Microstructure-Functional Behavior-Relationships in High Entropy Shape Memory AlloysThursday (27.09.2018) 10:00 - 10:15 S1/01 - A5 Part of:
High entropy alloys have received significant scientific attention during the last years, as they allow to investigate how chemical complexity affects elementary aspects such as phase stability, diffusion and deformation behavior. Recently, a new compositionally complex shape memory alloy (SMA) of type Ni-Cu-Co-Ti-Zr-Hf was introduced. This high entropy material, which is originally based on binary Ni-Ti, shows a reversible martensitic transformation at elevated temperatures. Preliminary results indicate that it provides a shape memory effect with relatively good stability. In the present study we investigate how the chemical complexity affects the martensitic transformation in NiTi-based shape memory alloys as well as mechanical behavior. We consider the high entropy shape memory alloys (HESMAs) and SMAs with compositions from HESMA subsystems with lower configurational entropy. The used alloys differ in their chemical composition of Ti, Zr, Hf, Ni, Cu, Co and were prepared by arc melting and subsequent heat treatments. In the future, it is planned to use induction melting for the production of HESMAs additionally. Characterisations focused on different compositions and microstructures are required to understand the martensitic transformations itself and its influence. Therefore, the basic properties of these new materials are determined by thermal, chemical and microstructural analyses. The phase transformation behavior of these produced alloys was characterized by differential scanning calorimetry (DSC). In addition, monotonic and cyclic-mechanical examinations, such as three-point bending tests and hysteresis tests in different temperature ranges provide informations on the functional fatigue and microstructural influences. The structural morphology of the different HESMA compositions were qualitatively determined by analysing via light microscopy and in-situ scanning electron microscopy during cooling tests. The results suggest that chemical complexity affects basic mechanical behavior in terms of fracture strength and strain, as well as critical stresses for twin boundary migration, which govern pseudoplastic deformation behavior.