W is usually used as plasma-facing components in nuclear fusion reactors because of its high melting point, low sputtering yield, high-temperature strength, and low tritium retention properties. On the other hand, Cu and its alloys show excellent thermal conductivity making them ideal as a heat sink material in reactors. Therefore, W-Cu layered composites have important applications in nuclear fusion reactors. Due to the immiscibility between W and Cu, direct alloying between them without using interlayer metals is critical for the preparation of such layered composites. In this study, a nanoporous active structure was used to induce and promote the direct alloying of the W-Cu system. Direct alloying consists of three steps. First, a nanoporous active layer is prepared on the surface of a W foil via two-step anodizing and deoxidized annealing in a hydrogen atmosphere. Second, a Cu coating layer is deposited on the nanoporous W by electroplating. Finally, the obtained W-Cu electrodeposited sample is annealed at temperatures close to the melting point of Cu (i.e., 980oC). The established thermodynamic model for the direct alloying of immiscible metal systems is used for the direct alloying of W and Cu based on a nanoporous active structure. There are two problems with this model. First, the surface energy results are arbitrary due to the selection of the number of surface atomic layers. Second, the unit scale in thermodynamic calculations. To solve these problems, the calculation methods for surface energy and pressure energy are improved in this work, which makes the thermodynamic calculation for the direct alloying of W-Cu based on a nanoporous active structure feasible. The results show that a nanoporous active structure is formed on the surface of W after nanotreatment. The characterization results of the W/Cu interface show that the diffusion distance between the two metals is about 27 nm and the direct alloying between W and Cu is successful. The average shear strength of the W-Cu layered composites was 101 MPa. This is a 16% increase compared with W-Cu layered composites without a nanoporous structure. The thermodynamic calculation results show that the surface energy of the W-Cu system is greatly improved due to the nanoporous active structure prepared on the W surface. The surface energy can be used as the main thermodynamic driving force for the direct alloying of W-Cu systems. There are different reasons why nanotreatment increases W surface energy. One reason is the increase of crystal planes with high surface energy via nanotreatment of the W surface, and another is the shape of the nanoporous structure.
Keywords：immiscible W-Cu system;
nanoporous structure;
direct alloying;
thermodynamic model;
surface energy
WANG Hanyu, LI Cai, ZHAO Can, ZENG Tao, WANG Zumin, HUANG Yuan. Direct Alloying of Immiscible Tungsten and Copper Based on Nano Active Structure and Its Thermodynamic Mechanism[J]. Acta Metallurgica Sinica, 2023, 59(5): 679-692 DOI:10.11900/0412.1961.2022.00003
式中，P的单位为N/m2，ρi (i = W、Cu)为密度(g/cm3)， (i = W、Cu)为摩尔质量(g/mol)。计算时应先统一各变量的单位，最后换算成kJ/mol。 式(6)的计算方法解决了 式(5)计算所获结果单位(kJ)与热力学模型中使用的能量单位(kJ/mol)不统一的问题。本工作对W-Cu直接合金化进行了有压力和无压力2种情况的计算。
本工作采用作者所在团队开发的W-Cu多体FS势函数[36]对W-Cu直接合金化的原子扩散过程进行分子动力学计算和实验验证。为了定量描述合金化过程中界面处(即W、Cu表面接触处)结构变化，基于分子动力学模拟结果计算了合金化过程中的W和Cu原子结构精细密度分布函数(fine-scale density profile)。精细密度分布函数描述了原子数量沿扩散方向(Z轴)的分布[37]，可以用来分析扩散界面处的微观结构特征，对应的计算公式如下[38,39]
Fig.1
Surface SEM images and corresponding EDS results (insets) (a, c) and cross-section SEM images of W plate (b, d) with nanoporous structure layer prepared through two-step anodizing process without (a, b) and with (c, d) deoxidation process
Fig.3
High-angle annular dark field (HAADF) image of W/Cu interface (a), atomic fraction along the red arrow marked in Fig.3a (b), and elemental mappings of W (c) and Cu (d)
Fig.4
HRTEM image of W/Cu interface (Inset I shows the fast Fourier transform (FFT) pattern from the darker area and inset II shows the brighter one, d—interplanar spacing)
