《Table 3.The volume fractions of O, B2, α2 phases, size of O phase, and microhardness of the deforme

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《Morphology and quantitative analysis of O phase during heat treatment of hot-deformed Ti_2AlNb-based alloy》

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The XRD patterns of the 950°C-solution-treated deformed alloys aged at different temperatures are illustrated in Fig.6.After the deformed specimen aged in the two-phase（B2+O）region，a mass of O phase appeared and the volume fraction of O phase was more than 64%（Table2）.The volume fraction of the phases was calculated from the peak intensity of the patterns by Rietveld method[36].The volume fraction of the O phase in the deformed specimens is higher than that in the undeformed one.A graph of phase content of the 950°C-solution-treated undeformed and deformed alloys versus aging temperature is shown in Fig.7.After deformation，crystal defects formed during deformation，such as dislocations and subgrain boundaries，would offer large heterogeneous nucleation sites for the acicular O phase[37–38]，resulting in the nucleation and growth of the acicular O phase along the crystal defects.With the elevating aging temperature，the phase content of O phase decreases and that of B2 phase increases，which indicates a transformation from O phase to B2 phase.The volume fraction of O phase decreased from 76.21%to 63.49%and80.51%to 64.12%for the undeformed alloy and deformed alloy，respectively，when the aging temperature rises from650°C to 800°C.This indicates that hot deformation restrains the transition from O to B2 at a low temperature.In other words，hot deformation accelerates the transformation process from B2 to O phase.Meanwhile，the volume fraction of theα2 phase is less than 10%both in the undeformed and deformed alloys.The transformation of the B2→α2 or O→α2 by mechanical displacement resulted in a fluctuation of theα2 content when the alloys were heat-treated in the phase regions containing O orα2 phase[39].Hence，it is difficult identify the variation rule of theα2 phase with the increasing aging temperature[40].