【作者】 武艳军；

【导师】 王经涛；

【作者基本信息】 南京理工大学 ， 材料加工， 2012， 博士

【摘要】 镁合金是最轻的金属结构材料,具有高的比强度、比刚度、减振性、导热性、可切削加工性和可回收性,因而被称为21世纪的“绿色工程材料”。尤其是随着汽车工业及电子行业的迅速发展,大量的镁合金构件应运而生,部分代替了塑料、铝合金甚至钢铁材料,预计镁合金将成为本世纪最重要的商用轻质金属结构材料之一。目前,铸造镁合金应用关键技术已取得较大突破,产业化已初具规模。研究结果表明,变形镁合金无论在强度还是韧性方面均远优于铸造镁合金。因此,变形镁合金必然为下一代新型镁合金的发展重点。作为结构材料,镁合金必然会受到循环应力／应变作用,为了其安全应用,需要了解其疲劳行为及失效机理。但是对镁合金疲劳性能的研究,一般采用唯象连续介质模型,忽略了镁合金独特的变形机制特点。AZ31镁合金独特的hcp晶格结构,导致孪生成为其重要的变形机制之一。但目前的研究,一般将位错滑移与孪生混合在一起研究,甚至忽略了孪生在疲劳开裂中的作用。事实上,孪生在一定条件下可以成为疲劳变形主要的变形机制,同时通过改变晶粒取向为位错的进一步滑移提供条件。因此,了解孪生在镁合金疲劳变形过程中的作用,可以更深刻的了解镁合金的疲劳变形行为。为此,本文以AZ31镁合金为研究对象,通过研究镁合金疲劳过程中孪生的影响因素入手,了解孪生在镁合金疲劳中的作用。对镁合金而言,影响孪生行为的因素主要包括织构、应变率、晶粒尺寸和预先引入的孪晶等。采用X射线衍射技术确定镁合金织构,沿特定取向切割试样,研究孪生与位错滑移各自对疲劳性能的影响规律；通过不同频率下的疲劳实验,产生不同的孪生量,分析孪生量对镁合金疲劳寿命的影响；ECAP变形细化镁合金,抑制孪晶的产生,研究细晶对镁合金疲劳性能的影响规律；采用预变形手段在镁合金中引入孪晶,研究初始孪晶对镁合金疲劳的影响。取得以下主要结论：(1)当镁合金以孪生为主要变形机制时,由于出现了大量的残余孪晶,试样具有较高的循环硬化行为；孪生主导试样具有较大的裂纹尖端循环塑性区,其尺寸大约是位错主导试样的的四倍,由此导致了孪生主导试样断口较为粗糙,而位错主导试样断口较为光滑,并出现了含有大量孪晶的小平面；由于粗糙裂纹面诱发的裂纹闭合效应,孪生主导试样具有较高的疲劳寿命。(2)恒应力幅对称高周疲劳条件下,随着疲劳的进行,能量参数逐渐降低至零,表明滞后环不对称性逐渐减轻；棘轮行为只存在于镁合金疲劳初期,随后出现棘轮安定,因此其对镁合金疲劳损伤影响不大；压缩应变峰值随疲劳进行逐渐变小,在约200周时反向,表现为拉伸应变,抑制了退孪生行为；疲劳裂纹易在材料表面孪晶带处形核,并以穿晶方式沿着孪晶界面扩展。(3)低于0.2%的应变幅下,加载频率提高可以延长镁合金疲劳寿命；而高于0.2%的应变幅时,频率对疲劳寿命影响不大。疲劳寿命的增加源于低应变幅高频率下,提高了孪晶的数量。(4)等径角变形AZ31镁合金在应变疲劳条件下表现出循环软化行为,且随着应变幅的增加,软化程度加剧；该软化行为可归因于疲劳过程中的微结构的不稳定性。等径角挤压变形造成了AZ31镁合金内部的缺陷沿着最后一道次的切变面分布,引起晶粒的再结晶,并使得再结晶晶粒的长轴沿着等径角挤压的切变面。等径角变形材料具有较小的疲劳应变能,由此提高了镁合金的低周疲劳寿命,其弹性应变幅、塑性应变幅与断裂时的载荷反向周次之间的关系可分别采用Basquin和Coffin-Manson公式描述。(5)无预压缩镁合金在拉伸阶段以位错滑移为主要变形机制,在压缩阶段以孪生为主要变形机制,而预压缩变形改变了拉伸和压缩阶段的变形机制,并导致了平均应力的改变。在高应变幅下,疲劳寿命取决于塑性应变幅。但在低应变幅下,位错运动的主要部分可逆,此时材料的疲劳寿命取决于平均应力。更多还原

