Study On The Microstructure And Properties Of Gamma TiAl And Beta/gamma TiAl By Cold Crucible Directional Solidification

TiAl-based alloys,as a new generation of high-temperature structural materials in the aerospace and automotive industries,have received significant attention due to their attractive properties,such as light weight,high strength and elevated temperature performances.However,as a kind of intermetallic compound,TiAl-based alloys have inherent brittleness,poor plasticity,low fracture toughness and thermal processing difficulties always restrict the further development of TiAl alloys.However,the performance of TiAl-based alloys strongly depends on the?₂/?lamellar orientation in the microstructure.However,it is difficult to achieve accurate control of the orientation of the alloy lamellar by the existing techniques.In this paper,Ti-45Al-2Cr-2Nb?at.%?and Ti-43.5Al-6V-1Cr?at.%?alloy directional solidification billet ingots with different process parameters were prepared by electromagnetic cold heading directional solidification technology.The effect of process parameters on the solidification structure and properties of the two alloys was investigated.Ti-45Al-2Cr-2Nb?at.%?alloy billet specimens were firstly prepared by changing the withdrawing rate.The surface quality and macro-microstructure of the specimens were observed,and their room temperature tensile and fracture toughness were tested.The results show that when the withdrawing rate is 0.2mm/min,the directional solidification specimens with good surface quality and orientation effect can be obtained.As the withdrawing rate increases,the surface quality and orientation effects gradually deteriorate.After observing the microstructure of the directional growth zone,it was found that the matrix of the directional solidification sample at different drawing rates was?₂/?lamellar structure,on the?₂/?lamellar substrate A small amount of B2 phase and a small amount of bulk?single phase are distributed.The elongation and fracture toughness of the specimen after directional solidification are higher than that of the original ingot,and the tensile strength is not much different from that of the original ingot.When the drawing rate is 1.6mm/min,the maximum tensile strength is 347 MPa,the drawing rate is 0.2 mm/min,the elongation reaches the maximum value 0.55%,and the fracture toughness reaches the maximum value 26.1 MPa·m^1/2when the drawing rate is1.0mm/min.Ti-43.5Al-6V-1Cr alloy prepared by cold crucible directional solidification has good surface quality.Observing the cross-section morphology of directional solidification specimens under different process parameters,it was found that the directional solidification specimens with better surface quality and orientation effect can be obtained with higher power and slower pull-out rates.The shape of the solid-liquid interface and the growth direction of the columnar crystals are greatly affected by the process parameters.When the withdrawing rate is constant and the heating power increases,the degree of depression of the solid-liquid interface decreases,and the angle between the growth direction of the columnar crystal and the longitudinal direction decreases.When the input power is constant and the withdrawing rate increases,the bottom of the solid-liquid interface of the billet samples gradually changes from a nearly flat interface to a concave one,and the included angle between the columnar crystal growth direction and the longitudinal direction increases.The Vickers hardness value and compressive strength are all higher than that of the original ingots.The tensile strength and fracture toughness values are different from the original ingots.As the input power increases,the compressive strength of the alloy gradually decreases.The tensile strength and fracture toughness are affected by the distance between the?2/?plate and the B2 phase.Under the combined action of the two,the tensile strength first increases and then decreases,and the fracture toughness gradually increases.When the drawing rate increases,the fracture toughness and Vickers hardness gradually increase,and the tensile strength and compressive strength increase first and then decrease.The three-point bending and tensile test fractures of the two component alloys were analyzed.It was found that the fracture modes of the alloys were mainly along the fractures of the sheet layer and the translaminate layer,and the fracture surface in the fracture layer often had a fractured step.The pattern characteristics of the river can be clearly observed in the fracture morphology of Ti-43.5Al-6V-1Cr alloy,indicating that the fracture mode of TiAl alloy is a typical brittle cleavage fracture.