Mechanism Study On Low ?CSL Grain Boundary Density Improvement Induced By Prior Stored Energy In Cold Deformatin For Nickel-based Superalloy

As a key component of the combustion chamber of marine diesel engine,exhaust valve is manufactured by electric upsetting and forging.During the narrow parameter range of thermal deformation for nickel-based superalloy,the twins and dynamic recrystallization coexist and synergistically interact resulting in mixed grain boundary network with special ones.The comprehensive properties of alloys can be improved significantly by promoting the proportion of low?CSL twin boundaries to optimize grain boundary characteristic distribution and realizing the goals of grain refinement and higher twins content.In this paper,the sub-grain boundaries with high dislocation density induced by prior cold deformation were obtained,and further to stimulate the nucleations of dynamic recrystallization and twins for the thermal deformation of Ni80A superalloy.The effects of cold and thermal deformation parameters on the evolutions of twins and dynamic recrystallization were analyzed to reveal the mechanisms of low CSL?3ⁿ grain boundary density improvement induced by prior cold deformation.Based on the experiment results of cold and thermal compressions and characterization of EBSD,the unified models of twins&dynamic recrystallization hybrid grain boundary,directly drived by cold and thermal deformation parameters,were established and solved for Ni80A superalloy.The multi-field and multi-scale finite element model of cold and thermal compressions was established by implanting the solved unified models of Ni80A superalloy in Msc.Marc software,and further being adopted to simulate the dynamic evolutions of twins and dynamic recrystallization in the whole process of prior cold compression,heating insulation and isothermal compression.The interactive correspondence between low CSL?3ⁿ grain boundary density and microstructure parameters were further revealed.Comparing simulated results with EBSD experimental results,the validity and reliability of the solved unified models of Ni80A superalloy and established finite element model were verified,and it can provide theoretical guidance for the regulation of low CSL?3ⁿ grain boundary density induced by prior cold deformation.The main contents and conclusions are as follows:(1)The axisymmetric cold compression experiments of Ni80A superalloy were conducted at the range of cold compressing rate of 10%?30%;and the isothermal compression experiments were carried out at temperatures of 1273?1473 K and strain rates of 0.001?10 s^-1 after 10%?30%cold compressing rate.The deformed samples were characterized by EBSD technology.(2)Based on EBSD characterization results,the microstructure evolutions at varieous cold compressing rates were analyzed,and the evolution mechanism of twins and dynamic recrystallization in thermal deformation varying with cold compressing rates were revealed.The sub-grain boundaries with high dislocation density induced by prior cold deformation were obtained and would become the nucleation region of dynamic recrystallization in thermal compression,which will reduce the critical stress/strain of dynamic recrystallization.And the dynamic recrystallization will be activated in advance,thus exerting the increase of recrystallization volume fraction and content of?3ⁿ grain boundary.(3)The evolutions of twins and dynamic recrystallization in temperatures of 1273?1423 K and strain rates of 0.001?10 s^-1 after 30%cold compressing rate were analyzed and revealed.Under different thermal deformation temperatures and strain rates,the evolution ofBLD_{?3 n} exhibites obvious dynamic interaction with the volume fraction of dynamic recrystallization,stored energy and average grain size,and BLD_(50)n3 increases with increasing stored energy and decreasing grain size,and higher BLD_{?3 n} with finer grains corresponds with lower temperatures and higher strain rates.Comparing with the microstructures without prior cold deformation,the microstructures were characterized as higher volume fraction of dynamic recrystallization more than80%,refined grain size and obviously increased low CSL?3ⁿ grain boundary density.(4)The evolution models of low CSL?3ⁿ grain boundary density,dynamic recrystallization and dislocation density in the whole process of prior cold compression,heating insulation and isothermal compression were established;based on the true stress-strain data and EBSD characterization results of Ni80A superalloy,the unified models of twins&dynamic recrystallization hybrid grain boundary were established and solved(5)Based on the solved unified models of Ni80A superalloy,the subroutines of microstructures evolution and the field succession of dislocation density were developed to establish the multi-field and multi-scale finite element model for cold and thermal compressions process of Ni80A superalloy.The evolutions and distributions of dislocation density,volume fraction of dynamic recrystallization,grain size and low CSL?3ⁿ grain boundary density were quantitatively analyzed and visualized.Furthermore,the interactive correspondence between low CSL?3ⁿ grain boundary density and microstructure parameters were further discussed.Meanwhile,the validity and reliability of the solved unified models of Ni80A superalloy and established finite element model were verified.