Mechanism of order-induced embrittlement in Ni-Ni₄Mo alloys

Intergranular embrittlement of Ni-Mo alloys by long-range ordering to Ni₄Mo was examined for an off-stoichiometric alloy and a stoichiometric alloy of known impurity contents. Both tensile properties and corrosion resistance in HCl were measured as functions of exposure time at 700°C. Various techniques employed for microstructural characterization and microchemical analysis included analytical electron microscopy, X-ray diffractometry and Auger electron spectroscopy. During exposure at 700°C, the yield-strength maxima in both alloys were reached before the ductility minima. Homogeneous matrix ordering was observed to cause a moderate loss of ductility and the fracture mode remained to be transgranular. However, a considerable loss of ductility, intergranular embrittlement and extensive intergranular corrosion attack were found to be associated with heterogeneous grain-boundary ordering, which occurred by a discontinuous mechanism. Discontinuous ordering resulted in molybdenum-depleted zones alongside grain boundaries as evinced from microchemical analysis and localized corrosion attack. It was concluded that the observed intergranular embrittlement was caused by highly localized deformation in the molybdenum-depleted zones.