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Volume 26A, No. 4, April 1995 This Month Featuring: Alloy Phases, Transformations, Transport Phenomena, Mechanical Behavior, Environment, Welding & Joining, Surface Treatment, Solidification. View April 1995 Contents.
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2-Al13CO4 Showing Fivefold Symmetry
2-Al13CO4 phase, whose a and c are about 2 = 2.618 ( = (1 + 
)/2) times greater than the respective parameters of the monoclinic Al13CO4, and its (100) and (001) twins has been reported earlier. Like its prototype structure, the monoclinic Al13Fe4/Al13Co4, this 2-Al13CO4 also shows a tendency to form multiple twins displaying fivefold symmetry. A transmission electron microscopic study of the twin mechanisms has been carried out. In addition to the well-known (001) and (100) glide twins, (20-1) glide twins have also been found. Alternate (100)-(20-1) or (100)-(001)-(100) twinning will yield twins displaying fivefold symmetry. The distribution of Co atoms in the pentagonal layer in 2-Al13CO4 and the tessellation of pentagons in the (100) and (20-1) glide twins have also been studied by high-resolution electron microscopy.
Communication: Phase Equilibria of Al-Rich Al-Cu-Mg Alloys
CHENG-CHIA HUANG AND SINN-WEN CHEN
Communication: Characterization of the Microstructure of Deformed Al-Zn-Cu Alloys by X-Ray Diffraction Line Profile Analysis
HIRANMAY PAL, S.K. PRADHAN, AND M. DE
0.1 mm) upon nitriding in a NH3/H2 gas mixture at temperatures in the range 803 to 853 K. Activation-energy analysis revealed that precipitation of AIN in the recrystallized specimens is associated with a Gibbs free energy barrier for the formation of a precipitate of critical size; the precipitation rate is controlled by both nucleation and growth. On the other hand, precipitation of AIN in the cold-rolled specimens occurs without a Gibbs free energy barrier for formation of a precipitate of critical size; the precipitation rate is controlled by growth with kinetics governed by volume diffusion of aluminum. Analysis of the total Gibbs free energy of formation of AIN in the 
-Fe matrix showed that in the case of the recrystallized specimens, the formation of incoherent AIN precipitates with a hexagonal crystal structure is favored. In the case of the cold-rolled specimens, containing a high dislocation density, the formation of coherent AIN precipitates with cubic crystal structure is favored, at least in the beginning of precipitation.
The Kinetics of Multilayered Titanium-Silicide Coatings Grown by the Pack Cementation Method
BRIAN V. COCKERAM and ROBERT A. RAPP
The halide-activated, pack cementation coating method is a viable technique for forming silicide diffusion coatings that provide oxidation resistance for titanium alloys at high temperature. In this study, growth rates were determined for the five-layered silicide coatings on commercially pure titanium grown at 950°C, 1050°C, and 1150°C using three halide activators. Solid-state titanium-silicon diffusion couple experiments were also made to determine the diffusional growth rates for the five silicide phases. Based on a model for growth of multiple layers, solid-state diffusion controls the growth rates in all of the silicide layers for coatings formed by packs with the less stable AlF3 and CuF2 activators. However, the growth of TiSi2 was influenced by gas-phase transport for coatings formed using the more stable MgF2 activator. The growth rates of the layers (TiSi2, TiSi, Ti5Si4, Ti5Si3, and Ti3Si) are parabolic; by assuming solid-state diffusion control, the diffusion coefficients in these phases were calculated.
