Investigation of the effects of solutes on the grain boundary stress relaxation phenomenon

An experimental investigation of grain boundary stress relaxation in copper binary alloys with nickel, silicon, aluminum, and silver comprising the range 0.03 to 1 atomic pct solute has been carried out. Two stress relaxation peaks of grain boundary origin have been ascertained and studied. The energy of activation and peak temperature for both peaks have been determined as functions of solute content and type of solute. It is suggested that this technique can be used as a measure of adsorbability of solute at grain boundaries as a consequence of the saturation effects observed in the measured properties in the vicinity of 0.1 atomic pct solute.     

温度梯度区域熔化对Al-Cu-Mg合金第二相析出及其性能的影响

采用SEM、EPMA、TEM以及显微硬度测试等手段,研究了温度梯度区域熔化(TGZM)对Al-Cu-Mg合金溶质原子浓度分布、第二相析出以及显微硬度的影响。研究表明:TGZM导致晶界两侧基体中的溶质原子浓度存在显著差异,靠近铸锭边部一侧基体的Cu、Mg溶质原子浓度明显高于靠近铸锭心部一侧基体的溶质原子浓度。在485℃的均匀化处理过程中,晶界两侧晶粒靠近边部一侧有大量第二相析出,而靠近心部一侧则很少或者几乎没有第二相析出。该现象致使同一晶界两侧区域的显微硬度值存在明显差别,并且这种差别无法通过长时间的均匀化处理得以完全消除,在微观上加剧了合金不同区域性能的差异。     

Nb对奥氏体热变形后等温回复的影响

采用热模拟实验研究了不同Nb含量的低C高Mn钢在800—900℃变形后奥氏体的回复特征,同时借助Fe-40%Ni-0.1%Nb(质量分数)合金揭示了回复过程中的位错演化及析出行为,建立了位错滑移及溶质拖曳机制的等温回复动力学模型,据此计算拟合了应力松弛曲线上回复实验数据,计算结果与理论分析及实验结果相符.实验及模拟结果表明,Nb溶质拖曳及析出均减慢回复过程,提高变形积累;与Nb溶质拖曳相比,析出能够更有效地延缓回复软化;Nb溶质拖曳通过升高回复激活自由能U0及减小激活长度来实现回复过程的延缓,提高溶质Nb含量,将升高Uo和减小激活长度.对于含Nb低C高Mn微合金钢,在道次间隔短的多道次热连轧精轧阶段,变形积累主要依靠Nb溶质拖曳作用,而对于轧制节奏较慢的中厚板精轧,轧制变形的积累依靠Nb溶质拖曳与析出的共同作用.     

Trapping of vacancies by rapid solidification

A model has been developed for the process of trapping of vacancies in rapid solidification of pure metals, which includes the effect of solute drag where vacancies play the role of solute. Within a reasonable range of parameter values it predicts that substantial trapping cannot occur unless the solidification velocity is 1 m/s or higher. It is demonstrated that the intrinsic mobility of the liquid/solid interface should not be evaluated without considering the effects of vacancy trapping and solute drag caused by vacancies. The model is applied to copper and unknown parameters are evaluated from information on the solidification velocity as a function of undercooling.     

Analysis of differential scanning calorimetric measurements performed on a binary aluminium alloy

A mathematical procedure is proposed to calculate fraction solid in the solidification interval of a binary alloy using cooling curves obtained with a power compensated Differential Scanning Calorimeter (DSC). The procedure takes advantage of using a validated thermodynamic model to enhance the accuracy of fraction solid evaluations, since the enthalpy of the phases vary according to their solute mass concentrations and temperature. Solute mass concentrations are deduced from the Brody-Flemings relationship to take into account the effect of solute diffusion kinetics occurring during the solidification. The procedure is applied to the fraction solid determination of aluminum-4.5% copper alloy submitted to different cooling rates. It was found that the fraction liquid remaining just before eutectic solidification was 2.45, 2.92 and 3.66 w% for a cooling rate respectively equal to 5, 10 and 60 K/min. The back diffusion parameter defined in the Brody-Flemings relationship was found to diminish as the cooling rate increases, as this is expected from basic kinetics consideration.     

