Elastic properties of Tin+1AlCn and Tin+1AlNn MAX phases
Elastic properties of Tin+1AlCn and Tin+1AlNn MAX phases
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DOI：
10.1002/adem.200800109
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发表时间：
2008-10-01
影响因子：
3.6
通讯作者：
McKenzie, David R.
中科院分区：
文献类型：
--
作者：
Cover, Myles F.;Warschkow, Oliver;McKenzie, David R.
MAX相是一类纳米层压板材料，具有陶瓷和金属性能的独特组合。MAX相类似陶瓷，因为它们坚硬，抗氧化，并且在超过1400℃的温度下保持坚固。MAX相的金属样特性表现在它们的可加工性，抗热冲击，高损伤耐受性以及导电性和导热性。[1,2]这种独特的性能组合表明它们是苛刻操作环境的结构材料。例如，要提高发动机的效率，就需要在比目前材料允许的温度高得多的温度下运行。由于生产纯相样品的困难，对MAX相性质的实验研究受到限制。MAX相采用一般化学计量Mn+ 1AXn，其中M为早期过渡金属，A为主族元素，X为碳或氮。MAX相独特性能的关键是它们的层状结构，其中碳化物/氮化物（Mn+ 1Xn）的板状结构被A元素的单原子层分开。这些材料吸引人的部分原因是大量的元素组合和成分(n)提供了广泛可调性的前景。因此，趋势分析和理论研究对于确定尚未通过实验制备的MAX相非常重要，并且还提供了对潜在结构/性质关系的基本理解。弹性性能是特别感兴趣的，因为它们是宏观性能的基础，如润滑、摩擦和可加工性。在这项工作中，我们使用第一性原理密度泛函理论计算来检查Tin+ 1AlXn （X= N， C）系列的弹性性能作为N的函数，目的是确定结合良好可加工性和高刚度的最佳成分。到目前为止，大多数弹性常数的趋势分析都局限于M2AX化学计量学（称为211相）[3-20]，大多数作者将他们的讨论局限于体积模量。只有几个例子，其中体积模量已经计算跨化学计量[5,19]，而其他弹性常数的化学计量依赖尚未被检查。这里我们考虑了Tin+ 1AlXn体系，并报告了n到4的化学计量量的弹性常数和模量。Mn+ 1AXn相的晶体结构如图1所示，显示了沿六边形晶格的c轴堆叠的M、A和X原子的单个原子层。一般的模式是一系列不同厚度的MX板（取决于n），它们被单层的a元素分开。MX板采用fcc型堆叠顺序（abcabc），每三层具有相同的面内位置。A元素周围的局部环境呈现hcp堆叠序列（aba），每二层重复；因此，A原子层在晶体中形成镜面。使用下标a， b， c表示平面内位置，前三个MAX相化学计量（n= 1,2,3）具有以下循环堆叠顺序的特征：
The MAX phases are a class of nano-laminate materials with a unique combination of ceramic and metallic properties. MAX phases mimic ceramics in that they are stiff, resistant to oxidation, and remain strong at temperatures exceeding 1400 C. The metal-like properties of MAX phases manifest themselves in their machinability, resistance to thermal shock, high damage tolerance, and electrical and thermal conductivity.[1, 2] This unique combination of properties suggests them as structural materials for demanding operating environments. Increasing the efficiency of engines, for example, requires operation at much higher temperatures than allowed by today’s materials. Experimental investigation of the properties of the MAX phases is limited by difficulties associated with producing phase pure samples. The MAX phases adopt the general stoichiometry Mn+ 1AXn, where M is an early transition metal, A is a main group element, and X is either carbon or nitrogen. Key to the unique properties of MAX phases is their laminate structure in which slabs of the carbide/nitride (Mn+ 1Xn) are separated by single atomic layers of the A element. Part of the attraction of these materials is the large number of elemental combinations and compositions (n) offering the prospect of a wide tunability of properties. Trend analyses and theoretical studies are therefore important to identify MAX phases that have not yet been prepared experimentally, and also provide a fundamental understanding of the underlying structure/property relationships. Elastic properties are of particular interest as they underpin macroscopic properties such as lubrication, friction, and machinability. In this work we use first principles density functional theory calculations to examine the elastic properties of the Tin+ 1AlXn (X= N, C) series as a function of n with the aim of identifying optimum compositions that combine good machinability with high stiffness. Thus far, most trend analyses of elastic constants have been confined to the M2AX stoichiometry (referred to as 211 phases)[3–20] with most authors limiting their discussion to the bulk modulus. There are only a few examples, where bulk moduli have been calculated across stoichiometries,[5, 19] while the stoichiometry dependence of other elastic constants has not yet been examined. Here we consider the Tin+ 1AlXn system and report calculated elastic constants and moduli for the stoichiometries n to 4. The crystal structures of the Mn+ 1AXn phases are illustrated in Figure 1, showing individual atomic layers of M, A, and X atoms stacked along the c-axis of a hexagonal lattice. The general pattern is a sequence of MX slabs of varying thickness (depending on n) which are separated by single layers of the A element. The MX slabs adopt a fcc-type stacking sequence (abcabc) with every third layer having the same in-plane position. The local environment around the A elements exhibits a hcp stacking sequence (aba) with every second layer repeating; thus, the layer of A atoms forms a mirror plane in the crystal. Using subscripts a, b, c to denote in-plane positions, the first three MAX phase stoichiometries (n= 1, 2, 3) are characterized by the following cyclic stacking sequences: