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金属学报, 2024, 60(2): 129-142 DOI: 10.11900/0412.1961.2023.00241

综述

含Zr镁合金晶粒细化机理与研究进展

刘勇,, 曾刚, 刘洪, 王煜, 李建龙

南昌大学 先进制造学院 江西省轻质高强结构材料重点实验室 南昌 330031

Grain Refinement Mechanism and Research Progress of Magnesium Alloy Incorporating Zr

LIU Yong,, ZENG Gang, LIU Hong, WANG Yu, LI Jianlong

Key Laboratory of Lightweight and High Strength Structural Materials of Jiangxi Province, School of Advanced Manufacturing, Nanchang University, Nanchang 330031, China

通讯作者: 刘 勇，liuyong@ncu.edu.cn，主要从事高性能镁合金成形研究

责任编辑: 肖素红

收稿日期: 2023-06-02   修回日期: 2023-11-30  

基金资助: 国家重点研发计划项目(2021YFB3501001)
国家自然科学基金项目(52061028)
江西省重大研发专项项目(20223BBE51021)

Corresponding authors: LIU Yong, professor, Tel: 13576087535, E-mail:liuyong@ncu.edu.cn

Received: 2023-06-02   Revised: 2023-11-30  

Fund supported: National Key Research and Development Program of China(2021YFB3501001)
National Natural Science Foundation of China(52061028)
Major Research and Development Projects of Jiangxi Province(20223BBE51021)

作者简介 About authors

刘 勇，男，1980年生，教授，博士

摘要

细晶强化是镁合金的主要强化方式。Zr是镁合金(不含Al、Si等元素)最有效的晶粒细化剂，通常以Mg-Zr中间合金的形式加入。如何调控Zr元素在Mg-Zr中间合金中的存在形态(颗粒Zr与溶质Zr)是实现含Zr镁合金晶粒有效细化的关键。本文综述了镁合金晶粒细化的理论研究，基于生长抑制理论与异质形核理论，探讨了溶质Zr与颗粒Zr细化镁合金的机理，指出了含Zr镁合金晶粒细化的工程应用瓶颈。从颗粒Zr与溶质Zr 2方面综述了镁合金晶粒细化的研究进展，提出了Zr细化镁合金晶粒的协同设计策略。最后，对Zr细化镁合金晶粒的发展趋势进行了展望。

关键词： 镁合金; 晶粒细化; 生长抑制; 异质形核; Mg-Zr中间合金

Abstract

Grain refinement stands out as the primary strengthening mechanism in magnesium alloys. Zr emerges as the most effective grain refiner for magnesium alloys in the absence of Al, Si, etc. Typically, Zr is introduced in the form of an Mg-Zr master alloy. The crucial factor for achieving effective grain refinement in magnesium alloys incorporating Zr lies in regulating the morphology of Zr elements in the Mg-Zr master alloy, distinguishing between particle Zr and solute Zr. This study presents the theoretical groundwork for grain refinement. Drawing upon the growth restriction theory and heterogeneous nucleation theory, the refinement mechanism of soluble Zr and particle Zr on magnesium alloys is discussed. The discussion also identifies the engineering application bottleneck associated with Zr-refined magnesium alloys. A comprehensive review of advancements in Zr-refined magnesium alloy research is conducted, encompassing particle Zr and solute Zr. This review highlights the synergistic design strategy proposed for Zr-refined magnesium alloys. Ultimately, the anticipated development trends for Zr-refined magnesium alloys is prospected.

Keywords： magnesium alloy; grain refinement; growth restriction; heterogeneous nucleation; Mg-Zr master alloy

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本文引用格式

刘勇, 曾刚, 刘洪, 王煜, 李建龙. 含Zr镁合金晶粒细化机理与研究进展[J]. 金属学报, 2024, 60(2): 129-142 DOI:10.11900/0412.1961.2023.00241

LIU Yong, ZENG Gang, LIU Hong, WANG Yu, LI Jianlong. Grain Refinement Mechanism and Research Progress of Magnesium Alloy Incorporating Zr[J]. Acta Metallurgica Sinica, 2024, 60(2): 129-142 DOI:10.11900/0412.1961.2023.00241

镁合金具有密度低、比强度和比刚度高、阻尼减震性能优良、电磁屏蔽性能好等优点，在航空航天、汽车交通及国防装备等领域应用越来越广泛，是理想的轻量化材料[1~11]。然而，由于其晶体结构为hcp结构，室温下独立滑移系少，表现出较差的塑性变形能力，同时，镁合金的结晶温度区间较宽、热导率低、收缩倾向大、晶粒粗化等现象较严重，在凝固过程中容易产生热裂、缩松等缺陷[12~15]。为提高镁合金的力学性能，改善热裂、缩松等铸造缺陷，可通过细化晶粒来调控合金的组织，改善镁合金强韧性与塑性变形能力[16~30]。

细晶强化是镁合金的主要强化方式，根据Hall-Petch公式，σ = σ0 + Kdgs-1/2 (其中，σ为多晶材料的屈服强度，σ0为单晶屈服强度，K为Hall-Petch系数，dgs为平均晶粒尺寸)，其中K随着Taylor指数的增加而增加，一般滑移系越少，Taylor指数越大，因此，对于hcp结构的镁合金，晶粒尺寸对其屈服强度的影响更为显著[31]。晶粒细化不仅能提高合金的力学性能，还能通过调控合金凝固过程的组织形态(枝晶形貌、共晶相组成、晶间残余液相等)降低合金的热裂倾向[32~41]，这对推动镁合金朝着薄壁、复杂、大型等趋势发展意义重大。Zr是镁合金(不含Al、Si等元素)最有效的晶粒细化元素之一，添加Zr可提高镁合金的强度、韧性、耐腐蚀性能及抗热裂性等[42~50]。研究Zr对镁合金晶粒细化的影响有利于进一步拓展镁合金的工业应用。工业上主要通过加Mg-Zr中间合金对镁合金进行细化，然而Zr在Mg-Zr中间合金中主要以颗粒态的形式存在，容易形成大量团簇，加速沉降，发生细化效果衰退，调控Zr元素在Mg-Zr中间合金中的存在形态是实现含Zr镁合金晶粒有效细化的关键。