Effect of interstitial carbon on the mechanical properties of electrodeposited bulk nanocrystalline Ni

Solid solution strengthening by carbon and sulfur in bulk nanocrystalline Ni was studied by electrodeposition and first-principles calculations. Bulk nanocrystalline Ni with a carbon content of 30–1600 ppm and a sulfur content of 140–1200 ppm was prepared using a sulfamate bath with different complexing agents and gloss agents. The hardness values of the bulk nanocrystalline Ni were scattered as the grain size decreased to ～12 nm with increasing carbon and sulfur content. It was found that the scatter could be explained by considering the effect of impurities such as solute atoms on the hardness of electrodeposited Ni, in addition to the Hall–Petch relationship. Thus, to determine the structure of Ni–C and Ni–S solid solutions and estimate the contribution of impurities to hardness, the enthalpy of solution and misfit strain were calculated by first-principles calculations. The results indicate that carbon exists as an interstitial solute atom in the Ni matrix, producing large misfit strains, and sulfur exists as a substitutional solute atom, inducing no significant changes. A model of solid solution strengthening due to interstitial solute atoms was developed by considering the interaction between mobile dislocations and solute atoms. This study has effectively divided the observed solid solution effect from the grain refinement effect in electrodeposited nanocrystalline Ni. The results of this study point to the origin of high-strength electrodeposited bulk nanocrystalline Ni.     

Dislocation motion in the presence of diffusing solutes: a computer simulation study

A discrete lattice, kinetic Monte Carlo model is developed to simulate the motion of an edge dislocation in the presence of interacting, diffusing solute atoms that have a misfit with respect to the matrix. The simulation self-consistently determines the solute concentration profile (in two spatial dimensions), as well as the associated dislocation velocity. The solute segregation profile around the moving dislocation is characterized at low velocity by a condensed solute cloud near and on one side of the dislocation core, a region depleted of solute on the opposite side and a diffuse solute (Cottrell) atmosphere further from the core. At high velocity, no condensed solute cloud forms. The relation between the dislocation velocity and the applied stress shows a low-velocity, solute drag branch and a high-velocity branch, typified by no solute cloud but with occasional solute trapping. At intermediate velocities, the dislocation stochastically jumps between these two branches.     

直流磁场对7075铝合金组织结构的影响

研究了在直流磁场下凝固的7075铝合金晶内溶质含量及凝固组织的变化。试验结果表明：与未施加磁场的试样相比，经过直流磁场处理后，7075铝合金中Zn、Mg、Cu3种溶质元素在晶粒内部的含量均增加。此外，直流磁场作用下，试样宏观组织的变化不明显，而晶内析出相的形貌和尺寸发生了显著变化，由无磁场时的粗大针状组织变为细小均匀的结构。探讨了磁场的作用机制。认为电磁场作用使得各溶质粒子对基体铝产生了相对运动，这种运动增强了溶质粒子在铝中的扩散，从而导致了晶内溶质含量的增加。     

Influence of solute segregation and drag on properties of migrating interfaces

Recent models on solute segregation and drag search for steady-state solutions of the diffusion equation in the region of a migrating interface and adjacent semi-infinite grains. Such solutions are limited to model massive transformations, for which the chemical composition of the parent grains and that of the product grains are the same. Simulation of diffusive transformations in transient systems with grains of different chemical composition is a hot issue. In the present model the steady-state solution of the diffusion equation is investigated only in the interface. The coupling with the diffusion equation in the adjacent grains is ensured by proper boundary conditions. For simulation of transient diffusive phase transformations it is very convenient if the interface can be treated as a sharp interface with a known effective mobility and prescribed boundary conditions at the interface. In this paper it is shown that solute segregation and drag can be taken into consideration by the effective mobility of a sharp interface and by proper boundary conditions at the sharp interface. The effective interface mobility reflects Gibbs energy dissipation due to rearrangement of solvent atoms and to trans-interface diffusion and drag of solute atoms in the migrating interface. Trans-interface diffusion and drag of solute atoms are also reflected by the jump of the chemical potential of the solute across the interface, which represents a boundary condition for coupling with the diffusion equation in the adjacent grains. Finally, it is shown that the incorporation of the effects of trans-interface diffusion and drag of solute atoms does not make the simulation more complicated provided that some necessary calculations are performed in preprocessing.     

