Effect of Grain Size on the Crystal Structure of Cobalt

The paper gives an account of measurements made on the lattice parameter of cobalt when the material contained crystal grains of different sizes as found in fine dust, filings and annealed solid rod. The stable structure of cobalt was found to depend upon the grain size: between room temperature and about 450°c when the grain size is very small, as in cobalt sponge, the stable structure is face-centred cubic; when the grain size is larger, as in a solid rod, the stable structure is close-packed hexagonal; when there is a range of grain size, as in fine filings a mixture of these two structures is observed. At temperatures above 450°c cobalt sponge is still face-centred cubic, and the filings and rod also now show face-centred cubic structure with the same lattice parameter (3.5370kx at 18°c). The lattice constants of the hexagonal close-packed structure at 18°c are: a = 2.5003kx, c/a = 1.6322.