Hot deformation behavior of Ti-55531 with the lamellarα initial structure was studied through single-stroke isothermal compression at the temperature of 750−825℃ and strain rate of 0.001−1 s−1. By further combining the stress-strain curves and the fragmentation and spheroidization of lamellarα, the critical strain corresponding to the onset of the fragmentation and spheroidization of lamellarα was analyzed. The results indicate that the flow stress increases with increasing strain rate or decreasing of deformation temperature. After yielding, the flow stressσ increases sharply, and then increases slightly till reaching the steady flow with the increase of strainε, in which dσ/dε has a minimum value corresponding to the critical strain. Before the critical strain, the flow softening mainly results by the rotation and buckling of lamellarα. When the strain is higher than the critical strain, the lamellar fragmentation and spheroidization have the dominant contribution to the flow softening. The critical strain increases with increasing strain rate and decreasing deformation temperature. The model for predicting the critical strain can be described asec¢=0.001 97×0.121[e&exp(392300/RT )] .%对具有初始针片α组织的Ti-55531合金进行单道次压缩变形，研究该合金在变形温度为750~825℃、应变速率为0.001~1 s−1条件下的热变形行为，并结合应力–应变曲线和针片α组织的破碎球化现象，分析针片α组织破碎球化的临界应变条件。结果表明：变形过程中流变应力随应变速率增大或变形温度降低而增大；合金发生屈服后，流变应力s随应变量ε增加先快速下降、后缓慢下降，直至趋于稳态流变，dσ/dε存在极小值，其对应的应变量为针片α组织发生破碎的临界应变量ec¢；应变量小于ec¢时，针片α组织旋转、弯折等引起的流变软化作用占主导，应变量大于ec¢时，针片α组织破碎球化引起的流变软化作用占主导；ec¢随应变速率增大及变形温度降低而增大，其预测模型可表示为0.121ec¢=0.00197[e&exp(392300/RT)]。
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