大块非晶临界冷却速率的非等温转变计算模型

在非等温转变理论以及非稳态形核理论基础上提出了计算大块非晶合金连续冷却转变曲线和 临界冷却速率的新模型,以用于评估合金的非晶形成能力. 依据此模型对Zr基和Pd基8种合金 进行了计算，计算结果与实验值符合较好. 计算结果表明，影响临界冷却速率的主要因素为 黏度、临界形核功和临界结晶分数. 随着黏度增大，临界冷却速率降低；随临界形核功增大 ，临界冷却速率急剧降低；随临界结晶分数增大，临界冷却速率起初降低较快，达到一定程 度后下降速率趋于缓慢. 关键词： 临界冷却速率 非晶形成能力 大块非晶 连续冷却转变曲线     

亚稳相的高压暴露

 很久以前,便有人指出,气态冷凝成固态时,要连续经历液相及各种高温相,才达到平衡结晶相。但是,液态及高温相往往需靠很大的冷却速度才能冻结下来,这在当时对绝大多数合金,是不可能的。近些年,随着超急冷等技术的进步,关于非晶等亚稳相得研究十分活跃。当超过一定临界冷却速度时,液态合金可固化为非晶态。虽然,亚稳结晶相较非晶应更容易冻结,但是,由于产生各种亚稳相所需的过冷条件各不相同,以及对冷却速度的选择不能是任意的,因此有时它们较非晶还难于形成。与液相凝固过程相似,非晶合金的晶化也服从构型最小重排原理,即在晶化完成之前,存在某些亚稳相变态阶段。但是,限于热力学上的不稳定性及动力学因素,在常压下这些亚稳相同样是难以发现的。作者根据对多种合金系的研究,提出高压暴露亚稳相的设想,并利用非晶等亚稳相的高压变态过程,将进行液态急冷时的速度控制方式,改为便于掌握的高压退火方式,来获得新亚稳相。本文对压力暴露亚稳相的原理和实践,加以论述。     

碳酸二甲酯与烷烃混合物的临界性质

本文基于低停留时间流动法建立了一套临界性质实验测量系统,该系统温度和压力的测量不确定度分别小于0.20 K和5.2 kPa。首次测量了碳酸二甲酯与正己烷、正庚烷、正辛烷、正壬烷和正癸烷五种烷烃的二元混合体系的临界性质,结果表明:各混合体系的临界温度随碳酸二甲酯混合比例的增加呈抛物线变化,临界压力随混合体系中临界压力较大物质混合比例的增加而增大。运用Redlich-Kister方程对以上混合体系临界性质进行了关联,临界温度和临界压力的平均绝对偏差分别小于0.04%和0.25%.     

调幅分解及形核对相分离作用机理研究

以SCN-30wt%H₂O, SCN-50 wt%H₂O和SCN-80 wt%H₂O三组透明体系, 在恒温场下实现了形核和调幅分解两种过程; 在此基础上, 施加温度梯度, 研究了第二相液滴的迁移运动规律. 结果表明, 相分离在临界成分体系以调幅分解方式进行, 在另外两种体系中以形核长大方式进行; 调幅分解与形核过程相比, 反应进行得更快, 液滴长大到同一尺寸所需时间仅为形核所需时间的1/3—1/2. 且临界成分体系有更大的不混溶间隙, 所以第二相液滴具有更多迁移时间, 揭示了偏晶体系相分离过程中在临界成分处易获得壳-核组织的内在机理. 在单向温度场中, 测量了不同半径的液滴迁移速率, 并且与理论Marangoni迁移速率值作比较, 发现液滴迁移速率和Marangoni理论迁移速率符合较好. 说明了在较好地抑制自然对流条件下Marangoni迁移对于相分离过程起主要作用.     

锗在高压下固化时的相演化及动力学成因

 报道了锗在高压下固化时亚稳相的形成和转化。通过原位观测锗在高压下固化过程中的电阻变化、差热信号，并结合X射线衍射分析，详细讨论了锗在高压下固化时的相演化行为。在我们所进行的实验条件下，当压力大于4 GPa时，熔态锗首先固化成金属锗，然后再向GeⅠ或GeⅢ相转变。还讨论了压力对锗的固化动力学参数的影响。     

Ti3Al合金凝固过程晶核形成及演变过程的模拟研究

采用分子动力学方法对Ti₃Al合金的形核机理进行了模拟研究,采用团簇类型指数法(CTIM),对凝固过程不同尺度的原子团簇结构进行了识别和表征,深入研究了临界晶核的形成和长大过程.结果表明,凝固过程体系包含了数万种不同类型的原子团簇结构,但其中22种团簇结构类型对结晶形核过程起关键性作用.在晶核的形成和长大过程,类二十面体(ICO)原子团簇、类BCC原子团簇和缺陷FCC及缺陷HCP原子团簇在3个特征温度点T₁ (1110 K), T₂ (1085 K)和T₃ (1010 K)时达到数量上的饱和,并根据数量和空间分布随温度的变化,得到了它们在形核和长大过程相互竞争的关系.跟踪平行孪生晶粒形成和长大的过程发现,临界晶核是由FCC原子构成的单相结构,并未观察到亚稳BCC相优先形核的过程;平行孪生结构是由FCC单相晶核在沿密排面逐层生长过程中形成的.结果还表明, CTIM相比于其他微观结构表示方法,能更为准确地揭示凝固过程微观结构的转变特征.     

