粗晶EW75镁合金变形孔洞的产生及愈合研究

通过金相显微镜(OM)、扫描电子显微镜(SEM)等测试手段,在Gleeble-1500热模拟机上研究了粗晶EW75镁合金热变形行为,变形温度为723 K、应变速率为0.05 s-1,最大变形程度为80%的条件下,根据结果分析了合金高温变形时的真应力-真应变曲线以及不同变形量的显微组织,揭示了合金在变形过程中孔洞产生及消失的机制。结果表明:铸态合金平均晶粒尺寸约为149μm,均匀化后合金平均晶粒尺寸达到197μm左右;真应力-真应变曲线呈现出典型的动态再结晶特征;变形量为40%,原始大晶粒被细小再结晶晶粒包围,呈现典型的"项链"状特征,在局部晶粒交结处出现孔洞,随着变形量的增加,孔洞先长大后变小,当变形量达到80%时,孔洞基本消失愈合,愈合区有细小的再结晶的晶粒,形成明显的愈合带;大尺寸晶粒间的相互协调性能较差是变形出现孔洞的主要原因,随着变形量的增加,再结晶比例的提高带来的变形协调性能增强,孔洞最终被压扁,重新接触的两表面存在较高的能量,最终发生完全动态再结晶是合金孔洞愈合机制。

Generation and Healing of Cavities in Coarse Grain EW75 Magnesium Alloy

The hot deformation behavior of coarse grain EW75 magnesium alloy was studied by optical microscopy,scanning electron microscopy.The experiment was performed on Gleeble-1500 at 723 K,strain rates 0.05 s-1 and a maximum strain of 80%.Combined with the results,true stress-strain curve and microstructure with different deformation degree of the alloy were analyzed;the mechanism of cavities generation and disappearance in the hot deformation process was revealed.The results showed that after homogenization treatment the grains grew up and mean grain size increased to 197 μm,compared with the as-cast alloy(149 μm).From the true stress-strain curve,the typical characteristics of dynamic recrystallization could be obtained;when the deformation degree reach to 40%,typical necklace microstructures was get and original large grains were encircled with recrystallized fine ones.Cavities appeared in the knot of grains which grew up first and turn down later with deformation degree increased.As deformation degree reached to 80%,cavities vanish and the fine recrystallized grains developed a banded structure which was observed in healing area.The poor compatibility between large grains was the main reason for the creation of cavities during deformation process.When the deformation degree increased,the volume fraction of dynamic recrystallization increases which contributed to promote the deformation compatibility.Cavities were squashed and re-encounter surfaces were energetically active.Fully dynamic recrystallization was responsible for cavities disappearance.