【作者】 曾桥石；

【导师】 蒋建中； 毛河光；

【作者基本信息】 浙江大学 ， 材料学， 2010， 博士

【摘要】 金属玻璃,作为非晶态玻璃家族的一个“新”成员(几十年历史),一方面,由于它优异的机械、磁学、电化学等性质而具有诱人的应用价值和前景；另一方面,它独特的无方向性金属键“无序”密堆结构,为包括玻璃结构、玻璃形成和熔体过冷液行为在内的一系列非平衡凝聚态领域内未解决的重大基础问题的研究提供了独特的模型体系和研究机遇。因此,近几十年来金属玻璃一直是材料科学研究的热点前沿。关于物质相变的研究直接关联到物质结构以及物质应用中的性能调制。“非晶多形态转变(相变)”在许多传统非晶态物质中的发现和深入研究,促进了人们对非晶态物质的认识。通常认为这种非晶多形态转变现象与其相应液体结构的起伏有关,对应着方向性键合的开放网络结构特性(低配位,一般小于6)。所以,人们认为在具有无方向性金属键密堆结构的金属玻璃中不太可能会存在非晶多形态现象。本论文的研究工作是以铈基稀土金属玻璃为模型体系,综合利用多种高压技术,发现了铈基金属玻璃中压力诱导的非晶多形态转变现象及其机理；研究了与金属玻璃的非晶多形态转变相关的各种物理性质的变化；探索了利用非晶多形态金属玻璃作为前驱体的新材料合成方法、机理以及新材料的结构形态。本论文得到的主要结果如下：(1)采用原位高压同步辐射X射线衍射技术研究了Ce32La32Al16Ni5Cu15大块金属玻璃的压缩行为,首次在大块金属玻璃中发现了从一个非晶态到另一个非晶态的非晶多形态转变现象。利用原位高压同步辐射X射线衍射技术,在液氦作为传压介质的极佳等静压条件下,研究了含有高浓度铈元素(Ce)的简单二元Ce75Al25金属玻璃的压缩行为,在Ce75Al25金属玻璃中进一步确认了一个从低密度非晶态到高密度非晶态的体积塌缩型非晶多形态转变现象。同时,采用原位高压Ce-L3边同步辐射X射线吸收谱技术,第一次直接从实验上揭示了含Ce金属玻璃中4f电子非局域化转变诱导其非晶多形态转变的机理。这种电子性质的非晶多形态转变与传统玻璃物质在压力下发生的由于配位数增加而导致的结构重排调整的非晶多形态转变现象完全不同,展示了一种独特的新型非晶多形态转变机理,拓展了无序体系中非晶多形态现象的研究。此外,Ce75Al25金属玻璃的结构因子中的预峰在压力下表现了奇特的难压缩性特征。这个现象可能促进我们对于与预峰相关的玻璃结构的认识。(2)通过原位高压高温同步辐射x射线衍射、常压及原位高压低温电、磁测量技术,研究了低密度非晶态和高密度非晶态Ce75Al25金属玻璃的热稳定性和电、磁性质及两种玻璃态之间性质的差异,通过与不含4f电子的La75Al25金属玻璃样品进行对比,再次确认了伴随非晶多形态转变而发生的物理性质突变的4f电子机制。据我们所知,本论文第一次在金属玻璃中观察到了压力调制的电阻温度系数从负到正的转变,以及成分和磁场调制的磁阻从正到负的转变。这些发现可能促进金属玻璃非晶多形态现象的应用,同时也可能为描述凝聚态物质中4f电子行为的理论研究提供特殊而有趣的模型体系。另外,在我国新建成的上海同步辐射微聚焦硬x射线光束站,我们第一次成功地开展了室温下的原位高压x射线衍射实验,发现了在Ce75Al23Si2金属玻璃中的高压诱导非晶多形态现象；揭示了微量元素掺杂对Ce75Al25金属玻璃的非晶多形态转变及其物理性质的影响。(3)利用高压同步辐射x射线衍射技术,我们进一步研究了高密度非晶态的Ce75Al25金属玻璃的高压行为,发现把它加压到25 GPa左右时,突然晶化形成了一种新型的fcc-Ce3Al取代型固溶体合金。根据休谟-饶塞里定律,Ce和Al元素在常温常压下,原子半径约差28%、电负性约差0.45,它们之间不可能形成固溶体合金。综合利用原位高压同步辐射x射线衍射、原位高压同步辐射x射线吸收谱以及第一性原理计算技术,我们发现是4f电子从局域态到巡游态的转变导致了Ce和Al的原子半径和电负性差异在高压下变得越来越小,使得体系满足了休谟-饶塞里定律从而形成新型的fcc-Ce3Al固溶体合金。当压力完全卸载后,尽管实验结果显示fcc-Ce3Al固溶体合金中的4f电子基本恢复到了局域态,但是其原子结构却在常压下可以保持稳定(至少在常温下的1年时间内),表明我们合成了一种全新的在常温常压下存在的“非休谟-饶塞里”固溶体合金材料。(4)利用高压原位同步辐射x射线衍射技术,我们在室温下发现了压力诱导的Ce75Al25金属玻璃多形态晶化现象。而且晶化转变的过程非常快,晶化相是和金属玻璃条带样品有固定晶体学取向关系的“单晶”结构。通过开展多种实验,基本证实了单晶的固定取向关系与金属玻璃样品内部的某种结构“取向”有关,而通过高分辨电镜和高分辨同步辐射X射线衍射应力分析,都没有发现样品中具有任何可以辨别的取向性(各向异性)的结构信息。结合第一性原理计算,我们提出了金属玻璃在快速冷却的制备过程中,可能保留(携带)了一定的长程取向性结构信息(可能是团簇的某种取向性堆积),而在室温下压力诱导的晶化可能是通过原子集体地局部调整而实现,能够不破坏金属玻璃样品结构中保留的取向信息,通过这种特殊的“单晶”晶化方式,重现了金属玻璃里可能携带的原子结构的各向异性信息。这种奇特相变的机理目前还不是很清楚,需要开展进一步的研究。但是这个实验在金属玻璃中发现和展示了一种全新的压力诱导非晶-单晶多形态晶化相变。这种新现象可能为单晶材料生长技术提供新的途径和思路。更多还原

【Abstract】 Metallic glass (MG), as a relative new member of glass family, offers novel mechanical, magnetic and electrochemical properties and has perspective potential applications in industry. Its unique nondirectional and densely packed structure provides an idea model system for studies of fundamental problems in condense matter physics, e.g. glass structure, glass forming, supercooled liquid behavior etc. It has been at the cutting edge of material researches for decades. The phase transitions in materials closely correlate with their structures and applications of switchable properties. The