Modeling for the structure evolution of alloys of the Al-Cu-Mg system during natural ageing

Natural ageing in alloys of the Al-Cu and Al-Cu-Mg systems is investigated and modeled. To determine the kinetic parameters, a method for measuring the electrical resistance is applied as the basic method. With the use of the Avrami equation in the differential form, a model for the structure evolution (changes in the volume fraction of the Guinier-Preston zones (GPZs) and the depletion of the solid solution) and the change in the yield strength for the alloys during natural ageing is constructed. In the case of the Al-Cu-Mg ternary system, two types of zones are present (the GPZ and the Guinier-Preston-Bagaryatskii zone (GPBZ)). In this case, the dissociation of the oversaturated solid solution at room temperature is described by a set of differential equations. The accuracy of the constructed model of the dependence of the yield strength on the alloy composition and the natural ageing time is 6%, which is within the error of experimental determination for this parameter.     

自然时效对7N01铝合金组织和性能的影响

通过电导率、力学性能测试和高分辨电镜分析研究了自然时效对7N01合金组织和性能的影响。结果表明:在自然时效中,由于过饱和固溶体不均匀析出与基体共格的析出相,合金电导率随着自然时效时间的延长逐渐降低,时效20 d后基本达到稳定状态。合金的强度随着时效时间的延长逐渐增大,在20 d后达到稳定,抗拉强度在400 MP以上,屈服强度在260 MPa以上,合金的延伸率在自然时效1 d后达到稳定值(约为15.5%)。稳定态合金的强化相主要为尺寸较小的GPⅡ区。此外,弯折试验表明合金具有良好的弯折性能,在自然时效10～60 d内未出现肉眼可见的裂纹。     

Precipitation hardening of HSLA steels

A process model to describe the strength contribution from precipitation hardening during coiling after hot rolling has been developed for V and Nb HSLA steels. Experimental measurements of ageing behaviour on the V steel were conducted on coil material which was received in an underaged condition. The size and composition of precipitates was examined on replicas using scanning transmission electron microscopy. The precipitates were observed to be V rich and a substantial increase in precipitate size occurred as a function of ageing time. The modelling approach developed by Shercliff and Ashby for aluminium alloys was extended to microalloyed steels. The model assumes particle coarsening is the rate controlling process and that the precipitates are initially sheared by dislocations with a transition to non-shearable precipitates at peak strength. After calibration of the model, good agreement was observed between the model predictions and experimental data. A model for ageing in a Nb HSLA steel was also developed using literature values for the ageing behaviour.     

A process model for age hardening of aluminium alloys—I. The model

Process modelling techniques are used to describe the changes in yield strength due to age hardening of heat-treatable aluminium alloys. A model for the isothermal ageing curve is developed. This is demonstrated for a number of alloys and the success of the approach is assessed. Applications and a new diagram, showing the variation of strength with temperature and time, are described in an accompanying paper.     

Strengthening of composites due to microstructural changes in the matrix

The addition of discontinuous silicon carbide (SiC) to aluminum (Al) alloys can result in a five-fold increase in the yield stress. The magnitude of the increase is obviously a function of the volume fraction and the particle size of the SiC. Previously, it was proposed that the strength increase due to SiC addition to Al alloys was the result of change in the matrix strength, i.e. an increase in dislocation density and a reduction of subgrain size. The data obtained from a series of experiments indicate that dislocation density increases with an increase in volume fraction of SiC and decreases with an increase in particle size. The subgrain size decreases as the volume fraction increases and increases as the particle size increases. There is a good correlation between the microstructural changes in the matrix and the changes in the yield stress of the composites.     

Use of a physical approach and artificial neural networks for the simulation of the relation between the yield strength of quenched Al-Si alloys and their structural characteristics

Models are developed to relate the yield strength of Al-Si alloys to their structural characteristics. The models are based on the physical theory of strength and artificial neural networks. The simulated and experimental yield strengths agree well. It is wise to use an artificial neural network to predict the properties of alloys whose structural parameters fall in the range of the learning sample.     

