China has the most abundant magnesium resources in the world. Magnesium and its alloys have the advantages of low density, high specific strength, good damping property, and exceptional electromagnetic-shielding and energy-storage characteristics. They are one of the most promising lightweight materials. The enhanced applications of magnesium alloys can save energy and reduce emissions and are significant to the new Chinese energy strategy. However, magnesium alloys have a hexagonal close-packed structure and exhibit relatively low ductility. A bottleneck in expanding the application of magnesium alloys is improving the ductility of magnesium alloys. For more than ten years, efforts have been made to improve the ductility and plastic deformation ability. Progress has been made in plastic-processing technologies of magnesium alloys. The novel alloy design theory “solid solution strengthening and ductilizing” and advanced preparation technologies such as “melt self-purification through varying temperature” have been established. Series of new magnesium alloys with good ductility and corresponding alloy grades have been developed, where the impurity content of iron can be reduced to below 10 × 10-6; the elongation was more than 60% for ultrahigh plasticity magnesium alloys and is above 10% for the ultrahigh-strength magnesium alloys (UTS > 550 MPa). New plastic-processing technologies, such as asymmetric extrusion, asymmetric rolling, asymmetric modification, cyclical multipass upsetting and squeezing, expansion control large ratio forging, and extrusion and forging composite forming, have been developed. These newly developed magnesium alloys and processing technologies weaken the basal texture in wrought magnesium alloys, improving the formability of sheets, tubes, profiles, and forgings and their product quality and reducing their product cost. These technologies have been successfully applied in the processing of magnesium sheets, pipe profiles, and forgings.

我国是世界上镁资源最为丰富的国家，镁及镁合金具有质轻、比强度高、阻尼减振、电磁屏蔽性能优良、储能特性好等优点，是最有潜力的轻量化材料之一，其推广应用对节能减排和能源转型战略具有重要意义。但镁合金具有密排六方晶体结构，塑性变形能力较差。如何改善镁合金的塑性变形能力是扩大镁合金应用的瓶颈问题之一。本文综述了10多年来，世界各国在改善镁合金塑性、提升镁合金塑性变形能力等方面所做的大量工作，及在镁合金塑性加工技术等方面取得的重要进展。发展了“固溶强化增塑”新型镁合金设计理论和“熔体变温自纯化”等关键制备技术，形成了一批高塑性变形镁合金材料和牌号，其中杂质Fe含量可降到10 × 10^-6以下，超高塑性镁合金延伸率可达到60%以上，超高强度(抗拉强度大于550 MPa)镁合金延伸率可以达到10%以上；开发了非对称挤压、非对称轧制、非对称改性、往复循环多道次镦挤开坯技术、扩收控制大比率锻造技术、挤锻复合成形技术等一批镁合金新型塑性加工技术。这些合金和技术使变形镁合金基面织构显著弱化，明显提高了变形镁合金板材、管型材和锻件的塑性成形能力和制品质量，产品成本大幅度降低，在板材、管型材和锻件制备加工中实现了成功应用。

经过50多年的发展, 中国已经成为世界上最大的Mg原材料生产国. 2000年, 师昌绪等5位院士联合向国家科技部提出了加速我国Mg工业发展的建议, 促成了科技部启动“十五”科技攻关重大专项“镁合金应用开发及产业化”, 拉开了我国大规模Mg材料研究和应用的序幕. 经过10年的发展, 我国在先进镁合金材料、表面处理技术和镁合金加工技术方面都取得了极大的进步, 正在从Mg生产大国向Mg研发应用的强国迈进.

基于Pandat相图设计了一种新型的Mg-0.2Ce-0.2Ca (质量分数，%)三元合金，经常规挤压变形后的屈服强度约364 MPa、总合金化含量约0.4%，实现了高强度、低合金化。对挤压过程中不同阶段的组织进行表征，发现Mg-0.2Ce-0.2Ca合金中的孪晶存在于挤压的整个阶段，表现出了高的孪晶迁移阻力，并且在挤压变形的中后期，部分动态再结晶晶粒沿着孪晶变体交割区域形核，导致孪晶界面比例显著降低。Mg-0.2Ce-0.2Ca合金在挤压变形的早期阶段即存储了大量<c + a>位错，这些位错的运动阻力大，因此位错主导的回复再结晶机制直至挤压变形的后期才大量启动，并直接促进了该阶段镁合金中高比例超细晶粒的形成。分析认为，Mg-Ce-Ca合金挤压过程中微观组织演变的主要原因是Ca元素的添加提升了Mg基体孪晶运动阻力，且Ce、Ca元素的共添加诱导了多系滑移。

