| The application of heat resistant magnesium alloys in automotive and/or aerospace industry has the advantages of energy saving and emission reduction. Si and Ca element are usually used to design heat resistance magnesium due to their low cost and the formation of stable compound when they are combined with Mg element. Therefore it has theoretical and practical significance to research the modification of Mg2Si phase and the creep properties of Ca-containing heat resistant magnesium alloys. In this paper, the effects of Bi, Sb and Sr addition on the modification and growth rule of the primary and eutectic Mg2Si phases of Mg-1.7Si alloy is investigated by the microstructure observations and the evolution of Mg2Si phase in three dimensions. A new Mg-5Al-2Sn-5Ca（ATX525） alloy with connected hard skeleton was designed by considering the microstructure and micro-hardness evolution of series of Mg-5Al-2Sn-x Ca（x=1, 2, 3, 4, 5, 6） alloys. The techniques for refining Ca Mg Sn phase and optimizing the microstructure of ATX525 alloy were obtained by choosing the temperatures and times in the range of 200-600℃ and 24-96 hrs for solid solution and 200℃ and 24-120 hrs for aging, respectively, through quantities of experiments. Based on which the creep properties of ATX525 alloy were investigated at 200℃and different creep stress. The creep properties for as-cast ATX525 alloy were also investigated in order to compare the creep properties of the alloy in different states. The results of above experiments are summarized as follows:（1） For Bi and Sr elements, they all have the effect on the modification of primary and eutectic Mg2Si phases. The difference between them is that the Sb can induce heterogeneous nucleation when Mg3Sb2 is formed in the liquid. The best modification effect on Mg2Si phase requires the proper additions of Bi and Sb in the range of 0.2-0.3% and 0.3-0.4%, respectively. While for primary and eutectic Mg2Si phases, Sr addition should be 0.5% and 1.0-2.0%, respectively. A spherical Mg2Si can be obtained by the addition of Bi or Sb element. However the addition of Sr element can not present such results.（2） The connected hard skeleton structure for ATX525 is perfectly integrated. The hard phases in the skeleton are composed of（Mg,Al）2Ca, Al2 Ca and Ca Mg Sn with good heat resistant properties. The eutectic and ending melting temperatures for the alloy are 543 ℃ and 610 ℃, respectively.（3） Ca Mg Sn phase can be refined by isothermal treatment. When the isothermaltemperature is in the range of 450-500℃, the dissolving and breaking processes can occurand no obvious volume fraction change for Ca Mg Sn phase in the matrix. When theisothermal temperature is 550℃, the Ca Mg Sn can be dissolved into the matrix. The refinedCa Mg Sn phase is obtained in the temperature range of 500-550℃.（4） Under the condition of creep temperature of 200℃ and loading stresses of 35, 50, 60and 75 MPa, respectively, the creep life, and the steady creep rate for as-cast ATX525 alloyis 1412, 170, 110 and 12 hrs, and 4310-8, 6.9310-7, 6.9310-7 and 2310-6s-1,respectively. The creep strain of 100 hrs for the samples loaded by 35, 50, 60 MPa is 0.04,0.05, 0.065%, respectively.Under the condition of creep temperature of 175℃ and loading stresses of 75, 85, and90 MPa, respectively, the creep life, and the steady creep rate for as-cast ATX525 alloy is127, 81, 110 and 69 hrs, and 1.25310-7, 1.67310-7 and 1.25310-6s-1, respectively. Thecreep strain of 100 hrs for the samples loaded by 75 MPa is 0.1%.（5） Heat treatment（500℃/24h） and ageing（200℃/96h） were employed for as-castATX525 alloy. Under the condition of creep temperature of 200℃ and loading stresses of 50,60 and 75 MPa, respectively, the creep life, and the steady creep rate for as-treated ATX525alloy is 510, 130 and 45 hrs, and 1.67310-8, 1.11310-7 and 2310-7s-1, respectively. Thecreep strain of 100 hrs for the samples loaded by 50, 60 MPa is 0.03, 0.06, respectively.（6） The hard skeleton phases are able to impede creep process to develop and result in stress concentration near the hard skeleton. Under low stress, the accumulation of micro-holes around the skeleton results in the separation between skeleton and matrix and therefore the creep fracture. We found that the hard skeleton in round particle morphology can effectively alleviate the stress concentration, reduce the degree of cracking and improve the creep life. |
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