Effect of Annealing on Hardness and the Modulus of Elasticity in Bulk Nanocrystalline Nickel

Experiments on hardness and the modulus of elasticity were conducted at room temperature on samples of electrodeposited (ED) nanocrystalline (nc) Ni that were annealed at temperatures ranging from 323 to 693 K (50 to 420 °C). The results showed the presence of three regions: I, II, and III. In region I (300 K (27 °C) < T < 350 K (77 °C)), the hardness and the elastic modulus remained essentially constant. In region II (350 K (77 °C) < T < 500 K (227 °C)), both the hardness and the elastic modulus increased. In region III (T > 500 K (227 °C)), the hardness dropped and then decreased with increasing grain size, whereas the modulus of elasticity approached a maximum plateau of ~240 GPa. It is suggested that while the increase in hardness in region II can be attributed in part to the formation of annealing twins, which serve as a source of strengthening, the decrease in hardness above 500 K (227 °C) is due to the occurrence of significant grain growth. The increase in the modulus of elasticity with increasing temperature in region II was attributed to the preferred orientation along (200) that was observed in the as-received samples and that continuously diminished with increasing temperature. In region III (T > 500 K (227 °C)), the preferred orientation disappeared and, a result, the modulus of elasticity approached a constant value of approximately 240 GPa.