The influence of annealing twinning on microstructure evolution

Abstract

This paper reports an experimental investigation on the effect of multiple twinning on the interface population in two low stacking-fault alloys. This is an important topic for grain boundary engineering because annealing twinning is the indirect cause of improved intergranular corrosion resistance in this class of materials. Proportions of Σ 3ⁿ (n = 1–5) boundaries were analysed in both a brass specimen and a superalloy specimen where the boundaries had been processed so as to be very mobile and less mobile respectively. When Σ 3 twin boundaries (as distinct from Σ 3 grain boundaries) are discounted, the Σ 3ⁿ distribution for both specimens had a peak at Σ 9, because Σ 3 + Σ 9 → Σ 3 occurs more frequently than Σ 3+Σ 9 → Σ 27. The distributions and reactions between various Σ 3ⁿ values are described and discussed in detail. A novel trace analysis procedure is used to extract information from Σ 3 boundaries to decide whether or not they are annealing twins, and so provide a convenient means to assess proportions of twin and non-twin Σ 3s. The data show unambiguously that a significant proportion of Σ 3s are not on 111, and these boundaries have on average higher angular deviations from the exact Σ 3 reference misorientation than do other Σ 3s. A population of Σ 3s which were vicinal to annealing twins were also recorded. These data support the contention that profuse annealing twinning produces concurrently many not-twin Σ 3s, which are pivotal in grain boundary engineering.