The effects of annealing treatment on mechanical behavior and microstructure in nanocrystalline nickel

Nanocrystalline (nc) materials are characterized by a grain size in the range 1-200 nm. Because of this characteristic, these materials exhibit unique microstructures in which the volume of grain boundaries is significant.;Nc-materials offer interesting possibilities related to many structural applications. In order to explore some of these possibilities, an understanding of the origin of mechanical properties such as strength, hardness, and ductility is essential. Mechanical properties in materials are very sensitive to their microstructure. One important process that can be used to manipulate the details microstructure in materials is annealing. In the present dissertation, annealing treatment was utilized to provide insight into the correlation between mechanical properties and microstructure. Primary among the mechanical properties that were selected for investigation are hardness, the elastic modulus, strength and ductility.;Experiments on hardness and the modulus of elasticity were conducted at room temperature on samples of electrodeposited (ED) nc-Ni that were annealed at temperatures ranging from 323 K to 693 K. The results showed the presence of three regions: regions I, II, and III. In region I (300 K < T < 350 K), hardness and the elastic modulus remained essentially constant. In region II (350 < T < 500 K), both hardness and the elastic modulus increased. In region III (T > 500 K), 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 500K 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 preferred orientation along (200) that was observed in as-received samples and that continuously diminished with increasing temperature. In region III (T > 500 K), preferred orientation disappeared and, a result, modulus of elasticity approached a constant value of about 240 GPa.;ED nc-Ni having an average grain size of 20 nm was annealed at 443 K for different holding times. An examination of the microstructure following annealing showed three important features. First, all annealed samples exhibited abnormal grain growth, which was manifested by the presence of large grains that were surrounded by regions of small grains (bimodal grain distributions). Second, annealing twins existed in the large grains of the samples that showed a bimodal grain distribution. Third, by estimating the density of annealing twin, it was found that annealing nc-Ni at 443 K resulted in a maximum twin density after 5h. Following annealing treatment, specimens with different volume fractions of twins were tested under uniaxial tension at 393 K and a strain rate of 10-4 s-1. The results showed that both strength and ductility in nc-Ni attained maximum values after annealing for 5h. The role of both bimodal grain distributions and annealing twins in enhancing ductility and strength was discussed.;The present results showed that both annealed ED nc-Ni having an average initial grain size of 20 and an average initial grain size 100 nm exhibited abnormal grain growth and that most annealing twins were induced in large grains. Despite this similarity, two differences were noted. First, in annealed nc-Ni having an average initial grain size of 20 nm, a large number of grains were still in the nanocrystalline range < 100 nm. By contrast, in annealed nc-Ni having an average initial grain size of 100 nm, most of the grains exhibited sizes larger than 100 nm. Second, the number of annealing twins in annealed nc-Ni having an average initial grain size of 100 was much higher than that in annealed nc-Ni having an average initial grain size of 20 nm. This observation suggests that the number of annealing twins is proportional to the distance of grain-boundary migration. After deformation, while both annealed 20 nm nc-Ni and 100 nm nc-Ni exhibited grain coarsening, several differences between the two grades of the material were noted observed. Primary among the differences was the observation that dislocation-dislocation interactions dominated in annealed 100 nm nc-Ni while grain boundary interactions were primarily evident in annealed 20 nm nc-Ni.