The influence of the fault energies on the anomalous mechanical behaviour of Ni3Al alloys

Single crystals of the intermetallic; L1(2) ordered alloy Ni78Al22 were deformed in compression at RT and at 400 degrees C, a temperature below and within the anomalous regime. Transmission electron microscopic (TEM) images were used to analyse the dislocation structures. At RT edge dipoles are prevailing (as in f.c.c. metals) whereas at 400 degrees C locked screws, screw dipoles and near screw dislocations bowed out on the cube cross-slip plane (010) are predominant. Their formation is caused by a gradual transition from ''normal'' octahedral cross-slip to the thermal activated cube cross-slip. By comparing the experimental weak-beam TEM images with computer simulations and using anisotropic elasticity theory the complete set of fault energies was determined: gamma(CSF) = 235 +/- 40 mJ/m(2), gamma(APB) (111) = 175 +/- 15 mJ/m(2), gamma(APB) (010) = 104 +/- 15 mJ/m(2) and gamma(APB) = 6 +/- 0.5 mJ/m(2). These values can be used to explain the shift of the anomalous increase of the yield stress to higher temperatures observed in Ni3Al as compared with Ni-3(Al, 1 at.%Ta). The value of R = gamma(APB)(111)/gamma(APB)(010) determines the driving force for cube cross-slip (by comparing the R values of the two alloys the reverse behaviour of the shift might be expected). The value of gamma(CSF) is the decisive parameter, it determines the dissociation width and therefore the constriction energy of the Shockley partials of the screws. A low value of gamma(CSF) reduces the thermal activation necessary for the formation of KW locks and screws bowing out on (010).