Deformation twinning in materials of the A 4 (diamond) crystal structure

Deformation under certain conditions causes twinning in silicon, germanium, gallium antimonide, indium antimonide and zinc blende which have the A 4 crystal structure. In material beneath hardness impressions formed at elevated temperatures a flow stress is superimposed upon hydrostatic compression; under these circumstances deformation twins form at temperatures between 0⋅44 and 0⋅74 of the absolute melting-points. Twins of the {111} type and, except in the case of zinc blende, of {123} type have been observed. Minor boundaries of thick growth twins may be {123} planes which also form boundary faces of etch pits. The only coherent interface possible between a {111} twin and matrix is a {111} plane; it is shown that among semi-coherent boundaries a {123} plane gives the best fit. The translational shears for twinning have been determined: for (111) twinning the shear is 0⋅4084a in the [11¯2] direction with (¯110) as the shear plane; for (123) twinning the shear is 0⋅6552a in [41¯2] direction with (1¯21) as the shear plane; where a is the parameter of the A 4 unit cell.