Towards optimization of grain-boundary structures in annealed nickel

Commercially pure and superpure nickel specimens have been subjected to the strain annealing treatments detailed in the Table. Fig. 1 shows the results from an electron backscatter diffraction study of the commercial purity specimens (C1 to C4). It is clear that the only boundary types that increase in frequency with increased thermal processing are {Sigma}3{sup n} (n=1,2,3) CSLs. An abundance of annealing twins was observed for sample C4, and is unique to this sample, and this accounts for the very high proportions of {Sigma}3s. Samples C1 to C3 were annealed under vacuum, whereas C4 was treated in air, and it is proposed that the dominant energy minimization mechanism has changed for grain/grain boundary rotation for samples C1 to C3, to annealing twin formation for sample C4. If the structure of a grain boundary evolves towards the exact misorientation of what is considered special CSL type, the special properties of that boundary will, in general, become more marked. {Sigma}3s have been shown to possess special properties. Fig. 2 shows the average deviation from exact misorientation for the {Sigma}3s in each sample, normalised by the Brandon criterion for maximum permitted deviation. This clearly indicates that the {Sigma}3s created by annealing twin formationmore » can be considered more special than those produced via grain/grain boundary rotations.« less