| High strength ?-titanium(?-Ti)alloy has good plastic processing performance and specific strength,so it is widely used in aerospace and automobile industry.With the rapid development of industry,high-strength ?-Ti alloy is required to have better matching of mechanical properties,so it is very important to improve the mechanical properties of ?-Ti alloys.The effects of duplex aging technique on the microstructure evolution and mechanical properties of the forged high-strength ?-Ti alloy Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe were investigated.Then summarized the strengthening mechanism of the alloy in different duplex aging.A systematic study on the microstructure evolution and mechanical properties of the alloy after low-temperature pre-aging,duplex aging in ascending order and duplex aging in descending order had been carried respectively.The effect of low-temperature(300?,350? and 400?)pre-aging on the microstructure evolution and mechanical properties of the alloy had been analyzed during the duplex aging in ascending order.The grain size of the alloy is about 200?m after ? solution treated.It was found that at the initial stage of low-temperature pre-aging,the ? phase and isothermal ? phase began to precipitate in the matrix.As the pre-aging time increased,the isothermal ? phase disappeared gradually,and the finally microstructure consists of acicular secondary ? and ? phase of the alloy.On the other hand,the strength of the alloy has few change when pre-aged at 300°C and 350°C.When pre-aged at 400°C,the strength of the alloy increased significantly with the prolongation of the pre-aging time,and the strength could reach to 1400 MPa when pre-aged for 2 hours,but the elongation sacrificed seriously.The effect of duplex aging in ascending order on the microstructure evolution and mechanical properties of ?-Ti alloy was studied.The alloy was pre-aged at low-temperatures firstly and then re-aged at a higher temperature.Compared with the single aging,it was found that the size of the secondary ? phase could be effectively refined and the volume fraction of the ? phase can be increased by duplex aging in ascending order.The duplex aging in ascending order can significantly increased the strength of the alloy.The strength of the alloy increased with the increasing of the pre-aging temperature and time,and it could be reached up to 1850 MPa,but the plasticity loss seriously.In addtion,by comparing the age hardening behavior of single aging condition,it is found that the hardness of the alloy was increased in the duplex aging in ascending order and the hardening rate parameters was accelerated.The duplex aging in ascending order can effectively increased the strength of the alloy,but the plasticity decreased obviously,so a duplex aging in descending order method was proposed for this problem.The effect of the duplex aging in descending order on the microstructure evolution and mechanical properties of the alloy was explored and studied.The alloy was pre-aged at higher temperature of 550? firstly and then re-aged at the lowtemperature of 350? and 400? respectively.The results shown that the microstructure of the duplex aging in descending order detected by scanning electron microscope has no significantly difference compare to the single aging.The XRD analysis found that the crystal lattice shrinked with increased the re-aging time under duplex aging in descending order conditions,and this phenomenon was more obvious at 400°C.It was found that the existence of interface ? and penetrating ? phase detected by transmission electron microscopy during duplex aging in descending order.Compared to single aging,the strength could be improved by duplex aging in descending order.With the aging condition of 550?/4h+400?/24 h,the ulmate strength of the alloy was increased to 1606 MPa with the elongation of 6.06%.The ability of strengthening by the duplex aging in descending order is slightly lower than duplex aging in ascending order,but the alloy has better plasticity.The analysis of hardening behavior during duplex aging in descending order shown that the hardness of the alloy gradually increased with the increasing of re-aging temperature and prolonging of re-aging time. |
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