Hot Working And Discontinuous Precipitation Behavior Of Ti-42Al-3Nb-1Mo-0.1B Alloy

Beta-gamma TiAl alloy has fine and uniform microstructure,excellent high temperature deformation ability and excellent machinability,which widens the processing window of TiAl alloy.However,the high temperature stability of beta gamma TiAl alloy decreases with the addition of a large number of stable elements.Under the condition of long-term thermal exposure,?₂/?lamellar structure will be discontinuous precipitation,which will lead to the destruction and decomposition of?₂/?lamellar structure,and deteriorate the mechanical properties of the alloy,which is not conducive to the large-scale engineering application of the alloy.Therefore,it is of great theoretical significance to systematically study the discontinuous precipitation behavior of lamellar structure for improving the comprehensive properties of alloys.In this paper,Ti-42Al-3Nb-1Mo-0.1B(at.%)alloy was prepared by vacuum consumable melting,and the alloy forging billet was prepared by forging process.The discontinuous precipitation behavior of as forged alloy during heat treatment was systematically studied,and the mechanical properties were improved by adjusting the microstructure.The solidification path of Ti-42Al-3Nb-1Mo-0.1B alloy is L?L+?????+?????+???+?+???₂+?/B2+?.After isothermal forging at 1260?and cooling forging at 1260?-1220?,the microstructure of the alloy is near lamellar structure,and the(?/B2+?)mixed structure distributes along the boundary of?₂/?lamellar crystal cluster,and shows obvious forging streamline.The burgers orientation relationship between the residual?/B2 and the adjacent?₂ phase at the lamellar grain boundary is destroyed during the forging process.The room temperature tensile strength of the as cast alloy is 659 MPa,and the room temperature elongation is very low,only 0.18%.After isothermal forging,the room temperature tensile strength of the alloy is significantly improved,reaching 886 MPa and 898 MPa,respectively.The high temperature tensile properties of the alloy forged at low temperature are generally better than those of the alloy forged at constant temperature.When the temperature is lower than the ductile brittle transition temperature T_BD,the tensile fracture morphology of the near lamellar structure shows the characteristics of lamellar and cleavage fracture.When the crack propagation direction is parallel to the lamellar interface or has a small inclination angle,the crack will propagate along the lamellar interface,and the lamellar and cleavage fracture will occur in the lamellar crystal cluster.When the temperature is higher than T_BD,ductile fracture dimples and tearing edges gradually appear at the tensile fracture.After crack initiation,it is not easy to expand outward,but will be passivated and gradually form micropores.Micropores are preferentially formed around the?/B2 phase.During the deformation process,a large number of micropores are connected and micropore aggregation fracture occurs.The microstructure of forged Ti-42Al-3Nb-1Mo-0.1B alloy after 950?/6 h/FC heat treatment has little change,and it is still composed of?₂/?lamellar clusters and?+?/B2 mixed structure at lamellar cluster grain boundary.As the temperature increases to 1000?,the shape of forging streamline disappears.When the temperature is 1100?,the microstructure changes from near lamellar structure to two-state structure,and there is obvious discontinuous precipitation phenomenon.Fine needle like?/B2 phase is precipitated continuously in?₂lamellar structure.Meanwhile,the lamellar structure undergoes cellular reaction(?₂+?)_L?(?+?)_E.The?₂/?lamellar structure is decomposed and equiaxed?and?/B2 grains are precipitated.Due to the bending and twisting of?₂/?lamellar structure during deformation,local stress concentration occurs.The stacking of dislocations at the stress concentration will continue to erode the?₂ lamellae,resulting in the local depression of the lamellae interface and the gradual precipitation of equiaxed?phase from?₂ phase.At the same time,a local enrichment area of Mo and Nb elements will appear around the precipitated?phase,which promotes the?grains to precipitate successively.The fully lamellar heat treatment process of forged Ti-42Al-3Nb-1Mo-0.1B alloy is1100?/6 h/FC+1300?/4 min/FC.After whole lamellar heat treatment,the grain size of lamellar clusters is relatively small,and the average grain size is about 56?m.The yield strength and tensile strength reach 703 MPa and 780 MPa respectively,while the elongation is only1.25%.With the increase of temperature to 850?,the strength of the sample decreases and the plasticity increases significantly.The yield strength and tensile strength decrease to 566MPa and 552 MPa respectively,and the elongation increases to 23.7%.When the whole lamellar structure is stretched below T_BD temperature,the whole lamellar structure starts to crack at the interface or grain boundary,and forms"Z"crack feature.When the deformation temperature is higher than T_BD,the ductile brittle transition occurs.The cracks germinate at the grain boundary and lamellar interface,and are easily passivated to form micropores during cleavage propagation,which hinders the crack propagation and improves the plasticity and toughness of the alloy.