Abstract
Several investigations of the effects of environment on the mechanical properties of coated superalloys are examined, and the results that have been obtained are discussed. The short–time mechanical properties of superalloys (especially cast alloys, which have their optimum mechanical properties in the as–cast condition) may deteriorate during the coating process (e.g. on thermal cycling during coating, during post–coating heat treatment, or by coating surface effects). The degradation of the properties of the base material leads to an increase in creep rate, so the elongation that can be tolerated before the coating starts to crack will be reached in a shorter time. In a corrosive environment, stresses may be enhanced by general corrosion, with consumption of the coating and a measurable reduction of the cross–section, or by selective corrosion attack after a crack is initiated in the coating and propagates rapidly along grain boundaries, or by both general and selective corrosion. The effects of general corrosion may be calculated, but the effects of selective corrosion cannot.
MST/286
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