【作者】 张松岭；

【导师】 胡茂良；

【作者基本信息】 哈尔滨理工大学 ， 材料与化工（专业学位）， 2025， 硕士

【摘要】 Mg-Al系合金具有良好的铸造性能和成型性,其中AZ91镁合金在Mg-Al系合金中最为常用,可以广泛应用于汽车、航空航天和电子等领域。然而AZ91镁合金组织中第二相较为粗大,因此其力学性能还存在较大的提升空间。添加稀土元素是一种有效改善AZ91镁合金组织与提升力学性能的方法。在Mg-Al系合金中,稀土元素的添加方式一般可分为单一添加和混合添加两种。通常在Mg-Al系合金中混合添加稀土元素形成的Al-RE相,较之单一稀土添加所形成的Al-RE相,具有更优异的微观组织。目前对于这两种不同添加稀土元素的方式所形成的Al-RE相的形貌对比的研究还较少,此外,含有混合稀土元素的Al-RE相中不同稀土元素之间交互作用的机制也有待深入研究。基于以上问题,本研究首先探讨了单一添加稀土La元素对AZ91镁合金组织与性能的变化,进一步针对含La元素的AZ91镁合金中Al-RE相形貌控制这一问题。在AZ91-2La镁合金基础上进一步添加稀土Gd元素研究组织与性能的变化,重点分析了Al-RE相貌的变化对性能影响、含单一稀土元素La的Al-RE相与含混合稀土元素La、Gd的Al-RE相的微观形貌及生长行为的不同。最后使用第一性原理计算深入研究了Al-RE相形貌转变的机理、稀土元素Gd和La的交互作用机制。本文表明了在Mg-Al-RE系合金中可以通过调整稀土元素种类和含量来控制Al-RE相的形貌以达到提升性能的要求,主要的研究结果如下:（1）AZ91镁合金的微观组织是由α-Mg和连续网状分布的Mg₁₇Al₁₂相组成,添加La元素,在组织中新生成了Al₁₁La₃相,抑制了Mg₁₇Al₁₂相生成并改变了其分布状态。随着AZ91镁合金中La元素含量的增多,针状形态的Al₁₁La₃相不断粗化变长,Mg₁₇Al₁₂相的体积分数不断减少。力学性能测试结果表明,随着La元素的增加,AZ91镁合金力学性能不断提升,这是由于在组织中生成了Al₁₁La₃相,当La元素含量为1.5wt.%时,镁合金的力学性能最优,当La元素含量超过1.5wt.%时,镁合金的力学性能将不再进一步提高反而下降,这是由于组织中Al₁₁La₃相粗化所导致。（2）在AZ91-2La镁合金中,进一步添加Gd元素能够有效的调控Al-RE相的形貌,可以将长针状结构的Al₁₁La₃相转变为短棒状结构的Al₁₁RE₃相,且分布密度增加。当Gd元素含量为0.6wt.%时,组织中开始出现块状的Al₂Gd相,且其尺寸随着Gd含量的增加逐渐增大。BFDH模型预测表明Al₁₁La₃和Al₁₁RE₃两相的生长行为模式不同,主要降低了（0 0 1）晶面的择优生长速率,从而导致长针状的Al₁₁La₃相转变为短棒状的Al₁₁RE₃相。转化后的Al₁₁RE₃相的形成进一步提高了镁合金在室温下的力学性能和在高温下的耐热性。（3）通过第一性原理计算和实验结果相结合深入研究了Mg-9Al-2La-Gd镁合金凝固过程中微观组织的演变过程。第一性原理计算研究了Gd对Al₁₁La₃相的生长的作用机制,计算结果表明,Gd原子能够稳定吸附在Al₁₁La₃的（0 0 1）表面,其中空位（V）为最稳定的吸附位点,Gd原子吸附在Al₁₁La₃的（0 0 1）表面对其表层结构影响最大。电子结构分析表明,Gd原子的吸附降低了Al和La原子电子轨道的态密度峰值,在Gd原子与Al、La原子之间形成了极性共价键,使得Al₁₁La₃（0 0 1）表面结构稳定性增强,表面能降低。此外,在镁合金中析出相的热力学稳定性顺序为:Al₁₁RE₃>Al₂Gd>Al₁₁La₃>Mg₁₇Al₁₂。更多还原