Rapid Graphitization of a Pulsed Laser Remelted Ductile Cast Iron during Multipass Overlap Melting
H.M. WANG and H.W. BERGMANN
As-cast ductile cast iron with an as-machined shiny metal surface was remelted with a high-power (1 kW) pulsed Nd:YAG laser using both single- and multipass overlap melt tracks. Changes in the microstructure of the underlying laser melted track caused by the transient overlap heating during multipass overlap remelting process were studied. The rapidly solidified metastable ledeburite structure of the underlying laser melted track was found to be rapidly graphitized during overlap remelting. The graphitized zone consists of a fully graphitized zone containing extremely fine graphite nodules and a partially graphitized zone containing extremely fine graphite nodules and undissolved cementite. The overlap ratios of the melt tracks were shown to have no noticeable influence on either the graphitized microstructure and the size of the graphitization zones. This newly observed rapid graphitization phenomenon is preliminarily discussed in terms of the microstructural characteristics of the rapidly solidified ductile iron and the unique heating behavior of pulsed laser beam to material.
Recrystallization of Molybdenum Wire Doped with Potassium-Silicate
MYOUNG KI YOO, YUTAKA HIRAOKA, and JU CHOI
The doping effect of the bubble formation oxide on the recrystallization of Mo wire was investigated. Five different wires of 1 mm in diameter were prepared through sintering, swaging, and drawing processes. Each wire was doped with various amounts of potassium (K) plus silicon (Si), i.e.,0, 0.028, 0.14, 0.28, and 0.49 by weight percent, and annealed for 30 minutes at given temperatures. To understand the overall recrystallization phenomena, changes in hardness and in optical microstructures were examined. Transmission electron micrographs were taken for the specimens in the as-drawn state and at the beginning of the decrease in hardness. And also, the relative excess resistivity was measured as a function of heating temperature to confirm the occurrence of the abnormal grain growth. During the grain growth, bubble dispersion was evaluated through fractography by scanning electron microscopy (SEM). Primary recrystallization started at 750°C regardless of the amount of dopants. For the specimens doped with 0.14 and 0.28 (K + Si), large elongated and interwoven grain structures indicating abnormal grain growth developed over 1400°C and 1600°C, respectively. For the specimens doped with 0.028 and 0.49 (K + Si), however, small equiaxial grain structure developed similar to pure molybdenum wire. Such a difference was understood through the relationship between grain structures and bubble dispersion parameters (the average bubble diameter, the bubble row density, the columnar bubble spacing, and the bubble row distance). It was concluded that two of the most important parameters to develop a grain structure of high aspect ratio were bubble row density and bubble row distance. At a high bubble row density, irregularity in bubble row distance induced the higher aspect ratio (length/width (L/W)) of grain.
+ ' mosaiclike structure. The formation of the + ' mosaiclike structure is attributed to an annealing effect during deposition. Matrix-lattice mismatches of 0.5 to 1 pct at the /' interface and {100} growth orientations of ' phase are deduced from microscopic observations. The formation of the banded structure is attributed to the high cooling rate that is inherent to RAD processing. Anticipated dispersoids, such as Y2O3, Al2O3, and Y3AI5O2 are identified using transmission electron microscopy (TEM). Dislocation pileups and grain boundary pinning are observed in the vicinity of oxide dispersoids. The origin and movement of dislocations in the as-deposited materials may be attributed to the residual stresses that originate from thermal gradients and the large amount of deformation experienced by the solid and semisolid droplets during impact. The preliminary results and analyses reported here suggest that the high thermal stability of the RAD processed Ni3Al using N2-15 pct O2 may be attributed not only to the hindering effect of oxide dispersoids on grain boundary migration, but also to the high cooling rate experienced by the droplets during atomization and the short annealing effect experienced by the material during deposition
Internal Hydrogen Embrittlement of a Ferritic Stainless Steel
J.-H. HUANG and C.J. ALTSTETTER
AL 29-4-2 ferritic stainless steel was cathodically precharged with hydrogen at 230°C in a molten salt electrolyte. Constant load crack growth tests were performed in air and in hydrogen gas at 108 kPa pressure at room temperature on both uncharged specimens and specimens containing 2 wt ppm hydrogen. The DC potential drop method was calibrated with optical measurements to continuously monitor the crack position, so that the crack velocity vs stress intensity relation could be calculated. From the test results, it was found that dissolved hydrogen does allow subcritical crack growth for tests in air, but its effect is masked by the external hydrogen for tests in hydrogen gas. The distribution of slip bands on the side surface indicates that the dissolved hydrogen causes localization of plasticity, with the strain concentrated in coarse slip bands. Comparing the embrittling effect of internal hydrogen with that of external hydrogen, it can be concluded that environmental hydrogen is far more damaging than internal hydrogen, at least in specimens containing 2 wt ppm hydrogen.
