EXAFS Studies on Local Atomic Structure in the Amorphous Mg_(65)Cu_(25)Gd_(10) Alloy

Local atomic structures of an amorphous Mg65Cu25Gd10 alloy and the structural changes by thermal annealing have been studied by extended X-ray absorption fine structure (EXAFS). The correlation between structural changes and mechanical properties for the Mg65Cu25Gd10 alloy has also been discussed. Results showed that Cu atoms around Gd in Mg65Cu25Gd10 lost rapidly during annealing , resulting in the segregation of Cu atoms. The coordination number NGd-Mg of Mg65Cu25Gd10 annealed at 373 K first diminished and then augmented with the increase of annealing time. The formation of polyhedral short-range order unit with coordination number of nearly 12 around Gd atoms is in favor of the improvement of mechanical properties. The chemical short-range order, not topological short-range order in the amorphous Mg65Cu25Gd10 alloy had obvious changes during annealing.     

Corrosion Behavior of Mg65Cu25-xZnxGd10 （x=0, 5）Metallic Glass

The effect of substitutional element Zn on corrosion behavior of Mg65Cu25Gd10 glass was investigated. The amorphous structure of Mg65Cu25-xZnxGd10 （x=0,5） alloys were examined by X-ray diffractometry and differential scanning calorimetry （DSC）. The dissolution rates of Mg65Cu25-xZnxGd10 （x=0, 5） metallic glasses in a 5 wt% NaCl solution with pH value of 7 were determined by a hydrogen evolution testing method. The corrosion behavior of these alloys was characterized using dipping tests with 5 wt% NaCl, in combination with electrochemical measurements and scanning electron microscopy （SEM）. Results show that the anti-corrosion ability of Mg65Cu25Gd10 alloy is significantly improved due to the addition of Zn. Possible mechanism responsible for the improvement is discussed.     

Preparation and mechanical behavior of Mg-based bulk metallic glasses and their matrix composite

Mg87-xCuxDy13(x=22,27,32) bulk metallic glasses (BGMs) with a diameter of 6-8 mm and in-situ Mg phase reinforced Mg70Cu17Dy13 BMG matrix composite with a diameter of 3 mm have been prepared by copper mould casting. The glass forming ability (GFA) of Mg-Cu-Dy alloys have been investigated by differential scanning calorimetry (DSC) and X-Ray diffraction (XRD) and tne mechanical properties have been measured. Results show that Mg87-xCuxDy13(x=22,27,32) alloys in the Mg-Cu-Dy alloy system exhibit excellent GFA, and Mg60Cu27Dy13 alloy has the largest GFA among these alloys. And In-situ Mg phase reinforced Mg70Cu17Dy13 BMG matrix composite exhibits some work hardening and a high fracture compressive strength of 702.38 MPa and some plastic strain of 0.81%. The improvement of the mechanical properties is attributed to the fact that the Mg phase distributed in the amorphous matrix of the alloy has some effective load bearing and plastic deformation ability to restrict the expanding of shear bands and cracks and produce its own plastic deformation.     

Microstructural characterizations and mechanical properties of Mg-8Sn-1Al-1Zn-xCu alloys

Microstructural characterization and mechanical properties of as-cast Mg-8Sn-1Al-1Zn-xCu（x=0wt%, 1wt%, 1.5wt% and 2.0wt%） alloys were studied by OM, Pandat software, XRD, SEM, DSC and a standard universal testing machine. The experimental results indicate that adding Cu to TAZ811 alloy leads to the formation of the AlMgCu and Cu3 Sn phases. Tensile tests indicate that yield strength increases fi rstly and then decreases with increasing Cu content. The alloy with the addition of 1.5wt% Cu exhibits optimal mechanical properties among the studied alloys. The improved mechanical properties can be ascribed to the second phase strengthening and fi ne-grain strengthening mechanisms resulting from the more dispersed second phases and smaller grain size, respectively. The decrease in ultimate tensile strength and elongation of TAZ811-2.0wt% Cu alloy at room temperature is ascribed to the formation of continuous AlMgCu and coarse Mg2 Sn phases in the liquid state.     

