THE INFLUENCE OF THE FAULT ENERGIES ON THE ANOMALOUS MECHANICAL-BEHAVIOR OF NI3AL ALLOYS

Single crystals of the intermetallic; L1(2) ordered alloy Ni78Al22 wer e deformed in compression at RT and at 400 degrees C, a temperature be low and within the anomalous regime. Transmission electron microscopic (TEM) images were used to analyse the dislocation structures. At RT e dge dipoles are prevailing (as in f.c.c. metals) whereas at 400 degree s C locked screws, screw dipoles and near screw dislocations bowed out on the cube cross-slip plane (010) are predominant. Their formation i s caused by a gradual transition from ''normal'' octahedral cross-slip to the thermal activated cube cross-slip. By comparing the experiment al weak-beam TEM images with computer simulations and using anisotropi c 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 c omparing the R values of the two alloys the reverse behaviour of the s hift might be expected). The value of gamma(CSF) is the decisive param eter, it determines the dissociation width and therefore the constrict ion energy of the Shockley partials of the screws. A low value of gamm a(CSF) reduces the thermal activation necessary for the formation of K W locks and screws bowing out on (010).