barriers between copper substrates and silver-copper eutectic alloys were tested at 800°C and 850°C, respectively. Growth of (Cu, Ni, Ag) ternary solid solution into the melt was observed at both temperatures. The growth pattern changed from cellular to dendritic as the temperature was increased from 800°C to 850°C. The nonplanar growth morphology can be explained in terms of constitutional supercooling in the melt. Kinetics of (Cu, Ni, Ag) solid solution growth were found to be controlled by interdiffusion at the interface of the nickel barrier and the growing solid-state phase. Local breakdown of the nickel diffusion barrier started once the (Cu, Ni, Ag) solid solution reached the copper substrate. Silver diffused from the silver-copper melt, through the ternary solid solution, dissolving copper and forming silver-copper liquid along copper grain boundaries. Ultimately, the nickel barrier was totally converted to the ternary solid solution, broke up, and floated into the liquid. Dissolution of the copper substrate occurred subsequently. A thin layer of chromium undercoating proved to be very effective in extending the protection time of the nickel diffusion barrier, due to the extremely low solubility of both copper and silver in chromium at these test temperatures.
Communication: Authors' Reply
, dual-phase 2 + , and fully lamellar (FL) 2 + TiAl intermetallic alloy microstructures have been performed. Nonlinear viscous primary creep deformation is modeled in each phase based on published creep data. Models were also developed that incorporate grain boundary and lath boundary sliding in addition to the dislocation creep flow within each phase. Overall strain rates are compared to gain an understanding of the relative influence each of these localized deformation mechanisms has on the creep strength of the microstructures considered. Facet stress enhancement factors were also determined for the transverse grain facets in each model to examine the relative susceptibility to creep damage. The results indicate that a mechanism for unrestricted sliding of lath boundaries theorized by Hazzledine and co-workers leads to unrealistically high strain rates. However, the results also suggest that the greater creep strength observed experimentally for the lamellar microstructure is primarily due to inhibited former grain boundary sliding (GBS) in this microstructure compared to relatively unimpeded GBS in the equiaxed microstructures. The serrated nature of the former grain boundaries generally observed for lamellar TiAl alloys is consistent with this finding.
Fracture Toughness of Polycrystalline NiAl from Finite-Element Analysis of Miniaturized Disk-Bend Test Results
S.J. ECK and A.J. ARDELL
The controlled-flaw method in conjunction with the miniaturized disk-bend test (MDBT) was implemented to determine the fracture toughness of polycrystalline NiAl. This procedure was previously used to measure the fracture toughness of completely brittle materials, so the present research extends the method to a material that exhibits a small amount of ductility prior to failure. The controlled- flaw method is based on the placement of a Vickers indentation in the center of the tensile side of the disks. In the MDBT, the specimens are disks 3 mm in diameter, and in this investigation, the disks ranged from 194 to 367 µm in thickness. Fracture initiated at the corners of the indentations for indentation loads exceeding 39 N. The fracture toughness was determined from an analysis of the dependence of fracture stress, 
f, on indentation load. In brittle materials, f can be calculated from the measured load at fracture, but this is not possible when the specimen deforms plastically prior to failure. The finite-element program NIKE2D was therefore used to calculate the stress during plastic deformation, using data on the tensile behavior of NiAl to model its deformation as an inelastic cylindrically symmetric plate. The fracture toughness of polycrystalline NiAl was measured as 6.41 ± 1.75 MPa
, which agrees well with independently measured values for similarly processed material. The relatively large uncertainty is associated with scatter in the experimentally measured yield stresses. The results of this investigation demonstrate that the controlled-flaw method can be used in conjunction with the MDBT and finite-element modeling to provide a reasonable estimate of the fracture toughness of a material with limited ductility, provided fracture initiates at the corners of the indentation.
