【作者】 高岩；

【导师】 王渠东；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2009， 博士

【摘要】 镁合金是目前实际应用中最轻的金属结构材料,具有比重小、比强度、比刚度高、导热导电性好、阻尼性、切削加工性好等优点。近年来人们相继开发了Mg-Y-Nd-Zr和Mg-Gd-Y-Zr等新型的高强耐热镁合金。随着Mg-RE合金的研究深入进行,很多研究者通过在合金中加入廉价的Zn来代替部分稀土,也获得了相近的力学性能。并且还发现加入少量的Zn,不仅可以调控Mg-RE系合金的时效析出组织;而且在适当的加入量和工艺条件下,Mg-RE-Zn系合金还产生了除沉淀相以外的新相或结构,即长周期有序结构(Long Period Stacking Ordered Structure,简称LPSO)。该结构使Mg-RE系合金表现出优异的室温和高温屈服强度、好的延伸率和高的应变速率超塑性。根据前期Mg-Gd-Y-Zr合金在高强耐热方面取得的实验结果,本文考虑在该合金体系的基础上,通过加入不同含量的Y元素(5-14wt.%)和不同含量的Zn元素(0.5-3wt.%)来研究它的微观组织和力学行为的关系,并在此基础上重点研究了Mg-10Y-5Gd-2Zn合金的热处理工艺、力学性能和高温蠕变行为。本文以Mg-(5-14)Y-5Gd-(0-3)Zn-0.5Zr合金为研究对象,采用电感耦合等离子直读光谱仪(ICP)、光学显微镜(OM)、差示扫描量热仪(DSC)、差热分析(DTA)、X射线衍射仪(XRD)、带能谱分析(EDAX)的扫描电子显微镜(SEM)和透射电子显微镜(TEM)等分析手段,通过硬度、室温和高温拉伸及拉伸蠕变性能等,系统地研究了不同Y含量、Zn含量和热处理工艺对Mg-(5-14)Y-5Gd-(0-3)Zn-0.5Zr合金的显微组织、力学性能和蠕变性能的影响;探讨了合金的强化机制,重点研究了时效析出相结构、长周期结构、形态、尺寸和分布的演变过程,为高性能稀土镁合金的进一步开发和应用提供理论和实践依据。研究结果如下:1.通过研究不同Y含量的Mg-(5-14)Y-5Gd-2Zn-0.5Zr合金发现:当Y≥8wt.%时,晶界处开始出现黑色的条状Mg12ZnY共晶相,并且晶内开始产生精细的条纹状LPSO结构相。随着Y含量的增加,晶界上的Mg24(GdYZn)5共晶相、黑色的Mg12ZnY相和晶内的精细条纹LPSO结构相都开始增多。铸态Mg-10Y-5Gd-0.5Zr合金主要由α-Mg过饱和固溶体、网状Mg24(GdY)5共晶相和花瓣状的Zr核组成。该合金加入不同含量的Zn后,组织发生了明显变化。晶内出现了层片状的精细条纹,随着Zn含量的增加,晶界上大块条状的Mg12ZnY相的数量也增加。铸态Mg-10Y-5Gd-2Zn-0.5Zr合金主要由α-Mg过饱和固溶体,Mg24(GdYZn)5网状共晶相, Mg12YZn大块条状共晶相和晶内的层片状精细条纹相组成。晶内的层片状精细条纹是6H′(ABCBCB′)类型的LPSO结构;为畸变的6H类型,其a轴和c轴夹角为88°。2.通过研究Mg-10Y-5Gd-0.5Zr合金在500-550℃和0-48h固溶过程中组织演变规律,优化出了535℃×16h的最优固溶工艺。在该工艺条件下,Mg24(GdY)5共晶相完全溶入了α-Mg基体,且晶粒没有明显长大。参考不同温度固溶处理的力学性能,优化出Mg-10Y-5Gd-2Zn-0.5Zr合金的最佳固溶工艺也为535℃×16h;而且还发现该合金在535-545℃固溶过程中,晶界的Mg24(GdYZn)5相全部溶入了基体,但是在晶界上依然残留着Mg12ZnY相。3.研究发现Mg-10Y-5Gd-2Zn-0.5Zr合金峰值硬度随着时效温度的上升而下降,过时效硬度下降幅度随着时效温度提高而加大。根据微观组织和力学性能的变化,优化出225℃×24h为合金最佳时效工艺。拉伸实验温度从室温升高到250℃的过程中,铸造T6态Mg-10Y-5Gd-2Zn-0.5Zr合金的抗拉强度只发生了微弱的降低,而当温度高于250℃时,合金的抗拉强度急剧下降,延伸率大幅度提高。铸造T6态Mg-10Y-5Gd-xZn-0.5Zr合金的常温和高温抗拉强度都明显高于WE54合金,高温抗拉强度尤其明显,其中Mg-10Y-5Gd-2Zn-0.5Zr合金在250℃和300℃的抗拉强度分别为326MPa和261MPa,远远高于同状态下的WE54合金。4.研究还发现Mg-10Y-5Gd-2Zn-0.5Zr合金在225℃时效24小时后,晶内析出β′相,该相具有底心正交晶体结构(a=0.640nm, b=2.223nm, c=0.521nm),与镁基体的取向关系为:(100)β′∥(2110)a, (001)β′∥(0001)a, [010]β′∥[1010]a。峰值时效时椭球形β′相是主要的强化相,对合金的强度贡献最大。此外,晶界上还残存着大量块状Mg12ZnY相,该相经过时效处理后结构没有明显变化;晶内仍然有6H’的LPSO结构相。5.通过对Mg-10Y-5Gd-2Zn-0.5Zr合金在温度(200-300℃)和应力(30-120MPa)条件下的高温蠕变测试发现,该合金在应力低于80MPa,温度低于250℃的范围内,蠕变性能随温度的升高下降幅度较少,合金的蠕变性能很好;当温度高于300℃而应力也高于80MPa时,合金的蠕变性能急剧变坏。对于铸造T6态Mg-10Y-5Gd-2Zn-0.5Zr合金比较适宜的使用温度不超过300℃,应力不超过80MPa。Y含量在10-12wt.%的合金具有最佳的抗蠕变性能。6.研究还发现,Zn元素可以显著提高Mg-10Y-5Gd-0.5Zr合金的高温蠕变性能。在300℃/50MPa条件下,铸造T6态Mg-10Y-5Gd-2Zn-0.5Zr的蠕变性能最好,稳态蠕变速率ε? min为6.60×10-8s-1,100小时的蠕变应变总量ε100仅为1.76%。合金在250℃和300℃指定应力范围的应力指数分别为2.3和5.1,在指定应力30MPa和50MPa下的表观激活能为分别为191.9KJ/mol和216.4KJ/mol。7.研究发现,铸态Mg-10Y-5Gd-2Zn-0.5Zr合金经过300℃/50MPa蠕变至稳态阶段(100h),晶内的LPSO结构相明显增多;而且在晶内和晶界还析出了一定量的粗大的平衡β相。晶界上依然存留着Mg12ZnY相。8.Mg-Y-Gd-Zr合金中加入一定量的Zn后,产生LPSO结构相,对于位错运动起了很大的阻碍作用。在高温蠕变过程中,开动非基面位错。LPSO结构相在位错的作用下内部发生了严重的晶格畸变,产生弯曲变形,这样的扭曲严重了阻碍了位错的运动,从而使合金的蠕变性能提高。9.铸造T6态Mg-10Y-5Gd-2Zn-0.5Zr合金在300℃/50MPa蠕变至稳态阶段(100h),晶内和晶界处析出了一定量的粗大的β相。β析出相与基体界面产生大量错配位错,阻碍了位错运动。在孪晶界和LPSO结构交互作用的位置可以看到,LPSO结构发生了偏转,形成了一定的角度。经测量,该角度约为4.1°。10.Mg-Y-Gd-Zn-Zr合金的高温蠕变变形机制为位错滑移和晶界滑移。晶界上的Mg12ZnY相也起到了钉扎晶界的作用。LPSO结构相和析出相的复合强化阻碍位错运动,提高了合金的蠕变抗力。更多还原

