Influence Of Multi-stage Heat Treatment On Microstructure And Strengthing-toughning Of TC21 Titanium Alloy

As a high strength and high toughness titanium alloy with good comprehensive properties,TC21 alloy is suitable for manufacturing various structural forgings and parts.Different morphology,grain size and intragranular structure in titanium alloy microstructure can determine the difference of its mechanical properties,therefore,effective microstructure control of TC21 alloy is an important means to improve mechanical properties.The current researches of TC21 alloy mainly focus on a single heat treatment,such as changing the cooling mode or simple solution+aging treatment,hence the quantity and morphology of the equiaxed?phase,lamellar?phase and dispersed?phase cannot be simultaneously controlled.There are few studies concerning the control of the morphology and quantity of?phase by multi-stage heat treatments,whereas the morphology and quantity of?phase play a crucial role in the performance of duplex titanium alloys.In this paper,multi-stage heat treatment experiments of TC21 titanium alloy were subjected to in?and?+?phase,respectively,to regulate the number and morphology of equiaxed?phase,lamellar?phase and dispersed phase,so as to study the effect of each heat treatment on the microstructure and mechanical properties.At last,the deformation behavior of the TC21 alloy was further studied by in-situ tensile.The slip band,crack initiation and propagation of?morphology?equiaxed/lamellar?with different volume fractions were observed in real time.The following main conclusions are drawn:1.When the TC21 alloy was carried out multi-stage heat treatments in?phase region,the first-stage treatment mainly regulated the morphology and size of the lamellar?phase;The second-stage treatment mainly regulated the quantity of secondary?phase,the tensile strength of the alloy increased,whereas the elongation decreased;The third-stage aging treatment mainly regulated the quantity of dispersed?phase,the tensile strength increased,and the elongation and impact toughness decreased.In a word,after three-stage treatment,the furnace cooling?FC?samples presentd better comprehensive properties.2.When the TC21 alloy was carried out multi-stage heat treatments in?+?phase region,the first-stage treatment mainly regulated the quantity and the thickness of equiaxed?phase,the quantity of equiaxed phase gradually decreased with the increase of cooling rate,the yield strength and tensile strength were not high,whereas the elongation and impact toughness were better;The second-stage treatment mainly regulated the quantity and the thickness of?lamelle,the tensile strength,yield strength increased and the elongation decreased;The third-stage aging treatment mainly regulated the morphology and quantity of dispersed?phase precipitated in the?_transphase,the tensile strength and yield strength of FC and air cooling?AC?were better than that of water cooling?WC?.In a word,after third-stage heat treatment,the FC samples presentd better comprehensive properties.3.According to the in-situ observation of different volume fractions of the?morphologies?equiaxed/lamellar?,the slip band was first concentrated in the equiaxed?phase and truncated at the?/?phase boundaries easily,whereas the slip band easily crossed the?lamella in the whole lamellar structure.The microcracks easily appeared in the equiaxed?or?/?phase boundaries.When equiaxed?and lamellar?coexisted,its strength and plasticity were better than those of fully lamellar microstructure.When the ratio of volume fraction of equiaxed?to lamellar?was about 3:4?S2?,the TC21alloy had better comprehensive properties.