Phase stability, defects and deformation mechanisms in the “exotic” intermetallic compounds Nb3Al and Cr2Nb

A review is presented of recent TEM investigations on two of the “exotic” intermetallics Nb₃Al and Cr₂Nb. In Nb-Al alloys diffraction data indicate that additional long range order is present in both the A15 and A2 phases. In A15 Nb₃Al, planar faults on {001} in have a displacement 1/4<021> and cannot move conservatively whilst extended screw dislocations with Burgers vectors <100> are dissociated on {012} cross-slip planes and are locked. In C15 Cr₂Nb grains deform by either dislocation glide or twinning. The dislocations glide on <110> {111} and are composed of widely separated Shockley partials due to the low stacking fault energy. Twinning occurs on only one <112> {111} system in any one grain probably due to autocatalytic nucleation. Both deformation modes require synchroshear but the individual components have not been observed directly.