【作者】 李伟；

【导师】 戎咏华；

【作者基本信息】 上海交通大学 ， 材料物理与化学， 2008， 博士

【摘要】 表面机械研磨处理(SMAT),作为一种利用剧烈塑性变形制备纳米材料的新技术,被广泛用于制备各种纳米晶金属材料,以期获得与传统粗晶材料不同的性能和行为。本文利用SMAT制备出Fe-30wt.%Ni合金(fcc)、纯Fe(bcc)、纯Ni(fcc)和纯Co(hcp)不同晶体结构的纳米晶金属材料,通过光学金相(OM)、X射线衍射(XRD)、透射电镜(TEM)、扫描电镜(SEM)、能谱仪(EDS)和差示扫描量热法(DSC)等多种表征方法,研究它们的微结构参数、热稳定性以及马氏体逆相变行为,为SMAT制备的纳米金属材料的实际应用提供理论指导,而且丰富了纳米材料中马氏体相变的研究内容。Fe-30wt.%Ni合金、纯Ni、纯Fe和纯Co经过表面机械研磨处理后,表面均发生剧烈塑性变形,随着深度的增加,晶粒尺寸和微观应变随深度呈梯度变化,并可获得10～20μm的纳米晶表层,根据TEM观测,最表层的晶粒尺寸小于10nm。用X射线单峰傅氏分析法对表面机械研磨处理后Fe-30wt.%Ni合金、纯Ni和纯Fe的微结构参数进行定量测定,结果表明材料在表面机械研磨处理后,晶粒尺寸迅速减小到纳米级,并可得到高的微观应变、位错密度和形变储存能,其中位错密度和形变储存能的数量级为1015-1016 m-2和106-107 Jm-3,均比拉伸变形样品的位错密度和储存能高一个数量级。通过对表面机械研磨处理后纳米晶Fe-30wt.%Ni合金的HRTEM观察表明,某些纳米晶粒内部包含一些由小角晶界分隔的亚晶粒,通过对HRTEM照片的一维傅氏变换可知纳米晶界和亚晶界上分布有高密度的位错,而亚晶粒内部位错密度较低。对纳米晶Fe-30wt.%Ni合金和纯Ni的热稳定进行了研究,两种材料在不同的退火温度下进行等温退火处理,用X射线衍射单峰近似函数计算了不同退火温度和时间下的纳米晶粒尺寸和微观应变,两种材料具有共同的晶粒生长特征。晶粒尺寸均在退火初期(前15min)生长速率较快,退火温度越高,退火初期的生长速率越快,退火后期(15min～120min)晶粒生长的速率变慢。微观应变均在退火的前15min左右下降较快,随后基本到达一个极限值,退火温度越高,到达的极限值越低。通过对纳米晶Fe-30wt.%Ni合金马氏体的原位加热TEM观察显示,亚晶粒的合并是退火初期晶粒尺寸快速增加的原因。对纳米晶Fe-30wt.%Ni合金和纯Ni晶粒生长动力学参数进行了计算,结果表明纳米晶Fe-30wt.%Ni合金和纯Ni的晶粒生长时间指数n分别约为0.1和0.14,说明纳米晶Fe-30wt.%Ni合金具有比纯Ni更慢的晶粒生长速率。在较低的温度下退火时(纯Ni低于250℃,Fe-30wt.%Ni合金低于350℃),两者的晶粒生长激活能均在30～40kJ/mol,说明晶粒生长均受晶界的重排导致的晶粒合并所控制。在较高的温度下退火时(纯Ni为250℃～450℃,Fe-30wt.%Ni合金为350℃～550℃),Fe-30wt.%Ni合金和纯Ni的晶粒生长激活能分别为176.8kJ/mol和121.3kJ/mol,分别对应于两种材料的晶界扩散激活能,这说明晶粒生长主要由晶界扩散所控制。两种材料在较高温度下退火均会出现某些晶粒的异常生长,这种现象是由晶粒尺寸分布的不均匀造成的。通过对文献中纳米材料逆相变试验和理论的分析,对课题组前期工作提出的纳米晶Fe-30wt.%Ni合金和Co金属马氏体逆相变行为基于晶粒尺寸效应的解释提出置疑。本文考虑了马氏体和奥氏体两相表面能的差异,建立了纳米晶马氏体逆相变的热力学表达式,通过在热力学表达式中加入形变储存能的影响,计算和分析了表面机械研磨处理制备的纳米晶Fe-30wt.%Ni合金和Co金属的马氏体逆相变的行为,计算的结果预示出两种材料的马氏体逆相变开始温度(As)应低于或接近对应的传统粗晶材料。DSC实验结果显示,表面机械研磨处理制备的纳米晶Fe-30wt.%Ni合金的As温度高于对应的粗晶样品,而纳米晶Co的As温度低于对应的粗晶样品。该实验结果与课题组前期实验工作一致,但Fe-30wt.%Ni合金的实验结果与理论预测相反,理论分析发现可能在SMAT处理中引起成分的变化而导致化学自由能的变化,实验进一步证明了在机械研磨过程中钢球中合金元素确实扩散入Fe-30wt.%Ni合金和Co金属,并导致表层成分的变化,从而使Fe-30wt.%Ni合金和Co金属的As温度不同于传统粗晶样品。通过磨去表层的合金元素扩散层,获得不受成分影响的纳米晶Fe-30wt.%Ni合金和Co金属,DSC实验结果显示两种材料的As温度均与对应粗晶样品接近,这与热力学模型的预测结果相符。在表面机械研磨处理的过程中,合金元素可在钢球和被处理样品之间发生互扩散,这种互扩散使Fe-30wt.%Ni合金和Co金属纳米晶表层的饱和磁化强度和居里温度等磁性能显著不同于相应的传统粗晶样品,由此预示出表面机械研磨处理不仅是一种表面纳米化技术,还是一种有效的表面改性方法,因此具有潜在的应用价值。更多还原

【Abstract】 Surface mechanical attrition treatment (SMAT), as a new technique producing nanocrystalline materials by severe plastic deformation, has been used to produce nanocrystalline surface layer in a variety of metallic materials, aiming at acquiring the properties and behaviors different from their coarse-grained counterparts. By means of SMAT, the different nanocrystalline surface layers of Fe-30wt.%Ni alloy(fcc), pure Ni(fcc), Fe(bcc) and Co(hcp) metals are obtained. By using optical microscopy (OM), X-ray diffraction (XRD), transmission electron microscopy (TEM) attached energy dispersive spectrometer (EDS), scanning electron microscopy (SEM) attached EDS and differential scanning calorimetry (DSC), the microstructural parameters, thermal stability and reversal martensitic transformation of nanocrystalline materials are investigated, which provides the theoretical instruction for the application of nanocrystalline materials produced by SMAT and enriches the research on reversal martensitic transformation of nanocrystalline materials.During SMAT, severe plastic deformation occurs in the surface layer of Fe-30wt.