Hf元素掺杂对TiSc合金调幅分解的影响
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Influence of Hf Doping on Spinodal Decomposition of TiSc Alloy
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通讯作者: 段慧超,副研究员,hcduan15s@imr.ac.cn,研究方向为金属材料的电子显微学分析
责任编辑: 姚金金
收稿日期: 2025-03-11 修回日期: 2025-05-12
| 基金资助: |
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Corresponding authors: DUAN Huichao, Tel:
Received: 2025-03-11 Revised: 2025-05-12
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作者简介 About authors
王昱坤,男,2000年生,硕士生
用透射电子显微镜、扫描电子显微镜和X射线衍射仪等手段表征掺杂不同含量Hf的TiSc合金的微观结构,研究了Hf掺杂对其调幅分解的影响。结果表明,随着Hf元素含量从0提高到10% (原子分数),TiSc合金调幅分解产生的片层组织的宽度呈增大的趋势,α-Ti和α-Sc调幅分解结构由片层分别转变为网格状和块状组织。转变为网格状组织可归因于Hf元素的添加降低了调幅分解的化学驱动力和Ti、Sc原子间的晶格错配度,使体系产生更大范围的成分波动和增大了调幅分解组织的片层宽度;转变为块状组织可归因于低扩散性Hf元素均匀固溶于TiSc合金降低了体系的扩散速率,使α-Ti和α-Sc的调幅分解结构发生转变。同时,随着合金片层组织宽度的增大其维氏硬度从375.37HV降低到281.11HV。这表明,Hf元素的添加改变了TiSc合金的微观组织结构从而影响其硬度。
关键词:
Spinodal decomposition enables the formation of continuous nanoscale dual-phase structures with periodic compositional fluctuations, a unique microstructure that significantly enhances mechanical properties such as strength, hardness, and creep resistance of alloys. Consequently, tailoring spinodal decomposition to optimize mechanical performance has emerged as a central objective in materials research. While compositional control is vital to this process, and elemental doping provides a precise strategy to regulate the decomposition behavior, the mechanism related with interactions between dopants and spinodal dynamics remains elusive. In this study, the evolution of nanoscale lamellar microstructures in Hf-doped TiSc alloys (0-10% Hf, in atomic fraction) was systematically investigated by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD), as well as Vickers hardness measurements. The results demonstrate that with the increase of Hf content from 0 to 10%, the lamellar structure width formed by spinodal decomposition in TiSc alloys exhibits a progressive growth trend. Meanwhile, the spinodal decomposition structures of α-Ti and α-Sc undergo morphological transitions from lamellar to interconnected network and blocky morphologies, respectively. The former phenomenon may be attributed to that both the chemical driving force for spinodal decomposition and the lattice mismatch between Ti and Sc atoms may be reduced by the addition of Hf, which may thermodynamically facilitate the occurrence of larger-scale compositional fluctuations within the system, as a consequence, the lamellar width of spinodal decomposition structures is then increased. The latter transformation may be raised from the homogeneous solid solution of low-diffusivity Hf atoms in the TiSc matrix, which significantly decreases the overall diffusion rate of the system, thereby promoting structural evolution in both α-Ti and α-Sc spinodal decomposition phases. Furthermore, the Vickers hardness continuously decreases from 375.37HV to 281.11HV with the widening of lamellar structures, indicating that Hf addition alters the microstructural characteristics and consequently affects the mechanical properties of TiSc alloys.
Keywords:
本文引用格式
王昱坤, 段慧超, 杜奎.
WANG Yukun, DUAN Huichao, DU Kui.