Tensile and compression properties of Zr-based bulk amorphous alloy at different temperatures

Since the discovery of the Au-Si metallic amorphous alloy ribbon in 1960 by Duwez and co-workers[1] by rapid quenching, rapid developments have been obtained on the preparation of the amorphous alloy with the rapid quenching technique. However, from 1960 to 1989, the amorphous alloy could be only produced into forms of ribbon, flake, wire and powder due to the limitation of the cooling rate (>105 K/s). In the late 1980s, the Inoue group at Tohoku University in Sendai, Japan, investigated th…     

Structural Properties and Energy Analysis of ZrxCu90-xAl10Ternary Metallic Glasses

Classical molecular dynamics(MD) were conducted to study the structure and energy distribution of Zr_xCu_90-xAl₁₀(x=20, 30, 40, 50, 60, 70) ternary alloys. When the Zr composition is 30%, the glass transition temperature reaches the maximum value and the Zr₃₀Cu₆₀Al₁₀ owns high glass forming ability(GFA). Analysis of the short-range structure shows that there are more low-energy Zr-centered polyhedron with high coordination number(CN...     

Mg-Cu-Zn-Y-Zr bulk metallic glassy composite with high strength and plasticity

Mg₆₅Cu₂₀Zn₅Y₉Zr₁ bulk metallic glass matrix composite with a diameter of 2 mm was produced by copper mold casting. Upon cooling the Mg₆₅Cu₂₀Zn₅Y₉Zr₁ melt, Mg₂Cu acicular crystalline phase precipitates uniformly with a size of about 20 μm long and 1 μm thick while the remaining melt undergoes glass transition. Room temperature compression tests revealed that the high fracture strength up to 830 MPa and the plastic strain of 2.4% before failure are obtained for the Mg-based bulk metallic glass matrix composite. The formation of the Mg₂Cu phase was proposed to contribute to high strength and plastic deformation of the material.     

Cooling Rate Dependence of Structural Order and Energy Landscape in Zr55Cu35Al10 Glass

论冷却速率对Zr55Cu35Al10合金结构和能量的影响

赵静锋,谢祥续,陈锋,狄凯龙,周雪峰*

（常熟理工学院 材料工程学院,江苏 常熟 215500）

中文说明：

经典分子动力学模拟被广泛应用于非晶合金的快速冷却过程研究。然而,模拟冷却速率比实验冷却速率高几个数量级。本文以Zr55Cu35Al10合金为例,发现在略低于玻璃转变温度的温度下加入等温退火,延长等温退火时间可有效降低冷却速率。用这种方法获得的非晶样品具有显著的能量稳定性和更有序的中短程序结构。

关键词：经典分子动力学模拟,冷却速率,等温退火,能量稳定性,短程有序,中程有序

     

退火处理对Ti-Zr-Cu-Be块体非晶合金力学性能的影响

采用铜模铸造法制备了直径为2mm的Ti55-xZr10+xBe27.5Cu7.5(x=0,10,20)块体非晶合金,并对其进行等温退火处理.利用X射线衍射(XRD)、扫描电镜(SEM)、差氏扫描量热仪(DSC)及压缩试验等方法研究了非晶合金的相结构、显微组织和热稳定性,以及退火处理对其力学性能的影响.结果表明:该系列合金在553 K及583 K下保温长达5 h时间内依然表现为非晶态.退火处理后,Ti35Zr30Be27.5Cu7.5合金屈服强度、断裂强度均提到了提高,其中在583 K下保温1 h后屈服强度、断裂强度分别达到了1 921、2 169 MPa;其塑性由处理前的3.47%提高到了6.57%.Ti45Zr20Be27.5Cu7.5合金在退火后其力学性能变化不明显.Ti55Zr10Be27.5Cu7.5合金随着退火温度及保温时间的增加其屈服强度、断裂强度及塑性均明显降低.     

