The Influence Of Annealing Temperatures And Cooling Rates On The Transformation Microstructural And Textural Characteristics Of Ti-6Al-4V Sheet

In this work,multiple characterization and analysis techniques including X-ray diffraction,electron backscatter diffraction,electron channeling contrast imaging and energy-dispersive spectrometry were jointly utilized to comprehensively investigate microstructural and textural characteristics of a hot-rolled Ti-6Al-4V(TC4)sheet after annealing in typical dual-phase(α+β)andβfields.A systematic investigation was conducted on the correlation between both microstructural and textural characteristics as well as mechanical properties of TC4 alloy under various cooling conditions.In addition,the variant selection and growth process of TC4 alloy during phase transformation were also further analyzed.Main conclusions are obtained as follows:(1)The as-received hot-rolled TC4 sheet has a typical dual-phase(α+β)microstructure,with theα-Ti as the major phase and short-rodβ-Ti(minority)uniformly distributed throughout theαmatrix.Most ofαgrains correspond to the un-recrystallized structures(dense LABs in their interiors)with a typical rolling texture(c//TD and<11–20>//ND).The denser LABs in the material suggest a relatively high dislocation density and stored energy introduced by the prior hot rolling.Besides,the rotation axes of LABs strongly concentrate on the<0001>direction,which could be largely attributed to preferential activation of prismatic<a>slip in hcp materials.(2)After annealing in theα+βregion,both the WC and AC specimens possess a mixed microstructure of primary bulkα(α_p)grains and transformed secondaryα(α_s)plates,and some untransformed residualβthin films(V-enriched)exist in the AC specimen.Plate structures are absent in the case of slow furnace cooling,with coarser equiaxedα(α_p)grains and residualβphase presented.For all the cooled specimens,the major textural feature is quite similar to that of the as-received material.After annealing at 880℃,the maximum texture intensity of the WC specimen is increased slightly compared with the as-received material but it is gradually decreased as along with intensified recrystallization textural component(90°,30°,0°)with decreasing cooling rates.Nevertheless,the maximum texture intensity of TC4 sheet annealed at 960℃ is increased with decreasing cooling rates,which should be attributed to(α_p)grain growth and(α_s)variant selection behaviors during theβ→αtransformation.For rapid cooling in water,a single high-temperatureβphase could generate twelve differentα_s variants by martensitic transformation and there appears a new textural component,leading to the globally weakened texture.(3)In regards to TC4 alloy heat-treated at 880℃,hardness values drop from 338.0HV for water cooling to 319.0 HV for furnace cooling,lower than that(366.1 HV)of the as-received specimen,which could be mainly attributed to the occurrence of recrystallization resulting in grain growth and reduced LABs.The hardening effect from the fine martensitic plates and residual thinβfilms is relatively limited.Noticeably,only the hardness value(379.7 HV)of the WC specimen annealed at 960℃ is higher than that of the as-received materials,which can be ascribed to combined contribution from grain refinement and solid solution of alloying elements during martensitic transformation.It is evident that hardness values are reduced from 342.5 HV for AC specimen to 327.2 HV for FC specimen with decreasing cooling rates,suggesting that increased grain sizes can account for softer hardness.(4)Theβ-cooled TC4 specimens in water,air and furnace exhibit twinned martensite,basket-weave and parallel-plate Widmanst?tten structures,respectively.The classical Burgers orientation relationship is obeyed during theβ→αtransformation at all cooling rates.After phase transformation,majorαtextural components((0°,60°,0°),(90°,30°,30°)and(90°,90°,30°))are completely different from those of the as-received material((0°,90°,0°)and(90°,30°,0°)).Slow cooling facilitates preferable nucleation ofαphases atβboundaries and strengthensα-variant selection,leading to greatly intensified transformation textures(the maximum density from 9.5 to 31.1 times of random).Rapid cooling weakens the transformation texture due to suppressedα-variant selection through easier nucleation insideβgrains.Furthermore,hardness values of theβ-cooled specimens drop from 416.8 HV for water cooling to 329.2 HV for furnace cooling.Quantitative analyses suggest that such hardness variation can be more related to differences in grain size between them,as compared to specific contributions from other microstructural and textural characteristics.