BEHAVIOR OF BORON IN POLYCRYSTALLINE AND MONOCRYSTALLINE NI3AL AND ITS EFFECT ON STRENGTH AT ROOM AND HIGH-TEMPERATURE

The behaviour of boron atoms in Ni3Al and its effect on strength properties has been systematically studied by means of metallography, X-ray diffraction, transmission electron microscopy, ion probe analysis, positron annihilation and atom probe-field ion microscopy. Results show that the yield strength of poly- and monocrystalline Ni3Al at 20 and 800-degrees-C increases with increasing boron content, mainly because of solid solution strengthening by boron atoms which occupy interstitial sites in the crystal lattice of Ni3Al. Its effect of filling the void-type defect and increasing the bond energy of nearest-neighbour pairs of Ni-Ni and Al-Al have also made some contributions to the strength properties. When the boron content exceeds its solubility in Ni3Al and reaches 2.22 at.%, the strength decreases markedly because of the formation of a boride eutectic which promotes crack nucleation and propagation. The rupture life of polycrystalline Ni3Al increases with increasing boron content, an important cause of which is the grain boundary strengthening effect of the boron atoms in addition to the above factors. The rupture lives of monocrystalline Ni3Al are longer than those of polycrystalline alloys with a similar boron content.