On the Functional Fatigue Behavior of High Temperature Shape Memory Alloys

Shape memory alloys have been intensively investigated in the last decades due to their unique properties. However, most traditional shape memory alloys lose their shape recovery ability above temperatures of about 70 °C. Thus, current research aims at establishing alloys which can be used at elevated temperatures up to about 400 °C. These alloys have been coined high temperature shape memory alloys (HTSMA). HTSMAs such as Co-Ni-Ga and Ti-Ta show fully recoverable transformation strains and excellent shape memory behavior at elevated temperatures, at least in single cycle tests. However, as HTSMAs will typically experience cyclic loads, the aspect of functional fatigue has to be addressed. Especially in the elevated temperature regime, where diffusion and precipitation may play a significant role during lifetime of a component, the aspect of cyclic stability is of crucial importance.

While functional degradation might be negligible at lower temperatures for Co-Ni-Ga and Ti-Ta, cyclic loading above a critical temperature will lead to degradation of shape memory and pseudoelastic properties as a result of interacting martensite variants and a shift of transformation temperatures due to the formation of additional phases generated during thermal cycling.

In order to shed light on the mechanisms that lead to functional degradation this study focused on the thermo-mechanical functional fatigue of Co-Ni-Ga and Ti-Ta HTSMAs under different loading conditions, i.e. isostress thermal cycling and pseudoelasticity, at elevated temperatures. In situ confocal laser scanning microscopy and in-situ scanning electron microscopy were carried out in order to highlight microstructural evolution as a function of temperature and cycles, especially to obtain crucial data about phase transformation processes and martensite variant interaction.