| With the continuous improvement of people's requirements for energy conservation and environmental protection,the research on lightweight materials becomes more in-depth.As the lightest metal structural materials,magnesium alloy possesses the high specific strength and stiffness,good damping performance,high thermal conductivity and strong electromagnetic shielding performance,so it has a wide application prospect in aerospace,automobile and electronic products.However,the low strength,poor heat resistance and corrosion resistance of magnesium alloy prevent its further application.The studies at home and abroad show that the mechanical properties and corrosion resistance of Mg-RE-Zn alloys can be improved by forming long period stacking ordered(LPSO)phase.After proper hot extrusion deformation,the properties of the alloys will be further optimized.At present,the RE elements that added in Mg-RE-Zn alloys are mainly Y,Gd,Er,Dy,Ce,etc.Relatively speaking,there are few reports about the addition of Ho.The Ho element with high solubility will promote the solution and precipitation strengthening effects,so as to enhance the mechanical properties of the alloys.Thus,in this paper,we selected the Ho as the additive element and designed as-extruded Mg-x Ho-2Zn(x=4,6,8 wt.%)alloys with different Ho contents.The effects of Ho content on the microstructure of the alloys were systematically studied;the mechanical properties of the alloys changed with the microstructure and the strengthening and toughening mechanism were studied;the corrosion behavior and its influencing factors of the alloys were studied.The conclusions were as follows:(1)The effects of Ho content on the grain size,phase composition,quantity and distribution of the alloys were studied.The results show that increasing Ho content can refine the grain size,change the phase composition and increase the volume fraction of the second phase of the alloys.The increase of Ho content is beneficial to the formation of 18R-LPSO phase.Mg-4Ho-2Zn and Mg-6Ho-2Zn alloy are mainly composed of ?-Mg and W phase,while Mg-8Ho-2Zn alloy is mainly composed of ?-Mg,W phase and 18R-LPSO phase.(2)The mechanical properties of alloys at room temperature and elevated temperature were analyzed by tensile test.The results show that the yield strength and ultimate tensile strength of the alloys increased with the increase of Ho content,while the elongation first decreases and then increases.With the increase of temperature,the yield strength and ultimate tensile strength of the alloys decrease,while the elongation increases.The Mg-8Ho-2Zn alloy containing LPSO phase has better heat resistance and the best comprehensive mechanical properties at room temperature and high temperature.(3)The tensile fracture morphology of alloys at room temperature and elevated temperature was observed by SEM.The results show that there are a certain number of dimples and cleavage facets on the fracture surface of the alloys at room temperature,exhibiting typical quasi-cleavage fracture.With the increase of temperature,the plasticity of the alloys gradually improved.When the temperature is 300 °C,there are lots of deep dimples on the fracture surface of the alloys,exhibiting the characteristic of ductile fracture.(4)The corrosion resistance of alloys was analyzed by immersion corrosion test.The results show that the corrosion resistance of Mg-Ho-Zn alloy decreased firstly and then increased with the increase of Ho content.The Mg-8Ho-2Zn alloy containing LPSO phase exhibits lower corrosion rate and less corrosion pits.However,the Mg-6Ho-2Zn alloy containing corrosion more W phase exhibits higher corrosion rate.The corrosion pits are connected with each other and the corrosion area extend to the whole surface.(5)The results of polarization curves and electrochemical impedance spectroscopy show that the three alloys have the same corrosion mechanism but different corrosion resistance.The Mg-8Ho-2Zn alloy has high corrosion potential and low corrosion current,which shows good corrosion resistance.While the Mg-6Ho-2Zn alloy has high corrosion current,which shows poor corrosion resistance. |
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