Tuning of organic magnetoresistance by reversible modification of the active material

We investigate the mechanism of the recently discovered phenomenon that the organic magnetoresistance effect can be enhanced by electrical device conditioning. In organic light emitting devices based on poly(paraphenylene vinylene) appropriate conditioning can increase the magnetoconductance values from 1% to 25% at 40 mT. While the conditioning procedure increases the magnetoconductance it leads to a reduction of the electro-optical device performance. The favorable effect of conditioning is non-permanent, leading to a slow reduction of the magnetoconductance values once the conditioning procedure has been turned off. We demonstrate that this relaxation process can be accelerated by thermal annealing of the devices. Higher annealing temperatures cause a stronger reduction of the magnetoconductance values as well as a more pronounced improvement of the previously reduced electro-optical performance. We attribute our results to a modification of the charge carrier transport properties in the bulk polymer material during conditioning and annealing. Changes in polymer morphology or in molecular conformation during conditioning are assumed to result in the formation of energetic trap states which are beneficial for the organic magnetoresistance effect. Thermal annealing reverses the material modifications, reduces the number of trap states in the material and thus decreases the magnetoconductance.     

钨中溶质原子铬与空位的相互作用的第一性原理计算研究

钨铬(W-Cr)合金具有优异的物理性能,被认为是面向等离子体材料(PFM)中最有潜力的候选材料。而合金元素对辐照损伤演化过程的影响规律是钨合金优化设计的关键,深入研究溶质原子与辐照缺陷(如：空位)的相互作用有助于理解辐照损伤演化的微观物理机制。本文基于第一性原理方法,对W-Cr合金的溶质原子Cr占位、Cr与空位的相互作用及Cr含量对其影响规律进行了计算研究。发现相比于间隙位置,原子Cr更易占据置换位置,W中溶质Cr有偏聚的趋势;在辐照环境下,空位和自间隙原子均易与溶质原子Cr相结合,易加速W中富Cr相的析出;随Cr含量的增加,体系的形成能线性增加,因而结构越发不稳定。研究还发现超胞中单原子Cr的第二近邻空位的形成能最低,且低于W的本征空位形成能,因而溶质Cr与空位之间存在微弱的吸引;对于同一Cr含量,不同构型的空位形成能及空位和Cr的结合能均不同,且Cr含量越高,数值越分散;随着Cr含量的增加,平均空位形成能及空位与Cr的平均结合能均略有下降,因而溶质原子Cr附近空位更易形成,空位浓度更高,同样也表明溶质Cr与空位有微弱的吸引。这些结论将有助于深入理解W中溶质元素存在条件下辐照缺陷演化过程的微观物理机制。     

Solute–vacancy binding of the rare earths in magnesium from first principles

The thermodynamic and kinetic properties of Mg–rare earth (RE) alloys are not widely known, despite increased research in recent years into their mechanical strengthening mechanisms. To aid in the development of new alloys and strengthening models, the solute–vacancy binding energies for all 17 RE elements in hexagonal close-packed (hcp) Mg are calculated by density functional theory (DFT) in several solute/vacancy configurations. As the REs can be considerably larger than Mg, special care is taken to ensure convergence with respect to supercell size, and the binding of other large, non-RE solutes is calculated in the large supercells employed in the current work. It is found that the light and heavy lanthanides have favorable and unfavorable nearest-neighbor solute–vacancy binding, respectively. The binding energies exhibit a strong linear correlation to the magnitude of the solute’s displacement towards the vacancy, suggesting that local lattice relaxation is a significant contribution to the binding energy. There are four notable exceptions to this trend: Na, Sb, Pb and Bi. Explicit calculation of the lattice relaxation energy due to the presence of the solute–vacancy pair confirms that local relaxation is an important component of the binding energy, particularly for the larger solutes. Finally, empirical predictions of the dilute mixing energy disagree with those predicted by DFT. New experimental measurements are necessary to resolve the discrepancies, as there are no published thermodynamic data of the REs in hcp Mg.     

