(精品)非晶金属塑料的研究

中 文 摘 要(b)(a)相对于金属玻璃而言，聚合物玻璃的玻璃形成能力高, 玻璃转变温度低, 过冷液体的稳定性也高，表现出非常优异的热塑性，因此聚合物玻璃（塑料）广泛应用于塑性成型工业，用于制作各种模型和零件。同聚合物玻璃相比，虽然金属玻璃具有优越的机械和电学等性能，但是，低的玻璃形成能力和差的可加工性能都极大的限制了金属玻璃的广泛应用。最近十年来，对金属玻璃的研究获得了很大的进展，人们可以在多个金属合金体系获得毫米甚至厘米级尺寸的块状金属玻璃。由于块状金属玻璃在过冷液相区也表现出如氧化物玻璃和塑料那样的粘性流变特性，因此，人们也自然会希望块状金属玻璃能像聚合物塑料那样, 在过冷液区对其进行变形和塑性加工。遗憾的是，大多数的块状金属玻璃的玻璃转变温度都很高，过冷液体的稳定性相对地说来也还比较低，对它们的塑性和应用研究都受到了限制。在本文的工作中，我们成功的开发了一种全新的铈基块体非晶合金材料。该非晶材料表现出很低的玻璃转变温度(Tg), 最低的可以达到68 oC (341 K)，接近于室温，和许多聚合物玻璃的Tg 相似如尼龙（大约43 C），甚至比一些典型的聚合物玻璃的Tg更低如聚氯乙烯(约75-100 C)。该材料和热塑性塑料一样可以在沸水中轻易的进行拉伸，压缩，弯曲，压印等各种塑性变形。而且，在机械强度和导电性能上，铈基块体非晶合金材料要比通常的聚合物材料要好得多。正是因为兼有金属和塑料两种材料的基本特征，我们把铈基块体非晶合金材料称之为非晶金属塑料(代表性成果1)。整个论文分为七章。第一章主要是提出全文的核心思想-开发聚合物型的低Tg的块体金属玻璃。大多数的块体金属玻璃的Tg都很高，一般都在 300600 的范围，远比聚合物玻璃的Tg要高。金属玻强度在聚合物玻璃，石英玻璃和普通玻璃中是最高的，它的Tg比聚合物玻璃要高，接近于普通玻璃如窗户玻璃（Tg为700左右）。另一方面，和塑料相比，块状金属玻璃在高温下的过冷液体的稳定性相对地说来也还比较低。正是由于高的Tg和相对低的热稳定性，人们对块体金属玻璃的过冷液体状态的研究和认识远不及聚合物玻璃。因此，我们希望开发出一种和聚合物玻璃的Tg一样低的块体金属玻璃。本文的第二章介绍这种独特的低Tg的块体金属玻璃金属玻璃的成分设计和合成。主要理论的依据就是通常的热力学判据和我们研究组多年来积累的弹性模量规则。本文考察了可能的十四个低熔点（500 KTm1300 K）的元素Mg、Al、Zn、Ge、Sr、Ag、Cd、Sn、Sb、Ba、La、Ce、Pr、Nd、Yb、Tl、Pb和Bi， 最终确定了以稀土金属元素Ce为基体组元，以Ce72Cu28 和Ce76Co24 这两个Ce基二元共晶中熔点最低的共晶成分为基本成分。通过其它组元的添加，我们在试验了首先合成出了低Tg的Ce-Al-Cu 三元块体非晶合金，试验上发现最小的Tg可达68（341 K）。总的来说在一个很大的成分范围，即在40% Ce %80 %, 5%Al %25 % 和10 %Cu %25 % （原子百分比）的范围内，Ce-Al-Cu合金都可以通过通常的铜模铸造技术获得临介尺寸1至3毫米的非晶合金。另外，本文还发现Ce-Al-Co 和Ce-Al-Ni 也表现出和Ce-Al-Cu 类似的玻璃形成的成分范围，但是其Tg要比对应的Ce-Al-Cu成分的玻璃要略高。通过混合稀土元素MM(由La, Ce,Pr和Nd混和组成)取代Ce, MM-Al-Cu合金也很容易形成块体金属玻璃。本文还采用元素掺杂的方法来进一步提高Ce-Al-Cu 三元合金的玻璃形成能力，同时还要保持其低的Tg。对于MP001（Ce70Al10Cu20）这个典型基体成分，最有效的添加元素是Fe, Co, Ni, Nb, Si, C 和 B， 0.1- 2 %（原子百分比）添加，最大的临介玻璃形成尺寸可以达到1厘米。而且，过去常被引用的原子尺寸效应等其他的玻璃形成机制，并不能解释本文的微量掺杂导致的玻璃形成能力的显著提高。用核磁共振技术发现，Co 的微量添加可以显著提高Ce-Al-Cu合金中Al为中心的局域原子团的对称性，这一重要的发现为理解和提高玻璃形成能力提供了新的思路和证据（本章主要内容见Appl. Phys. Lett. 85（2004）61； Acta Mater. 54(2006)3025；Phys. Rev. B，73（2006）092201；Phys. Rev. Lett. 99(2007)095501(代表性成果2)）。本文第三章对Ce基非晶金属塑料的基本特性作详细的分析。它的基本特性主要包括低的玻璃转变温度Tg，优良的近室温的塑性和导电性。最低的Tg在60 oC附近，多数都在100 oC 以下，这和许多聚合物塑料如尼龙和聚氯乙烯的Tg类似甚至更低。在从60 oC 到130 oC的宽的温度区间，金属塑料样品都表现出随着应变速率而变化的牛顿流体变形行为，即超塑性变形。可以在开水中方便的对金属塑料样品进行弯曲，拉伸，压缩和压印等变形（如图1所示）。当然，Ce基非晶金属塑料还是好的导体材料，导电能力是纯Ce元素的大约60 %。和塑料一样低的软化温度（Tg）和优良的粘塑性变形能力以及比塑料好的导电性。另外，Ce基非晶金属塑料其他性质，如稳定性，液体性质和声子软化特性本章也作了研究，结果发现，Ce基非晶金属塑料具有可靠的稳定性，至少能在室温下放置3个月不会晶化，等温晶化实验外推到室温的结果显示，可以在室温下放置大约200年而不发生晶化。动力学的方法证明Ce基非晶金(b)(a)图1 (a)金属塑料在开水中压印的直径为20mm的中国科学院物理研究所的所徽图案（a） 和中国传统的八卦图案（b）。属塑料还表现出强的液体行为。超声测量的结果显示，和晶体状态相比，Ce基非晶金属塑料的模量和声速显著降低，表现出明显的声子软化现象（本章主要内容见Phys. Rev. Lett. 94(2005)205502(代表性成果1); Phys. Rev. B 70 (2004)224208）。本文第四章是通过研究高压下Ce非晶金属材料的弹性性为，进一步理解和发现它的结构和性能特征。实验测量了从常压到0.5 GPa的纵波和横波波速以及密度的变化，并且计算出了压力依赖的弹性常数。和过去测量的氧化物玻璃以及块体非晶相比，Ce金属塑料在压力下的声速和模量变化规律和氧化物玻璃类似，是一种软化的模式（即压力增加，声速降低），而不同于其他的块体非晶的正常的硬化模式（即压力增加，声速增加）。相对应的，Ce非晶金属塑料的Grneisen常数和状态方程也是和氧化物玻璃类似，而不同于其它块体非晶。Ce非晶金属塑料的这种独特的压力行为在过去的块体非晶合金中没有观察过，这和它的独特的玻璃结构有关系。这和前面讲过的Ce非晶的特殊的玻璃形成能力以及液体行为应该是密切相关的。本文这一部分的高压结果进一步说明，Ce非晶金属玻璃中可能存在不同于一般金属健的共价键的微观结构（本章主要内容见Phys. Rev. B 72 (2005)104205）。测量非晶合金过冷液体的声速和瞬态模量是一件很困难的事情， 过去由于块体金属玻璃的Tg比较高，因此实验上对金属玻璃的玻璃转变过程的原位超声测量很少有报道。本文第五章利用Ce金属塑料靠近室温的Tg和过冷液体区间，测量了从室温到163 (436 K)温度范围的Ce非晶随着温度变化的纵波和横波波速，并且计算出了温度依赖的弹性常数。测量的温度区间横跨了玻璃态，玻璃转变过程和过冷液体状态，所以，能够完整的观测到超声波速和瞬时模量在玻璃态和过冷液体态以及玻璃转变过程中随着温度变化的规律。这一结果对于深入理解玻璃转变的本质有重要意义。结果发现，玻璃转变过程用自由体积理论来解释是不够的， 它不能够解释本章实验观测的纵波和横波波速在玻璃转变过程中不同步（温度范围不一致）的现象。在玻璃转变过程中，横波波速和切变模量的降低要远大于纵波波速和体模量的降低。自由体积理论无法解释纵波和横波之间的显著差别。超声测量的理论以及上一章压力下的结果都说明，Ce非晶合金的纵波波速对体积的变化比横波敏感。这一结果也说明，切变模量和横波声学支在玻璃转变过程中的变化不能用自由体积的变化来解释。本文还从实验上验证了 “推挤”模型中描述的切变模量G控制着过冷液体的弛豫时间的观点，发现“推挤”模型和“势能地形”理论在对过冷液体的Maxwell弛豫时间的描述上是等价的（本章主要内容见Phys. Rev. B 76 (2007) 012201）。