Microstructure And High Temperature Mechainical Properties Of Cold Crucible Directionally Solidified Titanium Aluminum Alloy

TiAl alloy has large elastic modulus,good high temperature oxidation resistance,corrosion resistance and good high temperature creep resistance.Compared with Ni-based superalloy,it has higher specific strength and is a kind of aerospace.An alloy with broad application prospects in the aerospace field.The addition of Nb elements to improve the performance of TiAl alloys is a common application.However,the high temperature mechanical properties of sorghum TiAl alloys have yet to be further studied.In this paper,TiAl-based alloys with different compositions were selected,and the solidification experiments were carried out by electromagnetic cold heading technology,including Ti-45Al-7Nb alloy,Ti-45Al-8Nb alloy and Ti-45Al-7Nb alloy with alloying elements Cr and Zr.A small amount of W,Cr,B element Ti-45Al-7Nb alloy,numbered Ti-45Al-7Nb,Ti-45Al-8Nb,Ti-45Al-7Nb(Cr,Zr),Ti-45Al-7Nb(W,Cr,B).The high temperature tensile properties and high temperature fracture toughness of the four alloys under the same process parameters were studied.The Ti-45Al-7Nb alloy was subjected to a high temperature endurance test.Explore the effects of different Nb content,different alloying elements and different microstructures on high temperature mechanical properties.The high temperature tensile test temperature was 700 ?,750 ?,800 ?,and 850 ?,and a tensile test at 900 ? was performed on the Ti-45Al-7Nb alloy.The high temperature fracture toughness test temperatures were700 ° C,750 ° C,800 ° C,and 850 ° C,respectively.From the perspective of the orientation effect,the order of preference is to Ti-45Al-7Nb,Ti-45Al-8Nb,Ti-45Al-7Nb(Cr,Zr),Ti-45Al-7Nb(W,Cr,B).Using the same process parameters,adding different gold elements has a greater impact on the orientation effect.Adding elements such as W,Cr,and B is not easy to orientate.The addition of a small amount of B makes the grain refinement noticeable,but the orientation effect is the worst.The stable growth zone microstructures of the four alloys include a layer of ?2+?,an equiaxed ? phase,and a B2 phase.Properly increasing the Nb content is beneficial to the high temperature tensile mechanical properties of TiAl alloys.When Cr and Zr alloy elements are added to the alloy,the high-temperature tensile properties of the sorghum TiAl alloy are greatly improved,and the temperature can be increased by 200 MPa or more at the same temperature.Compared with the strip-shaped B2 phase,B2 distributed at the interface of the lamellar layer is effective for improving the high-temperature tensile properties;in thecase where the number of B2 phases is similar,the bulk B2 phase is favorable for the lamellar group at high temperature.Coordinated deformation between them,so as to obtain higher strength and plasticity;under conditions perpendicular to normal stress,the B2 phase exhibiting a band distribution will have a higher high temperature fracture toughness value than the bulk B2 phase.In the Ti-45Al-7Nb alloy with obvious directional solidification effect,the KIC value of the slit parallel to the solidification direction is higher than the KIC value perpendicular to the solidification direction.It is shown that the length direction of the crystal has a higher resistance to crack growth.The extrapolation method was used to predict the permanent strength limit of the directionally solidified Ti-45Al-7Nb alloy at 700 °C,and the empirical equation was obtained.The fracture morphology of the long-term specimen with low stress at 700 °C has the existence of dimples.The sample showed ductile fracture;however,there was no dimple in the 700 °C high stress short-term sample.Using the obtained Larso-n-Miller model,the durability of Ti-45Al-7Nb alloy at 800 °C was predicted,and the prediction results were accurate.