锆基非晶合金在过冷液相区的氧化行为

发布时间：2019-02-26 18:02

【摘要】：针对非晶合金在过冷液相区进行热塑性成形时的氧化问题,本研究分析了加热条件、表面形貌和应力状态对Zr55Cu30Al10Ni5非晶合金氧化行为的影响,讨论了其在过冷液相区的氧化机理,并评价了氧化后非晶合金样品的综合性能。通过改变加热温度和保温时间,观察在不同加热条件下非晶合金的氧化行为。根据热重曲线和等温晶化曲线,发现其氧化动力学行为遵循多级抛物线速率定律,且其动力学行为转变是由晶化引起的。氧化层表面会形成由单质Cu和CuO组成的析出颗粒,氧化层的主要成分是t-ZrO2、m-ZrO2以及A1203。随着氧化程度的加剧,氧化层结构由单层向多层转变。非晶合金在过冷液相区的氧化速率是由离子扩散控制的,Cu离子向外扩散而O离子向内扩散。通过表面磨抛处理,使非晶合金获得不同的表面形貌,分析表面形貌对氧化速率的影响。研究表明,随着磨料粒径增大,表面析出颗粒的尺寸和数量升高,而氧化层厚度也随之增加。热动力学分析证明,粗糙表面对氧化的促进作用是由表面划痕尖锐度,而不是表面粗糙度直接决定的。从扩散动力学的角度,解释了粗糙表面对非晶合金氧化的促进机理。通过在氧化过程中对样品施加恒定的拉／压载荷,分析应力状态对非晶合金氧化行为的影响。研究证实,压应力可以抑制非晶合金的氧化,而拉应力则能促进非晶合金的氧化,但是两者的作用效果都比较有限。基于对氧化前后非晶合金样品微观组织和化学成分的分析,讨论其在过冷液相区的氧化机理。在氧化早期,氧化物通过晶格扩散以树枝晶形态生长。当基体层完全晶化以后,氧化层通过晶界扩散进行生长。Cu离子持续从氧化层中的晶界处向外扩散,导致氧化层中出现相互连通的缝隙。气态O2分子通过这些缝隙进入氧化层内部,一方面继续推进氧化层的生长,另一方面也导致在已形成的氧化层中出现新的亚层结构。采用纳米压痕、纳米划痕和动电位扫描评价氧化后非晶合金样品的微观硬度、摩擦系数和耐蚀性能,分析氧化对非晶合金性能的影响。研究表明,在氧化后,氧化层和基体层的微观硬度显著提高：表面摩擦系数下降,有利于降低成形过程中的流动阻力：腐蚀电位升高,表明其耐蚀性能得到了改善。综上所述,Zr55Cu30Al10Ni5非晶合金在过冷液相区的氧化会显著影响热塑性成形产品的表面形貌、尺寸精度以及综合性能。通过控制非晶合金热塑性成形过程中温度和时间等参数,不仅能避免氧化造成的负面影响,还可能在成形的同时实现产品的强化。
[Abstract]:In this paper, the effects of heating conditions, surface morphology and stress state on the oxidation behavior of amorphous alloy Zr55Cu30Al10Ni5 in the supercooled liquid phase region were studied, and the oxidation mechanism of the amorphous alloy in the supercooled liquid phase region was discussed. The comprehensive properties of the oxidized amorphous alloy samples were evaluated. The oxidation behavior of amorphous alloys under different heating conditions was observed by changing the heating temperature and holding time. According to the thermogravimetric curves and isothermal crystallization curves, it is found that the kinetic behavior of oxidation obeys the law of multistage parabolic rate, and the transformation of kinetic behavior is caused by crystallization. Precipitated particles consisting of Cu and CuO are formed on the surface of the oxide layer. The main composition of the oxide layer is t _ 2O _ 2, mg _ 2-ZrO _ 2 and A _ 120 _ 3. With the aggravation of oxidation degree, the structure of oxide layer changes from single layer to multi-layer. The oxidation rate of amorphous alloy in the supercooled liquid phase is controlled by ion diffusion. The Cu ion diffuses outward and the O ion diffuses inward. The surface morphology of amorphous alloy was obtained by surface grinding and polishing, and the effect of surface morphology on oxidation rate was analyzed. The results show that the size and quantity of precipitated particles increase and the thickness of oxide layer increases with the increase of abrasive particle size. It is proved by thermodynamic analysis that the effect of surface roughness on oxidation is directly determined by the sharpness of the surface scratch rather than the surface roughness. From the point of view of diffusion kinetics, the mechanism of promoting oxidation of amorphous alloy by rough surface is explained. The effect of stress state on the oxidation behavior of amorphous alloy was analyzed by applying constant tension / compression load to the sample during oxidation. It is proved that compressive stress can inhibit the oxidation of amorphous alloy, while tensile stress can promote the oxidation of amorphous alloy, but the effect of both is limited. Based on the analysis of microstructure and chemical composition of amorphous alloy samples before and after oxidation, the oxidation mechanism in the supercooled liquid phase region was discussed. In the early stage of oxidation, oxide grows in dendrite form through lattice diffusion. When the substrate layer is completely crystallized, the oxide layer grows through the grain boundary diffusion. The Cu ion continues to diffuse outward from the grain boundary in the oxide layer, resulting in the formation of connected cracks in the oxide layer. Gaseous O2 molecules enter the oxide layer through these gaps, on the one hand continue to promote the growth of the oxide layer, on the other hand, lead to the formation of a new sublayer structure in the formed oxide layer. The microhardness, friction coefficient and corrosion resistance of the oxidized amorphous alloy samples were evaluated by nano indentation, nano scratch and potentiodynamic scanning, and the effect of oxidation on the properties of the amorphous alloy was analyzed. The results show that after oxidation, the microhardness of the oxide layer and the substrate layer is significantly increased, and the friction coefficient of the surface decreases, which is beneficial to the reduction of the flow resistance during the forming process. The increase of the corrosion potential indicates that the corrosion resistance of the oxide layer and the substrate layer is improved. In conclusion, the oxidation of Zr55Cu30Al10Ni5 amorphous alloy in the supercooled liquid phase will significantly affect the surface morphology, size accuracy and comprehensive properties of thermoplastic forming products. By controlling the parameters such as temperature and time during thermoplastic forming of amorphous alloy, the negative effects caused by oxidation can not only be avoided, but also the strengthening of the products may be realized at the same time.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG139.8

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