Effect Of Bi Additions On Microstructure And Mechanical Properties Of Mg-Zn-Co Alloy

Precipitation hardening is one of the most effective method to improve the strength of Mg alloys. On the background of developing rare earth element-free Mg alloys, 1wt.%, 2wt.% and 3wt.% Bi were added to study the effect of Bi additions on age hardening behaviors and mechanical properties of Mg-6Zn-0.6Co alloy. Ultimately, Mg-Zn-Co-Bi alloy with high strength was made successfully. Scanning electron microscope(SEM), high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM) and other techniques were used to characterize the crystallography of strengthening phase, including precipitate types, dimension, number density, microstructure and orientation relationships(ORs) with matrix, which helps to optimize the design and application of Mg alloys. Furthermore, a mechanical property test was performed to establish the relations between the alloy composition, treatment process, microstructure and mechanical properties.The age hardening behaviors and the microstructures of Mg-Zn-Co-x Bi casting alloys were studied. With Bi additions increasing, the age hardening response of the alloy is improved gradually at 200oC. Compared with that of the Mg-Zn-Co alloy, the peak hardness increases from 68 HV to 78 HV with the optimal addition of 3.5 wt.% Bi, and the time to peak hardness is reduced to about 2.5h, which is attributed to the higher number density of rod-like '1?, homogeneously dispersed prismatic Mg3Bi2 plates, and the occurrence of Mg3Bi2 precipitates lying on pyramidal and basal planes, though their quantity is small. More quenched vacancies may have been retained since Bi has a large binding energy with vacancy, which promotes nucleation and growth of precipitates, thus resulting in an increased the number density of refined rod-like '1? in peak-aged stage and the fast coarsening rate in over-aged stage. The prismatic, basal and pyramidal Mg3Bi2 precipitates all have different ORs, during which two of them have never been reported before. The major facets of the cross-sections of three types of Mg3Bi2 precipitates are invariably parallel to23 gB i M0)0 0 1( plane and the preferred interface is rationalized by(near coincidence site) NCS model. Due to the chemical affinity with Bi and Zn, Co was segregated into Zn-enriched and Bi-enriched phase in the precipitates in grain interiors and intermetallics around the boundaries, without changing the phase crystal structure. Temperature can alter average aspect ratio of precipitates.Bi additions on microstructure and mechanical properties of the extruded Mg-Zn-Co-x Bi alloys were studied. After extrusion and T6, alloy with 3Bi exhibits an enhanced age hardening response where three types of Mg3Bi2 and more rod-like '1?were produced. Thus, superior mechanical properties with tensile yield strength of 322 MPa was attained. The yield strength is 15 MPa and 49 MPa higher than alloy without Bi in tention and compression, respectively. Simultaneously, the yield asymmetry is improved to some extent due to the enhanced precipitation hardening.