Microstructural Analysis of Inclusions in Six Nitinol Variants

Intrinsic Ti4Ni2Ox and TiC inclusions are known to affect the fatigue behavior of Nitinol particularly in its ultralong fatigue life; nevertheless, there are few statistical analyses to characterize the inclusion sizes and morphology to show such relationships quantitatively. This investigation characterizes the inclusions in Nitinol from six melt sources with scanning electron microscopy in the backscattered imaging mode in accord with the on-going ASTM Working Document (Determining the Porosity and Inclusion Content of Nickel Titanium Shape-Memory Alloys (NiTi SMAs) Using Automatic Image Analysis). Specifically, the Feret diameter, area fraction, and aspect ratio were measured from N = 36 images from each condition and both orientations. The results from two laboratories resulted in similar trends with respect to longitudinal and transverse inclusion characteristics. The Feret diameters were further analyzed according to ASTM E2283. In the longitudinal direction VAR Nitinol has extreme dimensions of > 270 µm, whereas the VAR/EBR Nitinol has < 8 µm inclusion dimensions.