晶粒细化和氧化物掺杂影响铝化物涂层热生长Al2O3相转变的本质原因

Al2O3-forming coatings have been most extensively applied for the metallic materials against high tmeperature oxidaiton. However, the oxidation behavior of these coatings is heavily affected by the process of the phase transformation of the formed scale of Al2O3 from the metastable phases as gamma and theta to the stable alpha phase.In the program proposed, various low-temperature aluminized Ni2Al3 coating system with different grain size level and containing and not containing the dispersed fine particles of an oxide (which has the hexagonal close-packed crystal structure being same to or different from that of alpha-Al2O3) will be developed based on our previous works. The formation and evolution of the metastable Al2O3 on the aluminide coatings oxidized for different time in air at temperatures 1000 to 1100oC will be investigated mainly by using analytical transmission electron microscope (TEM). The study will deeply reveal the fundamental mechanisms for the effect on the Al2O3 phase transformation by the grain refinement and various oxide dispersion and their interaction on the promotion in the growth of alpha-Al2O3, and establish the relationship of the grain refinement and oxide dispersion with the Al2O3 phase transformation kinetics and oxidation kinetics of the aluminide coating system. The result will provide powerful expermental evidences and useful theoretical guidance to increase the resistance of the Al2O3-forming coatings to high temperature oxidation through optimization in the composition and structure of the coatings.

Al2O3热生长型的高温涂层最广泛应用，Al2O3氧化层在高温环境下由亚稳态相(如gamma和thata)向稳态相（alpha）转变快慢显著影响涂层的氧化行为。本项目，申请人在前期工作的基础上制备晶粒度等级可控、有和没有氧化物弥散的Ni2Al3低温渗铝涂层，通过表征和分析不同晶粒度和不同氧化物（与稳态alpha相晶体学结构相同和不同）掺杂条件下的铝化物涂层的亚稳态Al2O3相的生长和演化过程，揭示晶粒细化与氧化物掺杂影响Al2O3相转变的内在机制以及它们促进稳态alpha相生长的协同效应，建立"晶粒细化+ 氧化物弥散－相转变动力学－氧化动力学"之间的本质联系，为Al2O3热生长型的高温涂层通过结构和成分优化改善氧化行为奠定实验和理论基础。

通过表征和分析不同晶粒度和不同氧化物（与稳态alpha相晶体学结构相同和不同）掺杂条件下的铝化物涂层的亚稳态Al2O3相的生长和演化过程，发现，铝化物涂层晶粒度从微米细化至亚微米后，其Al2O3氧化膜中gamma相明显减少，主要为alpha相，说明晶粒细化促进了后者的形成。进一步研究表明，晶粒细化导致初期生长的非稳态Al2O3膜中的缺陷（孪晶与层错）浓度高，这有利于此膜通过“切变-扩散”方式向alpha-Al2O3转变。另外，还发现晶粒细化促进alpha-Al2O3膜的生长可推广至TiAl合金。铝化物涂层弥散与alpha-Al2O3同晶型的Cr2O3能促进alpha-Al2O3的生长，同时，发现铝化物涂层表面预置Cr2O3纳米颗粒，可促进直接生长alpha-Al2O3，并提出了同型结构催化模型；铝化物涂层表面预置具有金红石结构的TiO2颗粒，也可直接促进alpha-Al2O3的生长，通过对微观结构的高分辨电镜（HREM）以及第一性原理模拟分析，证实TiO2具有与alpha-Al2O3相似的O的八面体亚点阵结构，这是促进alpha-Al2O3直接生长的根本原因。发现CeO2颗粒弥散对铝化物涂层的相转变无明显影响，但对Al2O3膜生长的扩散过程有影响。研究结果澄清了晶粒细化与氧化物掺杂影响Al2O3相转变的内在机制以及它们促进稳态alpha相生长的协同效应，为Al2O3热生长型的高温涂层通过结构和成分优化改善氧化行为奠定实验和理论基础。