本文论述了镁合金晶粒细化的理论，基于生长抑制理论与异质形核理论，探讨了溶质Zr与颗粒Zr细化镁合金的机理，指出了含Zr镁合金晶粒细化的工程应用瓶颈。从颗粒Zr与溶质Zr 2方面综述了镁合金晶粒细化的研究进展，提出了Zr细化镁合金晶粒的协同设计策略。最后，对Zr细化镁合金晶粒的发展趋势进行了展望，为含Zr镁合金晶粒细化提供参考与思路。

1 镁合金晶粒细化理论

镁合金晶粒细化理论的研究主要集中在2个方面：一是对晶粒细化理论模型的完善，包括用生长限制因子模型来解释溶质元素对晶粒尺寸的影响[51]，用自由生长模型来预估合适的形核剂尺寸与形核所需的临界过冷度[52]，以及用相互依存理论模型综合考虑溶质元素和异质形核颗粒对晶粒尺寸的影响[53]；二是利用晶体学特征预测潜在的形核粒子。晶格匹配一直被认为是潜在形核粒子成为有效形核粒子的关键因素[54]，根据形核粒子与基体的晶格错配度[55]、边与边匹配理论等[56,57]可高效筛选出潜在的晶粒细化剂，有效避免试错的盲目性。

1.1 生长抑制理论

溶质元素的偏析在晶粒细化中起重要作用，溶质过冷理论认为，溶质元素的偏析会导致固/液界面产生成分过冷，当溶质偏析产生的过冷度(ΔTC)大于形核临界过冷度(ΔTn)时，可通过促进新晶粒的形核而抑制原有晶粒的生长[58~61]。为了描述成分过冷对晶粒尺寸的影响，Easton和StJohn[59]提出了生长限制因子(Q)的概念，在二元合金系统中，Q可以表达为：

Q=∑miCi(ki-1)

式中，i表示不同的溶质元素，mi 为液相线的斜率，Ci 为溶质元素i的浓度，ki 为溶质平衡分配系数。溶质元素的Q越大，固/液界面生长速率越慢，晶粒生长的抑制能力越强，晶粒细化则越明显。表1[25,45,46,58]列出了镁合金中常用溶质元素的Q值，需注意的是，这里的Q值是基于Ci 为1.0% (质量分数)计算出的，即Q = m(k - 1)，实际合金一般为多元系统，该方程并不完全适用，但仍有借鉴意义。

表1   镁合金中常用溶质元素的生长限制因子(Q)[25,45,46,58]

Table 1  Growth restriction factors (Q) of common solute elements in magnesium alloys[25,45,46,58]

ElementQElementQ
Fe52.56[45]Nd3.557[25]
Zr38.29[45]Sm2.943[25]
Ca11.94[45]Pr2.909[25]
Si9.25[45]La2.895[25]
Ni6.13[45]Tb2.073[25]
Zn5.31[45]Eu2.490[25]
Cu5.28[45]Gd1.025[25]
Ge4.41[45]Ho0.860[25]
Al4.32[45]Dy0.828[25]
Sb0.53[45]Tm0.557[25]
Mn0.15[45]Er0.524[25]
Sr3.51[58]Lu0.123[46]
Ce2.74[58]Na6.878[46]
Sc3.96[58]Pd4.070[46]
Yb2.53[58]Co3.178[46]
Y1.70[58]Ag2.675[46]
Sn1.47[58]Cd2.644[46]
Pb1.03[58]Li2.034[46]

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1.2 自由生长理论

Greer等[52]研究发现，形核剂的尺寸对成核有显著影响，液态金属中异质形核的临界条件为dn ≥ 2r*，其中dn为颗粒直径(假设形核颗粒为球形)，r* 为晶核临界半径，超过此半径才可以形核。形核颗粒直径与ΔTn的关系为[52]：

dn=4σSLΔSvΔTn

式中，σSL为固/液界面能，ΔSv为单位体积的熔化熵(ΔSv是熔化潜热(Lv)与熔点(Tm)的比值)，这称为自由生长理论。由 式(2)可知，尺寸大的颗粒所需的临界过冷度小，更有可能成为形核位点。自由生长理论成功地解释了TiB2颗粒(尺寸2~5 μm)对铝合金[62]的细化、SiC颗粒对AZ31镁合金[63]的细化的颗粒尺寸效应。

1.3 相互依存理论

为了综合考虑溶质和形核剂对晶粒尺寸的影响，StJohn等[53]提出了相互依存模型。相互依存模型认为，先形核的晶粒在固/液界面前沿产生的溶质富集会给未成形的液相基体提供足够过冷度，促进下一个晶粒形核。该理论将成分过冷度与颗粒的形核过冷度结合起来，将dgs分为3部分，包括前一次形核区尺寸(Xcs)、固/液界面到成分过冷区域之间的扩散距离(Xdl)及无形核区到下一个有效形核点的距离(Xsd)，表达如下[53]：

dgs=Xcs+Xdl+Xsd

具体计算公式为[53]：

dgs=DzΔTnvQ+4.6DvC1*-CiC1*1-ki+Xsd

式中，D为溶质元素在液相中的扩散系数，zΔTn为促使下一个形核所需过冷度的增量，v为固/液界面的生长速率，C1*为距离固/液界面Xcs处的液相浓度。无形核区的存在可以很好地解释实际成核数量远少于外来形核颗粒数量的原因。在镁合金中添加Q值较高的溶质元素(如Zr)，能够缩小无形核区范围[53]，提高形核率，实现晶粒显著细化。

1.4 晶格匹配理论

晶格匹配理论适用于筛选潜在的异质形核剂，该理论认为，合适的异质形核剂需满足几个条件：(1) 化学性质稳定，不与基体发生反应；(2) 与基体存在良好的润湿性；(3) 与基体的界面能较低。影响界面能的主要因素包括晶格的匹配度、形核剂的尺寸、形核剂与基体电负差异性等，其中晶格匹配度直接影响形核剂的临界过冷度与粒子效应[64]。凝固理论认为，颗粒形核能力与颗粒/基体间的界面能息息相关，2者的晶格错配度越小，界面能越低，形核能力越强。

