| 摘要 | MN+1AXN相是一种三元层状可加工陶瓷，因为其综合了金属和陶瓷的优异性能，例如低密度、高模量、良好的导电和导热性能、优异的抗热震性能、可加工性等，受到了广泛的关注。根据N值的不同，MAX相可分为211相、312相和413相。迄今为止已经合成40余种211相，而312相和413相则相对很少。近年来，除了对单质的研究， MAX相固溶体也受到越来越广泛的关注。可以通过固溶处理来调整MAX相的强度、硬度和抗氧化等性能。本论文的大部分内容是研究MAX相的固溶体。其一是通过固溶调整材料的力学性能，达到强化的目的；其二是通过固溶制备N > 1的固溶体，为在原子尺度调整材料的性能提供可能性。
除此之外，本论文还研究MAX相中的Ti3SiC2相。最近的研究表明，Ti3SiC2可以作为第四代核反应堆或聚变堆的候选材料，它的工作温度是800-1000℃，所以把Ti3SiC2的韧脆转变温度降到1000℃以下是能否应用的前提条件。而纳米化有可能降低陶瓷材料的韧脆转变温度，因此我们研究了如何用高能球磨加热处理的方法制备纳米Ti3SiC2，以使纳米Ti3SiC2具有良好的综合性能。
综上所述，本研究主要分为以下几个方面：
第一，制备(Ti,V)2AlC、Ti2(Al,Sn)C和(Ti,V)2(Al,Sn)C固溶体并研究其显微结构、力学性能和电学性能。研究表明，用少量的V取代Ti可以提高Ti2AlC的强度和硬度，从而达到固溶强化的目的。其原因有二：首先，V比Ti多一个价电子，多余的价电子填充到M-A的p-d杂化轨道，从而增强了原来结合最弱的Ti-Al键。其次，由于V的原子半径比Ti小，取代后的固溶体晶格常数减小，价电子的密度会增加，所以强度也会增加。用Sn取代Al以后的固溶体，其强度和硬度在取代5%时略有下降，其后随着Sn含量的增加而提高，但是提高的程度低于(Ti, V)2AlC。其原因和(Ti, V)2AlC的类似，首先，Sn比Al多一个价电子，多余的价电子填充到M-A的p-d杂化轨道，从而增强了原来结合最弱的Ti-Al键，从而提高强度。其次，由于Sn比Al的原子半径大，取代后的固溶体晶格常数比Ti2AlC的大，这样使价电子的密度减少，使强度降低。而最终的性能取决于两个因素的综合影响，因此就出现了力学性能先减小后增加的现象。除了只在一个位置取代形成固溶体以外，我们还研究了同时在M和A两个位置取代形成的(Ti, V)2(Al, Sn)C固溶体。对力学性能的影响因素同上所述，其结果是在较小的固溶度下，强度提高的程度介于(Ti, V)2AlC和Ti2(Al, Sn)C之间，随着固溶度的增加，强度要高于相同V 和Sn 固溶度下的(Ti, V)2AlC和Ti2(Al, Sn)C。
第二，制备了(V, Cr)2AlC固溶体并研究了其显微结构、力学性能和电学性能。正如第一原理计算显示的一样，在V含量为50 at%时，(V, Cr)2AlC固溶体出现了固溶强化现象。而且，固溶体的制备工艺要比V2AlC和Cr2AlC的制备工艺简单。
第三，在(V, Cr)N+1 AlC N体系中，第一次发现了(V, Cr)3 AlC 2、(V, Cr)4 AlC 3、(V, Cr)5 Al2C 3等 N > 1的固溶体，用透射电镜对固溶体进行了原子尺度的表征。N > 1固溶体的发现为通过调整N值的大小来调整V-Cr-Al-C体系MAX相的性能提供了可能。
第四，利用高能球磨加热处理的方法，制备出纳米尺寸的Ti3SiC2。研究了球磨强度、球磨时间、原料的纯净度、金属Al添加和烧结时升温速率对制备纳米Ti3SiC2的影响，并最终在较低的反应温度1050℃制备出晶粒尺寸为70 nm的Ti3Si(Al)C2。 |
| 其他摘要 | Layered-ternary ceramics MN+1AXN phases have attracted increasing attention owning to its unique combination of merits of both metals and ceramics, such as low density, high modulus, good thermal and electrical conductivity, excellent thermal shock resistance and readily machinability. According to the N values, MAX phases include M2AX (211), M3AX2 (312) and M4AX3 (413) phases. More than forty 211 phases were reported until now. But only a few kinds of 312 and 413 phases were found recently. In addition to the single phases, the solid solutions of MAX phases also attracted more and more attentions. The mechanical and oxidation properties of MAX phases could be adjusted by solid solution treatment. In this dissertation, several solid solutions were fabricated in order to strengthen MAX phases which own relative lower hardness and strength.
As a member of MAX phases, Ti3SiC2 was intended for a structure material of fusion reactor or the 4th generation fission reactor. Previous results indicated that the ductile to brittle transition temperature (DBTT) of Ti3SiC2 was 1050-1200℃. Therefore, it is necessary to reduce the DBTT to 800-1000℃ which is the application temperature of Ti3SiC2 as a nuclear material. Nanostructuration could be a promising way to meet this requirement. Thus, another part of this dissertation focused on fabricate nanostructured Ti3SiC2 by high energy ball milling and heat treatment elemental powders Ti, Si, Al, and graphite.
(Ti, V)2AlC, Ti2(Al, Sn)C, and (Ti, V)2(Al, Sn)C solid solutions were fabricated by the in situ hot pressing/solid-liquid reaction synthesis process from elemental powders of Ti, V, Al, Sn and graphite. A systemically investigation on microstructure, mechanical and electrical properties demonstrated that small amount of V and/or Sn dopping could dramatically improve hardness and strength of Ti2AlC.
(V, Cr)2AlC solid solutions were synthesized based on the fact that there is a relationship between valence electron concentration (VEC) and mechanical properties. The flexural strength and Vickers hardness reach the highest value in (V0.5Cr0.5)2AlC solid solution like theoretic prediction; moreover, the synthesis process was simplified by solid solution treatment in V-Cr-Al-C system. Except (V, Cr)2AlC solid solutions, (V, Cr)3AlC2, (V, Cr)4AlC3, and (V, Cr)5Al2C3 phases were determined at first time using x-ray diffraction and high-resolution Z-contrast imaging in V-Cr-Al-C system. The discoveries of new (V, Cr)3AlC2, (V, Cr)4AlC3, and (V, Cr)5Al2C3 phases enrich the number of crystal structures of the MAX phase classification; moreover, they further imply possibility of tailoring the mechanical and electrical properties by crystal structure control.
Nanostructured Ti3SiC2 were synthesized by high energy ball milling and heat treatment elemental powders Ti, Si, Al, and graphite. Milling intensity, milling time, purity of raw material, Al addion and heating rate were tested in order to obtain single nanostructured Ti3SiC2. Finally, Ti3Si(Al)C2 was fabricated at 1050℃ with average grain size 70 nm. |
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