Effects of manganese dispersoid on the mechanical properties in Al-Zn-Mg alloys

In order to investigate the strengthening effect of manganese dispersoid in manganeseadded Al-Zn-Mg alloys, specially designed alloys with various manganese content were prepared and evaluated. The manganese dispersoid in the alloy is found to increase the strength significantly without losing much elongation. The strengthening effect originates from the fact that the manganese dispersoid behaves as the non-shearable particle which is composed of Al, Mn and Zn. It can be concluded that the improvement of mechanical properties by the manganese dispersoid without losing much elongation is due to the strengthening effect produced by the pinning action of the dispersoid on dislocation glide and the enhancement effect in elongation caused by a homogenization of slip. Meanwhile, the manganese dispersoid and soluble manganese element in the peak-aged manganeseadded Al-Zn-Mg alloys are observed not to influence the width of the precipitate-free zones (PFZ) and the kinetics of precipitates such as and GP zone (solute rich cluster).     

The effect of retrogression and re-ageing on the ductile fracture toughness of Al-Zn-Mg alloys containing different dispersoid phases

The effects of an incoherent and of a semi-coherent dispersoid phase upon the fracture toughness of a 7000-series AlZnMg alloys has been investigated. Standard crack opening displacement tests were successfully applied, the crack propagation being shown to be predominantly intergranular. The presence of manganese bearing incoherent dispersoids lowers the fracture toughness due to their effect of promoting microvoid formation. Retrogression and re-ageing led to a coarsening of the grain-boundary phases, but because the surface-to-surface separation of these particles remained approximately constant, there was no effect of this treatment, at constant yield stress, upon the fracture toughness. The toughness was only enhanced in the system (CZH) where retrogression and re-ageing led to a reduced yield stress.     

Hydrogen bubbles in embrittled Al-Zn-Mg alloys

Al-Zn-Mg alloys become embrittled during exposure to moist environments due to hydrogen penetration of grain boundaries. The result of this hydrogen penetration due to surface reaction with water vapour of both bulk specimens and electron-transparent thin foils, has been studied at high resolution in the JEM 100 C transmission electron microscope as a function of alloy composition and ageing treatment. In bulk specimens of alloys solution-heated, water-quenched, and aged in water-vapour-saturated air at 70° C, the hydrogen is in the form of a mobile atomic species which is transformed to bubbles of molecular hydrogen under the action of the electron beam. However, in electron-transparent specimens of aged alloys after exposure to water vapour the accumulated hydrogen is observed directly as bubbles. These bubbles take the form of hexagonal lenses bounded by {111} planes, and are associated with grain-boundary precipitates, particularly in over-aged microstructures, and with primary intermetallic particles in alloys containing sparingly soluble transition elements. The consequence of the observed hydrogen penetration of grain boundaries in promoting environmental debilitation of mechanical properties and stress-corrosion cracking of Al-Zn-Mg alloys is discussed.     

Al-Zn-Mg系铝合金的微合金化研究进展EI北大核心CSCD

Al-Zn-Mg系铝合金作为一种轻质高强合金在航空航天和交通等领域有着重要的应用。获得更高的力学性能以及更优的耐腐蚀性能是Al-Zn-Mg系合金的发展方向,因此需要进一步优化其微观组织。在合金成分和热处理制度调控空间有限的情况下,微合金化成为该合金性能改善的一种重要手段。本文简要总结了微合金化元素对Al-Zn-Mg系铝合金力学性能、热加工行为及耐腐蚀性能的影响,重点关注了微合金化元素在不同工艺阶段下形成的第二相颗粒能有效细化晶粒并强烈阻碍位错运动;讨论了热加工变形过程中钉扎晶界及亚晶界、抑制回复再结晶的作用;阐述了提高合金耐腐蚀性能方法的内在机理。最后对Al-Zn-Mg系铝合金微合金化的研究方向进行展望,深入理解微合金化元素间、主微合金元素间的相互作用机理,实现微合金化元素的精准、精确投放将是未来主要的研究内容之一。明确微合金化元素在热加工过程中对变形组织及位错组态的调控作用将对提高合金耐腐蚀性能提供借鉴。     

Static creep behaviour of Al-Zn-Mg and Al-Zn-Mg-Cu alloys

The creep behaviour of Al-Zn-Mg (7039) and Al-Zn-Mg-Cu (7075) alloys is evaluated at elevated temperatures (443T533 K and 483T563 K) under constant stresses between 49 and 123 MPa, respectively, in a custom-built creep testing facility. The measured activation energies of these alloys are 172–185 kJ mo^–1 and 248–272 kJ mol^–1. As the stress increases, the activation energy in both cases decreases due to the high density of dislocations. The average exponent values of these alloys are 7 and 9. The microstructure observation reveals that the dominant fracture mode of 7039 alloy is intergranular and that of 7075 alloy is transgranular.     

