Using Impact Nanoindentation to Investigate the High Strain Rate Response of Superelastic Shape Memory Alloys

There is interest in the performance of shape memory alloys (SMA) under high imposed strain rates. Interest centres mainly on NiTi alloys, which are being used in various applications that could involve rapid mechanical straining and there have been several studies of the superelastic response of this alloy to shock loading under uniaxial compression [1-4]. There is a need, however, for further investigation over a range of strain rates, and for loading scenarios that lead to more complex strain fields. Previous work [5] has already shown that nanoindentation data can be used to establish whether deformation is occurring via superelasticity (nanoindentation allows study of local mechanical characteristics in a versatile and cost-effective manner) and that, during nanoindentation, the superelastic response of shape memory alloys is sensitive to tip shape - since sharp indenters (i.e. conical) rapidly generate strains that exceed the superelastic strain regime. In contrast, when indented with a spherical indenter, the peak strains remain sufficiently low for superelastic behaviour to be probed. In this work, the strain rate sensitivity of the superelastic effect in an equiatomic NiTi alloy has been investigated (over a range of temperatures) using impact nanoindentation (spherical indenter), whereby high local strain rates are transiently generated by an accelerated indenter tip. Axisymmetric Finite Element simulations, employing the superelastic user sub-routine in ABAQUS/CAE accompany the experimental programme. Experimental data indicate that superelastic deformation can be detected in this way (throughout the superelastic temperature range), while measured deformation profiles and analytically predicted strain rates correlate closely with numerical predictions. Simulations have also been conducted to estimate the adiabatic temperature rise associated with impact indentation. The predicted rise in temperature is of the order of a few tens of degrees centigrade and some deductions are made about the possible consequences.