藉由鉭、氮添加於鋯-銅-鋁-銀金屬玻璃薄膜 以提升熱及機械特性

Effects of minor alloying on the mechanical properties of Al based metallic glasses

Bulk Metallic Glasses (BMGs) have been drawing increasing attention in recent years due to their scientific and engineering significance. A great deal of effort in this area has been devoted to developing BMGs in different alloy systems. BMGs based on certain late transition metals (e.g. Fe, Co, Ni, Cu) have many potential advantages over those based on early transition metals. These include even higher strength and elastic modulii, and lower materials cost, to name a few, which are highly preferable for a broad application of BMGs as engineering materials. Nevertheless, these ordinary-late-transition-metal-based BMGs generally have quite limited glass-forming ability (GFA). In particular, for the Ni-based and Cu-based alloys reported prior to this research, the maximum casting thickness allowed to retain their amorphous structures is only ~2 mm (or lower) and ~5 mm (or lower), respectively. During this research it was first found that certain quinary Ni-based alloys in the Ni-Cu-Ti-Zr-Al system can be cast into 5 mm diameter amorphous rods. This critical casting thickness is the highest for any reported Ni-based BMG’s indicating that these alloys are the easiest metallic glass formers based on Ni discovered to date. Secondly, certain binary alloys in the Cu-Zr and Cu-Hf systems were found to form bulk glasses with casting thickness as high as 2 mm. The discovery of these binary BMGs was very surprising since it had been widely considered that only multi-component (containing at least three elements) alloys could form bulk metallic glasses. These new binary BMGs provide interesting subjects for future theoretical studies such as molecular dynamics simulations since they possess both the simplicity of binary alloys and the good GFA of multi-component BMGs. In fact, these binary BMGs have led to the third but perhaps the most significant progress during this research, i.e., the discovery of a family of Cu-based BMGs in the Cu-Zr-Al-Y system which possess a critical casting thickness up to 1 cm. These quaternary Cu-based alloys, together with some complicated Fe-based alloys reported by two other groups during the course of this research, are the first centimeter level BMGs based on the ordinary late transition metals. This thesis first reviews the fundamentals related to BMG development, then reports in detail the formation and properties of the above-mentioned binary and multi-component BMGs based on Ni and Cu. A generalized geometric model for the critical-value problem of nucleation developed in this research is also presented.

The exceptional stability against crystallization of recently developed bulk metallic glasses, together with the non-contact measurement techniques to investigate their thermophysical properties, have opened the doors for detailed studies of these systems in the deep undercooled liquid state. The primary objective of this thesis is identification of reliable predictive indicators of glass-forming ability based on the measured thermophysical properties and intrinsic crystallization behavior of bulk metallic glasses. A number of bulk metallic glass-forming alloys having widely different glass-forming abilities are chosen for this investigation. All of the measurements are carried out using the noncontact high vacuum electrostatic levitation technique to avoid any heterogeneous nucleation effects from container walls or environment. The measured kinetic properties, viscosity, and free volume are found to have the most pronounced influence on glass-forming ability, while the thermodynamics show weak correlation with the trends in glass-formation. The glass-forming melts show orders of magnitude higher viscosity compared to pure metals. Among the glass-forming alloys, better glass formers have higher melting temperature viscosity, higher fragility, and show a smaller change in volume upon crystallization compared to poor glass formers. The melting temperature viscosity is correlated with volume change upon crystallization in accordance with Cohen-Grest free-volume theory for a wide variety of alloys. To quantify the glass-forming ability, Time-Temperature-Transformation (TTT) curves of a number of glass-forming melts are measured over a wide temperature range between their glass-transition and melting temperatures. A pronounced effect of overheating is observed on the undercooling levels and crystallization time scales. This is attributed to oxide particles acting as heterogeneous nucleation sites unless dissolved by overheating above their melting points. The TTT curves, obtained after eliminating the heterogeneous influences, are found to scale with their respective glass transition temperatures. The TTT curves are analyzed within the framework of nucleation theory to obtain a quantitative correlation between crystal-melt interfacial tension, melt viscosity, and glass forming ability. Finally, the crystallization behavior, microstructure, and melt viscosity of an in situ ductile phase reinforced amorphous matrix composite is investigated as a function of the processing temperature. The results for the in situ composite are compared with those of the monolithic bulk metallic glasses.

