Influence of inclusion size and void size on the rotary bending fatigue of next generation Nitinol materials

Due to the extraordinary properities, the superelastic Ni_50,8Ti_49,2Ti alloy is a popular material for a lot of medical applications. Especially the use of Nitinol in heart valve frames needs a high fatigue resistance of the material. Earlier work has shown that fatigue crack initiation mainly occured at oxide particles with particle/void assemblies. Material melted by EBR, known as HCF, exhibited a high fatigue life, however a clear correlation between fatigue behaviour and inclusion size and void size could not be illustrated so far. The present work is focused on the analysis of the size and shape of the oxid inclusions as well as the oxid particle/void assemblies in the HCF material. The result is compared to VAR melted material. To investigate this effect similar processed wires of both materials were tested in an extended quantitative metallographic evaluation with light- and scanning electron microscopy. At a sampling field area of 40000 µm² inclusion sizes and shapes were analyzed as well as the void sizes of both materials. A relationship between inclusion size and void formation could be found. Furthermore for investigation of the influence of inclusion size and void size on the fatigue life, fracture surfaces and outer wire sufaces of both materials were examined by scanning elektron microscopy after fatigue failure at rotary bending tests of 2,5% strain.