Yield strength of overaged alloys containing coherent ordered precipitates

The existing models for yielding of overaged alloys do not account for the observed strain rate sensitivity of the yield stress and predict a sharper decrease in stress as a function of particle size than the experimental observations. A dynamic model has been presented here to account for the yield strength of overaged alloys containing coherent ordered precipitates which takes into consideration the intrinsic characteristics of the precipitates. Concentric glide loops are supposed to form around precipitate particles. Their sizes as a function of applied stress have been computed. The stress on the innermost loop rises with increasing applied stress and when it exceeds the opposing force due to antiphase boundary, the loops shrink. The rate of shrinkage of loops has been calculated for different values of applied stresses. The yielding occurs when the external stress is high enough to account for the applied strain rate. The model has been applied to calculate the yield strength of an austenitic stainless steel containing γ′ precipitates. The antiphase boundary energy of the precipitates was measured by dislocation pair spacing method.The mode of variation of calculated yield stress values with particle size is predicted more accurately than by earlier models. The model qualitatively accounts for the observed sensitivity of yield stress to strain rate and antiphase boundary energy of the precipitates. Formerly Manager Research & Development, Alloy Steels Plant, Durgapur     

Yield strength of overaged alloys containing coherent ordered precipitates

The existing models for yielding of overaged alloys do not account for the observed strain rate sensitivity of the yield stress and predict a sharper decrease in stress as a function of particle size than the experimental observations. A dynamic model has been presented here to account for the yield strength of overaged alloys containing coherent ordered precipitates which takes into consideration the intrinsic characteristics of the precipitates. Concentric glide loops are supposed to form around precipitate particles. Their sizes as a function of applied stress have been computed. The stress on the innermost loop rises with increasing applied stress and when it exceeds the opposing force due to antiphase boundary, the loops shrink. The rate of shrinkage of loops has been calculated for different values of applied stresses. The yielding occurs when the external stress is high enough to account for the applied strain rate. The model has been applied to calculate the yield strength of an austenitic stainless steel containingγ’ precipitates. The antiphase boundary energy of the precipitates was measured by dislocation pair spacing method. The mode of variation of calculated yield stress values with particle size is predicted more accurately than by earlier models. The model qualitatively accounts for the observed sensitivity of yield stress to strain rate and antiphase boundary energy of the precipitates. Formerly Manager Research & Development, Alloy Steels Plant, Durgapur     

Mechanical properties of AlZnMg alloys

The yield stress and the microstructural parameters of GP zones and η′ precipitates as well as the temperature dependence of the yield stress were investigated in an Al-4.7 wt.% Zn-1.2 wt.% Mg alloy and in a series of the same base alloys with additional elements of Fe, Si, Cr and Zr. It was found that the GP zones formed either during RT or 50°C ageing after quenching are sheared by the moving dislocations upon deformation. By correlating the theoretical model of this cutting mechanism the particle strength, K, and the specific surface energy, Γ, necessary to cut through a unit area of a particle were determined. According to our results Γ is a linear, whereas K is a quadratic function of the average zone radius. The investigations have shown that in the case of η′ precipitates formed during ageing at 160°C the Orowan mechanism is effective in the dislocation movement. It was found that the largest yield stress increment can be expected by producing particles with an average radius of about 3 nm.The influence of strain rate and temperature on the yield stress of samples aged for different periods of time at RT was also investigated. The analysis of the experimental data has shown that the temperature dependent part of the yield stress, τ_T, decreases linearly with temperature up to a certain critical temperature, T_c which was found to be 260 K. The experiments have shown that the activation volume is linear whereas the activation enthalpy is a quadratic function of temperature. The largest values obtained for these parameters were 3,16 × 10⁻²⁷m³ and 0.58 eV. The thermodynamic analysis of the results have revealed that the interaction between dislocations and GP zones can be characterized by a square potential profile.     

混合实验法在W-Ni-Cu合金组分设计中的应用

采用极端顶点设计法设计W-Ni-Cu合金组分,将组分自变量与相对应的性能因变量(相对密度、显微硬度、抗弯强度)进行回归分析和规划求解,同时采用冷压烧结法制备不同组分的W-Ni-Cu合金,测定合金的密度、显微硬度和力学性能,研究组分对合金性能的影响。结果表明:回归方程复相关系数R~2=1,方程精确度高;合金性能随组分变化而呈规律变化;当Ni含量与Cu含量(均为质量分数)分别为3%和5%时,合金的综合力学性能最佳:相对密度为94.295%、显微硬度286.55、抗弯强度931.51 MPa。W-Ni-Cu合金的相对密度计算值与实验结果的误差为-0.45%~0.06%,显微硬度计算值与实验结果的误差为-8.48%~4.46%,抗弯强度计算值与实验结果的误差为-5.19%~4.15%。误差很小,说明混合实验和极端顶点设计法能优化W-Ni-Cu合金组分,并可靠预测合金性能。     

Predicting the stress-strain behavior of polycrystalline α-lron containing hard spherical particles

