Effect of Al-rich off-stoichiometry on the yield stress of binary Ni3Al single crystals

Stoichiometric binary single-phase Ni3Al single crystals (Ni-25 at.%Al) and A-rich single crystal-like Ni3Al alloys (Ni-26 at.%Al, Ni-27 at.%Al) have been successfully produced using the floating zone method (FZ). The Al-rich crystals consisted of single crystalline Ni3Al matrix and NiAl precipitated phase. The compressive yield stress of the as-grown crystals was measured at 293-1273 K for loading axes between [001] and [011]. All the crystals exhibited the yield stress anomaly. Compared to Al-rich crystals, stoichiometric Ni3Al exhibited higher peak temperature of the yield stress anomaly, lower yield stress below the peak temperature, but higher yield stress above the peak temperature. These features are thought to result from the combined effect of Ni3Al off-stoichiometry and NiAl second phase on the yield stress. The critical resolved shear stress (CRSS) on primary octahedral (111)[(1) over bar 01] slip system in the yield stress anomaly region was found to be orientation-independent for stoichiometric Ni3Al, but strongly orientation-dependent in Al-rich crystals. Thus, the Schmid law is obeyed in stoichiometric Ni3Al single crystals, whereas it is violated in off-stoichiometric crystals. (C) 1998 Acta Metallurgica Inc.