Surface hardening of Ni alloys by plasma nitriding was investigated by using tentative Ni binary alloys contained nitride forming elements such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Mn, Fe, Al, or Si at the nitriding temperature from 673 to 1073 K. Surface hardness was different depending on the types of alloying elements, their contents, and their nitriding temperatures. Higher hardness than HV500 was obtained in Ti, V, Nb, and Cr containing alloys at 823 to 873 K, but other alloys showed lower surface hardness than HV400. The elements Ti, V, Nb, and Cr were the effective alloying elements for the surface hardening of nitrided Ni alloys. From transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis, the nitrided layer was composed of fine precipitate particles in the matrix of the nitrided layer. At the lower nitriding temperature, these particles were metastable fine particles or Ginier-Preston (GP) zone having coherency with the matrix, and these fine particles induced large microstrain in the matrix. However, at the higher nitriding temperature, equilibrium nitride particles were precipitated and coherency with the matrix was decreased. Therefore, the hardening of Ni alloys by plasma nitriding was due to the microstrain induced in the nitrided layer by the precipitation of metastable particles or GP.