Magnesium rare-earth (Mg-RE) alloy castings with a large size and complex structure exhibit versatile prospects in critical aircraft, aerospace, and defense fields owing to their ultralow density, excellent specific strength, and high-temperature resistance. The grain refinement of cast Mg-RE alloys can significantly improve their strength, plasticity, toughness, and casting performance, which are critical for expanding their applications. In this work, the grain refinement mechanism of Mg alloys by introducing RE elements and heterogeneous particles is first discussed based on the classical theory of constitutional supercooling and heterogeneous nucleation. Various grain refinement technologies for Mg-RE alloy casting using chemical and physical methods are comprehensively summarized. Further, the influence of grain refinement on the casting performance, mechanical properties, and corrosion properties of Mg-RE cast alloys is thoroughly discussed. Finally, the deficiencies and development trends of the current grain refinement of Mg-RE alloys are discussed from the point of actual application requirements.

轻质高强耐热的镁稀土(Mg-RE)合金大型复杂铸件在航空航天、国防军工装备轻量化等方面展现出独特的优势。对铸造Mg-RE合金进行晶粒细化处理能够显著改善合金的强度、塑韧性以及铸造工艺性能，对拓宽其应用领域意义重大。本文首先基于成分过冷和异质形核，探讨了稀土元素及外加颗粒对镁合金晶粒细化的影响。归纳了适用于铸造Mg-RE合金的化学、物理细化方法及其作用机制，并系统论述了晶粒细化对铸造Mg-RE合金铸造工艺性能、力学性能及腐蚀性能的影响。最后面向Mg-RE合金的实际应用需求，对其细化处理方面存在的不足和发展趋势进行了探讨。

This paper briefly reviews the development and research history of strutures of the solid matters, and highlight two new strcutures of precipitates in Mg alloys found by our group recently. (1) The isothermally aged (Mg, In)2Ca "Laves phase" contains two separate unit cells promoting the formation of five tiling patterns. The bonding of these patterns leads to the generation of the present phase but without any six-fold rotational symmetry in a long-range on the (0001)L basal plane, constrainted by the Penrose geometrical rule, completely different from the known Laves phases. (2) The MgZn five-fold nanodomain structure is self-assembled by two separate unit cells (72° rhombus structure: MgZn2, and 72° equilateral hexagon structure: MgZn) under the Penrose geomotrical constraints, containing 2D five-fold symmetry locally and short-range ordered C14 and C15 Laves structures. These two special structures without any translational symmetry on the normal plane while periodical arrangement along the normal direction, are a new class of intermediate structures between crystals and quasicrystals. And thus, they does not belong to any crystals or 2D ordered structures in quasicrystals or quasicrystal approximants.

本文简要介绍了固态物质结构研究的发展历程,并着重介绍了作者课题组最近在凝聚态结构物质上的新发现。(1) 与传统的Laves结构不同,等温时效析出的(Mg, In)₂Ca “Laves相”包含2种晶胞单元,这2种单胞沿6个特定的方向排列,构成5种拼砌图案,这5种拼砌模型随机键合在一起,形成一个在(0001)_L基面长程范围内不具有6次旋转对称的“C14”型Laves相。(2) Mg-Zn二元合金等温时效析出的纳米棒状相是由2种最小结构单元,即72°菱形结构单元(MgZn₂)和72°等边六边形结构单元(MgZn),沿5个方向自组装而成的一个包含C14、C15等短程有序的Laves晶体相和二维5次旋转对称结构的纳米畴结构。(Mg, In)₂Ca Laves相和Mg-Zn 5次纳米畴结构沿法线方向具有周期性的原子排列,然而在法平面上不具有任何平移周期性,是一类既不属于晶体相也不同于任何已发现的准晶体或准晶近似相的二维有序结构。

