The Microstructure,Properties And Hot Deformation Behavior Of Extruded TNM Alloy

TiAl alloy,as a kind of lightweight high-temperature structural material with excellent comprehensive mechanical properties,is expected to partially replace the high-density Ni-based high-temperature alloy.TiAl alloy can be applied to high-temperature structural parts of engines,which makes TiAl alloy of broad application prospects.However,the difficulties of processing and performing have always restrained the manufacture of traditional TiAl alloys.In particular,there are few domestic research reports on the hot processing of beta-gamma TiAl alloys.In this paper,the TNM alloy with better hot workability was used as the raw material,and the extruded bar was heat-treated to analyze the microstructure evolution of the alloy under long-term heat preservation,while the thermal compression was used to analyze the thermal deformation behavior of the alloy,aiming to provide theoretical guidance for the subsequent isothermal forging of extruded TNM alloys.The difference between the structure of the core and the edge area of the bar was analyzed.The edge area is about 450 micro meters,which is comparably thin.The core area is a near-lamella structure,of which laminate size is between 100μm～200μm.The edge area is a dual-state structure,5μm-particle-size gamma crystals and 10～20μm laminate cluster.Two different forms ofγphase in the extruded TNM alloy were analyzed.One is the elongatedγphase after extrusion deformation,the other is the massiveγphase at the boundary.The massiveγphase consists of two forms,which are fine granularγphase precipitated fromβ₀and other one of comparably large size formed by recrystallization.The evolution of the structure after the heat treatment was analyzed.During the heat treatment in the two-phase zone,the massiveγphase around theα₂/γlayer flakes disappeared and the layer content increased.With the increase of temperature and holding time,the size of the flakes increased.Large,the content ofβ₀phase increases.When the heat treatment was performing in the three-phase zone,the alloy structure was a near-lamella structure.With the increase of temperature and holding time,the size of the lamellae did not change significantly,but the content and volume of the massiveγphase around the lamellae increases,while the content ofβ₀phase decreased.During air cooling,the（α₂+γ）needle-like Widmanstatten structure appeared in the alloy,and after heat treatment in the three-phase zone,α₂+γ→α₂+γ+β₀occurred at the boundary of the lamellae,forming a network structure.With the rise of annealing temperature and increase of holding time,the content of the network structure increases.After heat treatment in the two-phase region,a lenticularγphase precipitated in theβ₀phase.The mechanical properties of the TNM alloy in the extruded state and after heat treatment were analyzed.The tensile strength and elongation of the extruded alloy are significantly higher than that of the as-cast state,reaching more than 1 GPa.The tensile strength and elongation at high temperature are better than those of traditional TiAl alloys.Significantly increased,the brittle-ductile transition temperature also decreased significantly,with good high-temperature plasticity.After heat treatment,the hardness ofβ/β₀phase andγphase did not change much.The hardness ofα₂/γlayer cluster decreases with the increase ofγphase content and the coarsening of the layer;The tensile strength and yield has been improved after two heat treatments,though the brittle-ductile transition temperature increased.The thermal deformation behavior of the extruded TNM alloy was analyzed.From a macro point of view,the hot compressed sample is elliptical.The thermal activation energy of the extruded TNM alloy is 419.7387k J/mol.According to the hot working diagram,the alloy instability zone is in the range of 1100～1150℃+0.05～0.1s^-1.With the increase of deformation temperature,the content ofβ/β₀phase decreases;at1100℃/1150℃,the alloy is mainly dynamic recrystallization ofγphase,and at1200℃/1250℃,the alloy mainly consists ofαandβphases.Mainly recrystallization.At 1150°C,as the strain rate decreases,the dynamic recrystallization of theγphase is more sufficient.According to the hot working diagram and the deformation structure analysis,the optimal hot deformation condition of the extruded TNM alloy is1150～1200℃+0.01～0.05s^-1.