The twinning was observed in unreinforced pure nickel and all Ni–SiCNP composites. Fig. 6(a) exemplifies the latter, showing a bright-field TEM image of the Ni–SiCNP composite milled for 24 h. Here, distinct twin boundary planes are observed, approximating 500 nm in size. These twins are oriented along both reciprocal lattice directions in the [110] zone axis (Fig. 6(b)). It is generally accepted that grain growth occurs by the movement of grain boundaries instead of coalescence. With the combined effects of high temperature and high pressure during sintering, it is expected that the speed of nucleation and grain growth may be quite rapid due to the enhanced diffusivity of nickel atoms. When an occasional error occurs in the stacking sequence of atomic planes, several coherent twin boundaries can appear successively at a grain boundary corner[41].