Room-temperature deformation behavior of a directionally solidified β (B2)-(Ni-Fe-Al) intermetallic alloy

The room-temperature mechanical behavior of a directionally solidified columnar-grained, single-phase β (B2)-(Ni-20 at. pct Fe-30 at. pct A1) intermetallic alloy deformed along the “hard” 〈001〉 direction has been characterized. The 0.2 pct offset compressive yield stress was found to be comparable to that of 〈001〉 single crystals of stoichiometric NiAl. The dislocation substructure consisted of a preponderance of long, straight a〈111〉 screw dislocations on {112} planes, with cross-slip on {123} and {110} planes. The superpartials were not resolved by weak-beam imaging conditions, indicating that the antiphase boundary (APB) energy of NiAl is not reduced significantly by the Fe addition. The dislocation substructure was analyzed as a function of strain and compared to the dislocation substructure in 〈001〉 NiAl and body-centered cubic (bcc) metals deformed at low homologous temperatures. The debris left behind by a〈111〉 screw dislocations consisted of prismatic edge dipole loops 5 to 25 nm in diameter.     

Microstructural characterization of novel in-situ Al-Be composites

The microstructure of cast and extruded in-situ Al-Be alloys, of compositions of Be-37Al-3Ni (wt pct) and Be-34Al-2Ni-2Ag-2Si (wt pct), was investigated using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The study indicates that both the Be and Al phases are continuous. The Be phase has a coarse dendritic structure in the as-cast material. Fractographic analysis of failed tensile specimens tested at room temperature revealed basal-plane cleavage failure of the Be phase and ductile failure of the Al phase. A significant number of deformation twins were observed in the Be phase when the tensile loading axis was parallel to the Be dendrite growth axis. An additional fracture mode was observed in the samples tested at elevated temperatures. At elevated temperatures, decohesion of the Al-Be interface was observed on the fracture surface. This phenomena was observed to increase as the test temperature increased from 150 °C to 315 °C. A high density of dislocations with a tangled morphology were observed in the Al phase after the tensile test. These were determined to be associated with easy slip of 1/2〈101〉-type dislocations. The limited ductility of the Be phase was attributed to the predominant basal slip of 〈a〉-type dislocations, b = 1/3〈1120〉, and the lack of dislocations with 〈c〉 components. However, a significant number of dislocations with 〈c〉 components were found in localized areas of the Be phase after extrusion.     

The role of microstructure on strength and ductility of hot-extruded mechanically alloyed NiAl

Mechanical alloying followed by hot extrusion has been used to produce very fine-grained NiAl-based alloys containing oxide dispersoids. The dispersoids affect the progress of recrystallization during hot extrusion and contribute to the preservation of the 〈110〉 deformation fiber texture. The 〈110〉 texture enables the activation of 〈110〉 〈100〉 and 110 〈110〉 slip systems. The occurrence of 〈100〉 and 〈110〉 slip dislocations satisfies the von Mises criterion for general plasticity and is postulated to contribute to notable room-temperature compressive ductility of the mechanically alloyed (MA) materials. Another factor likely affecting the compressive ductility is the predominant occurrence of low-angle grain boundaries. The attractive dislocation — dispersoid interactions lead to a ductility trough observed at 800 K in the MA materials. The MA NiAl materials are strong at both ambient and elevated temperatures due to fine grain and the presence of dispersoids and interstitial atoms.     

