【作者】 乔军伟；

【导师】 李建平； 郭永春；

【作者基本信息】 西安工业大学 ， 材料加工工程， 2010， 硕士

【摘要】 镁合金因其比强度高、易回收等许多优点,在国防工业和民用工业中的应用日益广泛,引起国际材料界与工业界的广泛关注和研究。相图对于金属材料的研究与开发具有非凡的指导意义,通过相图可以对合金中的相组成做出确切的理论说明,使得研究者能够对合金的成分设计、热处理工艺的制定等做出合理和正确的判断。镁合金的强化行为主要集中在沉淀相强化方面,通过向纯镁中添加一些金属元素形成强化相进而起到改善各项性能的作用。所以,搞清楚金属元素对合金的作用显得尤为重要,对其所形成沉淀相的研究也显得尤为迫切。本文正是针对这一点切入,用CALPHAD(CALcuation of Phase Diagram)计算相图作为指导,向Mg-Gd-Y系合金中添加Zn元素,进而研究了Zn对该合金的主要影响。本文首先利用Pandat相图计算软件绘制了Mg-Zn、Mg-Gd-Y、Mg-3Gd-3Y-0.5Zr-xZn、Mg-6Gd-3Y-0.5Zr-xZn、Mg-9Gd-3Y-0.5Zr-xZn及Mg-12Gd-3Y-0.5Zr-xZn合金的富镁区域相图,并计算了各合金相比例随温度变化曲线。计算相图表明,Zn元素的加入促进了Mg24Y5相、Mg5Gd相的形成,合金液相线温度逐渐降低,MgZn相比例增大；而随着Gd含量的增加,促进了Mg5Gd相的形成,对Mg24Y5相、MgZn相影响较小。在相图计算基础上,采用差示量热扫描仪(DSC)测定了几种镁合金的相转变温度(Mg-xGd-3Y(x=3,6,9wt%)、Mg-9Gd-xY(x=1,5,7 wt%)、Mg-3Gd-3Y-xZn-0.5Zr(x=0,1,3,6wt%)和Mg-9Gd-3Y-xZn-0.5Zr(x=1,3,6 wt%))。结果表明,在简单的三元合金中,由于相区较大,各相转变点间隔较大,且相转变能量波动较大,试验测定值与预测值相差不大。当合金系多元化后,合金的高温相变仍可以测定,但是,随着合金相区增多,一些固相转变中相变潜热较小,采用差示量热扫描仪难以准确测定相变点。综合总体试验结果,表明利用Pandat软件所计算的合金相图是可靠的。最后,利用光学金相显微镜(OM)、扫描电镜(SEM)、能谱仪(EDX)和透射电镜(TEM)对部分合金进行了试验验证及观察。结果表明,MgZn相在铸态Mg-Zn二元合金中,主要存在于晶界处,且随着锌含量的增加而增加,经过520℃,8h+200℃,168h处理后,含锌较少的合金晶界处MgZn相基本消失,含锌量较多时,晶界处还有一定残留。在Mg-Gd-Y三元合金中,不论钆还是钇增加,合金中的第二相都增多,铸态合金中Mg-Gd相弥散分布,钆含量增多时会出现聚集,而Mg-Y相主要沿晶界分布。Gd含量9%以上的合金经过520℃,8h+400℃,168h处理后析出了短棒状的Mg5Gd相。对Mg-3Gd-3Y-xZn-0.5Zr、Mg-6Gd-3Y-xZn-0.5Zr、Mg-9Gd-3Y-xZn-0.5Zr和Mg-12Gd-3Y-xZn-0.5Zr四组Zn(x=0,1,3,6 wt%)含量不同的合金进行了对比分析,在铸态合金中,第二相都是随着锌含量的增加而增加,最后形成网状沿晶界分布；经过520℃,8h+200℃,168h处理后,第二相普遍有所减少,含锌量为6%的合金中,除了板条状的第二相,还产生了颗粒状的Mg(Zn,Gd,Y)相。随着Gd和Zn含量的增加,合金中第二相数量增多,对合金中晶粒的长大起到很好的抑制作用,Gd含量的增加对MgZn相的形成影响较小,而Zn的增加会促进Mg24Y5相、Mg5Gd相的形成,试验很好地验证了这一点,随着含Zn量的增加,在合金的晶界处产生了越来越多的富含Gd和Y的第二相。更多还原