Bramfitt[65]在Turnbull和Vonnegut[66]工作的基础上建立了点阵错配度模型，可用于计算不同原子排列的两相晶面间错配度，具体公式如下[65]：

δ(hkl)s(hkl)n=13∑i=13duvwsicos θ-duvwniduvwni×100%

式中，δ为异质形核颗粒和基体的低指数面上的晶面错配度，s表示基底，n表示形核颗粒，(hkl)为低指数晶面，[uvw]为低指数晶面上的低指数晶向，duvwsi和duvwni分别为沿对应晶面及晶向上的原子间距；θ为相应两晶向的夹角。该理论认为，当δ < 6%时，异质形核有效；当6% ≤ δ ≤ 12%时异质形核可能有效；当δ > 12%时，则无法作为异质形核核心。利用该模型计算出Mg2Si与Mg3Sb2间的面错配度约为2%[67]，Mg基体与Al4C3的面错配度为4.05%[68]，实验结果表明，Mg3Sb2与Al4C3对初生相具有良好的细化作用[67,68]，验证了该模型的准确性。目前，该模型已广泛用于解释镁合金添加异质形核剂的细化机理。

2 含Zr镁合金的晶粒细化机理

Zr细化镁合金可追溯到20世纪40年代，Saunders和Strieter[69]首次使用Zr对Mg-Th合金实现晶粒细化，对镁合金的发展做出了突出贡献。Zr的加入使纯Mg形成等轴晶，晶粒尺寸由毫米级细化到微米级[70]。Zr在镁合金中一般以2种状态存在，溶质Zr与颗粒Zr。溶质Zr指的是溶解进入α-Mg相的Zr，颗粒Zr指的是存在于熔体中但并未溶解进入α-Mg相的Zr粒子。颗粒Zr包括熔体中始终未溶解的Zr粒子与包晶反应再次析出的Zr粒子，由于现有表征技术很难区分这2种Zr粒子，同时包晶反应再次析出的Zr粒子数量较少，一般研究中的颗粒Zr是指始终未溶解的Zr粒子[71]。溶质Zr与颗粒Zr均能对镁合金产生明显的细化作用，但作用机制不同。

2.1 溶质Zr的细化机理

溶质元素的偏析在晶粒细化中起重要作用。在凝固过程中，由于液相中溶质分配的作用，使得固/液界面前沿的熔体实际温度(Tactual)低于溶质分配所决定的液相平衡温度(Tl)，从而产生成分过冷[71]。在成分过冷区，溶质原子的扩散速率慢导致晶粒长大的速率变缓，从而达到生长抑制效应，定量为Q。由表1[25,45,46,58]可知，元素Zr的Q值高达38.29，在镁熔体中加入Zr，溶质Zr在固/液界面前沿富集，形成强烈的成分过冷，抑制晶粒长大，同时为形核提供更高驱动力，提高形核率，实现晶粒细化。

溶质Zr对镁合金有显著的晶粒细化作用，Qian等[72]采用湿法化学分析法准确测得Mg-Zr合金中溶质Zr的含量，通过实验发现溶质Zr可以起到70%的晶粒细化作用。为进一步确认溶质Zr的细化作用，Qian等[73]采用不同坩埚对Mg-Zr合金进行了重熔实验，结果发现，在重熔时，当熔体静置一定时间后，Zr大量流失，晶粒尺寸明显粗化，组织不均匀性增加。重新搅拌后，对于含Fe坩埚，溶质Zr含量有所增加，但仍低于基体合金，晶粒尺寸有所细化，但仍比重熔开始时粗大，晶粒尺寸与溶质Zr含量有很大的相关性。对于不含Fe坩埚，再次搅拌后溶质Zr含量提高，晶粒尺寸细化，恢复到重熔开始时状态，溶质Zr含量的损失是可逆的，晶粒尺寸对溶质Zr的含量同样有很强的依赖性，如图1[73]所示。可见，溶质Zr含量越高，晶粒尺寸越小，说明溶质Zr有显著的晶粒细化作用。在搅拌作用下，富Zr镁熔体中Zr颗粒/熔体界面的扩散对Zr析出相的持续溶解起着至关重要的作用。对于含Fe坩埚，溶质Zr含量的损失是不可逆的，主要归因于Zr与Fe的反应损耗。

图1

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图1   重熔后镁合金中溶质Zr含量对晶粒尺寸的影响[73]

Fig.1   Effects of solute Zr content on the grain size of magnesium alloy after remelting[73]


Sun等[74]基于相互依存理论模型，得到了Q与合金晶粒尺寸的关系，发现合金晶粒尺寸与1 / Q呈线性相关。随着Q增加，1 / Q逐渐减小，晶粒尺寸也逐渐减小。因此，在镁合金中加入高Q值的溶质元素，可以显著细化晶粒尺寸。高Q值Zr的加入，使熔体粒子数密度显著增加，Xsd减小，相应无形核区长度(Xnfz)减小，细化效果增强。

为了保证溶质Zr的细化作用，希望其在合金中的含量达到最大溶解度。根据Mg-Zr相图，Zr在液态Mg中的最大溶解度在包晶点附近(0.45%)[75]。溶质Zr的生成是固体Zr粒子在液态镁合金中的溶解过程。影响固态Zr在镁液中溶解的因素主要4种。(1) 液态镁熔体的温度。根据Mg-Zr相图，温度在730~780℃时Zr在镁液中的溶解度恒定，一般认为熔体温度不是影响Zr溶解的主要因素，但温度的升高会缩短达到饱和溶解度所需要的时间。(2) 液态镁熔体的流动。加大金属液对流，例如对熔体施加搅拌，可以加快Zr在镁液中的溶解，减少Zr的自然沉降。(3) Zr粒子的尺寸。细小的Zr粒子由于比表面积大，与金属液接触面积大，因此溶解速率快[76]，同时，细小的Zr粒子有利于延缓Zr的沉降，增加Zr持续溶解的时间。(4) Zr粒子表面质量。颗粒表面的氧化膜也会影响固/液金属间的传热与传质，阻碍Zr粒子溶解。因此，希望获得细小、洁净的Zr粒子，以加速其在熔体中溶解，保证溶质Zr含量达到溶解极限。

