The reversibility of phase transitions in Ti/Co core/shell nanometre-sized particles

Molecular dynamics simulations have been employed to investigate the thermal response of unsupported Ti/Co core/shell particles with sizes between 2 and 10 nm. Co shells undergo a martensitic hcp-to-fcc phase transformation at temperatures dependent on the particle radius. Ti cores with radii of 2 nm or smaller also undergo the hcp-to-fcc transition. In the presence of Ti cores with radius larger than 2 nm, Co shells 2-4 nm thick exhibit hysteretic effects in the phase transformation. In particular, the transition temperatures under heating and cooling conditions are different. This determines the stabilization of the metastable fcc Co phase, related to the accumulation of lattice defects in the shell as a consequence of local atomic strains at the Ti/Co interface.