Effect Of Thermomechanical Deformation-induced Microstructure Refinement On Tensile Properties And Impact Toughness Of TC21 Titanium Alloy

TC21 alloy is a new type ofα+βtitanium alloy with high strength developed by China.It has been widely used in key load-bearing parts with high requirements for strength and durability in aerospace field.It is very important to study the dependence between microstructure and tensile properties of TC21 titanium alloy for the safe use of aerospace vehicles.In this paper,TC21 alloy with martensitic structure obtained after quenching was taken as the research object.Tensile properties of TC21 alloy were optimized by the combination of dual-phase region（860°C,900°C and 940°C）hot rolling deformation and medium temperature single-step aging treatment,and the influence law and potential mechanism of microstructure evolution on tensile properties and impact toughness were studied.The experimental data and reference are provided for the production and application of the alloy.Meanwhile,the effect of oxygen entry on the microstructure evolution and mechanical properties of martensitic and bimodal TC21 alloys was clarified.The main conclusions of this paper are as follows:（1）Compared with the tensile properties of TC21 alloy obtained through other processes,the 900°C rolled sample and 520°C aged sample in this work obtain better matching in strength and plasticity,which shows the superiority of using martensite as initial microstructure of the thermomechanical deformation treatment.（2）With the increase of rolling temperature,the volume fraction of primaryα（α_p）phase decreases gradually,the volume fraction ofβtransus(β_trans)microstructure increases significantly.The morphology of secondα（α_s）phase changes from short-rod shape to lamellar shape.The change of precipitate morphology ofα_sphase during high-temperature rolling process depends on the transformation of precipitate position from dislocation to interface.Compared with the rolled sample,the aging treatment promotes the precipitation of more fineα_Sphase,which is attributed to the compositional fluctuations based on the pseudo-spinodal concept,which greatly increases the formation rate ofα_sphase.Meanwhile,with the increasing of aging temperature,the volume fraction ofα_pphase decreases gradually,the volume fraction ofβ_transmicrostructure increases slightly,and theα_sphase becomes more coarser significantly.The coarsening ofα_sphase is attributed to the larger compositional fluctuations in the parentβphase.（3）With the increase of rolling temperature,the strength of the alloy gradually increases,which is attributed to the increase of the totalα_s/βphase interface caused by the increase ofβ_transmicrostructure.Compared with the 860°C and 900°C rolled samples,the higher strength of the aged samples is attributed to the increase ofα_s/βinterface due to the precipitation of more fineα_Sphase.Compared with the 940°C rolled sample,the increased strength of the aged sample is attributed to the precipitation of the short-barα_sphase and less deformation of theβ_transmicrostructure.In the range of 500°C to 600°C,the strength of the alloy decreases with the increase of aging temperature regardless of the hot rolling temperature.The main reason is that the volume fraction ofα_s/βphase interface decreases due to the coarsening ofα_sphase.The strength of the alloy decreases with the increase of aging temperature in the range of 500°C-600°C regardless of the change of hot-rolling temperature.The main reason is that the coarsening ofα_sphase leads to the decrease of the number ofα_sphase in the circle with equivalent diameter,and thus the volume fraction ofα_s/βphase interface decreases.Meanwhile,with the increase of aging temperature,the impact toughness of the alloy decreases gradually.（4）Taking 900°C rolling as an example,the hardness difference betweenα_pphase andβ_transmatrix was obtained by nano-meter hardness tester to determine the elongation.The results show that the hardness difference betweenα_pphase andβ_transmatrix is the smallest in as-received sample,and slightly increases in hot-rolled sample,while the maximum hardness difference is found in 520°C aging sample.The strain compatibility ofα_pphase andβ_transmatrix is the best in as-received sample,followed by the hot rolled sample and the aged sample.Therefore,the as-received sample has the best elongation,followed by the hot rolled sample and the aged sample.In the range of 500°C-600°C,the plasticity of the alloy decreases with increasing aging temperature,which is attributed to the decrease in the number ofα_pphase and coarsening ofα_sphase.（5）The depth of oxygen diffusion layer（ODL）increases first and then decreases with the increase of temperature regardless of the initial state of microstructure.The maximum ODL thickness was obtained at 960°C.This indicates that the abnormal diffusion of oxygen elements occur near the phase transition point,the transformation of crystal structure（α+β→β）is the most possible reason.The cross-section hardness of the alloy from the surface to the matrix gradually decreases when oxygen is absorbed at high temperature,the main reason is that the content of oxygen element gradually decreases.Moreover,the oxygen entry is able to promote the formation ofα_pphase and the precipitation ofα_sphase.（6）The strength and elongation of the 940°C hot rolled samples decreased after oxygen absorption during aging treatment.The strength decrease is attributed to the reduction in hardness due to reduced Mo concentration,which is caused by the entry of oxygen that"squeezed"Mo into the matrix.The decrease of elongation is attributed to the formation of the topmost oxide layer.