Effect of Nitrogen ion implantation on the cavitation erosion resistance and Cobalt-based solid solution phase transformations of HIPed Stellite 6.
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From the wide range of engineering materials traditional Stellite 6 (cobalt alloy) exhibitsexcellent resistance to cavitation erosion (CE). Nonetheless, the influence of ion implantation ofcobalt alloys on the CE behaviour has not been completely clarified by the literature. Thus, this workinvestigates the effect of nitrogen ion implantation (NII) of HIPed Stellite 6 on the improvement ofresistance to CE. Finally, the cobalt-rich matrix phase transformations due to both NII and cavitationload were studied. The CE resistance of stellites ion-implanted by 120 keV N+ions two fluences:5×1016cm−2and 1×1017cm−2were comparatively analysed with the unimplanted stellite andAISI 304 stainless steel. CE tests were conducted according to ASTM G32 with stationary specimenmethod. Erosion rate curves and mean depth of erosion confirm that the nitrogen-implanted HIPedStellite 6 two times exceeds the resistance to CE than unimplanted stellite, and has almost tentimes higher CE reference than stainless steel. The X-ray diffraction (XRD) confirms that NII ofHIPed Stellite 6 favours transformation of theε(hcp) toγ(fcc) structure. Unimplanted stelliteε-richmatrix is less prone to plastic deformation thanγand consequently, increase ofγphase effectivelyholds carbides in cobalt matrix and prevents Cr7C3debonding. This phenomenon elongates threetimes the CE incubation stage, slows erosion rate and mitigates the material loss. Metastableγstructure formed by ion implantation consumes the cavitation load for work-hardening andγ→εmartensitic transformation. In further CE stages, phases transform as for unimplanted alloy namely,the cavitation-inducted recovery process, removal of strain, dislocations resulting in increase ofγphase. The CE mechanism was investigated using a surface profilometer, atomic force microscopy,SEM-EDS and XRD. HIPed Stellite 6 wear behaviour relies on the plastic deformation of cobalt matrix,starting at Cr7C3/matrix interfaces. Once the Cr7C3particles lose from the matrix restrain, theydebond from matrix and are removed from the material. Carbides detachment creates cavitation pitswhich initiate cracks propagation through cobalt matrix, that leads to loss of matrix phase and as aresult the CE proceeds with a detachment of massive chunk of materials.
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Rekord utworzony: | 30 kwietnia 2021 11:23 |
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Ostatnia aktualizacja: | 1 stycznia 2023 23:06 |