Dissertation
Dissertation > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Non - ferrous metals and their alloys > Light non-ferrous metals and their alloys > Magnesium

Study on the ECAP Deformation of Different Second Phase in Mg-Al Based Magnesium Alloys and Its Effect on Performance

Author ZhouKangKang
Tutor WangHongXia
School Taiyuan University of Technology
Course Materials Science and Engineering
Keywords Mg-Al based alloys equal channel angular pressing (ECAP) second phase microstructures mechanical properties
CLC TG146.22
Type Master's thesis
Year 2013
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Mg-Al based alloy become the most widely used magnesium alloy, due to the advantage of low price and easy melting. However, β-Mg17Al12phase with a net structure form easily to reduce the room temperature strength of the alloy, and this phase is easy to soften at High temperature, lead to performance deterioration, limiting the wider application of Mg-Al based alloy.First, for eliminating the fragmentation effect of mesh structure second phase on the Mg-Al alloy matrix, ECAP technology was used to modify the morphology of Mg17Al12phase, and the ECAP deformation behavior of continuous and semi-continuous network Mg17Al12phases were studied. Furthermore, poor heat-resisting property of Mg-Al based magnesium alloy due to Mg17Al12phase soften at high temperature must be solved. Sb, Nd and Si elements were added to form high-temperature phases which can pinning grain boundary and dislocation thereby improving high temperature performance of Mg-Al alloy. However, the addition of Sb, Nd and Si resulted in the formation of coarse second phases with undesirable morphology, which limit the further improvement of high temperature property. In this way, ECAP process was conducted on these Mg alloys to modify the morphology of high temperature second phase, so as to obtain the grain refinement and β phase fragmentation for enhancing Mg-Al alloy. Six kinds of Mg-Al based magnesium alloys which containing different second phases were prepared in this experiment, and ECAP process was conducted on these magnesium alloys to modify second phase. The ECAP deformation of second phase was studied by OM、SEM、TEM and EDS methods. The tensile tests were carried out by a WDW-100KN tensile testing machine at room temperature,150℃and200℃, and the effect of second phase deformation on property of Mg-Al alloys was studied. The results are shown below:(1) Continuous and semi-continuous network second phases:The increase of Al content in the Mg-Al binary alloy promotes the mesh development of α-Mg17Al12phase. Continuous and semi-continuous network β-Mg17Al12phases were fragmented into small particles by four passes ECAP. But semi-continuous network β-Mg17Al12phase obtain the better distribution pattern than continuous network β-Mg17Al12phase. Continuous network β-Mg17Al12phase was still in reunion massive form. The room temperature tensile strength and elongation of MglOAl alloy (semi-continuous) after extrusion was higher than that of Mg20Al alloy (continuous).(2) Long needle-like second phase:the refining effect of cheap metal antimony and rare earth neodymium on Mg-Al alloy has no difference. The addition of Sb and Nd resulted in the long needle-like high temperature Mg3Sb2and Al11Nd3phases with the length of35-40μm. After ECAP, both of Mg3Sb2and Al11Nd3phases were refining into short rod-like phases with length of10μm, distributed in matrix uniformity. In addition, it’s significant to notice that the grain refinement and β phase fragmentation during ECAP processing was strongly promoted by the action of Sb and Nd addition. The tensile strength and elongation of Sb-containing Mg-Al alloy was195MPa and44%at the temperature of200℃. The tensile strength and elongation of Nd-containing Mg-Al alloy was173.2MPa and50.4%at the temperature of200℃. They are all higher than the relatively as-cast alloys and ECAPed alloys without Sb and Nd.(3) Coarse Chinese script and bone-like second phases:Coarse Chinese script Mg2Si phase have the size of90μm, appear in form of block、face-like and Chinese script, which often gathered together to form coarse Mg2Si community. Bone-like Mg2Si phase were appeared in shape of long bone、fish bone and small bone. The longest bone-like Mg2Si was50μm and the shortest was2μm. Particularly, these bones were very thin, having the size of0.4μm.they are well-distributed in matrix, and it provide very favorable conditions for the subsequent deformation processing. After ECAP, the shape of coarse Chinese script Mg2Si phase did not change, but with same passivation and little rupture on branching arm. Moreover, there are same cracks appearing in Mg2Si phase and it’s dangerous to alloy. The Mg9Al-1Si magnesium alloy with bone-like Mg2Si phase achieved better grain refinement and second phase fragmentation during ECAP processing. After ECAP, bone-like Mg2Si phases were fragmented into small particles with size of0.4μm. The Mg9Al-1Si magnesium alloy was reinforced by small matrix grain and uniformity distributed Mg2Si particles. In result, ECAPed Mg9Al-1Si magnesium alloy obtained the best performance at room and elevated temperature.

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