压力对非晶铜形成影响的分子动力学模拟

用分子动力学模拟法研究了Cu熔体在冷却过程中形成非晶的能力. 结果表明，压力使Cu熔体在冷却过程中形成晶体的倾向增加. 高压下冷却形成的Cu非晶中含有较多的晶态原子团，有序度较高.     

高压下Pd82Si18合金凝固过程中形成的亚稳相

 使用X射线衍射、差热分析、透射电镜及Ｘ射线能谱分析，研究了Pd₈₂Si₁₈合金在高压下的凝固特点及所形成的相的结构和稳定性。在5.5 GPa压力下以100 K/s的冷却速率淬火，得到了面心立方结构钯硅固溶体（MS-Ⅰ），其点阵常数为a=0.388 3 nm。与钯硅固溶体共存的还有另一未知结构亚稳相（MS-Ⅱ）。在700 K时上述亚稳相转变成平衡相Pd+Pd₃Si。     

高压下液态Ag-20 at%Ge合金的凝固及密排六方亚稳相的形成

 利用高压淬火方法，在液态Ag-20at%Ge合金的凝固过程中，对比液态急冷低四个数量级的冷却速率得到了单相密排六方结构亚稳相，其点阵参数为a=0.289 8 nm，c=0.470 8 nm，c/a=1.625。     

快速压致凝固法制备块体亚稳材料的研究进展

综述了快速增压使熔体凝固以获取非晶等亚稳相的方法和研究进展。介绍了自行研制的快速增压装置,展示了制备单质硫的大块非晶材料、单质硒的块体密实纳米材料、两种镧系基合金玻璃的块体材料,以及聚对苯二甲酸乙二醇酯(PET)、聚醚醚酮(PEEK)、等规聚丙烯(iPP)大块高分子亚稳材料的实验结果,证明了快速压致凝固法是制备块体亚稳材料的一种普遍可行的方法。最近的实验结果表明:快速压致凝固法获得的块体亚稳材料尺寸不受热传导率的限制;在快速压致凝固过程中,存在形成亚稳相的临界压力和临界增压速率两个限制条件。     

Formation of nano-crystalline and amorphous phases on the surface of stainless steel by Nd:YAG pulsed laser irradiation

Thin surface layers consisting of nano-crystalline and amorphous phases on the surface of stainless steel have been attained under the Nd:YAG pulsed laser irradiation. The phases and microstructures were investigated by X-ray diffraction (XRD) and high resolution transmission electron microscope (HRTEM). The phase compositions of the surface determined by XRD were α-Fe (ferrite) and γ-Fe (austenite) or only γ-Fe in the near surface region on the bases of the different laser power densities. The nano-crystalline grains with sizes of 4-100 nm could result from high cooling rate and crystallization in amorphous region by homogeneous and heterogeneous nucleation. The formation of the amorphous phase was attributed to the higher cooling rates.     

Phase transitions in condensed media at a finite rate of formation of a metastable state

The influence of the finite rate of formation of a metastable state on the kinetics of the first-order phase transition is analyzed. The conditions determined by the thermodynamic parameters and the cooling rate of the system under consideration are derived. Under these conditions, the formation of the metastable state can be treated either as an instantaneous process, when nucleation occurs at the end of the cooling stage, or as a slow process, when intensive nucleation of a new phase proceeds within the cooling stage. An equation describing the time and temperature that correspond to intensive nucleation of new-phase particles is obtained. The nucleation stage of the new phase takes place in the immediate vicinity of the temperature determined from this equation. All the other parameters, which determine the kinetics of the initial and transient stages of the phase transition, are calculated with respect to this temperature. As an example, all the relationships for a weak solid solution are presented.     