Deformation behavior of HCP Ti-Al alloy single crystals

Single crystals of Ti-Al alloys containing 1.4, 2.9, 5, and 6.6 pct Al (by weight) were oriented for 〈a〉 slip on either basal or prism planes or loaded parallel along the c-axis to enforce a nonbasal deformation mode. Most of the tests were conducted in compression and at temperatures between 77 and 1000 K. Trace analysis of prepolished surfaces enabled identification of the twin or slip systems primarily responsible for deformation. Increasing the deformation temperature, Al content, or both, acted to inhibit secondary twin and slip systems, thereby increasing the tendency toward strain accommodation by a single slip system having the highest resolved stress. In the crystals oriented for basal slip, transitions from twinning to multiple slip and, finally, to basal slip occurred with increasing temperature in the lower-Al-content alloys, whereas for Ti-6.6 pct Al, only basal slip was observed at all temperatures tested. A comparison of the critically resolved shear stress (CRSS) values for basal and prism slip as a function of Al content shows that prism slip is favored at room temperature in pure Ti, but the stress to activate these two systems becomes essentially equal in the Ti-6.6 pct Al crystals over a wide range of temperatures. Compression tests on crystals oriented so that the load was applied parallel to the c-axis showed extensive twinning in lower Al concentrations and 〈c+a〉 slip at higher Al concentrations, with a mixture of 〈c+a〉 slip and twinning at intermediate compositions. A few tests also were conducted in tension, with the load applied parallel to the c-axis. In these cases, twinning was observed, and the resolved shear for plastic deformation by twinning was much lower that that for 〈c+a〉 slip observed in compression loading. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science and Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Microscale and Mesoscale Crystallographic Textures of Nanocrystalline Ni-Based Electrodeposits

The microscale and mesoscale crystallographic textures observed in nanocrystalline Ni, Ni-20 pct Fe, and Ni-50 pct Fe electrodeposits are described. The nanosized grains are arranged in coarse mesoscale colonies. In the as-deposited state, the bulk texture of the Ni-20 pct Fe alloy displays a dominant 〈001〉 fiber parallel to the macroscopic deposition direction (DD). The grains are elongated along the 〈001〉 crystal lattice direction, which is mostly parallel to the local DD, producing a well-defined 〈001〉//DD fiber microtexture on a local scale. The grain misorientation histograms show some content of low-angle boundaries, frequently associated with the presence of grain clusters, and are dominated by high-angle boundaries with a possible enhanced frequency of ∑5 and ∑7 coincident site lattice boundaries and a significant content of ∑3 twin boundaries. For all three alloys, the coarsened grains, obtained by annealing, within the mesoscale colonies show a fiber mesotexture characterized by a 〈111〉 axis approximately perpendicular to the colony hemispherical growth surface (parallel to the local DD). It is surmised that a similar “cobblestone” mesotexture with a 〈001〉 fiber axis already exists in the as-received state, as previously proposed by a number of the present authors and as supported by the results on the Ni-20 pct Fe alloy presented here.     

Warm-temperature tensile ductility in Al-Mg alloys

Several binary and ternary Al alloys containing from 2.8 to 5.5 wt pct Mg were tested in tension at elevated temperatures (200 °C to 500 °C) over a range of strain rates (10⁻⁴ to 2.0 s⁻¹). Tensile ductilies of up to 325 pct were obtained in binary Al-Mg alloys with coarse grains deformed in the solute-drag creep regime. Under test conditions in which solute-drag creep controls deformation, Mg in concentrations from 2.8 to 5.5 wt pct neither affects tensile ductility nor influences strain-rate sensitivity or flow stress significantly. Strength is shown to increase with increasing Mg concentration, however, in the power-law-breakdown regime. The solute-drag creep process, which leads to superplastic-like elongations, is shown to have no observable grain-size dependence in a binary Al-Mg material. Ternary alloying additions of Mn and Zr are shown to decrease the strain-rate sensitivity during solute-drag creep, negatively influencing ductility. An important cause of reduced ductility in the ternary alloys during creep deformation is found to be a transition from necking-controlled failure in the binary alloys to cavitation-controlled failure in the ternary alloys investigated. An increase in ternary element concentration, which can increase the relative volume percentage of proeutectic products, increases cavitation.     

