Carbon Inclusions in NiTi SMAs: A Computational Thermodynamics Study

Binary NiTi alloys are one of the most successful shape memory materials today used in fatigue rated devices. Inclusions content represents a serious concern for fatigue life. Inclusions are generally the initiation sites of fatigue fractures and may negatively affect workability and mechanical properties. Typically, in nickel-titanium alloys the inclusions are titanium carbides, TiC, and intermetallic oxides (i.e. Ti₄Ni₂O_x).

In SAES Group we have developed a reliable procedure, combining FESEM-EDS technique with optical metallography, for inclusion quantitative analysis in commercial standard VIM/VAR NiTi alloys. The inclusion type, size and amount have been observed to strongly depend on the Ni/Ti ratio, the melting and thermo-mechanical processes.

In this work, we provide an interpretation and a theoretical model for carbide and oxide inclusions formation in NiTi alloys by means of computational thermodynamics.

A strong chemical reactivity of graphite crucible with NiTi melt is demonstrated to be responsible of carbon diffusion and contamination during vacuum induction melting process. In Ni-rich formulations (i.e. superelastic behavior), titanium carbides represent the main precipitates. In Ti-rich formulations (i.e. shape memory behavior), carbides become nucleation centers of a Ti₄Ni₂(C, O) phase, resulting in core-shell particle segregation.