Zr41 Ti14 Ni12.5 Cu10 Be22.5非晶合金冲击压缩行为理论与实验研究

为研究Zr_(41)Ti_(14)Ni_(12.5)Cu_(10)Be_(22.5)非晶合金的冲击压缩响应及物态方程,利用平板冲击试验,测试了受到速度为350 m/s～550 m/s的铜板冲击时,非晶合金试样的自由面粒子速度;采用阻抗匹配法得到了材料的雨贡纽参数,并利用理想混合物模型对材料的雨贡纽参数进行预估,分析对比了理论计算结果与实验结果;将求材料格林艾森物态方程转化为非线性最优化问题,提出了基于模拟退火算法的材料格林艾森物态方程计算方法,设计了退火参数表,并采用解析法计算了材料的布里希-默纳罕物态方程和三项式物态方程,对比研究了三种物态方程与实验结果.理论和实验结果表明,5～10 GPa压力范围内,Zr_(41)Ti_(14)Ni_(12.5)Cu_(10)Be_(22.5)非晶合金的零压体积声速为4 267 m/s,D-u曲线斜率为4.376,曲线斜率远大于一般金属材料,其雨贡纽极限强度为5.60 GPa左右;理想混合物模型仅适用于计算高压区Zr基非晶合金的雨贡纽参数,低压区理论计算结果与实验结果误差较大;在5～10 GPa压力范围内,格林艾森物态方程、三项式物态方程与实验结果吻合度较高,而布里希-默纳罕物态方程误差较大,不适用于计算材料状态参量.     

Point-piezomagnetic properties of the FeCuNbSiB alloy strips

The piezomagnetic properties of rapidly quenched Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy strips were investigated in as-quenched state and after annealing for 2 h in vacuum in the temperature range of 100–300 °C. The impedance of amorphous strips increases with frequency, and sensitively decreases with stress increasing, especially in the frequency range of 10–100 MHz. The impedance increases approximately linearly with frequency below the critical frequency, but begins to decrease non-linearly with frequency above the critical frequency. The higher the point compressive stress is, the smaller the critical frequency will be. The stability of piezomagnetic properties is very excellent. The impedance of amorphous strips after annealing, especially at 300 °C, decreases very strongly. The impedance and the absolute values of the sensitive degree of couple layers’ amorphous strips are lower than those of single layer strips. Funded by the National Natural Science Foundation of China (No. 0576014)     

Influences of melt spinning on electrochemical hydrogen storage performance of nanocrystalline and amorphous Mg2Ni-type alloys

In order to improve the electrochemical hydrogen storage performance of the Mg₂Ni-type electrode alloys, Mg in the alloy was partially substituted by La, and the nanocrystalline and amorphous Mg₂Ni-type Mg_20−x La _x Ni₁₀ (x=0, 2) alloys were synthesized by melt-spinning technique. The microstructures of the as-spun alloys were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical hydrogen storage properties of the experimental alloys were tested. The results show that no amorphous phase is detected in the as-spun Mg₂₀Ni₁₀ alloy, but the as-spun Mg₁₈La₂Ni₁₀ alloy holds a major amorphous phase. As La content increases from 0 to 2, the maximum discharge capacity of the as-spun (20 m/s) alloys rises from 96.5 to 387.1 mA·h/g, and the capacity retaining rate (S ₂₀) at the 20th cycle grows from 31.3% to 71.7%. Melt-spinning engenders an impactful effect on the electrochemical hydrogen storage performances of the alloys. With the increase in the spinning rate from 0 to 30 m/s, the maximum discharge capacity increases from 30.3 to 135.5 mA·h/g for the Mg₂₀Ni₁₀ alloy, and from 197.2 to 406.5 mA·h/g for the Mg₁₈La₂Ni₁₀ alloy. The capacity retaining rate (S ₂₀) of the Mg₂₀Ni₁₀ alloy at the 20th cycle slightly falls from 36.7% to 27.1%, but it markedly mounts up from 37.3% to 78.3% for the Mg₁₈La₂Ni₁₀ alloy.     

Effects of Ta addition on the microstructure and mechanical properties of Ti40Zr25Ni8Cu9Be18 amorphous alloy

The effects of Ta addition on the microstructure and mechanical properties of Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ bulk amorphous alloy were investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), scan electron microscopy (SEM) and compressive testing. As a result, the addition of Ta (0-8at%) prompted the successive precipitation of quasicrystalline phase, CuTi₂ phase and bcc β-Ti solid solution. Additionally, the addition of less Ta content (3at%-5at%) led to the formation of amorphous matrix/nanoquasicrystal/CuTi₂ complex phase structure; and nanoquasicrystals, as reinforcement precipitates, improved the fracture strength of Ti-Zr-Ni-Cu-Be-Ta alloys, which led to the high compressive fracture strength 1856 MPa of Ta5 alloy. With increasing Ta content (5at%-8at%), although the ductile dendritic β-Ti solid solution was precipitated, the strength and plasticity decreased to a great extent resulting from the growth of quasicrystalline phase and CuTi₂ phase.     