Warm-temperature tensile ductility in Al−Mg alloys

Several binary and ternary Al alloys containing from 2.8 to 5.5 wt pct Mg were tested in tension at elevated temperatures (200°C to 500°C) over a range of strain rates (10⁻⁴ to 2.0 s⁻¹). Tensile ductilities of up to 325 pct were obtained in binary Al−Mg alloys with coarse grains deformed in the solute-drag creep regime. Under test conditions in which solute-drag creep controls deformation, Mg in concentrations from 2.8 to 5.5 wt pct neither affects tensile ductility nor influences strain-rate sensitivity or flow stress significantly. Strength is shown to increase with increasing Mg concentration, however, in the power-law-break down regime. The solute-drag creep process, which leads to superplastic-like elongations, is shown to have no observable grain-size dependence in a binary Al−Mg material. Ternary alloying additions of Mn and Zr are shown to decrease the strain-rate sensitivity during solute-drag creep, negatively influencing ductility. An important cause of reduced ductility in the ternary alloys during creep deformation is found to be a transition from necking-controlled failure in the binary alloys to cavitation-controlled failure in the ternary alloys investigated. An increase in ternary element concentration, which can increase the relative volume percentage of proeutectic products, increases cavitation.     

Deformation of metastable betaTi-15Mo-5Zr alloy single crystals

Deformation modes have been investigated in metastable beta Ti-15Mo-5Zr alloy single crystals using both transmission electron microscopy techniques and multisurface trace analysis. {332} twinning and 〈111〉 crystallographic slip were observed to occur at an initial stage of deformation depending on deformation axis. {332} twinning occurs in a crystal whose tensile axis lies around 〈111〉, while 〈111〉 slip appears in a crystal having the tensile axis in the neighborhood of 〈001〉 to 〈011〉. The twinning system which possesses the maximum resolved shear stress is always operative in both tensile and compressive deformations. Single crystals of this alloy exhibit an asymmetry of the active slip plane and of the yield stress in a manner similar to other bcc metals and dilute alloys.     

High- temperature deformation properties of nial single crystals

The high-temperature deformation properties of stoichiometric NiAl single crystals have been studied in the temperature range from 850 °C and 1200 °C. We have established a basic data set for and have explored the high-temperature deformation characteristics of this intermetallic compound. The results provide a basis for determining the controlling mechanisms of high-temperature deformation. Constant stress tension creep and constant stress or constant strain rate compression experiments were conducted on crystals oriented with loading axes along the “hard,” [001] orientation, where no driving force exists for glide ofb = (001) dislocations, and along various “soft” orientations, [223], [111], and [110], where deformation can occur by the glide of these dislocations. In addition to these monotonie tests, high-temperature deformation transients were studied using stress relaxation, strain rate change, and stress change experiments. These transient deformation experiments were conducted in an effort to further elucidate the mechanisms that control high-temperature deformation of this material. The steady-state deformation properties of these differently oriented single crystals can be characterized by creep activation energies that all coincide, within experimental error, with the activation energy for diffusion of Ni in NiAl, 308 ± 10 kJ/mol. The stress dependence of steady-state deformation can be characterized with stress exponents that range from about 9 at 850 °C to about 4 at 1200 °C. At all temperatures and stresses, the soft oriented crystals creep about two orders of magnitude faster than the hard oriented crystals at the same stresses and temperatures. Soft oriented crystals loaded along [223] and [111] axes tested in both tension creep and constant stress or constant strain rate compression are found to deform at the steady-state rate from the very beginning of the deformation experiment. Crystals with these orientations exhibit virtually no evidence of strain hard-ening. Transients associated with stress changes suggest that deformation is limited primarily by the mobility of dislocations and not by dislocation interactions. These characteristics of deformation are consistent with the operation of easyb = (001) glide processes in these crystals. Crystals loaded along [110] exhibit small deformation transients which indicate both sluggish dislocation motion and some substructure formation. We speculate that cross-slip of dislocations from {110} to {010} planes is responsible for this effect. Deformation in hard oriented crystals provides evidence for both mo-bility and substructure controlled deformation. Creep in hard oriented crystals is characterized by a dramatic sigmoidal transient suggesting very low dislocation mobility. However, the strain hardening observed in monotonic tests and the transient responses suggest that deformation is also limited by a dislocation substructure that forms during deformation. These findings support the conclusion, explored fully in a forthcoming article, that creep deformation in the hard orientation is controlled by the motion and interaction ofb = (101) dislocations.     