Damping Characteristics of Ti50Ni49.5Fe0.5 and Ti50Ni40Cu10 Ternary Shape Memory Alloys
H.C. LIN, S.K. WU, and Y.C. CHANG
The damping characteristics of Ti50Ni49.5Fe0.5 and Ti50Ni40Cu10 ternary shape memory alloys (SMAs) have been systematically studied by resonant-bar testing and internal friction (IF) measurement. The damping capacities of the Bl9' martensite and the B2 parent phase for these ternary alloys are higher than those for the Ti50Ni50 binary alloy. The lower yield stress and shear modulus of these ternary alloys are considered to be responsible for their higher damping capacity. For the same ternary alloy, the Bl9/Bl9' martensite and R phase also have a higher damping capacity than does the B2 parent phase. In the forward transformations of B2 
R, R Bl9', and B2 Bl9' for Ti50Ni50 and Ti50Ni49.5Fe0.5 alloys, the damping capacity peaks appearing in the resonant-bar test are attributed to both stress-induced transformation and stress induced twin accommodation. The lattice-softening phenomenon can promote the stress-induced transformation and enhance the damping capacity peaks. The Ti50Ni40Cu10 alloy had an unusually high plateau of damping capacity in the Bl9 martensite, which is considered to have arisen from the easy movement of twin boundaries of Bl9 martensite due to its inherently very low elastic modulus. The peaks appearing in the IF test for the Ti50Ni40Cu10 alloy are mainly attributed to the thermal-induced transformation due to T 
0 during the test.
Novel Oxide-Dispersion-Strengthened Copper Alloys from Rapidly Solidified Precursors: Part 1. Microstructural Development
MICHAEL S. NAGORKA, CARLOS G. LEVI, and GLENN E. LUCAS
ZrO2, Y2O3, and rare earth oxides with related structures are attractive candidates for dispersion strengthening of copper alloys but pose significant processing challenges owing to the low solubility of the oxide-forming elements in Cu. It is shown that the problems may be circumvented by a synthesis approach coupling rapid solidification and internal oxidation, followed by standard powder metallurgy consolidation. Cu-Zr and Cu-Y alloys were melt spun into ribbons 
50 to 150-µm thick and internally oxidized at 1023 to 1223 K to yield 1 vol pct of ZrO2 or Y2O3 particles ranging in size from 
5 nm up to 150 nm. The coarser oxides result from direct oxidation of the intermetallic segregate, whereas the finer ones are generated by a dissolution-reprecipitation process. The relative proportions of fine and coarse oxides and the homogeneity of the distribution are related to segregation scale in the melt-spun ribbon and the relative permeabilities of oxygen and the oxidizable element in the alloy, which depend on the internal oxidation temperature. The oxide dispersoids were mostly cubic zirconia or cubic yttria and exhibited predominantly cube-on-cube orientation relationships with the matrix. Analysis of particle shapes revealed that the dominant interfaces are of the type {001}Ox {001}Cu and {
}Ox {}Cu and could be explained by image charge interaction concepts. Extrusion produced an elongated grain structure but no significant changes in the oxide distribution.
