J. B. Liu

Generalized planar fault energies and twinning in Cu–Al alloys

We report ab initio density functional theory calculations of generalized planar fault energies of fcc Cu–xAl (x=0, 5.0, and 8.3at.%) alloys. We investigate the effects of substitutional solute Al on the unstable intrinsic γus and twin γut stacking fault energies (SFEs). Our results reveal an increased tendency of Cu–Al to deform preferentially by twinning with increasing Al content, consistent with experiment. We attribute this mechanical behavior to appreciable lowering of the twinning barrier γut, along with the stable intrinsic and twin SFEs.

Recent advances in the study of structural materials compatibility with hydrogen.

Hydrogen is a ubiquitous element that enters materials from many different sources. It almost always has a deleterious effect on mechanical properties. In non-hydride-forming systems, research to date has identified hydrogen-enhanced localized plasticity and hydrogen-induced decohesion as two viable mechanisms for embrittlement. However, a fracture prediction methodology that associates macroscopic parameters with the degradation mechanisms at the microscale has not been established, as of yet. In this article, we report recent work on modeling and simulation of hydrogen-induced crack initiation and growth. Our goal is to develop methodologies to relate characteristics of the degradation mechanisms from microscopic observations and first-principles calculations with macroscopic indices of embrittlement. The approach we use involves finite element analysis of the coupled hydrogen transport problem with hydrogen-assisted elastoplastic deformation, thermodynamic theories of decohesion, and ab initio density functional theory calculations of the hydrogen effect on grain boundaries.

Energy pathways and directionality in deformation twinning

We present ab initio density functional theory calculations of twinning energy pathways for two opposite twinning modes, (111)[112¯] and (111)[1¯1¯2], in fcc materials to examine the directional nature of twinning which cannot be explained by classical twin nucleation models or the “twinnability” criterion. By accounting for these energy pathways in a multiscale model, we quantitatively predict the critical twinning stress for the (111)[1¯1¯2] mode to be substantially higher compared to the favorable (111)[112¯] mode (whose predicted stresses are in agreement with experiment), thus, ruling out twinning in the (111)[1¯1¯2] mode.