Deformation behavior in quenched and aged beta Ti-V alloys

The deformation characteristics of quenched and aged Ti-V alloys in the composition range 20 to 40 wt pct vanadium have been examined by optical metallography and transmission electron microscopy. A coarse lenticular deformation product similar in appearance to previously reported strain induced “martensites” was found to be associated with the occurrence of the omega phase. These features proved to be {112}〈111〉 twins. In the omega bearing alloys prolonged aging resulted in a transition of the deformation mode from twinning to slip at a point which corresponded either to the onset of embrittlement or alpha phase precipitation.     

Nonbasal deformation modes of HCP metals and alloys: Role of dislocation source and mobility

A review is presented on the role of dislocation cores and planar faults in activating the nonbasal deformation modes, 〈c+a〉 pyramidal slip and deformation twinning, in hcp metals and alloys and in D0₁₉ intermetallic compounds. Material-specific mechanical behavior arises from a competition between altemate defect structures that determine the deformation modes. We emphasize the importance of accurate atomistic modeling of these defects, going beyond simple interatomic energy models. Recent results from both experiments and theory are summarized by discussing specific examples of Ti and Mg single crystals; Ti-, Zr-, and Mg-base alloys; and Ti₃Al ordered alloys. Remaining key issues and directions for future research are also discussed. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM and TMS committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Effects of ternary additions on the thermoelastic martensitic transformation of NiAl

Nickel-rich β-NiAl alloys, which are potential materials for high-temperature shape-memory alloys, show a thermoelastic martensitic transformation, which produces their shape memory effect. However, the transformation to Ni₅Al₃ phase during heating of NiAl martensite can interrupt the reversible martensitic transformation; consequently, the shape memory effect in NiAl martensite might not appear after heating. The phase transformation process in binary Ni-(34 to 37)Al martensite was investigated by differential thermal analysis (DTA) method, and we found that the condition of reversible martensitic transformation was not the β → Ni₅Al₃ transformation, but rather the M → Ni₅Al₃ transformation occurring at 250 °C to 300 °C. Therefore, the transformation temperature of M → Ni₅Al₃ determined the highest operating temperature for the shape memory effect. For verifying the critical temperature, the phase transformation process was investigated for eight ternary Ni-33Al-X alloys (X=Cu, Co, Fe, Mn, Cr, Ti, Si, and Nb). Only Ti, Si, and Nb additions were found to be effective in dropping the M _s temperature, and they facilitated the shape memory effect in Ni-33Al-X alloys. In particular, the addition of Si and Nb raised the transformation temperature of M → Ni₅Al₃, a potentially beneficial effect for shape memory at higher temperatures. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

The creep-damage constitutive and life predictive model for nickel-base single-crystal superalloys

A two-state-variable creep-damage constitutive and life predictive model that has been built is discussed in this article. The cavitation-controlled damage mechanism and microstructural deg-radation, i.e., material damage mechanism, are considered. The latter is derived mainly from the rafting and derafting of the precipitate γ′. The model has been verified by the creep ex-periments of nickel-base single-crystal DD3 at 760 °C and 850 °C. The steady creep and tertiary creep can be predicted satisfactorily. The active slip systems are confirmed as octahedral 〈112〉 {111} based on the lattice rotation. The parameter C reflecting material damage mechanism depends on the crystallographic orientation and can be assigned to the value C _〈001〉 ^α along 〈001〉 crystallographic orientation and C _〈011〉 ^α along 〈011〉 orientation partially. The life in different crystallographic orientations can be predicted satisfactorily.     

Ductile-to-Brittle transition in 〈111〉 hadfield steel single crystals

The deformation mechanism and the character of fracture of 〈111〉 austenitic Hadfield steel single crystals are studied during tension in the temperature range 77–673 K by scanning and transmission electron microscopy. It is found that a change in the fracture mechanism from ductile to brittle fracture according to the fractography criterion takes place at a higher temperature than that determined from a change in the elongation to failure of the single crystals. The ductile-to-brittle transition in the Hadfield steel single crystals is shown to be related to a high level of deforming stresses induced by solid-solution hardening and to mechanical twinning.     