2.2 颗粒Zr的细化机理

颗粒Zr的细化机理主要是包晶反应机制与异质形核机制。Emley[77]最早提出了包晶反应理论，认为Zr粒子首先从熔体分离，溶于镁液形成富Zr镁固溶体。指出只有在包晶温度附近形成的富Zr粒子才能起到形核作用，其证据是经过Zr细化的镁合金存在富Zr晕圈，晕圈的中心存在一颗或者多颗纯Zr颗粒。然而，实践证明，当Zr含量低于包晶成分时，在合金中也出现了富Zr晕圈，这说明富Zr晕圈的形成可能存在其他机制。田倩[78]证实富Zr晕圈的存在既与包晶反应生成的Zr浓度较高的镁固溶体有关，也与因Zr粒子在液相中溶解扩散形成的溶质富集有关。异质形核机制认为，Zr与Mg都为hcp结构，2者的晶格常数非常接近，满足异质形核的晶体学条件，Zr可作为α-Mg的有效形核基底，促进形核，细化晶粒[79]。为了使颗粒Zr发挥好的细化效果，一方面要保证溶质Zr的含量，增加包晶反应再次析出的Zr粒子数量，另一方面要关注熔体中本身未溶解的Zr颗粒形态。

异质形核效应很大程度受颗粒尺寸与空间分布等因素影响。根据自由生长理论模型，异质形核颗粒尺寸必须满足一定条件才能作为有效形核核心，如 式(2)所示。Qian[79]研究了Zr颗粒尺寸对镁合金晶粒细化的影响。将α-Mg的热力学参数 (σSL = 0.115 J/m2，Lv = 5.898 × 108 J/m3，Tm = 649℃)代入 式(2)，得到：

ΔTn=0.719dn

式(6)表示颗粒尺寸与临界形核过冷度的关系，如图2a[79]所示。当形核颗粒尺寸为1~5 µm时，临界形核过冷度非常小，仅为0.14~0.72℃，说明Zr粒子临界形核功非常低，有利于形核。通过冷却曲线测的实际过冷度(0.15~0.58℃)，与理论计算结果非常接近。另一方面，将实际过冷度的结果代入 式(6)可计算出在该冷却条件下Zr颗粒的有效形核尺寸范围为1.24~4.79 μm。对Mg-1.0Zr (质量分数，%，下同)合金中Zr核尺寸进行观察与统计，结果表明大都集中在1~5 μm范围内，与计算结果接近，如图2b[71,79]所示，这说明能够作为异质形核核心的Zr颗粒尺寸范围为1~5 μm。此外，未观察到大于5 μm的富Zr核，说明其是无效Zr。

图2

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图2   Zr颗粒尺寸与镁合金临界形核过冷度的关系[79]及镁合金中Zr形核核心尺寸分布[71,79]

Fig.2   Relationship between Zr particle size and the minimum undercooling required for effective inoculation[79] (a) and size distributions of active Zr nuclei in magnesium alloys[71,79] (b)


普遍认为，有效形核Zr颗粒尺寸范围为1~5 µm，在这个区间内的Zr颗粒数密度最大时，合金细化效果最佳。孙明[71]研究了Mg-Zr中间合金Zr颗粒对镁合金组织的细化作用，对比了不同Mg-Zr中间合金细化的镁合金中形核Zr颗粒的分布，如图2b[71,79]所示。其中，中间合金A的溶质Zr含量(ZrS)和总Zr含量(ZrT)分别为0.42和0.46；中间合金B的ZrS和ZrT分别为0.44和0.55；中间合金C的ZrS和ZrT分别为0.45和0.59。可见，活性形核的Zr颗粒尺寸在小于5 μm范围内，其中1~3 μm居多(占比大于50%)；尺寸小于1 μm的颗粒占比10%以下，这些亚微米的Zr颗粒形核效力较低，不大可能充当有效形核核心，但它们更容易溶解形成溶质Zr，如2.1节所述，对晶粒细化同样有重要贡献。B、C中间合金尺寸为1~5 μm区间的粒子密度大于中间合金A，整体细化效果优于中间合金A。细化效果存在差异的根本原因在于中间合金内有效形核粒子数目的差别。然而，Tong等[80]研究认为亚微米级颗粒也可能成为镁基体的形核点。对Mg-Zr中间合金进行适当的预处理，提高了中间合金溶质Zr的含量，增加了熔体的成分过冷度，提高了形核驱动力，促使百纳米级Zr颗粒成为α-Mg的有效形核核心。

此外，Zr核与α-Mg的位向也会影响Zr颗粒的异质形核作用，Saha和Viswanathan[81]利用透射电镜观察了Mg-Zr合金α-Mg与Zr的位向关系，发现当2者为面错配度非常小的(0.1%~0.2%)位向关系时，Zr颗粒呈小平面(faceted)形态，这种形态颗粒更容易暴露出合适的形核面，利于形核。Saha和Viswanathan[81]观察到的位向主要是(0001)Mg//(0001)Zr和(1010)Mg//(1010)Zr，说明Zr基面{0001}和柱面{1010}适合作为α-Mg 的形核基底。

溶质Zr与颗粒Zr细化镁合金晶粒的机理不同，效果也有差异，普遍认为溶质Zr的晶粒细化作用优于颗粒Zr。Qian等[72]对比了溶质Zr和颗粒Zr细化纯Mg晶粒的效果，在纯Mg中加入一定量Mg-Zr中间合金，静置240或360 min后，由于熔体顶部的未溶解Zr颗粒大部分已经沉底，此时得到的溶质Zr与总Zr含量接近，因此认为静置一定时间后，样品的晶粒细化主要来自溶质Zr的影响，贡献了晶粒细化效果的70%。再次搅拌后，最终合金中的总Zr含量显著增加，但溶质Zr含量变化不大，换言之，再次搅拌后总Zr含量的增加主要是由于颗粒Zr含量的增加，晶粒细化效果的改善也主要归因于颗粒Zr含量增加，贡献了晶粒细化效果的30%。庞松[82]在研究砂型铸造的Mg-Gd-Y-Zr合金中也发现了类似的规律，合金的晶粒尺寸与Zr含量之间存在分段的线性关系：当Zr含量低于0.45% (质量分数，下同)时，合金晶粒尺寸随着Zr含量的增加而显著下降，但当Zr含量高于0.45%后，Zr含量对晶粒细化的效果变缓。Zr含量较低时，Zr的主要细化机制为成分过冷，Zr含量超过0.45%后，主要机制为异质形核。