Grain coarsening inhibited by solute segregation

It will be shown that grain boundaries are in a metastable thermodynamic equilibrium in the presence of solute atoms and, therefore, grain coarsening is stopped as there is no driving force. This is in contradiction to a generally accepted interpretation, where solute drag, i.e. zero mobility of the boundaries, stops grain coarsening. Based on the empirical relation between terminal solubility of a solute and its grain boundary segregation it can be shown that a two-phase mixture with solute atoms agglomerating in a precipitated phase will be the stable thermodynamic equilibrium state. However, if precipitation is kinetically hindered, the metastable equilibrium with a certain grain boundary area and a zero grain boundary energy is attained. Changes in this grain boundary area or grain size respectively are calculated as a function of temperature and compared with experimental findings.     

二元合金等轴枝晶生长相场模拟

在传统纯物质相场模型的基础上，对其中的体系自由能进行重新构造，得到一个紧密联系相图且综合考虑了传热、溶质扩散、界面能各向异性和界面动力学各向异性等适用于二元合金枝晶生长的新相场模型，并利用该模型结合一种能显著提高计算效率的“临界面点相场大梯度计算域控制法”，对AJ-7Si合金在初始过冷度分别为35K和50K的过冷熔体中等轴枝晶生长进行了模拟，得到许多与试验观察相符合的结果；同时在枝晶生长过程中跟踪枝晶主轴尖端，获得其尖端半径和尖端速率，发现随着凝固时间的增加，逐渐收敛，直到枝晶达到稳定生长，而且通过简化KGT模型计算的枝晶尖端的生长速率与模拟值相近。另外，就过冷度对等轴枝晶生长的影响进行了分析。     

A three-dimensional cellular automaton model of dendrite growth with stochastic orientation during the solidification in the molten pool of binary alloy

A three-dimensional (3D) cellular automaton model is developed for the prediction of dendrite growth with stochastic orientation during solidification process in the molten pool of binary alloy. An angle-information transfer method is proposed for improving cellular automaton technique to simulate the growth of the dendrites whose preferred growth direction owns stochastic misorientation with respect to the direction of the coordinate system. Dendrite morphologies and solute distributions of single dendrite growth and multi-dendrite growth are able to be obtained by the simulation using present model. The model is also employed to study the difference between two-dimensional simulation and 3D simulation on solute segregation and dendritic growth. Using the established model, 3D multi-columnar dendrites with stochastic crystallographic orientations can be obtained efficiently, and the competitive growth and impinging of dendrites can be reproduced in practice. The simulation results agree well with the experimental results.     

关于贝氏体形核和台阶机制的讨论--与徐祖耀院士等商榷

一些合金固溶化(或淬水)后在预贝氏体(孕育)期内等温淬火(或时效)，既保持高温时形成的晶体缺陷，又产生新的晶体缺陷。母相中由于溶质原子扩散而在缺陷处发生偏聚，则形成贫／富溶质微区，即类拐点(spinoclal—like)分解。当贫溶质微区成分作为Ms，其温度等于或高于等温淬火(或时效)温度时，贝氏体将以马氏体样切变形核，故贝氏体是在溶质原子扩散控制下切变形核。在TEM温台试验中未发现台阶生长机制，界面上巨型台阶是贝氏体增宽(厚)速度差异所致。     

Diffuse-interface modeling of solute trapping in rapid solidification: Predictions of the hyperbolic phase-field model and parabolic model with finite interface dissipation

