Effect of Aging Treatment on Microstructure Evolution and Strength and Ductility of Cold Forged Co-Cr-W-Ni-Mo Alloy
Cold-forged Co-Cr-W-Ni-Mo.alloy was aged at 400-900 ℃.The carbide,substructure,hcp phase and stacking faults evolution of Co-Cr-W-Ni-Mo alloy during aging were studied by SEM,TEM,XRD and chemical phase analysis,and the influence of the changes on strength and toughness were analyzed.The results show that the alloy aged at 600 ℃ for 2 h has high strength and hardness,while maintaining good ductility and toughness.Mf23C6 and M6C carbides are precipitated after aging.The precipitation temperature range of M23C6 is 600-800 ℃,and the precipitation temperature of M6C is 700 ℃.The precipitation of carbides leads to the increase in strength and hardness,but the precipitation of a large number of network M6C along the grain boundaries and deformation bands at 900 ℃ reduces the strength and ductility at the same time.When the alloy is aged at 600 ℃,the fcc phase reduces the energy,so as to keep stability by forming stacking faults,which increases the density of stacking faults and which improves the strength and ductility.The hcp phase reverses to fcc phase after aging at 800 ℃,which makes the strengthening effect of strain-induced martensitic transformation disappear.Meanwhile,recrystallization occurs at this temperature,which leads to the disappearance of cold deformation structure and a further decrease in strength.The recrystallization nucleation mechanism of the alloy is a combination of three nucleation mechanisms,i.e.protrusion.mechanism,subgrain boundary migration and subgrain merging.
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