二元液态金属AgxCu1-x快速凝固过程的分子动力学模拟研究

用quantum Sutton-Chen多体势对AgxCu1-x在冷却速率为2×1012K/s凝固过程的模拟,发现AgxCu1-x在x=35～65范围内具有形成非晶的能力,特别是对应于二元相图共晶点的Ag60Cu40在凝固过程中二十面体和其它多面体结构不仅数目大,而且还具有极为稳定的遗传性,而最容易得到非晶态合金.证实了合金化效应对非晶态合金的形成倾向和稳定性的关键作用.此外采用键对及原子多面体类型指数法对凝固过程中微观结构组态变化的分析,不但能说明二十面体结构在非晶态合金形成和稳定性中所起的关键作用,又有助于对液态金属的凝固过程、非晶态微观结构特征的深入理解.     

Ni-Al合金凝固过程的分子动力学模拟

利用分子动力学研究Ni3Al和NiAl合金在不同冷速下的凝固过程,分析冷却过程中不同温度下的偶分布函数、能量和体积的变化.研究表明:冷却速率为4×1013 K/s时,Ni3Al形成非晶结构;冷却速率为4×1011 K/s时,Ni3Al在1 100 K左右结构开始发生变化,最终形成晶体结构.冷却速率为4×1013 K/s时,NiAl形成非晶;冷却速率为4×1011 K/s时,NiAl在810 K形成部分的晶体结构.     

Nb、Ta等元素对Zr基大块非晶非晶形成能力及耐腐蚀性影响机理研究

  用计算机编程构造了Zr基非晶合金包含二十面体原子团簇的Zr₂Ni晶体相.用Zr₂Ni晶体相中以Ni原子为中心的二十面体原子团簇模拟Zr基非晶中二十面体团簇.应用实空间的递归方法计算了合金元素取代二十面体原子团簇中心和顶角位置原子时的局域态密度、二十面体原子团簇中中心Ni原子与其近邻合金元素Nb、Ta、Ti、V的总键级积分及团簇的费米能级.结果表明,Nb、Ta、Ti和V取代Zr原子后使其与Ni的键级积分有所降低,使合金的非晶形成能力下降,但Nb、Ta对合金的非晶形成能力影响不大;Nb、V取代Zr使费米能级升高,可使Zr基非晶容易钝化,提高耐蚀性;Ta对Zr基非晶的耐蚀性影响不大.而Ti使Zr基非晶的钝化能力下降.综合合金元素对非晶形成能力和耐腐蚀性的影响,Nb是最有益的合金元素,即在Zr基非晶中,通过加入少量的Nb元素,可以制备出具有较高耐腐蚀性的大块非晶.     

Modelling structural changes of a molten Cu55 cluster on cooling by molecular dynamics

应用分子动力学研究了一个熔融Cu55团簇冷却过程中变为二十面体的结构变化. 根据原子密度分布函数和对分布函数随温度变化的分析表明, 在降温过程中由于原子之间连续地交换位置, 团簇结构发生了三个阶 段的变化: 首先形成三壳层结构, 继之初步形成四壳层结构, 并在团簇内部开始形成由13个原子组成的二十面 体结构, 最终形成原子分布于4个壳层内具有二十面体结构的Cu$_{55}$团簇. 模拟中由原子密度分布函数确定了不同壳层的原子位置.     

熔融Cu55团簇在冷却过程中结构变化的分子动力学模拟

应用分子动力学研究了一个熔融Cu55团簇冷却过程中变为二十面体的结构变化.根据原子密度分布函数和对分布函数随温度变化的分析表明,在降温过程中由于原子之间连续地交换位置,团簇结构发生了三个阶段的变化:首先形成三壳层结构,继之初步形成四壳层结构,并在团簇内部开始形成由13个原子组成的二十面体结构,最终形成原子分布于4个壳层内具有二十面体结构的CU55团簇.模拟中由原子密度分布函数确定了不同壳层的原子位置.     

液态金属Ag6Cu4凝固过程中非晶转变的分子动力学模拟

用 quantum Sutton-Chen 多体势对液态金属Ag6Cu4凝固过程进行了分子动力学模拟研究. 在冷却速率为2×1012～2×1014 K/s范围内, Ag6Cu4总是形成非晶态结构, 在非晶态合金的形成和稳定性中起关键作用的是原子的二十面体结构. 采用键对及原子多面体类型指数法对凝固过程中微观结构组态变化的分析, 有助于对液态金属的凝固过程、非晶态结构特征的深入理解.     