Fig.6
SEM image of shear fracture surface (W side) of the W/Cu layered composite (a); EDS analysis results of the region marked with rectangles I (b), II (c), and III (d)
Fig.7
SEM image of shear fracture surface (Cu side) of the W/Cu layered composite (a), and EDS analysis result of the region marked with the rectangle in Fig.7a (b)
Table 2 Parameters used in thermodynamic calculation for the W-Cu direct alloying[25,42]
Element
V2/3
φ
γ
E
K
G
Sf
γS,0
ρ
M
Tm
cm2
(d.u.)1/3
V
GPa
GPa
GPa
105 m2
mJ·m-2
g·cm-3
g·mol-1
K
Cu
3.70
1.47
4.45
0.343
129.8
137.8
48.3
1.67
1825
8.96
63.55
1356
W
4.50
1.81
4.80
0.280
411.0
160.6
311.0
2.03
3675
19.32
183.84
3680
Note:V—molar volume of metal, nws—electron density at the boundary of the crystal cell, φ—electrical chemical potential of metal, γ—Poisson's ratio of metal, E—elastic modulus of metal, K—bulk modulus of metal, G—shear modulus of metal, Sf—surface area of 1 mol interfacial atoms, γS,0—specific surface energy of interfacial atoms, ρ—density of metal, M—molar mass of metal, Tm—melting point of metal
Fig.9
Calculated energy curves of W-Cu direct alloying processes based on nano active structure for the W-Cu systems (Δ—Gibbs free energy for the formation of W/Cu crystalline phase, Δ—Gibbs free energy for the formation of W/Cu amorphous phase, Einitial,W—initial energy of W-Cu system without nano-treatment, Einitial,nano—initial energy of W-Cu system with nano-treatment)
Fig.10
Origin shape of W nanoporous with convex structure (a), concave structure (b, c), structural diagram and cross-section of simplified geometric models of Figs.10a and c (d, e), and geometry model for the W plate without nano-treatment (f) (D—nanopore diameter, dsurf—the thickness of the surface layer in a metal that provides surface energy, h0—the height of a cylinder in the convex or concave structure, h1—the height of a cone in the convex or concave structure, h2—obtained from h1 ± dsurf, R1—the bottom radius of cylinder and cone in the convex or concave structure, R2—obtained from R1 ± dsurf, a—the side length of the model of W plate without nano-treatment)
Fig.11
Fine-scale density profiles (ρ(Z)) for W and Cu near and at the W/Cu interface constructed through direct alloying at different temperatures
本研究结果可以为核聚变堆用W/Cu PFCs的制备新方法研究提供参考。同时，不互溶金属体系在包括电池能源等在内的其他领域也有着广泛的应用，可为这些领域涉及的不互溶金属合金化问题研究提供参考。例如Cu-Li是一个比较典型的不互溶体系，由于热力学平衡态下2者之间不发生合金反应，Cu被大规模用于各类锂电池中作为负极集流材料，以保证在电池循环过程集流过程中不会发生电化学锂化形成合金相。但有研究[45]在原位TEM下观察到在纳米尺寸效应作用下，Cu和Li将发生合金化，即超小Cu纳米颗粒发生电化学锂化和CuLi x 非晶合金相形成将成为可能。并认为出现这种情况的原因在于Cu纳米颗粒导致的表面能显著增加所带来的反应活性极大增强，但并未给出热力学计算分析。而本工作所建立的基于纳米活性结构的不互溶金属合金化热力学模型可以解释Cu-Li发生合金化的原因，区别在于Cu-Li的合金化是通过电化学驱动原子运动释放表面能，而本工作中W-Cu是通过加热作用驱动原子运动释放表面能。
In our previous work, the joint between oxide dispersion strengthened copper alloy (ODS-Cu), GlidCop (R) (Cu-0.3 wt%Al2O3) and tungsten (W) demonstrated superior fracture strength (similar to 200 MPa). This joint was fabricated by the direct brazing method between W and ODS-Cu using BNi-6 (Ni-11%P) filler material without any intermediate layer. This method was named as the improved or the advanced brazing technique. In the present study, deformation and fracture behavior of the joint after the three-point bending test was investigated. At first, it was found that the crack initiation points were dominantly in the W bulk, although it was not clear that the crack initiated from grain boundary or not. Secondly, the crack propagation proceeded mostly in the W bulk but tended to deflect towards the bonding layer. These results interpret the strength of the bonding interface is superior and the present bonding technique is applicable for severe environments such as a high heat flux divertor component on the fusion reactor. Based on the above physical and technological understanding, we successfully fabricated the large scale divertor mock-up which has twenty-eight plates of W with each size of 20 x 20 x 5 mm(3).
PengS X, MaoY W, MinM, et al.
