| Mg-Gd-Y alloys, which exhibit superior tensile properties, fatigue resistance and creep resistance, offer very broad application prospects in the aerospace, weapons and transport vehicles automotive sectors compared with conventional high-performance rare earth magnesium alloys (Mg-4Y-3Nd and Mg-5Y-4Nd). It attracts widespread attention from researchers. The cost of Mg-Gd-Y alloy with high rare earth content is high, and this kind of alloy can’t be used widely in general industry. In regard to magnesium alloy with low Gd and Y content, it exhibits excellent mechanical and physical properties and is popular in civilian equipments. However, research on magnesium alloy materials with low Gd and Y content and its heat strengthened mechanism is not deep enough at home and abroad currently. Therefore, the study on microstructure and properties of Mg-Gd-Y alloys has important scientific significance and application value of engineering.In this paper, a series of alloys were prepared by metal casting based on Mg-(4,6wt.%)Gd-xY alloy. Then these alloys were solution treated at 520℃ for 0-24h and aged at 175-225 ℃ for 0-200h, respectively. The samples at different heat treatment states were preparated. The microstructure and phase composition of experimental alloys and the evolution of microstructure of alloys during solid solution and aging were analyzed by means of optical microscope (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and mechanical properties testing. In addition, it was found that the natural placement after squeezing has a significant influence on the microstructure, mechanical properties and subsequent artificial aging treatment of extruded GW102(Mg-10Gd-2Y-0.5Zn-0.5Zr) alloy. Thus, changes in the microstructure, tensile properties and subsequent artificial aging treatment of as-extruded GW102 alloy and its 3-year naturally aged counterpart were also comparatively investigated in this paper and following conclusions could be withdrawn.The as-cast microstructure of alloys Mg-4wt.%Gd-xY(x=2,3wt.%), Mg-6wt.%Gd-xY (x=2,4 wt.%) were composed of isometric a(Mg) crystals and divorced eutectic compound Mg5(Gd,Y) or Mg24(Gd,Y)s, and non-equilibrium crystallized phases Mg(Gd,Y) and Mg3(Gd,Y) existed in the as-cast Mg-6wt.%Gd-xY(x=2,4wt.%) alloys.With the increase of solution treatment time, the hardness of GW4x alloys decreased rapidly and unchanged essentially during solution treatment. While the hardness of GW6x alloys decreased rapidly and increased to maxima, and decreased essentially during solution treatment. When the same alloy was aged at 175-225 ℃, the lower aging temperature temperatures, the better effect of age-hardening. But the time required to reach peak aging increased. With increasing of Gd and Y content, the shapes of second phase gradually transformed from flaky shapes to semi-continuous irregular shapes, and the content of second phases increased gradually, and hardness of the alloy was improved significantly, and the change of hardness of the alloys was more significant at solid soluted states, and the age-hardening effect of the alloys was significant at the same temperature.The Mg-6wt.%Gd-2wt.%Y alloy was selected for further study. With the increase of solution treatment time, the size of the semicontinuous Mgs(Gd,Y) phases gradually decreased until dissolved during solution treatment. Rare-earth-rich phases(Mg(Gd,Y)2) with fcc structure was newly generated in the alloy at grain boundaries and inside grains after solution treatment. And it could hinder the growth of isometric a(Mg) crystals in the solution process. The age-hardening effect of GW62 alloy was significant at 175℃. The micro-structural evolution was as follows:firstly, the β" phase precipitated from a(Mg) during the under-aging stage (4-32h). Secondly, the β’ phase precipitated from a(Mg) up until peak aging was reached (32-100h). As aging time increase to 100h, the hardening hardness reached its maximum. At this stage, the precipitated phases were the β’ and β" phases. Finally, during over-aging(>100h), the size of β’ phase continually increased, and β’ phase transformed into the β1 and β phases at the same time.The natural aging time has a significant influence on the microstructure and mechanical properties of extruded GW102 alloy. The as-extruded alloy consisted of minute a(Mg) isometric crystals and intragranular slip bands. After natural aging for 3 years, the number of slip bands decreased significantly resulting in strain-free zones near the grain boundaries. In addition, the precipitation of β" phase during aging resulted in the increase of hardness and tensile strength by 4.9% and 4.2%, respectively. While it made the elongation slightly lower than that of the as-extruded alloy. The time to achieve the peak aging is significantly shortened when the alloy is aged at 200 ℃ after natural placement for 3 years, but the tensile strength in the peak aging is slightly lowed. |
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