Al—Li合金的晶界断裂

本文研究了两种晶粒组织的二元 Al-Li 合金拉伸性能与断裂行为。结果表明 Al-Li 合金力学性能与晶粒尺寸有关,其断裂行为决定于 PFZ 内平面滑移或晶界沉淀相与滑移的交互作用。     

Fe—Mn—Cr合金晶间腐蚀电子理论研究

为了从电子层面揭示Fe—Mn—Cr合金晶间腐蚀的物理本质,采用递归法计算了合金的原子埋置能、格位能、亲和能等电子结构参数,探索合金晶间腐蚀机理。研究表明：Cr在晶内稳定性很低,Cr在晶界和表面稳定性较高,基体中的Cr首先扩散到晶界,并通过晶界扩散至合金的表面。Cr元素减小费米能级差,抑制合金的晶间腐蚀。O-Cr间的亲和能为负数,表明氧与Cr之间有相互作用,生成Cr的氧化物。当氧化膜达到一定厚度可起到保护合金的作用。碳与Cr的亲和能也为负数,且其数值比氧与Cr间的亲和能更负。合金中碳优先与Cr形成化合物,在晶界析出,造成晶界贫Cr,使合金晶间腐蚀加重。     

Mechanical properties and deformation mechanisms of a novel fine-grained Mg-Gd-Y-Ag-Zr-Ce alloy with high strength-ductility synergy

Grain boundary precipitation and segregation play an important role in determining mechanical properties of Mg alloys. In the present work, we studied work focuses on the strengthening and deformation mechanism of coarse-grained(CG) and fine-grained(FG) Mg-Gd-Y-Ag-Zr-Ce alloy. The CG alloy is strengthened by means of age-strengthening with the formation of both basal plate γ" and prismatic plate β’ precipitates in the grain interior. While the strengthening of FC alloy is completed by intergranular alloying segregation and intragranular precipitates γ" and β’. The segregation of alloying elements at the grain boundary and formation of sub-micron particles can stabilize the grain boundary and suppress the intergranular deformation. Consequently, dislocations could be trapped near γ" and β’ precipitates in the grain interior. Unlike CG alloys, the FG alloys exhibit a heterogeneous transition from elastic to plastic deformation via the Lüders plateau. The rapid gliding dislocation multiplications and fine-grained size are necessary and sufficient conditions for the Lüders strains. Our work provides the insights on the evolution of fine-grained microstructure and helps for the design of Mg alloys with good mechanical properties.     

Intergranular fracture of Al–Li–Cu–Mg alloy resulting from non-equilibrium eutectic melting during solution treatment

Abstract

This investigation has examined intergranular fracture during heat treatment and deformation of an Al–Li–Cu–Mg alloy and of an Al–Li–Cu alloy. When solution treatment of the Al–Li–Cu–Mg alloy was initiated by rapid heating to temperatures ≥ 545°C, non-equilibrium eutectic melting of a grain boundary precipitate phase occurred and the liquid spread along grain boundaries as a thin film. On quenching, intergranular cracks were observed at grain boundaries into which a liquid film had penetrated during solution treatment. For less rapid heating rates, non-equilibrium eutectic melting did not occur and no intergranular cracks were observed after quenching. No evidence of non-equilibrium eutectic melting was observed in the Al–Li–Cu alloy irrespective of the rate of heating to 550°C. During tensile testing of as quenched and quenched and aged specimens of the two alloys, intergranular fracture occurred in most specimens, whether or not non-equilibrium eutectic melting had taken place during solution treatment, indicating that at least one additional mechanism of intergranular fracture was initiated by deformation.

MST/947     

Effect of cold working and ageing treatment on ductility of an Al-6.0% Zn-2.6% Mg alloy at 4.2 to 293 K

The effect of cold-working and ageing (CA) treatment on the ductility of an Al-Zn-Mg alloy was investigated at temperatures from 4.2 to 293 K. The ductility was improved by the CA treatment at 4.2, 77, 196 and 293 K. It was found that both the fracture strain and the reduction of area vary linearly with the area fraction of transgranular fracture. The improvement of ductility was thought to be due to the shape of the grain boundary, the formation of a subboundary and the scattering precipitate size in grains by which both the ledge- and the dimple-formation type intergranular fractures are difficult to occur. The increase of intergranular fracture with decreasing temperature is attributed to the increase of the difference between flow stresses in PFZ and in the matrix, the brittleness of grain boundary precipitates themselves, and the incoherent interface between grain boundary precipitates and the matrix. An intergranular fracture mechanism in the CA specimen is proposed.     

