METALLURGICAL AND MATERIALS TRANSACTIONS A
ABSTRACTS
Volume 28A, No. 3A, Special Issue 1997

This Month Featuring: Alloy Phases, Transformations, Transport Phenomena, Mechanical Behavior, Physical Chemistry, Environment, Surface Treatment, Solidification, Materials Processing. View Special Issue 1997 Table of Contents.

ALLOY PHASES

Decagonal Quasicrystal and Related Crystalline Phases in Mn-Ga Alloys with 52 to 63 At. Pct Ga
J.S. WU and K.H. KUO
A decagonal quasicrystal (DQC) and six related intermetallic phases with large unit cells have been found in binary Mn-Ga alloys with 52 to 63 at. pct Ga by means of transmission electron microscopy (TEM). As does the Al-Mn DQC, the Ga-Mn DQC also has a periodicity of 1.25 nm along its tenfold axis. However, its Mn content, determined by electron microprobe X-ray analysis (about 45 to 50 at. pct Mn), is much higher than that of the Al-Mn DQC (about 20 to 30 at. pct Mn). The compositions of the intermetallic phases are about 53, 56, 58, and 62 at. pct Ga, corresponding respectively to the unknown structures of MnGa (50.7 to 53.4 at. pct Ga), Mn5Ga6 (55 at. pct Ga), Mn5Ga7 (57.9 at. pct Ga), and Mn3Ga5 (62.9 at. pct Ga) given in the binary Mn-Ga phase diagram (Metals Hand book, T.B. Massalski, J.L. Murray, L.H. Benneft, and H. Baker, eds., ASM, Metals Park, OH, 1986, vol. 2, p. 1144). Their lattice types have been determined by selected area electron diffraction. The ferromagnetic Mn3Ga5 is tetragonal, a = 1.25 nm and c = 2.50 nm; Mn5Ga7 is orthorhombic, a = 4.57 nm, b = 1.25 nm, and c = 1.44 nm; Mn5Ga6 has two different but closely related orthorhombic unit cells, a = 1.26 nm, b = 1.25 nm, and c = 1.48 nm as well as a = 0.77 nm, b = 1.25 nm, and c = 2.36 nm; MnGa also has two different and related unit cells, one orthorhombic with a = 2.04 nm, b = 1.25 nm, and c = 1.48 nm and the other monoclinic with a = 2.59 nm, b = 1.25 nm, c = 1.15 nm, and 110 deg. All these orthorhombic phases have b = 1.25 nm, being the same as the periodicity along the tenfold axis of the Ga-Mn and Al-Mn DQCs. Moreover, all these six intermetallic phases give electron diffraction patterns displaying a pseudo-tenfold distribution of strong diffraction spots and are considered to be crystalline approximants of the Ga-Mn DQC.

TRANSFORMATIONS

Mössbauer Effect Study of Mechanically Alloyed and 1 Fe-Zn Intermediate Phases
ZHENTONG LIU and OSWALD N.C. UWAKWEH
The Mössbauer effect measurements of as-ball-milled mechanically alloyed Fe-Zn intermediate phases of (Fe3Zn10) and 1 (Fe5Zn21) compositions exhibit characteristic spectra consisting of triplets. Each is characterized with an Fe site with unusually large quadrupole splittings measuring 0.940 mm/s in the cubic Fe-Zn phases and, in addition, three other doublets. In the aged state, both compositions show spectra free of the Fe site with quadrupole splitting of 0.940 mm/s, designated as MSFe1 and MSFe1 and MS1Fe1, respectively. This suggests that the metastable states are similar, while their separate transformation paths to stable equilibrium lead to distinct crystal structures. The parameters for the aged states are compared with those reported in the literature for both phases.

Grain Boundary Mobility during Recrystallization of Copper
R.A. VANDERMEER, D. JUUL JENSEN, and E. WOLDT
Average grain boundary migration rates during recrystallization of cold-deformed copper were es timated from stereological measurements. In the same material, the instantaneous driving forces for boundary migration during recrystallization were calculated from calorimetric measurements of the release of the stored energy of cold work. The migration rate dependence on driving force was analyzed in the context of grain boundary migration rate theory, and within experimental error, a linear dependence was observed. The average mobility of grain boundaries migrating during recrys tallization of cold-worked copper at 121°C was calculated to be 6.31 x 10-10 (m4 s-1 MJ-1). This result was found to be consistent with single boundary, curvature-driven grain boundary mobilities measured in copper at higher temperatures. It was also demonstrated that the average grain boundary mobility was reasonably within the expectation (order of magnitude uncertainty) of the Turnbull single process model of boundary migration with a process akin to grain boundary self-diffusivity as the rate-controlling atomic mechanism.

