Recrystallization and martensitic transformation in nanometric grain size Cu-Al-Ni thin films grown by DC sputtering at room temperature

Abstract

We report the recrystallization of metastable hexagonal and body centered cubic BCC phase in free-standing 6 μm thick Cu-Al-Ni films grown by DC sputtering at room temperature. The results show that the BCC phase recrystallizes to L2_1 at around 533 K together with precipitation of γ2-phase. Annealing temperatures (533 K-623 K) at short times (0-30 min) produce systematic increment of the fraction of γ2-phase. Films with phase coexistence of hexagonal, γ2 and L2_1 structure display temperature driven martensitic transformation. This transformation is observed for samples with austenite grain size of around 30 nm. In all cases, the martensitic transformation temperature (MS) notoriously decreases (compared to bulk) and systematically increases with the annealing temperatures and annealing time. In addition, the films display extended transformation and retransformation ranges along with asymmetric hysteresis, which may be associated to restitutive forces due to elastic deformations at martensite - martensite and martensite - austenite interfaces. Annealing temperatures above 623 K produce fast precipitation to equilibrium α- and γ2-phases. The austenitic phase can be recovered after annealing over 1000 K and fast quenching in ice-water.