固溶前冷轧压下率对Cu-Ni-Si-Mg合金组织与性能的影响

为了优化Cu-Ni-Si-Mg合金的轧制工艺参数,利用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)、能谱(EDS)和X射线衍射(XRD)等手段,对固溶前不同冷轧压下率下的Cu-Ni-Si-Mg合金时效态的组织性能及拉伸断口进行了分析.结果表明:随着冷轧压下率的增加,Cu-Ni-Si-Mg合金的平均晶粒尺寸逐渐增加,屈服强度、硬度和延伸率先增大后减小,抗拉强度表现为先增大,后在710 MPa左右波动,再减小的趋势;压下率从94%增加到96%,晶粒尺寸从12μm增加到52μm,而抗拉强度变化较小,归因于低压下时Cu-Ni-Si-Mg合金的细晶强化和时效强化占主导,而大压下时第二相粒子均匀的时效析出弥补了因晶粒粗大而造成的抗拉强度损失;压下率为93%时,合金的断裂为单一的韧性断裂,压下率为97%时,合金的断裂由韧性断裂和沿晶脆性断裂组成.     

航空用7475-T7351铝合金厚板耐腐蚀性能

研究航空用7475-T7351铝合金厚板晶间腐蚀及剥落腐蚀性能,并利用金相和透射电镜分析该合金的腐蚀行为。结果表明:7475铝合金无明显晶间腐蚀,剥落腐蚀程度由表层的EA级递增至心部EC级。7475铝合金厚板发生剥落腐蚀主要是由于合金为片状组织,同时晶界存在由电偶腐蚀构成的通路,晶界腐蚀产物体积膨胀产生楔入力使晶间腐蚀沿着与表面平行的方向发展并逐步演变为剥落腐蚀。再结晶程度由表层到中心逐渐降低,晶粒长宽比增加,剥落腐蚀倾向增大,导致表层到心部的剥落腐蚀程度增加。     

The mechanism of elevated temperature intergranular cracking in heat-resistant alloys

Reheat or stress relief cracking phenomena have been reassessed in 2.25Cr1.5W heat-resistant alloys. During rupture test, time to intergranular failure increases with decreasing temperature and tensile stress and is shorter in the alloy containing a higher bulk content of phosphorus. Also the time to intergranular failure can be expressed by t = t₀·σ⁻ⁿ·exp(Q/RT) where t₀ is the proportional constant, n the stress exponent and Q the activation enthalpy. Matrix softening is accelerated under tensile stress and an active carbide growth occurs at grain boundaries oriented normal to the tensile stress direction. Because impurities segregate actively to dimples frequently observed at reheat intergranular fracture surfaces, the dimples are not micro-ductile fracture areas but the grain boundary carbide interfaces. The segregation concentration of the impurities is much higher at the grain boundary carbide interfaces than the carbide-free grain boundaries. The phosphorus segregation at the carbide interfaces of the alloy containing the higher bulk content of phosphorus is mainly replaced by the segregation of nitrogen, tin and tellurium in the alloy containing a lower bulk content of phosphorus. The elevated temperature intergranular cracking under tensile stress occurs finally due to the carbide-free grain boundary cracking following the decohesion of the grain boundary carbide interfaces.     

Corrosion of Artificial Aged Magnesium Alloy AZ80 in 3.5 wt pct NaCl Solutions

The corrosion morphologies of aged magnesium alloy AZ80 were investigated by immersion corrosion tests, scanning electron microscopy （SEM）, electrochemical measurement. The T5 heat treatment was carried out in a vacuum furnace, holding for 16 h at 177℃, and then cooling in air. The results showed intergranular corrosion （IGC） occurred as an aged AZ80 sample was immersed in 3.5 wt pct NaCI aqueous solution for 1 h and the narrow path attack progressed predominantly along the bulk β phase in the grain boundaries or took place in the eutectic areas. IGC was attributed to the network distribution of β phase along the grain boundaries, the depleted aluminium in the precipitation areas and the breakdown potential.     

Enhanced room-temperature tensile ductility of columnar-grained polycrystalline Cu-12 wt.%Al alloy through texture control by Ohno continuous casting process

The Ohno continuous casting (OCC) process is a practical way to control the solidification texture of Cu-12 wt.%Al alloy with a perfect < 001>β fiber texture along the solidification direction. Compared with the conventional randomly oriented polycrystalline Cu-12 wt.%Al alloy, the reorientation of β₁′ martensite and stress-induced phase transformation occurred at the same time within every columnar grain sharing the same [001]β orientation during tensile test, which would reduce the elastical and phase-transformational incompatibility and enhance the intergranular accommodation. As a consequence, a high tensile ductility up to 28% with transgranular fracture can be obtained for OCC columnar-grained Cu-12 wt.%Al alloy instead of intergranular fracture due to the incompatible stress at the grain boundary for randomly oriented polycrystalline Cu-12 wt.%Al alloy.     

