Computational and experimental studies of dislocation dynamics

Dislocation mobility in a Ni₃Al single crystal is studied as a means for understanding plastic flow in certain classes of intermetallics. This study is motivated largely by interest in the positive temperature dependence of the flow stress observed in some intermetallics. The high-temperature, high-strain-rate response of single crystal Ni₃Al is reported, based on experiments conducted using a newly-developed high-temperature Kolsky bar set-up. The dependence of dislocation velocity on resolved shear stress in Ni₃Al is reported for a wide range of dislocation velocities, based on quasi-static, four-point bending experiments and soft-recovery plate-impact experiments. To estimate dislocation velocities, an electrolytic etching process was developed for observing dislocation positions before and after a stress pulse is applied. The origins of the positive temperature dependence of the flow stress in Ni₃Al are investigated through lattice statics calculations using embedded atom method (EAM) potentials. Dislocation core structures are calculated for edge as well as screw superdislocations, lying on both {lcub}111{rcub} and {lcub}100{rcub} planes. The corresponding Peierls stresses are also calculated to understand the relatively sessile nature of screw dislocations that cross-slip onto {lcub}100{rcub} planes. To address the effects of temperature and inertia, the dislocation dynamics of edge dislocations in pure Al is modeled using molecular dynamics. The motion of an edge dislocation is analyzed for temperatures ranging from 10 K to 200 K and for stresses up to 5 GPa. The dependence of dislocation mobility on temperature is compared with that reported by others—based on ultrasonic experiments. A subsonic limiting velocity observed for the edge dislocation is shown to be predicted by a two-dimensional lattice-dynamics analysis. A modal analysis of a moving edge dislocation in Al confirms the existence of a localized normal (local) mode near the dislocation core when the dislocation is in an unstable configuration. Energy transfer between successive local modes is analyzed in an attempt to understand the origins of dislocation drag. To facilitate computer simulations of the response of materials with complex crystal structures (such as Ni₃Al), a revised quasicontinuum, formulation is presented. This formulation is used to study internal relaxation of basis atoms in Ni₃Al and Zr.