Recent interest in the work hardening of metal crystals containing a dispersion of hard particles has resulted in analytical expressions relating the work hardening to strain, particle diameter, and volume fraction as well as other material parameters. In this study, these models have been used to calculate the tensile stress-strain behavior of polycrystalline α iron containing dispersions of the intermetallic compound Fe₂Ta. The structural characteristics of the Fe-Ta alloys were thoroughly evaluated. The particle morphology was measured for randomness, mean particle diameter, standard deviation of the particle diameter, volume fraction, and planar interparticle spacing. Also, the matrix flow strength, composition, crystallographic randomness, dislocation morphology and grain size were evaluated. It was found that an Orowan type relationship as modified by Ashby satisfactorily described the yield strength as a function of the interparticle spacing and particle diameter. An experimental slope of 11.1 x 10^-5 kg-cm/mm² and a calculated slope of 9.75 x 10^-5 kg-cm/mm² were found. Both the Hart revised FHP work hardening model and Ashby’s model based on the generation of secondary dislocations were in good agreement with the experimental data. Hart’s revised FHP model required the use of empirically obtained values for the particle volume fraction which differed by a factor of 10 from the measured volume fraction and therefore is not suitable for predictive purposes. At tensile strains greater than 5 pct, the work hardening was characteristic of the matrix without particles; therefore, deviation between the experimental and calculated results based on Ashby’s model occurred at large strains. It is hoped that this study represents a step towards applying work hardening models to more complex polycrystalline alloys.     

Properties of shaped castings made of modern cast VML18 and VML20 magnesium alloys manufactured by new methods

The methods of casting of modern magnesium alloys (corrosion-resistant Mg–Al–Zn VML18 alloy and a high-strength Mg–Zn–Zr VML20 alloy) into the temporary molds made of cold-hardening mixtures and the molds produced by 3D printing are considered. The mechanical properties (ultimate tensile strength, yield strength, impact toughness), the corrosion properties, and the microstructure of the ingots are studied. The experimental results are used to choose the molds and the methods of casting of the parts of the control system of advanced aircrafts, which are made of modern cast magnesium alloys VML18 and VML20.     

The effect of microstructure and composition on the properties of vapour quenched AlCr alloys—II. Tensile properties

The effect of chromium and iron additions and of annealing and working on the microstructure and tensile properties of vapour quenched AlCr and AlCrFe alloys has been determined. Tensile strengths of the worked AlCrFe alloys were in the range 568–831 MPa. Chromium in solid solution or iron present as iron-rich precipitates increased the yield stress by 44.7 MPa/at.%Cr and 333 MPa/at.%Fe respectively. The contributions to the yield strength of AlCr alloys were solid solution 40% and dislocation density/cell size 60% and to the yield strength of AlCrFe alloys were solid solution 25%, iron-rich precipitates 42% and dislocation density/cell size 33%. Vapour quenching may allow the more efficient use of alloying elements in the strengthening of Al-alloys and greater flexibility in obtaining the desired combination of solute concentration, particle volume fraction and particle size.     

生物医用Ti6Al4V合金粉末注射成形工艺研究

钛及钛合金具有高比强度、低弹性模量、优良的耐蚀性和绝佳的生物相容性,但较差的加工性能大大限制了其应用范围。钛及钛合金金属粉末注射成形工艺克服了机加工、模压等传统加工工艺的缺点,结合传统粉末冶金和注塑成型的优势,实现了结构复杂的钛及钛合金产品低成本、大批量近净成形,提高了材料利用率。本文利用水溶性黏结剂和粉末粒度为16 μm和22 μm的商用球形Ti6Al4V合金粉制备了注射料和相应的试样,通过实验确定了气氛热脱黏结合真空烧结的最佳工艺,基于该工艺制备得到了两种注射料的烧结试样。结果表明:粉末粒度为16 μm注射料烧结件杂质含量未能满足外科植入用金属注射成形Ti6Al4V组件标准;粉末粒度为22 μm注射料烧结件物理化学性能如下,极限拉伸强度880 MPa,屈服强度830 MPa,延伸率13.2%,相对密度96.8%,氧质量分数为0.195%,氮质量分数为0.020%,碳质量分数为0.022%,该试样整体性能满足外科植入用金属注射成形Ti6Al4V组件标准。     

