It was proposed in the Paper I that the grain boundary of the polycrystalline Ni3Al compound is intrinsically weak due to the heterogeneous bonding environment at the grain boundary region. In this paper, the effect of the substitutional third elements on the mergranular embrittlement of the Ni3Al compound was systematically investigated at room temperature. Various kinds of third elements ranging from groups IIIa to Vb in the periodic table were selected and alloyed in the Ll2 structure range. First the phenomenological aspects such as the mechanical tests, metallographic and fractographic observations were described. It was shown that the third elements (Mn and Fe) which have the similar electronic chemical bonding nature with the Ni atom prohibited the grain boundary fracture when they substitute for the Al site. Several factors responsible for these phenomena were considered. As a consequence, the value of the valency difference between the third element and constitutive solvent atom substituted by the third element seems to control the grain boundary strength of the ternary Ni3Al compound. The grain boundary is strengthened when the value of the valency difference is positive. The possible fracture mechanism was proposed, based on the crystal structures connected with the electronic chemical bonding nature at the grain boundary region. The modification in that the electronic chemical bonding environment of the grain boundary region becomes more homogeneous by the alloying makes the grain boundary stronger.
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