Solid solution effects in the Ti2Al(CxNy) MAX phases: Synthesis, microstructure, electronic structure and transport properties

Abstract

Ti2AlCxNy solid solutions are synthesized using hot isostatic pressing. The X site solid solution effects are investigated, focusing on the microstructure, electronic structure and transport properties of the Ti2AlCxNy MAX phases. Combining X-ray diffraction with scanning electron microscopy and wavelength dispersive X-ray spectroscopy measurements, it is shown that solid solutions can be synthesized with well-controlled chemical compositions in the entire composition range (x=0–1). The measured a and c lattice-parameter values are shown to be in good agreement with those calculated using the cluster-expansion formalism. Combining electron energy-loss spectroscopy (EELS) and band structure calculations, it is demonstrated by Arroyave et al. that solid solution effects induce weak perturbations on the electronic structure. The solid solution effect thus results mainly in a rigid shift of the Fermi energy in a flat part of the electron density of states of Ti2AlC (or Ti2AlN). From these observations, the variations in two key parameters of the conductivity are rationalized. The relative variations in the residual resistivity in the whole composition range are shown to be small, which agrees well with the weak disorder evidenced by EELS. However, the values of the slope of the variation in resistivity vs. temperature are shown to vary quite significantly, evidencing a deviation from Matthiessen’s rule in these systems. By comparing the experimental data with calculations based on the semi-classical Boltzmann equation, it is demonstrated that the observed variations are due to a combination of band structure effects and changes in the scattering mechanisms. Finally, vacancies are also shown to have a prominent effect on the transport properties of Ti2AlCxNy solid solutions. In particular, both residual resistivity and dρ(T)/dT values are shown to be significantly higher than those of the corresponding stoichiometric compounds.