Two recently developed phase-field models, a hyperbolic model and a parabolic model with finite interface dissipation, are employed to study the solute trapping in a Si-0.25 at.% As alloy during rapid solidification. The hyperbolic model is applied at the nanometer scale of the interface width δ. The parabolic model is derived by a coarse-graining procedure and is intended to operate with mesoscopic resolution of the interface η. The coarse-graining numerical parameters, namely interface width η and the interface permeability P, are adjusted in the parabolic model to fit the segregation coefficient calculated by the microscopic model on the nanoscale. Based on the optimal sets of η and P selected at small interface velocity, a linear relation between their logarithm values is obtained. This logarithmic relation provides a theoretical basis for choosing the appropriate values of η and P in the numerical phase-field simulation in three spatial dimensions.     

Modeling of solute drag in the massive phase transformation

The role of solute drag in the massive phase transformation is evaluated through the dissipation of Gibbs energy by diffusion. As an introduction, the solute drag in grain boundary migration is first examined using the wedge-shaped energy function considered by Cahn and by Lücke and Stüwe. The effect of a diffusivity varying from a low value in the bulk to a high value in the center of the boundary is examined. It decreases the solute drag drastically. For the massive phase transformation it is demonstrated how the solute drag increases by the tendency for segregation and by a high diffusivity in the interface but it decreases if the diffusivity is lower than in the parent phase. The most important contribution to solute drag comes from the spike of solute atoms in the parent phase being pushed forward by the advancing interface. The spike is thus an obstacle for growth that must be broken through in order for diffusion-controlled growth to turn partitionless. The possibility of dynamic nucleation is also discussed.     

Reversal of the negative natural aging effect in Al–Mg–Si alloys

The effect of solute clustering on subsequent precipitation has been investigated for three different Al–Mg–Si alloys with similar Mg/Si ratios but different Mg + Si contents. Hardness measurements were carried out (i) during various natural aging (NA) times at room temperature, and (ii) after a fixed artificial aging (AA) heat treatment preceded by various NA times. Comparison between the two hardness plots stressed an intimate connection between solute clusters and precipitates: for the NA times investigated (between a few seconds and several months), transient phenomena with short (less than 10 h) lifetimes were observed, both occurring at earlier times and with higher relative importance for increasing solute concentration. Upon NA + AA, these transient phenomena were found to be responsible for the well-known “negative NA effect”, as evidenced by a close correlation between the respective lifetimes. In particular, the negative NA effect itself was found to be transient, and was reversed upon long storage times by the dominance of a process beneficial to precipitation. Transmission electron microscopy studies of selected alloy conditions ensured that the recorded hardness variations reflected variations in the alloy microstructure. Attempts were made to couple the transient clustering phenomena with solute strain field interactions. Preliminary calculations following this line of thought suggested a competitive nature of both Mg–Mg and Mg–Si interactions, in contrast with the usual view of Mg–Mg repulsion. An alloy hardness optimization procedure as a function of storage time, temperature and alloy composition has been suggested.     

低频电磁场对半连续铸造Al-Mg-Si-Cu合金显微组织的影响

采用低频电磁场半连续铸造工艺制备Al-Mg-Si-Cu铝合金铸锭,研究了低频电磁场对Al-Mg-Si-Cu铝合金铸锭组织的影响。结果表明,施加低频电磁场可以细化Al-Mg-Si-Cu铝合金铸锭的晶粒组织,减轻晶内元素的偏析程度。与常规半连续铸造相比,施加频率为20 Hz、电流为120 A的交变电磁场后,铸锭中的等轴晶组织增多,枝晶状组织减少,晶粒尺寸变得更加细小,铸锭边部和中心平均晶粒尺寸由常规半连续铸造的170和259μm分别降至133和187μm,同时有利于提高溶质元素在晶内的分布,很大程度上减轻了溶质元素的偏析。此外,低频电磁场的电磁振荡使得液穴内部温度场和流动场更趋均匀,溶质元素分配系数增大,结晶区间变小,抑制了树枝晶的生长,促进了Al-Mg-Si-Cu合金半连续铸锭中非枝晶组织的形成和溶质元素的强制固溶,并且抑制了溶质元素的偏析。