Microstructure and strength of B2-ordered NiAl containing L21-Ni2AlHf precipitates

Microstructural variations and correlated hardness changes during phase decomposition of B2-ordered NiAl(β) containing 2 mol% of Hf have been investigated by means of transmission electron microscopy (TEM), hardness and creep tests. At the beginning of aging, fine platelets of L21-type Ni2AlHf (Heusler phase) precipitate on the {111} planes of the matrix NiAl phase keeping a perfect lattice coherency. With longer periods of aging, Ni2AlHf precipitates lose their lattice coherency and change their morphology to spherical ones surrounded by misfit dislocations. Compressive creep tests clearly indicated that the creep rate decreases appreciably by the fine precipitation of Ni2AlHf phase. TEM observations of the interaction between dislocations and spherical precipitates revealed that the dislocations tend to be strongly attracted to the particle interfaces during the creep deformation.