Research On The Microstructure Evolution And Mechanism Of The Deformation And Heat Treatment Of CrCoNi Medium-Entropy Alloy

As a transformative multi-principal alloy,CrCoNi mediumentropy alloy exhibits outstanding combination of strength and toughness,which outperformed most alloys,and is even comparable with austenitic stainless steels and high nickel steels.This alloy has been expected as the most likely substitute for high temperature alloys.However,it has been found that the alloy suffers from the shortage of yielding strength at room temperature,the complex of atomic arrangement and chemical composition,the unexplored deformation and strengthening mechanisms.which would be difficult to improve the strength by regulating the microstructure,thus,significantly limit its further applications.In this project,to solve these problems,the recrystallisation and strengthening mechanisms during hot and cold deformation as well as subsequent heat treatment of equiatomic CrCoNi alloy are systematically investigated.The corrosponding microstructure and mechanical properties of the alloy under different conditions are also characterized in detail.The evolution of the recrystallisation drive force and microstructure in this alloy under different conditions and the reasons beneath them are analysed based on the dynamic recrystallisation process during hot deformation and the static recrystallisation process during annealing after hot deformation.The heterogeneous structure with three levels of grain size were tailoring by a combination of room temperature cold rolling and heat treatment,acheving the strengthening and toughening of this alloy.The following research works and results can be derived from the present work:(1)As an alloy with low stacking fault energy,the CrCoNi mediumentropy alloy undergoes continuous dynamic recrystallisation during thermal deformation rather than discontinuous recrystallisation.Moreover,the recrystallisation mechanism can be concluded as the subgrain boundaries within the original grains twisted to form large angle grain boundaries,thereby forming recrystallised grains within the deformed grains.Specimens that compressed at deformation temperatures(1000°C)and high strain rates(0.1/s,1/s)achieve a uniformly refined grain structure.During thermal deformation,the necklace structure,is generated by a continuous dynamic recrystallisation thickening band within the specimens under low strain rate(0.01/s).In addition,the rapid development of the strain gradient near the grain boundaries can not only accelerates the formation of the necklace structure,but also to accumulate a large number of dislocations at the grain boundaries.(2)As for the annealing process after thermal deformation,a significant amount of recrystallisation nucleation occurred during static recrystallisation without much grain growth when the specimens were compressed at low strain rates(0.01/s).By contrast,for specimens compressed at higher strain rates(1/s),the fine grains and recrystallized nuclei that remained after dynamic recrystallisation of thermal deformation were growing rapidly and becoming coarse during subsequent annealing.A large number of linear segmental annealing twins were generated during the annealing process,while some island-like twin boundaries appeared in specimens annealed at lower temperatures(650°C).The lower strain rate(0.01/s)during thermal deformation contribute to generation of a higher density of annealing twins.And the alloy achieves the highest twin density at 1000°C-0.01/s heat deformation and 750°C-1h heat treatment.(3)It has been proved that dislocation,stacking fault and twinning are all the deformation modes in CrCoNi medium-entropy alloy.These deformation mechanisms provide this alloy with excellent plastic deformation ability,allowing the alloy good fracture elongation even when partial recrystallized.Solid solution strengthening,grain boundary strengthening are found as the main method of providing major yield strength in this alloy,with solid solution strengthening providing ～68% of the yield strength in this alloy..The accumulated dislocations,HCP lamellae,nano-grains,nano-twins and nano-twin bundles,heterogeneous grain size structures and shear bands in the partial recrystallized condition are also provide additional strengthening effects to this alloy.As a result,the alloy is capable of achieving a yield strength of ～551 MPa even in the fully recrystallized situation.