Interfacial characteristics of high frequency induction brazed Cu and pre-metalized graphite joints

Oxygen-free copper and pre-metalized graphite were brazed using CuNiSnP braze alloy by high frequency induction heating method. Interfacial microstructures and reaction phases were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The strength and resistance of the joints were tested. It is found that when the brazing parameters are optimized, the structures of the joints are graphite/(Cu,Ni)/Ni(s.s)+Ni _x P _y /Cu₃P+Cu(s.s) (including Sn)+eutectic structures (Cu₃P+Ni₃P+Cu(s.s)/Cu (s.s)/Cu). When the temperature increases to 750 °C or the holding time prolongs to 300 s, the eutectic structures disappear and the amount of Cu₃P increases. The maximum shear strength of the joints is 5.2 MPa, which fracture at the interface of graphite and metallization. The resistance of the joints is no more than 5 mΩ.     

Influence of substituting Ni with Co on hydriding and dehydriding kinetics of melt spun nanocrystalline and amorphous Mg2Ni-type alloys

In order to improve the hydriding and dehydriding kinetics of the Mg₂Ni-type alloys, Ni in the alloy is substituted by element Co. The nanocrystalline and amorphous Mg₂Ni-type Mg₂Ni_1−x Co _x (x=0, 0.1, 0.2, 0.3, 0.4) alloys were synthesized by melt-spinning technique. The structures of the as-cast and spun alloys were studied with an X-ray diffractometer (XRD) and a high resolution transmission electronic microscope (HRTEM). An investigation on the thermal stability of the as-spun alloys was carried out with a differential scanning calorimeter (DSC). The hydrogen absorption and desorption kinetics of the alloys were measured with an automatically controlled Sieverts apparatus. The results demonstrate that the substitution of Co for Ni does not alter the major phase of Mg₂Ni but results in the formation of secondary phase MgCo₂. No amorphous phase is detected in the as-spun Co-free alloy, but a certain amount of amorphous phase is clearly found in the as-spun Co-containing alloys. The substitution of Co for Ni exerts a slight influence on the hydriding kinetics of the as-spun alloy. However, it dramatically enhances the dehydriding kinetics of the as-cast and spun alloys. As Co content (x) increases from 0 to 0.4, the hydrogen desorption capacity increases from 0.19% to 1.39% (mass fraction) in 20 min for the as-cast alloy, and from 0.89% to 2.18% (mass fraction) for the as-spun alloy (30 m/s).     

Influence of Mg3N2 powder on microstructures and mechanical properties of AZ31 Mg alloy

In order to obtain an effective and reliable grain refiner for Mg-Al alloys, 1% （mass fraction） Mg3N2 was added into AZ31 Mg alloy. The microstructures of the Mg alloys were studied by optical microscopy, scan electron microscopy and X-ray energy dispersive spectroscopy, and the mechanical properties were determined. The results show that adding a small amount of Mg3N2 to AZ31 Mg alloy can refine the grain size from 103 to 58 μm. The ultimate tensile strength and elongation of AZ31 Mg alloy are 174.1MPa and 8.3%, respectively. After the addition of 1% Mg3N2, the ultimate tensile strength and elongation of AZ31 Mg alloy are increased up to 198.7 MPa and 11.8%, respectively. The grain refinement mechanism is that AIN is formed after Mg3N2 is added. Both AIN and Mg phases are of HCP lattice structure, and the disregistry between Mg phases and AIN along （0001）Mg//（0001）AIN is 3.04%, which is very effective for heterogeneous nucleation.     

AFM research on Fe-based nanocrystal crystallization mechanism

The cross-section pattern of Fe-based alloy ribbon (Fe_73.5Cu₁Nb₃Si_13.5B₉) annealed at different temperatures was investigated by AFM (atomic force microscope), and the effect mechanism of Nb and Cu in Fe-based alloy ribbon annealing was analyzed with XRD diffraction crystal analysis technique and other research results. New concepts of encapsulated grain, Nb vacancy cluster, Nb-B atom cluster and so on were proposed and used to describe the formation mechanism of α-Fe (Si) nanocrystal. Finally, a three-phase (separation phase, encapsulated phase and nanocrystalline phase) interconnected structure model in Fe-based nanocrystalline alloy was established. Supported by the Natural Science Foundation of Zhejiang Province (Grant No. Y405021), Zhejiang Provincial Science and Technology Key Project (Grant No. 2006C21109) and Key Project of Science and Technology Research of China Ministry of Education (Grant No. 204059)     

