Suppression of environmental embrittlement in boron-free Ni3Al by unidirectional solidification

Abstract

Polycrystalline Ni[sub 3]Al has long been considered to be brittle, whereas single crystals of Ni[sub 3]Al are known to be ductile. Auger works have revealed that brittle intergranular fracture occurs even when the grain boundaries are free of embrittling impurities. Grain boundaries of Ni[sub 3]Al were, therefore, concluded to be intrinsically brittle. One of the authors recently developed a method to ductilize Ni[sub 3]Al by unidirectional solidification using a floating zone technique, which is called FZ-UDS. Binary Ni[sub 3]Al grown by FZ-UDS, showing a columnar-grained structure, and containing no ductility-enhancing elements, exhibited extensive tensile elongation over 50%. Even hyperstoichiometric Ni[sub 3]Al could be ductilized by FZ-UDS, while the addition of B, which is recognized as the most common and powerful ductility-enhancing element, is effective only in the hypostoichiometric composition. Although the Ni[sub 3]Al grown by FZ-UDS is known to exhibit a good ductility as mentioned above, it has not been studied yet whether the alloy is prone to environmental embrittlement or not. This study was undertaken to clear this point. The authors' results show that the stoichiometric NI[sub 3]Al grown by FZ-UDS exhibits no environmental embrittlement at room temperature; tensile tests in water or the tests at slow rate inmore » air did not reduce the ductility of the alloy.« less