【作者】 马奇；

【导师】 张静；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2011， 硕士

【摘要】 ZK60是目前商用镁合金中强度最高的一种,然而它还存在如绝对强度不够高,塑性不足等缺点。在改善合金塑性的同时,提高合金的强度是镁合金研究与应用中急需解决的关键问题之一。本课题在ZK60镁合金的基础上进一步提高Zn含量的同时,添加不同含量的稀土元素Er（wt%=0,0.5%,1.0%,2.0%,4.0%,下同）,在满足晶界形成一定量稀土相的同时,充分发挥Zn在基体中固溶强化作用,以期提高合金的综合力学性能。本文通过金相（OM）、X-射线衍射分析（XRD）、差热分析（DSC）、扫描电镜（SEM+EDS）和透射电镜（TEM）等实验手段研究了不同Er含量合金在铸态、挤压前热处理、变形态以及后续热处理态显微组织,分析了稀土Er在合金中的存在形式及对各种状态下显微组织和力学性能的影响。研究结果表明,Mg-9Zn-0.6Zr合金铸态组织主要由α-Mg和晶界上大量的Mg-Zn共晶相组成。随着稀土Er的加入,合金中形成了2种不同Zn/Er比的Mg-Zn-Er稀土相。随着Er含量的增加,第二相体积分数呈增加趋势。一定量的稀土Er具有细化铸态组织的作用:不含Er合金的枝晶间距为102μm,添加稀土Er含量达到2.0%时,合金组织最细小,枝晶间距为32μm。热挤压过程中镁合金发生了明显的动态再结晶。稀土Er加入到Mg-9Zn-0.6Zr合金中,在挤压过程中起到促进纳米级球状MgZn₂相动态析出的作用,有效强化了合金。随着稀土Er含量的增加,纳米级球状MgZn₂析出相成减小趋势。稀土Er的加入有利于合金均匀塑性变形和均匀再结晶,Er含量为2.0%时,其挤压态晶粒尺寸最小,平均长径为7⁸μm,平均短径为2³μm。其中Er含量为0.5%的合金力学性能为σb=366MPa,σ0.2=313MPa,δ=22%。对挤压态合金进行了固溶、固溶时效以及直接时效热处理。结果表明,稀土Er加入到Mg-9Zn-0.6Zr合金中,形成的含Er稀土相具有阻碍再结晶晶粒长大和提高合金的组织热稳定性的作用。挤压后在400℃固溶1.5h,不加稀土Er以及Er含量为0.5%合金发生完全再结晶;稀土Er含量大于1.0%时,合金再结晶不完全。随着固溶时间延长至12h时,Er含量为1.0%,2.0%,4.0%的合金发生完全再结晶,组织由晶粒平均尺寸为7⁸μm的等轴晶组成,其中Er含量为2.0%的合金组织最均匀。挤压后进行固溶时效处理,不含Er以及Er含量为0.5%的合金基体中析出了大量MgZn₂相,使得合金屈服强度较固溶态明显提高。而Er含量大于1.0%的合金屈服强度增加不明显。挤压后经直接时效,合金的屈服强度进一步提高,随着稀土Er的增加,增幅呈下降趋势,而塑性有所下降。这和基体中MgZn₂相的析出有关,随着稀土Er的增加,基体中MgZn₂相析出呈减少趋势。其中Er含量为0.5%的合金的力学性能为σb=372MPa,σ0.2=342MPa,δ=16%。对比挤压态和挤压后直接时效态合金发现,合金挤压过程中纳米级球状MgZn₂相的动态析出较时效过程中MgZn₂相的静态析出更加有优势,对提高合金屈强比的同时提高合金塑性更加有利。更多还原

【Abstract】 Be the highest strength in commercial magnesium alloys, ZK60 alloy also has the highest specific strength in all the magnesium alloys. However, it still exist some defect such as low-absolute strength, poor-plasticity. So, to improve the strength while the plastic of the magnesium alloys is improved, became the urgently key problem to be solved in research and applications of magnesium alloy.In the present paper, base on the ZK60 magnesium alloy, in order to meet precipitation strength by rare-earth phase and solution strength by Zn content in matrix, to improve the comprehensive mechanical property, Zn content was improved to 9%, meanwhile different Er（wt%=0,0.5%,1.0%,2.0%,4.0%） content were added. Optical microscope, X-ray diffraction analysis（XRD）, Differential Scanning Caborimetry（DSC）, scanning electron microscopy（SEM）, energy dispersive spectroscopy（EDS） and transmission electron microscopy（TEM） were used to study the microstructure of as-cast, heat-treatment before as-extruded, as-extruded and heat-treatment after as-extruded state. The forms of existence of Er in the alloys and its effect to microstructure and mechanical properties were studied under different conditions.The result indicate that the microstructure of as-cast Mg-9Zn-0.6Zr magnesium alloy was mainly consisted ofα-Mg and large amount of Mg-Zn eutectic phase. With Er addition, two types of thermally stable Er-bearing compounds with distinct Zn/Er ratio formed. The volume fraction of the second-phase particles increased with the increase of Er addition. The dendrite arm spacing gradually decreased with the increase of the Er addition, being the smallest at 2.0%Er, the average grain size is about 32μm（102μm without Er addition）.Dynamic recrystallization occurred obviously during hot deformation. The addition of Er could bring about homogenous deformation behaviour and recrystallization behaviour to the alloy and it also promote dynamic precipitation of nano-scale spherical MgZn₂ phase to improve the strength of matrix. With the Er addition, the volume of nano-scale spherical MgZn₂ phase decreased. With 2.0%Er addition, the grain size of as-extruded alloy is smallest with its the average long axis and short axis are in the range of 7⁸μm and 2³μm, respectively. The mechanical properties of Mg-9Zn-0.6Zr alloy with 0.5% Er content areσb=366MPa,σ0.2=313MPa,δ=22%.Solution, age and solution plus age treatment have been done to as-extruded alloys with different Er addition. It is suggested that Er addition can increase the microstructure stability drastically and the Er-bearing particles have hindered grain growth during the solution treatment, leading to the smaller grain size in the Er-bearing alloy. Solution treatment at 400℃for 1.5h was carried out after extrusion. Perfect recrystallization occurred in the alloys without Er and with 0.5%Er. With more Er addition, some non-recrystallized grains exist in the alloys. While extend solid solution time to 12h, alloys with Er content 1.0%,2.0%,4.0% were consisted of equiaxial grain with 7⁸μm in average scale. Because of a large amount of MgZn₂ phase were precipitated in age treatment at 200℃for 10h after solution treatment, the yield strength and the elongation of the alloys are increased. Age treatment after extrusion, the yield strength increased. With the Er addition, the increased range of the yield strength is decreased. It is ascribed to the precipitate of the MgZn₂ in the matrix. Compare with age treatment, the dynamic precipitation of fine MgZn₂ particles with a modified spherical morphology occurred during hot extrusion are more effective than static precipitation of MgZn₂ particles through subsequent long-time aging in improving the mechanical property. The mechanical properties of Mg-9Zn-0.6Zr alloy with 0.5% Er contentσb=372MPa,σ0.2=342MPa,δ=16%.更多还原