Formation of nanoscaled precipitates and their effects on the high-temperature shape-memory characteristics of a Ti50Ni15Pd25Cu10 alloy

Abstract

The effects of thermomechanical treatment on the microstructure and high-temperature shape-memory characteristics of a TiNiPdCu alloy were investigated. An unexpected precipitation behavior was identified in a Ti50Ni15Pd25Cu10 alloy. Very high densities of nanoscale precipitates of TiPdCu and Ti2Pd types were found to be formed in the thermomechanically treated Ti50Ni15Pd25Cu10 alloy. A spinodal type of decomposition process was expected to be the cause of the observed precipitation behavior. It was noticed that the preferential diffusion of Cu atoms towards the heterogeneous nucleation sites promoted the precipitation of TiPdCu-type precipitates, which in turn promoted the precipitation of fine Ti2Pd-type precipitates. These precipitates greatly increased the resistance against the transformation-induced plasticity and creep deformation, especially at high stresses and high temperatures, mainly because of the high-temperature stability of these precipitates. High densities of these nanoscaled precipitates caused an anomalous increase in hardness and retarded the martensitic transformation. It was expected that the current research results could be highly beneficial for the development of high-temperature shape-memory alloys stable at temperatures >773 K, while keeping the benefits of ease of fabrication. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.