M. Khantha

An atomistic study of the dislocation core structures and mechanical behavior of a model D019 alloy

Abstract As in many other intermetallics, the principal features of the plastic behavior of hexagonal D019 compounds appear to be controlled by the structure of dislocation cores. In the present paper we investigate the possible core configurations of the 〈 1 2 1 0〉 screw dislocations on basal and prism planes in a model D019 structure. First an N-body potential describing atomic interactions in a mechanically and structurally stable model D019 lattice is presented. This potential is then used to investigate the stability of planar faults on basal and prismatic planes and to calculate the core structures of the 1 3 〈 1 2 1 0〉 screw superpartials bounding antiphase boundaries on these planes. The possible microscopic origins of the basic characteristics of the plastic deformation of Ti3Al and Mn3Sn are then discussed in the light of these results.

Mechanism of yielding in dislocation-free crystals at finite temperatures—Part I. Theory

Abstract A new mechanism of dislocation generation that can be activated suddenly above a critical temperature is proposed. Unlike dislocation generation by Frank-Read type sources, this process is a thermally driven, stress-assisted cooperative instability of many dislocation loops (dipoles in two dimensions). The dislocation loops are formed by thermal fluctuations and are sub-critical in size at low temperatures. The small plastic strain associated with the loops invokes an effective decrease in the moduli that describe the linear relation between the applied stress and the total strain. The self-energy of a loop which forms in the presence of other loops is proportional to these effective moduli and, consequently, it is lower than the energy of an isolated loop. As the temperature increases, the density of thermally generated loops increases and the concomitant lowering of the effective moduli and the self-energy provides a feedback for its further increase. Ultimately, above a critical temperature, the free energy of the loops becomes negative and a collective unstable expansion of many loops occurs. In a solid which is not loaded, this instability corresponds to the Kosterlitz-Thouless model of a defect-mediated phase transition that occurs just below the melting temperature. However, under large applied loads, the instability appears well below the melting temperature. Above the critical temperature, the density of glissile dislocations increases precipitously and extensive yielding occurs. This paper analyzes this mechanism in two dimensions where dislocation loops are replaced by dipoles.

The brittle-to-ductile transition - I: A cooperative dislocation generation instability

Most materials (with the possible exception of f.c.c. metals) exhibit the well-known brittle-to-ductile transition (BDT) with increase in temperature. The variation of the fracture toughness, (K), with temperature is usually gradual in the brittle regime, but a dramatic rapid increase occurs over a narrow temperature range where the sharp crossover from brittle to ductile behavior takes place. The authors describe a new model that explains the dual characteristics of the BDT, namely, the massive dislocation generation at the transition temperature and the strain-rate dependence of this behavior. In the first part, the authors present a statistical mechanics based two-dimensional (2D) model of spontaneous dislocation generation leading to the BDT. The onset of ductile behavior corresponds to a thermally-induced cooperative instability of many small dislocation dipoles in the presence of an applied stress.

Mechanism of yielding in dislocation-free crystals at finite temperatures—Part II. Applications to the deformation of whiskers and the brittle-to-ductile transition

The cooperative dislocation generation model, described in Part I, is used to explain the onset of dislocation activity at the yield point in whiskers and at the brittle-to-ductile transition. This common approach helps to bring out the similarities between the two processes and the predictions of the model agree well with observations in both cases.

An atomistic study of dislocations and their mobility in a model D022 alloy

Abstract Since basic features of the plastic behavior of intermetallic compounds are often related to the structure of dislocation cores, we investigate here the cores of [1 1 0] and ( 1 2 )[11 2 ] screw dislocations in a model D022 alloy. First the stability of planar faults on {111} and {001} planes is analyzed and possible dislocation dissociations discussed. The only glissile dislocation with a planar core is the Shockley partial 1 6 [11 2 ] bounding a superlattice intrinsic stacking fault; however, the other superpartial bounding the same fault, 1 3 [11 2 ] , is sessile. Under the applied stress an intrinsic-extrinsic stacking fault pair is generated at the latter partial and this configuration may then act as a nucleus for mechanical twinning. Such fault pairs have been observed in Ni3V. Another deformation mode in the model alloy is the slip of 〈110〉-type dislocations but since these dislocations are sessile, this requires thermal activation. The major deformation modes of Ni3V and Al3Ti are, indeed, 〈112〉 twinning and, at high temperatures, 〈001〉{111} slip.

