Solid solution strengthening and fracture of nickel(3)aluminum single crystals

Two issues are addressed in this dissertation. The first is the mechanisms by which ternary additions strengthen Ni$\sb3$Al. The second one relates to the ductility and fracture behavior of single crystalline Ni$\sb3$Al and the arguments concerning the role of B in the ductility enhancement of polycrystalline Ni$\sb3$Al. Uniaxial tensile and compressive flow stress measurements at constant strain rate were performed on a binary Ni-rich Ni$\sb3$Al single crystal alloy and on Ni$\sb3$Al containing different additions of Hf, Ta, Zr, and B. Data were collected as a function of temperature, from $-196\sp\circ$C to 1100$\sp\circ$C, as a function of orientation of the stress axis, as a function of composition, as well as a function of the sense of the applied uniaxial stress. The flow stress anomaly was confirmed for all the compositions. The critical resolved shear stress (CRSS) for $\{111\}\langle 110\rangle$ slip was found to be orientation-dependent in the region of flow stress increase. Ternary elements increase the CRSS for octahedral slip but have much less effect on the CRSS for cube slip. The temperature of the maximum CRSS and the orientation of the stress axis for zero tension/compression asymmetry are both composition dependent. The low temperature CRSS is anisotropic and also composition-dependent. The T/C asymmetry, a measure of the rate of dislocation core transformations, is increased by ternary additions, especially Ta and Hf, in the region of flow stress increase. However, above the peak temperature the positive asymmetry seen in binary alloys disappears with the addition of ternary elements. Fitting of a functional form for the activation enthalpy for cross-slip with experimental values suggests the existence of a composition-dependent APB anisotropy. The mechanism giving rise to the solid solution strengthening is related to dislocation core effects rather than to lattice parameter or modulus mismatch effects only above room temperature. Tensile ductility tests on single crystalline Ni$\sb3$Al + B alloys showed that at room temperature, B additions increase the ductility, but the opposite occurs at elevated temperatures. The effect of B additions at room temperature indicates that a bulk effect should be added to the grain boundary strengthening effect of B when explaining the improvement in ductility of polycrystalline Ni$\sb3$Al due to B additions