TRANSPORT PHENOMENA

On the Blister Formation in Copper Alloys Due to the Helium Ion Implantation
D. MORENO and D. ELIEZER
A new approach to the blister-formation phenomenon is discussed by means of the mathematical solution on a uniformly loaded circular plate with clamped edges (circular diaphragm). In the present investigation, it was found that blister formation depends on the mechanical properties of the alloys and the near-surface concentration of the implanted gas, which itself is contingent on the crystallo graphic orientation by means of the stopping power of the implanted atoms. The reported model is based on the fact that at certain depths from the surface, the pressure in the cavities approaches the yield stress of the metal and blistering starts. The thickness of this thin film depends on the me chanical properties of the specific metal. Once a blister cavity is formed, the deformation of the thin film to form a blister cap depends on the buildup of pressure in the cavity contingent on the implanted dose. For the present model, it is sufficient to say that the thickness of the blister's cap cannot be correlated with the projected range of the implantation, as assumed by other authors. The implanted helium concentration needed to buildup enough gas pressure to create a blister at a depth which is close to the projected range is higher by 50 times than the gas helium concentration in the cavity. Experimental results, such as the fact that the blisters have burst at the edge of the circular skin, where the maximum stresses are developed, and the fact that at high implantation energy (large projected range), the bursting of the blisters occurs by multilayer caps, support the present model.

MECHANICAL BEHAVIOR

High-Temperature Crack Growth in 304 Stainless Steel under Mixed-Mode Loading Conditions
W.E. CHURLEY and J.C. EARTHMAN
High-temperature crack growth experiments have been conducted with 304 stainless steel specimens under mode I, mode II, and mixed-mode conditions. Crack growth rate and direction data for three different mixed-mode loadings have been analyzed to investigate the factors that control crack growth under mixed-mode conditions. The value of C* was calculated for mode mixities ranging from pure tensile to pure shear loading at the crack tip using a reference stress approach. Effective values of C* based on a damage mechanics approach were then calculated in an attempt to determine the multiaxial stress parameter that most accurately characterizes the stresses driving crack tip damage. The hydrostatic stress was found to be the stress parameter that best correlates the crack growth rate data for mode I, mode II, and mixed-mode loading conditions. The angle of growth for the mode I and mixed-mode conditions appears to be governed by both the maximum principal stress and the hydrostatic stress. However, the lack of tensile loads for mode II loading results in crack growth that is nearly collinear with the notch corresponding to the position of the maximum effective stress. Overall, the present results indicate that the hydrostatic stress is the most valid multiaxial stress parameter for predicting high-temperature crack growth in the present material under mixed-mode conditions.

Secondary Hardening and Fracture Behavior in Alloy Steels Containing Mo, W, and Cr
H. KWON, K.B. LEE, H.R. YANG, J.B. LEE, and Y.S. KIM
In 4Mo, 6W, 2Mo3W, 2Mo2Cr, and 3W2Cr alloy steels, which contain alloying elements, such as Mo, W and Cr, contributing to the secondary hardening by forming M2C-type carbide, the secondary hardening and fracture behavior were studied. Molybdenum had a strong effect on secondary hard ening, while W had a very weak effect on it but delayed the overaging. The MoW steel exhibited both moderately strong hardening and considerable resistance to overaging. On the other hand, the secondary hardening effect was diminished by the Cr addition, because the cementite of M3C type was stabilized at higher temperatures and the formation of M2C carbides was thus inhibited. Although the Cr addition had no merit in the secondary hardening itself, it eliminated the secondary hardening embrittlement (SHE). This was observed as a severe intergranular embrittlement due to the impurity segregation for the Mo and MoW steels and as a decrease in upper shelf energy for W steel, even in the overaged condition.

