Microstructure And Properties Of Mg-Al-RE-Ca Mg Alloy For Elevated Temperature Applications

When the Rare Earth resourse becomes limited and its market price keeps rising, the development of Mg alloys for elevated temperature application turns to be"competitive price, attractive properties at elevated temperature and good die-casting properties". The present study is devoted to replace Re in Mg alloy with cheap Ca. At the meantime, the method of Ca addition, melting process, gas shielding and melt purification techniques are expored in order to lay a sound emperimental and processing technical base for its future application.In the present study, the Mg-Al4-RE alloy specimes with Ca additions varing from 0.5-1.5 wt.% were prepared in industry conditions with simultanous evaluation of their die-casting properties. Then, the microstructure and mechanical properties of the alloys were systematically characterized with light microscopy, scanning electron microscopy, XRF analysis, static tensile test and creep resistance test. Results obtained are as follows.①Ca can be added into Mg-Al4-Re at 720°C at a recovery rate of 90%. Melt ignition can be effectively suppressed by SF6 + N2 shielding gas and melt quality meets the requirement of die-casting to produce homogeneous microstructure almost free of gas related porosity and hot tearing.②With the increase of Ca addition, the melt flow capability decreases. With the same rare earth concentration of 1.2 wt.%, the alloy with 1.2 wt.% Ca showes the best die-filling ability of 780 mm in length. Although the Ca containing alloys possess inferior die filling ability to AE42, but they are still able to meet the need of common castings production on die-filling capability.③The alloys are mainly consisits ofα-Mg matrix, fish bone-like Al2Ca and rice-like Al11RE3 that distributes on grain boundary. With the increase of Ca, the percentage of the needle-like Al11RE3 gradually decreases.④Due to brittleness of Al2Ca phase on the grain boundary, the alloys fail brittly under tension. As a consequence, the tensile properties of the alloys are inferior at room temperature, and the properties decrease with the increase of Ca concentration.⑤The Ca containing alloys exhibit superior elevated termpature tensile properties to AEC4112 alloy, in which, Mg-Al4-RE1-Ca1.2 demonstrates the best performance ofσ0.2= 128MPa,σb = 186MPa andδ= 8.3%. The morphology of the tensile fracture is decorated with quasi-cleavage features.⑥With the increase of Ca from 0.5% to 1.5%, the value of total creep decreases from 1.34mm to 0.34mm, significantly less than 1.85mm for AE42 alloy. The microstructure morphology and distribution of second phases show no observable changes post creep failure, indicating that the Ca addition hinders the sliding of grain boundary sliding and the movement of dislocations under test conditions.⑦Under the experiment conditions, the relationship between the steady creep rate of Mg-4Al-1Re-1.2Ca alloy, load stress and temperature is as follows:The main creep mechanism of the alloy is dislocation climbing and, of which, the stress exponent and apparent activation energy are 4.42 and 21.29 KJ / mol.