Title: Studying the residual macrostress and texture development upon low temperature nitriding of Co-Cr alloys

Abstract

Low temperature gaseous nitriding of Co-Cr alloys at 400°C with the initial hexagonal close-packed (hcp) crystal structure results in the development of supersaturated hcp phase with the incoming nitrogen atoms. The residual macrostress develops on the surface as a result of volume misfit between the nitrided case and the untreated core. This macrostress affects the nitriding behavior of differently-orientated hcp variants at the surface, in a way that their further transformation to face-centered cubic (fcc) phase is anisotropic. The intent of current research is to investigate the magnitude of this residual macrostress as well as the resulted texture on the surface of nitrided specimen. The stress measurements were carried out using stress analysis method by X-ray diffraction, so-call the Sin2ψ method, on a surface of specimen with the nitrided layer containing expanded hcp phase. The measured (101) lattice spacings against Sin2ψ depicts an oscillation, which can be ascribed to the crystallographic texture. In addition, re-orientation of surface adjacent grains with respect to their orientation in the core is realized using electron backscatter diffraction (EBSD). The lattice elongation perpendicular to the specimen ´s surface can be assumed similar to the deformation resulted from the uniaxial tension. In a case of a sever deformation, an obvious texture may be observed for the grains at the surface similar to the fibre texture that can be achieved after the uniaxial tension of hcp materials.