Study On Microstructure And Properties Of Directionally Solidified Magnesium Manganese Alloys

In the paper, the Mg-Mn based alloy is investigated by the method of zone melting liquid metal cooling(ZMLMC). And effects of different pulling rates and temperature gradients of directional solidification on the microstructure and mechanical properties of Mg-Mn based alloy is compared. Also under the the same experimental parameters, the effects of Al and Y on the Mg-Mn based alloy is explored.The results indicates that the process contains two stages in the directional solidification of Mg-Mn based alloy: competitive growth and stable growth. In the stage of competitive growth, the organization is not stable because of the unstable solid-liquid interface. In the beginning, the the morphology of organization is smallish columnar crystal. But with the crystal growing continually, the smallish columnar begins to merge into wide columnar crystal gradually, which illustrates the growth has become stable and simultaneously the grains grow to longer columnar crystal. In the stage of stable growth, the grains merge into columnar crystal against the direction of heat flow.When the temperature gradient is set, with the solidification rate increasing,the zone of supercooling ahead of solid-liquid interface is also increasing. The morphology of the crystal would change in this pattern:plane?cellular?dendritic. Also, with the solidification rate increasing, the nucleation becomes more, so the average size of the columnar becomes smaller.As the solidification rate is set, with the temperature gradient increasing, the zone of supercooling ahead of solid-liquid interface is decreasing, and the sequence of growth pattern of crystal is: cellular?dendritic?plane. So with the increasing of temperature gradient, the size of crystal is increasing.After directional solidification, the mechanical property of Mg-Mn based alloy was improved significantly. And with the solidification rate increasing, the mechanical property was increasing gradually. For Mg-1.5Mn, the ultimate tensile strength of alloy reaches to 187 Mpa, which is enhanced by 156%comparing to the as-cast alloy(73Mpa). And Mg-1.5Mn-Al, the ultimate tensilestrength of alloy reaches to 193 Mpa, which is improved by 124% comparing to the as-cast alloy(87Mpa). For Mg-1.5Mn-Y alloy, the ultimate tensile strength and elongation of the alloy reaches to 205 Mpa and 7.53%, which increased by127% and 59% as compared with the as-cast alloy(90Mpa,7.53%), respectively.Under the same experiment parameters, the average size of crystal of Mg-1.5Mn-Al and Mg-1.5Mn-Y are narrower than the Mg-1.5Mn. And Al and Y both can improve the mechanical property of Mg-Mn based alloy, especially Y.