| Magnesium alloys have been widely used in the fields of automobile industry, aerospace and other fields mainly owing to their low density, high specific strength. However, the poor strength of the magnesium alloys,especially high temperature strength, which have greatly limited the application of the magnesium. The low plastic forming of magnesium alloys,poor strength may attribute to the flowing reasons: its crystal structure is close-packed hexagonal structure, less slip system, grain orientation.Therefore, to control the microstructure and grain orientation is expected an effective way to improve its performance. Directional solidification as a new preparation process technology can gain a columnar crystal, which have a specific orientation, it can eliminate the longitudinal transverse grain boundary to improve mechanical performance.In this paper, the directional solidification sample rods have been successfully prepared with different Ca content(0%, 0.5%, 1.0%) based on the Mg-4Sm binary alloy. Then the Mg-4Sm-?Ca alloys were systematically investigated with different growth rate(25 ? m/s,50 ? m/s,100 ? m/s,200 ?m/s,1000?m/s), different temperature gradient(7K/mm,10K/mm,13K/mm). The microstructures and the fracture morphology were studied by means of the optical microscope, scanning electron microscopy(SEM). Analysis the species and distribution of the alloy, and the effect of growth rate, temperature gradient on the solid-liquid shape, microstructures, fracture morphology and the mechanical properties. The result as follows:The Mg-4Sm alloy is composed of ?-Mg and Mg41Sm5 phase, and after adding Ca a new Mg2 Ca phase generated. The microstructure of as-cast has many continuous coarse net-like dendrites. The structure changed from the dendrites to the columnar crystal after directional solidification. The fracture scanning indicate that the fracture pattern of the as-cast is brittle fracture, after directional solidification, the fracture pattern is brittle fracture with a little of ductile fracture, and the proportion of the ductile fracture reaches the maximum value at the growth rate of 50?m/s. With an increase of growth rats,the grain size dramatically decreases, however, when the growth rates reaches a certain value, the trend gradually weakened. It shows the finest grain size directionally solidified at 50?m/s. On the other hind, the increase of the growth rate also increased the discontinuity of the columnar, the faster the growth rate is, the more obvious the discontinuous is, especially the cellular-dendrite structure.Under the same growth rate, the higher the temperature gradient is, the better the uniform, stability, direction is. With an increase of growth rate, the tensile strength increased. The tensile strength reaches a maximum value 171 MPa,increased by 87.9% compared with the as-cast. During directional solidification,Sm, Ca atomic mainly move forward ahead of the columnar growth and form the second phase, which promote the formation of new crystal nucleus nucleation and hinder the original grain grow, as a result intensitify the discontinuous of the columnar crystal. What's more, the more Ca content is, the more obvious discontinuity is. In addition, with an increase of Ca content, the microstructures have a certain degree of refinement. The mechanical properties have also been improved. However, when the Ca content exceed 0.5%, the tensile properties began to reduce, it may be attributed to the effect of strong composition undercooling of the high content calcium intensitify the discontinuity of the crystal... |
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