%Ni alloy, pure Ni, Fe and Co metals. The grain size and microstrain present the gradient distribution with the increase of the depth. Nanocrystalline surface layers are determined as 10～20μm depth. Based on the TEM observation, the average grain sizes in the top layer decrease to less than 10nm. By means of XRD single-peak fourier analysis, the microstructural parameters of Fe-30wt.%Ni alloy, pure Ni and Fe metals are quantitatively measured. The results show that the grain size drops rapidly into the nanometer scale, and high values of root mean square (r.m.s.) microstrain, dislocation density and stored elastic energy are gained after SMAT. For example, the orders of magnitude for dislocation density and stored elastic energy are as high as 1015-1016m-2 and 106-107 Jm-3, which both exceed by one order of magnitude value in the tensile-deformed counterparts. The high resolution TEM (HRTEM) images of nanocrystalline Fe-30wt.%Ni alloy show that nanosized grain consists of some subgrains separated by low-angle grain boundary, and a large number of dislocations distribute at grain and subgrain boundaries, while few dislocations distribute in the subgrain interior, which makes grain and subgrain boundaries be in high-energy and non-equilibrium state.The thermal stability of nanocrystalline Fe-30wt.%Ni alloy and pure Ni are studied by annealed at different temperatures for different times. By using the the single line approximation analysis, the grain size and microstrain of different samples are determined. The results show that two kinds of nanocrystalline materials have common features of grain growth as follows. The grain size increases rapidly within the early stage of annealing (～15min), while it becomes slow during sequent annealing time (15min～120min). The higher the annealing temperature is, the faster grains grow at the early stage of annealing. The microstrain decreases rapidly within the first 15nm of annealing and decreases slowly during sequent annealing time. The higher the annealing temperature is, the lower the value of microstrain drops. Through the in-situ TEM observation, the incorporation of subgrains may be the main reason for the initially rapid grain growth. By the measurement of grain growth kinetics parameters of nanocrystalline Fe-30wt%Ni alloy and pure Ni, the value of time exponent, n, of Fe-30wt%Ni alloy and pure Ni are 0.1 and 0.14, respectively, indicating that the grain growth rate of Fe-30wt%Ni alloy is slower than that of pure Ni. When annealed in the low temperature (pure Ni:～250℃, Fe-30wt.%Ni alloy:～350℃), the activation energy, Q, of Fe-30wt%Ni alloy and pure Ni are both 30～40kJ/mol, suggesting that the grain growth is governed by incorporation of subgrains undergoing the rearrangement of the grain boundaries. When annealed in the comparatively high temperature (pure Ni: 250℃～450℃, Fe-30wt.