Precipitate formation in low-temperature nitrided cold-rolled Fe94Ni4Ti2 and Fe93Ni4Cr3 films

Using various experimental techniques, the formation and evolution of precipitates. the dilation of the matrix, and the nitrogen uptake and release were investigated in cold-rolled Fe94Ni4Ti2 and Fe93Ni4Cr3 materials subjected to nitriding in a gaseous mixture of NH3 + H-2 and to reduction in an H-2 atmosphere at low (300 degreesC) to moderate (600 degreesC) temperatures. Transmission electron microscopy (TEM) revealed that in both alloys the precipitates are thin platelets. In the Ti-containing alloy, the precipitates were also small in the lateral dimensions. The difference in size, aspect ratio of the precipitates, and misfit and coherency on the interface consistently explain the substantial differences in dilation of the matrix and nitrogen uptake for the Ti- and Cr-containing alloys under examination. The results provide evidence for the formation of mixed Fe-Ti-N precipitates (Guinier-Preston (GP) zones) in the early stages of nitriding.