Influence of Solid/Liquid Interfaces on the Microstructure and Stress-Rupture Life of the Single-Crystal Nickel-Base Superalloy NASAIR 100
XIPING GUO, HENGZHI FU, AND JIAHUA SUN
The [001] oriented single crystals of nickel-base superalloy NASAIR 100 with the planar, cellular, coarse-dendritic, and fine-dendritic solid/liquid (S/L) interfaces were prepared, respectively, and their microstructure and stress-rupture behavior at 1050°C were investigated in both as-cast and solution heat-treated conditions. It was found that in as-cast single crystals of NASAIR 100, microsegregation and /' eutectic produced in the solidification process increased, ' size decreased, and ' shape tended progressively to be cuboidal, with the successive transition of the S/L interface from planar to cellular, then to coarse-dendritic, and finally to fine-dendritic morphology. Furthermore, the solution temperature required to dissolve all as-cast ' and most of the /' eutectic increased with the aforementioned successive transition of S/L interfaces. The reprecipitated ', after solution heat treatment (SHT), was usually fine and cuboidal. However, some W-rich phase was present in the heat-treated dendritic single crystals. Both the planar and the cellular single crystals of NASAIR 100 exhibited no superiority in stress-rupture life, irrespective of the heat-treatment conditions. Instead, the single crystals with dendritic morphology possessed excellent stress-rupture lives, after heat treatment at 1320°C for 4 hours, followed by air cooling (AC). Perfect ' rafts with high-average aspect ratios formed during the stress-rupture tests of dendritic single crystals; in contrast, irregularly coarsening structures appeared in both the planar and cellular single crystals. The microstructure and solution behavior were illustrated in detail. Furthermore, the microstructural factors to affect the high-temperature stress-rupture life of the single crystals of NASAIR 100 were also analyzed.
The Effects of Composition and 
'/ Lattice Parameter Mismatch on the Critical Resolved Shear Stresses for Octahedral and Cube Slip in NiAlCrX Alloys
R.V. MINER
Prototypical single-crystal NiAlCrX superalloys were studied to examine the effects of the common major alloying elements, Co, Mo, Nb, Ta, Ti, and W, on yielding behavior. The alloys contained about 10 at. pct Cr, 60 vol pct of the ' phase, and about 3 at. pct of X in the '. The critical resolved shear stresses (CRSSs) for octahedral and primary cube slip were measured at 760°C, which is about the peak strength temperature. The CRSSoct and CRSScube are discussed in relation to those of Ni3 (Al, X) ' alloys taken from the literature and the '/ lattice mismatch. The CRSSoct of the + ' alloys reflected a similar compositional dependence to that of both the CRSSoct of the ' phase and the '/ lattice parameter mismatch. The CRSScube of the + ' alloys also reflected the compositional dependence of the '/ mismatch, but bore no similarity to that of CRSScube for ' alloys since it is controlled by the matrix. The ratio of CRSScube/CRSSoct was decreased by all alloying elements except Co, which increased the ratio. The decrease in CRSScube/CRSSoct was related to the degree in which elements partition to the ' rather than the phase.
Dislocation Structures in Zirconium and Zircaloy-4 Fatigued at Different Temperatures
LIN XIAO and HAICHENG GU
The development of characteristic dislocation structures in pure zirconium and zircaloy-4 fatigued under pull-push strain control as the testing temperature and the cyclic strain range varied was examined using a thin-foil transmission electron microscopy (TEM) technique. The slip planes and the twinning planes were determined by a standard stereographic trace analysis technique. The first-order prismatic slip {
}is the primary deformation mode in zirconium and zircaloy-4 fatigued from room temperature (RT) to 873 K. The pyramidal slip {
} is activated at 673 K and at high cyclic strain ranges, whereas the basal slip {0001} only appears in those specimens fatigued at 873 K. The {}, {
}, and {
} types of twins were detected in specimens fatigued at RT. Twinning becomes less frequent as the testing temperature increases. The schematic map of the cyclic deformation modes as a function of the plastic strain range and the test temperature is described. The dislocation configurations in fatigued pure zirconium specimens evolve from a planar arrangement to a cell structure as the test temperature and the strain range increase. For zircaloy-4, the fatigued dislocation structure is parallel dislocation lines at RT, cells at 673 K, and two sets of approximate mutually perpendicular dislocation bands at 873 K, respectively. Finally, the fatigued dislocation-structure evolution map with the cyclic strain range and the test temperature are qualitatively established for zirconium and zircaloy-4, respectively. The effect factors on the fatigue mechanism and the thermodynamic and dynamic criteria of the dislocation-pattern evolution are discussed.
Communication: Effects of Temperature on the Fatigue Crack Growth of an Al-Li 8090 Alloy with ' Microstructure
CHONG SOO LEE, KI JONG PARK, DAOMING LI, and NACK JOON KIM
Communication: the Antiphase Boundary Energies of L12 Ordered Ni74.5Pd2Al23.5 Alloy
J. SUN, C.S. LEE, and J.K.L. LAI
Communication: Dislocation/Particle Interactions in '(NiAl) Precipitation Strengthened Ferritic Fe-19Cr-4Ni-2Al Alloy
S.M. ZHU and S.C. TJONG
= 2.31 to 9.24. Evidence shows that intense simple shear deformation promotes dynamic or continuous recrystallization by subgrain rotation. The effects of the number of extrusion passes and deformation route for Cu 101, and the deformation route after four passes for Al 3003, are studied. Increasing the number of ECAE passes in Cu 101 causes strength to reach saturation and grain refinement stabilization after four passes (true strain of 4.68), and subgrain misorientation to increase as the number of passes increases. For multipass ECAE with billet orientation constant (route A) or rotated 90 deg between all passes (route B), two levels of structures are created inside the original grains: shear bands (first level) and very fine subgrains (second level) within the shear bands. For a billet rotation of 180 deg between passes (route C), an unusual event is observed. At each even numbered pass, shear bands nearly disappear and only subgrains are present inside the original grains. Route B gives the highest strength, whereas route C produces a more equiaxed and stable microstructure. Subsequent static recrystallization increases the average grain size to 5 to 10 µm.
