Nanoindentation of Ti50Ni48Fe2 and Ti50Ni40Cu10 shape memory alloys

In the present paper we use nanoindentation to investigate two shape memory alloys, austenitic Ti50Ni48Fe2 and martensitic Ti50Ni40Cu10 which both show two step martensitic transformations on cooling from the high temperature regime. No such two step features were observed during nanoindentation. Load controlled nanoindentation experiments were performed using a Berkovich indenter using maximum loads ranging from 0.5 to 85 mN resulting in maximum indentation depths between 30 and 1350 nm. For small maximum indentation loads below 5 mN, longer indentation times result in larger indentation depths (austenitic Ti50Ni48Fe2). At loads of about 1 mN the indentation curves show an abrupt change in slope which we suggest to be related to an interaction of the deformation and failure of a thin oxide layer and the formation of stress induced martensite (austenitic Ti50Ni48Fe2) or the de-twinning of martensite (martensitic Ti50Ni40Cu10). Nanohardnesses and shape recovery ratios strongly increase with decreasing indentation depths below 400 nm. For larger indentation depths, constant values are observed (austenitic Ti50Ni48Fe2: nanohardncss 2.45 GPa/recovery ratio 0.25, martensitic Ti50Ni40Cu10: nanohardness 1.6 GPa/recovery ratio 0.4).