Effect Of Magnetic Field On Solidified Structure And Mechanical Properties Of Mg₉₇Y₂Cu₁ Alloy Reinforced By Long Period Ordered Structure

Magnesium alloys, as a kind of environmental materials, have been widely used in aeronautic, automotive, electronic industries in recent years. Magnesium alloys reinforced by long period ordered structure show excellent room temperature and high temperature performance, and its mechanical properties are much higher than traditional commercial magnesium alloy. It is of vital importance to control the solidified structure and crystal texture of the alloy in order to improve its mechanical properties. Magnetic field treatment during solidification of alloys is attached more and more importance by peopole, which has the advantages of pollution-free, convenient operation and significant effect. In order to improve mechanical properties of magnesium alloy reinforced by long period ordered structure, pure magnesium and Mg97Y2Cu1 alloy were employed as raw materials for this study. Solidified structure, crystal texture and mechanical properties of the alloy with DC magnetic field(DCMF) treatment, pulsed magnetic field(PMF) treatment and compound treatment of DCMF and PMF were studied, and the related mechanism was also discussed. The major conclusions are as follows:1) When the strength of DC magnetic field is at 0-1.0T, with the increase of magnetic field strength, grain size of pure magnesium increases gradually. Crystal texture of pure magnesium can be influenced significantly. The(0002)plane texture is weakened and the)0110(-plane texture is enhanced. The tensile strength of pure magnesium decreases and its elongation increases obviously. The maximum elongation of pure magnesium is increased by up to 544%, compared with the sample without DCMF treatment.2) Solidified structure of Mg97Y2Cu1 alloy can be improved significantly by DCMF. When the magnetic field strength is at 0-1.2T, with the increase of magnetic field strength, grain size of the alloy decreases firstly, then increases, while volume fraction of second phase increases at first, then decreases, and the turning point is 0.3T. The(0002)plane texture is weak and confused. The)0110(-plane texture enhances gradually. The(112 0)-plane texture enhances at first, then weakens, and the turning point is 0.9T. The tensile strength and elongation of the alloy increase gradually in general. When the magnetic field strength is 0.9T, comprehensive mechanical properties of the alloy are the best. Tensile strength and elongation of the alloy are increased by up to 96.6% and 61.1%, respectively, compared with the sample without treatment.3) Primary phase of Mg97Y2Cu1 alloy can be refined greatly with PMF treatment. Meanwhile, morphology and volume fraction of the second phase are changed. Primary phase of the alloy changes from well-developed dendrites into fine equiaxed grains or rosette crystals. When the magnetic field strength is at 0-0.25 T, with the increase of magnetic field strength, primary phase of the alloy refines gradually, and the size and volume fraction of second phase increase. When the pulse frequency is at 1-5Hz, effect of pulse frequency on primary phase and second phase is not obvious. When the pulse frequency reaches 10 Hz, the refinement effect of primary phase is the best, the distribution of second phase is the most uniform and the volume fraction of second phase is the highest. When the pouring temperature is at 650-750℃ or the mold temperature is at 20-600℃, grain size of the alloy decreases gradually with the increase of the pouring temperature or the mold temperature. Crystal orientation of the alloy along(0002) plane is inhibited, and new crystal orientation of the alloy along(1011)-plane is formed. With the increase of magnetic field strength or pulse frequency, overall the tensile strength and elongation of the alloy increase gradually. When the magnetic field strength is 0.25 T, tensile strength and elongation of the alloy are increased by up to 42.5% and 106.0%, respectively, compared with the alloy in traditional condition.4) Solidified structure of the alloy can be improved significantly by compound treatment of DCMF and PMF. The improvement effect by compound treatment is more efficient than DCMF treatment or PMF treatment. Primary phase of the alloy is changed into fine equiaxed grains or rosette crystals. Distribution of second phase is more uniform and continuous, and its volume fraction increases. When the pulsed magnetic field strength is at 0-0.25 T or pulse frequency is at 1-10 Hz, grain size of the alloy decreases gradually with the increase of pulsed magnetic field strength or pulse frequency. When the mold temperature is at 20~600℃, with the increase of mold temperature, grain size of the alloy decreases firstly, then increases, and the turning point is 400℃. When the pouring temperature is at 650~750℃, with the increase of pouring temperature, grain size of the alloy increases at first, then decreases, and the turning point is 720℃. With the increase of magnetic field strength or pulse frequency, the tensile strength and elongation of the alloy increase gradually. When the magnetic field strength is 0.25 T, tensile strength and elongation of the alloy are increased by up to 146% and 113%, respectively, compared with the alloy under DCMF.