Research On Microstructrues And Mechanical Properties Of Ti-22.5Al-xNb Alloys

Different Ti₂AlNb ingots were prepared by Non-consumable vacuum arc melting furnace. Chemical compositions of ingots were detected by XRF. Microstructrues observation of homogenized, heat treated, high temperature oxidized samples was carried out by OM, XRD, SEM and EPMA methods. Hardness tests were performed on a HV-1000Z Vickers hardness tester. Room temperature compression properties were measured using an Instron-5569 testing machine. The high temperature oxidation resistance of the alloys was evaluated respectively. Results were presented as follow:The microstructures of different positions of the ingot were different, they were dendrite or equiaxed grains, but the phases of homogenized were (B2+O+α₂). The mechanical properties of homogenized Ti-22.5Al-xNb alloys showed that if x≤25 the Vickers hardness, yield strength, compressive strength, maximum compressive strain increased with x increasing; The Room temperature compression strength and maximum compressive strain of Ti-22.5Al-25Nb alloy were 2240.51MPa, 39.92%, respectively. If x>25, the Vickers hardness, compressive strength, maximum compressive strain were all lower than those of Ti-22.5Al-25Nb alloy. The mechanical properties of Ti-22.5Al-29Nb alloy were better than that of Ti-22.5Al-27Nb alloy. The compression fractures of Ti-22.5Al-xNb alloys were mainly cleavage fracture.Heat treatment showed that when the alloys were water quenched at 1050℃the phases were all (B2+α₂+O). The appearance of the O phase was that the cooling rate was quickly in the air. When quenched at 990℃, the alloys were also B2+α₂ +O, but the O-phase diffraction peak increased and the peak intensity was significantly enhanced. When it was 760℃, the alloy is composed of a large number of O-phase and B2 phase but a small amount ofα₂ phase. The left ofα₂ phases were due to the shorter holding time which made the transformation ofα₂ phases could not be completed. The mechanical properties of Ti-22.5Al-29Nb after three different heat treatment revealed that with the decreasing quenching temperature,the hardness, compression strength and the maximum compressive strain increased. This is mainly affected by the phase composition and phase content.Through the analysis of oxidation dynamic curves, surface morphology , phase composition and the thickness of the oxidized films, it was obvious that the law of Ti-22.5Al-xNb alloys was the same when oxidized at 700℃and at 800℃. With the increasing of Nb, the oxidation resistance of the alloys decreased. In the high Nb content of Ti-22.5Al-xNb alloys,Nb⁺⁵ could provide excess quasi-free electrons and even could form Nb oxides which made the surface oxides could not protect the alloys. With the increase of oxygen activity,the oxygen diffusion capacity increased. All of these made the oxidation resistance of Ti-22.5Al-xNb alloys decreased.