Strain rate dependent micromechanical properties of NiTi shape memory alloys: Laser powder bed fusion versus casting

Abstract

As the strain rate influences the loading sensitivity of the metallic materials over time, it is very important to assess the associated plastic behavior exposed to a diverse strain rate. Strain rate sensitivity is a fundamental property used to assess the controlling mechanisms of plastic deformation and the susceptibility of a material to creep. In materials with high strain rate sensitivity, plastic deformation can occur even under the application of a small stress value with relatively low rates of strain. Because of the diverse use of NiTi shape memory alloys in small size applications, it would be crucial to understand how strain rates, locally and on small scale, influence plastic deformation, especially in superelastic conditions. Thus, proper and optimal implementation of their functional properties can be achieved. In this paper, the local strain rate sensitivity of heat-treated NiTi samples in the additive manufactured and cast conditions have been investigated employing depth-sensing indentation testing technique at ambient temperature subjected to various indentation loading rates. To this end, using a self-similar pyramidal (Berkovich) indenter, the materials are loaded with different indentation loading rates of 1, 5, 10, and 50 mN/s to a peak load of 200 mN and then unloaded. Upon conducting the indentation tests, the extracted results including indentation load-depth curves, indentation stress-depth curves, and indentation strain rate sensitivity values have been analyzed and discussed for the cast and additive manufactured materials. Experimental results demonstrate that the hardness values increase linearly with the increase in indentation loading rate while the elastic modulus stays relatively constant. These findings are important in understanding the local and small-scale deformation behavior of additively manufactured NiTi in which properties are location-dependent, as compared with the cast counterparts.