Elevated temperature compressive properties of N-doped NiAl

The elevated temperature properties of NiAl slightly enriched with approx900 appm nitrogen by atomizing the aluminide to powder under a nitrogen atmosphere have been determined. Compression samples were machined from hot extruded material and tested in air between 1100-1400K under both constant velocity and constant load conditions. It appears that N in solid solution contributes little to the creep strength of B2 nickel aluminide. Excess N leading to the formation of AlN and Al(O,N) particles, however, can have a pronounced effect on creep behavior. These fine second-phase particles stabilize a small grain structure which, in turn, can improve or reduce the mechanical strength, depending on the deformation conditions. Under certain test conditions, high-angle grain boundaries can break away from the particles and overall grain growth occurs, leaving behind a network of AlN and Al(O,N) particle. This network of particles is very effective in anchoring a small, stable subgrain structure that provides elevated temperature strength without being subject to undesirable, weakening grain-boundary deformation mechanisms. Overall, the results provide further evidence that creep in NiAl is dislocation-climb controlled which involves subgrain formation and that stabilization of subgrains will improve mechanical strength in the manner proposed by Sherby et al.