调幅分解,能使材料的成分周期性变化且具有特定波长结构的连续纳米双相组织。这种独特的微观结构能显著提高材料的强度、硬度、抗蠕变性等力学性能[1,2]。因此,调幅分解可用于设计性能优异的先进材料。调控调幅分解强化材料的力学性能,一直是国内外科研工作者的重点。相关研究涉及多种合金体系,包括Ti基合金[3,4]、Al基合金[5,6]、Cu基合金[7~9]、Fe-Mn记忆合金[10,11]以及双相钢[12~14]等。Ti基合金体系的Ti-6554[15,16]、Ti-55531[17]和Ti-10V-6Cu[18]等亚稳态β-Ti合金中都存在调幅分解。Yang等[19]在Ti-Al-Mo-V-Cr-Zr合金中构建了β+β'调幅分解片层结构,通过周期性分布的纳米结构阻碍了位错运动使其力学性能显著提高。An等[20]利用调幅分解机制在难熔高熵合金Hf-Nb-Ti-V体系中生成了具有周期性晶格畸变的β'相,在保持高强度的同时显著改善了合金的室温拉伸延展性。Hua等[21]将调幅分解机制引入Zr-Nb-Ti中熵合金体系,使其屈服强度显著提高并降低了弹性模量。同时,许多研究者也在探究调幅分解影响材料性能的机理。Cahn[22,23]建立了预测调幅分解微观结构的理论模型,提出了调幅分解强化理论[24]。这个强化理论指出,调幅分解在合金中的强化作用是通过共格组织之间的内应力与位错相互作用实现的。Mott和Nabarro[25]认为,屈服强度取决于调幅分解组织的内应力场。值得注意的是,作为固态相变过程中的一种扩散型相变,调幅分解与传统的形核-长大方式不同,母相的过饱和固溶体在一定温度下通过原子上坡扩散引起的成分涨落产生结构相同、成分不同的两个新相,两相组织根据相变动力学呈现片层状或网格状等不同形貌[26,27]。因此,成分起伏是调幅分解的原始驱动力,成分调控在调幅分解中占据主导作用。Park等[28]在铁质中熵合金中引入Cu和Al元素,通过调幅分解结构抑制了应变局域化,在保持良好伸长率的同时使其机械强度提高。Röyset和Ryum[29]在Al-Zn-Mg合金中添加微量Sc元素促进了调幅分解过程生成了纳米级析出相,显著提高了合金的强度和抗腐蚀性能。Guo等[30]在Cu-Ni-Zn合金体系中引入P元素,通过增大溶质与溶剂原子尺寸的差异诱导合金调幅分解而使其硬度提高。
1 实验方法
1.1 TiSc系合金的制备
成分为Ti50Sc50(下标为原子百分比)的母合金锭的制备:依据母合金的配比计算所需纯度高于99.99%(质量分数)的块状Sc、Ti的质量,将这些原料放入真空电弧炉中进行电弧熔炼。为了确保合金成分均匀,在熔炼过程中将合金锭翻转熔炼5~6次。对制备出的合金锭进行真空封管处理,以防其在后续热处理过程中氧化。从合金锭上截取试样进行均匀化处理:将试样加热到1200 ℃保温5 h后水淬。根据Ti-Sc相图[31],调幅分解温度为1050 ℃。在水淬冷却的短时间内完成调幅分解。
对TiSc母合金和原子分数分别为2%、5%和10%的Hf元素进行电弧熔炼,制备TiSc-Hf合金锭,记作TS-xH (x = 2, 5, 10)合金。将TS-xH合金锭真空封管,进行均匀化热处理和淬火处理,温度和时长与母合金锭的热处理参数相同。
1.2 TiSc系合金的表征
用线切割机将合金锭切成厚度为1000 μm的薄片试样。用Tescan Clara扫描电子显微镜背散射电子模式(SEM-BSE) 表征薄片试样。用Bruker D8 Discover X射线衍射仪(XRD)测试试样的XRD谱,2θ为5°~90°。用FEI Tecnai F30透射电子显微镜明场像(TEM-BF)模式表征试样的显微组织并根据能量色散X射线谱(EDS)分析其元素分布。使用Qness Q10A+全自动硬度计测量试样的显微硬度,载荷为0.1 kg,每隔1 mm等间距打20个点。
2 实验结果
2.1 Hf元素掺杂后合金结构的变化
图1给出了不同Hf元素含量TiSc合金的SEM-BSE图像。可以看出,4种试样中均有由α-Ti相(灰白色)和α-Sc相(黑色)交替分布(图中箭头标出)构成的调幅分解结构。未掺杂Hf元素的TiSc合金中Ti、Sc片层组织高度细化,沿着同一方向排布,表明其倾向于形成高度有序的调幅分解结构。Hf元素的添加,使TiSc合金的调幅分解发生了变化。从图1b可见,TS-2H试样中α-Ti与α-Sc片层的宽度增加。随着Hf含量提高到5% (图1c),α-Ti和α-Sc两相组织进一步粗化,α-Ti形成局部连通的网状结构将α-Sc包裹。在Hf含量为10% (图1d)的试样中α-Ti形成了连续的网格状组织,而α-Sc则转变为圆角方块状。
图1
图1
不同Hf元素含量TiSc合金的SEM背散射电子图像
Fig.1
SEM-BSE images of TiSc alloys with different Hf element contents (a) TiSc, (b) TS-2H, (c) TS-5H, (d) TS-10H
图2给出了不同Hf元素含量TiSc合金的微观结构。可以看出,在TEM-BF像中Ti和Sc两相的衬度与扫描电镜背散射电子成像结果不同:α-Ti呈现黑色,而α-Sc呈现灰白色(图2中箭头所示)。从Ti片层宽度的统计图(图3)可见,随着Hf含量的提高TiSc合金中Ti片层(Sc片层与Ti片层宽度接近)的宽度显著增大。未掺杂Hf元素的Ti片层(图2a)的平均宽度为20.50 nm,TS-2H试样(图2b)中Ti片层的平均宽度为44.02 nm。Hf含量提高到5%,Ti片层的平均宽度增大到56.11 nm (图2c);Hf含量为10%时网格状Ti组织的平均宽度显著增大到357.03 nm (图2d),且Ti组织宽度的分布范围较广。
图2
图2
不同Hf元素含量TiSc合金的TEM明场像
Fig.2
TEM-BF images of TiSc alloys with different Hf element contents (a) TiSc, (b) TS-2H, (c) TS-5H, (d) TS-10H
