Measurement and modelling of the nanoindentation response of shape memory alloys

A nickel–titanium shape memory alloy was subjected to nanoindentation over a range of temperature (up to 200 °C), such that the starting material was either predominantly martensitic or largely composed of the parent phase. The load–displacement data were interpreted to give information about whether the imposed strain was being at least partly accommodated by the martensitic phase transformation, i.e. whether superelastic deformation was taking place. This interpretation was assisted by finite element simulation of the evolving strain field under an indenter, with or without the superelastic deformation mechanism being operative. It is concluded that the nanoindentation response can be used to determine whether the material is capable of exhibiting superelastic deformation, provided appropriate procedures are employed. Spherical indenters are more suitable than sharp tips. A relatively low value for the remnant indent depth ratio (depth after unloading/depth at peak load) is indicative that superelasticity is occurring. The procedure was found to be viable with a small radius (10 μm) spherical indenter, so it can be employed to explore local variations in superelastic response.