Experimental investigations on mechanical properties, smart functional capabilities and corrosion behavior of friction stir welded Nitinol shape memory alloys

Abstract

Shape memory alloys (SMA) are smart materials having distinctive behaviour such as shape memory effect and pseudoelasticity. A reversible solid state displacive phase transformation from austenite to martensite is the reason behind the smart behaviour [1,2]. SMAs are metallic alloys and categorised as NiTi based alloys (NiTi, NiTiCu), copper based alloys (CuZnAl, CuAlNi) and iron based alloys (FeNiCoTi, FeMnSi). The SMAs exhibit high actuation energy densities and have capabilities to recover their shape under large applied loads. However, the actuation frequencies are comparably low for SMAs due to the associated thermal inertia or slow dissipation of heat from the material during cooling. SMA thin films can exhibit better actuation frequencies compared to bulk materials due to the increased surface area to volume ratio. SMAs can exist in two phases having three different crystal structures viz. twinned martensite, detwinned martensite and austenite. The martensite is a low temperature phase while the austenite is stable at high temperatures. The phase transformations have characteristic start and finish temperatures and gives information about the operating range of an alloy. Ms, Mf, As and Af are the characteristic temperatures of the martensite and austenite transformations.