Novel Oxide-Dispersion-Strengthened Copper Alloys from Rapidly Solidified Precursors: Part 2. Creep Behavior
MICHAEL S. NAGORKA, GLENN E. LUCAS, and CARLOS G. LEVI
Yttria- and zirconia-dispersion-strengthened copper alloys produced by hot pressing and hot extrusion of internally oxidized melt-spun Cu-0.33 at. pct Y and Cu-0.32 at. pct Zr ribbons were subjected to compressive creep tests at 923 and 973 K. Creep strengths and stress exponents were higher for the Cu-ZrO2 alloy than for Cu-Y2O3, and both were higher than those of pure copper. Comparisons of the creep properties with published data for pure copper along with microscopic evidence indicated that at least two creep mechanisms were operating in these alloys. These are attractive dislocation/particle interactions in the matrix and particle-inhibited diffusional creep. The experimental data at low stresses could be described reasonably well by the Arzt-Ashby-Verral model for particle-restricted diffusional creep, using plausible values for the structure-related parameters. Fitting the higher stress creep data to the Rösler-Arzt model of dislocation/particle interaction resulted in values of the relaxation parameter (k) within the bounds predicted by the theory. The estimated k values for Y2O3 and ZrO2 are in the vicinity of 0.8, compared with 0.9 for the -Al2O3 dispersoids in conventional oxide-dispersion-strengthened (ODS) Cu. The analysis suggests that these alternate dispersoids with fluorite-related structures may interact more effectively with dislocations during creep.
Cracking Mechanisms in Thermally Cycled Ti-6AI-4V Reinforced with SiC Fibers
S.H. THOMIN, P.A. NOËL, and D.C. DUNAND
A titanium alloy (Ti-6Al-4V) reinforced with continuous SiC fibers (SCS-6) was thermally cycled between 200°C and 700°C in air and argon. The composite mechanical properties deteriorate with an increasing number of cycles in air because of matrix cracks emanating from the specimen surface. These cracks also give oxygen access to fibers further resulting in fiber degradation. The following matrix cracking mechanisms are examined: (1) thermal fatigue by internal stresses resulting from the mismatch of thermal expansion between fibers and matrix, (2) matrix oxygen embrittlement, and (3) ratcheting from oxide accumulating within cracks. Matrix stresses are determined using an analytical model considering stress relaxation by matrix creep and the temperature dependence of materials properties. Matrix fatigue from these cyclically varying stresses (mechanism (1)) cannot solely account for the observed crack depth; oxygen embrittlement of the crack tip (mechanism (2)) is concluded to be another necessary damage mechanism. Furthermore, an approximate solution for the stress intensity resulting from crack wedging by oxide formation (mechanism (3)) is given, which may be an operating mechanism as well for long cracks.
Interfaces in Continuous Filament-Reinforced Al2O3/NiAl Composites
L. WANG, K. XU, R.R. BOWMAN, and R.J. ARSENAULT
Interfacial structures in a continuous Al203 filament-reinforced NiAl composite were investigated by transmission electron microscopy (TEM). A graphite phase, which is an artifact of the composite fabrication procedure, decorates the interfacial region of the composite. The presence of the graphite is believed to play a role in both the low interfacial bond strength in the as-fabricated composite and the further reduction in bond strength after 10 thermal cycles in the temperature range of 373 to 1373 K. In regions where the graphite phase was not present, there appeared to be an intimate bond between the NiAl matrix and the Al2O3 filaments. Simulation of TEM diffraction contrast images based upon a three-dimensional (3D) finite element analysis was employed to investigate the nature of the residual strains in regions along the interface. The simulations suggested that radial residual strains within the Al2O3 filaments were randomly distributed along the interface. These strains are believed to be related to dislocation nucleation in the NiAl, which results from the relaxation of the thermally generated residual stresses.