Superplastic Behavior of Spray-Deposited 5083 Al
YUE WU, LINDA DEL CASTILLO, and ENRIQUE J. LAVERNIA
A 5083 Al alloy was synthesized using spray deposition processing with N2 as the atomization gas. It was noted that the grains that were present in as-spray-deposited 5083 Al were equiaxed with an average size of 15.2 µm. The matrix of the material was supersaturated with Mg and Mn. The as- spray-deposited microstructure contained irregular pores with porosity in the range of 0.1 to 5.4 vol pct, depending on spatial location in the preform. The spray-deposited alloy was thermomechanically processed using extrusion and multiple-pass warm rolling to reduce grain size and close porosity. It was observed that spray-deposited 5083 Al exhibited superplasticity following thermomechanical processing by extrusion followed by rolling. Superplasticity was observed in the 500°C to 550°C temperature range and 3 x 10-5 to 3 x 10-3 s-1 strain rate range. The corresponding strain-rate- sensitivity factors were in the 0.25 to 0.5 range and increased with decreasing strain rate. A maximum elongation of 465 pct was noted at 550°C and 3 x 10-5 s-1. The spray-deposited 5083 Al, thermomechanically processed by direct rolling, exhibited superplasticity in the same temperature and strain rate ranges as those for the extruded and rolled materials. The superplastic elongation of the spray-formed and rolled material was relatively low, being in the range of 250 to 300 pct. The deformation behavior is discussed in light of the presence of porosity in the microstructure.
Fabrication of Al2O3-Reinforced Ni3Al Composites by a Novel In Situ Route
S. NOURBAKHSH, O. SAHIN, and H. MARGOLIN
A novel in situ technique has been used to fabricate an Al2O3-reinforced Ni3Al matrix composite. The composite was prepared by first incorporating a low volume fraction of continuous Al2O3 fibers in a Ni3Al alloy containing 0.34 at. pct Zr. Pressure casting was used to embed the fibers. Casting resulted in partial reduction of the Al2O3 fiber by the Zr present in the matrix and the formation of a layer of ZrO2 on the surface of the fibers. The final composite was then prepared by air annealing the precursor composite at 1100°C for 10 days. Air annealing led to the formation of networks of Al2O3 around the fibers. The matrix in the immediate vicinity of the networks consisted of Ni3Al particles in a matrix of disordered -Ni(Al). The Al2O3 networks raised the yield and tensile strength of the material by 35 and 18 pct, respectively. The composite had a tensile ductility of 14 pct.
Microstructure and Mechanical Behavior of the NiAl-TiC In Situ Composite
Z.P. XING, J.T. GUO, Y.F. HAN, and L.G. YU
The microstructure, interfaces, and mechanical properties of NiAl-matrix composites reinforced by 0 and 20 vol pct TiC particles have been examined. The composites were prepared by the hot-press-aided exothermic synthesis (HPES) technique. Portions of the HPES-processed samples were hot isostatically pressed ("hipped") at 1165°C/150 MPa for 4 hours or annealed at 1400°C for 48 hours. In the as-fabricated state, TiC particles were generally polygonal and faceted, and the interfaces between TiC and NiAl were atomically flat, sharp, and generally free from any interfacial phase. At least two orientation relationships between TiC and NiAl were observed. In some cases, thin amorphous layers existed at NiAl/TiC interfaces. After "Hipping," the TiC particles tended to become round and the TiC/NiAl interfaces became overlapped. Annealing at 1400°C for 48 hours did not affect the microstructure or the interfacial structure of the composite in most cases. The compressive yield strengths (YSs) from room temperature to 1100°C of the composite were considerably higher than that of the monolithic NiAl. At 980°C, the tensile YS of the composite was approximately 3 times that of the monolithic NiAl. In addition, the ambient fracture toughness of the composite was 50 pct higher than that of the monolithic NiAl.
Communication: Interfacial Bonding In Spray-Formed Metal Matrix Composites
M. DE SANCTIS, R. VALENTINI, and A. SOLINA
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