RESISTOMETRIC STUDY OF IRON-BASED ALLOYS NITRIDED BY CONSTANT ACTIVITY AGING
YANG, MING MING
Doctor of Philosophy
Constant activity nitriding experiments were performed in Fe-V and Fe-Mo alloys. The nitriding was accomplished aging the alloys in a NH(,3)/H(,2) atmosphere at temperature between 400(DEGREES)C and 600(DEGREES)C, during which fine substitional-interstitial clusters are formed. These coherent clusters are small platelets lying on 100 ferrite matrix planes. During the nitriding process, these clusters form first near the surface of the sample. For the Fe-V alloys, nitriding proceeds by the advance of a interface bounding region with and without clusters. This results in a hardness profile which is discontinuous at the interface. The rate of the hardness front movement in Fe-V can be predicted by internal oxidation equation. For the Fe-Mo alloys, a distinct interface is not observed, and the hardness varies in a continuous fashion through the cross section of the sample. No particular model has been developed for those alloys. The resistivity rises during the nitriding for both Fe-V and Fe-Mo alloys. In Fe-V alloys, the nitrided region and unnitrided region are separated by a sharp boundary. Thus, the resistivity can be calculated by analogize this system to a pair of resistor connected in parallel. The resistivity at full saturation of nitrogen were explained mainly by formation of the coherent clusters in the alloys. It is a function of density, size distribution and average size of the clusters. From the resistivity maxima upon full saturation at different nitriding temperatures, we can understand the size distribution and average size of the clusters are function of nitriding temperature, while density of the clusters and total volume of the strain field are function of concentration of alloy elements and nitriding temperature. The static displacement around clusters has been estimated, from resistivity data, and equal 0.68(ANGSTROM) for a full nitrided Fe-3.0at.%Mo.