Energetic and kinetic considerations of grain boundary engineering of Ni(3)Al.

摘要

Grain boundary design is a microstructural control technique which has demonstrated success in improving toughness, resistance to intergranular stress corrosion cracking and reducing tendency to intergranular fracture in several materials including nickel and nickel alloys. Driven by an energetic preference for twin boundaries and low {dollar}Sigma{dollar} grain boundaries, a twin limited structure in which 2/3 of the boundaries are special grain boundaries is possible. Twin formation is a complex function of energetic factors such as stacking fault energy, and kinetic factors such as grain boundary mobility both of which may be altered by segregation and alloying, as well as geometrical interactions between the grains. The effect of ordering on the development of microstructures during strain annealing has not been studied in detail.; Nickel aluminide is a high temperature structural material which has not yet achieved its full market potential because of high susceptibility to intergranular fracture. Grain boundary design is a possible processing route which may reduce this problem.; Simulation of grain boundaries in Ni and Nickel Aluminide have revealed that while average grain boundary energies are similar, {dollar}rm Nisb3Al{dollar} has a smaller energetic preference for twin and other low {dollar}Sigma{dollar} grain boundaries than Ni. An energetic criterion defining a special grain boundary has been developed and applied to Ni and Ni{dollar}sb3{dollar}Al. Twin boundary energy was found to be significantly larger than for Ni. These results indicate that Ni{dollar}sb3{dollar}Al should have a lower tendency for twinning.; Strain annealing was successful in increasing the frequency of twin boundaries in Ni{dollar}sb3{dollar}Al from a recrystallized value of 31% to 47% after three strain annealing treatments. Deformations in the range of 5% to 7%, annealing at 1050{dollar}spcirc{dollar}C and anneal times of 15 minutes generated the best grain boundary character distributions, with the lowest low angle grain boundary frequency, lowest random boundary frequency and highest twin boundary frequency. The frequency of {dollar}Sigma{dollar}5-29 was little affected by the heat treatments. The results obtained from recrystallization are consistent with both the energetic and kinetic models of twin formation.; To achieve further improvements in grain boundary character distributions in this material the energetic preference for low {dollar}Sigma{dollar} boundaries and twin boundaries needs to be re-established. Likewise, grain boundary mobility, which is critical to twin growth needs to be increased to achieve further gains. However, it has been demonstrated in this work, that even when energetics and kinetics are somewhat unfavourable to twinning, significant improvements in the grain boundary character distribution are achievable.