The effect of a ductile second phase on the room temperature mechanical behavior of directionally solidified beta (B2) nickel-aluminum and nickel-aluminum-iron alloys.

Abstract

The effect of ductile $\gamma$ (fcc) or $\gamma\sp\prime$ (L1$\sb2$) second phases on the room temperature mechanical behavior of $\langle001\rangle$ oriented $\beta$ (B2) Ni-Al and Ni-Al-Fe alloys is studied. The model materials used had the following nominal compositions (in at.%): Ni-30Fe-20Al and Ni-30Al and were prepared by directional solidification. Room temperature tensile ductility of up to 12% and fracture toughness (K$\sb{\rm Q}$) of up to 30 MPa$\surd$m is seen in these multiphase alloys, whereas the single phase $\langle001\rangle$ $\beta$ is known to have $\sim$0% tensile ductility at room temperature. The enhancement in ductility and toughness is primarily due to nucleation of mobile $\langle100\rangle$ dislocations in the $\beta$ phase as a result of slip transfer from the ductile second phase. The enhancement in ductility and toughness is less pronounced in alloys where the less mobile $\langle111\rangle$ superdislocations are nucleated in $\beta$ phase as a result of slip transfer. The slip transfer process across the $\beta/\gamma$ interface boundaries is studied in detail. The nucleation of $\langle100\rangle$ dislocations in the $\beta$ phase, even though the loading axis is parallel to $\langle001\rangle$, is explained by the high ratio of the critical resolved shear stress (CRSS) of $\langle111\rangle$ slip to the CRSS of $\langle100\rangle$ slip. The reduction of this ratio by bcc precipitates in $\beta$ phase favors $\langle111\rangle$ slip. A part of the ductility and toughness enhancement in these multiphase alloys is due to extrinsic mechanisms such as crack bridging and blunting. These results indicate that alloying to include creep strengthening phases/solutes in these tough $\beta/\gamma\sp\prime$ microstructures may allow the development of a multiphase NiAl based alloy for high temperature structural applications.