| This study investigated the structure-property relationships of a newly developed Ni-based cast superalloy and a {dollar}gamma{dollar}-based TiAl alloy.; The investigation on the superalloy concentrated on understanding the effect of grain size and precipitate morphology on crack growth under creep and low cycle fatigue conditions at elevated temperatures. A series of mechanical tests conducted on fine and coarse grain samples revealed significant differences in mechanical properties. Fine grain samples had a much lower fracture toughness compared to coarse grain samples. In addition, when holding periods at the peak load were imposed, the fatigue crack growth rate of the fine grain material accelerated, in sharp contrast to the constant crack growth of the coarse grain material. Analysis of the samples before and after testing for microstructural changes and crack growth mechanisms indicated the importance of grain size and microstructure on high temperature properties. For example, a high volume fraction of large {dollar}delta{dollar} plates significantly weakened the precipitate strengthening effect. Also, the {dollar}delta{dollar}-rich regions acted as preferred crack initiation sites and crack propagation paths.; The mechanical behavior of a {dollar}gamma{dollar}-based TiAl alloy on variations in solidification rate, post-cast heat treatment and testing temperature was determined. After mechanical testing, samples were examined for deformation-induced defects and fracture surface morphology. These analyses were used to correlate the macro-mechanical behavior to the micro-mechanism of deformation. TEM examination revealed that, in the tested samples, equiaxed {dollar}gamma{dollar} grains had the highest defect density and the {dollar}alphasb2{dollar} plates had the lowest. The role of {dollar}alphasb2{dollar} and its contribution to the plastic deformation of the alloy were discussed based on the defect configuration. It showed that {dollar}alphasb2{dollar} itself had little direct participation in the deformation process. However, its appearance indirectly assisted the deformation of {dollar}gamma{dollar}. This conclusion supported the bond theory which attributes the brittleness of the TiAl intermetallic to its covalent bond, and explained the ductility behavior for a wide composition range of {dollar}gamma{dollar}-based TiAl alloys. |
