Influence Of Pre-strain And Thermo-mechanical Treatment Of Next Generation Nitinol Materials in Rotary-Bending Fatigue

It is well known in the industry, that Ti-rich secondary phases, as well as the ΔT are influencing the fatigue behavior of the final product [1,2,3]. Smaller DT results in better fatigue life [4]. Futhermore in recently published works it was shown, that pre-strain [5] and thermomechanical treatments [6] can also improve fatigue life. This paper works on the effect of pre-strain and thermomechanical treatment of different low inclusion Nitinol and tries to clarify if the influence of the pre-straining on fatigue behavior without the fatigue limiting effect of secondary phases. To investigate and to distinguish the influence of secondary phases on fatigue behavior, Nitinol material which has been produced according to EBR processes, was used in the test as reference. This material is almost inclusion free, and the size is far below the 10 µm. This material was processed into 1 mm wires with a grain size around 20 µm. The fatigue behavior itself is determined in a Rotary Bending Test for pre-strained wires between 2% and 10% implemented below and over A_f, thus in the martensitic and austenitic condition. The test temperature is selected in a way, that the difference between test temperature and active A_f-temperature is always constant.