【Abstract】 Magnesium alloy have a wide application in Defense-industry and Civil-industry for its advantage as high strength weight ratio, easy recycling, etc.Phase diagram is important for metal material design, which can explain the phase composition of alloy by theory, and allow researcher to make reasonable and correct judgments about design the composition of the alloy, heat treatment process, etc. To strengthen magnesium alloy by the precipitates strengthening phase, some metal elements was added in pure magnesium. It is particularly important that find out the influence of metal element on the alloy. So,the research on the precipitate process is urgent. In this paper, the CALPHAD phase diagram was used to studied the influence of Zn on Mg-Gd-Y alloy system.First of all, The Mg-Zn, Mg-Gd-Y, Mg-3Gd-3Y-0.5Zr-xZn, Mg-6Gd-3Y-0.5Zr-xZn, Mg-9Gd-3Y-0.5Zr-xZn, Mg-12Gd-3Y-0.5Zr-xZn alloys phase diagram and phase ratio varies with temperature was calculated with a phase diagram calculation software Pandat.The result shows that The addition of Zn element in Mg-Gd-Y alloy promoted the formation of Mg5Gd and result to the fall of liquid temperature.The increase of Gd content, promoted the formation of Mg5Gd phase and Mg24Y5 phase, but less affected to the MgZn phase.In the foundation of CALPHAD, the phase-chage temperature of Mg-xGd-3Y (x=3,6,9wt%),Mg-9Gd-xY(x=1,5,7wt%),Mg-3Gd-3Y-xZn-0.5Zr(x=0,1,3,6wt%)and Mg-9Gd-3Y-xZn-0.5Zr(x=1,3,6 wt%) alloys, was tested with a DSC instrument to verified the calculated phase diagram. The result shows that tested result close to that calculated in ternary alloys, because of the large phase region, the broad interval between phase transition point and larger phase transition energy fluctuations. But, in complicate alloys, with the increase of phase region amount, a number of solid-phase transitions energy are too small to test by DSC, some process could not be well verified by this method. Anyway, the Pandat soft is practical and reliable.At last, the microstrcture of some MG-Gd-Y-Zn system alloy was observed with the OM, SEM, EDX, and TEM.The results show that, MgZn phase mainly distributes in the grain boundaries in the as-cast Mg-Zn binary alloys, and manifold with the zinc content increases. After 520℃for 8h and 200℃for168h treatment, MgZn phase almost disappeared in less Zine content alloy, but in mass zinc content alloy, there is a certain residue in the grain boundary. In the Mg-Gd-Y ternary alloy, no matter gadolinium or yttrium to increase, the second phases were increased. Mg-Gd phase diffuse distribution in cast alloy, aggregated when the Gd content increased. Mg-Y phase mainly distributed along the grain boundary distributed. Gd content with more than 9% after treatment at 520℃for 8h for 400℃,168h, short rod-like Mg5Gd phase was found. Four groups of different Zn (x=0,1,3,6 wt%)content alloy, Mg-3Gd-3Y-xZn-0.5Zr, Mg-6Gd-3Y-xZn-0.5Zr, Mg-9Gd-3Y-xZn-0.5Zr, Mg-12Gd-3Y-xZn-0.5Zr were compared and analyzed. In the as-cast alloy, the network-like second phase manifolded with the zinc content increases, and distributed along the grain boundary eventually. After 520℃for 8h and 200℃for 168h treatment, the second phase general decline in the 6% zinc content alloy, plate shape,strip shape also a granular Mg (Zn, Gd, Y) phase was found. With the increase of Gd and Zn content, the second phase increased, and inhibited the grain growth. Gd content was less affect to formation of MgZn phase,the increase in Zn content would promote the formation of Mg24Y5 phase, Mg5Gd phase. Test result well confirm that with the increase in the amount of Zn element a number of rich Gd and Y second phase appear in the grain boundaries.更多还原