%Ni alloy: 350 ℃～550℃), the activation energy, Q, of Fe-30wt%Ni alloy and pure Ni are 176.8kJ/mol and 121.3kJ/mol, suggesting that the grain growth is governed by the grain boundary diffusion. Abnormal grain growth are both observed during annealing of Fe-30wt%Ni alloy and pure Ni, which can be attributed to the non-uniformity of the grain size distribution.Based on the experimental and theoretical analysis on reversal martensitic transformation of nanocrystalline materials reported in the literatures, some errors are corrected in the explanation of the reversal martensitic transformation of nanocrystalline Fe-30wt.%Ni alloy and Co previously suggested by our research group. Considering the difference of surface energies of martensite and austenite, a thermodynamic expression of the reversal martensitic transformation in nanocrystalline materials is established and is used for SMAT nanocrystalline Fe-30wt.%Ni alloy and Co by the addition of the store energy term. The theoretical calculation and analysis predict that the start temperatures of reversal martensitic transformation, As, of nanocrystalline Fe-30wt.%Ni alloy and Co are both lower than or close to those of their coarse-grained counterparts. The experimental results from DSC show that As of nanocrystalline Fe-30wt.%Ni alloy is higher than that of conventional coarse-grained alloy, while As of nanocrystalline Co metal is lower than that of coarse-grained Co, which are consistent with previous experimental results in our group. However, the experimental result of Fe-30wt.%Ni alloy is contrary to the prediction from the thermodynamic expression in this paper. The theoretical analysis suggests that the chemical free energy change resulting from the composition deviation during SMAT may be responsible for the increase of As. The further experimental results show that alloying elements do diffuse from steel balls into the Fe-30wt.%Ni alloy and Co metal during SMAT, leading to the composition deviation of surface layer from their original compositions, in turn resulting in the different As from the conventional coarse-grained samples. By removing the surface layer of nanocrystalline Fe-30wt.%Ni alloy and Co with 5μm thickness, the effect of diffusion of alloying elements on As are eliminated. The DSC results show that As temperatures of nanocrystalline Fe-30wt.%Ni alloy and Co are very close to those of their coarse-grained samples, which agree with the predicted results from the thermodynamic expressions.The diffusion of alloying elements can occur between steel balls and the treated sample during SMAT. The diffusion of alloying elements leads to the remarkably different saturation magnetization and Curie temperature of nanocrystalline Fe-30wt.%Ni alloy and Co from their conventional coarse-grained counterparts, suggesting that SMAT is not only a surface nanocrystallization technology, but also an effective alloying method for surface modification and thus has potential application in practice.更多还原