综上所述，Zr对镁合金晶粒细化的贡献由溶质Zr和颗粒Zr共同决定，2者发挥细化协同效应，如图3所示。在凝固过程中，溶质Zr对α-Mg晶粒的生长有强烈的抑制作用，而颗粒Zr则可作为异质形核点，2者综合产生了更高形核率及更细的晶粒尺寸。Zr对晶粒细化的效果最终取决于Zr在镁熔体中的最大溶解量及有效形核尺寸范围内的Zr粒子数量。

图3

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图3   镁合金Zr细化晶粒协同策略

Fig.3   Synergistic strategy of Zr on grain refinement of magnesium alloy (During solidification, solute Zr strongly inhibits the growth of α-Mg grains, while particle Zr acts as a heterogeneous nucleation core, and the combination of both produces higher nucleation rate and finer grain size)


3 含Zr镁合金晶粒细化工程应用瓶颈

工业上主要通过添加Mg-Zr中间合金对镁合金细化，这种方式操作简单、细化效果好，加入0.5%左右的Zr就能使合金获得显著的细化效果。然而Zr在Mg-Zr中间合金中主要以颗粒态的形式存在，传统的Mg-Zr中间合金往往组织粗大、不均匀，颗粒团聚明显，容易发生细化效果衰退。产生衰退的原因主要有：(1) Zr的密度大，约为Mg的4倍，在静置条件下，Zr易在镁熔体中发生自然沉降[83]，同时，大密度团聚状的Zr颗粒容易与熔体中的精炼剂、夹杂物等混合，加剧沉降；(2) Zr的化学活性比较强，会与镁液中的Fe、Mn、Si等杂质元素和O2、H2O、H2等气体发生反应损耗。Zr的大量流失不仅使合金容易发生细化衰退，也导致Zr的收得率低(< 30%)，增加企业生产成本，限制了其工程应用。

为探寻更好的Zr细化工艺，有研究[71]采用向镁熔体加入低熔点Zr盐的方式，这种方式可以达到与Mg-Zr中间合金相当的细化效果，成本较低，但面临几个难题：(1) Zr盐一般通过与镁熔体在高温条件下置换反应生成Zr，反应的副产物与残余Zr盐很难分离；(2) 副产物会影响Zr颗粒的纯净度，进一步影响Zr颗粒的异质形核效果；(3) Zr盐化学活性强，吸水易潮解，储存、运输等也比较困难。因此，目前Zr细化工艺仍集中在添加Mg-Zr中间合金。

Mg-Zr中间合金中Zr颗粒尺寸较大，团聚明显，有效形核尺寸区间的颗粒少，大尺寸Zr颗粒在熔炼过程中不但难于分散、溶解，反而会因为密度较大而发生沉降，降低Zr的细化效果。图4a为工业上应用Mg-Zr中间合金的显微组织。可见，Zr团簇明显，粒径达毫米级。图4b为Zr颗粒粒径分布图。可见，尺寸< 5 μm的Zr颗粒占比低，仅约为15%，说明潜在异质形核颗粒少，不利于Zr细化。针对中间合金团簇多、颗粒大、易沉降等特点，有必要研究Zr在熔体中的沉降行为，探明Zr的内在损耗机制。

图4

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图4   工业上应用Mg-Zr中间合金的显微组织及Zr颗粒尺寸分布

Fig.4   Microstructure (a) and Zr particle size distri-bution (b) of Mg-Zr master alloy for industrial application


Qian等[83]系统研究了Zr的沉降行为，通过观察颗粒Zr的形貌发现，随着熔体静置时间的延长，颗粒Zr逐渐沉降，熔体中剩余的Zr粒子普遍呈孤立状态。基于Stokes沉降公式推导了Zr颗粒在镁液中沉降距离(S)公式[83]：

S≈g(ρZr-ρMg)dn218ηt

式中，g为重力加速度，ρZr和ρMg分为Zr与镁熔体的密度，η为镁熔体黏度，t为沉降时间。由 式(7)可知，Zr合金化温度越高、颗粒尺寸越大，则Zr颗粒的沉降距离越大。通过理论数学模型预测了不同尺寸颗粒的沉降行为，dn ≥ 10 μm的未溶解Zr粒子能够在1 h沉降约1000 mm，dn ≤ 1 μm的未溶解Zr粒子仅仅轻微沉降约20 mm。可见，Zr颗粒在Mg熔体中的沉降受粒子尺寸影响较大。

Zr沉降还受熔炼工艺的影响，研究[79]表明，随着熔体静置时间的延长，Zr逐渐沉降，细化效果衰退。当静置时间达120 min时，dn > 1.08 μm的Zr颗粒基本沉底，有效异质形核颗粒减少，晶粒粗化，并趋于稳定；再次搅拌熔体后，Zr颗粒重新溶解到熔体中，晶粒再次得到细化，如图5[79]所示，实验结果符合 式(7)的理论数学模型。

图5

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图5   在780℃下沉降和再次搅拌对Zr细化镁合金晶粒尺寸的影响[79]

Fig.5   Effects of settling and restirring on inoculation of magnesium by zirconium at 780oC[79]


本课题组针对Zr细化镁合金的工业生产背景，对Zr的沉降行为做了相关研究。图6为镁合金熔体Zr颗粒尺寸与沉降距离的关系。可以看出，在该工艺条件下(熔体静置2 h，温度从790℃降到700℃)，dn> 30.42 μm的Zr颗粒2 h的沉降距离约为2000 mm，而熔炼炉的高度约为2000 mm，这意味着dn> 30.42 μm的Zr颗粒完全沉降，因而造成Zr收得率低、细化效果不佳。同时发现，纳米级Zr颗粒的沉降距离仅约为2 mm。为了获得好的Zr细化效果，须首先改善Mg-Zr中间合金的质量，尽可能细化中间合金中的Zr颗粒尺寸，优化Zr颗粒分布，这样既可以促进Zr的溶解，保证高的溶质Zr含量，又可以避免熔炼过程中Zr的沉降损耗，还可根据中间合金Zr颗粒的尺寸、形态优化熔炼工艺。