【Abstract】 Magnesium alloys are among the lightest metallic materials for structural applications. Due to high specific strength and rigidness, as well as good machinability and recyclability, magnesium alloy has been known as the 21st century ’green’ engineering material. In recent years, with the rapid progress of automotive and electronic industries, a number of magnesium alloy components have been manufactured to replace those made from plastics, aluminium alloy and steel ones. It can be expected that magnesium alloys will become the most important structural materials in commercial metal materials. Wrought magnesium alloys can exhibit the higher strength and better plasticity than cast magnesium alloys, and show the more significant potential in further applications of magnesium based materials. As structural materials in service, magnesium alloys are usually subjected to repeated reverse loading, and therefore the cyclic deformation behavior of these materials needs to be studied in detail for safety reasons. However, many investigations were based on the macroscopic fatigue properties, neglecting the deformation characteristics of magnesium alloys. Due to hcp crystal lattice, twinning and slip are important to deformation of magnesium alloys. And in some cases, twinning may be the main deformation mechanism. As such, it is important to understand the role of twinning in fatigue process.So in this dissertation, the widely used AZ31 wrought magnesium alloy are chosen as model material to understand the role of twinning. In fact, some factos influence the behavior of twinning, such as texture, strain rate, grain size and initial twins. According to the initial texture of the extruded AZ31 plate investigated by X-ray diffraction, samples were cut along different directions to discriminate the role of twinning and slip in fatigue process. Under different frequencies, the degree of twinning is different and its effect on fatigue properties was analyzed. Magnisum alloy can be fined by equal channel angular pressing (ECAP), which suppress the activation of twinning. And the fatigue properties of ultrained AZ31 can be understood. Initial twins can alter the deformation mechanism under fatigue process, and the influence of initial twins on fatigue properties can be acquired by pre-deformation. From this work, the following conclusions are drawn:(1) The low-cycle tension-tension fatigue properties of extruded Mg-3%Al-1%Zn alloy plate have significantly different features in twinning-dominated samples and dislocation-dominated samples. The twinning-dominated samples show more pronounced cyclic hardening and longer fatigue life than those of the slip-dominated samples. The elongated lifetime of the twinning-dominated samples may be due to the roughness-induced crack closure, according to the calculated reverse plastic zone size.(2) A number of uniaxial stress-controlled cyclic loading experiments were conducted on extruded AZ31 magnesium alloy, in order to investigate the influence of tension-compression asymmetry on fatigue properties. The results show that the systeresis loops exhibit asymmetry during initial fatigue cycles, but this asymmetry vanishes after 200 cycles. The peak compressive strain gradually decreases, and at about 200 cycles, it reverses to tensile strain. Fatigue crack initiates at the twin bands in the surface, and the crack propagates along with specific twin boundaries. Due to texture and deformation mechanism, twining and detwinning behaviors are often observed in the fatigue process, which leads to the hysteresis loop asymmetry.(3) Fully reversed strain-controlled tension-compression fatigue tests were carried out at frequencies of 1Hz and 10Hz in ambient air to investigate the frequency effect. When the strain amplitude was lower than 0.2%, the fatigue life exhibited a positive correlation with loading frequency, and the activity of twinning was increased at 10Hz. When the strain amplitude was higher than 0.2%, significant twinning was observed both at both frequencies, and the fatigue life was found to be independent of frequency. The possible reasons for this frequency-related fatigue lifetime may be due to the dependence of twinning upon loading frequency and strain amplitude.(4) The low-cycle tension-compression fatigue tests were performed at ambient temperature on ultrafine grained AZ31 magnesium alloy processed by equal channel angular pressing. All samples exhibited cyclic softening, and the softening effect increased with increasing total strain amplitude, which may be due to the instability of microstructure. Observations by optical microscope revealed that pronounced recrystallization occurred, and the direction of larger axis of recrystallized grains was nearly 45°with respect to the loading axis. A model is proposed to account for the recrystallization, based on the characteristic distribution of defects introduced by equal channel angular pressing. Compared with the conventional extruded AZ31 alloy, the ECAP processed AZ31 alloy has lower hysteresis strain energy and leads to enhanced fatigue lives. And the dependences of the strain fatigue life on plastic strain amplitude and elastic strain amplitude can be described by the Coffin-Manson and Basquin equations, respectively.(5) The fatigue properties of a AZ31 magnesium alloy was investigated in both thermomechanically treated extruded and pre-compression conditions. For pre-compression materials, twinning was the dominant deformation mechanism during the tensile loading, and slips were the dominant during the compressive loading, which induced different mean stresses in extruded and pre-compressin materials. Expermental results show that in high strain amplitude, the fatigue lifetime was controlled by plastic strain amplitude which could lead to accumulated cyclic damage. On the other hand, in low strain amplitude, it is invalid for most of dislocation slips are reversal, and the fatigue lifetime was controlled by mean stress.更多还原