对具有片层α相的Ti-17合金圆饼在两相区进行5种变形程度的等温锻造及固溶时效处理,用定量金相学方法研究了片层α相在热加工过程中的演变规律.研究发现:α相的厚度随着应变量的增大而增大;变形程度及圆饼的变形区域对α相形态的变化有很大影响.随变形程度的增大,α相的形态参数Feret Ratio (Feret_(max)/Feret_(min))的分布曲线在Feret Ratio介于1.5～2.5区间出现单峰,且峰值不断增大,大Feret Ratio的α相比例逐渐减小.圆饼中心位置的Feret Ratio分布曲线具有较大的峰值.大变形使片层α相的等轴化程度提高,改善了圆饼的组织均匀性.等效应变对组织中不同形态α相分布的影响曲线表明:球化α相比例随着等效应变的增大呈单调增大,增大速率表现为慢-快-极慢的分阶段特征;近等轴状的α相比例先快速增长后不断减少;片层状和大片层状α相比例随着等效应变增大呈单调减少.片层α相的临界球化与完全球化的等效应变分别约为0.4与1.0.
To quantitatively research the microstructural evolution of lamellar α phase in titanium alloys, large size cakes of Ti-17 alloy with initial lamellar microstructure were isothermally-forged at different strains in α+β phase field followed by solid-solution and aging treatment. The changes of α thickness and morphology in different locations (center, midradius, edge) of the cakes deformed to different strain levels were measured and the imposed strain was estimated using finite element analysis. It is found that increasing of forging strains causes an increase in thickness of α phase. The modification of α morphology depends strongly on the forging strains and the locations in the cakes. The distribution curves of α morphology parameter Feret Ratio were characterized by no peak at lower strains, and a single peak appears at Feret Ratio 1.5-2.5 at larger strains. Moreover, the peak values increased with increasing of the height reduction, and the distribution curves at the center have the largest peak values. Increasing of forging strains will cause the high-Feret-Ratio lamellae to decrease but low-Feret-Ratio α to increase noticeably. Sufficiently imposed strains also can improve the microstructure homogeneity in different locations of the cakes. The effect of the effective strains on the percentage of α phase with different morphologies is also quantificationally summarized. The average strains for initiation of globalization are of the order of 0.4 and those for completion are of the order of 1.0, which are lower than that of conventional α+β titanium alloys.
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