Effects of swift heavy ions at different fluencies on WC-6Co hard metal alloy

Abstract

Tungsten carbide hard metal alloy with 6% by weight cobalt was studied before and after irradiation at different fluencies with 167 MeV132Xe ions. Raman spectroscopy, X-ray diffraction, neutron diffraction and positron lifetime spectroscopy were employed in order to assess the microstructural evolution in the material upon irradiation fluence increase. Analysis of the Raman spectrum for the pristine, non-irradiated material unveils that the surface is composed of a graphite-like phase and highly oxidized tungsten atoms spread in the carbon matrix. All characteristic peaks of tungsten carbide (WC) and possible cobalt phases are either missing or strongly overlapped in all Raman spectra. Bonding between tungsten and oxygen atoms broke upon irradiation and total deoxidation of the surface is detected for the two highest fluencies investigated at 5 × 1013 ions/cm2 and 3,83 × 1014 ions/cm2. Increasing the irradiation dose causes amorphization of the carbon phase on the surface accompanied by “up and down” trend of change in carbon cluster size. The Raman spectra analysis also unveils, that molecular nitrogen (N2) from the atmosphere penetrates the carbon matrix upon irradiation. The results from the X-ray and neutron diffraction reveal that the main phase in the material is δ-WC and also give information about changes of the lattice parameters with increasing fluence. Reorganization of the induced point defects to dislocation defects as a function of the irradiation dose is discussed, but no phase transition of the main δ-WC phase is detected. Steady increase of compressive internal stress with increasing irradiation dose is noted by XRD. The tendency is not monotonic and the stress leans towards saturation at the highest fluence, with the highest value of −5.26 GPa. The Positron lifetime spectroscopy measurements show the presence of short lifetime component ranging from 170 ps to 190 ps, interpreted as small vacancy clusters. The intensities of the different positron lifetime components vary with the irradiation dose non-monotonically.