Influence of Nb addition and process parameters on the microstructure and phase transformation behavior of NiTiNb ternary shape memory alloys fabricated by laser powder bed fusion

NiTiNbx (x=1,3,5) alloys were fabricated by laser powder bed fusion (L-PBF) of powder mixtures containing prealloyed NiTi and elemental Nb powders. Upon Nb addition, the dependence of the martensite transformation temperatures (MTTs) on the L-PBF process parameters was significantly altered. In case of the NiTiNb1 alloy, the MTTs increased as the laser energy density was increased, whereas it decreased in case of the NiTiNb5 alloy. It is suggested that the net effect of Nb addition is to lower the MTTs, while Ni evaporation has the opposite effect. The latter contribution is predominant in alloys with low Nb addition (1 at.%), while the Nb-related effects become the major influencing factor in the high-Nb alloys (5 at.%). In the case of the NiTiNb3 alloys, the factors that promote and suppress MTTs are almost completely compensated, and the MTT is rather stable despite of the large variation of L-PBF process parameters.