【作者】 刘先兰；

【导师】 曾苏民； 刘楚明；

【作者基本信息】 中南大学 ， 材料科学与工程， 2012， 博士

【摘要】 镁合金具有密度小,比强度高,优良的减振性、电磁屏蔽性以及易回收利用等优点,在航空航天、汽车、电子等领域具有很大的应用潜力,被誉为21世纪最具发展前途的金属结构材料。纯镁的阻尼性能好,但强度低。通过合金化、塑性变形、热处理等强化方法可提高镁合金的强度,但对阻尼性能有不同程度的影响。目前对镁合金的阻尼机制存在多种不同的观点,对高强度镁合金的阻尼性能及阻尼机制的研究更未深入。本文采用光学显微镜(OM)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、机械动态分析仪(DMA)等检测手段,系统地研究了Mg-Zn-Nd-Cd-Zr合金的组织、力学性能和阻尼性能,并对Mg-Zn-Nd-Cd-Zr合金的强化机制和阻尼行为进行了较全面的分析。系统地研究了ZK60-2Nd1Cd合金的应力-应变特征,建立了流变应力本构方程为f=1.579×1011[sinh(0.0123σ)]5.098exp[-160.1×103/(RT)]。合金在应变速率为0.001s-1～1s-1,变形温度为300℃-420℃范围内的真应力-真应变曲线具有动态再结晶的特征,其高温变形过程是一种受热激活过程控制的塑性变形过程。系统地研究了Cd、Nd、Zn的变化对ZK60合金的组织和力学性能的影响,分析了Mg-Zn-Nd-Cd-Zr合金的强化机制。合金通过固溶强化、细晶强化、第二相强化等强化机制,提高了强度,改善了塑性。铸造合金晶界上连续网状分布的T相使合金ZK60-2Nd1Cd的晶粒细化,屈服强度提高,但抗拉强度降低。挤压后合金的抗拉强度和屈服强度分别为310MPa和290MPa,伸长率为16%；随着Nd含量增加,第二相T相和W相增多,晶粒更加细化,挤压态合金ZK60-2.5Nd1Cd的的抗拉强度和屈服强度增加到355MPa和300MPa,伸长率为10%;随着Zn含量降低,合金中的T相消失,W相增加,合金的强度降低。T5时效处理,合金的极限抗拉强度和屈服强度相比挤压态合金得到提高,经过140℃×34h时效后,合金ZK60-2.5Nd1Cd抗拉强度和屈服强度分别为385MPa和320MPa,伸长率为10%。系统地研究了合金元素Zn、Nd、Cd对合金Mg-Zn-Nd-Cd-Zr合金的室温阻尼性能的影响,分析了Mg-Zn-Nd-Cd-Zr合金的阻尼机制。室温下,合金元素Cd使铸造合金Mg-Zn-Zr基体中的固溶原子浓度增加,减小了弱钉扎点间距,降低合金的应变无关阻尼；Nd元素的加入,使Mg-Zn-Cd-Zr合金产生了T相和W相,使基体中的Zn原子浓度降低,合金的应变无关阻尼有所增加。在一定的应变振幅条件下,Mg-Zn-Nd-Cd-Zr合金的应变相关阻尼比Mg-Zn-Zr合金的高,随着Zn含量的降低,合金的应变相关阻尼相应增加。较高振幅下,固溶处理和挤压态合金Mg-Zn-Nd-Cd-Zr的应变相关阻尼均比铸造合金的阻尼小。系统地研究了Mg-Zn-Nd-Cd-Zr合金的高温阻尼性能。合金Mg-Zn-Zr的阻尼-温度曲线上有与溶质原子有关的P1峰和与晶界驰豫有关的P2峰；合金Mg-Zn-Nd-Cd-Zr的P1峰变得很小,且随着Zn含量的增加而向高温区移动；固溶处理后,Zn含量低的合金的P1峰被抑制；Nd含量高的三种合金在铸造、固溶及挤压后都没有出现P2峰,且挤压后合金的P1阻尼峰也完全被抑制。当温度超过某一临界温度时,合金的高温阻尼随温度增加而迅速增长,含Zn量高的合金阻尼增加得更快。提出了强钉扎、次强钉扎和弱钉扎的位错钉扎脱钉模型,该模型中的次强钉扎点可以是难脱钉的位错、固溶原子团或者第二相化合物质点。试验合金Mg-Zn-Nd-Cd-Zr的阻尼机制符合位错型阻尼机制,在较高应变振幅下出现的阻尼平台可以用强钉扎、次强钉扎和弱钉扎的位错钉扎脱钉模型来解释。建立了Mg-Zn-Nd-Cd-Zr合金的综合性能指数α与抗拉强度(屈服强度)和阻尼的关系模型；建立了铸态合金的综合性能指数α与合金的晶粒大小d的数学模型,表达式为：α=4639.63-95.42d+0.58d2。更多还原