On the effects of thermomechanical processing on failure mode in precipitation-hardened aluminium alloys

This paper concentrates on the influences of thermomechanical processing on fracture behaviour of Al-Mg-Si-Cu (AA6061) alloys. Important factors are grain boundary structure and extent of matrix- and grain boundary precipitation. Large grain boundary phases in the as-air-cooled alloy, explains its much smaller fracture strain with respect to the water-quenched alloy. With increasing artificial ageing time, the bulk fracture strain of the air-cooled alloy exhibits a minimum. This is due to grain boundary precipitate growth and coarsening affecting the fraction of strain confined to the grain boundary region. For the recrystallized microstructure aged to peak strength, the fraction of intergranular fracture is much larger for the air-cooled alloy. This can be understood on the basis of a much wider precipitate free zone and a smaller grain boundary precipitate volume fraction for the air-cooled alloy, increasing the fraction of strain confined to the grain boundary region by about one order of magnitude with respect to the water-quenched alloy. A much coarser distribution of intermetallic phases in the extruded microstructure is responsible for a larger degree of slip localization. This enhances the tendency for shear- and intergranular fracture, reducing the ductility and thus the fracture strain with respect to that of the recrystallized microstructure.     

THE INFLUENCE OF TEST TEMPERATURE ON IMPACT FATIGUE CRACK GROWTH BEHAVIOR IN QUENCHED AND TEMPERED Cr-Mo ALLOY STEELS WITH DIFFERENT PRIOR AUSTENITE GRAIN SIZES

Abstract— Impact fatigue tests were carried out using a rotating-disk type impact fatigue testing machine. The influence of prior austenite grain size, ductile-brittle transition temperature and test temperature on impact fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel in which the prior austenite grain size was varied from 8–3 to 25-4 μm. The results in impact fatigue tests were compared to those under non-impact conditions. The crack growth rates associated with striation formation were insensitive to the change in prior austenite grain size, ductile-brittle transition temperature and test temperature regardless of impact and non-impact fatigue. When the material was in the brittle condition, impact fatigue gave rise to a transition from striation formation to intergranular and cleavage cracking. Such a transition will result in the acceleration of crack growth rate. The Paris Law exponent values in impact fatigue were reasonably expressed by the ratio of test temperature to ductile-brittle transition temperature.     

Failure analysis of brass rotor bars

Cracked rotor bars with characteristic chemical composition of 90Cu–10Zn are investigated in this paper. A brazing technique was employed to join these rotor bars with rings in a motor assembly factory. However, frequent edge cracks were observed after this joining process. Intergranular facets of these cracked bars revealed brittle fracture. Mechanical tests of the bars showed lack of ductility and strength, and the bars were found to have a higher lead (Pb) content. In addition, the season cracking susceptibility of copper alloy that can result in intergranular brittle fracture was assessed. The metallurgical structure revealed a very large grain size (150 μm) in the copper alloy. Non-uniform thermal stresses in the heating process triggered cracking of the bars owing to an insufficiency of material strength. The large grain size accounted for the failures.     

On preventing HAZ cracking in laser welded DS Rene 80 superalloy

Abstract

The heat affected zone (HAZ) cracking behaviour in a laser beam directionally solidified (DS) Rene 80 nickel based superalloy subjected to preweld heat treatments was studied. The HAZ cracks in the alloy are grain boundary liquation cracks caused by liquation reaction of both non-equilibrium secondary solidification product, MC carbides and equilibrium solid state reaction product, γ′ precipitates. In contrast to theoretical prediction based a preweld heat treatment that reduced grain boundary liquid film thickness did not result in a lower HAZ cracking, which can be related to concomitant reduction in the ability of the base alloy to relax welding stress. In addition, formation of intergranular M₅B₃ boride particles in preweld alloy appeared to have aided cracking susceptibility by lowering grain boundary liquation temperature and widening the brittle temperature range in the HAZ during cooling. Based on the analysis of the results, application of a new preweld heat treatment that prevents the formation of the intergranular borides and induces moderate base alloy hardness resulted in a nearly crack free HAZ in laser welded DS Rene 80 superalloy.     