本文第六章主要是探索铈基非晶金属塑料的可能的应用，结果证明它是一种理想的微纳米加工材料。作为直接的微米压印成型材料它的显著特点是比一般塑料的强度高，但是和塑料软化点类似，所以不需要高的温度和能量就可以方便的实现高精度的几个微米甚至纳米尺度的压印成型。借助于现代的FIB加工技术，铈基非晶金属塑料样品因为导电性和高的强度，也是优良的纳米加工材料，至少可以在100纳米尺度上进行图形和器件的制作，并且可以用扫描电镜来原位观察，这点也是一般绝缘材料难以做到的。总之，本文所提出的金属塑料的概念还是很初步的，对它的认识和研究也是初步的。我们首先在Ce基合金体系中实现了金属塑料的概念，并不意味着这是唯一的体系。希望本文的工作能够起到抛砖引玉的作用，希望能够看到更多的兼有塑料和金属两种材料特征的新型材料被开发出来。关键词： 块体金属玻璃，非晶金属塑料，合成，性能，潜在应用Study on the amorphous metallic plasticZhang Bo ABSTRACTCompared with metallic glasses, polymeric glasses have a much more wide range of applicability because they exhibit higher glass forming ability (GFA), lower glass transition temperature (Tg) and a more stable supercooled liquid region than those of metallic glasses. The thermoplastic nature of glassy polymers is extensively exploited in molding and printing. For metallic glasses, however, engineering applications have been limited because of the limitation of alloys size and the lack of workability and machinability. Although many metallic glasses (the so-called bulk metallic glasses (BMGs) based Zr, Ti, Cu, Co, Pd and so on are now available in bulk form, similar exploitation of their viscous flow in the supercooled liquid state is still impeded by higher Tg and lower resistance to crystallization. In the present dissertation, we developed a new BMG family based on rare earth metal cerium (Ce). These Ce-based BMGs exhibit extremely low Tg down to about 68 oC (341 K) close to room temperature (RT) and similar to those of many polymeric glasses such as nylon and polyvinyl chloride (PVC). These materials also show excellent deformability like thermoplastics and can be readily elongated, compressed, bended and imprinted in hot water. The Ce-based BMGs also show superior GFA and high resistance to crystallization over a wide temperature range above Tg. Yet the mechanical and electrical properties of these BMGs are, for some applications, far superior to those of polymers. The Ce-based amorphous alloys have unique combined properties of thermoplastic and metallic behaviors at temperatures close to room temperature, thus can be regarded as amorphous metallic plastics. The title of the present dissertation, “amorphous metallic plastic”, directly comes from this point. The whole dissertation is composed of seven chapters. In the first chapter, we extracted the idea of developing polymerlike metallic glasses from the backgrounds in both BMGs and polymeric glasses. Compared with BMGs, polymeric glasses have low Tg around RT, excellent deformability, superior GFA and stable supercooled liquid state. While BMGs generally show higher Tg (usually higher than 300 ), excellent mechanical properties and electrical conductivity. Due to the high Tg, the exploitation and investigation of the supercooled liquid region (Tx, defined by the difference between Tg and the onset crystallization temperature Tx ) in BMGs are still very limited compared with those in polymeric glasses. Therefore, a new metallic material with combined properties of polymeric glasses (like low Tg and thermoplasticity) and metallic glasses (high strength and electrical conductivity) is of considerable importance for both of fundamental researches and potential applications. In Chapter 2, the design of BMGs with low Tg and the selection and preparation of Ce-based alloys were described. From a thermodynamic point, Tg is roughly proportional to the melting temperature Tm for a metallic glass. For most of