Cooperative generation of dislocation loops and the brittle-to-ductile transition

A mechanism of cooperative generation of dislocation loops above a critical temperature in loaded solids is described. We model the massive dislocation activity which commences near the crack tip at the brittle-to-ductile transition temperature in terms of this mechanism and show that it can explain the main features of the transition.

Strain-rate dependent mechanism of cooperative dislocation generation: application to the brittle-ductile transition

Abstract A strain-rate dependent mechanism of cooperative dislocation generation in loaded solids above a critical temperature is described. The massive dislocation activity, which commences near the crack tip at the brittle-to-ductile transition temperature is modeled in terms of this mechanism. The strain-rate dependence of the critical temperature arises from the glide of both pre-existing dislocations and dislocations which are ‘thermally nucleated’ below the critical temperature by the cooperative process. Depending on their relative contributions, the apparent activation energy associated with the brittle-to-ductile transition temperature is either equal to or larger than the activation energy for dislocation motion. We compare the predictions of the model with observations in TiAl.

Temperature-Dependent Onset of Yielding in Dislocation-Free Silicon: Evidence of a Brittle-To-Ductile Transition

An investigation of the brittle-to-ductile transition (BDT) in silicon has been conducted on essentially dislocation-free silicon test specimens made by photolithography. No pre-cracks or additional dislocation sources were introduced into the samples. Three-point bending tests of the samples reveals a well defined transition from brittle fracture of the specimens to complete yielding near 732°C at a crosshead displacement rate of 0.1 mm/min. Limited plasticity is observed below 732°C but is insufficient to prevent crack propagation suggesting that yielding is not dislocation mobility limited. Instead the transition may be controlled by the nucleation of a sufficient density of dislocations. Further support comes from the results of experiments conducted at temperatures below 732°C in which samples were rapidly pre-loaded within the linearly elastic regime, then immediately retested. This rapid pre-loading results in a lower transition temperature. This would not be expected if dislocation mobility controlled the BDT. Instead, it is believed that the transition only occurs when a sufficient density of dislocations has nucleated within the sample. In these experiments, the pre-loading event may increase the dislocation nucleation rate. The source of the dislocations in these defect free samples is still under investigation.

Dislocation generation instability and the brittle-to-ductile transition

Abstract We propose a new model for the brittle-to-ductile transition based on a statistical mechanics description of dislocation generation. We begin with a summary of the deformation and fracture behavior of titanium trialuminides to illustrate the need for a new approach to understanding the ubiquitous brittleness of intermetallic compounds. We then describe the important features of the new model, which is applicable not only to intermetallic compounds but also to the wider class of crystalline materials that exhibit the brittle-to-ductile transition. In two dimensions the onset of the ductile behavior corresponds to a cooperative dissociation instability of many dislocation dipoles driven primarily by thermal fluctuations and assisted by the applied stress. The mutual interactions between the dipoles are taken into account using the Kosterlitz-Thouless concept of thermally induced dislocation screening. The predictions of the transition temperature for several materials are discussed.

Atomistic modeling of planar faults, dislocations and twins in a non-cubic intermetallic compound

This paper describes the nature of planar faults, atomic configurations of dislocation cores and a mechanism for the nucleation of twins in a model DO22 intermetallic compound. The results are in good agreement with TEM observations of dislocations and twins in Ni3V, which crystallizes in this structure. The calculations are also helpful in understanding the general deformation features of DO22 compounds.