Microstructure And Mechanical Properties Of Cryorolled Invar Alloy Thin Strips

Invar alloy is prestigious for the excellent low expansion property and widely used in precision instruments,aerospace and other fields.Cryorolling can significantly increase the dislocation density and provide more recrystallization nucleation sites for the subsequent annealing process,causing a significant influence on grain refinement in the deformed metals.In this paper,the manufacturing procedure of cryorolling + annealing was proposed.The microstructures and mechanical properties during cryorolling and subsequent annealing processes were investigated to provide the theoretical basis and technological support for producing of high strength invar alloy.The main work and the experimental results of this paper were listed as follows:(1)At first,the linear thermal expansion coefficients of 4J36 and 4J32 alloys were measured.The invar sheets were prepared by hot rolling,solid solution treatment,cryogenic treatment,cold rolling and annealing,the microstructure and mechanical properties were.investigated,as well as provided the initial materials for the cryorolling experiment.After solid solution treatment at 800 ? for 30 min + 90%reduction of cold rolling + annealing at 670? for 5 min,4J36 alloy sheets with the average grain size of 9.6?m,yield strength of 292 MPa,tensile strength of 464 MPa and the total elongation of 32.5%were produced.After solid solution treatment at 800? for 30 min + 90%reduction of cold rolling + annealing at 720? for 5 min,4J32 alloy sheets with the average grain size of 9.3?m,yield strength of 289 MPa,tensile strength of 457 MPa and the total elongation of 33.5%were also produced.A certain amout thermally induced a'-martensites were transformed in 4J32 alloy with lower stacking fault energy(SFE)after cryogenic treatment,resulting in about 10? decrease of recrystallization temperature.By solid solution treatment at 800 ? for 30 min + 90%reduction of cold rolling + annealing at 710 ? for 5 min,4J32 alloy sheets with average grain size of 6.1 ?m,yield strength of 325 MPa,tensile strength of 491 MPa and the total elongation of 33.4%was obtained.(2)After cryorolling with different reductions,the microstructures and mechanical properties of 4J36 alloy during deformation process were studied.The effect of initial grain size on the micro structure and mechanical properties of the cryorolled 4J36 alloy was also investigated.Then the cryorolled 4J36 alloy thin strips were annealed and the sketch map showing the recrystallization microstructure evolution was plotted.The results showed that both the deformation twins and deformation induced ?'-martensites were formed during cryorolling,inducing a great influence on the microstructure and mechanical properties of,4J36 alloy.More effective deformation zones existed in the sample with small initial grain size,resulted in a large number of defects formed in 4J36 alloy after cryorolling.After solid solution treatment at 800 ? for 30 min + 90%reduction of cryorolling + annealing at 550? for 30 min,the cryorolled 4J36 alloy thin strips were fully recrystallized with an average grain size of 4.6?m,the yield strength and tensile strength were 378 MPa and 550 MPa respectively,and the total elongation was 35.2%.Compared with the cold rolling experiment results,both the yield strength and tensile strength were improved 86 MPa.(3)The microstructures and mechanical properties of 4J32 alloy during cryorolling were studied,the grain refining mechanism during annealing process were also analyzed.The results showed that more ?'-martensites were transformed in 4J32 alloy during cryorolling and the mechanical properties of the tested alloy were significantly improved.After solid solution treatment at 800? for 30 min + 90%reduction of cryorolling + annealing at 650? for 30 min,the cryorolled 4J32 alloy thin strips were fully recrystallized with an average grain size of 4.9?m,the yield strength and tensile strength were 370 MPa and 546 MPa respectively,and the total elongation was 34%.Compared with the cold rolling experiment results,the yield strength and tensile strength were improved 81 MPa and 89 MPa respectively.While,the average grain size of 4J32 alloy was failed to be further refined compared with 4J36 alloy,which was mainly resulted from the higher recrystallization temperature of the deformed austenite in 4J32 alloy.(4)The SFE of the invar alloy and the deformation temperature during cryorolling process were calculated by thermodynamic model and rolling temperature rise model respectively,the deformation mechanisms of invar alloy during cryorolling were analyzed.The calculation results revealed that the SFE of 4J36 alloy and 4J32 alloy was 15.6 mJ/m2 and 4.8 mJ/m2 at-196? respectively.The deformation work and friction heat played an important role on the deformation temperature during cryorolling and the deformation temperature was rapidly increased with the accumulation of cryorolling reductions,causing the deformation mechanisms of stress assisted a'-martensite transformation,strain induced a'-martensite transformation,deformation twinning and dislocation slipping co-existed in the cryorolled 4J36 alloy.The decrease of SFE inhibited the deformation twinning and strain induced ?'-martensite transformtion during cryorolling process.Therefore,only dislocation slipping and stress assisted a'-martensite transformation were triggered in the cryorolled 4J32 alloy.With the progress of cryorolling,the morphology of a'-martensite transformed in 4J32 alloy was gradually changed from plated to irregular shaped.(5)The influence of cryorolling on the recrystallization texture of invar alloy was analyzed.The results revealed that there were obvious differences in the recrystallization texture of 4J32 alloy and 4J36 alloy.The recrystallization texture of the cold rolled 4J36 alloy sheets were mainly Cubic texture,while Copper and Goss textures for cryorolled alloy due to the formation of deformation twins and deformation induced a'-martensites.No obvious recrystallization texture was detected in cold rolled 4J32 alloy sheets after annealing,but Goss texture was the major recrystallization texture for cryorolled one due to the formation of a large amount of deformation induced a'-martensites.The manufacturing procedure of cryorolling + low temperature annealing was proposed in this study and a grain refinement and mechanical properties improvement were achieved in invar alloy,which provided the theoretical basis and technological support for producing of high strength invar alloy.