Gd添加量对Zn-1.2Cu-1.2Mg锌合金组织和性能的影响

研究了Zn-1.2Cu-1.2Mg-xGd（x=0,0.1,0.25,0.5,质量分数,%）锌合金微观组织、在模拟体液中的腐蚀行为和力学性能。结果表明,锌合金组织主要由Zn固溶体和Mg₂Zn₁₁金属间化合物组成。当Gd添加量为0.5%时,形成了GdZn₁₂化合物。锌合金的硬度随Gd添加量的增加而增加,在0.5% Gd添加量时,硬度达到最大值为1530 MPa。在0.25% Gd添加量时锌合金具有高的抗拉伸强度,而在0.1% Gd添加量时锌合金具有最低的腐蚀速率。     

Microstructure and properties of strip cast AZ91 Mg alloy

A study has been conducted on the microstructure and mechanical properties of the strip cast AZ91 Mg alloy. The microstructure of the as-cast strip is characterized by a fine equiaxed dendritic structure. There is a variation of secondary dendrite arm spacing throughout the thickness of the strip, showing the smallest value at the wheel surface and the largest value at the center. The distribution of Mg₁₇Al₁₂ particles is also not uniform in the as-cast strip. The microstructure of the solution treated strip consists of fine Mg grains and Al−Mn particles in the matrix with no Mg₁₇Al₁₂ particles. T6 treatment of the strip results in the precipitation of Mg₁₇Al₁₂ particles, the volume fraction of which decreases from the wheel side to the center of the strip. The strip cast AZ91 Mg alloy has the best combination of tensile properties in the T4 condition. It is believed that the good tensile properties of the T4 treated strip are due to the presence of Al−Mn particles, which induce homogeneous deformation.     

AZ91镁合金的挤压和析出硬化行为(英文)

挤压比为4:1,将铸态AZ91镁合金分别在250,300和350℃下进行挤压,随后进行析出硬化处理(T6)。经过热挤压和析出硬化处理后,铸态AZ91镁合金中粗大的和偏析Mg17Al12析出相被细化并均匀分布在α-镁基体中。在不同的挤压温度下合金中发生了部分或全部动态再结晶。经挤压后,该合金的极限抗拉强度从铸态的190MPa增加到570MPa。AZ91镁合金的时效硬化特征与晶粒尺寸有关。在250、300和350℃下以4:1的挤压比挤压该合金后,获得峰值硬度的时效时间分别为35、30和20h。SEM观察到在AZ91基体中存在均匀细小的Mg17Al12析出相。     

Grain refinement of Mg-Al binary alloys inoculated by in-situ oxidation

Utilizing oxide inclusion to induce heterogeneous nucleation event is an available method to achieve grain refinement. In this study, Mg-Al binary alloys were refined by inoculation of in-situ oxidation process. Results show that MgO and MgAl₂O₄ phases are primary oxide products for Mg-xAl alloys inoculated by in-situ oxidation. For pure Mg and Mg-1Al alloy, MgO is the only oxide product. MgAl₂O₄ is another oxide product for Mg-xAl alloy as Al content increases to 3 wt.%. For Mg-3Al alloy, average grain size significantly decreases from 1135 to 237 μm, with a high grain refining ratio of 79.1%. Both MgO and MgAl₂O₄ possess nucleating potency for α-Mg grain. MgAl₂O₄ exhibits a higher nucleating potency due to the lower misfit with α-Mg. The grain refinement of Mg-xAl alloys inoculated by in-situ oxidation process is attributed to heterogeneous nucleation events of α-Mg grains on MgO or MgAl₂O₄ particles.     

Microstructural evolution and mechanical performance of cast Mg-3Nd-0.2Zn-0.5Zr alloy with Y additions

This work investigated the effects of different Y additions (0, 1.5, 3.0 and 4.5 wt.%) on the microstructural evolution and mechanical performance of cast Mg-3Nd-0.2Zn-0.5Zr alloy. The results show that as the Y content increases, the key secondary phases in as-cast alloys change from the Mg₁₂Nd type to the Mg₂₄Y₅ type. Meanwhile, the number density of Zn-Zr particles in the grains of as-quenched alloys gradually decreases. HAADF-STEM observations of peak-aged samples reveal that element Y is greatly enriched in the globular β′ precipitates, leading to a significantly increased volume fraction and promoted precipitation kinetics of β′ precipitates, resulting in enhanced strength of the alloy. Tensile tests reveal that, with the addition of 4.5 wt.% Y, the yield strength of the base alloy is substantially increased by 88 and 61 MPa after being aged at 200 and 225 °C under peak-aged conditions, respectively.     

