The effect of covalency on the deformation mechanisms in intermetallic compounds

Abstract

It has been found that the effects of covalency on the anisotropy of Peierls stresses plays a significant role on the mobilities of dislocations in intermetallic compounds, and therefore influences the ductility exhibited by these materials especially at room temperature. In way of example, brief notes are given for the cases of TiAl and Ti3AL For TiAl, the microstructure of samples deformed at room temperature consists mainly of dislocations of the type b=<011], with the attendant superlattice extrinsic stacking-faults, see Fig.l; also, in some grains twinning has occurred. The density of dislocations with Burgers vectors parallel to <110] is extremely low; their line directions tend to be fairly close to screw orientation. For compression at 600°C, the deformation microstructure tends to be dominated by dislocations with b=1/2<110], as shown in Fig.2(a), and most of the dislocations observed of this type were found to be lying in screw orientation. In agreement with previous work, at the higher temperatures dislocations with b=<101] were observed to be dissociated partly on the {100} planes, the two inclined sets of straight dislocations in Fig.2(b), corresponding to Kear-Wilsdorf locks. These various results, including the line directions of the dislocations, are interpreted on the basis of the effect of covalent bonding on the anisotropy of the Peierls stresses (following) and therefore dislocation mobilities in this compound. The lack of ductility exhibited at room temperature is attributed to a lack of mobility of dislocations with b=1/2<110] caused by covalency, and the formation of sessile configurations by dissociation of dislocations with b=<101]. The increase in ductility of TiAl at higher temperatures is attributed to the operation of the slip systems 1/2<110]{111}, where dislocations with b=1/2<110] become significantly more glissile because of thermal activation, and also to an increased amount of twinning (apparently above 600°C) since the twinning dislocations (with b=1/6<112]) also are expected to be more glissile. When samples of this compound are produced containing 0.4at.%Er, the action of the rare earth addition is to internally getter the interstitial impurities. In this case, the deformation microstructure is dominated by dislocations with Burgers vectors with b=1/2<110], a typical area being shown in Fig.3. This is consistent with a reduced anisotropy of charge density about the Ti atoms, and therefore reduced effects of covalency.