The formation of multi-domain precipitates in a NiW alloy

The formation of twinned Ni₄W precipitates in a NiW alloy has been studied. On prolonged aging the dominant morphology found is one where two perpendicular twin related variants of Ni₄W form plate-shaped precipitates with a near (100)_Ni habit plane. The orientation relationship between the Ni₄W variants and the Ni matrix leads to a macroscopic shape change with the (100) habit plane interface nearly invariant. In addition each domain has a simple shear associated with it, which can be accomodated either elasticity or by arrays of screw dislocations. Elastic accommodation leads to a strain contrast effect which is similar to that observed at screw dislocations lying normal to the foil surface.