On the effect of texture on kinking non-linear elasticity of MAX phases and MAX-reinforced Mg matrix composites
Authors:   , Amini Shahram , Barsoum Michel W. 
Published: July 2021

The MAX phases and hexagonal metals, among many other plastically anisotropic solids with c/a ratios > 1.5, have been recently classified as kinking nonlinear elastic (KNE) solids - the signature of which is the formation of fully reversible, hysteretic stress-strain loops during cyclic loadings. Herein, a unique and novel class of Mg matrix composites reinforced with Ti₂AlC was fabricated, for the first time, by spontaneous melt infiltration. The ~ 35 nm Mg grains that constituted the matrix of these composites were exceptionally stable: repeatedly heating the composite to 700 °C - 50 °C above the melting point of Mg - remarkably did not lead to any coarsening. At 380±20 MPa, 700±10 MPa and 1.5±0.5 GPa, the ultimate tensile and compressive strengths and Vickers hardness of these composites, respectively, are significantly greater than other pure Mg-composites reported in the literature, a fact attributed to nanocrystalline nature of the Mg-matrix. The advantages of melt infiltration as a processing technique are affordability and ease of scalability. To make these composites, all one needs to do is melt bulk Mg above a porous preform of Ti₂AlC; nature does the rest. Because kinking is a form of plastic instability, orienting the Ti₂AlC grains, prior to infiltration, with their basal planes parallel to the loading direction led to exceptionally high values of dissipated energy per unit volume per cycle, Wd. At 450 MPa, Wd of these composites with this texture was found to be ~ 0.6 MJ/m3, believed to be the highest ever reported for a crystalline solid. Counterintuitively, the Wd's of bulk Ti₂AlC samples, in which the basal planes are normal to the applied load were higher than those in which the basal planes were loaded parallel to the loading axis. In the case of the composites, the relatively softer Mg phase in between the Ti₂AlC grains allows the latter to kink, while in the Ti₂AlC bulk samples the majority of the plate-like grains of Ti₂AlC seem to be constrained by the minority grains. In-situ neutron diffraction experiments suggest that the minority grains are kinking significantly more than the majority grains. The KNE microscale model - previously developed to explain kinking nonlinear elasticity - is in excellent agreement with the experimental results obtained in this work. The model is capable of calculating the critical resolved shear stresses, CRSS, of basal plane dislocations making up the incipient kink bands - the reversible micro-constituent responsible for the nonlinearity and Wd. The model can also estimate the Taylor factor that, as far as we are aware, is the first time that factor has been measured experimentally. The Taylor factor - that ranged from 1.8 to 3.0 - was found to be a function of texture and grain size. In summary, this work shows that the MAX-metal composites and their monolithic MAX phase counterparts fabricated herein are all KNE solids and their KNE behavior is a strong function of texture, and characterized by the formation of fully reversible hysteretic stress-strain loops under uniaxial cyclic compression.