Processing and Quality Control of Binary NiTi Shape Memory Alloys

Abstract

Binary alloys based on the intermetallic system NiTi are at present the commercially most important shape memory materials on the market. Especially in the field of medical technology the NiTi alloys (NiTiNOLA) had their breakthrough during the past years as implant or instrument e.g. for minimal invasive surgery. The improved understanding of the metallurgical principles was certainly the basis for this success. But probably more important were the achievements related to the reliable fabrication of NiTi with tight tolerances and high reproducibility. As the temperature of the human body varies only within a very narrow window the characteristics temperatures of a shape memory alloy have to be adjusted and controlled very carefully to meet the unique requirements of the medical product. On the other hand, the constant temperature of the human body is a very good prerequisite for the application of shape memory alloys in general as all effects related to the martensitic phase transformation are very sensitive to changes of environmental temperature (Fig. 1). Therefore, most pro-perties of NiTiNOL depend strongly on the temperature [i]. These facts show clearly the importance of an advanced quality control concept for NiTi alloys because once the tight tolerances were achieved, the market situation for these unique materials in the field of medical implants and instruments became very interesting and promising.