The Impact Of Heat Treatment On Microstructure Transformation And Mechanical Properties Of Casting TiAl Alloy

As a new type of lightweight high temperature materials,gamma-based TiAl alloy's density is only half of the nickel-based alloy.Gamma-based TiAl alloy also has high specific strength,high stiffness,good corrosion resistance and wear resistance.It can replace nickel-based alloy materials to make some aerospace structure components that will lead to the decrease of weight.In this paper,we choose the second generation TiAl alloy Ti-48Al-2Cr-2Nb(4822)as the canditate to investigate the effects of different heat treatment temperature,heat treatment time and types of cooling on the transformation of the alloy and conclude the regulation of microstructure transformation.The properoties of room temperature tensile and fracture toughness were tested.The main research results and innovation are as follows:1 Finding that the microstructure type of casting TiAl alloy is extremely sensitive to heat treatment temperature and cooling rate.It is easy to get basket-weave microstructure with fast cooling rate,but the volume fraction of lamellar microstructure is low.As the cooling rate reduces,lamellar microstructure's volume fraction increases and the basket-weave microstructure transforms into near lamellar microstructure.The initial microstructure will transform into basket-weave microstructure,then near lamellar microstructure and finally the fully lamellar microstructure.2 Testing the TiAl alloy's fracture toughness of different microstructure:fine fully lamellar microstructure,small basket-weave microstructure,small near lamellar microstructure,massive fully lamellar microstructure,small duplex microstructure and small near gamma microstructure.Their fracture toughness Kic value are 15.4 MPa-m1/2,12.81 MPa·m1/2,11.32 MPa·m1/2,10.5 MPa·m1/2,8.32 MPa·m1/2 and 8.1 MPa·m1/2.3 The structure of TiAl alloy has an obvious influence on crack growth.The lamellar microstructure's ability to resist crack growth is higher than the gamma single phase grain.The grain boundaries of fine fully lamellar have high binding force.The lamellae of small basket-weave microstructure make up a net,which small gamma single phase grains are in.This will make small basket-weave microstructure's crack growth resistence higher than small near lamellar microstructure.There are some lamellae in duplex microstructure that makes duplex microstructure's crack growth resistance higher than near gamma microstructure.