分子动力学法对Cu-Ag合金熔化及凝固过程的模拟

借助镶嵌原子势(EAM)利用等压等温的分子动力学方法(NPT-MD)模拟共晶Cu40Ag60合金的熔化,凝固过程.利用径向分布函数和对分析技术研究Cu40Ag60台金在不同的冷却速率(1×1011,1×1012,5×1012,1×1013。及1×1014 K/s)下的非晶形成能力和微观结构演化.研究发现Cu40Ag60合金形成非晶态需要极大的冷却速率.模拟中的结构分析揭示了合金在冷却过程中微观结构的演化规律     

ZnxAl100-x合金快凝过程中微结构演变特性的分子动力学模拟

采用分子动力学方法对液态Zn_xAl_100-x（x=25,50,75）合金的快速凝固过程进行了模拟,并通过键型指数与原子团类型指数表征和分析了凝固过程中熔体微观结构的演变特性.结果表明,在冷速1×10¹²K/s下,3种成分的Zn-A1合金都形成了以1551键对和二十面体（12 0 12 0 0 0）基本原子团为主体的非晶结构,并且在玻璃转变温度T_g附近熔体中1551键对和二十面体基本原子团的数目最多;随Zn含量增加,合金玻璃转变温度T_g、非晶形成能力和化学短程序参数降低.对应不同成分的合金,Zn和A1呈现不同程度的偏聚与团簇化趋势.     

液态Ca70Mg30合金快速凝固过程中纳米团簇结构形成演变特性模拟

采用分子动力学方法对液态Ca70Mg30合金快速凝固过程中纳米团簇结构的形成和演变特性进行模拟。采用原子团类型指数法(CTIM)对凝固过程中纳米团簇结构的演变进行分析。结果表明：系统在5×1011 K/s的冷速条件下形成以1551、1541和1431为主的非晶态结构,非晶转变温度约为530 K;(120120)二十面体基本原子团对系统非晶结构的形成起到决定性的作用,并且原子半径较小的Mg原子更容易占据二十面体基本原子团中心原子的位置;同时,纳米团簇主要是通过中等尺寸团簇的合并而形成,纳米级大团簇的形成演变过程呈现出类似于非晶晶化过程的3个阶段式的变化。     

甩带速率对U_(66.7)Co_(33.3)合金非晶形成的影响

针对U_(66.7)Co_(33.3)二元合金,采用小型亚稳合金炉制备样品,结合X射线衍射和热分析技术,研究了不同甩带速率对该合金形成非晶相的影响。结果表明,该合金在50 m/s的甩带速率下几乎形成完全非晶相,在25 m/s速率下出现了体心四方结构的U_6Co相,在20 m/s以下主要形成了U_6Co相;提高合金熔体的冷却速率可有效增大非晶形成的驱动力,促进非晶相形成。     

Local atomic structure of Pd–Ni–P bulk metallic glass examined by high-resolution electron microscopy and electron diffraction

Structural fluctuation in a Pd₄₀Ni₄₀P₂₀ bulk metallic glass is investigated by transmission electron microscopy and electron diffraction. Local atomic ordered regions with a fcc-(Pd,Ni) type structure was sharply imaged by a high-resolution electron microscopy (HREM) attached with a Cs-corrector. Interference function for the glassy state was obtained from electron-diffraction intensity profiles using energy-filter and imaging-plate techniques. We used a reverse Monte Carlo (RMC) simulation method to develop a realistic structure model. The model consists of a dense-random-packing structure, in which an fcc ordered region with Pd, Ni, and P atoms was embedded. The structure model is consistent with the diffraction and HREM results. In Voronoi polyhedral analysis of the RMC simulated structure, P-centered (Pd,Ni)-P trigonal prisms are found primarily in the matrix structure embedding the fcc-cluster. Around Pd and Ni atoms deformed-fcc type polyhedra were frequently observed. From these local structural features, nanoscale phase separation was revealed to occur during the glass formation.     

Energetic analysis for optimum amorphous compositions in binary systems

Quantifying glass forming ability of metallic glasses concerns the thermodynamic, the kinetic and the structural aspects. From the thermodynamic point of view, under the consideration of the driving forces for the formation of crystalline phases from supercooled liquids, the formation of crystalline phases can be regarded as the opposite of the retention of the amorphous structure on cooling, and the optimum glass forming compositions can be related to the local minima of the driving force-composition curves. Based on the optimized thermodynamic parameters from literature, the calculated results in the Cu–Hf, the Cu–Zr, the Ni–Zr, the Nb–Ni, the Ti–Ni and the Pd–Si binary systems are compared with experimental measurements, with special attentions to the off-eutectic glass forming compositions, the composition dependent amorphous stabilities and the systems with wide optimum glass forming ranges.     

EFFECT OF HEAT OF MIXING ON FORMATION AND STABILITY OF Zr-BASE METALLIC GLASSES

用熔融自旋技术制备了Zr_(1-x)M_x(M=Fe,Co,Ni,Cu和Pd)和Cu_(67.5)Ti_(32.5)合金玻璃体。用变换转鼓切线速度的方法,确定了上述合金形成玻璃态的临界切线速度为31m/s。所有合金玻璃体经X射线衍射分析证明为玻璃态结构。用示差扫描量热器,以不同的升温速率测量了它们的热行为。在Fe_(24)Zr_(76),Ni_(24)Zr_(76)合金玻璃体中呈现出一个放热峰,而Co_(21)Zr_(79),Pd_(24.5)Zr_(75.5),Cu_(45)Zr_(55)和Cu_(67.5)Ti_(32.5)则有两个或三个放热峰。用Kissinger技术确定了各个合金玻璃的结晶激活能。最后讨论了由Miedema模型计算的二元合金的混合热对合金玻璃体形成和稳定性的影响。     

