Confirming the Composition of Shape Memory Alloys by Microstructural Characterization

It is well known that the properties of shape memory alloys, such as binary Ni-Ti (nitinol), are very sensitive to composition.[1-3] Especially on the Ni-rich side of stoichiometry, the martensite/austenite phase transformation temperature decreases abruptly when Ni is increased only a few tenths of a weight percent.[4] In ternary high temperature shape memory alloys (HTSMAs) containing Pt, Pd, or Hf, this compositional sensitivity can cause even greater shifts in transformation temperature, potentially hundreds of degrees C.[2,3] Figure 1 shows the Austenite-start temperatures of several heats of Ni-Ti-Pt alloys, measured by differential scanning calorimetry (DSC), as a function of aim Pt concentration. Some samples exhibit comparatively low As temperatures. The target composition for all specimens was slightly Ti-rich (50.5 at. % Ti). However, slight changes in the melt composition due to Ti oxidation, inhomogeneity, or other effects can sometimes push the alloys to the (Ni,Pt)-rich side of stoichiometry. While DSC provides a good measure of phase transformation temperature, it is only indirect evidence of the compositional effect. In addition, the small discrepancies in matrix composition that lead to property changes are often masked within the experimental error of bulk chemical analysis methods. To definitively confirm the (Ni,Pt,Pd)-rich or (Ti,Hf)rich character of a specific lot of material, microstructural characterization is necessary.