图6

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图6   镁合金熔体Zr颗粒尺寸与沉降距离的关系

Fig.6   Relationship between Zr particle size and settling distance (S) in magnesium alloy melt (Crucible depth is 2 m, melt from 790oC to 700oC, cooling rate 0.75oC/min, cooling time 120 min. Inset is a partial enlarged view of the box. dn—particle diameter)


4 含Zr镁合金晶粒细化协同设计策略

在工业生产中，为改善细化效果、提高Zr收得率，一般会采用提高Zr合金化温度、延长保温时间、施加熔体搅拌等措施，以促进颗粒Zr通过扩散、溶解等方式向溶质Zr转变和延缓颗粒Zr沉降。但过长的熔体保温时间，甚至是在高温下搅拌会导致熔体强烈氧化，恶化熔体质量，产生细化与净化相互制约的问题。如何解决上述应用瓶颈、实现含Zr镁合金晶粒的有效细化，关键是调控Zr元素在Mg-Zr中间合金的存在形态。基于Zr的细化机理，颗粒Zr和溶质Zr具有细化协同效应，改善Zr细化效果的设计策略应从颗粒Zr与溶质Zr 2方面出发。

4.1 颗粒Zr的形态与分布

异质形核效应很大程度受颗粒尺寸与空间分布等因素影响。通过挤压[71]、轧制[84]、搅拌摩擦加工(FSP)[85,86]、等通道挤压(ECAE)[87]等大塑性变形工艺对Mg-Zr中间合金进行预处理，能够分散Zr颗粒团簇、细化Zr颗粒尺寸及改善Zr颗粒的空间分布。

孙明[71]尝试利用挤压、轧制工艺对Mg-30Zr中间合金进行预处理，观察了不同工艺下的Zr粒子形态与分布，对比了不同中间合金的晶粒细化效果，如图7[11]所示。结果表明，挤压工艺(挤压棒材、挤压板材)未明显改善Mg-Zr中间合金中Zr粒子的形态与分布，挤压工艺处理后只有少部分Zr团簇被破碎。经多道次轧制后，团簇Zr粒子破碎为细小粒子，Zr粒子分布明显改善，尤其增加了尺寸5 µm以下粒子的比例，产生更多有效形核Zr颗粒，对Mg-10Gd-3Y-0.5Zr合金的细化效果提升了30%。Qian等[84]将接收态Mg-Zr中间合金热轧成薄板，改善了Zirmax®合金中Zr颗粒的形态、分布。在热轧的作用下，大的团簇状Zr颗粒明显减少，尺寸为0.5~5 µm之间的粒子比例明显增加(超过50%)。轧制态Zirmax®合金中Zr颗粒形态、分布的改善使Mg-0.5Zr合金的晶粒细化效果提升了10%。

图7

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图7   不同Mg-Zr中间合金的组织形貌及使用不同中间合金细化的Mg-10Gd-3Y-0.5Zr合金晶粒尺寸[71]

(a) extruded rod-EX1 A (ED—extrusion direction)

(b) extruded sheet-EX2 A

(c) rolled sheet-RL7.5-0.5 A (RD—rolling direction)

Fig.7   Microstructures of Mg-Zr master alloys treated by different processes (a-c) and grain size of Mg-10Gd-3Y-0.5Zr alloy refined by different master alloy (d)[71]


Wang等[86]通过FSP技术对Mg-30Zr中间合金进行预处理，结果发现，在FSP作用下，中间合金产生严重的塑性变形，团簇状Zr颗粒破碎成细小的颗粒。与原始铸态相比，经FSP处理的中间合金具有更多尺寸小于5 μm的Zr颗粒。Viswanathan等[87]通过ECAE技术对Mg-30Zr中间合金进行预处理。结果表明，经ECAE处理后，晶粒被拉长，Zr颗粒沿晶界扩散。原本在晶界处的大的Zr颗粒与团簇被大量剪切，破碎成细小的单个颗粒，粒径分布得到改善，小颗粒(尺寸0.5~2 μm)的数量明显增加。经ECAE预处理后的中间合金对Mg-2Zn-0.1Zr合金的晶粒细化效果明显提升。

4.2 溶质Zr含量

目前对改善含Zr镁合金晶粒细化效果的研究主要集中于对中间合金中颗粒Zr的改性，通过大塑性变形或熔体处理技术改善Zr颗粒的尺寸、形态及分布，提高异质形核率。从溶质Zr的角度出发、提升Zr细化作用的相关研究较少，究其原因主要为：Zr在Mg基体中的溶解度有限，如前文所述，其最大溶解度仅为0.45%，通过工艺来实现溶质Zr提升的难度大。近期，Tong等[80]从提升溶质Zr含量出发，创新地提出了一种Mg-Zr中间合金预处理方法。采用超高频脉冲重熔(UHFP-TIGR)技术对Mg-Zr中间合金进行预处理，提升了Mg-Zr中间合金的溶质Zr含量及其细化效果。采用UHFP-TIGR技术，利用焊接熔池小、温度高的特性，实现“高温溶解+包晶与快冷促进Zr固溶”，使中间合金的溶质Zr含量大幅提升，从而提高溶质Zr的偏析能力。一方面，对晶粒生长抑制作用增强；另一方面，提高熔体成分过冷度，即提高熔体的形核驱动力，促使大量百纳米级颗粒Zr成为有效形核位点，提高形核率。2者综合作用使Zr中间合金细化能力大幅提升，为高效Zr晶粒细化剂的协同设计与制备提供了好的范例。

