Microstructure and Tribological Properties of Laser Clad gamma/Cr7C3/Tic Composite Coatings on gamma-TiA1 Intermetallic Alloy

In order to improve the wear resistance of the gamma-TiAl intermetallic alloy, microstructure, room- and high-temperature (600 degrees C) wear behaviors of laser clad gamma/Cr7C3/TiC composite coatings with different constitution of NiCr-Cr3C2 precursor-mixed powders have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS), block-on-ring (room-temperature) and pin-on-disk (high-temperature) wear tests. The responding wear mechanisms are discussed in detail. Results show that microstructures of the laser clad composite coatings have non-equilibrium solidified microstructures consisting of primary hard Cr7C3 and TiC carbides and the inter-primary gamma/Cr7C3 eutectic matrix, about three to five times higher average microhardness compared with the TiAl alloy substrate. Higher wear resistance than the original TiAl alloy is achieved in the clad composite coatings under dry sliding wear conditions, which is closely related to the formation of non-equilibrium solidified reinforced Cr7C3 and TiC carbides and the positive contribution of the relatively ductile and tough gamma/Cr7C3 eutectics matrix and their stability under high-temperature exposure.

Numerical simulation of temperature field evolution of metallic alloy surface due to pulsed laser treatment

This paper performed a numerical simulation on temperature field evolution for the surface layer of a metallic alloy subjected to pulsed Nd:YAG laser treatment. The enthalpy method was adopted to solve the moving boundary problem, I.e. Stefan problem. Computational results were obtained to show the temperature field evolution. Effects of latent heat and mushy zone width on the temperature field were investigated. The results also show very high values of temperature gradient and cooling rate, which are typical characteristics during the solidification process.

Glass Transition and Thermal Stability of Hard Magnetic Bulk NdAlFeCo Metallic Glass

Glass transition and thermal stability of bulk Nd60Al10Fe20Co10 metallic glass were investigated by means of dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The glass transition temperature, not revealed by DSC, is alternatively determined by DMTA via storage modulus E' and loss modulus E" measurement to be 498 K at a heating rate of 0.167 K s (-1). The calculated reduced glass transition temperature (T-g/T-m) is 0.63. The large value of T-g/T-m of this alloy is consistent with its good glass-forming ability. The crystallization process of the metallic glass is concluded as follows: amorphous --> amorphous + metastable FeNdAl phase --> amorphous + primary delta-FeNdAl phase --> primary delta-phase + eutectic delta-phase + Nd3Al + Nd3Co. The appearance of hard magnetism in this alloy is ascribed to the presence of amorphous phase with highly relaxed structure. The hard magnetism disappeared after the eutectic crystallization of the amorphous phase. (C) 2002 Elsevier Science B.V. All rights reserved.

Nonequilibrium microstructures and phase evolutions of an iron-based laser clad coating

The microstructural and compositional features of the laser-solidified microstructures and phase evolutions occurring during high temperature tempering were investigated by using analytical electron microscopy with energy dispersive X-ray analysis. The cladded alloy, a powder mixture of Fe, Cr, W, Ni and C with a weight ratio of 10:5:1:1:1, was processed with a 3 kW continuous wave CO2 laser. The cladded coating possessed the hypoeutectic microstructure of the primary dendritic gamma-austenite and interdendritic eutectic consisting of (gamma+M7C3). The gamma-austenite is a nonequilibrium phase with extended solid solution of alloying elements. And, a great deal of fine structures, i.e., a high density of dislocations, twins, and stacking faults existed in austenite phase. During high temperature aging, the precipitation of M23C6, MC and M2C in austenite and in situ transformation of M7C3(+gamma) --> M23C6 and M7C3+gamma --> M6C occurred. The laser clad coating revealed an evident secondary hardening and superior impact wear resistance.

Rapidly solidified nonequilibrium microstructure and phase transformation of laser-synthesized iron-based alloy coating

The rapidly solidified microstructural and compositional features, the precipitation and transformation of carbides during tempering, and the impact wear resistance of an iron-based alloy coating prepared by laser cladding are investigated. The clad coating alloy, a powder mixture of Fe, Cr, W, Ni, and C with a weight ratio of 10:5:1.1.1, is processed using a continuous wave CO, laser. Microstructural studies demonstrate that the coating possesses the hypoeutectic microstructure comprising the primary dendritic gamma-austenite and interdendritic eutectic consisting of gamma-austenite and M7C3 carbides. gamma-Austenite is a non-equilibrium phase with an extended solid solution of alloying elements. During high temperature tempering at 963 K for 1 h, the precipitation of M23C6, MC and M2C carbides in austenite and in situ carbide transformation of M7C3 to M23C6 and M7C3 to M6C respectively are observed. In addition, the microstructure of the laser-clad coating reveals an evident secondary hardening and a superior impact wear resistance.