Crystallographic Analysis of the Influence of Stress State on Earing Behavior in Deep Drawing of Face-Centered Cubic Metals
S.Y. LI, X.M. ZHANG, H.Z. ZHOU, and G. GOTTSTEIN
The Tucker method of earing analysis is modified by introducing the normalized radial strain to represent the radial anisotropic deformation in deep drawing. By means of this method associated with different assumptions on the radial strain contribution of slip, the influence of stress state in the flange of blank on earing behavior is analyzed in detail for some important ideal orientations in face-centered cubic (fcc) metal sheets. The results show that with the change of stress ratio, the ear profiles predicted by the total-slip (TS) model change very slightly for all the orientations, while those by the partial-slip (PS) model change significantly for the (100)[001], (100)[011], (110)[001], and (110)[] orientations. In both predictions, the change of earing extent with stress ratio is usually large for small stress ratio, i.e., near the outer edge of the blank, and different for various orientations. A simple method is proposed and applied to predict the cup profiles of some aluminum sheets for deep drawing.

Room-Temperature Deformation Behavior of Directionally Solidified Multiphase Ni-Fe-Al Alloys
A. MISRA and R. GIBALA
Directionally solidified (DS) + ( + ') Ni-Fe-Al alloys have been used to investigate the effect of a ductile second phase on the room-temperature mechanical behavior of a brittle <001>-oriented (B2) phase. The ductile phase in the composite consisted of a fine distribution of ordered ' precipitates in a (fcc) matrix. Three microstructures were studied: 100 pct lamellar/rod, lamellar + proeutectic , and discontinuous . The matrix in the latter two microstructures contained fine- scale bcc precipitates formed due to spinodal decomposition. Room-temperature tensile ductilities as high as 12 pct and fracture toughness (KQ) of 30.4 MPa were observed in the 100 pct lamellar/rod microstructure. Observations of slip traces and dislocation substructures indicated that a substantial portion of the ductility was a result of slip transfer from the ductile phase to the brittle matrix. This slip transfer was facilitated by the Kurdjumov-Sachs (KS) orientation relationship between the two phases and the strong interphase interface which showed no decohesion during deformation. In microstructures which show higher values of tensile ductility and fracture toughness, <100> slip was seen in the phase, whereas <111> slip was seen in the phase in the microstructure which showed limited ductility. The high ductility and toughness are explained in terms of increased mobile dis location density afforded by interface constraint. The effect of extrinsic toughening mechanisms on enhancing the ductility or toughness is secondary to that of slip transfer.

Dry Sliding Wear Response of Some Bearing Alloys as Influenced by the Nature of Microconstituents and Sliding Conditions
B.K. PRASAD
An attempt has been made in this study to examine the dry sliding wear response of a leaded-tin bronze, an aluminum bronze, and a conventional zinc-based alloy under varying applied pressure and speed conditions. Different characteristics of the microconstituents of the alloys have been cor related with that of their wear behavior. The study clearly indicates that the influence of the mi crostructural features greatly changes with the sliding conditions. It also has been observed that in order to attain good wear characteristics, a material should comprise an optimum level of lubricating, load bearing and ductile microconstituents, and, above all, thermal stability. Room temperature prop erties in fact play rather a secondary role in this context.

The Effect of Hydrogen on the Fracture Toughness of Alloy X-750
DOUGLAS M. SYMONS and ANTHONY W. THOMPSON
The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147 MPa. Charging with hydrogen decreased the fracture toughness, Klc, to 52 MPa at a rapid loading rate and further decreased the toughness to 42 MPa for a slow loading rate. This is consistent with the rate-limiting step for the embrittlement process being hy drogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.

Mechanisms of Ambient Temperature Fatigue Crack Growth in Ti-46.5Al-3Nb-2Cr-0.2W
B.D. WORTH, J.M. LARSEN, S.J. BALSONE, and J.W. JONES
Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti-46.5Al-3Nb-2Cr-0.2W (at. pct), heat treated to produce duplex and lamellar microstructures. Fatigue crack growth tests were conducted at 23°C using computer-controlled servohydraulic loading at a cyclic frequency of 20 Hz. Several test methods were used to obtain fatigue crack growth rate data, including decreasing-load-range-threshold, constant-load-range, and constant-Kmax increasing-load-ratio crack growth control. The lamellar microstructure showed substantial improvement in crack growth resistance and an increase in the threshold stress intensity factor range, Kth, when compared with the behavior of the duplex microstructure. The stress ratio had a significant influence on crack growth behavior in both microstructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic characterization of fatigue crack propagation modes indicated a highly tortuous crack path in the fully lamellar microstructure, compared to the duplex microstructure. In addition, limited shear ligament bridging and secondary cracking parallel to the lamellar interfaces were observed in the fully lamellar microstructure during fatigue crack propagation. These obser vations were incorporated into a model that analyzes the contribution of intrinsic vs extrinsic mech anisms, such as shear ligament bridging and roughness-induced crack closure, to the increased fatigue crack growth resistance observed for the fully lamellar microstructure.