Joining of tungsten to CuCrZr alloy with Cu-TiH2-Ni filler and Cu interlayer
[J]. Int. J. Refract. Met. Hard Mater., 2019, 79: 31
Direct alloying is difficult to be realized in an immiscible Mo-Ag system with a positive formation heat due to the absence of thermodynamic driving force at equilibrium. In this work, a direct alloying method is developed to realize the direct alloying between Mo and Ag and construct Mo-Ag interface. The direct alloying method was mainly carried out through a direct diffusion bonding for Mo and Ag rods at a temperature close to the melting point of Ag (TmAg). Then the microstructure and phase constitution of the as-constructed Mo-Ag interface are characterized. The results show that Mo-Ag metallurgical bonding interface has been constructed successfully, indicating that a direct alloying in the immiscible Mo-Ag system has been realized. Additionally, mechanical tests are carried out for the Mo-Ag joints prepared through the direct alloying method. The test results show that the average maximum tensile strength of the joints is about 107 MPa. The effect of alloying parameters on the tensile strength is also discussed, which shows that there is an effective temperature range for the direct alloying between Mo and Ag. Lastly, an improved thermodynamic model that considers the formation of Mo-Ag crystalline and amorphous phase is presented to reveal the thermodynamic mechanism of the direct alloying. Combining the calculation and differential scanning calorimetry (DSC) tests results, the Gibbs energy diagram for the direct alloying is obtained. It is confirmed that the co-release of storage energy and surface energy can serve as the thermodynamic driving force to overcome the effect of positive formation heat and lead to direct alloying for Mo-Ag systems.
PanX C, ZhangJ, HuangY, et al.
Construction of metallurgical interface with high strength between immiscible Cu and Nb by direct bonding method
W/Cu joining is key for the fabrication of plasma-facing compounds of fusion reactors. In this work, W and Cu are joined through three steps: (1) hydrothermal treatment and reduction annealing (i.e., nano-treatment), (2) Cu plating and annealing in a pure H2 atmosphere, and (3) W/Cu bonding at 980 °C for 3 h. After nano-treatment, nanosheets structure can be found on the W substrate surface. The tensile strength of the W/Cu joint prepared via nano-treatment reaches as high as approximately 93 MPa, which increases by about 60% compared with the one without nano-treatment. The microhardness curves exhibited continuous variations along the W/Cu interface. The TEM images show that the W/Cu interface is compact without any cracks or voids. This work may also be applied for enhancing bonding strength in other immiscible materials.
ZhaoC, LiF, ChenY Y, et al.
Joining of oxygen-free high-conductivity Cu to CuCrZr by direct diffusion bonding without using an interlayer at low temperature
Materials based on binary immiscible metal systems are widely used in aerospace, nuclear fusion engineering, electronic packaging, anti-armor weapons and other fields. However, due to the positive formation heat and the large differences in the properties of the component, the direct alloying of binary immiscible metals and the preparation of the corresponding materials are very difficult. Varieties of methods have been developed for direct alloying of binary immiscible metals at home and abroad, and the thermodynamic and diffusion mechanism of these methods have been studied. In this review, firstly the principle and thermodynamic mechanism of mechanical alloying, physical vapor deposition and ion beam mixing, as well as their applications in binary immiscible metal powder alloys and nano-multilayer films are reviewed. Then the irradiation damage alloying (IDA) and high-temperature structure induced alloying (HTSIA) methods that are proposed and developed by our group are introduced. Besides, the principle, interfacial microstructure, thermodynamic mechanism, diffusion mechanism and application of these two methods were described in detail. Finally, the development trend of the research on alloying of binary immiscible metals is proposed.
W-Cu laminated composites are critical materials used to construct nuclear fusion reactors, and it is very important to obtain direct alloying between W and Cu at the W/Cu interfaces of the composites. Our previous experimental studies showed that it is possible to overcome the immiscibility between W and Cu and obtain direct alloying when the alloying temperature is close to the melting point of Cu. Because the W-Cu interatomic potentials published thus far cannot accurately reproduce the alloying behaviors of immiscible W and Cu, an interatomic potential suitable for the W-Cu system has been constructed in the present study. Based on this potential, direct alloying between W and Cu at high temperature has been verified, and the corresponding diffusion mechanism has been studied, through molecular dynamics (MD) simulations. The results indicate that the formation of an amorphous Cu layer at the W/Cu interface plays a critical role in alloying because it allows Cu atoms to diffuse into W. The simulation results for direct alloying between W and Cu can be verified by experimental results and transmission electron microscopy observations. This indicates that the constructed W-Cu potential can correctly model the high-temperature performance of the W-Cu system and the diffusion mechanism of direct alloying between W and Cu.
GanX L, XiaoS F, DengH Q, et al.