Manipulation of relaxation processes in a metallic glass through cryogenic treatment

Tunable elastic heterogeneity caused by deformation-induced magnetization in flexible metallic glass

Tailoring microstructure of metallic glass for delocalized plasticity by pressure annealing: Forward and inverse studies

Extreme conditions involve low or high temperatures (> 1500 K), high pressures (> 30 MPa), high strains or strain rates, high radiation fluxes (> 100 dpa), and high electromagnetic fields (> 15T). Material properties under extreme conditions can be extremely different from those under normal conditions. Understanding material properties and performance under extreme conditions, including their dynamic evolution over time, plays an essential role in improving material properties and developing novel materials with desired properties. To understand material properties under extreme conditions, we use molecular dynamics (MD) simulations with recently developed reactive force fields (ReaxFF) and traditional embedded atom methods (EAM) potentials to examine various materials (e.g., energetic materials and binary liquids) and processes. The key results from the simulations are summarized below. Anisotropic sensitivity of RDX crystals: Based on the compress-and-shear reactive dynamics (CS-RD) simulations of cyclotrimethylene trinitramine (RDX) crystals, we predict that for mechanical shocks between 3 and 7 GPa, RDX is the most sensitive to shocks perpendicular to the (100) and (210) planes, while it is insensitive to those perpendicular to the (120), (111), and (110) planes. The simulations demonstrate that the molecular origin of anisotropic shock sensitivity is the steric hindrance to shearing of adjacent slip planes. Mechanisms of hotspot formation in polymer bonded explosives (PBXs): The simulations of a realistic model of PBXs reveal that hotspots may form at the nonplanar interfaces where shear relaxation leads to a dramatic temperature increase that persists long after the shock front has passed the interface. For energetic materials this temperature increase is coupled to chemical reactions that eventually lead to detonation. We show that decreasing the density of the binder eliminates the hotspots or reduces the sensitivity. Cavitation in binary metallic liquids: We demonstrate the stochastic nature of the cavitation process in binary metallic liquids, and that classical nucleation theory can predict the cavitation rate if we incorporate the Tolman length derived from the MD simulations. Synthesis the single metallic glass on amorphous substrate: We show that single component metallic glasses (SCMGs) can be synthesized by thermal spray coating of nanodroplets onto an amorphous substrate (ND-AS). The key requirements to form the SCMGs are the rapid cooling rates and the amorphous substrates. Carbon and hydrogen phases under extreme conditions: we report on the use of electron force fields (eFF) in characterizing the Hugoniot relationships of carbon, which includes consecutive phase transitions also captured by experiments, as well as the Hugonoit states of hydrogen centered at various initial densities compared to experiments and the predictions of other theories.

利用强流脉冲离子束(High Intensity Pulsed Ion Beam，HIPIB) 模拟核聚变装置中的瞬态高热负荷环境，离子束成分为Cn+(70%) 和H+(30%)、加速电压为250 kV，研究金属玻璃Zr53Al23.5Cu5.9Co17.6 和金属W在不同参数的HIPIB 辐照下结构和性能的变化规律以及损伤行为。XRD显示辐照后金属玻璃均保持非晶相为主要结构，金属W中有应力产生。SEM观察在辐照次数为3 和10 次时金属玻璃和金属W表面都没有明显的辐照损伤现象；当辐照次数增加到100 和300 次后，金属玻璃表面出现了花瓣状形貌和小球，金属W表面则出现了裂纹。纳米压痕仪测量辐照后金属玻璃的表面纳米硬度随辐照次数的增加逐渐降低。Zr 基金属玻璃具有较好的耐辐照性能，对HIPIB 辐照时产生强的热应力的缓冲能力比金属W好. High intensity pulsed ion beam(HIPIB) technology was used for simulating the transient heat load conditions of fusion reactor. The ion beam was mainly composed of Cn+(70%) and H+(30%) at an acceleration voltage of 250 kV. We investigated the changing rule of the structure and performance and damaged behavior of metallic glassZr53Al23.5Cu5.9Co17.6 and W metal under different number of pulses. XRD analysis showed that the metallic glass remained making amorphous phase as its main structure after HIPIB irradiation, while stress were produced in W metal. SEM analysis concluded that there was no apparent irradiation damage on the surface of metallic glass and W metal when the irradiation frequency was 3 and 10 times. While the irradiation frequency increased to 100 and 300 times, “petal”-shaped and balls appeared on the surface of metallic glass, and cracks appeared on the surface of W. Nanoindentor showed that nano-hardness for the surface of metallic glass after irradiation reduced gradually with the increase of the frequency of irradiation. Zr-based metallic glass had a better resistance under HIPIB irradiation. The buffering capacity of Zr-based metallic glass in connection with HIPIB irradiation-induced thermal stress is superior to W metal.

Metallic glasses (MGs) are susceptible to embrittlement due to structural relaxation. Microalloying has been explored as a strategy to improve the mechanical properties of MGs. Here it is shown that microalloying with several elements can mitigate embrittlement due to relaxation and even increase plasticity in annealed states. Changes in the glass states with relaxation were studied using X-ray diffraction and differential scanning calorimetry. The mechanical properties of the as-cast and relaxed states were characterised by macroscopic mechanical testing, microindentation, and nanoindentation. Relaxed samples with improved plasticity show more shear events and lower propagation velocities during indentation. Transmission electron microscopy micrographs showed no evidence of crystallization in the annealed samples, suggesting the improvements are due to structural features of the glassy state. This microalloying approach should translate to other alloy systems and aid the wider use of MGs.

