Hot Deformation Behavior And Microstructure And Mechanical Properties Of Ti-44Al-4Nb-4V-0.3Mo-Y Alloy

TiAl based alloy is one of the most promising high-temperature light structural materials used in aerospace and automobile industry,due to its low density,high strength,superior oxidation and creep resistant properties.Beta gamma TiAl alloy has proved to possess good hot-working ability.In this paper,five button ingot with a nominal composition of Ti-44Al-(0,4Nb,4V,2Mo,4Mo)respectively were prepared by non-consumable vacuum arc melting and the influence of Nb,V,Mo on the microstructure of Ti Al alloy were studied.According to this,a beta gamma TiAl alloy ingot with a nominal composition of Ti-44Al-4Nb-4V-0.3Mo-Y was fabricated by induction skull melting,then,microstructure,properties,hot deformation behavior and forging technology of this alloy was systematically investigated.The ? phase stabilizing ability of Nb,V,Mo is Mo>V>Nb.No B2 phase formed when 4at% of Nb were added in the Ti-44 Al alloy,but,when 4at% of V or 2at% of Mo were added,some B2 phase was retained and divided the large lamellar colonies into small colonies.When the content of Mo increased to 4at%,a widmannstatten structure formed.The as-cast Ti-44Al-4Nb-4V-0.3Mo-Y alloy has a near lamellar structure,but after HIPing and homogenization annealing,it transformed into a near ? structure.At 850?,which is higher than the ductile-brittle transition temperature of this alloy,the elongation of this alloy increased to 13% rapidly.Ti-44Al-4Nb-4V-0.3Mo-Y alloy possess a good oxidation resistant ability,The mass gain is only 1.92 mg/cm2 after cyclic oxidation at 800? for 80 h.After heat treatment at ?+? two phase region,? phase precipitated from the ?/B2 phase following the Blackburn relationship with ? phase and Burgers relationship with ? phase simultaneously,finally formed coarse near lamellar structure.After heat treatment at ?+?+? three phase region,fine near ? structure,duplex structure and near lamellar structure formed respectively with the increase of the temperature.After heat treatment at ?+? two phase region,most of the ? phase were eliminated and formed a full lamellar structure.Hot deformation behavior and microstructure evolution of Ti-44Al-4Nb-4V-0.3Mo-Y alloy were investigated by isothermal compression test.This alloy has a good hot-working ability,but it still maintains a high strength at service temperature of TiAl alloy.Hot deformation activation energy and constitutive equations were calculated according to the thermal simulation result.There are two typical microstructure after deformation at elevated temperature,and DRX was more sufficient at the strain rate of 0.01s-1.Ti-44Al-4Nb-4V-0.3Mo-Y alloy were forged at different parameter.Fine NL structure with a grain size about 20?m formed after forging at 1250?/0.02s-1,and the room temperature tensile properties of the as-forged alloy increased obviously compared with the as-cast alloy,the elongation and the tensile strength increased to 1.9% and 842 MPa respectively.After forged at 1200?/0.02s-1 or cooling forging,a fine mixed microstructure of ? phase and B2 phase formed,but the room temperature tensile properties of the as-forged alloy were still poor.B2 phase can restrain the expansion of the crack,but,the fracture toughness of as-forged NL structure is superior to that of the as-forged finer ? and B2 mixed structure.