Ca,Sr对AM80镁合金显微组织和高温蠕变性能的影响

采用扫描电镜(SEM)、X射线衍射仪(XRD)和高温蠕变试验机等实验手段研究了碱土元素Ca,Sr对AM80镁合金显微组织和力学性能的影响.结果表明:在AM80合金中复合添加0.2%Sr和0.5%~2.5%Ca,Ca、Sr元素可逐步细化合金的铸态组织,Ca原子与Al原子优先结合在晶界处生成了高熔点相Al2Ca,抑制了低熔点相-βMg17Al12的形成.AM80合金在高温蠕变过程中,-βMg17Al12相在晶界处存在连续析出和非连续析出2种形式.-βMg17Al12相非连续析出并且垂直于晶界,造成合金蠕变性能较差.当在合金中复合添加Ca,Sr后,高熔点相Al2Ca是主要的晶界强化相,替代低熔点的-βMg17Al12相,从而减少-βMg17Al12相的非连续析出,抑制了晶界滑动,改善了合金的高温蠕变性能.当Ca质量分数增加到2.5%时,合金的高温蠕变性能最优.

Microstructure and Creep Properties of Casting AM80 Alloy Containing Ca and Sr

The effects of alkaline elements Ca and Sr additions on the microstructure and mechanical properties of AM80 magnesium alloy were investigated by using scanning electron microscopy(SEM),X-ray diffraction(XRD) and creep properties testing at elevated temperature. The results show that the microstructures of the as-cast alloys are refined by adding 0.2%Sr and 0.5%～2.5%Ca. A high-melting point Al_2Ca phase dispersedly formed at the grain boundaries in the AM80 alloy due to the priority combination of Ca and Al, thus the formation of β-Mg_(17)Al_(12) phase was reduced. The mechanical properties of the alloys were also improved due to the existence of the addition of Ca and Sr. The β-Mg_(17)Al_(12)phase had two kinds of precipitation forms, which were non-continuous precipitation and continuous precipitation during creep process. The non-continuous β-Mg_(17)Al_(12)phase precipitated at the grain boundaries and was perpendicular to the grain boundaries, decreasing the creep properties of AM80 magnesium alloy. The elevated temperature creep properties of the alloys were greatly improved because the low-melting point β-Mg_(17)Al_(12) phase was replaced by high-melting point Al_2Ca phase, which prevented the grain boundaries sliding. The creep mechanism was best when the mass fraction of Ca was 2.5%.