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

Abstract

In this work, a Zr702 sheet was subjected to β 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 β 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°–5° and 85°. Analyses on kernel average misorientations reveal that there exist very low residual strains in the β-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 β air cooling, which can be attributed to grain coarsening and the scattered orientations associated with the slow β → α 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.