Investigation On Microstructure Evolution And Mechanical Properties Of Heat-Treated Ti-47.5Al-3Nb-1.5/3.5Cr Alloy

TiAl alloy has become one of the most promising aerospace materials due to its low density,good high temperature strength,excellent oxidation resistance and creep resistance,and it is considered to be promising to replace Ni-based superalloys in a series of fields.In this paper,two new component alloys(Ti-47.5Al-3Nb-1.5/3.5Cr alloys)are prepared by modifying the Nb and Cr contents of Ti-48Al-2Nb-2Cr(at.%)alloy which is a typical?-Ti Al alloy.Firstly,based on the existing heat treatment work of Ti Al alloy,three different heat treatment processes are selected to systematically study the heat treatment regime in the(?+?)two-phase zone.The microstructure of Ti-47.5Al-3Nb-1.5/3.5Cr alloy is studied by changing the holding temperature,holding time and cooling mode,and the dependence of the mechanical properties on the microstructure of Ti-47.5Al-3Nb-1.5/3.5Cr alloys is investigated.Finally,the phase transformation and lamellar structure formation of Ti-47.5Al-3Nb-1.5/3.5Cr alloys during cooling process at different cooling rates are analyzed by high temperature in-situ observation.The as-cast Ti-47.5Al-3Nb-1.5Cr alloy is mainly composed of lamellar?₂+?,and a small amount of massive?phase is distributed on the boundary of the lamella.After holding for a period of time in the(?+?)two-phase zone,the Nb and Cr elements in Ti-47.5Al-3Nb-1.5r alloy are redistributed uniform,and the interlamellar spacing is refined.Moreover,the microstructure and mechanical properties of the alloy show that the tensile strength and elongation at fracture can be effectively improved by refining the interlamellar spacing and solid solution strengthening.The combined effect of interlamellar spacing refinement and solid solution strengthening in the heat treatment sample obtained by air cooling after holding at 1350?for 60min is the best.Thus,the heat treatment sample has the best mechanical properties,its tensile strength increased from 372.54MPa to 533.86MPa,and the elongation at fracture increased from 0.70%to 1.04%.The as-cast Ti-47.5Al-3Nb-3.5Cr alloy is mainly composed of lamellar?₂+?,and a small amount of massive?and B2 phases are distributed unevenly at the boundary of the lamella.In the heat treatment experiment of Ti-47.5Al-3Nb-3.5Cr alloy,when the heat treatment temperature is close to the?phase solid solution precipitation temperature(T_?-sol),the interlamellar spacing of the alloy is obviously coarsened.However,when the heat treated sample is held at 1400?for 60min and following air cooling,the interlamellar spacing is refined and the size of lamellar colonies is reduced.In addition,uniform B2 phases play a precipitation strengthening role in the matrix.Therefore,the air-cooled sample obtained good tensile strength and elongation at fracture after holding at 1400°C for 60min,which are 359.54MPa and 2.04%,respectively.Compared with the tensile strength of 405.06MPa and elongation at fracture of 0.66%of the as-cast alloy,heat treatment can significantly improve the elongation of the alloy.In addition,the results show that solid solution strengthening is the main reason for the increase of Vickers hardness.After heat treatment,the Vickers hardness of the sample increases from 352.6Hv in the as-cast to 465.2Hv.The phase transformation and lamellar formation of Ti-47.5Al-3Nb-1.5/3.5Cr alloys at different cooling rates are investigated by laser confocal in-situ observation.Different cooling rates effect the lamellar precipitation temperature and interlamellar spacing of the alloy.The higher cooling rate results in the lower lamellar precipitation temperature,which means greater supercooling degree is required to promote the formation of the lamellar.At the same time,higher cooling rate will also lead to smaller grain sizes of?and?phases formed in the cooling process,and the lamellar can be refined in the subsequent solid phase transformation.In addition,the cooling process of Ti-47.5Al-3Nb-3.5Cr alloy is relatively slow at the cooling rate of 6?/min.Thus,the more Cr element in the alloy has more diffusion time at high temperature,resulting in the formation of the steps between?and?lamellar.Subsequently,the?lamellar continues to grow in the thickness direction through the movement of the steps,resulting in excessive interlamellar spacing of the alloy.