Microstructure Evolution Behavior And Mechanism Of Ti₂AlNb-based Alloy Processed By Multi-directional Forging

With the continuous development of aerospace industry,high-temperature structural materials with high specific strength are increasingly used in modern aerospace equipmen.Ti₂AlNb-based alloys have high specific strength,good high-temperature tensile and fatigue strength,and become the most potential lightweight high temperature structural materials which can be used at 650?800?.However,due to the intrinsic brittleness of Ti₂AlNb-based alloys,hot working deformation is needed to improve the structure and properties of the alloy.At present,hot working in three-phase zone and above temperature zone is mostly used to improve the initial structure of the alloy,and it always requires heat treatment assistance.This processing method is costly,and the phase transformation involved in the process is complex and difficult to control steadily.There are few studies on the hot working process of Ti₂AlNb-based alloys under large deformation,especially the severe plastic deformation.The preparation of ultrafine grains by severe plastic deformation is of great significance and research value for improving the strength-plasticity matching at room temperature and the high temperature properties of the alloys.On the other hand,the B2+O two-phase structure is considered to have the best comprehensive properties.Therefore,it is important to explore the severe plastic deformation of Ti₂AlNb-based alloys in the low temperature(B2+O)phase region and the effect of deformation on the microstructures evolution.In order to obtain the strain accumulation rule of Ti₂AlNb-based alloy multi-directional forging in B2+O phase and explore the reasonable deformation parameter interval,the deformation process at 800?was simulated by finite element method.The effective strain distribution of Ti₂AlNb-based alloy under unidirectional compressing has obvious geometric symmetry.The cumulative effective strain of Ti₂AlNb multi-direction forging is strictly linearly related to the number of compression times.The larger the unidirectional compression strain is,the faster the eqffective strain accumulates.With the increase of forging cycles,the dispersion of effective strain increases,and the spatial distribution of effective strain tends to be non-uniform.The most suitable deformation parameters are:strain rate 0.02s^-1,single step reduction 40%,deformation cycles 3.The microstructural evolution of Ti₂AlNb-based alloys before and after deformation was studied,including morphology,grain boundary orientation difference,phase evolution,refinement of deformation substructure and grain boundary evolution.The results show that the grains of Ti₂AlNb-based alloys are remarkably refined after multi-directional forging,but with the increase of deformation cycles,the grain size decreases rapidly and then tends to be stable.The transition from low-angle grain boundaries to high-angle grain boundaries can be observed clearly.The amount of?₂ phase decreases and O phase increases after multi-directional forging in(B2+O) phase region,and the alloy phase composition is mainly B2+O and some residual?₂ phase.The micro-strain increases sharply after deformation,but its increment decreases gradually with the increase of forging cycles.The dislocation density increases first and then decreases slightly,which is related to the dynamic recrystallization process.The effect of multi-directional forging on the spheroidization of?₂ and O phase slab structure was analyzed,and the spheroidizing mechanism was discussed.It is found that the spheroidizing mechanism of O phase dynamic globularization is mainly shear fracture mechanism.The degree of spheroidization increases and the grain size decreases gradually with the increase of strain,and the microstructure tends to be uniform and finely equiaxed.