DISLOCATION ENERGIES FOR AN ANISOTROPIC CUBIC-CRYSTAL CALCULATIONS AND OBSERVATIONS FOR NIAL

The line energies of straight dislocations as a function of the Burgers vector and line direction have been calculated for the cubic crystal NiAl using the method developed by Eshelby, Read and Shockley. Plots of the energy E are shown for the three different Burgers vectors, [001], [110] and [111], which have been observed in NiAl single crystals deformed at high temperatures. Plots of the derivatives partial derivative E/partial derivative theta and partial derivative E/partial derivative phi of the energy with respect to a variation in line direction, and the line tension Tare also shown. The line energies for b = [001] dislocations differ significantly from the isotropic case. The b = [001] screw dislocation is no longer an energy minimum, but an absolute energy maximum. Several energy minima can be identified. Line tension calculations show that for all three Burgers vectors a line direction with negative line tension exists. The computed line energies, line tensions and derivatives are compared with dislocation configurations after creep deformation of NiAl single crystals. Transmission electron microscopy observations show dislocation reactions leading to Burgers vectors of the type [110] and [111], and fully developed networks built up from [001] Burgers vectors. The observed line directions and Burgers vectors are in good agreement with the calculated line energies and line tensions. From a knowledge of E, partial derivative E/partial derivative theta, partial derivative E/partial derivative phi, the line direction and the glide plane, a nodal displacement which lowers the overall energy of a nodal configuration can be calculated. These nodal force calculations indicate how the nodes were moving at the time that the creep test was terminated.