等通道转角挤压高铝镁合金的微观组织和力学性能
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摘要
镁合金是很有吸引力的最轻的一类金属结构材料,在汽车、机车和航空工业上有广阔的应用前景。镁是密排六方结构,只有有限的滑移系,可塑性差。另一方面,又缺乏有效的强化相,限制了镁合金比强度的大幅度提高。细化晶粒成为目前优先考虑用来提高镁合金强度和塑性的一种有效手段,等通道转角挤压工艺(ECAP)容易实现强塑性变形,从而达到细化晶粒的目的。
本文对Mg10Al、Mg15Al、Mg20Al、Mg25Al四种铸态高铝镁合金在280℃使用φ=90°的模具进行了4道次Bc路线挤压,并选择了经4道次挤压后强度最高的Mg15Al进行了8道次Bc路线挤压。采用金相显微镜(OM)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)分析挤压前后高铝镁合金的微观组织变化,比较挤压前后高铝镁合金的硬度和室温拉伸性能。
结果表明:四种镁合金经过4道次挤压后,原先存在于α-Mg晶粒边界的网状β-Mg_(17)Al_(12)细化为亚微米的等轴颗粒,分布于α基体中;在β相较少的区域,α-Mg的晶粒尺寸由原先的50μm细化至1μm,而在双相混合较均匀的区域,晶粒尺寸被细化至200nm以下,比常规镁合金(AZ61,AZ91)经等通道转角挤压后的组织明显细小,这是由于高铝镁合金在高温挤压过程中除α-Mg基体相外,存在较多β-Mg_(17)Al_(12),两相相互制约,显著降低各相的(动态)再结晶速率,从而容易获得比常规Mg-Al系合金细小的多的组织。
经挤压后,镁合金的硬度、强度和伸长率均有大幅度提高。其中Mg15Al获得了最高的强度,由挤压前的150MPa上升至268MPa,其伸长率由0.05%上升到7.4%;Mg20Al经挤压后强度由42MPa上升至209MPa,伸长率由近似0%变为3.3%;Mg25Al经挤压后经挤压后强度由54MPa上升至182MPa,伸长率由近似0%变为1.3%。随着Al含量的变化,上述合金中相组成比例明显不同,在Mg10Al中Mg_(17)Al_(12)的含量相对较少,等通道挤压过程中组织细化效果不显著,对合金的强化效果也较小,而当Al含量过高时,如在Mg25Al中,β-Mg_(17)Al_(12)相所占比例过多,反而不利于合金性能的改善,故Mg15Al经挤压后获得了最高的强度。断口分析表明,断裂方式由挤压前的解理断裂转变为挤压后的准解理断裂,并在断口上出现了大量的韧性撕裂棱。对比4道次和8道次挤压后的力学性能,发现8道次挤压后镁合金的强度反而下降,伸长率则上升,结合他人研究结果,这种现象可能与镁合金中形成了织构有关。本文研究预示着,Al含量在15%～20%的二元铸态高铝镁合金经等通道转角挤压可获得良好的力学性能。在此基础上,仿照常规Mg-Al系合金,我们可以预期,如果在上述二元合金中添加其它合金元素,如Zn、Mn等,则其性能有望得到更进一步的提高。
Magnesium alloys are one of the lightest structural materials for application in aerospace and other transportations. However, magnesium exhibits poor ductility due to its HCP structure with limited slip systems. Moreover, the increasement in their specific strength is limited because of the lack of effective reinforcement measurements. Grain refinement has been one of the preferringly considered measures in improving the strength and plasiticity of magnesium alloy. Equal channel angular pressing (ECAP) provides a technique for producing ultra-fine grained bulk materials in the submicrometer or manometer range by imposing severe plastic deformation.
In this paper, the binary magnesium alloys with high aluminum content Mg10Al, Mg15Al, Mg20Al and Mg25Al were cast and then ECAPed for 4 passes by route Bc using a die with 0=90°at 280℃. The alloy Mg15Al after ECAP shows the highest strength, which was futher ECAPed for another 4-pass. Their microstructures before and after ECAP were investiagted by Optical microscopy(OM) scanning electron microscope(SEM) and transmission-type electron microscope(TEM), and the mechanical properties were measured by hardness test and tension at room temperature and compression at 250℃.
