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X. Zhao^a, F. Neves^b, J. B. Correia^b, K. Liu^a, F. M. Braz Fernades^c, V. Koledov^d, S. von Gratowski^d, S. Xu^e, J. Huang^a

^aSchool of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

^bUnidade de Energia Solar, Laboratório Nacional de Energia e Geologia, Estrada do Paço do Lumiar 22, 1649-038 Lisboa, Portugal

^cCENIMAT/i3N, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Portugal

^dKotelnikov Institute of Radio Engineering and Electronics of the Russian Academy of Sciences, 125009 Moscow, Russia

^eDepartament de Física, Universitat de les Illes Balears, Cra Valldemossa, km. 7.5, Palma de Mallorca. Spain

In this work the brittle nature of NiTi₂ was used to promote a better efficiency of the mechanical activation process of NiTi alloys. Starting with (NiTi₂-Ni) powder mixtures resulted in the formation of a predominant amorphous structure after mechanical activation at 300 rpm for 2 h. A sintered specimen consisting mainly of NiTi phase was obtained after vacuum sintering at 1050 ºC for 0.5 h. The produced NiTi phase exhibited the martensitic transformation behavior. Using elemental Ti powders instead of pre-alloyed NiTi₂ powders, the structural homogenization of the synthesized NiTi alloys was delayed. Performing the mechanical activation at 300 rpm for the (Ti-Ni) powder mixtures gave rise to the formation of composite particles consisting in dense areas of alternate fine layers of Ni and Ti. No significant structural modification was observed even after 16 h of mechanical activation. Only after vacuum sintering at 1050 ºC for 6 h, the NiTi phase was observed to be the predominant phase. The higher reactivity of the mechanically activated (NiTi₂-Ni) powder particles can explain the different sintering behavior of those powders compared with the mechanically activated (Ti-Ni) powders. The obtained results demonstrate an effective way to optimize the mechanical activation process of NiTi alloys.

Fig. 1 - Typical macro- and microstructures images of the a, b (NiTi2–Ni) and c, d (Ti–Ni) vacuum sintered specimens.

Reference

X. Zhao, F. Neves, J. B. Correia, K. Liu, F. M. Braz Fernandes, V. Koledov, S. von Gratowski, S. Xu, J. Huang

Journal of Materials Science 53 (2018) 13432 –13441

doi: 10.1007/s10853-018-2560-5