Study On Aging Precipitation Behavior And Mechanical Properties Of Ti-6554 Alloy

In this paper,scanning electron microscopy（SEM）,X-ray diffraction（XRD）and transmission electron microscopy（TEM）were used to characterize the effect of microstructure changes on the tensile properties of Ti-6Cr-5Mo-5V-4Al（Ti-6554）alloy under different heat treatments,and the precipitation sequence during solution at 800℃and aging was investigated.The effect of microstructure on fracture toughness and fracture mechanism at different solution temperatures was explored by the same aging treatment after the solid solution at 770°C and 800°C respectively（580°C with the best toughness match）.The conclusions are as follows:（1）Ti-6554 alloy has more primary phase（α_p）and smaller grain size after solution at 770℃for 2h than at 800℃for 2h.Solution at 800℃for 2h,more phases（α_s）are precipitated during subsequent aging.After solution and aging at different solution temperatures for 1-12 h,the strength of the alloy increases first and then decreases with the aging time.With the extension of aging time,theα_sprecipitation increases and the strength increases.When theα_sprecipitation reaches a certain amount,the secondary phase tends to become coarser and shorter,but the strength decreases.The ductility of the alloy increases with aging time.At the same solution temperature,when the aging time is extended from 1h to5h,the strong plasticity of the alloy has a synergistic improvement,which is because the alloy precipitates from unevenα_sto uniform and fineα_Sin the matrix.The strength of the alloy after solution at 770℃for 2h and aging at 580℃for 1h is worse than that of the alloy after solution at800℃and aging at 580℃for 1h.This is because the large amount of PFZ produced by the alloy after solution at 770℃and aging at 580℃for 1h is detrimental to the plasticity of the alloy.（2）Ti-6554 alloy exhibits obvious aging hardening phenomenon.After solution at 800℃for 2h,theβsupersaturated solid solution is decomposed and the second phase is precipitated during aging.The aging process of the alloy has experienced typical under-aging,peak aging and over-aging stages,and the aging groups at different temperatures show different aging hardening laws and aging peaks.Under low temperature aging（300℃-450℃）,the hardness of Ti-6554 alloy increases with aging time,and the alloy will not reach the peak aging after 120 h.With the increase of aging temperature,the peak aging time and peak hardness decrease.The precipitation sequence of Ti-6554 alloy at 800℃is as follows:400-450℃:β+α_p+ω_ath→β+α+ω_iso500-550℃:β+α_p+ω_ath→β+α+ω_iso→β+α580-600℃:β+α_p+ω_ath→β+α（3）When the alloy is solution at 800℃for 2h and aged at 400℃for 8h and 48h,there is little difference in the microstructure of the samples,which areα_pphase;However,when the aging time was extended to 120h,a small amount of fine short needle-likeα_sphase appeared in the samples.With the increase of aging temperature,the volume fraction ofα_sphase precipitates increases and the phase appearance becomes shorter and coarser at the same aging time.When the aging time is extended to 12h at 550℃,needle-likeα_sphase precipitates inside the alloy.When the aging time is extended to 48h,moreα_sphase is generated and its size increases.Compared with the samples aged at 400℃and 500℃,the precipitation time ofα_sphase at580℃is croaser and the appearance ofα_sphase is shorter,indicating that higher aging temperature can promote the precipitation ofα_sphase.（4）Aging at 400°C-550°C will cause the precipitation of theωphase,resulting in an increase in the strength of the alloy and a decrease in plasticity.And the relationship ofω,αandβphase to is（0001）ω//（11^<sub>20）α//（11^<sub>1）β,（0^<sub>110）ω//（1^<sub>100）α//（112）β,（11^<sub>20）ω//[0001]α//[1^<sub>10]β.（5）The alloy with large volume fraction ofα_pand small volume fraction ofα_sphase has good plasticity,severe deformation and good toughness during fracture.However,the fineα_sphase cannot prevent the crack propagation,so the crack propagation path is straight,the energy loss is low,and the toughness is poor.Therefore,the increase of crack propagation path zigzag and good plasticity are beneficial to increase the K_ICvalue.（6）βgrain orientation affects the propagation of cracks,which tend to slip on slip systems with higher Schmidt factors.Crack deflection inside the grain may be attributed to coarseα_s.There are 47 figures,7 tables and 116 references...