The main results can be summarized as follows: After 4 passes pressing, the network ofβ-Mg_(17)Al_(12) existing at the grain boundary ofα-Mg changed into submicron equiaxed particles which not very uniformly distributed into the matrex. The grain size of theα-Mg was refined from 50μm to 1μm in the areas with shortage ofβ-Mg_(17)Al_(12); while in the areas where a- and (3-phase coexist the grain size was refined below 200 run which was far smaller than the conventional magnesium alloys (AZ61,AZ91) after ECAP pressing. The mutual restriction between the two phases a andβgreatly reduced the speed of their dynamic recrystallization during ECAP.
The mechanical properties, such as hardness, tensile strength and elongation, of the high-aluminum magnisium alloys were all improved significantly after ECAP. The ECAPed Mg15Al exhibited the highest strength 268MPa and the elongation was improved from 0.05% to 7.4%; For Mg20Al, the tensile strength was improved from 42 MPa to 209 MPa, and the elongation from near 0% to 3.3%; The strength of Mg25Al improved from 54MPa to 182MPa, and the from near 0% to 1.3%. According to the microstructures of Mg alloys, it can be thought that the content of Mg_(17)Al_(12) in Mg10Al is low, so the grain refinement is not obvious, and so is the strengthening. When the content of Al was too high, Mg25Al for instance, the fraction of Mg_(17)Al_(12) phase is too large to improve the mechanical properties of the alloy. The fracture morphology showed that fracture type changed from cleavage fracture to quasi-cleavage afer pressing, and there was large quantity of tough torn grain in the fractures. Comparing the mechanical properties of the specimens sufferd 8 passes pressing with that of the 4 passes pressing we found that the strength decreased while the elongation increased, the possible reason was the formation of texture. The present work indicates that the excellent mechanical properties can be obtained in the as-cast binary high-aluminum magnisium alloy with Al content ranging 15-20 % after ECAP processing. It is expected that these properties can be further improved by alloying with other elements such as Zn, Mn etc. as that employed in the conventional Mg-Al alloy system.
本文对Mg10Al、Mg15Al、Mg20Al、Mg25Al四种铸态高铝镁合金在280℃使用φ=90°的模具进行了4道次Bc路线挤压,并选择了经4道次挤压后强度最高的Mg15Al进行了8道次Bc路线挤压。采用金相显微镜(OM)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)分析挤压前后高铝镁合金的微观组织变化,比较挤压前后高铝镁合金的硬度和室温拉伸性能。
结果表明:四种镁合金经过4道次挤压后,原先存在于α-Mg晶粒边界的网状β-Mg_(17)Al_(12)细化为亚微米的等轴颗粒,分布于α基体中;在β相较少的区域,α-Mg的晶粒尺寸由原先的50μm细化至1μm,而在双相混合较均匀的区域,晶粒尺寸被细化至200nm以下,比常规镁合金(AZ61,AZ91)经等通道转角挤压后的组织明显细小,这是由于高铝镁合金在高温挤压过程中除α-Mg基体相外,存在较多β-Mg_(17)Al_(12),两相相互制约,显著降低各相的(动态)再结晶速率,从而容易获得比常规Mg-Al系合金细小的多的组织。
经挤压后,镁合金的硬度、强度和伸长率均有大幅度提高。其中Mg15Al获得了最高的强度,由挤压前的150MPa上升至268MPa,其伸长率由0.05%上升到7.4%;Mg20Al经挤压后强度由42MPa上升至209MPa,伸长率由近似0%变为3.3%;Mg25Al经挤压后经挤压后强度由54MPa上升至182MPa,伸长率由近似0%变为1.3%。随着Al含量的变化,上述合金中相组成比例明显不同,在Mg10Al中Mg_(17)Al_(12)的含量相对较少,等通道挤压过程中组织细化效果不显著,对合金的强化效果也较小,而当Al含量过高时,如在Mg25Al中,β-Mg_(17)Al_(12)相所占比例过多,反而不利于合金性能的改善,故Mg15Al经挤压后获得了最高的强度。断口分析表明,断裂方式由挤压前的解理断裂转变为挤压后的准解理断裂,并在断口上出现了大量的韧性撕裂棱。对比4道次和8道次挤压后的力学性能,发现8道次挤压后镁合金的强度反而下降,伸长率则上升,结合他人研究结果,这种现象可能与镁合金中形成了织构有关。本文研究预示着,Al含量在15%～20%的二元铸态高铝镁合金经等通道转角挤压可获得良好的力学性能。在此基础上,仿照常规Mg-Al系合金,我们可以预期,如果在上述二元合金中添加其它合金元素,如Zn、Mn等,则其性能有望得到更进一步的提高。