Effects of Nitrogen on the Glass Formation and Mechanical Properties of a Ti-Based Metallic Glass

Effects of nitrogen addition on glass formation and mechanical properties of the Ti42.5Cu40Zr10Ni5Sn2.5metallic glass were systematically investigated. It was found that a small amount of nitrogen addition facilitated the glass formation by suppressing formation of the competing eutectic structure. Unlike large atomic size elements such as Hf and Pd which usually deteriorate specific strength, nitrogen can also increase the specific strength of the current Ti-based BMGs. The results are not only helpful for understanding glass-forming ability in general, but also useful in developing cost-effective, high-performance Ti-based bulk metallic glasses with enhanced glass-forming ability.     

Ni- and Cu-free Ti-based metallic glasses with potential biomedical application

In the present work Ti–Fe–Si and Ti–Fe–Si–X (X = Zr, Pd, Ge) glassy alloys are discussed as potential biomedical materials. Depending on composition and experimental conditions these alloys possess glassy, quasicrystalline or crystalline structure. The glassy state and crystallization behavior of the melt spun ribbons were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and the Hank's solution was used as simulated body fluid for corrosion tests. Ternary Ti–Fe–Si alloys near the Ti₆₅Fe₃₀Si₅ eutectic point were prone to form quasicrystals if the cooling rate was not high enough to retain amorphous structure. The compositions on the steeper side of the eutectic point could be vitrified. The results indicate that small additions of Zr can have a positive effect on glass formation, while additions of Ge, Pd may have a detrimental effect by promoting crystallization. Ti–Fe–Si and Ti–Fe–Si–Zr alloys exhibited high corrosion properties, superior to that of pure Ti and most of Ti-based glassy alloys reported in the literature. Being free of Ni and Cu this group of alloys may be considered for possible biomedical application.     

KINETIC STUDY ON FORMATION OF BULK GLASS STATE Zr52.5Ni14.6Al10Cu17.9Ti5 ALLOY

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原子有序度对Zr55Al10Ni5Cu30非晶合金等温晶化过程的影响

通过改变母合金铸锭的重熔次数和熔体冷却速率获得3种具有不同原子有序度的Zr55Al10Ni5Cu30块体非晶合金,并采用差示扫描量热仪研究了原子有序度对该合金等温晶化过程的影响。结果表明,在等温晶化条件下,原子有序度的增加使Zr55Al10Ni5Cu30块体非晶合金孕育期明显减小;对晶化相的形核和长大机制几乎没有影响,均为形核速率随时间增加的扩散控制的三维长大;使Avrami指数出现一定程度的减小。     

Formation and crystallization of bulk Pd_(82)Si_(18) amorphous alloys

1　INTRODUCTIONMetallicglassisregardedasastatethatisofdis orderunlikecrystalalloyswith periodicatomstruc ture.Soitshowsexcellentcapabilitiesofsoftmag netism ,mechanics ,corrosionresistance ,etc .How ever ,mostofamorphousalloyswereproducedbyus ingrapidsolidificationmethodssuchassplatquench ing ,meltspinning ,andsoon ,withcharacteristiccoolingratesof 10 4 10 6 K/s .Becauseamorphousal loysarepreparedwithsilk ,powderandribbon ,itisgreatlylimitedinengineeringapplication .Recently ,severalbulk…     

块体非晶合金在低温下的剪切带特征

通过宏观洛氏压痕实验在低温(193K)下对玻璃转变温度和本征脆性具有显著差别的5种典型非晶合金(Mg65Cu25Gd10,La55Al25Cu10Ni5Co5,Pd43Cu27Ni10P20,Zr47.9Ti0.3Ni3.1Cu39.3Al9.4,Fe52Cr15Mo9Er3C15B6)的剪切带特征进行了研究。考查了剪切带间距和数量与归一化温度和材料本征脆性的关联,并根据放射状剪切带的夹角讨论了不同合金的压力敏感系数及其影响因素。     

Evidence of refilled chamber gas pressure enhancing cooling rate during melt spinning of a Zr_(50)Cu_(40)Al_(10) alloy

The influence of the refilled gas pressure on the glass forming behaviour of one of the best ternary glass forming alloys Zr_(50)Cu_(40)AI_(10) was studied for the melt spinning process.The amorphicity of as-quenched ribbons was characterized by X-ray diffraction(XRD) and differential scanning calorimetry(DSC).The refilled chamber atmospheric pressure is crucial to the cooling rate of melt spinning.At high vacuum,at pressure less than 0.0001 atm,fully crystalline fragments are obtained.Monolithic amorphous ribbons were only obtained at a gas pressure of 0.1 atm or higher.The extended contact length between thecribbons and the copper wheel contributes to the high cooling rate of melt spinning.Higher chamber gas pressure leads to more turbulence of liquid metal beneath the nozzle;therefore,lower pressure is preferable at practical melt spinning processes once glass forming conditions are fulfilled.