Unraveling the microstructural heterogeneity and plasticity of Zr50Cu40Al10 bulk metallic glass by nanoindentation

An amorphous phase in Nd–Fe–Al system was formed in an extremely wide composition range of 0 to 90 at% Fe and 0 to 93 at% Al by melt spinning. Based on the information on the amorphous formation, ferromagnetic Nd90−xFexAl10 bulk amorphous alloys with high coercive force at room temperature were obtained by a copper mold casting method. The maximum diameter of the cylindrical amorphous samples with a length of 50 mm is about 7 mm for the 20%Fe alloy and about 4 mm for the 30%Fe alloy. Neither glass transition nor supercooled liquid region is observed in the temperature range before crystallization, being different from previous bulk glassy alloys exhibiting a wide supercooled liquid region before crystallization. The onset temperature of crystallization (Tx) and melting temperature (Tm) are measured to be 778 and 863 K, respectively, for the Nd70Fe20Al10 alloy. The resulting reduced ratio of Tx⁄Tm is as high as 0.90 and the temperature interval between Tx and Tm is as small as 85 K. The extremely high Tx⁄Tm and small ΔTm(=Tm−Tx) values are the reason for the achievement of the large glass-forming ability. The bulk amorphous Nd70Fe20Al10 alloy has a ferromagnetism with the Curie temperature (Tc) of about 600 K which is much higher than the highest Tc (about 480 K) for the Nd–Fe binary amorphous alloy ribbons. The remanence (Br) and intrinsic coercive force (iHc) for the bulk Nd60Fe30Al10 alloy are 0.122 T and 277 kA/m, respectively, in the as-cast state and 0.128 T and 277 kA/m, respectively, in the annealed state for 600 s at 600 K. The Br and iHc decrease to 0.045 T and 265 kA/m, respectively, for the crystallized Nd60Fe30Al10 sample consisting of Nd+Al2Nd+δ phases and the maximum hard magnetic properties are achieved in the amorphous state. The hard magnetic properties for the bulk amorphous alloys are presumably due to the homogeneous development of ferromagnetic clusters with large random magnetic anisotropy. The finding of the bulk amorphous alloys exhibiting hard magnetic properties at room temperature is promising for the future development as a new type of metallic amorphous permanent magnet.

The aim of the present paper is to analyse the formation and stability of metallic glasses. Attention is focused to the history of the progress on new glassy alloys with specific applications: from soft magnetic Fe‐based ribbons obtained by melt spinning or planar flow casting, through metallic glasses acting as precursors of nanocrystalline precipitates embedded in an amorphous matrix, towards the finding of new multicomponent metallic alloys with large glass‐forming ability that open a new area of development and applications of bulk metallic glasses. The glass formation process is discussed in terms of the structure of the resultant material and in terms of the thermodynamics and kinetics of the process. The development of structural criteria for glass formation is based upon the notion that the free energy of particularly stable glassy structures will be low enough to make the driving force for crystallization negligible. Both structural and energetic criteria point to an enhanced glass‐forming ability in multicomponent alloys. Kinetic criteria allow to differentiate between the ease of glass formation from an undercooled melt (or from a mixture of crystalline grains, as in solid state amorphization) and the stability of the glassy state versus devitrification or crystallization. Key kinetic parameters are viscosity and, more specifically in multicomponent systems, diffusivities of the different elements in the crystalline, glassy and metastable liquid state, respectively. The formation of nanostructured materials from controlled thermal treatment and their temperature stability is discussed in terms of the concentration gradients developing at the interface between the nanocrystals and the amorphous matrix. The qualitative influence of several parameters in the glass‐forming ability and glass‐stability as well as their link to the empirical criteria that have been successfully employed to predict the glass‐forming ability are presented.

Workflow for a reproducible and accurate determination of free volume in metallic glasses by differential scanning calorimetry

Effect of ion irradiation in an Al90Fe2Ce8 metallic glass

One of the central themes in materials science is the structure-property relationship. In conventional crystalline metals, their mechanical behaviour is often dictated by well-defined structural defects such as dislocations, impurities, and twins. However, the structure-property relationship in amorphous alloys is far from being understood, due to great difficulties in characterizing and describing the disordered atomic-level structure. Herein, we report a universal, yet simple, correlation between the macroscopic mechanical properties (i.e., yield strength and shear modulus) and a unique characteristic structural length in metallic glasses (MGs). Our analysis indicates that this characteristic length can incorporate effects of both the inter-atomic distance and valence electron density in MGs, and result in the observed universal correlation. The current findings shed lights on the basic understanding of mechanical properties of MGs from their disordered atomic structures.