| Gamma-TiAl alloys are potential replacements for conventional titanium and nickel-based alloys due to their high specific stiffness and good oxidation resistance at intermediate temperatures. One obstacle to implementation of these alloys is their relatively low ductility and the variation in that property. This work is an attempt to quantify ductility variability and understand the microstructural variables that affect it. It was found that a three-parameter Weibull analysis of tensile data using yield strength as the minimum failure strength isolates the variation in ductility from that in yield strength, and provides a useful way to quantify this variability. The properties of cast gamma-TiAl alloys are consistent with a volume-distributed flaw population, including exhibiting the expected reduction in strength with increased sample volume. Although other factors affect both strength and average ductility, the variability appears to be controlled by grain size alone; alloys with different microstructures but similar grain sizes had similar Weibull moduli. Fracture surface examinations indicated that the ‘flaws’ are intrinsic in nature; the fracture origins were generally not associated with pores, inclusions, or second phase particles. Instead, the typical origin was located in a small region that had undergone extensive plastic deformation. A technique, called AMaSD, was developed to accurately measure small-scale surface strains and identify the microstructural features associated with these areas of local high strain. These studies found that several factors affect the development of strain localization, including relative grain orientation, segregation, and phase distribution, but the relative importance of these factors are difficult to quantify. Finally, the cast structures of these materials were examined using texture measurements and statistical segregation studies, since texture and segregation had been identified as factors that could affect the distribution of high strain regions. Both alloy composition and cooling rate were found to affect the cast texture, but most alloys examined had textures consistent with solidification as single-phase hcp alpha growing along the c-axis. Slow cooling and high levels of aluminum promote alpha [1 0 - 1 0] growth instead, and it appears that high levels of niobium can promote [1 0 0] bcc beta-phase solidification. |
