Hot Isostatic Pressing of NiTi-X (X=Ag, Nb, W) Alloys for Coupling Elements

Due to their unique properties, shape memory alloys (SMAs) based on NiTi are highly attractive for functional engineering applications like coupling elements. Adding a third element is a well known measure to influence the transformation behaviour. In the case of coupling elements, a broad hysteresis between austenitic and martensitic phase transformation is the key of function. Widening of the hysteresis can be achieved by adding a ductile phase with limited solubility in the NiTi-matrix. Elemental Ag, Nb and W are promising candidates to fulfill this aim. A homogeneous distribution of the ductile phase is prerequisite for reproducible mechanical properties and transformation behaviour. In the present work, hot isostatic pressing (HIP) was found to be a preferred method for manufacturing such alloys starting from pre-alloyed NiTi powder and adding Ag, Nb or W as elemental powders. For comparison, these alloys are difficult to prepare by conventional melting due to strong differences in melting points. Interdiffusion phenomena, mechanical properties as well as potential of these alloys to be used for coupling elements are discussed.