Continuous-cooling α-to-γ transformation behaviour of Ti–45.5 at.% Al–0.05 at.% B alloy

The continuous-cooling transformation behaviour of Ti–45.5?at.%?Al–0.05?at.%?B alloy was quantitatively measured using a real-time resistivity–temperature–time measurement apparatus operating under a high vacuum. The addition of a small amount of B does not significantly alter the α–γ-phase equilibria but significantly raises the α–γ lamellar start temperature of Ti–45.5?at.%?Al alloy at most cooling rates. Furthermore, it markedly increases the critical cooling rate for the ordering reaction. The effect of B addition, which greatly stabilizes the lamellar structure up to a fast cooling rate, is to accelerate the lamellar formation kinetics; the lamellar spacing was nevertheless distinctively larger in a B-doped alloy. This is because lamellae in B-doped alloy nucleate heterogeneously on titanium borides at the grain boundary; the borides are effective nucleation sites particularly since local Ti depletion can occur near the interface of the growing titanium borides during cooling. In the absence of B addition, the lamellar structure starts to form only at temperatures below T ₀, suggesting that a large undercooling is required for the nucleation of lamellae even at the grain boundaries. On the other hand, the B addition greatly retards the kinetics of the α-to-α₂ ordering reaction by markedly increasing its critical cooling rate without a large change in the ordering temperature. This is believed to be due to its tendency to segregate strongly to the antiphase boundaries.     

Monte-Carlo study of scaling exponents of rough surfaces and correlated percolation

In spallation neutron sources, liquid mercury, upon adsorbing the proton beam, is exhibited to large thermal and pressure shocks. These local changes in the state of mercury can cause the formation of unstable bubbles in the liquid, which can damage at their collapse the enclosing the liquid solid material. While there are methods to deal with the pressure shock, the local temperature shock cannot be avoided. In our paper we calculated the work of the critical cluster formation (for mercury micro-bubbles) together with the rate of their formation (nucleation rate). It is shown that the homogeneous nucleation rates are very low at the considered process conditions even after adsorbing several proton pulses, therefore, the probability of temperature induced homogeneous bubble nucleation is negligible.     

Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas

We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H₂ or H₂S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c     

Ab Initio computer simulation of the early stages of crystallization: application to Ge(2)Sb(2)Te(5) phase-change materials

By virtue of the ultrashort phase-transition time of phase-change memory materials, e.g., Ge(2)Sb(2)Te(5), we successfully reproduce the early stages of crystallization in such a material using ab initio molecular-dynamics simulations. A stochastic distribution in the crystallization onset time is found, as generally assumed in classical nucleation theory. The critical crystal nucleus is estimated to comprise 5-10 (Ge,Sb)(4)Te(4) cubes. Simulated growth rates of crystalline clusters in amorphous Ge(2)Sb(2)Te(5) are consistent with extrapolated experimental measurements. The formation of ordered planar structures in the amorphous phase plays a critical role in lowering the interfacial energy between crystalline clusters and the amorphous phase, which explains why Ge-Sb-Te materials exhibit ultrafast crystallization.     

非晶物质中的临界现象

非晶态材料有着复杂的原子结构(短程有序、长程无序)和特殊的物理性质,其临界现象和相变问题一直受到学术界关注.非晶合金,又称为金属玻璃,是一种新型的非晶态材料,具有很高的强度和优异的弹性.从微观的角度来看,非晶合金可以看作是一个多粒子系统.临界现象的研究对认识和理解多粒子系统之间的相互作用有深刻的意义.本文主要讨论非晶合金中的临界现象,包括非晶合金从制备过程、微观结构到宏观的力学性能以及磁性方面存在的临界现象,并分析这些临界现象之间的内在联系,进而深入理解非晶合金的微观结构对其宏观性质的影响.这为认识非晶合金的形成本质,提高服役可靠性,探索具有实际应用价值的非晶合金提供理论依据.     

Deep metastable eutectic nanometer-scale particles in the MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system

Laboratory vapor phase condensation experiments systematically yield amorphous, homogeneous, nanoparticles with unique deep metastable eutectic compositions. They formed during the nucleation stage in rapidly cooling vapor systems. These nanoparticles evidence the complexity of the nucleation stage. Similar complex behavior may occur during the nucleation stage in quenched-melt laboratory experiments. Because of the bulk size of the quenched system many of such deep metastable eutectic nanodomains will anneal and adjust to local equilibrium but some will persist metastably depending on the time–temperature regime and melt/glass transformation.     

On the formation of amorphous layers by interdiffusion in zirconium-copper system

Formation of amorphous phase by solid state reaction has been observed in bulk copper-zirconium couples. Transmission electron microscopy (TEM), Auger electron spectroscopy (AES) and differential scanning calorimetry (DSC) techniques have been employed to detect and study the amorphous phase. The experimental observations have been explained with a model, which considers the breakdown of the a-zirconium lattice due to the solute enrichment beyond a certain limit where the nucleation of the amorphous phase becomes thermodynamically possible. The present-experiments convincingly show that amorphous phase can form in bulk diffusion couples made from large-grained well-annealed material and conclusively establish that the presence of crystal defects in high concentrations is not a pre-requisite for the formation of amorphous phase.