Magnesium alloys are one of the lightest structural materials for application in aerospace and other transportations. However, magnesium exhibits poor ductility due to its HCP structure with limited slip systems. Moreover, the increasement in their specific strength is limited because of the lack of effective reinforcement measurements. Grain refinement has been one of the preferringly considered measures in improving the strength and plasiticity of magnesium alloy. Equal channel angular pressing (ECAP) provides a technique for producing ultra-fine grained bulk materials in the submicrometer or manometer range by imposing severe plastic deformation.
In this paper, the binary magnesium alloys with high aluminum content Mg10Al, Mg15Al, Mg20Al and Mg25Al were cast and then ECAPed for 4 passes by route Bc using a die with 0=90°at 280℃. The alloy Mg15Al after ECAP shows the highest strength, which was futher ECAPed for another 4-pass. Their microstructures before and after ECAP were investiagted by Optical microscopy(OM) scanning electron microscope(SEM) and transmission-type electron microscope(TEM), and the mechanical properties were measured by hardness test and tension at room temperature and compression at 250℃.
The main results can be summarized as follows: After 4 passes pressing, the network ofβ-Mg_(17)Al_(12) existing at the grain boundary ofα-Mg changed into submicron equiaxed particles which not very uniformly distributed into the matrex. The grain size of theα-Mg was refined from 50μm to 1μm in the areas with shortage ofβ-Mg_(17)Al_(12); while in the areas where a- and (3-phase coexist the grain size was refined below 200 run which was far smaller than the conventional magnesium alloys (AZ61,AZ91) after ECAP pressing. The mutual restriction between the two phases a andβgreatly reduced the speed of their dynamic recrystallization during ECAP.
The mechanical properties, such as hardness, tensile strength and elongation, of the high-aluminum magnisium alloys were all improved significantly after ECAP. The ECAPed Mg15Al exhibited the highest strength 268MPa and the elongation was improved from 0.05% to 7.4%; For Mg20Al, the tensile strength was improved from 42 MPa to 209 MPa, and the elongation from near 0% to 3.3%; The strength of Mg25Al improved from 54MPa to 182MPa, and the from near 0% to 1.3%. According to the microstructures of Mg alloys, it can be thought that the content of Mg_(17)Al_(12) in Mg10Al is low, so the grain refinement is not obvious, and so is the strengthening. When the content of Al was too high, Mg25Al for instance, the fraction of Mg_(17)Al_(12) phase is too large to improve the mechanical properties of the alloy. The fracture morphology showed that fracture type changed from cleavage fracture to quasi-cleavage afer pressing, and there was large quantity of tough torn grain in the fractures. Comparing the mechanical properties of the specimens sufferd 8 passes pressing with that of the 4 passes pressing we found that the strength decreased while the elongation increased, the possible reason was the formation of texture. The present work indicates that the excellent mechanical properties can be obtained in the as-cast binary high-aluminum magnisium alloy with Al content ranging 15-20 % after ECAP processing. It is expected that these properties can be further improved by alloying with other elements such as Zn, Mn etc. as that employed in the conventional Mg-Al alloy system.
引文
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