Thermocompression Dynamic Recrystallization Behavior Of V - 5Cr - 5Ti Alloy

V-Cr-Ti alloy, as a candidate for the fusion reactor first-wall structural materials, has been extensively studied for over 20 years. Thermal processing is a key step for V-Cr-Ti alloy which control the material utilization and yield, but researchers’understanding on thermal process is not deep enough. In order to recognize the high temperature microstructure evolution of alloy and to provide necessary data to optimize thermal processing, the dynamic recrystallization (DRX) behavior of V-5Cr-5Ti alloy (wt.%) under hot compression was studied. The experiment was performed on a Gleeble-3800 machine based on the’strain freeze’method and the testing temperatures are in the range of 1100-1400℃ and strain rates are 0.01,0.1 and 1.0s-1. The microstructure was characterized by using of electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Results show that:Reduce prior grain size or lower Zener-Hollomon parameter can effectively inhibit shear zone and intergranular cracks and improve the deformation uniformity of V-5Cr-5Ti alloy. Depend on deformation conditions, DRX grains may nucleate at the original grain boundary, deformation bands and shear bands. High temperature flow stress decreases with increasing temperature and decreasing strain rate and initial grain size.Vary deformation conditions (Z-H parameters and strain amount), discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX) can start in alloy, and the corresponding physical processes including strain-induced grain boundary migration and subgrain rotation. Compression can promote the formation of a strong <001> orientation of DRX grains, increasing temperature or decreasing initial grain size can accelerate this transition process. The formation mechanism of <001> fiber texture includes oriented nucleation, dislocation slip and grain rotation, but the dominate one is oriented nucleation.DRX grains’grew up showing an approximate parabolic law. Both the grown line rate and nucleation rate of DRX grains show a rise-peak-decline tendency. The empirical mathematical relationships between average grain size Dm, dislocation cell size d, critical dislocation density pb and other materials parameters with Z-H value were obtained. Initial DRX transition kinetics approximately obey the Avrami model and the Avrami parameters n were determined as 1.5-2.1.Dislocations are mainly loacted at dislocation cell walls, and dislocation slip can form slip bands and dislocation network. The average dislocation density are in the range of (0.6-5.5)×1014m-2. Stress exponent n and physical activation energies Qp were measured as 4.87 and 375.89 kJ mol-1, respectively. The value of physical activation volume Vp are between 120-700b3 and which has a positive correlation with temperature.The{110}<111> and {112}<111> type stacking fault energy (ys) of pure V were calculated as about 540 mJ m-2 and mJ m-2, respectively. Both ys increasing with Cr content increases and decreasing with Ti content increases; Ti element can segragate on the stacking fault and format Suzuki atmosphere. There are stable stacking faults and twins in hot compressed alloy.