Influence Of High Strain Rate Deformation On Microstructure and Mechanical Properties Of Martensitic NiTi Shape Memory Alloys

In addition to the shape memory effect and superelasticity, shape memory alloys (SMAs) also exhibit high damping capacity in its martensitic state, which effectively converts mechanical energy into heat. Three primary mechanisms are responsible for the high damping capacity in SMAs: a) internal friction, which results from atomic ‘shuffling’ at the boundaries between austenite and martensite in austenitic SMAs or martensitic variants in martensitic SMAs, b) martensitic twin reorientation, i.e. all variants reorient to a preferred single variant orientation under an applied deformation, resulting in about 8% fully recoverable strain, c) stress-induced martensite, which results from phase transformation from austenite to martensite that occurs at the stress plateau in austenitic SMAs, contributes about 5% fully recoverable strain. Only the first two mechanisms operate in fully martensitic SMAs. Due to their excellent ability of attenuating and dissipating energy, as well as their high yield stress and strong resistance to corrosion, SMAs are promising candidates for damping applications which reduce vibrations and thus reduce potential structural damage to instruments and even large infrastructures. Examining the relationships between microstructure and mechanical properties in martensitic SMAs and their effect on the mechanisms which control damping will lead to further improvements and aid in the development of potential damping applications. In the present research, the compressive mechanical behavior of martensitic NiTi-based SMAs from low (1×10^-3) to very high strain rates (1×10³) is experimentally investigated and compared with the corresponding microstructures from optical microscopy and scanning electron microscopy with energy dispersive spectroscopy and electron backscatter diffraction.