【Abstract】 Magnesium alloys are the lightest structural materials with high specific strength, good electric conduction, thermal conduction, damping capacity, electromagnetic shielding, formability, as well as easy recycled. Recently, Mg-Y-Nd-Zr and Mg-Gd-Y-Zr alloys were development. Many researchers found adding some cheaper zinc into the Mg-RE alloys brought great changes. Zinc can control the precipitate phase and induced the LPSO (Long Period Stacking Ordered Structure). It can exhibit excellent room and high temperature mechanical properties and creep properties. Based on the results of Mg-Gd-Y-Zr alloys, Zn (0.5wt.%-3wt.%) were added into the alloys. The microstructure and mechanical properties of Mg-10Y-5Gd-2Zn-0.5Zr alloy were researched.Several Mg-(5-14)Y-5Gd-(0-3)Zn-0.5Zr alloys were prepared. Effects of variant content of Y and Zn, heat treatment and thermal-mechanical process on the microstructure, mechanical properties and creep resistance were mainly investigated, by computer data collection system, optical microscopy (OM), image analysis apparatus with a image analysis software, X–ray diffract meter (XRD), inductively coupled plasma analyzer (ICP), Differential Thermal Analysis (DTA), Differential Scanning Calorimeter (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with energy dispersive X-ray analyses (EDAX) and micro-diffraction etc.. The strengthening mechanism of the alloys was analyzed and discussed, and micro-structural evolution during aging, including the morphology, structure, size and distribution of the precipitates, was studied in detail. The purpose of the present work is to provide theoretical and practical results for the development of high-performance magnesium- rare earth alloys. The main conclusions can be summarized as follows:1.The as-cast Mg-10Y-5Gd-0.5Zr alloy contains the majorαphase which is supersaturated Gd + Y solid solution in Mg matrix; Mg24(GdY)5 eutectic phase which looks like narrow island morphology and has higher Gd + Y content than the matrix; and the intra-crystalline zirconium rich cores. Adding 2wt.% Zn to Mg-10Y-5Gd-0.5Zr alloy leads to form a long-period stacking-ordered structure via a conventional casting method. The structure is a 6H′-type (ABCBCB′) which is a distorted stacking order from an ideal hexagonal lattice of 6H-type. The angle between the c- and a-axis is estimate to be approximately 88°.2. At 535℃for 16h, the Mg24(GdY)5 net-work of second phase was completely dissolved, and only remained some Mg-Y-Gd cuboid-shaped compound. The optimum solid-solution condition of Mg-10Y-5Gd-0.5Zr is 535℃/16 h. And the optimum solid-solution of Mg-10Y-5Gd-2Zn-0.5Zr is also 535℃/16 h. The peak hardness was obtained at about 225℃for 24h.3. The strengths of cast-T6 Mg-10Y-5Gd-2Zn-0.5Zr decline very slowly from room temperature to 250℃. However, at the During the room temperature to 250℃or more, the strengths steeply decrease. The instant tensile strengths of cast-T6 Mg-10Y-5Gd-xZn-0.5Zr alloys are remarkable superior to those of WE54. A very high strength of cast-T6 Mg-10Y-5Gd-2Zn-0.5Zr alloy with UTS=326MPa, and UTS=261MPa which is at 250℃and 300℃are remarkable superior to those of WE54.4.The present work has investigated theβ′precipitates are formed within grains when the alloy is aged at 225℃for 24h. The globular shapeβ′precipitates are formed in the under aged state, which is corresponding to the highest strength of the alloy. The precipitates coalesce, and the plate shapeβ′precipitates are formed lying in the {2110 } habit planes with increasing ageing time. This intermediate phaseβ′has a base-centered orthorhombic structure (a=0.640nm, b=2.223nm,c=0.521nm), the