Atomistic simulations of diffusional creep in a nanocrystalline body-centered cubic material

Molecular dynamics (MD) simulations are used to study diffusion-accommodated creep deformation in nanocrystalline molybdenum, a body-centered cubic metal. In our simulations, the microstructures are subjected to constant-stress loading at levels below the dislocation nucleation threshold and at high temperatures (i.e., T > 0.75T_melt), thereby ensuring that the overall deformation is indeed attributable to atomic self-diffusion. The initial microstructures were designed to consist of hexagonally shaped columnar grains bounded by high-energy asymmetric tilt grain boundaries (GBs). Remarkably the creep rates, which exhibit a double-exponential dependence on temperature and a double power-law dependence on grain size, indicate that both GB diffusion in the form of Coble creep and lattice diffusion in the form of Nabarro–Herring creep contribute to the overall deformation. For the first time in an MD simulation, we observe the formation and emission of vacancies from high-angle GBs into the grain interiors, thus enabling bulk diffusion.     

Mechanisms of martensitic phase transformations in body-centered cubic structural metals and alloys: Molecular dynamics simulations

Two different mechanisms of the stress-induced martensitic phase transformation at the crack tip in body-centered cubic (bcc) structural metals and alloys have been studied by molecular dynamics simulations. For cracks with 〈1 0 0〉 crack fronts, the bcc (B2) to face-centered cubic (fcc) (L1₀) phase transformation along the Bain stretch occurs. Whereas for cracks with 〈1 1 0〉 crack fronts, either the bcc (B2) to fcc (L1₀) or the bcc (B2) to hexagonal close-packed (hcp) transformation is the candidate. We have found that the combination of local stress and crystal orientation plays an important role in the mechanism of the martensitic transformation. Thus a simple way to determine the mechanism of the martensitic transformation is developed. The complicated deformation behaviors at the crack tip in bcc iron and B2 NiAl are discussed in terms of this method.     

Tensile properties of nanocrystalline tantalum from molecular dynamics simulations

The tensile behaviors of nanocrystalline tantalum are studied using molecular dynamics simulations. The results show that the elastic modulus increases linearly with density. The flow stress decreases with decreased grain size, but increases with increased strain rate or decreased temperature. A strain rate sensitivity of ∼0.14 is derived from the simulations with a resultant activation volume of ∼1b³ associated with plastic deformation. Grain rotation, grain boundary sliding or migration, dislocation motion and intergranular activities are observed in the deformation process. Twinning is regarded to be a secondary mechanism. Stress-induced phase transitions from body-centered cubic to face-centered cubic (fcc) and hexagonal close-packed (hcp) structures take place locally, and the hcp structure is a derivative of the fcc structure. The higher the strain rate, the further delayed the phase transition. Such phase transitions are found to occur only at relatively low-temperatures and are reversible with respect to stress.     

Material simulations with tight-binding molecular dynamics

Tight-binding molecular-dynamics has recently emerged as a useful method for atomistic simulation study of realistic materials. The method incorporates electronic structure calculation into molecular dynamics through an empirical tight-binding Hamiltonian and bridges the gap between ab initio molecular dynamics and simulations using empirical classical potentials. This article reviews some achievements and discusses some recent developments in materials simulations with tight-binding molecular dynamics.     

A dislocation dynamics study of the transition from homogeneous to heterogeneous deformation in irradiated body-centered cubic iron

Low temperature irradiation of crystalline materials is known to result in hardening and loss of ductility, which limits the usefulness of candidate materials in harsh nuclear environments. In body-centered cubic (bcc) metals, this mechanical property degradation is caused by the interaction of in-grown dislocations with irradiation defects, particularly small dislocation loops resulting from the microstructural evolution of displacement cascades. In this paper, we perform dislocation dynamics simulations of bcc Fe containing various concentrations of dislocation loops produced by irradiation in an attempt to gain insight into the processes that lead to hardening and embrittlement. We find that a transition from homogenous to highly localized deformation occurs at a critical loop density. Above it, plastic flow proceeds heterogeneously, creating defect-free channels in its wake. We find that channel initiation and size are mediated by loop coalescence resulting from elastic interactions with moving dislocations.     

Orientation stability in equal channel angular extrusion. Part I: Face-centered cubic and body-centered cubic materials

Stability of crystallographic orientations is a key aspect in the characterization and understanding of texture evolution during plastic deformation. In this study, a rate-dependent crystal plasticity model was applied to investigate orientation stability during equal channel angular extrusion (ECAE) of face-centered cubic (fcc) and body-centered cubic (bcc) crystals. The stability of experimentally observed ideal orientations was examined according to lattice rotation fields computed at and around the orientations. It is shown that these ideal orientations are meta-stable under rate-sensitive conditions, and their stability generally increases with the decrease of strain rate sensitivity. The results also reveal a well-preserved duality in the lattice rotation and orientation stability between the two types of crystal structure. The stability results simulated at low strain rate sensitivities agree well with the experimental observations in one-pass ECAE of Al and Cu single crystals. In Part II of the paper, this analysis is extended to hexagonal materials.     