【Abstract】 Magnesium alloys have a great application potential in the aerospace, automotive, electronics, and other industrial fields with the characteristics of low density, high specific strength, excellent anti-vibration capacities, good electromagnetism shield properties, and easy recovery utilization etc, which is known as the most promising metal structure material in the21st century. Pure magnesium possesses extraordinary high damping capacity, but the low tensile strength. Some experimental results have shown that addition of elements, heat treatment and plastic deformation could efficiently improve the strength, and effect on the damping capacity of magnesium alloys. There exist some different views on the damping mechanism of magnesium alloys, especially to the high strength magnesium alloys such as Mg-Zn-Nd-Cd-Zr alloys. The microstructures, the phase constitution of the Mg-Zn-Nd-Cd-Zr magnesium alloy were analyzed by optical microscopy(OM), X-ray diffraction (XRD), scanning electron microscopy(SEM), transmission electron microscopy (TEM). The mechanical properties were tested by tensile test and hardness test. The strain dependent and temperature dependent low frequency damping capacities of these alloys were studied by DMA. The strengthening mechanism and damping behavior of these Mg alloys were revealed comprehensively and systematically.The flow stress behavior of ZK60-2NdlCd alloy was studied using hot simulator. The constitutive equation of flow stress can be expressed as ε=1.579x1011[sinh(0.0123ρ)]5098exp[-160.1×103/(RT)]. The stress-strain curves of ZK60-2Nd1Cd alloys at the strain rate within0.001～1and temperature within300℃-420℃showed a typical dynamic recrystallization characteristic. The hot deformation Was controlled by activation energy.The effect of Cd, Nd, Zn on the microstructures and mechanical properties of ZK60magnesium alloy were studied, and the strengthening mechanisms of Mg-Zn-Nd-Cd-Zr alloy were analysed. Excellent strengthen and plasticity are obtained by solution strengthening, fine grain strengthening, and the second phase strengthening. The tensil yield strengthen is increased and the ultimate tensile strength is decreased of ZK60-2Nd1Cd alloy as-cast owing to the continuous net T phases along the grain boundaries. The ultimate tensile strength and tensile yield strength of ZK60-2Nd1Cd as-extrusion are315MPa and290MPa, and the elongation is16%. The ultimate tensile strength and yield strength of ZK60-2Nd1Cd as-extrusion are315MPa and290MPa, and the elongation is10%, which is owing to additional Nd. The lower the Zn content is, the more decrease of the strength. There are W phases not T phases in the alloy. The ultimate tensile strength and tensile yield strength of ZK60-2.5Nd1Cd alloy are385MPa and320MPa, and the elongation is16%through aging heat-treatment.The effect of Cd, Nd, Zn on the damping capacities of Mg-Zn-Nd-Cd-Zr alloy at room temperature were studied, and the damping mechanisms of Mg-Zn-Nd-Cd-Zr alloy were analysed. At the room temperature, addition atoms of Cd reduced the the mean length of dislocation segments between minor pinns, then the strain-independent damping decreased. T phases or/and W phases exist in the alloy with addition atoms of Nd, and decrease the Zn content, which improve the strain-independent damping.The strain-dependent damping are increased by the increasing of movable dislocation density caused by the decreasing Zn content on certain strain, and the strain-dependent damping of Mg-Zn-Nd-Cd-Zr alloy as-extrusion and as-solution are lower than that of alloy as-cast on the higher strain.The damping capacities of Mg-Zn-Nd-Cd-Zr alloy at high temperature were studied. There are damping peaks of P1and P2in Mg-Zn-Zr alloy, P1induced by the interaction between dislocations and the point defects in the crystal lattice of Mg, and P2caused by the grain boundaries sliding at high temperature. The damping peaks of P1of Mg-Zn-Nd-Cd-Zr alloy are less than that of Mg-Zn-Zr alloy, and the temperature of the P1exist in the higher temperature region with the increase of Zn content. P1peak of the alloys with lower Zn content will be inhibited by solution heat-treatment. There no P2peak in the alloys as-cast, as-sulution, as-extrusion with higher Nd content, and the P1peak will be inhibited by extrusion. Exceeding a certain critical temperature, the high damping values are increasing with the temperature. The higher the Zn content of the alloy is, the higher the temperature damping value is.A new model of a dislocation line pinned by major pins, intermediate strength pins and minor pins was put forward, which could explain the damping flat or the damping maximum values on damping-strain spectrum of the experimental alloys. The intermediate strength pins maybe dislocation, the uneven distribution of the very small amount of point defects, or compounds particle with low melting point.Correlation of mechanical properties and damping properties of Mg-Zn-Nd-Cd-Zr alloy was analysed. The connection models among comprehensive performance index a, the ultimate tensile strength (or tensile yield strength) and damping capacities of alloys as-cast and as-extrusion were builded, respectively.A mathematics model between comprehensive performance index a of Mg-Zn-Nd-Cd-Zr alloy as-cast and the size of grain was builded, the equation can be expressed as α=4639.63-95.42d+0.58d2.更多还原