Microstructure and Properties of As-Cast Mg–2.0Sn–1.0Zn–1.0Y–0.3Zr Alloys at Different Extrusion Ratios

Herein, the effect of hot extrusion with different extrusion ratios (λ = 6, 8, 10, and 12) on the microstructure evolution and properties of as-cast Mg–2.0Sn–1.0Zn–1.0Y–0.3Zr magnesium alloys, using optical microscopy (OM), scanning electron microscopy (SEM), immersion corrosion and electrochemical corrosion experiment, and tensile testing, is investigated. The results show that the Mg₁₄SnY and Mg₆SnY precipitated phases exist in the alloy before and after extrusion. After hot extrusion, the second phase of the alloy is broken into particles along the extrusion direction, whereas the grain size is significantly reduced, and dynamic recrystallization and deformed grains exist in the microstructure. The mechanical properties of the extruded alloy improve, but the corrosion resistance weakens. When the extrusion ratio is λ = 10, the extruded alloy exhibits relatively good mechanical properties and corrosion resistance. The corrosion behaviors of the extruded alloys are affected by both the grain size and galvanic corrosion. In the initial stage of corrosion, intergranular corrosion plays a major role in reducing the corrosion resistance of the extruded alloys. With prolonged corrosion time, galvanic corrosion has a more significant effect on weakening the corrosion resistance of the extruded alloys.     

Microstructure,microhardness and corrosion susceptibility of friction stir welded AlMgSiCu alloy

Microstructure, microhardness and corrosion susceptibility of friction stir welded joint in an AlMgSiCu alloy were investigated. It was found that the joint exhibits different corrosion susceptibility among the microstructural zones. The base material is the most susceptible to intergranular corrosion because of the presence of continuous cathodic precipitates (Si and Q phases) at grain boundaries and the precipitate free zone along the grain boundaries. The coarsening of intergranular precipitates and the precipitation of Q′ phases in the grain bodies reduce intergranular corrosion susceptibility but introduce pitting corrosion in the heat-affected zone. The significant elimination of intergranular corrosion both in nugget zone and thermo-mechanically affected zone is related to the low volume fraction of intergranular precipitate. Microhardness variations depend on the evolution of intragranular precipitates. The dissolution and/or coarsening of the strengthening precipitates result in the softening within the welded zone.     

Fracture of polycrystalline Fe−2.3%V−0.12%P alloy

Fracture experiments on a polycrystalline Fe-2.3mass%V-0.12mass%P alloy show that the fracture mode is directly related to the grain boundary segregation of phosphorus. Comparison of the experimental data with theoretical simulations showed that intergranular fracture spreads along those grain boundaries, which contain more than 17 at.% of segregated phosphorus.     

The influence of Ca on the corrosion behavior of new die cast Mg–Al-based alloys for elevated temperature applications

In this study, the corrosion behavior of Mg–5Al alloys with different quantities of calcium was investigated by DC and AC polarization and immersion tests. The alloys’ microstructure was investigated by optical microscopy and SEM with EDS. It was found that increasing the Ca content from 0.9 to 1.8 wt% improves the corrosion resistance of the alloy; the corrosion resistance of the alloy with 1.8 wt% Ca equals that of the base Mg–5Al alloy. Ca addition causes a reduction of the average grain size, which increases corrosion resistance as the precipitates located at grain boundaries are more continuous and act as a barrier to the advance of corrosion. The precipitates located at grain boundaries contain Al and Ca and are much less cathodic than the β phase, thus making the microgalvanic effect in the specimen less effective.     

奥氏体钢晶间腐蚀机理电子理论研究

采用递归法计算了奥氏体钢的杂质掺入能、格位能、亲和能等电子结构参数,建立电子参数与奥氏体钢晶间腐蚀关系,探索晶间腐蚀机理。研究表明：奥氏体钢中S和P由晶内向晶界扩散,恶化晶界稳定性,且使电子从晶界流向晶内,造成晶界与晶内产生电位差,形成微电偶结构,发生晶间腐蚀。C和Cr具有较强亲和力,能在奥氏体钢晶界形成（Cr,Fe）23C6相。Ni提高C的格位能,降低C在奥氏体中的固溶度,促进（Cr,Fe）23C6的形成与长大,增加奥氏体钢的晶间腐蚀敏感性。Nb和Ti降低C的格位能,抑制（Cr,Fe）23C6的形成,提高奥氏体钢的抗晶间腐蚀能力。     

The control of grain boundary segregation and segregation-induced brittleness in iron by the application of a magnetic field

Recycling of iron and steel becomes an universally important issue from the viewpoint of energy and resource saving. Impurity elements like Sn and Cu tend to accumulate in steels by repeated recycling and remarkably degrade mechanical properties of recycled iron alloys due to segregation-induced intergranular embrittlement. The goal of this work is to study the potential of magnetic annealing for the control of grain boundary segregation and intergranular embrittlement in iron alloy. This paper reports several important findings regarding the effect of magnetic annealing on segregation-induced brittleness in iron-tin alloy. Of particular importance is the observations that the concentration of tin at grain boundaries in iron is decreased by magnetic annealing and fracture toughness of iron-tin alloy is drastically improved to the level as high as pure iron.