Diffusion dominated process for the crystal growth of a binary alloy

The pure diffusion process has been often used to study the crystal growth of a binary alloy in the microgravity environment. In the present paper, a geometric parameter, the ratio of the maximum deviation distance of curved solidification and melting interfaces from the plane to the radius of the crystal rod, was adopted as a small parameter, and the analytical solution was obtained based on the perturbation theory. The radial segregation of a diffusion dominated process was obtained for cases of arbitrary Peclet number in a region of finite extension with both a curved solidification interface and a curved melting interface. Two types of boundary conditions at the melting interface were analyzed. Some special cases such as infinite extension in the longitudinal direction and special range of Peclet number were reduced from the general solution and discussed in detail.

分析金属熔滴快速凝固过程的双倒易边界元方法

本文发展了非线性边界条件相变传热过程的轴对称双倒易边界元方法，数值模拟了金属熔滴在快速冷却条件下的快速凝固过程。分别研究了在微重力落管和落塔中及喷射成形过程中金属熔滴的快速凝固过程，得到了过冷度，再辉时间，温度变化及相变界面随时间的变化等数值结果。

连续冷却多阶段快速凝固过程起始形核温度的计算

基于快速凝固的瞬态形核理论和对连续冷却条件下整个温度区间的瞬态形核数和晶核体积积分，导出了连续冷却多阶段快速凝固条件下过冷熔体中各析出相起始形核温度的计算方法。假定界面传热处于牛顿冷却方式，对Ni－Al合金平面流铸条带形成过程中析出的化合物相起始形核温度做了计算。

Cracks in saturated sand

The formation mechanism of water film (or crack) in saturated sand is analyzed numerically It is shown that there will be no stable "water film" in the saturated sand even if the strength of the skeleton is zero and no positions are choked. The stable water films initiate and grow if the choking state keeps unchangeable once the fluid velocities of one position decreases to zero in a liquefied sand column. A simplified method for evaluating the thickness of water film is presented according to a solidification wave theory. The theoretical results obtained by the simplified method are compared with the numerical results and the experimental results of Kokusho.

超分辨近场结构中二元共晶合金掩膜的工作机制

超分辨近场结构(super-RENS)技术通过在传统光盘结构中插入掩膜结构而实现近场超分辨,是目前最具实用化前景的超高密度光存储技术之一,其中掩膜层的近场光学特性是决定其光存储性能的关键。利用三维时域有限差分法(3D-FDTD)对合金掩膜的近场光强分布进行了数值仿真和分析,提出二元共晶合金薄膜在激光作用下形成的规则微结构可能是以其作为掩膜层的超分辨近场结构光盘产生较高信噪比(SNR)的原因。

Thermal, spectroscopic and laser properties of Yb3+Na+: CaF2 single crystals

The effects of Na+ doping level on the thermal conductivities, absorption and emission spectra, and fluorescence lifetimes of Yb3+ ,Na+ :CaF2 crystals were systematically studied. Sites structure, covalent force, and crystal field strength of Yb3+ :CaF2 crystals were markedly varied by codoping Na+ as charge compensator. The 2.0at% Yb3+ and 3.0at% Na+-codoped CaF2 crystal was demonstrated to operate in diode-pumped passively mode-locking scheme. Transform-limited 1 ps laser pulses were obtained, showing the crystal capable of producing ultra-short laser pulses. (c) 2006 Elsevier B.V. All rights reserved.

2种离子固化剂改善黄土抗剪强度和抗渗性的研究

为了探讨CONAID和LUKANG 2种离子固化剂的固土性能及其影响因素,对不同固化剂掺量、养护龄期、压实度和含水率的固化黄土进行了直剪试验和渗透试验。研究结果表明:随着固化剂掺量、养护龄期和压实度增大,2种固化土的内摩擦角和黏聚力呈上升趋势,渗透系数呈下降趋势,且规律比较接近。其中LUKANG固化剂对黄土抗剪强度和抗渗性的改善效果优于CONAID固化剂。建议在施工过程中选用LUKANG固化剂,掺量宜取0.01%。为了达到更好的抗剪和抗渗效果,应尽量延长固化土的养护龄期,增加压实度。

