【作者】 韩富银；

【导师】 梁伟；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2012， 博士

【摘要】 镁合金作为目前可应用的最轻的金属结构材料,具有优异的室温性能,但当温度超过120℃时,抗蠕变性能显著下降,其应用受到了极大的限制。因此,提高镁合金的高温性能尤其是高温抗蠕变性能,是镁合金研究开发的重要方向。目前工业化的耐热镁合金主要是稀土耐热镁合金和镁基复合材料,这两种合金由于价格昂贵,难以适应民用工业的需要,开发新型低成本耐热镁合金成为目前耐热镁合金研究的热点。目前镁合金结构件主要是铸造镁合金,其强度低,塑性差。变形镁合金具有高强度和高伸长率等优点,能够满足更高的设计要求。将等通道转角挤压(ECAP)技术应用于耐热镁合金的研发,系统的研究ECAP技术对耐热镁合金组织及性能的影响规律,为开发低成本,综合性能优良的耐热变形镁合金材料奠定理论基础。本文选用Mg-Al系和Mg-Zn系中较典型的AZ61和ZK60合金,加入较多量元素Si形成高温增强相Mg2Si来提高其耐热性,制成AZ61-4Si和ZK60-4Si耐热镁合金,再利用ECAP技术细化合金组织,提高合金力学性能。采用金相显微镜(OM)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)及能量分散光谱仪(EDS)分析了材料的微观组织；采用电子万能拉伸实验机测试了材料的室温力学性能,并测试了高温蠕变性能。对ECAP细化镁合金的机制、镁合金拉伸变形及断裂机制、Mg2Si相形貌对镁合金室温及高温蠕变性能的影响机制进行了探讨。研究结果表明：(1)ECAP可显著细化镁合金组织。AZ61-4Si合金经4道次变形基休晶粒山铸态300μm细化为10μm,汉字状Mg2Si由铸态的最大70μm细化为7μm；8道次变形使基体进一步细化为6μm, Mg2Si颗粒细化为5μm。ZK60-4Si合金经4道次变形基体晶粒由铸态400μm细化为55μm,汉字状Mg2Si由铸态的最大76μm细化为18μm；8道次变形使基体进一步细化为25μm, Mg2Si细化为极细小颗粒。ECAP细化镁合金机制为大塑性引起的连续动态再结晶。(2)由于ECAP细化了实验合金组织,使合金力学性能得到大幅度提高。AZ61-4Si合金经4道次变形屈服强度提高128%,抗拉强度增加89%,伸长率提高340%；8道次后,屈服强度进一步提高；但抗拉强度与伸长率略有降低。ZK60-4Si合金经4道次变形后屈服强度提高90%,抗拉强度提高63%,伸长率提高203%；ECAP8道次后,屈服强度略有下降,抗拉强度有较大幅度提高,实验合金硬度随挤压道次增加而增大(3)实验合金铸态组织中Mg2Si相为汉字状,拉伸断裂时,a-Mg/Mg2Si相界面微裂纹成为主要的裂纹源,且裂纹易沿a-Mg/Mg2Si相界面扩展,导致材料发生脆性断裂。ECAP使Mg2Si相碎化为颗粒状,合金的断裂为微孔形核的韧性断裂机制。(4)经ECAP后合金AZ61-4Si和ZK60-4Si高温抗蠕变性能均得到显著提高,其机制是大量高温稳定相Mg2Si颗粒分布于晶内和晶界,既阻碍了晶内位错运动,又阻止了晶界滑移。更多还原

【Abstract】 As the lightest structural metallic materials applied currently, magnesium alloys show excellent room temperature performance. However, when the temperature exceeds120℃, the creep resistance significantly decreases and its application is extremely limited. Enhancing the high temperature properties of magnesium alloys, especially high-temperature creep resistance, is an important direction for the research and development of magnesium alloys. The industrialized heat-resistant magnesium alloys are mainly rare earth heat-resistant magnesium alloys and magnesium matrix composites which are difficult to meet the need of the civilian industry due to the expensive price. The research emphasis of heat-resistant magnesium alloys is focus on the development of new low-cost heat-resistant magnesium alloys recently. The magnesium alloy structural components which are mainly made of cast magnesium alloy show lower strength and poor plasticity. Deformation magnesium alloys with high strength and high elongation, etc. can meet higher design requirements. Thus, using equal channel angular processing (ECAP) technology in the R&D of heat-resistant magnesium alloy and systematically investigating the effect law of ECAP technology on the microstructure and properties of heat-resistant magnesium alloy can establish the basis for the development of new heat-resistance magnesium alloys with low-cost and excellent combination properties.In this study, typical AZ61magnesium among Mg-Al system and ZK60alloy among Mg-Zn system was selected as matrix materials. Higher amounts of element Si was added to form high temperature phase Mg2Si for improving its heat resistance. The AZ61-4Si and ZK60-4Si heat-resistance magnesium alloys were made. ECAP technique was used to refine the alloys microstructure and to improve its mechanical properties. Optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to analyze the alloys microstructure. Electronic universal tensile testing machine was used to test the alloys room temperature mechanical properties and high temperature creep resistance. The magnesium alloys refinement mechanism by ECAP, magnesium alloy tensile deformation and fracture mechanisms and the effect mechanism of Mg2Si phase morphology on the room temperature mechanical properties and high temperature creep resistance of the magnesium alloys were studied. The research results are shown as follows:(1) The microstructure of magnesium alloys can be significantly refined by ECAP. After4-pass ECAP, the matrix grains of AZ61-4Si alloy are changed from300μm of the as cast alloy to10μm. Chinese script type Mg2Si particles are changed from the maximal70μm of the as cast alloy to7μm. After8-pass ECAP, the matrix grains are further refined to6μm and the Mg2Si particles to5μm. The matrix grains of ZK60-4Si alloy are changed from400μm for as cast alloy to55μm. Chinese script type Mg2Si particles are changed from the maximal76μm of the as-cast alloy to18μm after4-pass ECAP. The matrix grains are further refined to25μm and the Mg2Si particles are very small particles after8-pass ECAP. The refinement mechanism of magnesium alloy by ECAP is continuous dynamic recrystallization caused by large plastic deformation.(2) The mechanical properties of the experimental alloy improve due to the microstructure refinement effect by ECAP. The yield strength, tensile strength and elongation of AZ61-4Si alloy after4-pass ECAP increase by128%,89%and340%, respectively. The yield strength of the8-pass ECAP AZ61-4Si alloy further improves while the tensile strength and elongation slightly reduce. The yield strength, tensile strength and elongation of ZK60-4Si alloy after4-pass ECAP are improve by90%,63%and203%, respectively. The yield strength of the8-pass ECAP ZK60-4Si alloy slightly decreases while the tensile strength increases significantly. The hardness of the experimental alloy increases with the pressing times increasing.(3) Mg2Si phase is shown with Chinese script type in the as-cast microstructure of the experimental alloy. The micro-cracks in the a-Mg/Mg2Si phase interface become the main crack source. The crack propagation easily along the a-Mg/Mg2Si phase interface can result in the brittle fracture of the materials. However, the Mg2Si was broken into grainy by ECAP. The fracture of the alloy shows a ductile fracture mechanism with nucleation model.(4) Both the high temperature creep resistances of the AZ61-4Si alloy and ZK60-4Si alloy improve remarkably by ECAP. The mechanism is that a large number of high-temperature stable phase of Mg2Si particles are distributed in the grain interior and grain boundaries, which not only hinder the intragranular dislocation movement, but also prevent grain boundary sliding.更多还原