| 其他摘要 | The advantage of -TiAl based alloys, in terms of low density, specific high temperature strength, creep resistance and oxidation resistance, makes them as the most attractive candidate materials for use in motor engine of automotive industry. The effects of boron, niobium and oxygen on the microstructure, mechanical properties and phase transformation in TiAl alloys have been studied by means of optical microstcopy (OM), scaning electrolic microscopy (SEM), transparent electrolic microscopy (TEM), x-ray diffraction (XRD), and tensile testing at different temperature.
Boron content has a great effect on the microstructure of the Ti-46.5Al-8Nb alloys, with increasing of the boron content, the grain size decreases and the interlamellar spacing coarsens. The interlamellar spacing () shows linearity relationship with grain size d-1/2. The borides morphologies are ribbon and blocky, with increasing of boron content, the volume fraction of borides increases and the type configuration of borides changes from TiB to TiB2. The effect of boron on the transus temperature can be ignored. Boron increases the start temperature of lamellar formation and reduces the under cooling of lamellar formation, so boron promotes lamellar formation. The tensile properties at room temperature and high temperature are improved when the boron content increases. The room temperature ultimate tensile strength (UTS) shows Hall-Petch dependence on grain size. The trend of the variation of microstructure with different cooling rates is the same in the alloys with and without boron addition varying from massive gamma (m) to lamellar, but the boron addition increases the critical cooling rate to obtain these microstructures. Boron can promote the to m transformation during water quenching (WQ), while oil quenching (OQ) boron can promote the to lamellar transformation. Boron can suppress the to w (Widmanstätten lamellar microstructure) transformation during OQ and air cooling (AC). The microstructures of Ti-46.5Al-8Nb after being heat treated at 1340℃, 1300℃ and 1250℃ for two hours and water quenched were complex microstructures(lamellar, m, ordering 2, and equiax ), complex microstructures (lamellar, ordering 2, and equiax ), and duplex microstructure, but for Ti-46.5Al-8Nb-0.7B(0.7B) the microstructures were NFL (Nearly full lamellar), NFL, and NFL individually. The effect of boron on phase transformation in TiAl alloy has been investigated. It has been found the volume fraction of 2 phase of 0.7B alloy in the same holding time is less than that of 0B alloy. Because the nucleation of phase on the grain boundaries is very small and the growth is much difficult. Boron has no effect on the solidus and increases the liquidus. The precipitation temperature of borides increases with the increasing of boron content. For Ti-46.5Al-8Nb-0.5B(0.5B) and 0.7B alloys，borides precipitate at liquid phase area, but for Ti-46.5Al-8Nb-0.2B(0.2B) alloy, borides precipitate at L+ two phase field. Boron influences the L®b transformation, refining the grain size of b phase to refine the lamellar grain size.
Oxygen refines the grain size of Ti-48Al-0.8B exhaust valve. The critical oxygen content is between 3000 and 3300 wt. ppm. The microhardness (H) and the 2 volume fraction (Fv2) increase with increasing oxygen content. 1B9 exhaust valve has the optimum balance of properties in recycling exhaust valves. The water quenching microstructures of exhaust valves with different oxgen content are all lamellar and oxygen content is the main factor of m transformation.
With increasing the niobium content, Ti-48Al-0.7B-5Nb(5Nb) exhaust valve has the minimum grain size. The reason is boron produced additional undercooling at solidification front and pinning by borides. The shape of borides in four alloys is the ribbon. The exhaust valve with smaller grain size has the shorter ribbon of borides. The volume fraction of 2 phase and hardness increases with Nb content increasing. 5Nb exhaust valve has the optimum balance mechanical properties. 0Nb and 5 Nb alloys water quenching from phase area, Nb content, oxygen content and grain size are the factors to affect the to m transformation. |
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