| Magnesium alloy with long-period stacking ordered?LPSO?structure phase has a wide application prospect in the field of high performance heat-resistant magnesium alloys due to its unique structure,superior strength and plasticity.However,conventional as-cast Mg-Gd-Zn alloys are priori to form coarse W eutectic phase but formation of the LPSO phases through long time high-temperature solid solution heat treatment.The coarse LPSO phases have a bad influence effect on magnesium alloy.In this paper,based on the fact of Ca refined Mg alloys microstructure,the W phases have been almost entirely transformed into18R LPSO structure with the different amounts of Al element addition in as-cast Mg-Gd-Zn alloys.The formation and transformation of LPSO phases have an important effect on the mechanical properties of the alloys in the as-cast,heat treatment and hot extrusion deformed process.It has an important theoretical and practical significance for the preparation of low-cost high strength and toughness magnesium alloys with LPSO phases reinforced.In this paper,Mg96-x6-x Gd2.5Zn1Ca0.5Alx?at.%??x=0,0.25,0.5,1.0?alloys were prepared by conventional casting method under the protection of argon gas.It was investigated that the influence on the formation of LPSO phase and the change of its mechanical properties in as-cast Mg-Gd-Zn alloy at room temperature by adding Al.A small amount of Al addition reduces the stacking fault energy in the as-cast alloy,which promotes the precipitation of the LPSO phases.With the increase of the content of Al,the morphology of the LPSO structure transforms from fine sheet to massive block gradually,the well-developed dendrites of thea-Mg matrix was blocked by LPSO and shift to smooth equiaxed crystal gradually.The mechanical properties of the alloy increased first and then decreased with the increase of the content of Al.The as-cast Mg95.75Gd2.5Zn1Ca0.5Al0.25.25 alloy had the best comprehensive mechanical properties with maximum tensile strength,yield strength and elongation were 216MPa?152MPa and 6.7%,respectively.The mechanical properties of the as-cast Mg95.75Gd2.5Zn1Ca0.5Al0.25.25 alloy increased by 47.6%?40.3%and116%compared with Mg96Gd2.5Zn1Ca0.5.5 alloy with a yield strength of 103MPa,tensile strength of 154MPa and elongation of 3.1%,respectively.The best solid solution temperature of Mg95.75Gd2.5Zn1Ca0.5Al0.25.25 alloy was determined by the investigation of heat treatment parameters such as temperature,time and cooling method,and the transformation pattern of LPSO phase in the alloy was investigated and further regulate the morphology,quantity and distribution of the LPSO phase by solid solution heat treatment.It was found that W phase at the grain boundary transformed into18R LPSO phase and coarse 18R LPSO phase at the grain boundary melted into the magnesium matrix and form a supersaturated solid solution under high temperature for long time,precipitation of layer 14H LPSO phase in the crystal under the the furnace cool slowly.Gd and Zn atoms precipitated from the solid solution and diffused into the dislocation plane promotes the precipitation of 14H LPSO phase.Therefore,cooling slowly method was more conducivetotheprecipitationof14H LPSOphase.Thehardnessofthe Mg96-xGd2.5Zn1Ca0.5Alx?at.%??x=0,0.25,0.5,1.0?alloys showed tendency decreasing first and then increasing after solution treatment.And the hardness of Mg95.75Gd2.5Zn1Ca0.5Al0.25.25 alloy was the lowest.The optimal extrusion process was established by examining the influence of initial microstructure and extrusion temperature on the microstructure transformation and mechanical properties of the LPSO phase reinforced Mg95.75Gd2.5Zn1Ca0.5Al0.25.25 alloy during the extrusion process.The microstructure transformation and mechanical behavior of Mg96-x6-x Gd2.5Zn1Ca0.5Alx alloys during extrusion were investigated.It was found that the direct hot extrusion of the as-cast Mg95.75Gd2.5Zn1Ca0.5Al0.25.25 alloy was conducive to precipitation of 14H LPSO phase,but had an inhibitory effect on the dynamic recrystallization.The 14H LPSO phase precipitated in the heat treatment process had promoting effect on the dynamic recrystallization process of the alloy and the tiny dynamic recrystallized grains improve the plasticity alloy greatly.The maximum tensile strength and yield strength of the extruded Mg95.75Gd2.5Zn1Ca0.5Al0.25.25 alloy increased and the elongation also showed an increasing trend.The submicron grains were appeared in the Mg95.75Gd2.5Zn1Ca0.5Al0.25.25 alloy after extruded at360°C.The alloy had a typical bimodal grain size structure and its room temperature yield strength,tensile strength and elongation were 382MPa,438MPa and 9.4%,respectively.The element of Al may participated in the formation of LPSO structure with the increase of the content of Al,and it preliminarily inferred that its chemical composition was consistent with Mg9Gd1?Zn,Al?1.The bulk 18R LPSO structure had better plasticity than Mg9Gd1?Zn,Al?1compared with the bulk 18R LPSO(Chemical composition,Mg12Gd1Zn1)in Mg-Gd-Zn alloys. |
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