Fatigue deformation of TiAl base alloys

The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti₃Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 10⁶ cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation. S. M. L. SASTRY was formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

Tensile ductility of several commercial aluminum alloys at elevated temperatures

One experimental and five commercial aluminum alloys were tested in tension at elevated temperatures (225 °C to 500 °C) over a range of strain rates (2×10⁻⁵ to 10⁻¹ s⁻¹). The experimental alloy contained 5 wt pct Zn with a balance of Al. The commercial alloys included AA 5182, 5754, 7150, 6111, and 6022. Two 5182 materials were examined, one produced by standard ingot-processing methods and the other by continuous casting. The 5754 and 5182 alloys exhibited a deformation regime consistent with solute-drag creep for values of diffusivity-compensated strain rate less than 10¹³ m⁻². Within this regime, the 5754 and ingot-metallurgy 5182 materials exhibited tensile ductilities up to 140 pct. The continuously cast 5182 material exhibited lower ductility in this regime than the 5754 and ingot-metallurgy 5182 materials, despite similar stress exponents. Ductility was reduced in the continuously cast 5182 because of significant dynamic grain growth and cavitation. The 7150, Al-5Zn, 6111, and 6022 materials exhibited significantly higher stress exponents and lower tensile ductilities than the 5000-series materials.     

Evolution of textures in zirconium alloys deformed uniaxially at elevated temperatures

Texture evolution inα-Zr due to uniaxial deformation at 923 to 1123 K was investigated in crystal-bar Zr and Zr-2.5Nb. The temperature range selected corresponds to the two-phase (α +β) field in the Zr-2.5Nb alloy. It was found that uniaxial compression causes a progressive rotation of the (0002) plane normals away from the compression direction and away from the compression plane. In the crystal-bar Zr, the compression texture consists of a [0001] fiber tilted 30 deg from the compression axis. By contrast, in Zr-2.5Nb, a [0001] fiber with an angular spread of 30 deg is obtained. The effect of theβ phase present in Zr-2.5Nb at the temperatures investigated was evaluated by testing a Zr-20Nb alloy in compression. The β-phase texture consisted of a weak 〈111〉-〈00l〉 double fiber. Comparison of this texture and the textures observed in Zr-2.5Nb indicates that theβ →α transformation takes place by the growth of pre-existing a grains and not according to the Burgers mechanism. This transformation has, therefore, no direct effect on the α-phase texture after cooling to room temperature from the (α +β) field. Uniaxial elongation by swaging of Zr-2.5Nb produces a dual fiber. Similar results are obtained in hot extruded rods. Modeling of the development of textures in the α phase was performed using linear programming and employing relaxed constraint (RC) models (“curling” for tension and ”pancake” for compression) implemented for hexagonal close-packed (hcp) grains. It is assumed that prismatic, basal, and 〈c +a〉 pyramidal slip were the active deformation modes at high temperatures. It is shown that these models reduce the activity of the pyramidal slip systems to realistic values, in contrast to the full constraint (FC) approach, where most of the deformation is accommodated by 〈c +a〉 slip. Microstructural evidence is presented regarding the occurrence of ”curling” during uniaxial elongation. Formerly Graduate Student with the Department of Metallurgical Engineering, McGill University     

Deformation and strength behavior of two nickel-base turbine disk alloys at 650 °C

