On the effect of texture on the mechanical and damping properties of nanocrystalline Mg-matrix composites reinforced with MAX phases

Herein, we report on the effect of texture on kinking nonlinear elasticity, damping capability, Vickers microhardness and ultimate compressive strengths of nanocrystalline (NC) Mg-matrix composites reinforced with 50 vol.% Ti2AlC. Because all composite and bulk Ti2AlC samples tested herein traced fully reversible, reproducible, hysteretic stress–strain loops during uniaxial cyclic compression tests, they were classified as kinking nonlinear elastic (KNE) solids. When the results were analyzed using our recently developed microscale incipient kink band (IKB) model, the various relationships predicted among the three independently measured values – stress, nonlinear strain and dissipated energy per cycle per unit volume, Wd – were exceptionally well adhered to. In all composites, and despite very different loops’ shapes and sizes, the critical resolved shear stresses of basal plane dislocations in Ti2AlC, calculated from the model fell within the narrow range of 37.7 ± 0.5 MPa. The same was true for the reversible dislocation density that fell in the quite narrow range of 1.1 ± 0.3 × 1014 m−2, suggesting the presence of an equilibrium state to which all systems migrate. Because kinking is a form of plastic instability, it was hypothesized that orienting the Ti2AlC grains, prior to infiltration, with their basal planes parallel to the loading direction should lead to exceptionally high values of Wd. Indeed, at 450 MPa, Wd of the composite with this texture was found to be ≈0.6 MJ/m3, a value that exceeds the previous record of ≈0.4 MJ/m3 at 475 MPa reported for a randomly oriented composite. At 700 ± 10 and 380 ± 20 MPa, the ultimate compressive and tensile strengths of the composites were higher than those reported in the literature, Mg–Ti3SiC2 and Mg–SiC composites, in which the Mg-matrix grains were not at the nanoscale.