The thermal stability of surface deformed zirconium

Abstract

This study examined the microstructural and mechanical changes in shot-peened zirconium resulting from extended exposures to temperatures between 200 and 300°C. The peening hardens the surface region which extends to about 300 μm from the surface. It was discovered that the outermost (10–50 μm) shot-peened surface decreases in hardness from approximately 230 VHN (DPH) to about 220 VHN after 560 h at 200°C. The same drop in hardness was observed after about 5.5 h at 300°C. Further drops in hardness with time were observed at 300°C to a minimum surface hardness of 206 VHN after 560 h. Longer times at 200 and 300°C did not appear to significantly affect the hardness. These decreases in hardness were determined from hardness depth profiles before and after heat treating the zirconium to various times from 0.5 to 4458 h. The hardness between 150 and 300 μ (the less deformed region) was approximately unaffected by the thermal treatments. The grain dislocation structure of the shot-peened zirconium was examined in the as-peened as well as the annealed conditions using polarized light optical and transmission electron microscopy. These observations are consistent with an explanation of decreased hardness based on static recovery without any static recrystallization.