PHYSICAL CHEMISTRY

Reaction Diffusion and Phase Equilibria in the V-N System
C. TEICHMANN, W. LENGAUER, P. ETTMAYER, J. BAUER, and M. BOHN
The formation of phase bands in in situ diffusion couples of the V-N system was studied by the reaction of vanadium sheet with pure nitrogen within the temperature range 1100°C to 1700°C and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase bands of -V2N and -VN1-x develop. The morphology of the -V2N/-V(N) interface depends on the saturation state of the - V(N) core. If the nitrogen content in -V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the -V(N) phase, the interface is planar. Electron probe microanalysis (EPMA) was used to measure the diffusion profiles within the couples. The homogeneity regions of the nitride phases were established and the phase diagram accordingly corrected. From the growth rates of the phase bands, the mean composition-independent nitrogen diffusivities in -V2N and - VN1-x were derived. These diffusivities follow an Arrhenius equation with activation energies of 2.92 (-V2N) and 2.93 eV (-VN1-x). By using -VN1-x as a starting material and a low nitrogen pressure during annealing, it could be shown that the direction of nitrogen diffusion can be reversed, i.e., -V2N is formed on the surface of the couple as a result of out-diffusion of nitrogen.

ENVIRONMENT

Room-Temperature Diffusion in Cu/Ag Thin-Film Couples Caused by Anodic Dissolution
DENNY A. JONES, ALAN F. JANKOWSKI, and GAIL A. DAVIDSON
Thin, 100-nm films of first silver and then copper were deposited consecutively onto inert substrates by magnetron sputter deposition. Constant anodic current densities were applied at room temperature to dissolve the outer copper film to varying depths. The 50Cu/50Ag interface, derived from the auger electron spectroscopic concentration-depth profile, initially moved into the copper toward the outer dissolving surface, indicating enhanced diffusion of copper into silver. After longer times at all anodic current densities, the interface reversed and moved back toward the underlying silver-rich layer, indicating that eventually diffusion of silver into copper predominated. The reversal time was in versely proportional to the anodic current density. These effects are explained by anodic formation of subsurface vacancies which migrate as divacancies to the copper/silver interface where they affect interface movements by the well-known Kirkendall mechanism. Calculated diffusivities up to 10-12 cm2/s at maximum anodic current densities of 900 µA/cm2 are dramatically above any that are normally observed at room temperature.

SURFACE TREATMENT

X-Ray and Transmission Electron Microscopy Investigation of Strain in a Nitrided Steel: No Evidence of Plastic Deformation
L. BARRALLIER, R. SOTO, J.-M. SPRAUEL, and A. CHARAI
In order to characterize the nitrided layers of steels, the X-ray diffraction (XRD) technique has been used coupled with transmission electron microscopy (TEM) observations. The TEM observations of the nitrided layers were made using a cross-sectional specimen preparation technique. Using the Warren-Averbach approach to the analysis of the diffraction peak broadening and TEM investiga tions, the existence of several precipitates of nitrides was brought to the fore, but no evidence for dislocations was found. To improve the calculation of the Fourier coefficient, the diffraction peaks were described in reciprocal lattice by a mixed Cauchy-Gauss curve. In fact, our investigations show that the broadening of the diffraction peaks of nitrided layers is strongly dependent on the size of the coherently diffracting domains. For nitriding, this result confirms that the residual stresses are only attributable to the precipitation of nitrides. In contrast, the microstructural state obtained after mechanical surface treatments, such as shot peening, is very different; the incompatibility of the plastic strains is the origin of the stress field.

