【作者】 赵辉；

【导师】 杨川；

【作者基本信息】 西南交通大学 ， 材料学， 2016， 硕士

【摘要】 化合物层因其优良的力学性能和耐蚀性能得到广泛的应用,由于化合物层的厚度很薄,无法单独测试化合物层的性能。本文对Q235钢进行650℃× 3h+450℃× 2.5h+400℃×2h的低温气体多元共渗工艺得到120μm的化合物层,并对制得的超厚化合物层进行了组织分析、物相分析、显微硬度分析和纳米压痕测试、阻尼性能和磁性能测试、中性盐雾腐蚀性能和极化性能测试,以期全面系统地表征化合物层的综合性能。研究发现,经过多元共渗处理后,表面组织依次为氧化层、化合物层和扩散层,硬度大幅度提高。化合物层的主要物相为Fe2N相,内部含有少量Fe3N相,其中饱和磁化强度Ms(Fe2N)<Ms(Fe3N)<Ms(Fe),随着化合物层所占比例的增加,饱和磁化强度Ms降低。阻尼性能有所提高,阻尼系数η升高,相比原材提高了 2倍。当材料全部转化为化合物层后,其耐蚀性大幅度提升,经过900h以上的中性盐雾腐蚀试验,其保护等级仍为10级,保护等级时间是普通多元共渗处理的试样的几十倍。通过对比自腐蚀电位Ecorr、阳极塔菲尔斜率βa,多元共渗处理后,材料的腐蚀性能随着化合物层厚度比例的增加而增加,与中性盐雾腐蚀试验结果一致。通过第一性原理计算分析了化合物层的磁学性能,计算了 Fe2N相、Fe3N相和α-Fe相的磁矩,发现Fe原子磁矩依次升高,同时计算了 Fe2N相、Fe3N相和α-Fe相的电子结构,通过分析其态密度来分析各相的磁性能与磁化强度Ms规律一致。通过对比多元共渗处理后试样的拉伸数据和断裂方式,计算出化合物层的强度为990MPa。探究了多元共渗在裂纹愈合中的应用,发现125μm裂纹也可通过超厚多元共渗方法进行愈合,愈合区域硬度与基体硬度相差不大,应力集中很小,愈合后,材料的强度和塑性都大大提高,并进一步探究了裂纹愈合的机理。更多还原

【Abstract】 The compound layer formed on nitrided surface shows an extensive application due to its excellent mechanical properties and corrosion resistance.But the thickness of the compound layer is very thin,thus its properties cannot be tested exactly.In order to characterize comprehensive properties of the compound layer systematically,a thick compound layer of 120μm was produced by the low temperature gas multicomponent thermochemical treatment of 650℃×3h+450℃×2.5h+400℃×2h.The microstructure,phase,hardness,damping capacity,magnetic property,neutral salt spray corrosion and polarization performance of the compound layer were measured in this paper.After low temperature gas multicomponent thermochemical treatment,an oxide layer was formed on the treated surface followed with a compound layer and diffusion layer underneath.The hardness was improved significantly.The main phase of compound layer is Fe2N with a trace of Fe3N.With the increase of proportion of the compound layer,damping capacity was promoted while saturation magnetization exhibits an opposite tendency.The time of neutral salt spray corrosion is more than 900h under the maximum protection level of 10.By comparing the corrosion potential and the anode Tafel slope,the thicker the compound layer is,the larger the corrosion resistance obtains.The magnetic moment and property of Fe2N,Fe3N and α-Fe phase was calculated by first principles method.The results shows that the magnetic moment of Fe atoms increased with the decrease of N concentration.The electronic structure of those phases was also calculated.The magnetic properties were analyzed by density of state and the calculated results agreed well with the experimental data.Through comparing the tensile data and fracture mode of different samples,the tensile strength of compound layer with the value of 990MPa was calculated by theoretical analysis.This paper studied the application of low temperature gas multicomponent thermochemical treatment in crack healing.The crack whose width is 125μm can be healed by this treatment method.The difference of hardness between healing area and matrix is very small,which indicates the minimum stress concentration.After healing treatment,the tensile strength and plasticity increased significantly.Meanwhile,the healing mechanism was clarified.更多还原