Effects of test orientation on fracture and fatigue crack growth behavior of third generation as-cast Ti–48Al–2Nb–2Cr

• Effects of load ratio on fatigue crack growth of as-cast TiAl investigated.
• Large colony size and large lamellar spacing was demonstrated.
• Little effects of sample orientation on fatigue and toughness.
• Very large effects of load ratio on fatigue threshold, Paris slope.
• Roughness of fracture surface quantified and related to load ratio effects.

The effects of changes in test orientation and load ratio on the room temperature fracture and fatigue crack growth behavior of as-cast Ti–48Al–2Nb–2Cr titanium aluminide was investigated to determine the presence of any anisotropy in mechanical properties. As-cast samples were tested in the longitudinal and transverse directions to the casting direction at room temperature in air. Load ratios ranging from R = 0.1 to R = 0.9 were used in the fatigue tests in order to determine its effects on the threshold for fatigue cracking, the Paris law slope, and fatigue crack instability toughness, Kc, in addition to determining both notched and fatigue-precracked values for toughness. Optical metallography and SEM fractography were used to document the effects of orientation on the fracture path and morphology. Significant effects of changes in load ratio were obtained on the fatigue threshold and Paris law slope, while its effects on Kc and the effects of sample orientation were found to be minimal. These are rationalized by considering microstructural effects on the properties measured and are compared to similar materials processed via different techniques.