Grain Boundary Sliding in the Presence of Grain Boundary Precipitates during Transient Creep
X.-J. WU and A.K. KOUL
A constitutive rate equation for grain boundary sliding (GBS), in the presence of grain boundary precipitates, is developed. Langdon's GBS model is modified by incorporating physically defined back stresses opposing dislocation glide and climb and by modifying the grain size dependence of creep rate. The rate equation accurately predicts the stress dependence of minimum creep rate and change in activation energy occurring as a result of changing the grain boundary precipitate distribution in complex Ni-base superalloys. The rate equation, along with the mathematical formulations for internal stresses, is used to derive a transient creep model, where the transient is regarded as the combination of primary and secondary stages of creep in constant load creep tests. The transient creep model predicts that the transient creep strain is dependent on stress and independent of test temperature. It is predicted that a true steady-state creep will only be observed after an infinitely long time. However, tertiary creep mechanisms are expected to intervene and lead to an acceleration in creep rate long before the onset of a true steady state. The model accurately predicts the strain vs time relationships for transient creep in IN738LC Ni-base superalloy, containing different grain boundary carbide distributions, over a range of temperatures.
Critical Analysis of Alloy 600 Stress Corrosion Cracking Mechanisms in Primary Water
R. RIOS, T. MAGNIN, D. NOEL, and O. de BOUVIER
In order to study the mechanisms involved in the stress-corrosion cracking (SCC) of Alloy 600 in primary water the influence of the relevance of physicochemical and metallurgical parameters was assessed: hydrogen and oxygen overpressures, microstructure, and local chemical composition. The obtained results show that even if the dissolution/oxidation seems to be the first and necessary step responsible for crack initiation and if hydrogen effects can also be involved in cracking neither a dissolution/oxidation model nor a hydrogen model appears aufficient to account for cracking. Moreover, fractographic examinations performed on specimens fracture surfaces lead to the fact that attention should be paid to a cleavagelike microcracking mechanism involving interactions between corrosion and plasticity at the vicinity of grain boundaries. A corrosion-enhanced plasticity model is proposed to describe the intergranular and transgranular cracking in Alloy 600.
). However, following high-temperature exposure in oxidative environments, aluminum is depleted from the coating. Aluminum depletion in turn, leads to destabilization of the phase, resulting in the formation of a characteristic lathlike , -derivative microstructure. This article presents a transmission electron microscopy study of the formation of the lathlike , -derivative microstructure using bulk nickel aluminides as model alloys. In the bulk nickel aluminides, the lathlike microstructure has been found to correspond to two distinct components: L10-type martensite and a new derivative. The new derivative is characterized and the conditions associated with the presence of this feature are identified and compared with those leading to the formation of the L10 martensitic phase.
On the Application of Magnetomechanical Models to Explain Damping in an Antiferromagnetic Copper-Manganese Alloy
S. LADDHA and D.C. VAN AKEN
The Smith-Birchak model for magnetoelastic damping was successfully applied to model the damping observed in an antiferromagnetic Cu-48Mn-1.5Al (wt pct) alloy. Antiferromagnetic domains were developed by solution treatment at 820°C and subsequent aging at 400°C for 4, 10, and 16 hours. Damping capacity and dynamic elastic modulus were measured as a function of strain amplitude and temperature. A maximum in the strain-amplitude-dependent damping was obtained for the 4-hour-aged sample for which a magnetostriction constant, , equal to 4.65 x 10-4, was derived. An exact fit for the Smith-Birchak model was obtained at low strains, whereas the model predicted lower damping than was observed for strains greater than 1.1 x 10-3. This discrepancy was attributed to an additional damping mechanism at high strain amplitudes, i.e., dislocation damping. A magnetostriction constant equal to 3.23 x 10-4 was also calculated based upon the Nel temperature and the observed microstructure.
