Just another WordPress site - Ruhr-Universität Bochum

Abstract

In the present work, we demonstrate how advanced ingot metallurgy can contribute to a better understanding of functional and structural properties of NiTi shape memory alloys (SMAs). In the first part of our work, we investigate the effect of Ni-concentration on the martensitic transformation. NiTi ingots with precisely controlled compositions were produced through an optimized arc melting procedure. It was found that increasing Ni-concentrations are associated with decreasing transformation temperatures. As a striking new result, the Ni-content also affects the hysteresis width of NiTi SMAs. This observation can be rationalized on the Ball and James theory; a higher Ni-concentration is associated with a better crystallographic compatibility between the high and the low temperature phase, and thus with a lower nucleation barrier for the martensitic transformation. In the second part ofthe present work, we investigate how impurities (small oxygen and carbon levels) affect fatigue lives of pseudoelastic NiTi SMAs. The presence of oxygen and carbon in NiTi results in the formation of oxides and carbides. NiTi SMA wires with different oxygen and carbon levels, but very similar microstructures and functional/mechanical properties were prepared through arc melting, rolling and wire drawing in combination with different heat treatments. Fatigue experiments were performed using a self-designed bending rotation fatigue test rig. It was observed that inclusions, which are associated with small voids, represent critical defects that facilitate crack initiation. However, a reversible stress-induced martensitic transformation can reduce mechanical stresses, which is beneficial for fatigue lives.