Abstract
Iron-nitrogen martensite was prepared by the gaseous nitrogenization of iron in the austenite-phase field, followed by quenching. Low- and high-nitrogen contents (lathvs plate morphology) were investigated. Optical microscopy, transmission electron microscopy and diffraction, and differential thermal analysis were employed to study the crystallography and the tempering behavior of the martensites. The orientation relation between austenite and martensite, the habit plane and types of (post-) transformation twins were determined. Tempering-induced changes in hardness and release of heat were related to structural changes as revealed by electron diffraction. Differences with analogous iron-carbon martensites were discussed. Aging at room temperature leads to nitrogen diffusion-controlled precipitation of coherentα″-Fe16N2 platelets, which process is completed after about one day. Aging at higher temperatures (up to about 475 K) results in incoherentα″particles, which, at temperatures above 460 K, exhibit small deviations (≃18 deg) from the usual {001}α′/α habit plane.
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