Nitrogen absorption by Fe-1.04at.%Cr alloy: uptake of excess nitrogen

By dedicated pre-nitriding (at 580 C in an ammonia/hydrogen gas atmosphere) and de-nitriding (at 470 C in a hydrogen gas atmosphere) experiments, performed on Fe-1.04at.%Cr alloy, it could be demonstrated that the uptake of ‘‘excess’’ nitrogen by the nitrided ferritic matrix is not due to the presence of iron in chromiumnitride precipitates, as it was suggested previously. The determination of nitrogen-absorption isotherms for these pre-nitrided and de-nitrided Fe-1.04at.%Cr alloy specimens revealed that the total amount of excess nitrogen in the alloy is composed of two parts: (a) nitrogen adsorbed at the precipitate/matrix interface, and (b) nitrogen dissolved interstitially in the ferrite matrix strained by the misfit between (coherent) the CrN precipitates and the matrix.