Influence of nickel concentration on multi-scale mechanical properties and wear behavior of NiTi alloys processed via laser powder bed fusion

Nickel – titanium (NiTi) alloys are a promising class of advanced materials that exhibit unique mechanical properties, enabling their use in a wide range of industrial applications. Powder bed fusion laser beam (PBF-LB), one of the prominent additive manufacturing processes, has been widely utilized for fabricating NiTi alloy parts. However, the effect of Ni concentration on the tribological behavior of NiTi alloys has not previously been examined. In this study, two NiTi alloys were successfully fabricated using PBF-LB: Ni 51.1 Ti 48.9 at.% alloy and Ni 49.8 Ti 50.2 at.% alloy. The microstructure, microhardness, nano-scale properties and reciprocating wear behavior of these alloys were systematically investigated. Microstructural analysis revealed fine equiaxed structures interspersed with columnar dendrites in the Ni 49.8 Ti 50.2 at.% alloy, whereas slightly coarser equiaxed cells were observed in the Ni 51.1 Ti 48.9 at.% alloy. The Ni 49.8 Ti 50.2 at.% alloy also exhibited higher micro and nanohardness compared to the Ni 51.1 Ti 48.9 at.% alloy. Both as-printed NiTi alloys demonstrated a difference in the coefficient of friction (COF), with Ni 51.1 Ti 48.9 at.% alloy achieving a slightly lower COF of 0.72, compared to 0.76 for Ni 49.8 Ti 50.2 at.%. However, the as-printed Ni 49.8 Ti 50.2 at.% alloy exhibited superior wear resistance, which correlated well with the micro and nanohardness, hardness to elastic modulus ( H/E ) ratio and strain hardening capacity. Ex-situ analysis indicated that the improved wear resistance of the Ni 49.8 Ti 50.2 at.% alloy was primarily attributed to the stable tribolayers along the wear track, emerged as a dominant wear resistance mechanism, which was absent for the Ni 51.1 Ti 48.9 at.% alloy. These results suggest that the lower Ni content alloy (Ni 49.8 Ti 50.2 at.%) is a better candidate for tribological interfaces under heavy loading conditions, making it suitable for various industrial uses.