Atomistic simulations of the Fe(001)-Li solid-liquid interface
For the immiscible Mo/Cu system with a positive heat of mixing (ΔHm > 0), building metallurgical bonding interfaces directly between immiscible Mo and Cu and preparing Mo/Cu laminar metal matrix composites (LMMCs) are very difficult. To solve the problem, a new alloying method for immiscible systems, which is named as irradiation damage alloying (IDA), is presented in this paper. The IDA primarily consists of three steps. Firstly, Mo is damaged by irradiation with multi-energy (186, 62 keV) Cu ion beams at a dose of 2 × 1017 ions/cm2. Secondly, Cu layers are superimposed on the surfaces of the irradiation-damaged Mo to obtain Mo/Cu laminated specimens. Thirdly, the irradiation damage induces the diffusion alloying between Mo and Cu when the laminated specimens are annealed at 950 °C in a protective atmosphere. Through IDA, Mo/Cu LMMCs are prepared in this paper. The tensile tests carried out for the Mo/Cu LMMCs specimens show that the Mo/Cu interfaces constructed via IDA have high normal and shear strengths. Additionally, the microstructure of the Mo/Cu interface is characterized by High Resolution Transmission Electron Microscopy (HRTEM), X-ray diffraction (XRD) and Energy Dispersive X-ray (EDX) attached in HRTEM. The microscopic characterization results show that the expectant diffusion between Mo and Cu occurs through the irradiation damage during the process of IDA. Thus a Mo/Cu metallurgical bonding interface successfully forms. Moreover, the microscopic test results show that the Mo/Cu metallurgical interface is mainly constituted of crystalline phases with twisted and tangled lattices, and amorphous phase is not observed. Finally, based on the positron annihilation spectroscopy (PAS) and HRTEM results, the diffusion mechanism of IDA is discussed and determined to be vacancy assisted diffusion.
DuJ L, HuangY, LiuJ W, et al.
Irradiation damage alloying for immiscible alloy systems and its thermodynamic origin
Construction of an n-body potential for revealing the atomic mechanism for direct alloying of immiscible tungsten and copper
3
2021
... 本工作采用作者所在团队开发的W-Cu多体FS势函数[36]对W-Cu直接合金化的原子扩散过程进行分子动力学计算和实验验证.为了定量描述合金化过程中界面处(即W、Cu表面接触处)结构变化，基于分子动力学模拟结果计算了合金化过程中的W和Cu原子结构精细密度分布函数(fine-scale density profile).精细密度分布函数描述了原子数量沿扩散方向(Z轴)的分布[37]，可以用来分析扩散界面处的微观结构特征，对应的计算公式如下[38,39] ...
Atomistic simulations of the Fe(001)-Li solid-liquid interface
1
2014
... 本工作采用作者所在团队开发的W-Cu多体FS势函数[36]对W-Cu直接合金化的原子扩散过程进行分子动力学计算和实验验证.为了定量描述合金化过程中界面处(即W、Cu表面接触处)结构变化，基于分子动力学模拟结果计算了合金化过程中的W和Cu原子结构精细密度分布函数(fine-scale density profile).精细密度分布函数描述了原子数量沿扩散方向(Z轴)的分布[37]，可以用来分析扩散界面处的微观结构特征，对应的计算公式如下[38,39] ...
Atomistic characterization of the chemically heterogeneous Al-Pb solid-liquid interface
1
2012
... 本工作采用作者所在团队开发的W-Cu多体FS势函数[36]对W-Cu直接合金化的原子扩散过程进行分子动力学计算和实验验证.为了定量描述合金化过程中界面处(即W、Cu表面接触处)结构变化，基于分子动力学模拟结果计算了合金化过程中的W和Cu原子结构精细密度分布函数(fine-scale density profile).精细密度分布函数描述了原子数量沿扩散方向(Z轴)的分布[37]，可以用来分析扩散界面处的微观结构特征，对应的计算公式如下[38,39] ...
Atomic scale analysis of the corrosion characteristics of Cu-Li solid-liquid interfaces
1
2018
... 本工作采用作者所在团队开发的W-Cu多体FS势函数[36]对W-Cu直接合金化的原子扩散过程进行分子动力学计算和实验验证.为了定量描述合金化过程中界面处(即W、Cu表面接触处)结构变化，基于分子动力学模拟结果计算了合金化过程中的W和Cu原子结构精细密度分布函数(fine-scale density profile).精细密度分布函数描述了原子数量沿扩散方向(Z轴)的分布[37]，可以用来分析扩散界面处的微观结构特征，对应的计算公式如下[38,39] ...
Molecular dynamics simulations of the characteristics of Mo/Ti interfaces
