Abstract
A correlation was made between long-range ordering to Ni4Mo (A4 → D1a superlattice) in a Ni-28wt % Mo alloy and its corrosion properties. Emphasis was placed upon the effects of ordering on: (i) microchemical composition including grain-boundary chemistry, and (ii) plastic deformation behaviour. Analytical electron microscopy and Auger electron spectroscopy were utilized for microstructural characterization and microchemical analysis. Corrosion testing in the ordered state revealed a considerable increase in the corrosion rate in boiling 20% HCl and also less resistance to stress corrosion cracking in boiling 10% HI, all relative to the disordered state. Molybdenum-depleted zones were detected in the ordered microstructure near the Ni4Mo-matrix interfaces, at antiphase boundaries and alongside grain boundaries. Examination of the tensile deformation substructure indicated that ordering had lowered the stacking-fault energy of the alloy. It was concluded that the observed degradation in the chemical stability of the alloy in the ordered state was due to inhomogeneities in microchemical composition and a low stacking-fault energy.
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