Thermal stability of a submicrocrystalline structure formed by high-pressure torsion in Ni and Ni–2 % Cr alloy

Abstract

The main problem of submicrocrystalline (SMC) materials formed as a result of large plastic deformation is their thermal stability. The large stored energy and the formation of strongly disordered microcrystallites in the structure lead to a decrease in the recrystallization onset temperature and, therefore, possibly decrease the structure stability. In the work, severe plastic deformation by high-pressure torsion and annealing of pure nickel and an alloy containing 2 at. % chromium were carried out. The structure of both deformed and annealed material was studied by scanning and transmission electron microscopy. The dependence of hardness on the square root of true strain and structure evolution were analyzed to identify the boundaries of the stages of structural states. The energy stored during deformation was estimated using differential scanning calorimetry by the amount of absorbed heat energy. The author studied the behaviour of materials during annealing depending on the stored strain energy at the SMC structure stage. Three stages of structural stats were identified in pure nickel: cellular, mixed, and SMC structure, while in the alloy containing 2 at. % chromium, a cellular structure stage was not detected. A decrease in the stored strain energy was found at the stage of the SMC structure for both materials. Alloying nickel with 2 at. % chromium increases its thermal stability, which increases the temperature when the grain growth becomes intensive by 150 °C. The amount of stored strain energy affects grain growth in the alloy containing 2 at. % chromium, whereas in pure nickel no effect was detected. In the Ni–Cr alloy, greater stored energy corresponds to larger recrystallized grain size.

Keywords

• nickel
• ni–cr alloy
• high-pressure torsion
• submicrocrystalline structure
• stored strain energy