Investigation of microstructure and hardness evolution in N36 alloy subjected to gradient deformation and annealing

A simple batch-processing method has been proposed to prepare large-sized zirconium alloys with gradient microstructure. Specifically, a wedge-shaped N36 alloy (Zr-1Sn-1Nb-0.3Fe) sample was hot-rolled to produce a continuous deformation gradient ranging from 0 to 90 %. The samples were subsequently annealed at different temperatures ranging from 500 °C to 650 °C for 2 h to explore the effects of annealing on the hardness and microstructure. The results showed that the hardness increases proportionally with the hot-rolling strain below 60 %, but increases much more significantly when the strain reaches 90 %. Annealing results in a hardness decrease and the higher annealing temperature induces a larger decrease, while the regions with higher hot-rolling strain retained higher hardness after annealing. The as-rolled alloy exhibits a dynamically recrystallized microstructure consisting of equiaxed grains with a high dislocation density. Annealing results in a significantly decreased dislocation density and a bigger grain size. After annealing, the regions with higher rolling strain exhibited larger grain sizes, higher recrystallization degrees, lower dislocation densities. However, the second-phase particles (SPPs) in the more severely deformed regions exhibited a larger size and higher precipitate volume fractions, which contributed to the higher hardness. Both the rolled and annealed alloys displayed {0001} <11$\overline{2}$0 > and {0001} <01$\overline{1}$0 > textures.