Prof.  Natalia  Resnina
Saint-Petersburg State University,  Russia

Title: Mechanical behaviour of low-, medium- and high-entropy Ti-Hf-Zr-Ni-Cu-Co shape memory alloys

Abstract:

The mechanical behaviour was studied in the The Ti_51-2xHf_xZr_xNi_49-2xCu_xCo_x and Ti_50-2xHf_xZr_xNi_50-2xCu_xCo_x alloys (X = 1, 5, 10 or 17 at. %) which were characterized by various configuration entropy. Twelve alloys were manufactured in a vacuum arc furnace or in electron beam furnace under an argon atmosphere. All alloys included the B2 phase undergoing the martensitic transformation and the secondary phase: Ti₂Ni-type or Ni₄Ti₃-type depending on the concentration of Ni group atoms (Ni, Cu, Co). The samples were deformed by tension up to break with intermediate unloading at 100 ^oC (at which the alloys were in the austenite state), at 22 ^oC (at which the alloys were in austenite, martensite or mixture state depending on the chemical composition) and at - 100 ^oC (at which the low and medium entropy alloys were in the martensite state, while the high-entropy alloys were in the austenite state). It was found that the senary Ti-Hf-Zr-Ni-Cu-Co alloys can be deformed by martensite reorientation, stress-induced martensitic transformation and dislocation slip as in the binary NiTi alloys. The activation of the deformation mechanisms depends on the position of the deformation temperature compared to the temperatures of the martensitic transformations. The yield limit for dislocation slip, the strength in the martensite and austenite state as well as the strain up to failure in the martensite and austenite states were measured. It was found that an increase in the X value from 1 to 10 at. % increased the yield limit for dislocation slip in the austenite phase. An increase in the X value from 10 to 17 at. % decreased the yield limit for dislocation slip. The strength in the austenite state changed on a rise in the X value in the same manner as limit for dislocation slip in the austenite state. It was observed that an increase in the X value decreased the strain up to failure in the martensite or austenite states. If the deformation occurred by reversible mechanisms such as martensite reorientation or stress induced martensitic transformation, the strain up to failure was larger than after plastic deformation. The fracture surface was studied by scanning electron microscopy in all samples. It was found that an increase in X value from 1 to 5 at. % dramatically changed the fracture mechanisms from ductile to brittle due to the suppression of the dislocation slip. Further increase in X value did not affect the brittle fracture. It was found that the high-entropy alloys demonstrated the perfect superelasticity at extremely low temperatures (-100 ^oC or less). The reasons for the influence of the configuration entropy on the mechanical behaviour of the senary Ti-Hf-Zr-Ni-Cu-Co alloys was discussed.

This study was supported by Russian Science Foundation (# 22-19-00169). SEM studies were carried out using the equipment of Resource Centre for Nanotechnology of Saint Petersburg State University.

Biography: