Al-based Metallic Glasses Research Articles

Revealing hidden supercooled liquid states in Al-based metallic glasses by ultrafast scanning calorimetry: Approaching theoretical ceiling of liquid fragility

铝基非晶合金具有密度低、强度高、耐腐蚀等诸多优异性能; 然而, 铝基非晶合金形成能力差, 一般需要非常高的冷却速率, 这限制了铝基非晶合金的应用. 玻璃形成理论认为形成能力与过冷液体密切相关. 但在一般升温测量时, 铝基非晶合金不显示玻璃转变或过冷液体, 而是直接变成晶态. 目前为止, 关于铝基非晶合金的玻璃转变和过冷液体属性仍然是未知的. 本文采用超快速差热分析方法(Flash DSC)使得升温速度达到 10000 K s−1, 测量了20余种常见铝基非晶合金的玻璃转变行为和过冷液体特征. 发现铝基非晶合金普遍具有很高的液体脆度系数( m ), 其中某些成分 m >160, 已经接近理论上预测的脆度系数上限 m ~175. 通过系统研究这些成分的形成能力, 发现铝基非晶合金的玻璃形成能力与脆度系数成反相关, 而且这种相关不是线性的. 只有 m m 才会对玻璃形成能力有明显影响; 相反, m >100的玻璃形成力普遍较弱, 而且随 m 变化不显著. 因此, 过高的液体脆度系数可能是铝基非晶合金形成能力差的一个重要原因.

Flash DSC determination of the delay time for primary crystallization and minor alloying effect in marginal Al-based metallic glasses

The delay time for primary crystallization plays an important role in the understanding of the transport and nucleation behaviors during crystallization in metallic glasses. In this work, a novel method via Flash DSC is demonstrated to measure the delay time for primary crystallization in Al-based metallic glasses. In a typical Al-based metallic glass of Al87Y7Fe6, the delay times were established corresponding to different temperatures. Transmission electron microscopy tests before and after delay time were conducted to further verify the validity of the measured delay time. The effect of a 1 at.% substitution by Cu on the delay time of Al88Y7Fe5 was investigated to determine the effect of minor alloying on the glass formation ability. The significant changes in the primary crystallization temperatures were demonstrated to be directly related to the corresponding changes in the delay time. The results indicate that minor alloying that is designed to increase the delay time is an effective strategy to enhance the glass formation ability.

Effect of quenching temperature on local ordering in Al85Ni5Co2Y8 metallic glass

The effect of quenching temperature on local atomic structure of Al85Ni5Co2Y8 metallic glass has been investigated in the present study. With increasing quenching temperature from 1273 K to 1573 K, the amount of medium range order (MRO) tends to decrease, while the structural unit size remains almost constant. The higher amount of MRO leads to movement of the crystallization nose of time-temperature-transformation (TTT) curve to a longer time scale by raising the viscosity of the liquid. On the other hand, quenched-in nuclei are present in the samples with higher quenching temperature, since the probability of intersecting the crystallization nose during cooling becomes higher with increasing quenching temperature. As a result, typical two types of as-quenched microstructure can be obtained, i.e. fully amorphous structure with higher amount of MRO and that with embedded quenched-in nuclei as well as relatively lower amount of MRO. The sample quenched at 1273 K without quenched-in nuclei exhibits the most fragile behavior, indicating that the fragility can be affected by the quenching temperature as well as the alloy composition in Al-based metallic glass.

Recent Topics on the Structure and Crystallization of Al-based Glassy Alloys

Al-based glassy alloys (metallic glasses) have been of great scientific and technological interest as a high specific strength and high corrosion resistant materials. However, the low glass-forming ability (GFA) is still a choke point that greatly influences the applications. In order to further comprehend the glass formation mechanism and investigate the possibility of enhancing the GFA, it is important to obtain the definite information on the atomic scale glassy structures in correlation with properties and to study the composition dependence of crystallization and properties of Al-based metallic glasses. The purpose of this paper is to summarize the structure characteristics and crystallization behavior in conjunction with alloy component in Al-based metallic glasses and to investigate the possibility of synthesizing an Al-based bulk metallic glass with better engineering performances. This may assist to learn the structural features systematically and understand the development and current status for Al-based metallic glasses.

Investigation of the nucleation delay time in Al-based metallic glasses by high rate calorimetry

During isothermal annealing at temperatures lower than the crystallization onset temperature, Tx, measured during continuous heating, the Al88Y7Fe5 metallic glass undergoes a period of delay time (τ) before the primary crystallization reaction. The delay time is an important parameter, which is closely related with the transport behavior and can be used to calculate the attachment frequency coefficient and steady state nucleation rate. Previously, τ was usually determined from the nanocrystal number density vs. annealing time plot through tedious TEM measurements. Now, a more effective approach to measure τ has been developed by analyzing the Tg shift resulting from annealing with high rate differential scanning calorimetry (FlashDSC). With the increase of annealing time (tannealing), Tg shifts to higher temperatures and in the Tg vs. tannealing plot, there is a break point in slope. Before this break point, no Al nanocrystals could be detected by transmission electron microscopy (TEM) but after this break point, Al nanocrystals were identified by TEM. Thus, the break point time corresponds to the onset of Al nanocrystal formation and the delay time. The underlying mechanism is unveiled through the matrix composition measurement by energy dispersive spectroscopy (EDS). The analysis reveals that before the break point the amorphous matrix composition is unchanged by annealing so that Tg shifts solely due to the increased glass stability through a relaxation process. After the break point, the precipitation of Al nanocrystals induces a composition change in the amorphous matrix so that Tg shifts to higher temperatures because of both the relaxation and the composition change effects.

Influence of yttrium on surface chemistry and stability of passive film in Al-based binary metallic glasses

The effect of yttrium on the electrochemical polarization behavior, composition and semiconductor property of passive film in Al-Y glasses have been studied. It was found that the corrosion potential and passive current density are sensitive to the minor variation of yttrium content. The high Y-containing samples possess a homogeneous single layer passive film rich in yttrium. The passive film exhibits a p-type semiconducting behavior and the carrier density tends to be reduced with increasing yttrium due to the formation of YAlO3. Higher yttrium content makes the passive film inert to electron transfer and the metastable pitting are inhibited.

Fabrication of Metallic Glass Powder for Brazing Paste for High-Temperature Thermoelectric Modules

Metallic glass (MG) offers the advantage of outstanding oxidation resistance, since it has disordered atomic-scale structure without grain boundaries. We fabricated Al-based MG ribbons (Al84.5Y10Ni5.5) by a melt spinning process. We evaluated the adhesion strength of interfaces between the Al-based MG and a Ni-coated Cu electrode formed under various conditions at high temperature. In addition, we attempted to optimize the process conditions for pulverizing MG ribbons to < 100 micrometers by combining high-energy ball milling and planetary milling. We confirmed that the electrical resistivity of the Al-based MG ribbon was substantially reduced after annealing at high temperature (over 300°C) due to crystallization.

Melt fluxing to elevate the forming ability of Al-based bulk metallic glasses

Salt-fluxing treatment is an effective technique to improve the glass-forming ability (GFA) of bulk metallic glass (BMG)-forming melts, as demonstrated before in Pd- and Fe-based systems. However, it has been challenging to develop similar fluxing protocol for more reactive melts, such as Al-rich BMG-forming systems. Here we design new fluxing agents, from a thermodynamics perspective that takes into account combined effects of physical absorption and chemical absorption (reaction) between the fluxing agents and oxide inclusions. MgCl2-CaCl2 composite salts were selected, and their fluxing effects were systematically studied on an Al86Ni6.75Co2.25Y3.25La1.75 alloy, the best BMG-forming composition reported thus far for Al-rich alloy systems. The oxygen content was found to continuously decrease in the master alloy with increasing cycles of salt-fluxing treatment, with chlorate products on the surface suggesting concurrent physical absorption and chemical reaction. The fluxing treatment developed has enabled a record critical size (diameter) of 2.5 mm for Al-based BMGs. Our finding is thus an advance in developing highly desirable Al-based BMGs, and also provides guidance for designing processing protocol to produce larger-sized BMGs in other reactive systems.