Microstructure and mechanical properties of ultrafine grained Mg15Al alloy processed by equal-channel angular pressing

An as-cast magnesium alloy with high Al content Mg15Al was subjected to equal-channel angular pressing (ECAP) through a die with an angle of ϕ=90° at 553 K following route B_c. It is found that the network β-Mg₁₇Al₁₂ phases in the as-cast Mg15Al alloy are broken into small blocks and dispersed uniformly with increasing numbers of pressing passes. Moreover, many nano-sized Mg₁₇Al₁₂ particles precipitate in the ultra-fine α-Mg matrix. The grains are obviously refined. However, the grain structure is inhomogeneous in different areas of the alloy. The average size of the primary phase α-Mg is reduced to about 1 μm while grains of around 0.1–0.2 μm are obtained in some two-phase areas. With additional ECAP passes (up to 8), coarsening of the grains occurs by dynamic recovery. Room temperature tensile tests show that the mechanical properties of Mg15Al alloys are markedly improved after 4 ECAP passes. The ultimate tensile strength and elongation to failure increase from 150 MPa to 269.3 MPa and from 0.05% to 7.4%, respectively. Compared with that after 4 passes, the elongation to failure of the alloy increases but the strength of the alloy slightly decreases after 8 ECAP passes. Fracture morphology of the ECAP-processed alloy exhibits dimple-like fracture characteristics while the as-cast alloy shows quasi-cleavage fractures.     

Gaseous and electrochemical hydrogen storage kinetics of as-quenched nanocrystalline and amorphous Mg2Ni-type alloys

The nanocrystalline and amorphous Mg₂Ni-type Mg₂Ni_1?x Co _x (x = 0, 0.1, 0.2, 0.3, 0.4) alloys were synthesized by melt quenching technology. The structures of the as-cast and quenched alloys were characterized by XRD, SEM and HRTEM. The gaseous hydrogen storage kinetics of the alloys was measured using an automatically controlled Sieverts apparatus. The alloy electrodes were charged and discharged with a constant current density in order to investigate the electrochemical hydrogen storage kinetics of the alloys. The results demonstrate that the substitution of Co for Ni results in the formation of secondary phases MgCo₂ and Mg instead of altering the major phase Mg₂Ni. No amorphous phase is detected in the as-quenched Cofree alloy, however, a certain amount of amorphous phase is clearly found in the as-quenched alloys substituted by Co. Furthermore, both the rapid quenching and the Co substitution significantly improve the gaseous and electrochemical hydrogen storage kinetics of the alloys, for which the notable increase of the hydrogen diffusion coefficient (D) along with the limiting current density (I _L ) and the obvious decline of the electrochemical impedance generated by both the Co substitution and the rapid quenching are basically responsible.     

Crystallization kinetics and magnetic properties of Fe73.5Si13.5B9Cu1Nb1V2 nanocrystalline powder cores

Amorphous ribbons of the alloy Fe_73.5Si_13.5B₉Cu₁Nb₁V₂ were prepared by the standard single copper wheel melt spinning technique in the air atmosphere. The crystallization kinetics of amorphous ribbons was analyzed by non-isothermal differential scanning calorimetry (DSC) measurements. The crystallization activation energies of amorphous ribbons calculated by using Kissinger model were 364 and 337 kJ/mol for the first and the second crystallization, respectively. The Avrami exponent n was calculated from the Johnson-Mehl-Avrami (JMA) equation. The value of the Avrami exponent showed that the crystallization mechanism in the non-isothermal primary crystallization of amorphous ribbons was all shapes growing from small dimensions controlled by diffusion at decreasing nucleation rate. The variation of soft magnetic properties of nanocrystalline Fe_73.5Si_13.5B₉Cu₁Nb₁V₂ alloy powder cores as a function of milling times has been investigated. It is found that the effective permeability of the cores shows high frequency stability and decreases with the increase of milling times. The quality factor increases with increasing frequency in lower frequency range, and reaches a maximum at the frequency of 80 kHz then decreases gradually with increasing frequency.