Formation and Suppression of Channels during Upward Solidification of a Binary Mixture
Factors Affecting Pressure Infiltration of Packed SiC Particulates by Liquid Aluminum
Microstructure and Crystallographic Texture of Strip-Cast and Hot-Rolled Austenitic Stainless Steel
Gravity Segregation of Complex Intermetallic Compounds in Liquid Aluminum-Silicon Alloys
SURESH V. GARIMELLA, JAMES P. McNULTY, and LEI Z. SCHLITZ
Characteristics of the mushy zone were experimentally investigated in the upward solidification of a binary mixture. The formation of channels and techniques for their suppression were explored. The experiments were carried out in a rectangular test section using aqueous ammonium chloride as the phase-change material at hypo- and hypereutectic concentrations ranging from 15 to 33 wt pct of salt. The cold-plate temperature was varied in the range of -60°C to -14°C. Transient temperature profiles as well as the positions of the liquidus and solidus interfaces were obtained. The mushy-zone characteristics, the nature and distribution of channels, and the associated fluid flows were studied as a function of initial solution concentration and cold-plate temperature. The application of low-amplitude vibration to the test cell was found to reduce the mushy-zone thickness and the number of channels for all concentrations; channeling was almost completely suppressed at the lower concentrations. Channel suppression was influenced by vibration amplitude rather than frequency, with the larger-amplitude vibration being the more effective. The vibration results suggest that the formation and sustenance of channels is influenced more by developments within the mushy zone than by bulk liquid behavior.
J. NARCISO, A. ALONSO, A. PAMIES, C. GARCÍA-CORDOVILLA, and E. LOUIS
Pressure infiltration is being currently used to evaluate the wettability of ceramic particles and fibers by liquid metals. The objective of this work is to investigate the effect of type and surface condition of the particulates and infiltration atmosphere on pressure infiltration of packed SiC particulates by pure liquid aluminum. Fourteen SiC particulates from five different suppliers, of green (high purity) and black (low purity) types, were used. Two of the particulates were subjected to either low- or high-temperature heat treatments in order to either burn organic residues or promote oxidation. Infiltrations were carried out in air, argon, argon/hydrogen, and nitrogen/hydrogen atmospheres. The results indicate that the threshold pressure does not depend appreciably on the type of particulate. A heat treatment at 600°C improves the infiltration performance of particulates having a large amount of organic residues, whereas oxidation at 1000°C decreases the threshold pressure. The results also illustrate the effects of the infiltration atmosphere; in particular, it is shown that inert or reducing atmospheres improve wettability.
D. RAABE
The microstructure and texture of a strip-cast as well as a hot-rolled austenitic stainless steel (18 pct Cr, 8.5 pct Ni) are investigated by the use of optical metallography and quantitative X-ray texture analysis. In the hot band, a homogeneous microstructure is revealed together with a through-thickness texture gradient consisting of a weak cold rolling type of texture in the center layer and a shear texture close to the surface layers. The result is discussed in terms of the through-thickness shear profile that is generated during hot rolling. In the strip-cast material, a random orientation distribution as well as the development of martensite close to the center layer is attributed to the impingement and deformation of the films that are solidified on the surfaces of the casting rolls. The texture close to the surface is attributed to the growth selection of {001}<uvw> oriented grains.
S.G. SHABESTARI and J.E. GRUZLESKI
Primary crystals of intermetallics that are rich in iron, manganese, and chromium form at temperatures above the liquidus, and because their density is higher than that of liquid aluminum, they cause gravity segregation in the melt. Segregation may occur either in the mold at slow cooling rates or in the bulk liquid in furnaces or ladles. The kinetics of settling of these intermetallic compounds in a melt of Al-12.5 pct Si having 1.2 pct Fe, 0.3 pct Mn, and 0.1 pct Cr has been studied. Sedimentation was investigated at 630°C for settling times of 30, 90, and 180 minutes in an electric resistance furnace. The effect of settling time and height of melt on the volume percent, number, and size of intermetallic compounds was studied by image analysis. The volume percent of intermetallics increases with distance from the melt surface. Both the number of particles and the average size increase during sedimentation. The rate of settling varies with position in the melt due to depletion of intermetallics near the surface and an increase near the bottom. The settling velocities obtained experimentally were compared with terminal velocities calculated by Stokes' law. Good agreement was generally found. The settling speed of intermetallics reaches the terminal velocity at very short times and very close to the liquid surface. Stokes' law is therefore applicable to virtually all locations within the melt.
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Metallurgical and Materials Transactions B, Volume 26B, April 1995
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