Fatigue-crack growth of small cracks in a directionally-solidified nickel aluminide with molybdenum additions

Abstract

In recent years, aluminide intermetallic compounds, such as NiAl and TiAl, have been contemplated for potential high-temperature structural use in aero-engine applications as possible replacement for the nickel-base superalloys. However, like many intermetallics, its application is severely compromised by its very low ductility and toughness properties at ambient temperatures; moreover, NiAl displays lower strength at elevated temperatures. Accordingly, much alloy design and microstructure research on NiAl has been focused on attempts to improve the low-temperature ductility, fracture toughness and high-temperature strength of this intermetallic. However, despite progress in the toughening of NiAl-based materials under monotonic loading, nothing is known about their fatigue-crack growth properties under cyclic loads. This is important since it is known from studies on other ductile-phase toughened intermetallics ({gamma}-TiAl, Nb{sub 3}Al, MoSi{sub 2}) that extrinsic toughening (crack-tip shielding) mechanisms such as crack bridging are far less potent, and indeed degrade substantially, under cyclic loading due to premature fatigue failure of the ductile phase. Moreover, for many engine applications such as turbine blades, it is the fatigue properties of small flaws (typically below {approximately} 500 {micro}m in size), especially at near-threshold stress intensities, that are limiting with respect to lifetime considerations. Accordingly, it is the objective of themore » current note to present the first results on the fatigue behavior of NiAl aligned-eutectic composites. Specifically, the near-threshold fatigue-crack propagation properties of physically-small (< 400 {micro}m) surface cracks in one such alloy, namely a DS NiAl-9Mo (at.%), are examined as a function of the load ratio, with the objective of discerning the salient mechanisms affecting crack-growth behavior.« less