【Abstract】 Mg-Al alloys exhibit excellent casting properties and formability,with AZ91magnesium alloy being the most commonly used in the Mg-Al system.This alloy has wide applications in automotive,aerospace,and electronics industries.However,the second-phase particles in the microstructure of AZ91 magnesium alloy are relatively coarse,leaving significant room for improvement in its mechanical properties.The addition of rare earth elements is an effective method to improve the microstructure and enhance the mechanical properties of AZ91magnesium alloy.In Mg-Al alloys,the addition of rare earth elements can generally be categorized into single-element addition and mixed-element addition.Typically,the Al-RE phase formed by the mixed addition of rare earth elements exhibits a better microstructure compared to that formed by the addition of a single rare earth element.Currently,there is limited research comparing the morphology of Al-RE phases formed by these two different methods of adding rare earth elements.Moreover,the interaction mechanisms between different rare earth elements in Al-RE phases containing mixed rare earth elements require further investigation.Based on the above issues,this study first investigates the effects of single rare earth element La on the microstructure and properties of AZ91 magnesium alloy.It then explores the control of Al-RE phase morphology in AZ91 magnesium alloy containing La.Further,rare earth element Gd is added to AZ91-2La magnesium alloy to study the changes in microstructure and properties.The research focuses on analyzing the impact of changes in Al-RE phase morphology on properties and compares the microstructure and growth behavior of Al-RE phases in alloys containing single rare earth element La and mixed rare earth elements La and Gd.Finally,first-principles calculations are conducted to investigate the mechanisms underlying the transformation of Al-RE phase morphology and the interaction between Gd and La rare earth elements.The results of this study show that in Mg-Al-RE alloys,the morphology of Al-RE phases can be controlled by adjusting the type and content of rare earth elements to achieve performance enhancement.The main findings are as follows:（1）The microstructure of AZ91 magnesium alloy consists ofα-Mg and a continuous network distribution of the Mg₁₇Al₁₂ phase.With the addition of La,a new Al₁₁La₃ phase forms,which inhibits the formation of the Mg₁₇Al₁₂ phase and alters its distribution.As the La content in AZ91 magnesium alloy increases,the acicular Al₁₁La₃ phase gradually becomes coarser and longer,while the volume fraction of the Mg₁₇Al₁₂ phase continuously decreases.Mechanical property tests indicate that the mechanical performance of AZ91 magnesium alloy steadily improves with increasing La content due to the formation of the Al₁₁La₃ phase.The alloy exhibits optimal mechanical properties at a La content of 1.5wt.%.However,when the La content exceeds 1.5wt.%,further increases in mechanical properties are not observed;instead,properties decline due to the coarsening of the Al₁₁La₃phase in the microstructure.（2）In AZ91-2La magnesium alloy,the addition of Gd effectively regulates the morphology of the Al-RE phase,transforming the long acicular Al₁₁La₃ phase into short rod-like Al₁₁RE₃ phases with increased distribution density.When the Gd content reaches 0.6wt.%,blocky Al₂Gd phases begin to appear,and their size increases with higher Gd content.BFDH model predictions indicate that the growth behavior of Al₁₁La₃ and Al₁₁RE₃ phases differs,mainly reducing the preferred growth rate of the（0 0 1）crystal plane,resulting in the transformation of the long acicular Al₁₁La₃ phase into short rod-like Al₁₁RE₃ phases.The formation of the transformed Al₁₁RE₃ phase further improves the mechanical properties and heat resistance of the magnesium alloy at both room temperature and high temperature.（3）The evolution of the microstructure during the solidification process of Mg-9Al-2La-Gd magnesium alloy was deeply studied through a combination of first-principles calculations and experimental results.The first-principles calculations investigated the role of Gd in the growth mechanism of the Al₁₁La₃phase.The results showed that Gd atoms can stably adsorb on the（0 0 1）surface of Al₁₁La₃,with vacancies（V）being the most stable adsorption sites.The adsorption of Gd atoms has the greatest impact on the surface structure of Al₁₁La₃（0 0 1）.Electronic structure analysis indicated that the adsorption of Gd atoms reduced the peak density of states of the electronic orbitals of Al and La atoms,forming polar covalent bonds between Gd and Al,La atoms.This interaction enhanced the surface stability of Al₁₁La₃（0 0 1）and lowered the surface energy.Additionally,the thermodynamic stability order of the precipitates in the magnesium alloy is as follows:Al₁₁RE₃>Al₂Gd>Al₁₁La₃>Mg₁₇Al₁₂.更多还原