Synthesis and characterization of solid solutions of MAX phases

The MAX phases, best described as polycrystalline nanolaminates, deserve attention in today's world because they combine some of the best properties of ceramics and metals. While considerable work exists on selective MAX phases, to date, little is known about MAX solid solutions. This work is focused on the synthesis and characterization of A- or X-site substituted MAX solid solutions.; In general, substitutions on the A sites led, either to solid solution softening or to a monotonic change in properties from one end member to the other. The Vickers microhardness values of the solid solution compositions were in between those of the end members, indicating that no hardening is operative in these systems. Substitutions on the A-sites resulted in a reduction in the residual resistivity ratios and the phonon conductivities as compared to the end members, indicating that solid solution scattering of both phonons and electrons was occurring.; Substitutions on the X sites, on the other hand, resulted in an increase in the elastic moduli and a concomitant reduction in the coefficients of thermal expansion with respect to the end members. This behavior, combined with the higher values of Vickers microhardness clearly indicated solid solution hardening. Somewhat surprisingly, the phonon conductivities of Ti3Al(C 0.5N0.5)2 and Ti2AlC0.5N 0.5 were higher than the respective end members. This enhancement was attributed to the stiffening of the structure as reflected in the higher shear moduli than that of the end members.; All the MAX phases tested herein were kinking nonlinear elastic solids. Substitutions on the X-sites significantly influenced the nonlinear kinking behavior under compression. The threshold stress (sigmac) required for kinking and the critical resolved shear stresses needed for dislocations to glide along the basal planes were observed to increase considerably for the solid solutions compared to the end members. The effects of substitutions on the A-sites on the kinking behavior were quite mild relative to those of the X-site substitutions.