基于纳米量热技术的金属微滴和非晶颗粒极端非平衡相变研究

发布时间：2018-07-03 13:39

本文选题：纳米量热 + 金属微滴　；参考：《上海大学》2017年博士论文

【摘要】：随着现代社会的发展和科学技术的进步,金属材料在极端非平衡条件下的相变越来越引起人们的重视。纳米量热作为一种新兴的量热技术,具有高达107 K/s的加热冷却速度,可以原位获取金属材料在极端非平衡条件下的相变行为。相对于传统热分析技术,其热容灵敏度在纳米量级,可以检测到微弱的相变信息。除热力学和动力学外,纳米量热结合其它表征手段,可以对相变进行全方面的研究。这不仅有助于揭示极端非平衡相变的内部机理,还可以为新材料开发和改性提供直观的实验证据。本文在前期非平衡形核研究的基础上,利用Sn基合金微滴和Ce_68Al_10Cu_20Co_2(at.%)大块非晶在极端冷却和加热条件下的凝固、玻璃化转变、晶化以及熔化现象进行了系统研究差示快速扫描量热仪(Differential fast scanning calorimetry,DFSC)实现了单个Sn基合金微滴在10~4 K/s级别的原位冷却,凝固组织得以细化,析出相分布均匀。在Sn3.5Ag微滴中,15000 K/s的冷却速度可以获得0.22 Tm的过冷度。借助于聚焦离子束(Focused ion beam,FIB)和高分辨透射电子显微镜(High resolution transmission electron microscopy,HRTEM),成功对其凝固组织进行了表征。在此条件下,微滴析出相都在100 nm以下,没有大块针状Ag_3Sn的形成。快速凝固中,b-Sn基体和Ag_3Sn之间形成纳米扩散偶并存在高达10~9/m的浓度梯度,可以降低形核驱动力,提高过冷熔体的稳定性,形成SnAg非晶。在Sn_3.0Ag_0.5Cu微滴中,通过对20000 K/s冷却速度下“冻结”的不同尺寸、不同形貌析出相的表征,阐明了Ag_3Sn的生长机理。结果表明,晶粒内部Ag_3Sn形成初期为球形,其((?)02)晶面作为择优生长面沿[(?)03]方向生长,变为棒状。但是受限于生长时间,尺寸仍在纳米级别。纳米量热实现了Ce_68Al_10Cu_20Co_2大块非晶从0.083 K/s到14000 K/s加热速度下的晶化动力学研究。加热速度增加,晶化温度升高,晶化激活能降低,传统Kissinger方程不再适用。通过计算玻璃化转变温度和熔点之间的晶体生长速度,发现了低温熔体中生长速度和粘性流动发的去耦合,即不再遵循Stokes-Einstein方程,而Ediger关系可以在低温段表述此变化,即Dμη~(-0.865)。结合经典形核理论可以获得Ce_68Al_10Cu_20Co_2过冷熔体的形核率,以此判断加热和冷却过程中晶化行为的不对称性,即加热晶化由生长控制,冷却晶化由形核控制。由此进一步排除Ce晶体首先析出的可能性。随着加热速度的提高,Ce_68Al_10Cu_20Co_2晶化路径发生变化,因此普通差示扫描量热仪(Differential scanning calorimetry,DSC)和快速加热下的晶化产物差异明显。在Ce_68Al_10Cu_20Co_2金属玻璃中,以Al原子为中心的二十面体为基本原子结构。相对于直接形成晶体,亚稳的Al_13Co_4准晶与非晶具有更高的结构相似性,因此会首先析出,作为金属玻璃微结构和最终晶化产物的过渡相。纳米量热设备极高的冷却速度可以实现Ce_68Al_10Cu_20Co_2非晶的原位制备。当冷却速度从100 K/s增大到50000 K/s,先后获得晶体、晶体-非晶混合组织以及完全非晶三种不同凝固组织。通过后续再加热可以确定凝固组织微结构对玻璃化转变、晶化以及熔化的影响。研究表明,金属玻璃形成由笼统地抑制晶化进一步细分为形核和晶体生长两方面。具体而言,10000 K/s的冷却速度可以抑制冷却过程中晶化的发生,即传统意义上的临界冷却速度。而在50000 K/s的冷却速度下,不仅晶化被完全抑制,均匀形核的发生也被限制。原位冷却形成的Ce_68Al_10Cu_20Co_2非晶在玻璃化转变温度附近退火10-3-10~4 s,实现了微结构的有序化转变。通过计算再加热过程中的晶化焓和整体潜热,实现了对等温形核和晶化动力学的定量分析,结合结构表征,证实了Ce_68Al_10Cu_20Co_2的二次晶化机制。在等温退火过程中产生的有序团簇表现出了明显的尺寸效应,形成低温熔化峰。玻璃态的晶化由均匀形核和非均匀形核共同作用,而过冷熔体中,异质形核成为影响晶化的主要因素。当非晶中形成大量纳米晶,二者界面会存在高致密度的原子团簇,可从整体上提高残余非晶的玻璃化转变温度,即提高其动力学稳定性。
[Abstract]:With the development of modern society and the progress of science and technology, the phase transition of metal materials in extreme non equilibrium conditions has attracted more and more attention. As a new calorimetric technology, nano calorimetry has a heating cooling rate of up to 107 K/s, and the phase transition behavior of metal materials in the extreme non equilibrium condition can be obtained in situ. In the traditional thermal analysis technology, its heat capacity sensitivity is at the nanometer scale, and the weak phase change information can be detected. In addition to thermodynamics and dynamics, nano calorimetry combined with other characterization methods can make full research on the phase transition. This not only helps to reveal the internal mechanism of the extreme non equilibrium phase transition, but also can be developed and modified for the new material. On the basis of the previous nonequilibrium nucleation research, this paper systematically studies the differential rapid scanning calorimeter (Differential fast scanning) using Sn based alloy micro droplets and Ce_68Al_10Cu_20Co_2 (at.%) bulk amorphous alloy under extreme cooling and heating conditions, glass transition, crystallization and melting phenomena. Calorimetry, DFSC) realized the in-situ cooling of a single Sn based alloy microdrop at the 10~4 K/s level. The solidification structure was refined and the precipitation phase was uniformly distributed. In the Sn3.5Ag microdrop, the cooling rate of 15000 K/s could be obtained by the cooling rate of 0.22 Tm. N transmission electron microscopy, HRTEM), the solidification structure has been successfully characterized. Under this condition, the precipitates of the microdroplets are below 100 nm, and no large acicular Ag_3Sn is formed. In the rapid solidification, the nano diffusion couple between the b-Sn matrix and Ag_3Sn is formed and the concentration gradient of up to 10~9/m can be found, which can reduce the driving force of the nucleation. The stability of the supercooled melt formed SnAg amorphous. In the Sn_3.0Ag_0.5Cu droplet, the growth mechanism of Ag_3Sn was clarified by the characterization of the different sizes of freezing at 20000 K/s cooling rate and the precipitation of different morphologies. The results showed that the formation of Ag_3Sn in the grain was spherical at the initial stage, and ((?) 