Manipulation of thermal and mechanical stability by addition of multiple equiatomic rare-earth elements in Al-TM-RE metallic glasses

In the present study, we demonstrate that the addition of multiple equiatomic rare-earth elements (REs) in the order of electronegativity in Al84Ni10RE6 (RE = La, Ce, Nd, Gd, Y) metallic glasses (MGs) can result in stabilizing supercooled liquid region and destabilizing shear avalanches via controllably induced compositional complexity. In detail, the addition of multiple equiatomic REs leads to a relatively fragile glass, but interrupts easy crystallization behavior in Al84Ni10(LaCeNdGdY)6MG, which causes a larger supercooled liquid region and higher thermoplastic formability. Furthermore, the increased structural instability reduces cut-off size of self-organized shear avalanches and increases the number of chaotic shear avalanches, which can distribute the applied strain more homogeneously, resulting in enhanced intrinsic ductility. These results provide a guideline on how to design promising Al-based MGs with high thermal stability and improved mechanical stability, which is a key step to developing ductile Al-based bulk metallic glasses through thermoplastic forming process.

Characterization of Al/crystallized Al-based metallic glass composites produced by repeated roll bonding process

Devitrified Al84Gd6Ni7Co3 glassy particles have been used to reinforce Al-matrix composites through repeated roll bonding (RRB) process. Microstructural characterization of the produced composites after various rolling cycles was performed by scanning electron microscopy. Mechanical properties of the fabricated composites were evaluated by the tensile and microhardness tests. The results indicate that the RRB process is successful to produce composites with the negligible amount of flaws and porosity, and it is followed by homogeneous distribution of Al84Gd6Ni7Co3 particles in the Al matrix after nine rolling passes. Elongation of the composites improves significantly upon RRB cycles and the tensile strength and microhardness of them increase more than two times compared to unreinforced Al. According to fractography results, the enhanced mechanical properties are correlated with formation of excellent bonding at the interface of Al84Gd6Ni7Co3 particles and Al matrix. The theoretical values of composites hardness and yield strength calculated based on iso-strain model show a good agreement with respect to the experimental results.

In-situ synthesis of novel Al-Fe-Si metallic glass coating by arc spraying

A novel in situ forming Al-Fe-Si metallic glass coating comprising partial nanoscale particles embedded in Al-based metallic glass matrix was synthesized by arc spraying process. The coating is adhering well and very compact with porosity of less than 2%. The volume fraction of metallic glass is 74.9% approximately. The onset crystallization temperature of the coating is 359 °C. The coating exhibits a high hardness (H = 4.25 GPa) and reduced Young's modulus (E = 235 GPa), together with good H/E, H2/2E and elastic recovery (η value). Dry sliding wear testing of the coating shows that the relative wear resistance of the coating is about 2.9 times that of the Al-6061 alloy under the same testing conditions. The dominating wear mechanism of the coating is a combination of brittle fracture delamination coupled with oxidative wear under dry sliding conditions. Meanwhile, potentiodynamic studies of the metallic glass coating in 0.6 M NaCl solution displays better corrosion resistance than that of Al alloyed coatings and EQ70 marine steel. In addition, the reasons actualizing the enhancement of the mechanical properties and corrosion resistance were elucidated.

Fast decolorization of azo dyes in both alkaline and acidic solutions by Al-based metallic glasses

The advanced efficiency of metallic glasses in degrading organic water pollutants has attracted wide interests. Yet the decolorization efficiency decreases in alkaline solutions which remains a challenge for their application. In this work, the performance of Al91-xNi9Yx (x = 0, 3, 6, 9 at.%) metallic ribbons in degrading azo dye in aqueous solutions at wide pH conditions is studied. It is surprising to find that the reaction activity of Al-based metallic glass in alkaline and acidic azo dye solution is about 1.5 and 189 times higher than that in neutral solutions, respectively. The low reaction activation energy and formation of nano-porosity on the surface of metallic glass are responsible for the high reactivity in alkaline and acidic solutions. The reaction activity can be further enhanced by modifying the alloy composition. These findings suggest that the Al-based metallic glasses hold promising potential in degrading azo dyes solutions, especially in alkaline and acidic environments.

