Effects of β Air Cooling and Subsequent Cold Rolling on Microstructure and Hardness of Zr702 Sheet

In this work, a Zr702 sheet was subjected to beta air cooling and then rolled to 15% reduction at room temperature, with their detailed microstructural characteristics characterized by electron channeling contrast imaging and electron backscatter diffraction techniques. Results show that after the beta air cooling, the prior equiaxed grains in the as-received material are completely transformed into Widmanstatten structures featured by coarse plates with typical phase transformation misorientations between them. The subsequent 15% rolling allows both slip and twinning (especially the {10-12} type) to be activated readily, leading to significant grain refinement and the appearance of misorientation angle peaks around 3 degrees-5 degrees and 85 degrees. Analyses on kernel average misorientations reveal that there exist very low residual strains in the beta-air-cooled specimen while they are markedly increased after the 15% rolling and preferably distributed near low-angle and twin boundaries. Hardness measurements show that the specimen hardness is evidently decreased from ~ 199 to ~ 170 HV after the beta air cooling, which can be attributed to grain coarsening and the scattered orientations associated with the slow beta -> alpha transformation. For the 15%-rolled specimen, however, effective grain refinement by twinning and denser low-angle boundaries jointly lead to ~ 35% hardness increment to ~ 228 HV. Graphic