胶体体系的固化及分离过程的计算机模拟

胶体体系是自然界很重要的组成部分，广泛应用于工业生产和日常生活中。胶体体系在固化的过程中，会形成有序的晶体结构或无序的玻璃态结构。对胶体的结晶和玻璃化的认识对研究相变理论，纳米材料以及自组装有着重要的意义。事实上，在这一领域存在着许多悬而未决的问题。另一方面，胶体的分离也是胶体科学研究中不可或缺的步骤。特别是近年来微流器件技术的进步为流体动力学色谱分离方法带来了进一步发展的机会，这为胶体的分离提供了非常有效的实验手段。然而，由于实际应用过程的复杂性，理论方法还不足以深入探讨胶体粒子体系固化和分离的本质。作为一种可以直接反映物理过程的研究方法，计算机模拟成为一种有效的研究胶体粒子固化和分离的手段。 硬球模型是一种最简单的胶体粒子体系，能够很好地描述具有体积排斥作用的胶体粒子的行为，因此，对于胶体粒子体系固化问题，我们采用分子动力学方法研究了硬球粒子体系的固化过程。此外，对于胶体和高分子体系的分离过程，我们主要采用耗散分子动力学（dissipative particle dynamics DPD）进行研究。论文的主要内容包括： 1)采用硬球分子动力学模型，研究了胶体颗粒在不同的压缩速率下得到的堆积态的结构异质性。我们应用Lubachevsky-Stillinger方法，在不同的压缩速率下得到了一系列的结构受限的硬球堆积状态。这些体系的有序度随着压缩速率的减小而增加，而体系的密度则是先增加后减小又增加，在中间压缩速率时出现最小值。通过测量体系中的局域序参量分布，粒子位置的均方根涨落分布和直接观察受限粒子的位置，发现这些状态中包含有无规区域和结晶区域，存在显著的结构异质性。特别的是低密度状态中许多小的有序区域互相连接成贯穿体系的逾渗网络，说明晶区之间的结构不匹配是生成这些状态的重要机理。 2)用DPD方法模拟了线性高分子链和星形高分子链在穿越纳米孔道的输运过程。对复合管道中小管的半径分别为2.5 和2.0 的两种情况进行了研究。在各种情况下我们都发现了存在一个临界流量Jc，只有当流量大于Jc的时候高分子才能进入纳米孔洞。对于线性高分子，长链的高分子进入同样粗细的小管所需要的临界流量比短链大。线性链和星形链在半径是2.0小管子中的临界流量比通过半径2.5的管子的临界流量大，或者近似相同。在半径2.5的小管中，星形高分子的临界流量要大于同等尺寸的线性高分子。此外，我们没有观察到不同接枝数的星形高分子的临界流量之间的差异。出现这种情况的原因可能是DPD模型中粒子间的相互作用势能很软，所以不能很好地体现出不同接枝数的星形高分子在位阻上的差异。 3）我们利用DPD方法模拟了圆柱形管道中线性高分子链的输运过程。比较了模拟得到的高分子保留时间和DG理论预测值之间的差异。发现分子量越大的高分子对DG理论的偏离越显著。计算了高分子链在流动过程中伸长的情况，发现了链的伸长率S与Weissenberg number 有着明确的标度关系。与狭缝中的情况类似，对于圆柱型管道中流动的线性高分子，如果采用修饰过的Peclet number，则能够更好地满足Aris-Taylor分散的标度关系。 4)我们建立了纳米粒子在DPD模拟的动力学模型，研究了在本体溶液中纳米粒子的自扩散系数和速度关联函数对与胶体粒子的半径定性关系。发现纳米粒子的扩散系数随粒子半径的增加而减小。纳米粒子的半径越大，其速度关联函数的衰减越慢，同样大小纳米粒子的质量越大，其速度关联函数衰减得越慢。我们进一步模拟了在狭缝中纳米粒子的输运过程。通过观察流体受限方向的流速曲线，我们发现由于与器壁之间的体积排斥作用，使得纳米粒子远离流体的边界。对Aris-Taylor分散的研究结果表明，在λ不大的时候，应用不同的Peclet numbers 对分散方程结果的影响不大。

Fabrication of nano-sized grains by pulsed laser surface melting

In this work, the formation and characterization of nano-sized grains on the modified surfaces of GCr15 and H13 steels have been investigated. The material was processed by pulsed laser surface melting (LSM) under different depths of de-ionized water. The microstructures and phases of the melted zones were examined by x-ray diffraction, environmental field emission scanning electron microscopy and high resolution transmission electron microscopy. The results indicate that LSM under water can successfully fabricate nano-scaled grains on the surfaces of steel, due to the rapid solidification and crystallization by heterogeneous nucleation. The elemental segregation of chromium and activated heterogeneous nucleation mechanism of austenite in liquid metal play a key role in the formation of nano-sized grains at high cooling rates. This one-step technique provides us a new way to prepare uniform nano-scaled grains, and is of great importance for further understanding the growth of nano-materials under extreme conditions.

Thermal analysis of microstructural evolution of Al2O3 surface modified layers

Solidification behavior and microstructural evolution of surface modified layers in plasma cladding technique are studied via numerical simulations. Both the coupling effect of temperature and solid volume fraction are considered in the proposed thermal analytical model, by which the transient temperature distributions are calculated and the shape of melting pool is determined. Furthermore, we perform microscopic thermal analysis on the nucleation and growth behaviors of ceramic hardening phases and dendrites, as well as the kinetics of related two-phase flow systems. By comparing with experimental observations, the evolution mechanisms of the morphology of Al2O3 ceramic hardening layer are explained. Based on the above results, a relationship among the scanning velocity of plasma stream, dendritic growth rate and the advancing speed of solid/liquid interface is found, and an energy criterion is proposed for predicting the pushing/engulfing transition of ceramic particles by grain growth fronts. (C) 2009 Elsevier B.V. All rights reserved.