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 equiaxedαphase,lamellarαphase and dispersedαphase cannot be simultaneously controlled.There are few studies concerning the control of the morphology and quantity ofαphase by multi-stage heat treatments,whereas the morphology and quantity ofαphase 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 inβandα+βphase,respectively,to regulate the number and morphology of equiaxedαphase,lamellarαphase 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 ofαmorphology（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 inβphase region,the first-stage treatment mainly regulated the morphology and size of the lamellarαphase;The second-stage treatment mainly regulated the quantity of secondaryαphase,the tensile strength of the alloy increased,whereas the elongation decreased;The third-stage aging treatment mainly regulated the quantity of dispersedαphase,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 inα+βphase region,the first-stage treatment mainly regulated the quantity and the thickness of equiaxedαphase,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 ofαlamelle,the tensile strength,yield strength increased and the elongation decreased;The third-stage aging treatment mainly regulated the morphology and quantity of dispersedαphase precipitated in theβ_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 theαmorphologies（equiaxed/lamellar）,the slip band was first concentrated in the equiaxedαphase and truncated at theα/βphase boundaries easily,whereas the slip band easily crossed theαlamella in the whole lamellar structure.The microcracks easily appeared in the equiaxedαorα/βphase boundaries.When equiaxedαand lamellarαcoexisted,its strength and plasticity were better than those of fully lamellar microstructure.When the ratio of volume fraction of equiaxedαto lamellarαwas about 3:4（S2）,the TC21alloy had better comprehensive properties.