Improving the Low Temperature Ductility of NiAl

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Abstract
As part of a study aimed at developing a ductile NiAl-based alloy, ingots of Ni-Fe-Al alloys were cast and hot extruded to rods. The purpose of the iron additions was two-fold viz; to produce a change in the slip vector from to and, in one alloy, to add a L12-structured ductile second phase. Extruded Ni-20A1–3OFe was two-phase, containing a pro-eutectic B2 phase in a fine lamellar structure (< 0.5 μm spacing) of B2+L12phases. Room temperature tensile testing of both single extruded and double extruded alloys resulted in 8% and 22% plastic elongation and yield stresses of 850 and 760 MPa respectively. Fracture in both cases occurred by ductile tearing of the eutectic and transgranular cleavage of the proeutectic phase at 1350 MPa. The ductility in double extruded condition is higher than that reported earlier in rapidly solidified wires by Inoue et al., J. Mat. Sci.,19, (1984), 3097. By comparison, extruded single-phase B2-structured Ni- 30AI-2OFe exhibited a fracture strength of 780 MPa, no plasticity and a mixture of intergranular fracture and transgranular cleavage. This is in contrast to earlier work by Inoue et al. (ibid) where a yield stress of 400 MPa, 5% plastic strain and a mixture of dimple and intergranular fracture was reported.