DISLOCATION VELOCITIES IN NI3AL SINGLE-CRYSTALS

The average velocities of dislocations moving on octahedral and cube p lanes in Ni3Al single crystals have been measured as a function of the resolved shear Stress (RSS) in the temperature range from 293 to 1133 K by the etch-pit technique. The RSS dependence of the velocities can be expressed by an equation of the form upsilon = upsilon(0)(tau/tau(0 ))(m) in all cases. Some of the parameters associated with the disloca tion mobility are deduced from the experimental data. For the octahedr al slip, the exponent m lies between 20 and 31 and the activation area A = (81-99)b(2). For cube slip, m = 12-16 and A* = (56-84)b(2). Thre e temperature domains have been defined where different glide systems operate. In the first domain the dislocation can move only on the octa hedral plane, in the second domain it can move on both octahedral and cube planes, and in the third domain it can move only on the cube plan e. It is found that the velocity of dislocations moving on the octahed ral plane exhibits anomalous behaviour, consisting of a positive tempe rature dependence of the RSS as required for a constant dislocation ve locity and a tension-compression velocity asymmetry. No obvious anomal ous velocity behaviour for the cube glide system was observed. All the se results suggest that positive temperature dependence of the critica l resolved shear stress and the tension-compression flow asymmetry in L1(2) Ni3Al can be directly related to the behaviour of individual dis location velocities.