图3
图3
不同Hf元素含量TiSc合金中Ti片层宽度的分布
Fig.3
Statistics of Ti lamellae width of TiSc alloys with different Hf element contents
2.2 掺杂Hf元素后TiSc合金中元素的分布
为了深入研究Hf元素掺杂对TiSc合金调幅分解行为的影响,用EDS系统分析了合金组织中Ti、Sc和Hf元素的分布。如图4所示,α-Sc和α-Ti两相表现出显著的调幅分解特征,即两相交替分布。虽然Hf元素的含量不同,但是Hf元素在两相中的分布均匀,未观察到明显的浓度梯度和偏聚。
图4
图4
不同Hf元素含量TiSc合金的EDS分析
Fig.4
EDS analysis of TiSc alloys with different Hf element contents (a) TS-2H, (b) TS-5H, (c) TS-10H
图5
图5
Hf元素在不同合金中的占比以及不同合金中α-Ti和α-Sc的Ti/Sc比值
Fig.5
Proportion of Hf element in different alloys (a) and Ti/Sc ratio of α-Ti and α-Sc in different alloys (b)
图6给出了不同Hf元素含量TiSc合金的XRD谱。可以看出,随着Hf元素的含量从0提高到10%,TS-xH合金中(10
图6
图6
不同Hf元素含量TiSc合金的XRD谱
Fig.6
XRD patterns of TiSc alloys with different Hf element contents
2.3 Hf元素掺杂后力学性能的变化
图7给出了不同Hf元素含量TiSc合金的维氏硬度。可以看出,合金的硬度与Hf含量呈负相关:未掺杂Hf元素的TiSc合金的平均硬度为375.37HV;掺杂2%Hf元素的TS-2H合金的平均硬度降低到365.92HV;TS-5H合金的平均硬度进一步降低到338.77HV;Hf元素含量为10%的TS-10H合金,其维氏硬度降低到281.11HV。这表明,掺杂Hf元素使TiSc合金的调幅分解组织粗化而导致合金的硬度降低。
图7
图7
不同Hf元素含量TiSc合金的维氏硬度
Fig.7
Vickers hardness of TiSc alloys with different Hf element contents
3 讨论
3.1 添加Hf元素对合金结构的影响
其中K为相关比例常数(取决于同类和异类原子对键能的差异),|f''|为化学驱动力,E为弹性模量,Vm为摩尔体积。Hf的原子半径介于Ti与Sc的原子半径之间且其扩散性较差,对E和Vm的影响较小。这表明,本体系中调幅分解的特征波长主要决定于|f''|和η。Hf元素的均匀固溶使体系的混合熵增加,在焓变较为稳定的情况下自由能的降低使自由能曲率变化变缓,即|f''|减小,从而允许更大范围的成分波动[38],即调幅分解特征波长增大。同时,随着Hf元素含量的提高α-Ti与α-Sc两相的晶面间距差值减小。Hf的原子半径(0.158 nm)介于Ti的原子半径(0.146 nm)与Sc的原子半径(0.164 nm)之间,使Hf元素固溶在α-Ti和α-Sc基体中产生相反的晶格变化:在α-Ti中引起晶格膨胀,而在α-Sc中则引起晶格收缩。这降低了Ti与Sc原子间的晶格错配度η,使调幅分解波长增大。以上双重机制的共同作用,使合金在调幅分解时更倾向于形成波长更大的结构,即调幅分解产生的组织片层更宽。
掺杂Hf元素后α-Ti与α-Sc调幅分解结构由片层转变为网格状和块状组织,主要是扩散速率降低所致。原子扩散较快时,溶质原子能在较短时间内长距离迁移。Ti、Sc原子迅速向特定区域扩散使某个方向上的成分较快出现明显的浓度差异,溶质原子优先在这个方向上坡扩散聚集使组织在该方向呈现出明显的层状特征而形成片层组织。而扩散速率较低时Ti、Sc原子难以长距离扩散而在较小范围内波动聚集,成分波动在多个方向上较为均衡而形成网格状组织。这种网格状组织,验证了Zhang等[39]的模拟结果。
3.2 添加Hf元素对合金性能的影响
表示合金强度与片层宽度的关系。其中σy为屈服应力,σ0为晶格对位错滑移的抵抗力,d为调幅分解产生的组织片层宽度,ky为通过层状边界产生位错所需临界应力的相关参数。维氏硬度可换算为
其中A为硬度与强度换算的比例系数(A ≈ 2.78)[41]。依据
图8
图8
维氏硬度与平均片层宽度的关系
Fig.8
Variation of Vickers hardness and average lamella width with the content of Hf element
4 结论
(1) 随着TiSc合金中Hf元素含量的提高,α-Ti片层的厚度和TiSc合金片层组织的宽度增大。Hf元素的添加改变了体系的化学驱动力和Ti、Sc原子间的晶格错配度,从而增大了合金片层的宽度。
(2) Hf元素在TiSc合金中的均匀固溶使α-Ti和α-Sc调幅分解结构由片层转变为网格状与块状组织。低扩散性Hf元素的掺杂降低了合金体系的扩散速率,改变了调幅分解组织的形貌特征。
(3) 随着Hf元素含量的提高片层的宽度增加和维氏硬度显著下降。Hf元素的添加通过调控微观结构而影响TiSc合金的硬度。
