Isotope study reveals atomic motion mechanism for the formation of metal whiskers in MAX phase

Abstract

Spontaneous growth of metal whiskers has been studied for around 70 years, but still resists interpretations. Herein, the extrusion of whiskers from MAX phases, a group of nanolaminate ceramics, is surveyed using a combination of experiment and simulation methods. It was once believed that the excess A in MAX substrate provided element source for whiskers, while in this work, however, direct evidence from isotope experiment of Ti2SnC/120Sn demonstrates that the extruded Sn whiskers are composed of the atoms from both the excess Sn and the lattice Sn. Moreover, the Ti2AC/A’ (A, A’ = Sn, Ga) crossover experiment indicates that the extrusion of A whiskers is always companied with the substitution process of excess A (or A’) for lattice A atoms, with the driving force of which concluded to be the chemical potential gradient (or reduced formation enthalpy). Accordingly, a lattice diffusion based atomic motion mechanism is proposed in this work, to explain the whiskering behaviors in MAX phase.