Atomistic Deformation Mechanisms in Advanced Structural Materials

Twinning in Hexagonal Close-Packed Alloys

Deformation twinning impacts the mechanical properties of a material by influencing its strength, work hardening, ductility, and toughness. Twinning plays an important role in the mechanical behavior in hexagonal-close-packed (HCP) materials, where the number of operative dislocation slip systems is limited. The study of twin nucleation and growth in HCP materials is of broad interest in the field of structural metals and alloys. By using density functional theory theory and molecular dynamics simulations, the kinetics and thermodynamics of twinning behaviors can be investigated.

Dislocation Climb and Cross-Slip Kinetics

Fundamental understanding of mechanical behavior of materials beyond the initial yield point is crucial for a variety of structural applications. This requires elucidation of the kinetics of various processes involving dislocations, which are defects that govern deformation in metals.

 

Dislocation climb is a mechanism fundamental to high temperature deformation processes, annealing of quenched-in defects, and irradiated defect absorption in metals. In FCC metals, the dislocation climb process can be slower than expected from diffusion kinetics due to limited jog availability. Using a dislocation climb theory informed by atomistic calculations, we have shown that the climb efficiency of a straight edge dislocation in FCC metals is low until high homologous temperatures are reached (especially if the stacking fault energy is low). The results of this work are anticipated to be an important contribution for alloy design and mesoscale modeling centered around creep.

Estimate of the climb efficiency of a straight edge dislocation in pure Al as a function of temperature and vacancy concentration.
Estimates of the ratio of dynamic recovery of a Ni-10% Al alloy in a random versus a short-range ordered alloy as a function of temperature and stress.

Dislocation cross-slip is a process which allows a screw dislocation to switch glide planes. The rate at which this process occurs can influence the microstructure obtained in a metal during deformation as well as the work-hardening rate. This is especially apparent in solid solution alloys which exhibit short-range ordering, as the suppression of cross-slip leads to a phenomenon known as planar-slip. Using atomistic simulations, we have calculated cross-slip energy barrier distributions in a variety of short-range ordered environments in a Ni-10% Al solid solution alloy. We find that the presence of a defect called a diffuse anti-phase boundary has a strong influence on these distributions, and we also use these distributions to estimate that the dynamic recovery rate (an important consideration for work-hardening) is much lower in an alloy with short-range order than without. These findings are likely applicable to most solid solution FCC alloys, including high-entropy alloys.

This research is sponsored by the Office of Naval Research.