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AEM examinations of a NiAlCo alloy of composition Ni-29 at.% Al-21 at.% Co after room temperature compression show that the microstructure consists of a twinned tetragonal matrix (L10, marked A in Figure 1a) and ordered fcc gamma prime precipitates (L12, marked B in Figure 1a) along grain boundaries. The compressive yield strengths of this alloy at room temperature and 760°C are 754 MPa and 163 MPa respectively. It also has superior room temperature ductility as compared to binary NiAl. An addition of 5 at.% Ti at the expense of Ni was made to this alloy in order to increase the yield strengths. The quarternary alloy shows compressive yield strengths of 976 MPa and 403 MPa at room temperature and 760°C, respectively, indicating that the Ti addition is having the desired effect.Comparison of the microstructures of the two alloys after room temperature compression (Figures la and lb) shows that the Ti containing alloy has a smaller grain size. X-ray diffraction data indicate that the gamma prime volume fraction increases from 10% to 20% as the result of the Ti addition. Titanium was also found to stabilize the B2 matrix (marked A in Figure lb) as no tetragonal L10 phase was found. All precipitates along grain boundaries were identified by micro-diffraction to be gamma prime.

Cobalt-modified NiAl alloys are being studied for their potential for room temperature ductility and toughness. An alloy of Ni - 29.3 a/o Al - 36.7 a/o Co has shown improved toughness and ductility with respect to binary NiAl alloys due in part to a stress-induced martensitic transformation. Furthermore, the cobalt additions have altered the slip behavior to {110}<111> type from {110} <001> for binary NiAl alloys. Hafnium was added to improve the alloy's relatively poor creep resistance and high temperature strength. Hf was found to be insoluble in the NiAlCo alloy and formed precipitates with a hexagonal structure. The Hfmodified alloy had improved high temperature strength. In addition, the Hf apparently changed the creep mechanism resulting in poorer creep resistance at low temperatures, but improved creep resistance at higher stresses and temperatures.