Unraveling Frequency Effects in Shape Memory Alloys: NiTi and FeMnAlNi

With the presence of internal interfaces such as the austenite-martensite interface and the internal twin boundaries in the martensite, shape memory alloys (SMAs) can be employed in passive/active damping applications. Due to the latent heat of transformation, a temperature rise/drop during a load/unload cycle is expected to dynamically couple with the mechanical response of the SMA and influence the stress levels of forward/reverse transformation and thus the hysteretic area (i.e. the dissipated energy). Additionally, the temperature change per cycle is a function of loading frequency due to momentary heat transfer effects. To this end, for the first time, we demonstrate a rate insensitive shape memory alloy system, Fe43.5Mn34Al15Ni7.5 which also exhibits near-zero temperature dependent stress-strain response. Contrastingly, we show that Ni50.8Ti, which is widely used commercially, is highly rate sensitive. With straightforward in situ experiments, complemented with thermomechanical modelling, we pinpoint the key material parameter which dictates frequency sensitivity. The corresponding results are then discussed in the light of different mechanisms contributing to the damping capacity of SMAs.

Automated summary

Shape memory alloys can be used in damping applications, with temperature changes during load/unload cycles affecting their mechanical response. Fe43.5Mn34Al15Ni7.5 shows rate insensitivity, while Ni50.8Ti exhibits high rate sensitivity.