the bulk metallic glass formers, the commonly used reduced glass transition temperature Trg (=Tg /Tm, Tm is the melting point or liquidus temperature) are larger than a value of 0.6. Thus, if Tg for a glass is between 300 and 400 K, its Tm should locate at temperatures around 500 670 K. Meanwhile, the often-sited eutectic principle is also a useful guider to look for the possible glass forming composition. From these thermodynamic considerations, the base element for the possible metallic glass with low Tg should be among the elements Mg, Al, Ca, Zn, Ge, Sr, Ag, Cd, Sn, Sb, Ba, La, Ce, Pr, Nd, Yb, Tl, Pb and Bi, whose Tm are all in the range of 5001300 K. For Ge, Sr, Cd, Ba and Tl, they are not conventionally used metals, thus are not taken into account. With respect to the rest 14 elements, we need to check the eutectic temperatures of the binary eutectic compositions based on them. Here, we only simply examine the binary eutectic alloys having the lowest Tm among all the binary eutectic compositions based on each element. Sn, Pb and Bi have the binary eutectic alloys with the lowest Tm below 500 K, but they are too low to form glasses above RT using conventional casting methods. For Mg- and Ca- based alloys, they have already been reported to form BMGs, whereas their Tg are still higher than 373 K (100). Al-based alloy system is one of the most difficult metallic families to form bulk glasses, and can only make melt-spin glasses with high Tg of about 200 . In Au-based alloys, Au-Si was the first reported metallic glass with a very low Tg of 43 , but was unable to keep its glassy state at ambient temperature for more than 3 hours. Thus, Zn- and Ce -based alloys should be considered since they own the lowest eutectic Tm among the rest binary eutectic compositions. After these analyses, we tried to prepare metallic glasses based on Zn and Ce using copper the mold casting method. Fortunately, we succeeded in preparing Ce-based bulk glasses with extremely low Tg (down to 68 /341 K) and the starting compositions are the eutectic Ce-based alloys Ce72Cu28 and Ce76Co24, whose Tm are the same lowest value of 424 /697 K in all the Ce-based binary eutectic alloys. By adding Al element into the basic alloys Ce72Cu28 and Ce76Co24, a series of Ce-based BMGs were prepared. In general, we found that the ternary Ce-Al-M (M=Cu, Co, Ni) alloys can form fully glassy rods with 1-3 mm in diameter in a wide composition range of 40-80 at. % Ce, 5-25 at. % Al, and 10-25 at. % Cu. Replacing Ce by misch metal (MM, a natural mixture of La, Ce, Pr, and Nd), MM based MM-Al-Cu bulk metallic glasses can also be readily prepared. By selecting appropriate minor additions (about 1 - 5 at.%) of elements X (X represents a series of elements such as Fe, Co, Ni, Nb, Si, C and B), the quaternary Ce-Al-Cu-X alloy system can be easily cast into glassy cylindrical rods with a diameter even larger than one centimetre. It is also found that the often-sited empirical criteria like the atomic size effect for BMG formation cannot interpret the formation and addition effect on GFA in the metallic glasses. The striking effect and mechanism of the microalloying on the GFA of the metallic glasses are investigated（For this chapter, the main related results have been published in Appl. Phys. Lett. 85（2004）61; Acta Materialia 54(2006)3025, Phys. Rev. B，73（2006）092201, and Phys. Rev. Lett. 99(2007)095501）.Thermal, mechanical and electrical properties of Ce-based BMGs are summarized in