Pinning of dislocations by solutes in NiAl

Four alloying elements Zr, Hf, B and C added in intermetallic NiAl were found to enhance yield strength. The large increment of yield strength observed cannot be explained by conventional mechanisms such as particle strengthening and solid solution hardening. A new approach based on break away of edge dislocations from Cottrell atmosphere is proposed. The Cottrell atmosphere, as originally proposed for interstitial atoms in the neighborhood of the edge dislocation, is extended for both substitutional and interstitial solutes, with Zr and Hf being substitutional atoms replacing Al and B and C interstitials. This quantitative model yields results in good agreement with the experimental data for the substitutional impurities, but for interstitial solutes the calculated values are smaller than the observed increase in yield stress. The embrittlement of NiAl by these solutes is also discussed in terms of dislocation pinning and the increase of yield stress.