Two powder metallurgy nickel-base turbine disk alloys, RENE’95* and KM4, were studied for strength and deformation behavior at 650 °C. Two classes of microstructures were investigated: unimodal size distributions of γ′ precipitates with particle sizes ranging from 0.1 to 0.7 μm and commercially heat-treated structures with bimodal or trimodal size distributions of γ′ precipitates. The strength and deformation mechanisms were heavily influenced by the microstructure. In both alloys, deformation during compression tests consisted of a combination of a/2〈110〉 antiphase boundary (APB)-connected dislocation pairs and a/3〈112〉 partials creating superlattice intrinsic stacking faults (SISFs). In unimodal alloys, the fault density increased with decreasing particle size and decreasing strain rate. These trends, observed in compression testing, are consistent with earlier studies of similar alloys, which were tested in creep. As the γ′ size was reduced, the nature of the faults changed from being isolated within single precipitates to being extended across entire grains. Commercially heat-treated alloys, containing a bimodal distribution of γ′ particles, exhibited significantly more faulting than unimodal alloys at the same cooling γ′ size. This augmentation of the faulting in commercial alloys was apparently due to the presence of the fine, aging γ′ particles. The two typical commercial heat treatments (supersolvus and subsolvus) resulted in different deformation structures: the subsolvus behavior was similar to that of unimodal alloys with γ′ sizes between 0.2 and 0.35 μm, while the supersolvus deformation was similar to that of unimodal alloys with the 0.1 μm γ′ size. These differences were attributed to differences in the size of the fine, aging γ′ particles. Creep deformation in a commercially heat-treated material at 650 °C occurred solely by SISF-related mechanisms, resulting in a macroscopic slip vector of 〈112〉. The effects of alloy chemistry, APB energy, and microstructure on the deformation and mechanical behavior are discussed in detail, and possible effects of the faulting mechanisms on the mechanical behavior are explored. Finally, models for yield strength as a function of microstructure for bimodal alloys with large volume fractions of precipitates are found to be in need of development. RENE′95 is a trademark of General Electric Company, Fairfield, CT.     

Recrystallization and grain growth of cold-drawn gold bonding wire

Recrystallization and grain growth of gold bonding wire have been investigated with electron back-scatter diffraction (EBSD). The bonding wires were wire-drawn to an equivalent strain greater than 11.4 with final diameter between 25 and 30 μm. Annealing treatments were carried out in a salt bath at 300 °C, and 400 °C for 1, 10, 60 minutes, and 1 day. The textures of the drawn gold wires contain major 〈111〉, minor 〈100〉, and small fractions of complex fiber components. The 〈100〉 oriented regions are located in the center and surface of the wire, and the complex fiber components are located near the surface. The 〈111〉 oriented regions occur throughout the wire. Maps of the local Taylor factor can be used to distinguish the 〈111〉 and 〈100〉 regions. The 〈111〉 oriented grains have large Taylor factors and might be expected to have higher stored energy as a result of plastic deformation compared to the 〈100〉 regions. Both 〈111〉 and 〈100〉 grains grow during annealing. In particular, 〈100〉 grains in the surface and the center part grow into the 〈111〉 regions at 300 °C and 400 °C. Large misorientations (angles >40 deg) are present between the 〈111〉 and 〈100〉 regions, which means that the boundaries between them are likely to have high mobility. Grain average misorientation (GAM) is greater in the 〈111〉 than in the 〈100〉 regions. It appears that the stored energy, as indicated by geometrically necessary dislocation content in the subgrain structure, is larger in the 〈111〉 than in the 〈100〉 regions.     

Dislocation structure behind a shock front in fcc perfect crystals: Atomistic simulation results

Large-scale molecular dynamics simulations are used to investigate the dislocation structure behind a shock front in perfect fcc crystals. Shock compression in both the 〈100〉 and 〈111〉 directions induces dislocation loop formation via a sequential emission of partial dislocations, but in the 〈100〉 case, this process is arrested after the first partial, resulting in stacking-fault loops. The large mobility of the bounding partial dislocations results in a plastic wave that is always overdriven in the 〈100〉 direction; the leading edges of the partials are traveling with the plastic front, as in the models of Smith and Hornbogen. In contrast, both partials are emitted in 〈111〉 shock compression, resulting in perfect dislocation loops bounded only by thin stacking fault ribbons due to the split partial dislocations. These loops grow more slowly than the plastic shock velocity, so new loops are periodically nucleated at the plastic front, as suggested by Meyers. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.