Alternative Sample Preparation of Co-28Cr-6Mo to Avoid Strain-induced Phase Transformation

The allotropic transformation, characteristic of cobalt-based alloys, occurs at around 970 ºC in Co-28Cr-6Mo alloys. When subjected to fast cooling, such alloys can maintain the high temperature cubic centered phase at room temperature, resulting in a CFC metastable matrix. This metastable condition leads to a nucleation of the hexagonal phase (the stable one), which is induced by deformation or isothermally. In general, Co-based alloys are submitted to solution heat treatment plus aging to control both the precipitation of carbides and the nucleation of the hexagonal phase. When the study of this type of alloy is conducted in the metastable condition, it is extremely important do not induce the hexagonal phase during sample preparation. Traditionally, the metallographic route preparation is carried out using electropolishing to avoid the deformation-induced phase transformation. The need for specialized equipment and hazardous electrolytes to perform electropolishing limits its use. Therefore, the aim of the present work is to propose the use of an adaptation to traditional metallographic techniques in order to prepare Co-28Cr-6Mo alloy samples. To this end, the samples prepared by three different routes were analyzed by X-Ray diffraction, including Rietveld refinement, as well as EBSD in order to identify and quantify the phases present in the structure. A control cold rolled sample was also analyzed. The results showed a significant reduction in the HCP phase fraction, strain-induced during grinding, after the application of the alternative preparation method proposed. Further studies might be useful to validate the present methodology.

Keywords:
sample preparation; metallography; XRD; Rietveld; EBSD