Deformation twinning in nanocrystalline metals

Deformation twins have been oberved in nanocrystalline Al processed by cryogenic ball-milling and in nanocrystalline Cu processed by high-pressure torsion at a very low strain rate. They were formed by partial dislocations emitted from grain boundaries. This paper first reviews experimental evidences and atomistic simulation results on deformation twinning and partial dislocation emissions from grain boundaries and then discusses recent analytical models on the nucleation and growth of deformation twins. These models are compared with experimental results to establish their validity and limitations. This paper was presented at the International Symposium of Manufacturing, Properties, and Applications of Nanocrystalline Materials sponsored by the ASM International Nanotechnology Task Force and TMS Powder Materials Committee on October 18–20, 2004 in Columbus, OH.     

Nanostructures generated by explosively driven friction: Experiments and molecular dynamics simulations

Dynamic friction experiments between aluminum and steel were conducted using an explosively driven tribotester, which allows friction testing with high velocity and high pressure within a very short time. Characterization of the sliding surfaces showed material transfer, severe plastic deformation of the softer material, evidence of high strain and strain rate, and nanostructure formation at the surface and subsurface. Such friction-induced phenomena have frequently been reported. However experimental results obtained in a few microseconds are rarely described. In particular, formation of a thin (～1 μm) and uniform nanostructure within such a short time suggests that a new mechanism is needed to explain the nanostructure formation. Molecular dynamics simulations of the sliding of hard/soft crystals reveal nanometer size vorticity at the sliding interface. This results in atomic-scale flow and mixing that contribute to a disordered and nanostructured surface layer, as observed in experiments.     

Short-to-medium range order of Al-Mg metallic glasses studied by molecular dynamics simulations

Molecular dynamics simulations of Al-Mg metallic glasses with a wide composition range have been conducted. We made use of a variety of analytical methods to study their amorphous structure. Pair distribution functions were constructed to determine the interatomic distances, and we found good agreement with reported simulation and experimental results. Coordination number analyses revealed that Mg atoms are more likely to serve as neighbour atoms in Al-centred icosahedral clusters in the middle concentration range or to form larger polyhedra at the Al-rich end. The presence of the 155-type pairs demonstrated that icosahedral ordering is predominant in the Al-Mg alloy. Results of Voronoi tessellation showed that Al-centred icosahedra are abundant in most compositions of the Al-Mg alloy and will lead to the formation of short-range order. The icosahedral clusters are highly shared with one another forming the medium-range order and there are less than 3% isolated icosahedra in most of the compositions of the Al-Mg alloy. Finally, the sharing schemes of icosahedra are represented by the splitting of the second peak of each pair distribution function.     

Interstitial loop strengthening upon deformation in aluminum via molecular dynamics simulations

The formation of prismatic interstitial loops during plastic deformation, as well as their interaction with dislocations, is systematically investigated in aluminum, using molecular dynamics simulations. First, direct dislocation interaction is responsible for producing various types of defects, typically vacancies and their clusters and interstitial loops. Secondly, small interstitial loops act as obstacles to dislocation gliding. When close to each other, a loop either decorates a dislocation with jogs or drags it elastically. The mobility of a dislocation decorated by a loop is significantly reduced. Depending on the relative position of the dislocation and the loop, the Peierls stress can increase several hundred times. The present work shows a complete picture of dislocation–loop interaction with atomic-scale details, which provides reliable information for parameterizing dislocation–debris interactions in constitutive models.     

Effect of Mn addition on one-dimensional migration of dislocation loops in body-centered cubic Fe

Elucidation of the one-dimensional (1-D) motion of dislocation loops is important for describing the microstructural development of materials under irradiation. In this study, the effect of Mn on radiation-induced microstructure evolution in body-centered cubic Fe was experimentally investigated by focusing on the migration of dislocation loops. Pure Fe and Fe–1.4Mn alloy were irradiated with Fe³⁺ ions to introduce dislocation loops. In pure Fe, inhomogeneous distribution of loops in the vicinity of the residual dislocation was observed. However, in Fe–1.4Mn, isolated dislocation loops were homogeneously distributed in a high number density. In situ transmission electron microscopy during annealing revealed that 1-D motion of dislocation loops occurred in pure Fe at 623 K, while 1-D motion of dislocation loops occurred minimally in Fe–1.4Mn annealed at temperatures below 773 K. These results indicate that 1-D motion of dislocation loops play a key role in producing the differences in the microstructures between pure Fe and Fe–1.4Mn. In pure Fe, dislocation loops were mobile and trapped in the strain field of a dislocation, leading to the formation of loop decoration of dislocations. However, in Fe–1.4Mn, dislocation loops were less mobile and dislocation loops were homogeneously formed in high density in the matrix. The migration of dislocation loops by Mn solute is strongly suggested as one of the key mechanisms of microstructure development in irradiated Fe–Mn alloy.     