Mechanical properties of Al-Zn-Mg alloys investigated by microhardness measurements

One of the most simple and economic methods of testing the mechanical properties of alloys is the microhardness measurement. In the present paper we report on the results and the interpretation of experiments carried out on a series of AI-Zn-Mg alloys prepared from high purity base materials. The following results were obtained: (a) the incremental microhardness,HV, was related to the microhardness,HV, of high purity Al and can be given asHV=264(C_mg–0.25C_zn) wherec _mg andC _zn are the concentrations of Mg and Zn respectively, in the as-quenched state after solution heattreatment, (b) the ultimate tensile strength and the microhardness were correlated by the approximation:HV3_u.HV was investigated in the light of the average radius and the volume fraction of zones forming at room temperature. On the basis of the micromechanism of plastic deformation further evidence was found to show that the shearing mechanism is responsible for strengthening by GP zones in AI-Zn-Mg alloys.     

Factors leading to grain-boundary fatigue crack propagation in Al-Zn-Mg alloys

Fatigue crack propagation experiments have been carried out at low load amplitudes with a high purity and a corresponding commercial purity Al-Zn-Mg alloy. When the high purity alloy was tested in laboratory air, cracks were often seen to propagate along the grain boundaries. Particularly in the peak aged condition, this alloy is highly susceptible to failure by intercrystalline cracking. However, with dry nitrogen as the test environment, the crack was observed to propagate preferentially along shear bands within individual grains. In the commercial purity alloy, grain-boundary crack propagation was not observed for either laboratory air or dry nitrogen atmospheres. The proportion of intercrystalline cracking in laboratory air could be lowered for the high purity alloy by a thermomechanical treatment.     

Enhanced mechanical properties of Al–Si–Cu–Mn–Fe alloys at elevated temperatures through grain refinement and dispersoid strengthening

The effect of Ti content on the microstructure and mechanical properties of heat-treated Al–Si–Cu–Mn–Fe alloys was investigated. It was found that the mechanical properties increased with the increase of Ti content. This was attributed to the refinement of grain size, the increased amount of T (Al₂₀Cu₂Mn₃), the α-Fe (Al₁₅(FeMn)₃(CuSi)₂) precipitated particles, and the decrease in Al₂Cu. At an elevated temperature of 300°C, the heat-treated Al–Si–Cu–Mn–Fe alloy with 0.5% Ti demonstrated the best mechanical properties, which are superior to those of commercial aluminium alloys. The yield strength contribution at 300°C was quantitatively evaluated based on the dispersoid, solid solution, and matrix contributions. It was confirmed that the main strengthening mechanism in the experimental alloys was the dispersoid strengthening.     

Fatigue characteristics of pulsed MIG-welded Al-Zn-Mg alloy

Pulse-current MIG welding of Al-Zn-Mg alloy was carried out using an extruded section of base material and Al-Mg (5183) filler wire. During welding the pulse parameters such as the mean current and pulse frequency were varied and their effect on the geometry and porosity content of weld deposit as well as the fatigue life of the weldment was studied. The pulse parameters were found to affect significantly the geometry and porosity content of weld deposit and consequently the fatigue life of the weldment. For a comparative study, weldments were also prepared by using conventional continuous current MIG-welding process, where welding currents equivalent to the mean currents of pulsed process were used. The fatigue life of the weldment was correlated with the geometry and porosity content of weld deposit.     

