High temperature deformation of NiAl matrix composites

The intermetallic compound, NiAl, has many attractive properties as a high temperature structural material. However, its lack of creep resistance prevents practical applications. Adding ceramic reinforcements, such as TiB₂ particles, Al₂O₃ particles or whiskers can significantly improve the strength of binary NiAl at high temperatures. However, the increase in the yield stress of the discontinuous NiAl matrix composites as compared with monolithic NiAl is difficult to explain. The purposes of this research were to understand the deformation mechanisms which cause the increase in strength achieved by adding TiB₂ particles, Al₂O₃ particles or whiskers to NiAl, and to recognize the principles of the deformation process in NiAl matrix composites. In order to accomplish these objectives, mechanical properties and thermal activation parameters in NiAl matrix composites with different types, shapes and sizes of reinforcements have been systematically evaluated. Microstructures and dislocation structures in NiAl matrix composites have also been thoroughly characterized before and after deformation. It was found that the size of the reinforcement had a large influence on the microstructures of the composites, and the nominal activation energies for all the composites were the same and within the range of the activation energy of self-diffusion for pure NiAl. This indicated that the deformation mechanism was the same for all NiAl matrix composites. The thermally activated motion of jogged screw dislocations is postulated as the rate controlling mechanism of the deformation of NiAl matrix composites. However, this non-conservative motion of jogged screw dislocation theory requires only the appearance of vacancy producing (VP) jogs. A simple model which is based on the cross slip of screw dislocations in NiAl is proposed to account for the occurrence of VP jogs. A formulation of strain rate vs stress for the VP jogged screw dislocation model was derived. By computer simulation, it was found that this mechanism was capable of predicting the temperature dependence of the yield stresses of NiAl composites. It was further concluded that the reinforcement addition only increased the non-thermally activated component of the yield stress.