此外，动力学细化法常用于含Zr镁合金的晶粒细化，该技术依赖于激活合金熔体中本身存在的形核颗粒，通过向熔体施加机械搅拌[88~90]、外加物理场[91~100]等促进晶核的形成，并使已形成的枝晶破碎，提高形核率。其本质也是通过外加物理场改善Zr颗粒尺寸、形态、分布或者提高溶质偏析能力，提升过冷熔体形核驱动力。受此启发，有研究[88~100]通过对含Zr镁合金熔体引入强剪切(intensive melt shearing，IMS)、超声场(ultrasonic treatment，UST)、脉冲磁场(pulsed magnetic field，PMF)、脉冲电场(pulsed electric current，PEC)等物理场，有效分散了Zr颗粒在熔体中的分布，提高了溶质Zr含量，实现了含Zr镁合金的晶粒细化效果提升。把熔体处理技术与Zr细化结合起来，一方面，可以发挥其本身的细化晶粒作用；另一方面，可通过这些处理技术引入外加物理场，使熔体中颗粒Zr更加细小、均匀分散，更好发挥颗粒Zr异质形核作用。另外，通过外加物理场，还可以促进颗粒Zr的扩散、溶解，提高溶质Zr含量，增加熔体的成分过冷度，提高形核驱动力。

强剪切熔体工艺是为了在凝固前对熔体进行预处理而开发的，强剪切作用后的凝固导致晶粒显著细化[88~90]。MgO与Zr颗粒通过强剪切的分散作用可以作为强有效的异质形核剂[90]，Peng等[90]发现，经过强烈的熔体剪切，MgO薄膜分散为细小MgO颗粒，同时MgO粒子数密度增加。当加入低含量Zr (0.1%)时，Zr原子的扩散速率大幅度提高，MgO颗粒表面能够吸附大量Zr原子并形成数纳米厚度的Zr层。由于Zr与α-Mg的晶格常数更为接近，Zr吸附层降低了MgO与α-Mg的错配度，进一步提高了MgO粒子的形核能力，使Mg-0.1Zr合金晶粒尺寸由(321 ± 30) μm细化到(127 ± 15) μm。

超声处理由于其空化效应和声流效应[91~93,99,100]，可以使金属中已形成的枝晶发生重熔与破碎，形成大量细小、弥散分布的形核核心，从而改善细化效果。Nagasivamuni等[93]采用超声波处理，研究是否可以通过UST使Zr颗粒分散更均匀、减少Zr颗粒沉降以提高颗粒细化效率。结果表明，Zr 细化与超声处理相结合可大幅提高铸造镁合金的细化效果，在液相线温度以上施加超声波可明显降低熔体内的温度梯度，促进晶粒在过冷熔体中的形核；空化与声流效应促进了Zr颗粒的分散、破碎与溶解，不仅减少了Zr沉降，还提高了熔体中溶质Zr含量。

在凝固过程中施加脉冲磁场、脉冲电场等，能够在熔体中产生Lorentz力，使金属液发生对流，从而获得均匀的温度场与浓度场，达到细化合金的目的[94~98]。汪彬等[96]研究了脉冲磁场对Mg-Gd-Y-Zr合金晶粒细化效果的影响。结果表明，施加脉冲磁场使晶粒尺寸得到明显细化，当磁场频率为5 Hz时，晶粒细化效果最佳，晶粒尺寸为37 μm，相比不加脉冲磁场时细化效果提升了43%。细化效果提升的主要机制为：(1) 在磁场对流作用下，有效形核Zr颗粒更加细化、均质化，促使更多Zr形核；(2) 脉冲磁场导致熔体磁过冷，提高形核驱动力。庞松[82]创新性地提出了Zr与脉冲电场复合细化砂型铸造Mg-Gd-Y合金的方法，有效提高了Zr的细化能力。复合处理在仅Zr细化的基础上提高合金中α-Mg的起始形核温度与形核过冷度，提高了形核驱动力，促使更多颗粒Zr成为有效形核点，细化α-Mg晶粒。

综上所述，对含Zr镁合金晶粒细化的改善取得了一定的进展(如表2[71,80,82,84,86,87,90,93,96]所示)，但仍存在局限性。(1) 对于Mg-Zr中间合金大塑性变形预处理，在预处理过程中，Mg-Zr中间合金由于其高硬度且低延展性，容易发生破碎，对模具要求较高，成本高。(2) 对于熔体处理技术，在强熔体剪切过程中，镁熔体容易被氧化，导致整个铸件的性能和组织不均匀，同时增加了熔体精炼难度；在超声处理时，振动仅限于声纳极附近的熔体，改善效果有限，同时，空化效应会对超声辐射杆端面造成腐蚀，导致超声振动的效果大幅减弱，既缩短了辐射杆的寿命，也增加了污染熔体的风险；脉冲磁场、电场等仍限于理论研究，在实际工程应用中的适用性不高。(3) Zr细化的改善多集中于颗粒Zr的改善，缺乏有效的工艺来提升溶质Zr含量等。在晶粒细化过程中，当中间合金不足以达到理想的合金化效果时，Zr还可能发生沉降，要实现Zr的完全分散和溶解仍具有挑战性。单一预处理方式对改善Zr细化镁合金的效果有限，后续可重点关注复合细化方法[101]，发挥颗粒Zr与溶质Zr的协同效应，当然，还需考虑预处理工艺对于大批量工程应用的适用性。

表2   不同处理工艺下镁合金细化效果对比[71,80,82,84,86,87,90,93,96]

Table 2  Comparisons of magnesium alloy refining effects by various treatment processes[71,80,82,84,86,87,90,93,96]

Treatment

Process

Alloy

Casting conditionHolding time / minAverage grain size / μm

Improvement

rate / %

Ref.

UntreatedTreated
Mg-Zr refinerUHFP-TIGRMg-9Gd-3Y-0.5ZrSteel ladle5924254[80]
treatedRollingMg-10Gd-3Y-0.5ZrSteel ladle15866030[71]
RollingMg-0.5ZrSteel ladle3016214510[84]
FSPMg-3Nd-0.2Zn-0.6ZrSteel ladle1514011121[86]
ECAEMg-2Zn-0.1ZrMetallic mold-13111016[87]
Mg alloy meltIMSMg-0.1Zr-30321*12760[90]
treatedUSTMg-1.0Zr--2949465[93]
PMFMg-Gd-Y-ZrGraphite crucible-653743[96]
PECMg-10Gd-3YSand cast-1158923[82]

Note: * is the grain size of pure Mg without intensive melt shearing. UHFP-TIGR—tungsten inert gas arc re-melting combined with ultra-high frequency pulsing, FSP—friction stir processing, ECAE—equal channel angular extrusion, IMS—intensive melt shearing, UST—ultrasonic treatment, PMF—pulsed magnetic field, PEC—pulsed electric current

