Research On Microstructures And Properties Of Magnesium Alloy With Rare Earth

To meet the demand of corrosion-resistant and high-strength magnesium alloy,this paper was engaged in the study of the effects and influence mechanisms of Rareearth （RE） to the corrosion-resistant performance and mechanical properties. Twokinds of alloys with different RE contents were involved to study the influence ofcasting, heat treatment and hot-extrusion deformation processes to the microstructuresand properties of the alloys.By investigating the influence mechanism of RE elements to thecorrosion-resistant performance of Mg alloy, it shows that, the voltage between theMg matrix and the RE second phases is decreased and the driving force of microelectrical-couple corrosion became weak and corrosion rate was dropped; thecorrosion-resistant performance was improved by the RE elements through the meansof optimizing the structure of the oxidation film to increase the film density, refiningthe grain size, changing the composition, shape and distribution of the second phases.The approaches of guaranting the stability of the RE Mg alloy, reducing the fluximpurities and gas purifying the melt were helpful to improve the corrosion-resistantperformance of the RE Mg alloy. Among the three methods of the squeeze casting,metal mold casting and sand casting, the RE Mg alloys prepared by the squeezecasting were provided with the finest grain size, the best mechanical properties andcorrosion-resistant performance.The investigation to the compressive stress-strain behavior of two kinds of theRE Mg alloys showed that the RE Mg alloys belong to the materials being sensive topositive strain rate, and the hot-compression plastic deformation of the RE Mg alloyswas up to the thermal activation.The investigation to the Mg-Nd-Gd-Zn-Zr alloy shows that, the RE Mg alloyswith favorable corrosion-resistant performance and casting processing performancecould be achieved by composition design and optimization, and the prepared alloy canbe provided with comprehensive properties by reasonable heat treatment. Because oflow-content RE, the precipitate strengthening effect of aging for the extruded alloywas not predominant, and the interaction between the dislocations and the RE secondphases was not obvious.The microstructures of the high RE-content as-cast Mg-xGd-3Y-Zn-0.5Zr alloys were consists of α-Mg and βphases（Mg₅Gd（Y,Zn））, and with the increase of the Gdcontent, the grain size of the alloys became finer and the eutectic phases on the grainboundaries transformed from continuous network-shape distribution to discontinuousdispersion distribution and the volume fraction of eutectic phases were also increasedcorrespondingly. During solution treatment, theβphases at the grain boundariesdescomposed and transferred into the block-shaped and lamellar-shaped14H-LPSOstructure phases. These LSPO structure phases were composed of elements of Mg, Zn,Gd and Y, and Zn was the key element for the formation of14H-LPSO structurephases. With the solution time further prolonged, the block-shaped phases alsogradually transferred into lamellar-shaped14H-LPSO structure phases. With the Gdcontent added, the peak hardness increased and the time to reach the peak hardnesswas shortened. With the temperature raised, the peak hardness decreased and the timeto reach the peak hardness was shortened. For Mg-8Gd-3Y-Zn-0.5Zr alloy, the β’phases precipitated and dispersed at200℃aging temperature, but β’ phases were notobserved when aging at250℃. For Mg-12Gd-3Y-Zn-0.5Zr alloy, the size of β’ phaseswas increased and the quantitiy of β’ phases was reduced when aging temperature wasraised, the distributed high-density β’ phases were contributed to aging hardeningeffects at200℃and225℃. When aging at250℃, the precipitated phases of theMg-12Gd-3Y-Zn-0.5Zr alloy were composed of β’ phases and β1phases, and the β’phases were gradually transformed into β1phases with the aging time prolonged.The investigation to the mechanical properties of cast Mg-xGd-3Y-Zn-0.5Zralloys shows that, when Gd content is not below10wt%, the ultimate tensile strengthincreased with the temperature raised between room temperatue and200℃, andreached peak value near200℃and then dropped gradually. At room temperature, thefracture mode of Mg-xGd-3Y-Zn-0.5Zr alloys was cleavage fracture and the plasticityof the alloys was dropped with the increase of Gd content. The fracture mode of thealloys was quasi-cleavage crack at elevated temperature.The grain size of the Mg-xGd-3Y-Zn-0.5Zr was obviously refined after hotextrusion and the aging strengthening effect of the extruded alloys was predominant.The peak values of the mechanical properties were492MPa （ultimate tensile strength）and11%（elongation rate）. They were endued with favorable plasticity.