Research for Brazing Materials of High-Temperature Thermoelectric Modules with CoSb3 Thermoelectric Materials

Metallic glass (MG) can be a candidate for an alternative brazing material of high-temperature thermoelectric modules, since we can expect both a lower brazing temperature and a high operating temperature for the junction from the MG brazers. Another advantage of MG powders is their outstanding oxidation resistance, namely, high-temperature durability in atmosphere. We fabricated three compositions of Al-based MGs—Al-Y-Ni, Al-Y-Ni-Co, and Al-Y-Ni-Co-La—by using the melt spinning process, and their Tgs were 273°C, 264°C, and 249°C, respectively. The electrical resistivity of the Al-Y-Ni MG ribbon dropped significantly after annealing at 300°C. The electrical resistivity of crystallized Al-Y-Ni reduced down to 0.03 mΩ cm, which is an order of magnitude lower than that of the amorphous one. After the MG ribbons were pulverized to sub-100 μm, the average particle size was about 400 μm.

Effect of substituting elements on thermal stability and glass-forming ability of an Al-based Al Ni Er metallic glass

Abstract Addition of a small amount of La and Co enhances the stabilization of supercooled liquid and glass-forming ability (GFA) of an Al85Ni7Er8 metallic glass. The Al85Ni5Co2Er6La2 alloy exhibits large supercooled liquid region of 40 K, high GFA with a critical thickness of 450 μm, and the viscosities on the order of 107−109 Pa s in the supercooled liquid state. The Al-based glassy nano-scale patterns were formed by imprinting, indicating its good thermoplastic formability. The possible mechanism for the improvement of the supercooled liquid stability of the Al Ni Er alloy by addition of La and Co elements was discussed in terms of the crystallization behavior.

Enhanced mechanical properties due to nanocrystallization by isothermal annealing in Al85Ni9Er6 glassy alloy

In present work, isothermal annealing has been used to enhance the mechanical properties of Al85Ni9Er6 glassy alloy. Compared to 3.55 GPa, the hardness of the melt-spun alloy, the hardness of the alloy after annealed for 60 min and 90 min increased to 5.7 GPa and 5.55 GPa, respectively. The appearance of many fcc-Al nanocrystals lead to significant hardening. During the annealing, the nanocrystal size does not change much, but nanocrystal number increases between 50 min and 90 min at 519 K, which inferred that the mechanical properties of Al-based metallic glasses may be influenced by crystal number more than crystal size. The mechanical properties evolution of the metallic glass with the annealing time is due to combined effects of variations of the free volume and fcc-Al nanocrystals in the amorphous matrix.

Preparation of Al&lt;sub&gt;72&lt;/sub&gt;Ni&lt;sub&gt;8&lt;/sub&gt;Ti&lt;sub&gt;8&lt;/sub&gt;Zr&lt;sub&gt;6&lt;/sub&gt;Nb&lt;sub&gt;3&lt;/sub&gt;Y&lt;sub&gt;3&lt;/sub&gt; Amorphous Powder and Bulk Material

Al-based amorphous materials are believed to be a promising light weight and high strength material, but the application has not yet been realized due to difficulties in fabricating bulk materials resulted from their quite weak glass forming ability. In order to overcome this difficulty, a solid state route was studied to fabricate bulk Al-based metallic glass. An amorphous Al72Ni8Ti8Zr6Nb3Y3 powder was first fabricated by mechanical alloying, and then the microstructure, glass-forming ability and crystallization behavior were characterized. The powder shows a wide supercooled liquid region (81K), and high activation energy for crystallization (312.6kJ/mol). By this powder, a bulk material was fabricated through high pressure (2GPa) sintering (673K and 723K). The microhardness of bulk sample is as high as 1215Hv, providing a solid basis for fabricating bulk materials and applications.