SOLIDIFICATION

Convection and Channel Formation in Solidifying Pb-Sn Alloys
MICHAEL I. BERGMAN, DAVID R. FEARN, JEREMY BLOXHAM, and MARGARITA C. SHANNON
A suite of experiments on the dendritic solidification of Pb-Sn melts has been carried out. The first goal has been to quantify the longitudinal macrosegregation, and hence the convective vigor through the dendritic ("mushy") zone during solidification, as a function of the mushy zone Rayleigh num ber. The mushy zone Rayleigh number Ram is a ratio of the driving compositional buoyancy force to the retarding Darcy frictional force. The second goal has been to characterize the formation of convection channels as a function of Ram. In a fixed furnace, the melts were program cooled and solidified from beneath, at various cooling rates. Two different temperature gradients were examined. Each pairing of cooling rate and temperature gradient results in a different Ram. As expected, the measured longitudinal macrosegregation increased with Ram. The vestiges of convection channels on a solidified ingot surface (which we call "freckle trails") were observed for all conditions except for the most rapid cooling rate with the smaller temperature gradient (i.e., the smallest Ram) and for the slowest cooling rate with the larger temperature gradient (i.e., the largest Ram). Under the latter solidification conditions, the vestiges of convection channels in an ingot interior (which we call "chimneys") were observed. Chimneys were not observed in other ingots. When present, the number of freckle trails decreased and the width of the trails increased with increasing Ram. The trails became more diffuse as well. It appears that Ram may control channel characteristics as well as convection and the resulting macrosegregation. There appear to be two critical values, a lower one for surface freckle trails and a higher one for interior chimneys. Conditions at the Earth's inner-outer core boundary (ICB) may be those exhibiting high Ram convection, so that convection channels, if they exist, could be as large as several hundred meters in width.

On the Transition from Pushing to Engulfment During Directional Solidification of the Particle-Reinforced Aluminum-Based Metal-Matrix Composite 2014+10 Vol Pct Al2O3
U. HECHT and S. REX
Microstructural evolution in the commercial aluminum-based metal matrix composite 2014 + 10 vol pct Al2O3 was investigated during directional solidification with planar interface within the initial transient. Investigations were directed toward phase formation and phase distribution, with special emphasis on the critical conditions for the transition from particle pushing to engulfment. In situ nucleation of intermetallic particles, identified as being the complex silicides (Fe, Mn)3Si2Al15, their growth, pushing, and subsequent engulfment are shown to be interactively coupled to the pushing and engulfment of the inert alumina particles. The experimental conditions for engulfment of the inert particles are in good agreement with predictions according to the "critical velocity" model of Pöxchke and Rogge, the critical velocities ranging from 0.3 to 1.0 µm/s, due to the effect of the solutal field. This indicates that in castings with equiaxed dendritic solidification patterns, the radial growth velocities are not necessarily responsible for pushing the particles into the interdendritic spaces. For the intermetallic particles, the dependence of the critical velocity on particle size is not linear as for inert particles, but deflected upward for increasing size. This is probably due to the fact that they act as a sink for certain species of segregated solute atoms, meaning that size and solutal distortions are reactively coupled.

The Influence of Convection during Solidification on Fragmentation of the Mushy Zone of a Model Alloy
C.J. PARADIES, R.N. SMITH, and M.E. GLICKSMAN
Experiments have been conducted to observe fragmentation events in a model alloy (succinonitrile and acetone) solidifying in the presence of forced convection in the superheated melt. Measurements of fragmentation rates have been made, and an attempt was made to relate the results to the con trollable parameters of the system. A microscope-video system recorded the mushy zone-melt in terface, and the fragmentation process and fragmentation rates could be determined from a frame-by-frame analysis of the video images. Experiments were conducted for varying cooling rates, overall temperature differences, melt flow rates, and for two different concentrations of acetone (1.3 and 6.1 wt pct). Significant dendritic fragmentation occurred for all runs. In addition, the influence of buoyancy forces is clearly evident from particle motion near the mushy zone-melt interface. Fragmentation rates appear to correlate well with the magnitude of particle velocities near the inter face, with increasing fragmentation being associated with higher particle velocity magnitude (either in the same or the opposite direction to the mean flow) for the 1.3 wt pct acetone mixture. However, the correlation is quite different for the higher concentration. The relationship between these results and the possible mechanisms for fragmentation are discussed. Although it appears that either con stitutional remelting or capillary pinching are likely of importance, hydrodynamic shear forces or some other mechanism as yet undiscovered cannot be completely discounted, although circumstantial evidence suggests that mechanical shearing is inconsistent with observations made both here and already in published literature. The results provide a step in the development of solidification models that incorporate fragmentation processes in the mushy zone as an important mechanism of grain refinement and a potential source of macrosegregation in ingots and large castings.