Chapter 3. All these Ce-based BMGs have exceptionally low Tg (about 341 K 439 K) comparable with those of typical polymers such as Nylon and PVC, and have superlasticity in a wide temperature window near RT. TheTx of these Ce-based BMGs are all among the range of 40 80 K. Such a wide window makes it suitable for deformation in the viscous supercooled liquid state. We experimentally demonstrated that many of these glasses could be easily elongated, compressed, bended and printed in near boiling water by hand (as shown in Fig.1). The true stress-train curves (b)(a)Fig.1 Articles for the badge of the institute of physics of chinese academy of sciences (a) and the chinese traditional Eight Diagrams (b). These articles with 20mm in diameter are cheaply imprinted on samples of Ce-based BMG in near boiling water.for Ce-based BMGs suggested that they exhibited superplasticity in large temperature and stain rate ranges. The compressive strength at RT for Ce-BMGs is about 500 MPa, which is comparable and even higher than those of high strength Al and Mg commercial alloys. Compared with polymeric glasses, the elastic constants (Yongs modulus E and bulk modulus K are about 30 GPa and the shear modulus G 12 GPa) and mechanical strength of Ce-based BMGs are about several times larger than those of typical polymers like Nylon and PVC. The thermal stability of Ce-based glasses is also examined over a wide range of temperatures above Tg by a DSC isothermal annealing method. The measured time temperature- transformation (TTT) diagram gives an Arrhenius extrapolation with a predicted lifetime at 20 of 1010 s ( about 200 years). DSC isothermal measurements also demonstrated that a Ce-based BMG sample stored at room temperature( 20-38 ) for 3 month is still glassy. In contrast to the insulation properties of typical polymers, Ce-based BMGs are metallic conductors with an electrical resistivity of 119 mW-cm. These combined properties give the reasons why Ce-based BMGs can be regarded as amorphous metallic plastics (For this chapter, the main related results have been published in Phys. Rev. Lett. 94(2005)205502）.In Chapter 3, other physical properties including liquid fragility and the phonon softening behaviors in the glassy state relative to the crystallized state were also investigated in a Ce-based Ce70Al10Ni10Cu10 BMG sample. Using DSC scanning methods at different rates, liquid fragility parameter m of the BMG is determined to be 21, suggesting a strong liquid behavior concerning the temperature dependence of viscosity. From Angells classification, the Ce-based glass forming liquid belongs to so-called “strong ” liquids like the oxide glass SiO2. The acoustic velocities (longitudinal velocity vl and shear velocity vs), elastic constants (E, G, and K) and Debye temperature (qD) of the glassy and crystallized states of Ce70Al10Ni10Cu10 BMG sample were measured by the ultrasonic technique. The large changes in vs(21%), vl(13%), E(43.5%), G(47%) K(22.9%) and qD(20.6%) and small change in density(0.6%) between the glassy and crystallized states for the Ce-based BMG can be observed. The remarkably large changes of elastic moduli in the BMG relative to its crystallized state mean that markedly softening of long-wavelength transverse as well as longitudinal acoustic phonons in the BMG. However, only the markedly softening of the transverse elastic modulus (normally 20-35% change in G) can be observed in various other BMGs. This unusual softening concerning