Detwinning mechanisms for growth twins in face-centered cubic metals

Using in situ transmission electron microscopy, we studied the stability of growth twins. We observed the rapid migration of incoherent twin boundaries (ITBs), indicating that nanotwins are unstable. Topological analysis and atomistic simulations are adopted to explore detwinning mechanisms. The results show that: (i) the detwinning process is accomplished via the collective glide of multiple twinning dislocations that form an ITB; (ii) detwinning can easily occur for thin twins, and the driving force is mainly attributed to a variation of the excess energy of a coherent twin boundary; (iii) shear stresses enable ITBs to migrate easily, causing the motion of coherent twin boundaries; and (iv) the migration velocity depends on stacking fault energy. The results imply that detwinning becomes the dominant deformation mechanism for growth twins of the order of a few nanometers thick.     

Effect of deformation mode and grain orientation on misorientation development in a body-centered cubic steel

Strain-induced misorientation development was studied in an IF steel as a function of strain for two deformation modes, plane strain compression and simple shear. Using electron back-scattered diffraction, orientation maps of “large” areas were obtained, from which several individual grains associated with the principal texture components could be extracted so that only intragranular misorientations could be estimated for these orientations. It was observed that the increase of the misorientation angle was more prominent in simple shear than in plane strain compression and that the orientation influence was different for each mode. Considering texture evolution as a possible source of misorientation development, the lattice spin tensor was estimated with the Taylor model for the two deformation modes; both reorientation axis and angle were compared with misorientation angle and axis. The striking concordance of both quantities allows us to conclude that there is a direct contribution of texture evolution to misorientation accumulation with strain.     

Influence of Pb segregation on the deformation of nanocrystalline Al: Insights from molecular simulations

Molecular dynamics straining simulations using a two-dimensional columnar model were run for pure Al with grain sizes from 5 to 30 nm, and for 10 nm grain size Al–Pb alloys containing 1, 2 and 3 at.% Pb. Monte Carlo simulations showed that all the Pb atoms segregate to the grain boundaries. Pb segregation suppresses the nucleation of partial dislocations and twins during straining. At 3 at.% Pb, no dislocations or twins are observed throughout the straining history. It also appeared that Pb tends to segregate to the same locations in grain boundaries that were favorable for partial dislocation emission. Grain boundaries with Pb segregates were very robust against dissociation during straining compared to pure Al. The yield stress determined from stress–strain curves showed a decrease with increasing Pb content, supporting a similar observation for the hardness change measured on nanocrystalline Al–Pb alloys.     

Deformation mechanisms and tensile superplasticity in nanocrystalline materials

Nanocrystalline materials provide a unique opportunity to investigate deformation mechanisms at an extremely fine microstructural scale. An intriguing question has been whether the deformation mechanisms scale with grain size to the nanocrystalline range or whether there are fundamental changes/transitions. The observations of low-temperature and high-strain-rate superplasticity in nanocrystalline materials with some unique features open up new possibilities for scientific and technological advancements. R.S. Mishra earned his Ph.D. in metallurgy at the University of Sheffield in 1988. He is currently adjunct assistant professor at the University of California at Davis. Dr. Mishra is also a member of TMS. S.X. McFadden earned his B.S. in materials engineering at California Polytechnic State University, San Luis Obispo, in 1996. He is currently a research associate at the University of California at Davis. Mr. McFadden is also a member of TMS. R.Z. Valiev earned his Dr.Sci. in solid-state physics from the Institute for Materials Science, Academy of Sciences of the USSR, Kiev, in 1984. He is currently the scientific director and professor at the Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Russia. Dr. Valiev is also a member of TMS. A.K. Mukherjee earned his D. Phil. in materials science at Oxford University in 1962. He is currently a professor of materials science at the University of California at Davis. Dr. Mukherjee is also a member of TMS.     

Deformation mechanisms in nanocrystalline palladium at large strains

The mechanical behaviour and microstructure evolution of nanocrystalline palladium was investigated. Material with an initial grain size ～10 nm was prepared by inert gas condensation. Instrumented high-pressure torsion straining was used to characterize the flow stress during plastic deformation to shear strains up to 300. A change in primary deformation mechanism was induced by stress-induced grain growth. For grain sizes <40 nm, grain boundary mediated processes (shear banding, grain boundary sliding and grain rotation) controlled the deformation, with dislocation slip, twinning, and grain boundary diffusion providing the accommodation. For larger grain sizes, the operative deformation mechanism was dislocation slip.