STRENGTH AND DUCTILITY OF NI3GA WITH AND WITHOUT BORON

Experiments have shown that upon alloying Ni-rich Ni3Ga with 0.11 at.% boron the contribution of the grain boundaries to both the yield stre ss and the hardness is reduced by about 40%. Correspondingly, the duct ility is increased from about 2% elongation to about 10% elongation, a lthough the fracture mode remains predominantly intergranular. In dry oxygen or under reduced pressure, the ductility of the boron-free allo y, increases to about 3%, as it does when the alloy is strained in air at a high rate (equal to or greater than(1)0(-2) s(-1)). Auger electr on spectroscopy revealed an enrichment of boron at grain boundaries in the doped alloy. In situ TEM straining experiments showed that in bot h alloys slip is transmitted from grain to grain through the nucleatio n of dislocations at the heads of dislocation pile-ups. Slip dislocati ons are comprised of pairs of a/3 [211] superlattice partials coupled by a superlattice intrinsic stacking fault. The effects of boron on th e mechanical behaviour are explained in terms of the nucleation of dis locations at the heads of pile-ups and the accommodation/transmission of slip at/across grain boundaries.