为了研究Mg-Gd-Zn-Zr合金中的长周期堆垛有序(LPSO)结构的形成及其演化, 对铸态、固溶态和时效态Mg_96.32Gd_2.50Zn_1.00Zr_0.18合金的显微组织进行了观察. OM, SEM和TEM观察表明, 合金铸态组织由α-Mg固溶体、晶内层片状14H-LPSO结构和晶界处树枝状α-Mg+β-(Mg, Zn)₃Gd共晶相组成; 500 ℃/35 h固溶处理后, 晶界处发生β→X的固态相变, 层片状X相也具有14H-LPSO结构; 再经200 ℃/128 h 峰时效处理后, 晶内析出椭球状$\beta^{\prime}$相和片状$\beta_{1}$相与14H-LPSO结构共存. 室温拉伸实验和Vickers硬度测试表明, 500 ℃/35 h+200 ℃/128 h处理改善了合金的力学性能, 抗拉强度为290.7 MPa, 屈服强度为162.5 MPa, Vickers硬度为108.0 HV, 延伸率为10.4%. 力学性能的改善与晶界、晶内的14H-LPSO结构及时效相等复合强韧化作用有关.

Mg-13.1Gd-1.6Ag-0.4 Zr (wt%) alloy was either iso-thermally extruded at 350 °C or differential-thermally extruded with respectively pre-heated billet at 500 °C and die at 350 °C. The iso-thermal extrusion leads to a near fully recrystallized structure and a [0001]//ED (extrusion direction) texture. In contrast, the differential-thermally extruded alloy develops a bimodal-grained structure composed of fine equiaxed recrystallized grains and coarse elongated unrecrystallized grains with a 011ˉ0//ED texture. The differential-thermally extruded alloy has a higher number density of precipitates after post-extrusion ageing than that of the iso-thermally extruded counterpart. Moreover, precipitation in the differential-thermally extruded alloy is further enhanced with cold rolling before ageing. Finally, the alloy obtains room temperature tensile yield strength of 421 MPa and ultimate tensile strength of 515 MPa via differential-thermal extrusion, cold rolling and ageing, mainly ascribed to the coupled strengthening from the bimodal-grained structure and enhanced precipitation. Strength of the alloy is noticeably higher than those of Mg-Gd(-Y)-Ag extruded alloys with similar compositions reported previously and is comparable to those of other high-strength Mg wrought alloys. The findings suggest that differential-thermal extrusion plus strain ageing is a suitable approach for achieving high strength in age-hardenable Mg alloys.

Large-scale Mg-8Gd-4Y-1Zn-Mn (wt.%) alloy ingot with a diameter of 315 mm and a length of 2410 mm was prepared through semi-continuous casting. Chemical composition, microstructure and mechanical properties at different locations of the samples with as-cast, T4 and T6 heat-treated states, respectively, were investigated. No obvious macro segregation has been detected in the high-quality alloy ingot. The main eutectic structures at all different locations are composed of α-Mg, Mg3RE-type, Mg5RE-type and LPSO phases. At the edge of ingot, the unusual casting twins including $\text{ }\!\!\{\!\!\text{ 10}\bar{1}\text{2 }\!\!\}\!\!\text{ }$ extension twins and $\text{ }\!\!\{\!\!\text{ 10}\bar{1}\text{1 }\!\!\}\!\!\text{ }$ compression twins were observed due to the intensive internal stress. In T4 heat-treated alloy, the micro segregation was eliminated. The remained phases were α-Mg and LPSO phase. Combined with the remarkable age-hardening response, T6 samples exhibits improved mechanical properties at ambient temperature, which derives from the dense prismatic β' precipitates and profuse basal γ' precipitates.

以Mg-8Gd-3Y-0.5Zr (GW83K)镁合金为研究对象，探索通过调控混晶显微组织来提高其力学性能的途径。将具有混晶组织的合金抽象为一种不同尺寸的晶粒嵌入到晶界中的颗粒复合体模型，在该模型中基体相为晶界，不同的颗粒相是不同尺寸的晶粒；然后基于Taylor关系的非局部塑性理论修正不同粒子嵌入到基体后对基体性能的影响，建立具有混晶组织特征的有限元平面应变细观力学模型。采用该模型模拟了含有混晶组织的GW83K镁合金在拉伸条件下的应力-应变行为，模拟结果对比实验数据验证了模型的有效性。在此基础上，通过真实时空相场模拟出的不同退火工艺下的混晶组织作为力学模型的几何输入参数，建立“工艺参数-混晶组织-力学性能”之间的关系。模拟结果对比发现，具有混晶组织的GW83K镁合金强度随平均晶粒尺寸的变化符合Hall-Petch关系，粗晶含量和分布显著影响着合金的塑性，当合金在623 K下退火90 min后对应的混晶组织可在保持高强度的同时有效提升塑性，为混晶组织的设计提供了参考。