| Ternary Ti-Al-Nb alloys based on intermetallic compound Ti2AlNb exhibit superior performance on specific strength,density,high temperature strength and creep resistance.This can be attributed to strengthening effect of the ordered orthorhombic Ti2AlNb phase(O phase).But applications of Ti2AlNb-based alloys is quite limited due to their brittleness at room temperature(RT).Microstructure of such alloys is quite sensitive to heat treatment regimes,so it's significant to optimize heat treatments during processing.This study is to investigate variation of microstructure and properties under different heat treatments in the Ti2AlNb-based alloy with a nominal composition of Ti-22Al-25Nb.Solution treatment at B2 field was adopted based on a thermal analysis.Firstly,effect of cooling rates on precipitates and hardness of the alloy was studied.Then mechanism of the whole precipitation process during slowing cooling was thoroughly investigated.In the final chapter,alloys with totally different microstructures were acquired by various prior heat treatments.Then effect of microstructure on creep resistance and tensile properties at room temperature was studied.Meantwhile we explored variation of microstructure and RT tensile properties of the alloys during long-term aging treatment.Here are some important conclusions:(1)Cooling rate can effectively control the precipitation of O phase and?2 phase in the Ti-22Al-25Nb alloy.When the cooling rate is slowed to 20oC/min,?2 phase on the grain boundary can grow into continuous coarse plates.When the cooling speed is greater than 40 oC/min,?2 phase on the grain boundary can be effectively inhibited;(2)In the slow cooling process,there exists a shear precursor during the transformation of B2??2,the?2 phase tends to nucleate in the precursor area.(3)Annealing at(O+B2+?2)or(B2+?2)phase regions can greatly promote the growth of?GB on the grain boundary and?WGB that extended into matrix,which is harmful to the toughness of the alloy at room temperature.(4)Solution treatment at high temperature of single-phase region causes coarsening of grains in the alloy.This brought great damage to the toughness of the alloy,as a compensation the grain boundary migration rate in the creep test was reduced.Bimodal(O+B2)structure has the best toughness at room temperature but the worst creep resistance at the same time.(5)In the process of long-term aging at 650°C,phenomena such as the decomposition of?2 phase,reprecipitation of?/B2 phase from O phase matrix and discontinuous(cellular)precipitation B2??+O were observed. |
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