Plastical Deformation And Microstructural Formation Of Icosahedral Quasicrystalline Reinforced Mg-Zn-Y Alloys

Magnesium and magnesium alloy are becoming more and more attractive for many engineering structural applications owing to their low density and high specific strength. However, the application of most magnesium alloys has been greatly restricted due to their poor strength, oxidation and creep resistance. In order to extend the application of magnesium alloys, many methods have been reported to improve mechanical properties.Icosahedral quasicrystalline phase (Ⅰ-phase) that has attractive mechanical and physical properties attributed to their unique atomic structure, such as high strength, high hardness at elevated temperature and low friction coefficient has been reported in many alloy systems. Recently, it has been reported that the Mg-Zn-Y alloys containing Ⅰ-phase as a secondary solidification phase exhibit good mechanical properties at room temperature and elevated temperature, which provide a good method for the development and application of new magnesium alloys.In this paper, several Mg-Zn-Y alloys with different contents of AlTiC master alloys were fabricated by using the conventional casting method. The effect of the AlTiC master alloy contents on as-cast microstructure and mechanical properties was studied. The grain refinemnt mechanism of the AlTiC master alloy and its effect on amounts and morphology of quasicrystals were also discussed. Based on the as-homogenized ZW61alloy and ZW61-0.15AlTiC alloy, the influence of different extrusion ratio and extrusion speed on the microstructure, texture and mechanical properties was investigated, and the refinement mechanism, fracture behavior and strengthening mechanism were studied. Two as-extruded magnesium alloys with good comprehensive mechanical properties were heat treated, and the effect of different heat treatment processes (T4, T5and double-step ageing treatment) on microstructure and mechanical properties was discussed.The results showed that the addition of AlTiC master into Mg-Zn-Y alloy not only could refine the microstructure, but also could increase the amounts of the Ⅰ-phase. Mg-Zn-Y alloy with proper AlTiC master alloy had good comprehensive mechanical properties. After the as-cast Mg-Zn-Y alloys with AlTiC master alloys were homogenized, the amounts of the second phases increased obviously and distributed more uniformly and the mechanical properties were further improved.Different extrusion ratios and different extrusion speeds could cause the as-homogenized ZW61alloy and ZW61-0.15AlTiC alloy grain refinement and the second phases were broken and mainly distributed along the direction of extrusion. The ultimate tensile strength (UTS), elongation and impact toughness of the as-extruded magnesium alloys were increased remarkably compared with the as-cast and as-homogenized ones.There was a good relationship between UTS, elongation of the two magnesium alloys and the intensity of basal PF with the small extrusion ratio, and the change of elongation was most associated with the average Schmid factors m. Extruded at large extrusion ratio, the relationship between UTS, elongation and the intensity of basal PF was complicated. The tendency of UTS of the two alloys with different extrusion speeds could be represented by maximum intensity of basal PF. And the tendency of elongation was still associated with the average Schmid factors m.As-extruded magnesium alloys were solid solution treated at different temperature for2h, the elongation increased obviously at350℃. However, the UTS and elongation decreased a little at other temperature. Solid solution treated at375℃for different time, the changes of the UTS and elongation had the similar tendancy, that is, with the increase of time, the properties decreased first and then increasd, and further decreased.In general, the mechanical properties of ZW61alloy and ZW61-0.15AlTiC alloy are lower than the as-extruded after artificial ageing treatment.The mechanical properties of the two alloys decreased after double-step ageing treatment. However, the comprehensive mechanical properties increased remarkably after direct double-step ageing treatment without solid solution treatment.