Research On Forging And Microstructure And Mechanical Properties Of TiAl Alloy Containing β/B2Phases

TiAl alloys are very promising structural materials for making structures andparts working at elevated temperatures in aerospace field, due to their low density,high specific strength, good high-temperature burning resistance and oxidationresistance, excellent creep and fatigue properties. However, the poor hotdeformability, low room-temperature ductility and fracture toughness limit theirpractical engineering applications. Beta-gamma TiAl alloys are deemed as a newgroup of TiAl alloys that attract scientists strong research interests because of theirexcellent hot deformability and wide hot processing condition window. In this paper,two types of beta-gamma TiAl alloys with nominal compositions of Ti-43Al-9V-Yand Ti-43Al-5V-4Nb （at.%） were prepared and then undergone high temperatureforging. Microstructure and mechanical properties of the two TiAl alloys wereinvestigated systematically.A large size Ti-43Al-9V-Y ingot （Ф220mm×800mm） was prepared by vacuumconsumable electrode arc furnace remelting （VAR）. As-cast Ti-43Al-9V-Y alloyconsist of massive γ phase and β/B2phase, as well as a few α₂and YAl2phase. Alarge size Ti-43Al-9V-Y alloy pancake （Ф500mm×46mm） with a total deformationstrain of80.8%was prepared using specially canned forging technology carried outat1175°C and at a strain rate of0.01s^-1. The as-forged Ti-43Al-9V-Y alloy showeduniform duplex microstructure consisting of γ/（β/B2）/α₂lamellar crystal clusterswith the size of approximately30⁵0μm, and block-like γ phase and β/B2phaseprecipitating at the boundaries of the lamellar colonies. Different microstructures ofTi-43Al-9V-Y alloy were obtained by different heat treatment techniques. These γand β/B2phases gradually transformed into γ/（β/B2）/α₂lamellar clusters withincreasing of heat treatment temperature and duration. A fullly γ/（β/B2）/α₂lamellarcluster microstructure was obtained when annealing at1350°C/8h.Tensile properties of Ti-43Al-9V-Y alloy with different processing states weretested. The as-forged Ti-43Al-9V-Y alloy exhibits excellent room temperaturetensile properties, with an ultimate tensile strength of863MPa, yield strength of698MPa and an elongation of2.0%. The ultimate tensile strength and yield strengthincrease while the elongation decrease with increasing of the test temperature. At700°C, the ultimate tensile strength and yield strength are693MPa and589MPa,respectively, and the elongation reaches12.0%. After heat treatment of1350°C/8h,Ti-43Al-9V-Y alloy with fully-lamellar microstructure has an ultimate tensilestrength of863MPa, yield strength of698MPa and elongation of1.5%at roomtemperature. As the tensile temperature increases to700°C, the ultimate tensilestrength and yield strength decrease to571MPa and539MPa respectively, while the elongation increases to2.0%. By means of the three-point bending tests, it is foundthat the as forged Ti-43Al-9V-Y alloy with duplex microstructure shows the bestfracture toughness, its KICvalue is high up to22.5MPa·m^1/2, while forfully-lamellar microstructure after1350°C/8h heat treatment, its KICvalue is21.2MPa·m^1/2. In situ tensile test was performed on as-forged Ti-43Al-9V-Y alloy. Theresults show that cracks deflect when meeting β/B2phase during propagation in thisalloy. After the alloy was heat-treated at1350°C for8h, γ/（β/B2）/α₂fully-lamellarmicrostructure was obtained. It is found that when the angle between the lamellarlayer interface and the crack is small, the crack deflects towards the direction oflayer interface and propagates along layer interface, resulting in interlayerdelamination. When a crack propagates to a layer interface which has a relativelylarge angle with the crack, the crack may propagate across the lamellae or along thelamellae interface, depending on β/B2lamellar size. If the size of β/B2lamellae issmall, fracture across the lamellar will occur; if the size is large, fracture along thelamellae interface will happen. Nano-hardness test shows that the nano-hardness ofγ phase, γ/（β/B2） lamina and β/B2phase is4.44GPa,4.78GPa and5.25GPa,respectively, in as-forged Ti-43Al-9V-Y alloy. The hardness of γ phase is close to thatof β/B2phase.High-cycle fatigue tests show that the fatigue strength of as-forgedTi-43Al-9V-Y alloy is481.9MPa and415.3MPa at700°C and750°C, respectively.After1350°C/8h heat treatment, the fatigue strength at700°C and750°C decreasesto437.1MPa and392.4MPa, respectively. Fatigue fracture morphology analysisshows that fatigue crack nucleates at the internal shrinkage pores, or inclusions.Fatigue crack propagation region has fatigue crack, whereas fatigue instantaneousfault zone has the micro characteristics of the static tensile fracture. Hightemperature creep performance test results show that the creep time of as-forgedTi-43Al-9V-Y alloy at700°C is371h,182h and74h, corresponding to stress of200MPa,200MPa and250MPa, respectively. At700°C, when the stress is200MPato250MPa, the stress constant is3.0088. When the stress is in the range of250MPa to300MPa, the stress index is3.943at this temperature. Creep activationenergy is26.68kJ·mol-1under the stress of250MPa. High temperature creepdeformation mechanism is dislocation glide and grain boundary sliding.The rapid sintering of elemental powders and forging technology without packwere studied systematically. In addition, as strain rate is about0.01s^-1, step forgingdeformation of30%and60%at1300°C was employed to fabricate Ti-43Al-5V-4Nb（at.%） alloy forging stock with density of98.9%. This forging stock is mainlycomposed of γ, α₂, β/B2phases and not fully diffused Nb. Test results show that thetensile strength and elongation of Ti-43Al-5V-4Nb alloy forging stock at roomtemperature are442.96MPa and0.27%, respectively. Many kinds of heat treatmentprocesses were performed on Ti-43Al-5V-4Nb alloy forging stock and different microstructures were obtained.1300°C/1h heat treatment leads to the formation ofγ/α₂fully-lamellar microstructure. The tensile strength of535MPa and elongationof0.52%in the alloy with fully-lamellar microstructure are obtained. With theincrease of test temperature, tensile strength declines and elongation increases. At700°C, tensile strength and elongation are582MPa and11.00%, respectively.