02) as the preferred growth surface along [(?) 03] direction] The crystallization kinetics of Ce_68Al_10Cu_20Co_2 bulk amorphous from 0.083 K/s to 14000 K/s was studied. The heating rate increased, the crystallization temperature increased, the crystallization activation energy decreased, and the traditional Kissinger equation was no longer applicable. The crystal growth rate between the glass transition temperature and the melting point has been found. The decoupling of the growth speed and the viscous flow in the melt is found, that is, the Stokes-Einstein equation is no longer followed, and the Ediger relationship can be expressed at the low temperature section, namely, D UA ~ (-0.865). The form of the Ce_68Al_10Cu_20Co_2 supercooled melt can be obtained by combining the classical nucleation theory. The nucleation rate is used to determine the asymmetry of crystallization behavior during heating and cooling, that is, the heating crystallization is controlled by the growth and the cooling crystallization is controlled by the nucleation. Thus the possibility of the first precipitation of the Ce crystal is eliminated. With the increase of the heating speed, the crystallization path of the Ce_68Al_10Cu_20Co_2 is changed, so the common differential scanning calorimeter (Different The crystallization products of the ial scanning calorimetry, DSC) and the rapid heating are distinct. In the Ce_68Al_10Cu_20Co_2 metal glass, the twenty surface body centered on the Al atom is the basic atomic structure. Compared with the direct formation of the crystal, the metastable Al_13Co_4 quasicrystal has a higher structural similarity with the amorphous, so it will first precipitate, as a metal glass. The transition phase of the glass microstructures and the final crystallization products. The high cooling rate of the nano calorimeter can be prepared in situ of the Ce_68Al_10Cu_20Co_2 amorphous. When the cooling rate is increased from 100 K/s to 50000 K/s, the crystal, crystal - amorphous and completely amorphous three different solidification structures are obtained. The subsequent reheating can be confirmed. The effects of solidification microstructure on glass transition, crystallization and melting are determined. The study shows that the formation of metallic glass is further subdivided into two aspects: nucleation and crystal growth. Specifically, the cooling rate of 10000 K/s can inhibit the crystallization of the crystal during the cooling process, that is, the critical cooling rate in the traditional sense. At the cooling rate of 50000 K/s, not only the crystallization is completely suppressed, but the homogeneous nucleation is also restricted. The Ce_68Al_10Cu_20Co_2 amorphous formed by the in-situ cooling is annealed 10-3-10~4 s near the glass transition temperature, which realizes the ordering transformation of the microstructures. The quantitative analysis of temperature nucleation and crystallization kinetics, combined with structural characterization, confirmed the two crystallization mechanism of Ce_68Al_10Cu_20Co_2. The ordered clusters produced in the process of isothermal annealing showed a significant size effect and formed a melting peak at low temperature. The crystallization of the glass state was combined with homogeneous nucleation and non-uniform nucleation, while the supercooled melt was different. The mass nucleation is the main factor affecting the crystallization. When a large number of nanocrystals are formed in the amorphous, there will be a high density cluster of atoms in the two interface, which can improve the glass transition temperature of the residual amorphous from the whole, that is, to improve its dynamic stability.
【学位授予单位】：上海大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG111

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