the elastic phonons of the Ce-based BMG may result from the intrinsic structural feature of the Cerium base that is of variable electronic structure and dual valence states, and only small amount of energy is required to change the relative occupancy of the electronic levels. The small density change (0.6%) between the glassy and crystallized states suggests that the Ce-based BMG contains less density of free volume and has more random packed microstructure. This result is in good accordance with the strong liquid behavior concerning the temperature dependence of viscosity of Ce-based BMG (see Phys. Rev. B 70 (2004)224208).In Chapter 4, the pressure dependent elastic properties of Ce-BMG were investigated. It is a very common view that metallic glasses should become stiffer with the application of pressure. In this chapter, we reported unusual responses of elastic properties to high hydrostatic-pressure in a Ce70Al10Ni10Cu10 BMG sample. The density and acoustic velocities under hydrostatic-pressure (up to 0.5GPa) of the BMG have been in situ measured by using a pulse echo overlap method. The pressure derivatives of acoustic velocities (vl, and vs) and Grneisen parameters of the BMG are anomalously negative and comparable with those of silicate glasses such as fused quartz. The volume of the BMG changes upon pressure (i.e. the equation of state (EOS) also in a very similar way to that of typical oxide glasses but quite different from that of the conventional BMGs. The experimental results may provide evidences for the existence of special covalent bonded short-range ordering structures in the BMG, which should be primarily responsible for such unusual elastic responses of the BMG to pressure. The results throw new light for us to understand the complicate elastic properties and microstructures as well as their relations of metallic glasses (see Phys. Rev. B 72 (2005)104205).In Chapter 5, we report in-situ temperature (T) dependent acoustic and instantaneous elastic properties around Tg (343 K) of a Ce68Al10Cu20Co2 BMG. We found that the T dependences of transverse velocity vs and longitudinal velocity vl are distinctively different not only in the decrease magnitude but also in their corresponding softening temperature around glass transition. The softening T of vs corresponds well with calorimetric Tg in the glass, while the softening of vl occurs at about 10 K above calorimetric Tg. As a result, as T approaching Tg, the shear modulus G shows steep decrease while bulk modulus K undergoes weak T dependence, which suggests that the G be an important factor that controls the alteration in the configuration structure in the glass transition, and the T-dependent G can accurately describe the change of the viscosity and fragility in the glass-forming liquid. The results also support the models of the shear interaction/energy dominated glass transition. The results might shed light on the characteristics of processes involved in the glass transition and the rheological properties of glass-forming liquids (see Phys. Rev. B 76 (2007) 012201).In Chapter 6, we demonstrated that Ce-based metallic plastics (Ce-based BMGs) have good workability in the micro-/nanometer scales and thus have potential industrial applications such as thermomechanical data storage by forming nanoindentations through Joule heating of scanned nano-tips. For the application of micro- or nano-imprintability, there is an advantage in the increased precision possible when the