摘要
与MAX体系中研究最为广泛的Ti3SiC2和Ti3AlC2相比,Cr2AlC拥有更优异的高温抗氧化性和耐腐蚀性能,已成为高温结构件和涂层的优选材料。但是相对于传统的结构陶瓷(如Al2O3,SiC),Cr2AlC的强度有待提高。为了增强Cr2AlC材料的力学性能,拓宽其工程应用范围,固溶改性被视为一种有效手段。
本文利用密度泛函理论的第一性原理软件包VASP对Cr2AlC及其Cr2AlSiC固溶体的取代位置,固溶度和相关性能开展计算研究,通过实验验证了计算结果的正确性,为合成相关固溶体提供理论依据,并对实验中出现的强化现象从理论上进行解释。
本研究工作首先集中于Cr2AlC和Cr2AlSiC固溶体的模型确立,晶体结构和形成能的计算。选取Cr2AlC原胞,并对其进行优化。在Cr2AlC模型的基础上,令Si原子取代其中的部分Al原子,通过最低能量法则,建立起不同Si固溶度的固溶体模型,再对Cr2AlSiC的晶体结构和形成能进行计算。计算结果表明：随着Si原子的加入,晶格常数a基本保持不变,而c呈逐渐递减的趋势。并通过进一步计算键长变化发现晶格常数c的缩短主要是由Cr-Si键长短于Cr-Al键长的影响所致。计算得到的Cr2AlSiC固溶体的形成能都为负值,说明固溶体在能量上趋于稳定。其次,利用实验手段合成了Cr2AlSiC固溶体,证实了计算中固溶体能够稳定存在的结论。其晶格常数a=2.85A,c=12.83A,而相对应的计算值为a=2.85A,c=12.58A,误差小于5%,再次证明了选取模型的可靠性。
本研究进一步计算Cr2AlC和Cr2AlSiC固溶体的弹性模量、态密度和电荷密度来分析Cr2AlC的性能特点。材料拥有的高弹性模量和机械强度,是因为保持了Cr-C之间的强共价键；而它优异的抗高温氧化性和耐腐蚀性是由于穿插在Cr6C层之间的Al原子层与Cr6C层结合较弱,容易在外界作用下,断键并通过空位迁移到表面形成致密的Al2O3层,从而保护基体免受进一步的氧化和腐蚀。通过对Cr2AlC和Cr2AlSiC的计算值进行对比,总结出Cr2AlSiC强化的机制是由于Si原子比Al原子多一个核外电子,从而增强最外层电子的杂化作用,引起固溶体的总电荷密度集中,键强得到提高,达到强化效果。
通过对Cr2AlC及其Cr2AlSiC固溶体的计算,全面了解其结构性能特点,通过实验证实了计算结果的可靠性,并从理论上解释了强化现象。本研究的开展,对其它MAX材料的成分设计也同样具有指导意义。
Compared with the most widely studied Ti3SiC2and Ti3AlC2in MAX family, Cr2AIC possesses excellent oxidation resistance and corrosion resistance. It is a promising candidate as high-temperature structural components and coating materials. However, the strength of Cr2AIC is not so high compared to that of the conventional structural ceramics such as Alumina and Silicon Carbide. Thus, improvement of Cr2AIC with tunable properties by solid solution strengthening is a very interesting task for its potential applications.
Based on density functional theory, first-principle has become a popular method for the computation of materials. In the present study, the structure, formation energy, mechanical and electronic properties of Cr2AIC as well as Cr2AlSiC solid solution have been investigated by using the VASP package of the first-principle. In addition, the solid strengthening mechanism has also been explained in calculation theory.
The research firstly focused on the construction of Cr2AIC and Cr2AlSiC solid solution models and calculation of the structure and the formation energy. At first, we chose a Cr2AIC primitive cell, optimized its lattice constants. Final models have been constructed by replacement of Si atoms with Al in Cr2AIC using the minimum energy law to determine the location of the Si atoms solute. The structure and formation energy of those Cr2AlSiC models were calculated. The results show that that the lattice constant a keeps unchanged, while c decreases lineally. The decrease of c is mainly resulted from that the bond length of Cr-Si is shorter than that of Cr-Al. In addition, all of the calculated formation energies of Cr2AlSiC solid solution are negative, indicating the stability of those solid solutions. We synthesized the corresponding Cr2AlSiC solid solution, and proved the conclusion of the calculation. In addition, the lattice constants of a=2.85A, c=12.83A, which are close to the calculated values of a=2.85A, c=12.58A. The deviation between calculated and experimental values is smaller than5%, indicating the reliability of the model.
The calculation of elastic modulus, density of states, and electron density distribution of Cr2AIC and Cr2AlSiC solid solution has been further performed to analyze the performance of Cr2AIC. The result shows that the high elastic modulus and high mechanical strength results from the strong Cr-C covalent bonds via the comprehensive calculation of density of states and electron density distribution. Its excellent high-temperature oxidation resistance and corrosion resistance is due to the weak bonds between Al layers and the Cr6C octahedrons. These weak bonds make Al atoms easily escape to the surface of materials under some special conditions, which will formulate a dense Al2O3layer and protect the substrate from further oxidation and corrosion. Compared the results of Cr2AIC and Cr2AlSiC, it can be concluded that the solid solution strengthening mechanism of Cr2AlSiC is due to the extra electron of Si atom than Al atom, which increases the outermost electron bond filling and the total charge density of Cr2AIC. The above mechanism improves the bond strength, and further enhances the strength of Cr2AIC.
Based on the calculation of Cr2AIC and Cr2AlSiC solid solution, a comprehensive understanding of the relationship between their structural and characteristics has been made, and solid strengthening mechanism has been obtained. This research has guidance for designing other MAX materials.
引文
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