MATERIALS PROCESSING

An Investigation of the Effect of Texture on the High-Temperature Flow Behavior of an Orthorhombic Titanium Aluminide Alloy
P.D. NICOLAOU and S.L. SEMIATIN
The effect of mechanical and crystallographic texture on the flow properties of a Ti-21Al-22Nb (at. pct) sheet alloy was determined by conducting uniaxial tension and plane-strain compression tests at temperatures between 900°C and 1060°C and strain rates between 10-4 and 10-2 s-1. Despite the presence of noticeable initial texture, all of the mechanical properties for a given test temperature and strain rate (i.e., peak stress, total elongation to failure, strain-rate sensitivity, and normal plastic anisotropy) were essentially identical irrespective of test direction relative to the rolling direction of the sheet. The absence of an effect of mechanical texture on properties such as ductility was explained by the following: (1) the initially elongated second-phase particles break up during tension tests parallel to the rolling direction of the sheet, thereby producing a globular morphology similar to that noted in samples taken transverse to the rolling direction; and (2) failure was flow localization, rather than fracture, controlled. Similarly, the absence of an effect of mechanical texture on strain-rate sensitivity (m values), normal plastic anisotropy (r values), and the ratio of the plane strain to uniaxial flow stresses was rationalized on the basis of the dominance of matrix (dislocation) slip processes within the ordered beta phase (B2) as opposed to grain boundary sliding. Aggregate theory predic tions supported this conclusion inasmuch as the crystallographic texture components determined for the B2 phase ((001) [100] and () [110]) would each produce identical r values and uniaxial and plane-strain flow stresses in the rolling and transverse directions.

Consolidation of Nanostructured Metal Powders by Rapid Forging: Processing, Modeling, and Subsequent Mechanical Behavior
G.R. SHAIK and W.W. MILLIGAN
Fe-10Cu powders containing 20-nm grains were produced by attritor milling of elemental powders in argon. A rapid powder forging technique was developed to consolidate the powders into fully dense compacts while maintaining nanoscale grain sizes. Grain growth during the consolidation was controlled by reducing the time of exposure at elevated temperature to a few minutes or less, a technique which is applicable to all materials and does not necessitate the addition of dispersoids. This was achieved by heating green compacts quickly using an induction heater, and then forging and rapidly cooling them back to room temperature. Forging was conducted in a protective argon atmosphere to limit contamination. Fully dense compacts were produced at relatively low tempera tures, mainly due to the accelerated creep rates exhibited by the nanostructures. Transmission electron microscopy and X-ray diffraction analysis found an average grain size of 45 nm in the fully dense samples forged at 530°C. Indications are that finer grain sizes should be attainable by using slightly lower temperatures and higher pressures. The success of the technique (compared to hot-isostatic pressing ("hipping")) is due to both reducing time at elevated temperatures and applying relatively high pressures. Microhardness tests revealed a significant strengthening effect due to grain size refinement, following a Hall-Petch relation. Compression testing at room temperature showed no strain hardening during plastic deformation, which occurred by shear banding. High strengths, up to 1800 MPa, were obtained at room temperature. Compression testing at 575°C revealed a significant strain rate dependence of mechanical behavior and also the possibility of superplastic behavior. Power-law creep was observed at 575°C, with very high steady-state creep rates on the order of 50 pct/s at 230 MPa. The consolidation process was successfully modeled by slightly modifying and applying the Arzt, Ashby, and Easterling (AAE) hot-isostatic press (HIP) model. The experiments and modeling indicated that creep was the dominant densification mechanism in these materials, even at relatively low temperatures and high loading rates. The results of this investigation suggest the possibility of a commercially viable nanostructured metal, which is easily processed to large strains at moderate temperatures, yet maintains high strength at room temperature without the necessity of heat treatment or mechanical working.

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