Multiphase modelling of the growth kinetics of precipitates in Al-Cu alloys during artificial aging

ABSTRACT

In the aluminum industry, the needs of predictability of the kinetics of precipitation during the artificial aging processes increase as the targeted applications require the maximisation of properties at the lowest costs possible. In this regard, kinetics modelling can be helpful to design the heat treatment processes. Despite using many fitting parameters, available models show a lack of fitting with experimental data, especially for the apparent heat capacity measured at high temperatures by a differential scanning calorimeter (DSC). To address this issue, a mixed-mode model was recently developed for isothermal heat treatment, whereas non-isothermal heat treatment must be considered to compare the calculated results with those measured by isochronal heating in a DSC. In this contribution, the model is extended to non-isothermal heat treatments. To this end, the growth kinetics pathway and sequence of precipitation in a binary Al-Cu alloy have been simulated, optimising the pre-exponential factor and the activation energy of the interfacial mobility of the secondary phases. This calibration of the interfacial mobilities allowed a very good reproduction of the evolution of the apparent heat capacity with temperature. The model and calibrated interfacial mobilities were then used to compute the size evolution of θ′ precipitates in an Al-4?wt%Cu. The isothermal growth rates calculated at 4 temperatures were in good agreement with those measured and reported by independent researchers. The good predictability of the model indicates that the assumptions made were suitable and well funded, especially regarding the growth rates of embryos emerging from the subcritical growth regime.     

Influence of grain boundaries on the distribution of components in binary alloys

Physics of the Solid State - Based on the free-energy density functional method (the Cahn–Hilliard equation), a phenomenological model that describes the influence of grain boundaries on the...     

Effect of solute atoms on grain boundary sliding in magnesium alloys

The effect of solid-solution alloying on grain boundary sliding (GBS) was investigated using pure magnesium and six kinds of Mg–X (X?=?Ag, Al, Li, Pb, Y and Zn) dilute binary solid solutions with an average grain size of 10?µm. A sharp increase in damping capacity caused by GBS was observed above a certain temperature. The temperature at which a sharp increase in damping capacity occurred depended on the alloying element. The addition of Y and Ag markedly increased the onset temperature (more than 100?K) for a sharp increase in damping capacity, whereas the addition of Zn, Al and Li slightly increased the onset temperature (less than 50?K) as compared with that for pure magnesium. Tensile tests at a temperature of 423?K revealed that the higher the onset temperature, the lower the strain rate sensitivity of the flow stress. It is suggested that the former elements (Y and Ag) are more effective in suppressing GBS in magnesium alloys than the latter ones (Zn, Al and Li). The suppression of GBS was associated with low grain boundary energy, and the extent to which the energy is reduced depended on the alloying element. It was suggested that the change in the lattice parameter (the so-called c/a ratio) affects the grain boundary energy, and thus, the occurrence of GBS.     

Calculation of the binding energies of complexes formed by magnesium and zinc impurity atoms with vacancies in dilute aluminum alloys

Expressions are obtained for calculating the binding energy of an arbitrary complex formed from impurity atoms and vacancies. Calculations are made for various complexes formed from magnesium and zinc impurity atoms and vacancies in aluminum. The results obtained are used to study the role of vacancies in the formation of Guinier-Preston zones. The need to allow for the lattice relaxation is demonstrated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 99–104, August, 1980.     

ZA27合金晶界处铁、稀土元素的有序化与交互作用

为从理论上揭示铁、稀土元素在锌铝合金晶界处的行为本质，建立了ZA27合金中α相大角度重位点阵晶界模型，利用递归法(Recursion)计算了晶界的电子结构(状态密度、费米能级、结构能).用晶界结构能定义合金的团簇能(有序能)，并计算了偏聚铁及稀土晶界的团簇能.计算结果表明：铁、稀土元素在锌铝合金晶界处团簇能为正值，不能形成团簇，具有有序化倾向，趋于形成稳定的金属间化合物.铁与稀土元素在晶界形成负电中心，降低晶界的费米能级. 关键词： 稀土 晶界 电子结构 有序化     

ZA27合金中稀土及铁的晶界偏聚与交互作用

根据分子动力学理论建立了液态ZA27合金的原子集团，结合计算机编程构造出ZA27合金α相与液相共存时的原子构形及α相大角度重位点阵晶界模型.利用递归法计算了铁、稀土元素 固溶 于晶粒内，游离于固液相界面及其在α相晶界处的环境敏感镶嵌能，计算了铁、稀土元素与 Al 的键级积分.由此得出：铁、稀土处于固液相界区比在晶内更稳定，解释了铁、稀土在α相内溶解度很小，结晶时富集于固液相界前沿液体中，从而导致凝固结束后铁、稀土元素偏聚于晶界，并形成成分复杂的稀土化合物的事实. 关键词： 稀土 晶界 递归方法 电子结构     

Influence of solute cloud and precipitates on spatiotemporal characteristics of Portevin-Le Chatelier effect in A2024 aluminum alloys

In this paper, solute concentration and precipitate content in A2024 aluminum alloy are adjusted by solution treatment (ST) at different temperatures and tensile experiments on these treated specimens are carried out. It is found that the temperature of solution treatment (ST temperature) has a remarkable influence on the amplitude of the serrated flow and the propagation characteristics of shear bands. These results are due to the effects of solute atoms and precipitates on dynamic strain aging (DSA). When ST temperature is higher than 300~℃, solute concentration is relatively high and solute cloud is a key factor of DSA. When ST temperature is lower than 300~℃, precipitate content is relatively high and the mechanism of DSA is determined by precipitates.