polyamorphic transitions in traditional amorphous materials have improved and extended our knowledge of amorphous matter. Generally, polyamorphic transitions occur in open-network glasses (coordination number<6) linking with the structure fluctuation in their relevant liquids. Thus, in principle it was thought that no polyamorphic transition would occur in MG because of its non-directional densely packed structure. In this thesis, by applying the state-of-the-art in-situ high-pressure synchrotron x-ray technologies to Ce-based MG as a model system, we discovered the polyamorphism in the Ce-based MG, as well as its underlying mechanism, and the emergent properties accompanying with the polyamorphic transitions. The results from this work also promise a new material synthesis method under high pressure in polyamorphous MGs. The main results are summarized as follows.(1) With in-situ high-pressure synchrotron x-ray diffraction (XRD) techniques, we investigated the compression behavior of Ce32La32Al16Ni5Cu15 bulk metallic glass (BMG), an amorphous-to-amorphous polyamorphic transition was discovered in BMG for the first time. Using in-situ high-pressure synchrotron XRD and x-ray absorption spectroscopy (XAS) techniques; we experimentally confirmed a low density amorphous (LDA) to high density amorphous (HDA) polyamorphic transition in a binary Ce75Al25 MG with high Ce concentration under hydrostatic pressure condition. Densification in this novel type of polyamorphic transition is dictated by the Ce 4f electronic transition from localized to itinerant state, which causes volume collapse. This transition is fundamentally different from the normal structural polyamorphism, in which coordination number changes and topological rearrangement of atoms occurs. These results in MGs extend our understanding about polyamorphism and may promote the searching for polyamorphism in other densely packed MGs, which could have pressure-tuned electron transitions. In addition, it was found that the pre-peak in the structure factor of Ce75Al25 MG showed less compressible behavior under pressure, which may improve our understanding of pre-peak in MGs.(2) Combining in-situ high-pressure and high-temperature energy dispersive XRD with in-situ high-pressure, low-temperature, four-probe resistance and magnetic measurements, we investigated the thermodynamic stability, electronic transport and magnetic properties accompanying the polyamorphic transition between LDA and HDA in Ce75Al25 MG. Compared with the La75Al25 MG sample, the change of properties was determined to associate with the 4f electron delocalization in Ce75Al25 MG. To the best of our knowledge, this is the first time in MGs that a pressure-tuned temperature coefficient of resistance (TCR), composition and magnetic field-tuned magnetoresistance were observed to change from negative to positive values. These obtained results