orientation relationship betweenβ′and the matrix phase is: (100)β′∥( 2110 )a, (001)β′∥(0001)a, [010]β′∥[1010]a. Some Mg12ZnY phases was remain on the grain boundary and the 6H′phase was not changed in the grain.5. The research was focused on the creep properties of Mg-10Y-5Gd-xZn-0.5Zr alloys at high temperature. It is found that the creep resistance was very good when temperature is lower 250℃and the stress is lower 80MPa; but when the temperature is higher than 300℃and the stress is higher than 80MP, the creep properties was serious deteriorated. So the cast-T6 Mg-10Y-5Gd-2Zn-0.5Zr can not be used on above 300℃. In all the Mg-Y-Gd-Zn-Zr alloys, the content of 10-12 wt.% Y alloys have excellent higher temperature creep properties.6. Influences of Zn addition on microstructure and mechanical properties at room and elevated temperatures up to 300℃have been investigated. It can be seen that: 2% element Zn had remarkable improved the creep properties of the Mg-10Y-5Gd-0.5Zr alloy at room and elevated temperatures. For the cast-T6 Mg-10Y-5Gd-2Zn-0.5Zr alloy, at 300℃/50MPa, the steady-state creep rate is 6.60×10-8s-1 and the creep strain after the creep life of 100 hours is 1.76%. The stress exponent at 250℃and 300℃is 2.3 and 5.1, and the apparent activation energy value is 191.9KJ/mol and 216.4KJ/mol when stress was 30MPa and 50MPa.7. Furthermore, the LPSO phase increased after crept at 300℃/50MPa for 100 hours , the quantity and density also increased which is the first time discovered . The purpose of the present work is to provide theoretical and practical results for the development of high-performance magnesium-heavy rare earth alloys. The Mg12ZnY phase was still at the grain boundary, its structure was not changed. Also it can be seen that the large numbers of plate shape equilibriumβphases with bcc crystal structure precipitate along the {1010 } habit planes in the grain.8. Through the analysis of creep data and the TEM results, it is found that zinc element can induce the stacking fault energy debased, and the 6H’-LPSO structure within grains prevent the dislocation movement. During the creep test, some basal dislocation changed non-basal dislocation. The 6H’-LPSO structure brought out serious crystal lattice aberration which hampered the dislocation movement, So the creep properties were increased.9. It is found that the large numbers of plate shape equilibriumβphases hampered the dislocation movement. The inhibition of the basal ship in the Mg-Y-Gd-Zn-Zr alloy is due to the formation of Mg12ZnY phase and the LPSO structure. The deformation twin is deflected in the formation of Mg12ZnY phase and the LPSO structure. The base plane trace of LPSO phase is inclined with constant angle about 4.1°. The stacking fault energy of Mg-Y-Gd-Zn-Zr alloy is quite low due to high Y and Zn additions. During the deformation, the stacking faults can be easily introduced in and the dislocation would be accumulated at the front of stacking fault region.10. The creep deformation mechanisms of Mg-Y-Gd-Zn-Zr alloy were investigated systematically by TEM. According to the results, dislocation slip and the grain boundary slip were the main deformation mechanisms. The Mg12ZnY phase pins up the grain boundary and barrage the grain boundary movement. The LPSO structure andβphases contribute to the strengthening of the alloys and hampered the dislocation movement.更多还原