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[J].It is shown that the free energy of a volume V of an isotropic system of nonuniform composition or density is given by : NV∫V [f0(c)+κ(▿c)2]dV, where NV is the number of molecules per unit volume, ▿c the composition or density gradient, f0 the free energy per molecule of a homogeneous system, and κ a parameter which, in general, may be dependent on c and temperature, but for a regular solution is a constant which can be evaluated. This expression is used to determine the properties of a flat interface between two coexisting phases. In particular, we find that the thickness of the interface increases with increasing temperature and becomes infinite at the critical temperature Tc, and that at a temperature T just below Tc the interfacial free energy σ is proportional to (Tc−T)32.
Spinodal decomposition in Fe-25Cr-12Co alloys under the influence of high magnetic field and the effect of grain boundary
[J].Fe-Cr-Co alloys precipitate nanosized α1 particles through spinodal decomposition, and their magnetic performance is susceptible to influence by the shape and arrangement of α1 particles. We studied spinodal decomposition during the heat treatment of Fe-Cr-Co alloys by both experimental and numerical simulation. Fe-Cr-Co alloys were fabricated first by directional solidification, followed by thermomagnetic treatment in a high magnetic field (HMF) and step aging. The experimental results show a spinodally decomposed structure consisting of nanosized α1 particles. The applied HMF caused the α1 phase to change into a rod-like shape. Moreover, a feather-like structure was observed near the grain boundary (GB), with slim α1 rods regularly arranged along the direction perpendicular to the GB. With the shape change and alignment of the α1 phase in the HMF, Fe-Cr-Co alloys show magnetic coercivity that is superior to those of samples without an HMF. To reveal the influence of HMF on phase transformations and the effect of GB, we conducted phase-field simulations of spinodal decomposition in the Fe-Cr-Co alloy. A migrating GB contributes to the elongation and arrangement of the α1 phase in the regions where the GB has passed. Thus, the α1 phase is arranged as parallel rods that are perpendicular to the GB. This GB effect consists of the effect of enhanced atomic mobility and the elastic energy. The α1 rods are elongated along the direction of HMF. The simulation results indicate that the feather-like structure is caused by a combined effect of both the GB and HMF. It is shown that the model generates microstructures which are qualitatively similar to those observed experimentally.
The role of ordered domains and slip mode of α2 phase in the plastic behaviour of TiAl crystals containing oriented lamellae
[J].
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