The flow stress of many L12 ordered alloys has a very unusual temperature dependence: the flow stress increases with increasing temperature. This unusual behavior is related to the nature of dislocation dissociation and core structure. The flow stress increase is the result of thermally activated cross slip of  screw dislocations to the (010) plane which is accompanied by a transformation of the dislocation core from a glissile to a sessile form. Thus dislocations which are mobile on (111) planes become immobile after cross-slip into (010) planes. The dependence of the flow stress on temperature, orientation and sense of the applied uniaxial stress will be discussed in the light of this cross slip model for Ni3Al, Ni3Ga and for γ/γ′ nickel base superalloys.
The response of Ni3Al to cyclic plastic strains (plastic strain controlled fatigue) will also be shown to be in accord with the cross slip model. The mean stress in such a test becomes compressive or tensile, depending on the orientation of the sample, even though the net plastic strain is zero after each cycle.
The strengthening of Ni3Al by ternary additions will also be discussed. It will be shown that ordinary solid solution strengthening models are not applicable but that the cross slip model can also be applied.
Finally, it will be shown that dislocation core simulation studies predict that there should also be a class of L12 ordered alloys that show a “normal” flow stress-temperature behavior, i.e., the flow stress increases at low temperatures. The results of our studies on Pt3Al will be used to illustrate this behavior.