Study of Recovery and Recrystallisation in Folded BCC, FCC and HCP Sheet Samples

Abstract

Microstructural changes due to isothermal heat treatment in the bent regions of cold folded nickel alloy (Ni200), mild steel, and Titanium (Ti6-4) sheet samples, followed by the EBSD (electron back-scattered diffraction) technique are reported. Large strain gradients that vary from being tensile to compressive exist in the bent regions and have been used to study their influence on recovery and recystallisation processes. Isothermal heat treatment at 600°C in the Ni200 and steel sheets results in recrystallisation process to start in regions with the highest strains and progresses towards the centre of folded regions. Grain growth was observed to be dependent on both the sign of the strain and its level. Generally larger grain sizes were associated with tensile strains in both Ni200 and steel. Abnormal grain growth occurred after prolonged heat treatment in the Ni200 sample in regions with low levels of tensile strains found in the middle of the sheet thickness. Although complete recrystallisation was not observed in the steel sample significantly large grains formed in regions with low strain levels. Only recovery processes were observed in the Ti6-4 samples. Some twinning was also observed in Ti6-4 that was confined in the regions with compressive strains. Crystallographic textural changes were observed during this study and will be reported in future publications.