Research On The Microstructure And Mechanical Properties Of Ti-44Al-6V-3Nb-Y Alloy Pancake And Sheet

Due to the excellent high temperature properties and low density, TiAl based alloys are attractive for use in aerospace. Understanding the correlations of processing conditions with the resulting microstructures and corresponding mechanical properties is of prime importance to obtain TiAl based alloys with expected service performance. In this study, the microstructures and tensile properties of Ti-44Al-6V-3Nb-Y (at.%) alloy pancake and sheet after canned forging and pack rolling were investigated systematically. All these works will provide theoretical and technical supports for improving the fabrication techniques of TiAl alloy sheets.By means of X-ray diffraction analysis and SEM observation, this TiAl pancake after canned forging exhibits inhomogeneous deformation microstructures, which can be ascribed to the temperature drop and friction between billet and outer pack, as well as the anisotropic plastic flow of lamellar colonies during forging. TEM results indicated that plenty of dynamically recrystallizedγgrains generated during forging, high density of dislocations and several substructures (subgrains and sub-boundary structures) were also observed. According to EBSD measurement, the microstructure in large deformation zone was further characterized. The deformation microstructures are composed of dominant DRX grains ofγ(83%) and B2 (16.1%), with grain size between 1μm and 8.5μm. The microstructure features dominant high angle boundaries (78.8%), with 21.2% content of low-engle boundaries. several deformation substructures and twins are observed as well. Additionally, in the as-forged alloy, the deformation texture is found to be random. After canned forging, The bend strength of current alloy increases from 1058MPa to 1635MPa, the fracture toughness reaches up to 22.4MPa·m1/2.The microstructures and tensile properties of the as-forged alloy after 950℃annealing andαphase domain heat treatment were investigated. After 950℃/6h annealing, the mislocation density and substructures decreased dramatically. And the fully lamellar microstructure was gained after treatment of 1325℃/1h/FC+ 900℃/6h/FC. The tensile strength of FL microstructures at room temperature and 700℃are 633MPa and 690MPa, with elongation of 2.37% and 15.06%, respectively. Both the as-forged and fully lamellar microstructures display yield stress anomaly (YSA), with yield strength increases as temperature raises, which can be mainly ascribed to dislocation cross slip and the resulting dislocation locking.The Ti-44Al-6V-3Nb-Y alloy sheet has 5.5% moreγphase, but less B2 than as-forged alloy. Current TiAl sheet is characterized by dominant dynamically recrystallizated microstructures, except some refined lamellar colonies in the end area, which are primarily caused by the temperature drop during canned forging and subsequent pack rolling. Meanwhile, by EBSD measurement, the DRX grains produced by prior forging are found to grow up apparently. Comparing to the as-forged alloy, TiAl sheet features uniform microstructures except for the B2 phase-enriched surface layers caused by element diffusion during hot rolling. The sheet has weak {100}<010> cube texture. Tensile tests suggested that the as-rolled alloy has lower room temperature tensile strength than as-forged one, with yield strength of 520MPa and ultimately strength of 598MPa. The strength degradation after rolling are primarily caused by the coarsening of both the grains and the lamellar interspacing, as well as the elimination of dislocation and substructures. Different microstructures of the as-rolling alloy, duplex structure (DP), nearly lamellar structure (NL), fully lamellar structure (FL), were attained using different heat treatment technology respectively. The as-rolled alloy was found to have the highest compression strength and microhardness.