The influence of the chemical composition of the Ti-Hf-Zr-Ni-Cu-Co shape memory alloys on the structure and the martensitic transformations

Twelve senary alloys were manufactured with different concentrations of doping elements (Hf, Zr, Ni, and Cu) that varied from 1 to 17 at%, and the concentration of the Ni equivalent (Ni, Cu, and Co) atoms changed from 49 to 51 at%. It allowed the production of Ti-Hf-Zr-Ni-Cu-Co alloys with various configuration entropies. It was found that, regardless of the entropy value, all alloys crystallised in the B2 phase, in which the lattice parameter increased upon a rise in the doping elements’ concentration (X parameter) and was hardly affected by the Ni group concentration ([Ni] = Ni + Cu + Co). In the alloys with low and medium entropy, the chemical composition of the B2 phase was homogeneous and close to the composition of the alloy. In the alloys with high entropy, the core and edge of dendrite cells in the matrix were characterised by different concentrations of doping elements, while the concentration of the Ni group was constant. Besides the B2 phase, the secondary phase existed in all alloys, whose type [Ti] ₂[Ni] or [Ni] ₄[Ti] ₃ depended on the [Ni] value, as was observed in the binary NiTi alloys. It was found that the Ti-Hf-Zr-Ni-Cu-Co alloys with low and medium entropy underwent the B2 ↔ B19′ martensitic transformation on cooling and heating. An increase in the [Ni] and X values initiated the formation of the strain nanodomains on cooling before the forward transformation. In the high-entropy alloys with X = 10 at%, the strain nanodomains formed on cooling to − 180 °C without the formation of the B19′ phase. In the alloys with X = 17 at%, strain nanodomains, and martensitic transformation were not found on cooling, but the martensitic transformation was initiated by loading at − 100 °C. It was shown that the variation in the Ni group concentration affected the martensitic transformation parameters only in the low-entropy alloys. In the medium- and high-entropy alloys, the concentration of the doping element had a more significant effect on the transformation than the [Ni] value.