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5 结语与展望

Zr是镁合金(不含Al、Si等元素)最有效的晶粒细化元素。添加少量Zr，就能使镁合金获得显著的细化效果。Zr对镁合金晶粒细化的贡献由溶质Zr和颗粒Zr共同决定。在凝固过程中，溶质Zr对α-Mg晶粒的生长有强烈的抑制作用，而颗粒Zr则可作为异质形核点，2者综合产生了更高形核率及更细的晶粒尺寸。Zr对晶粒的细化效果最终取决于Zr在镁熔体中的最大溶解量及有效形核尺寸范围内的Zr粒子数量。调控Zr元素在Mg-Zr中间合金中的存在形态是实现含Zr镁合金晶粒有效细化的关键。

随着铸件朝着薄壁、复杂、大型的趋势发展，对于镁合金不但要求其具有好的力学性能，还要求其具有优良的铸造性能。通过Zr细化镁合金能同时提高力学性能与铸造性能，对发展镁合金的应用具有重要意义，但受限于Zr收得率低、容易出现细化效果衰退等因素影响，其发展和推广仍面临挑战：(1) Zr细化镁合金的机理研究不够完善，虽然目前普遍认为Zr细化镁合金的主要机制归因于生长抑制理论与异质形核理论，但某些机制仍存在争议，如Zr晕圈的形成机制，后续可考虑采用同步辐射等原位表征技术追踪观察合金的异质形核与晶粒生长过程，进一步完善Zr细化镁合金机制；(2) 关于颗粒Zr与Mg基体间位向关系对晶粒细化作用的研究相对较少，可探究Zr颗粒/Mg基体不同位向关系对晶粒细化的影响规律，为筛选合适的晶粒细化剂作参考；(3) 工业应用的Zr细化方式严重依赖于Zr的合金化温度与搅拌工艺等，增加了熔体净化的难度，后续应重点探究适用于工业化应用的Mg-Zr中间合金预处理工艺，避免通过升高温度和增加搅拌等方式来达到细化目的。单一预处理方式对改善Zr细化镁合金的效果有限，可重点关注复合细化方法，改善Zr元素在Mg-Zr中间合金的存在形态，使颗粒Zr尺寸细小、分散均匀，溶质Zr含量尽可能达到溶解极限，以发挥颗粒Zr与溶质Zr的协同效应，实现含Zr镁合金晶粒的有效细化。

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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.

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Research on ultrasonic treatment (UST) of aluminium, magnesium and zinc undertaken by the authors and their collaborators was stimulated by renewed interest internationally in this technology and the establishment of the ExoMet program of which The University of Queensland (UQ) was a partner. The direction for our research was driven by a desire to understand the UST parameters that need to be controlled to achieve a fine equiaxed grain structure throughout a casting. Previous work highlighted that increasing the growth restriction factor Q can lead to significant refinement when UST is applied. We extended this approach to using the Interdependence model as a framework for identifying some of the factors (e.g., solute and temperature gradient) that could be optimised in order to achieve the best refinement from UST for a range of alloy compositions. This work confirmed established knowledge on the benefits of both liquid-only treatment and the additional refinement when UST is applied during the nucleation stage of solidification. The importance of acoustic streaming, treatment time and settling of grains were revealed as critical factors in achieving a fully equiaxed structure. The Interdependence model also explained the limit to refinement obtained when nanoparticle composites are treated. This overview presents the key results and mechanisms arising from our research and considers directions for future research.

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Magnesium - Zirconium master alloys are the dominant grain refiner for a range of commercial magnesium alloys that exhibit good elevated temperature properties. However, the grain refining efficiency achieved by the master alloy is relatively low because Zr particles quickly settle to form sludge. In this study ultrasonic treatment (UST) is applied to investigate whether the grain refinement efficiency can be improved by increasing the Zr solute content, producing a more uniform dispersion of Zr particles and minimizing sludge formation. It was determined that, for a range of Zr contents from 0.2 to 1.0 wt% Zr, UST of the melt above the liquidus temperature decreased the grain size due to an increase in the amount of solute Zr and the number of Zr particles. For Zr contents less than 0.5 wt% the grain size decreased further when UST was applied from above to below the liquidus temperature to include the onset of nucleation of alpha - Mg grains. Additionally, the application of UST substantially decreased the amount of sludge which increased the number density of Zr particles and improved their likelihood of successful nucleation. The Interdependence nucleation model was successfully applied to predict the grain size of each casting condition where the parameter (z) relates to the temperature gradient which in turn is affected by the acoustically-induced convection. A lower value of z when UST is applied facilitates nucleation and the survival of grains. The factors affecting grain refinement under each condition are evaluated in terms of solute and particle Zr contents, cavitation and acoustic streaming and their effect on nucleation and as-cast grain size.

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The researches on development, application and solidification microstructures of high performance magnesium (Mg) alloys have received considerable interest recently. The solidification microstructures of Mg alloys can be effectively controlled by using directional solidification technology, rapid solidification technology and the application of external field during solidification, and thus the enhanced comprehensive mechanical properties of the materials are obtained. The current researches on solidification microstructure controlling of high performance Mg alloys by using directional solidification technology, rapid solidification technology and electromagnetic stirring were reviewed. Finally, the development trend on the controlling of solidification microstructure was proposed.

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高性能镁合金的开发应用与凝固组织控制是当前的研究热点。采用定向凝固技术、快速凝固技术以及在镁合金凝固过程中施加外场能够有效地控制镁合金的凝固组织,从而改善材料的综合力学性能。本文主要综述了定向凝固技术、快速凝固技术以及电磁搅拌3种方式对高性能镁合金的凝固组织进行控制的研究现状。最后,展望了凝固组织控制的发展趋势。

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介绍了传统单一物理场对金属凝固组织影响的研究概况，阐述了在此基础上几种新发展起来的复合物理场对金属凝固组织的影响及应用情况。由于复合物理场良好的控制效果，为了更好地实现晶粒细化，提出目前复合物理场研究进展中的一些问题，最后对今后复合物理场的发展进行了展望。

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