Deformation mechanisms in B2 aluminides: shear faults and dislocation core structures in FeAl, NiAl, CoAl and FeNiAl

Although aluminides with the B2 crystal structures have good properties for high temperature applications, the strong ordered bonds that make them durable at high temperature also make them too brittle at room temperature for industrial fabrication. In order to better understand this lack of ductility, molecular statics simulations of planar fault defects and dislocation core structures were conducted in a series of B2 aluminides with increasing ordering energy (FeAl, NiAl, CoAl). The simulation results in NiAl were compared with in-situ straining observations of dislocation motion.

The dislocations simulated were of (100) and (111) types. The simulations results obtained indicate a strong influence of the planar fault energies on the mobility of the dislocations. As the cohesive energy increases from FeAl to CoAl, antiphase boundary and unstable stacking fault energies increase resulting in more constricted dislocation core spreadings. This constriction of the cores decreases the mobility of dislocation with planar core structures and increases the mobility of dislocations with non-planar cores.

The (100) screw dislocations were found with planar cores in {110} planes for FeAl, NiAl and CoAl. For very high APB values, the cores were very compact, as predicted by the Peierls- Nabarro model. As the APB energies decrease, increasingly two dimensional spreading of the cores was observed and ultimately dislocation dissociation into partials. As a result of the deviation of the stable planar fault energy from the APB fault, the partials were not exact 1/2(111) but deviate to the point corresponding to the actual minima of the γ-surfaces for these compounds. Alloying NiAl with Fe was found to promote the dissociation of the (100) dislocation.

The in-situ straining of a single crystal of NiAl only revealed the motion of (100) dislocations. Both in-situ observations and atomistic simulations agreed on the zig-zag shape of the (100) dislocation with an average screw orientation. In this configuration, the mobility of the dislocation is severely reduced.