Toughening of nickel aluminide (NiAl) composites

Nickel aluminide (NiAl) is a candidate material for high temperature (above 1000°C) structural application in aerospace engine components due to its attractive properties such as high melting point (1638°C), moderate density (5.85 g/cm3) and excellent oxidation and corrosion resistance up to 1300–1400°C. However, NiAl exhibits brittle behavior with low ductility (less than 1–2%) and fracture toughness ($\sim$5–7 MPa√m) at room temperature. This makes NiAl unsuitable as a structural material. The principal objective of the current study was to improve the fracture toughness of NiAl by reinforcement with a second phase and to study the toughening mechanisms in general for brittle matrix composites. Three different kinds of reinforcement architectures were considered: NiAl composites reinforced with ductile layer (vanadium and Nb-15Al-40Ti), NiAl composites reinforced with partially stabilized zirconia (2 mole % yttria stabilized zirconia - YSZ), and hybrid NiAl composites reinforced with both YSZ and ductile phases (molybdenum particulates and vanadium layers). Ductile layers can improve the fracture toughness of brittle phase primarily by the crack bridging mechanism, while YSZ has the potential to improve the fracture toughness of the matrix through the mechanism of stress-induced transformation toughening. The effects of thickness of two different ductile layers (vanadium and Nb-15A1-40Ti) on the resistance curve behavior of the layered composites were studied. The results showed that steady-state toughness in these composites increases with the increasing thickness of ductile layer. Toughening analysis was performed in the framework of large-scale bridging toughening mechanisms, which provide good agreement between the experimental results and theoretical calculations. Different responses of the layered composites under monotonic loading and cyclic loading were compared. In addition to traditional analytic toughening models, finite element analysis was conducted to elucidate the crack/microstructure interactions in the layered MAIN composites. The resistance-curve behavior of the NiAl/YSZ composites was studied and significant toughening was achieved in these composites. The stress-induced transformation in the NiAl/YSZ composites was studied using Raman spectroscopy and the shielding contributions from transformation toughening were estimated using both dilatational and dilatational plus shear formulations. It was found the dilatational models alone usually underestimate the experimentally measured toughness increments. In the final part of the current study, synergistic toughening of NiAl composites was explored using two model hybrid toughened composites—NiAl/YSZ/Mo _(p) composites and layered NiAl/YSZN composites. The significant improvement of initiation toughness and resistance-curve behavior was quantified using both linear superposition concepts and upper and lower bound synergistic toughening analysis. The results showed encouraging promise of engineering synergistic composites to achieve maximum toughening with optimal microstructure.