Improvement of elevated-temperature strength and recrystallization resistance via Mn-containing dispersoid strengthening in Al-Mg-Si 6082 alloys

The precipitation behavior of Mn-containing dispersoids in Al-Mg-Si 6082 alloys with different Mn contents(0, 0.5 and 1.0 wt%) during various heat treatments(300–500℃) was investigated. The effects of dispersoids on elevated-temperature strength and recrystallization resistance during hot-rolling and post-rolling annealing were evaluated. The results showed that the dispersoids in the Mn-containing alloys(0.5 and 1.0%) began to precipitate at 350℃ and reached the optimum conditions after 2–4 h at 400℃. However, the dispersoids coarsened with increasing holding time at temperatures above450℃. After the peak precipitation treatment at 400℃ for 2 h, the yield strength at 300℃ increased from 28 MPa(base alloy free of Mn) to 55 MPa(alloy with 0.5% Mn) and 70 MPa(alloy with 1% Mn), respectively, demonstrating a significant dispersoid strengthening effect at elevated temperature. In addition,the dispersoids were thermally stable at 300℃ for up to 1000 h holding owing to its relative high precipitation temperature(350–400℃), leading to the superior constant mechanical performance at elevated temperature during the long service life. During hot rolling and post-rolling annealing, the presence of a large amount of dispersoids results in the higher Zener drag PZcompared with base alloy and then significantly improved the recrystallization resistance. The alloy containing 0.5% Mn exhibited the highest recrystallization resistance among three experimental alloys studied during the post-rolling process,likely resulted from the lower coarsening rate of dispersoids and the lower dispersoids free zone.     

Stress corrosion behaviour of Al-Zn-Mg alloys based upon microchemical surface reactions

Divergences between the chemical compositions of the natural oxide layers on pure aluminium and of alloys containing magnesium have been shown with the aid of SIMS measurements. Scanning and transmission electron micrographs and electron microprobe measurements indicate the sensitizing effect of the thermal oxide layer to water vapour and this is substantiated by SIMS studies. From published data and this investigation a further elaboration of the possible explanation for stress corrosion is proposed.     

The determination of tensile properties from hardness measurements for Al-Zn-Mg alloys

The correlations between tensile properties (yield strength, ultimate tensile strength and uniform strain) and indentation hardness are studied for two types of Al-Zn-Mg alloys. The reasons why Tabor's equations do not well fit the experimental data when the strain-hardening coefficient is larger than 0.3 are discussed. New equations for the determination of tensile properties from hardness measurements are theoretically derived and found to be in excellent agreement with the experimental data for Al-Zn-Mg alloys. The equations areT _u=(H _v/c ₂)[4.6(m–2)] ^m–2 and _y=(H _v/C ₂)^{1-(3–m> )} +25 (m–2), whereT _u and _y are ultimate tensile strength and yield strength,H _v is Vicker's hardness number,m is Meyer's hardness coefficient,E is Young's modulus,c ₂ is a constant about 2.9 in magnitude. In these equationsT _u, _y,H _v andE are all expressed in kg mm^–2.     

Surface tension of binary and ternary aluminium alloys of the systems Al-Si-Mg and Al-Zn-Mg

The surface tension and density of liquid binary and ternary aluminium alloys of the systems Al-Si-Mg and Al-Zn-Mg (Si, Mg and Zn contents less than 19, 8 and 20 wt %, respectively) have been measured by means of the maximum bubble pressure method. A semi-empirical theory, which relates the surface tension to bulk thermodynamic properties, is used to calculate the surface tension of the binary alloys and discuss the experimental data. For the ternary alloys, the present results indicate that in the range of compositions explored here, the properties of the ternaries can be obtained from those of the binaries. Comparison with results previously reported by other authors is made.     

Uniform texture in Al-Zn-Mg alloys using a coupled force field of electron wind and external load

Cylindrical Al-Zn-Mg alloys were processed by electroplastic compression with forced air cooling.Compared to a simple compression process,an unequal intensity of {110} <111> was obtained,and other textures were eliminated by electroplastic compression,that is,electroplastic compression can promote a uniform texture.The various textures formed in different regions along the radial direction under a simple compression process were illuminated by analyzing the relationship between the crystal rotation and stress state.Furthermore,the interaction between the electrons and dislocations was studied in electroplastic compression.The electrons enhanced {110} <111> by promoting slipping of the dislocations when the Burgers vectors of the dislocations were parallel to the drift direction of the electrons.However,the electrons also inhibited crystal rotation by pinning the dislocations with the Burgers vectors perpendicular to the drift direction of the electrons.Therefore,textures other than {110} <111>have difficulty forming under electroplastic compression.The effect of the current energy on the texture(enhancement or attenuation) was in accordance with the law of conservation.The results provided reasonable explanations for the test phenomena.