Novel Al-based metallic glass coatings by Cold Gas Spray

Al-based metallic glass powder was deposited by high pressure Cold Gas Spray technology onto Aluminum-7075-T6 alloy. Cold Gas Spray parameters such as stand-off distance, gas pressure and gas temperature were varied in order to investigate the optimal conditions to obtain better mechanical and corrosion properties of as-sprayed coatings. X-ray diffraction and differential scanning calorimetry were used to study the amorphous nature of feedstock powders and coatings, while optical microscopy, scanning electron microscopy and Vickers microhardness were employed to determine thickness, porosity and hardness. Wear and corrosion measurements were carried out on optimized Al-based metallic glass coatings. Sliding wear behavior was evaluated using a ball-on-disk tribometer, while the corrosion behavior was studied through electrochemical measurements in NaCl solution at room temperature. The results show an improved wear resistance in the order of 200% higher and higher corrosion resistance than Al-7075-T6 substrate. Present results prove that Al-based metallic glass coatings are a good option to protect Al-alloys in corrosive and wear environments.

Evaluation of glass formation and critical casting diameter in Al-based metallic glasses

Abstract Critical cooling rate R c for a series of Al-based glass formers, as well as cooling rate T ′ for copper-mold casting Al-based bulk metallic glass (BMG) samples, have been calculated. It is found that the glass forming ability (GFA) is reflected by the change of R c value, and Al 86 Ni 6 Y 4.5 Co 2 La 1.5 alloy, the best glass former in Al-based systems, holds the lowest R c of about 3.01 × 10 3 K/s. The T ′ for the rod sample is determined by casting diameter D . When synthesizing Al-based BMGs, with the D increases from 1 mm to 3 mm, the value of T ′ decreases from 7.48 × 10 3 K/s to 0.83 × 10 3 K/s. Based on these calculations, the critical casting diameter D c for a certain Al-rich multi-composition could be predicted. Further, for the composition Al 86 Ni 6 Y 4.5 Co 2 La 1.5 , bulk glassy sample with D c up to 1.5 mm has been obtained, confirming the theoretical prediction.

High plastic Zr–Cu–Fe–Al–Nb bulk metallic glasses for biomedical applications

Four Zr–Cu–Fe–Al-based bulk metallic glasses (BMGs) with Zr contents greater than 65at% and minor additions of Nb were designed and prepared. The glass forming abilities, thermal stabilities, mechanical properties, and corrosion resistance properties of the prepared BMGs were investigated. These BMGs exhibit moderate glass forming abilities along with superior fracture and yield strengths compared to previously reported Zr–Cu–Fe–Al BMGs. Specifically, the addition of Nb into this quaternary system remarkably increases the plastic strain to 27.5%, which is related to the high Poisson’s ratio and low Young’s and shear moduli. The Nb-bearing BMGs also exhibit a lower corrosion current density by about one order of magnitude and a wider passive region than 316L steel in phosphate buffer solution (PBS, pH 7.4). The combination of the optimized composition with high deformation ability, low Young’s modulus, and excellent corrosion resistance properties indicates that this kind of BMG is promising for biomedical applications.

Local constriction around minor elements in Al86Ni7X1Y6 metallic glass (X: Ag, Au, Pt)

The local structure around Ag, Au and Pt atoms in Al86Ni7X1Y6 (X: Ag, Au, Pt) bulk amorphous alloy was studied by X-ray absorption spectroscopy. Strong bond-shortening and a concomitant reduction of Al nearest neighbors around Ag, Au and Pt atoms compared to a dense random packing of hard spheres (based on the metallic radii of the elements) were observed suggesting strong covalent (Ag, Au, Pt) — Al bonds. This is ascribed to charge transfer from first-shell Al atoms to the X atoms resulting in s–p hybridization of the bonds. This causes opposite changes of the effective radii of X and Al atoms. Our results suggest that models using the metallic radii of elements are not appropriate to describe Al-based metallic glasses which contain Ag, Au or Pt atoms.