will trigger more studies to discover polyamorphous MGs with interesting properties and potential applications. Moreover, they could be an interesting model system for the investigation of 4f electron behaviors. Additionally, we conducted the first successful in-situ high-pressure XRD experiment at the 15U, SSRF, which revealed another MG system having polyamorphic transition under pressure and minor alloying, e.g., Si, could modify the transition pressure and properties of LDA and HDA MGs.(3) Using in-situ high-pressure synchrotron XRD, we further investigated the high-pressure behavior of HDA Ce75Al25 MG. We discovered a crystallization of a new face-centered cubic (fcc) solid solution alloy from HDA Ce75Al25 MG at about 25 GPa. Normally, the Ce and Al with big radii difference (28%) and electronegativity difference (0.45) could not form substitutional solid solution alloys according to the Hume-Rothery rules. Synchrotron XRD, Ce L3-edge XAS, and ab-initio calculations revealed that the pressure-induced volume collapse and 4f electron delocalization of Ce reduced the differences in atomic size and electronegativity between Ce and Al and brought them within the Hume-Rothery limit for substitutional alloying. The novel alloy remained after complete release of pressure which was also accompanied by the transformation of Ce back to its ambient 4f electron localized state and reversal of the volume collapse, resulting in a novel non-Hume-Rothery alloy at ambient conditions.(4) Utilizing in-situ high-pressure synchrotron XRD, we discovered a novel pressure-induced "single crystal-like" crystallization in Ce75Al25 MG at room temperature. The transition was very fast and the crystalline fcc phase showed a fixed orientation with the starting Ce75Al25 MG ribbon sample, which was confirmed to be attributed to the intrinsic structure of the MG ribbon sample by many diagnostic XRD experiments. High resolution electronic microscopy and high resolution XRD results showed no inhomogeneity or stress existing in the starting Ce75Al25 MG ribbon sample. With the help of ab-initio calculations, we proposed that some kind of long range orientation structure was inherited during the melt-spinning synthesis process of the Ce75Al25 MG ribbon sample (possible some kind of cluster packing orientation). The pressure-induced "single crystal-like" crystallization in Ce75Al25 MG may not need diffusion process, rather collective adjustments, thus the hidden long range orientation in MG ribbon sample was awakened by such a transition to a single crystal. The origin of this novel phenomenon is not clear yet and needs further studies. However, these results obtained showing a novel type of pressure-induced single crystal-like polymorphic crystallization in MG, may promise a new method for synthesizing single crystal materials with controlled orientation from non-equilibrium systems.更多还原