Indentation is a powerful technique for investigation of local mechanical characteristics. For example, the Oliver and Pharr approach allows hardness and Young modulus to be obtained from the unloading curve. The current work studies the superelastic characteristics of shape memory materials under indentation loading, in particular comparing the responses of the same material in both the superelastic and non-superelastic states, making use of a hot stage.

The temperature A_f, which is the temperature at which a shape memory material is fully transformed to the superelastic state, is used here to describe the transition temperature between these two states: this is a simplification, but not an oversimplification!

Figure 1: Indentation traces from a nickel titanium sample loaded above and below A_f (~75 ^oC), using a Berkovich indenter.

Indentation with a Berkovich tip yielded conventional results, with little evidence of any superelastic effect (see Figure 1). Data from above and below A_f show similar recovery of the indent on unloading, with a slightly more pronounced recovery during the final stages of unloading above A_f. With a pointed indenter, the strain immediately under the tip is so large that it exceeds the levels that can be accommodated entirely by transformation to martensite, so some conventional plastic deformation occurs and the behaviour approaches that expected in the absence of a superelastic effect. However, further from the tip the strain levels are lower and may be accommodated entirely by superelastic deformation, generating slight extra recovery.

Figure 2: Indentation traces from a nickel titanium sample loaded above and below A_f (~75 ^oC), using a spherical indenter.

With a spherical indenter, however, a more marked difference was observed between the behaviour above and below A_f (see Figure 2). Both the degree of recovery of the indent and the shape of the loading/unloading curve are different. This is qualitatively consistent with the differences observed during tensile testing. Under a spherical indenter, when above A_f, the strain may be low enough for the superelastic effect to accommodate all the strain. On unloading, this strain is recovered, through the reverse transformation to the parent B2 structure, leading to a much higher recovery in depth than in the parent material at room temperature.