Micromechanism of Deformation in Nanolayered Ternary Carbides

Abstract

MAX phases are a family of ternary carbides and nitrides with unique layered hexagonal crystal structure and a combination of strong and weak atomic bonds within it. These materials are endowed with some extremely useful properties of both metals and ceramics. The weakly bonded MX-A interlayers in the MAX phases facilitate crystallographic slip, cleavage, buckling of layers and kinking, which contribute to their anisotropic mechanical behavior. Thus, it is important to understand the single-crystal level mechanical behavior of MAX phases. To this end, grain level nanoindentation are first carried out on a polycrystalline MAX phase. The results show that the propensity of both crystallographic slip and cleavage vary nonlinearly with the orientation of the basal planes, however, due to the constraint imposed by the neighboring grains, the propensity of crystallographic slip is found to be always greater than cleavage. Next, extensive micropillar compression tests are carried out to directly characterize the mechanical behavior of free-standing single crystals of a MAX phase. The results show that depending on the crystallographic orientation, the micropillars of a MAX phase either undergo only non-classical (non-Schmid) crystallographic slip, non-classical crystallographic slip followed by cleavage or cleavage without any appreciable crystallographic slip. The non-classical crystallographic slip is found to be a result of the strong dependence of crystallographic slip on both, the resolved shear stress, and the stress normal to the slip plane. The micropillar compression studies are then extended to unravel the effects of stacking and M-site elements on the crystallographic slip behavior of MAX phases.