Slow strain rate testing of alloy 800 in molten nitrate salts

ABSTRACT:
An experimental technique has been developed to examine the interaction between deformation and the exposure of certain high temperature structural alloys to oxidizing molten salt environments. The experimental program involved performing a series of long-term tensile tests over a wide

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range of strain rates. Fracture strain reduction in area and ultimate strength (UTS) were monitored as parameters indicative of an alloy's susceptibility to enviornmental degradation. For Incoloy Alloy 800 tested at 600°C in the salt medium and at initial strain rates between 2 x 10-7 sec-1 and 1 x 10-5 sec-l no appreciable loss of ductility, as measured by reduction in area, was observed relative to control specimens tested in air at the same temperature and strain rates. Similarly, fracture strain and UTS were essentially unaffected by exposure to the oxidizing environment. The structure of the oxide film formed by contact of the specimens to the molten salt was affected by the imposed continuous deformation. Deformation resulted in an oxide which was thicker than that formed on undeformed saltexposed surfaces. While this difference in oxide thickness was a measurable phenomenon, the increased rate of oxidation was not great enough to appreciably alter alloy mechanical behavior in the salt compared to that measured in air. In addition, fine subsurface cracks were observed in those oxides formed on deformed surfaces.

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