难熔高熵合金:制备方法与性能综述

从加工方法、微观结构以及各类性能三方面介绍了难熔高熵合金(Refractory high-entropy alloys,RHEAs),最后对难熔高熵合金的发展和未来进行了展望。以MoNbTaVW为代表的难熔高熵合金在高温下表现出优于传统镍基高温合金的压缩屈服强度,且屈服强度随温度的变化更加缓慢,高温力学性能优异;以MoNbTaVW、MoNbTaTiZr、HfNbTiZr等为代表的难熔高熵合金,与商用高温合金、难熔金属、难熔合金以及工具钢相比,展现出更优的耐磨性能。以W₃₈Ta₃₆Cr₁₅V₁₁合金为代表的难熔高熵合金在辐照后,除了析出小颗粒第二相外,不存在位错环缺陷结构,抗辐照性能优异。提出了难熔高熵合金未来发展的两大方向:建立高通量的实验和计算方法继续探索更多的难熔高熵合金组成和结构模型;探索多场耦合环境下难熔高熵合金的服役行为。      

Solidification Microstructure and Mechanical Properties of Cast Magnesium-Aluminum-Tin Alloys

The solidification microstructure and mechanical properties of as-cast Mg-Al-Sn alloys have been investigated using computational thermodynamics and experiments. The as-cast microstructure of Mg-Al-Sn alloys consists of α-Mg, Mg₁₇Al₁₂, and Mg₂Sn phases. The amount of Mg₁₇Al₁₂ and Mg₂Sn phases formed increases with increasing Al and Sn content and shows good agreement between the experimental results and the Scheil solidification calculations. Generally, the yield strength of as-cast alloys increases with Al and Sn content, whereas the ductility decreases. This study has confirmed an early development of Mg-7Al-2Sn alloy for structural applications and has led to a promising new Mg-7Al-5Sn alloy with significantly improved strength and ductility comparable with commercial AZ91 alloy.     

Neural network prediction of mechanical properties of porous NiTi shape memory alloy

Abstract

A multilayer back propagation learning algorithm was used as an artificial neural network tool to predict the mechanical properties of porous NiTi shape memory alloys fabricated by press/sintering of the mixed powders. Effects of green porosity, sintering time and the ratio of the average Ti to Ni particle sizes on properties of the product were investigated. Hardness and tensile strength of the compacts were determined by hardness Rockwell B method and shear punch test. Three-fourths of 36 pairs of experimental data were used for training the network within the toolbox of the MATLAB software. Porosity, sintering time and particle size ratios were defined as the input variables of the model. Ultimate strength and hardness were the outputs of the model. Results indicated that seven neurons in the hidden layer yielded the minimum normal error. The modelling outcomes confirmed the feasibility of the model and its good correlation with the experimental information.     

A process model for friction welding of AlMgSi alloys and AlSiC metal matrix composites—II. Haz microstructure and strength evolution

The present investigation is concerned with the development of an overall process model for the microstructure and strength evolution during continuous drive friction welding of AlMgSi alloys and AlSiC metal matrix composites. In Part II the heat and material flow models presented in the first paper (Part I) are utilized for prediction of the HAZ subgrain structure and strength evolution following welding and subsequent natural ageing. The modelling is done on the basis of well established principles from thermodynamics, kinetic theory and simple dislocation mechanics. The models are validated by comparison with experimental data, and are illustrated by means of novel mechanism maps. These show the competition between the different process variables that contribute to microstructural changes and strength losses during friction welding of AlMgSi alloys and AlSiC metal matrix composites.     

Microstructural effects on the tensile and fracture behavior of aluminum casting alloys A356/357

The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on secondary dendrite arm spacing (SDAS), Mg content, and, in particular, the size and shape of eutectic silicon particles and Fe-rich intermetallics. In the unmodified alloys, increasing the cooling rate during solidification refines both the dendrites and eutectic particles and increases ductility. Strontium modification reduces the size and aspect ratio of the eutectic silicon particles, leading to a fairly constant particle size and aspect ratio over the range of SDAS studied. In comparison with the unmodified alloys, the Sr-modified alloys show higher ductility, particularly the A356 alloy, but slightly lower yield strength. In the microstructures with large SDAS (>50 μm), the ductility of the Sr-modified alloys does not continuously decrease with SDAS as it does in the unmodified alloy. Increasing Mg content increases both the matrix strength and eutectic particle size. This decreases ductility in both the Sr-modified and unmodified alloys. The A356/357 alloys with large and elongated particles show higher strain hardening and, thus, have a higher damage accumulation rate by particle cracking. Compared to A356, the increased volume fraction and size of the Fe-rich intermetallics (π phase) in the A357 alloy are responsible for the lower ductility, especially in the Sr-modified alloy. In alloys with large SDAS (>50 μm), final fracture occurs along the cell boundaries, and the fracture mode is transgranular. In the small SDAS (<30 μm) alloys, final fracture tends to concentrate along grain boundaries. The transition from transgranular to intergranular fracture mode is accompanied by an increase in the ductility of the alloys.