晶粒细化对金属氧化膜的力学行为的影响

Grain refinement helps to alloy metals to thermally grow oxide scales with greatly enhanced adhesion. The fundamental reason, however,remains to be clarifed. In the program proposed, pure Ni samples with the mean grains of micron-, submicron- and nano-size, respectively, will be developed. The mechanical behavior of the NiO scales on the different grain-sized Ni during oxidation in air at temperatures of 800-1000 oC and during cooling will be investigated and compared using the deflection apparatus which has been developed based on the principle of the one-side deflection in combination with an optical(laser beam) cantilever technique, together with both the analysis of the scaling kinetics using thermal balance and the microstructrual investigation of the formed scale,the underlying Ni substrate and interface between the scale and the substrate using scanning electron micorscope (SEM) and transmission electron microscope (TEM). The study will elucidate the variation of the mechanical behavior of the oxide scale with the grain refinement of the Ni substrate，and establish the relationship of the Ni grain refinement with the internal stresses and scaling kinetics of the oxide scale formed and microstructure of the oxide scale and the scale/Ni interface. The result will deeply disclose the effect of the Ni grain refinement on the generation and relaxation mechanism of the internal stresses in the formed oxide scale. It will provide credible experiemtal evidences and useful theoretical guidance to the tailoring of grain size for thermally forming a highly adhesive oxide scale on metals.

晶粒细化能提高金属氧化膜的粘附性，其原因尚不清楚。本项目，用"单面弯曲+光杆法" 研究微米、亚微米和纳米晶等级的金属Ni镍分别在800-1000oC空气中氧化和冷却过程的力学行为、并辅以相应的氧化膜生长动力学，氧化膜、金属基体及两者界面结构的显微分析，阐明晶粒细化影响氧化膜力学行为的规律，建立"晶粒细化-氧化膜内应力（生长应力和热应力）-氧化膜生长动力学-氧化膜及膜/基体金属界面结构"之间的联系，深入揭示晶粒细化影响氧化膜内应力产生和释放机制；为金属材料热生长"高粘"氧化膜而进行晶粒细化提供实验数据和理论指导。

利用“光杆”原理改进的“氧化弯曲”法，通过比较粗晶Ni和细晶Ni在升温至800oC、恒温以及随后降温过程中的挠度变化，表明Ni晶粒细化尤其纳米晶化能致使NiO膜生长应力降低，但热应力增大；进一步在细晶Ni中掺杂CeO2纳米颗粒发现，氧化时NiO膜生长应力增大、但冷却时膜内热应力降低。通过TEM和SEM等分别对不同晶粒度的Ni及掺杂CeO2颗粒后热生长NiO膜在氧化过程中的结构演化进行分析，解释了Ni纳米晶化以及CeO2掺杂影响NiO膜力学行为的根本原因，即Ni纳米晶化通过促进Ni2+在NiO膜中的外扩散降低膜的横向生长和致密度而降低生长应力，通过形成晶粒较粗大的NiO膜使其在冷却过程中不易蠕变而增大热应力，纳米晶Ni掺杂CeO2后，可通过抑制占主导的Ni2+扩散而促进NiO膜的横向生长而增大生长应力，通过细化NiO膜晶粒促使膜在冷却时易蠕变而降低热应力。研究结果澄清了晶粒细化以及稀土氧化物掺杂对金属氧化力学行为的影响，为金属材料热生长“高粘”氧化膜而进行晶粒细化提供了理论和实验依据。