摘要
微合金化是提高铝合金综合性能的有效方法之一。本文采用传统的铸锭冶金法制备含Er的Al-4.5Mg-0.7Mn-0.1Zr-0.4Er合金。借助硬度测试(HBS)、金相显微组织观察(OM)、X射线衍射分析(XRD)、扫描电镜(SEM)与能谱分析(EDS)、透射电镜观察(TEM)等分析测试手段,研究了退火制度对Al-4.5Mg-0.7Mn-0.1Zr-0.4Er合金的力学性能与微观组织的影响,深入分析了Er在合金中的存在形式、与合金元素的交互作用以及对合金的强化机理,并初步研究了实验合金的疲劳极限和平面应力断裂韧性,分析了Er对疲劳极限和平面应力断裂韧性的作用及机理。
研究退火温度和时间对合金力学性能的影响,结果表明,退火温度对合金力学性能影响很大,而退火保温时间对合金力学性能影响程度小。冷轧变形量为81.3%的合金在不同温度下退火2小时,在125℃～225℃之间,强度下降缓慢,225℃～275℃之间强度急剧下降,延伸率显著上升,275℃之后强度几乎不再发生变化。结合硬度法和金相法确定了合金的再结晶起始温度为225℃,再结晶终了温度为275℃。通过实验对比研究表明,冷轧板在125℃退火1个小时,合金的综合力学性能最佳。
按照国标GB/T3075-1982,测试了合金室温下的条件疲劳极限。合金板材在应力比R=0.1,循环寿命为10~7条件下的疲劳极限为293.6MPa。与传统的5083板材相比,实验合金的疲劳极限有了很大的提高。用扫描电镜对疲劳断口进行观察和分析,结果表明:疲劳裂纹起源于试样侧表面,因为在循环载荷作用下,该处存在应力集中,导致裂纹萌生和扩展。随着退火温度的提高,合金的疲劳寿命显著下降。按照HB5261-83,采用紧凑拉伸试样,测试了出合金板材的平面应力断裂韧性KC为51.92MPa·m~(1/2)。
对合金不同退火温度的试样进行了SEM和TEM观察。发现合金中除了α-Al基体、Al_6Mn相外,合金中存在大量的Al_3(Er,Zr)粒子,大小为几十纳米,这些粒子与基体成共格关系,并且弥散分布于基体中,起到了析出强化的作用。在退火过程中,这些第二相粒子并没有长大粗化,说明它们具有很好的高温稳定性。这些纳米级粒子能够强烈钉扎位错和亚结构,对晶界迁移及晶粒的长大有显著的抑制作用,阻碍了再结晶的形成。
The addition of trace mircoalloying element to aluminum alloys is one of those effective means to improve the synthesis properties of the aluminum alloys. In our group’s earlier research, rare-earth element Er have been proved to have positive effects on the structure and properties of Al-5Mg and Al-Mg-Mn alloys. In this thesis, Al-4.5Mg-0.7Mn-0.1Zr-0.4Er alloy was prepared by using the Metallurgy cast processing. By means of hardness testing, tensile properties measurement, optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS), effects of annealing temperature and time on microstructure and mechanical properties of Al-4.5Mg-0.7Mn-0.1Zr-0.4Er alloy were investigated. The existing forms of Er and interactions with other alloying elements in the alloy, the strengthening and refining mechanism were profoundly studied. The effects of Er on the fatigue propertity and the plane ssress fracture toughness of the alloy were also discussed.
It was shown that annealing temperature has a significant affect on the mechanical properties and microstructure of the experimental alloys while the annealing time has little affect on the mechanical properties and microstructure of the experimental alloys.The mechanical properties of the alloys changed a little when annealed at the temperatures below 225℃at which temperture only recovery happens. Mechanical properties of the alloys vary strongly only for the annealing temperatures of 225～275℃with the recrystallization. The second phase particles containing Er do remarkably inhibit the coarsening of recrystallized grains. When the alloy was annealed at 125℃for 1 hour, the optimized mechanical properties were obtained.
According to GB/T3075-1982, the high cycle fatigue behavior of Al-4.7Mg-0.7Mn-0.1Zr-0.4Er alloy was studied using an up-and-down method. Fatigue strength of the alloy is determined as 293.6MPa by the up-and-down method calculation under the conditions of stress ra tio R=0.1 and cycles N=107 which was higher than the traditional 5083 alloy. The fatigue fracture morphology was examined by means of Scanning Electron Microscopy (SEM) and the crack propagation process was also analyzed. Results show that the crack was initiated on the surface of the samples where stress concentration existing under the cyclic lording.
According to HB5261-83, the plae-sress frature toughness(KC) was tested using the CS sample. The KC was determined as 51.92MPa·m~(1/2) which may be lower than its true value because bucking appeared during the testing process. The frature morphology was examined by means of SEM.
A large amount of Al_3(Er,Zr) particles can be observed in researched alloy, which were high dispersed and coherent with the Al matrix. They are responsible for two mechanisms in the experimental alloy: precipitation strengthening and find grain strengthening. These particles can pin the dislocation and substructure, hinder the dislocation moving and transffering of substructure. Hence Al_3(Er,Zr) is the important strengthening phase in experimental alloy. The Al_3(Er, Zr) remarkably inhibit the coarsening of recrystallized grains.
引文
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