Microstructure characteristics of resistance spot welds of AZ31 Mg alloy

Abstract

The experimental investigation was carried out to study the weld microstructure of resistance spot welding of AZ31 Mg alloy 1 mm thick. A fine and homogeneous non-equilibrium microstructure of globular α grains, surrounded by eutectic mixtures of α and β (Mg₁₇Al₁₂), was achieved. The thermal–electrical–mechanical analysis model was employed to simulate the thermal history and the temperature gradient. It was found that a combination of the welding conditions and the particular thermophysical properties of the AZ31Mg alloy established a uniform temperature distribution throughout the weld pool and this thermal condition is ideal for nucleation throughout the melt metal and equiaxed grain structure forming.     

The Microstructure and Tensile Properties of a Newly Developed Mg–Al/Mg<Subscript>3</Subscript>Sb<Subscript>2</Subscript> In Situ Composite in As-Cast and Extruded Conditions

The microstructures and tensile properties of Mg–x wt%Al–y wt%Sb alloys have been studied where x/y ratio was 1 and Sb(Al) contents were 5, 10, 15 and 20 wt%, respectively. The results indicated that by increasing Sb(Al) content, not only the crystals of primary Mg₃Sb₂ alter from small flake-like particles to polygonal or needle-like morphology, but also the eutectic structure changes from semi-continuous network in Mg–5Al–5Sb to continuous network in Mg–20Sb–20Al alloy. The results obtained from thermal analysis revealed different peaks related to the formation of Mg₃Sb₂ as primary phase and eutectic structure containing Mg₁₇Al₁₂?+?Al₃Mg₂ intermetallic phases. Further results also revealed that Sb(Al) additions change the solidification performance of the material by depressing the Mg₃Sb₂ nucleation temperature, reducing solidification range and widening eutectic area. Tensile testing results showed that with the increase in Sb (Al) content, ultimate tensile strength (UTS) and elongation values of the alloys are decreased in as-cast condition. But, significant improvement in the UTS and elongation values of the extruded specimens was attributed to the severe fragmentation of intermetallic phases and well distributed fine particles in the matrix which provided proper obstacles for dislocation motion. It was interesting to note that the fracture behavior of intermetallic particles was found to be different, while Mg₃Sb₂ was ductile, intermetallic compounds in eutectic regions were brittle.     

Microstructure and mechanical behaviour of cold metal transfer welded Mg/Al dissimilar joint using wire AZ31 as filler metal

Mg/Al dissimilar butt joint was produced by modified cold metal transfer process using wire AZ31 as filler metal. The energy input characteristics and the microstructure and mechanical behaviour of the joint were investigated. Microstructural analysis shows that a diffusion Mg–Al intermetallic compounds interface layer formed along the weld boundary near Al substrate. The interface layer consisted of three intermediate layers from Al substrate to weld metal: Mg₂Al₃ layer, Mg₁₇Al₁₂ layer and Mg₁₇Al₁₂?+?α-Mg solid solution eutectic layer. The tensile strength of the welded joint was 38.4?MPa, which was fairly dependent on the lowest strength of the three intermediate layers. The brittle fracture occurred primarily within the thinnest Mg₂Al₃ intermediate layer adjacent to Al substrate.     

Effects of TiC additions on as cast and heat treated microstructures and mechanical properties of ZA84 magnesium alloy

Abstract

The effects of TiC additions on the as cast and heat treated microstructures and mechanical properties of ZA84 Mg alloy are investigated. The results indicate that, after adding 0·5 wt-%TiC to ZA84 alloy, the as cast microstructure of the alloy is refined. At the same time, the distribution of second phases in the alloy becomes relatively uniform and the quasi-continuous networked second phases in the alloy become discontinuous. As a result, the as cast tensile properties of the alloy are improved. In addition, after solutionisation at 345°C, parts of the Mg₃₂(Al,Zn)₄₉ phases in the ZA84 alloy without TiC modification still exhibit quasi-continuous distribution. The Mg₃₂(Al,Zn)₄₉ phases in the ZA84 alloy treated with 0·5 wt-%TiC change to disconnected particles with an angular morphology and gradually spheroidise. Under the optimum heat treated conditions, the ZA84 alloy treated with 0·5 wt-%TiC exhibits higher heat treated tensile properties than the ZA84 alloy without TiC modification.     

Microstructure and Mechanical Properties of a Ag Micro-Alloyed Mg-5Sn Alloy

Effects of 1.5 wt.% Ag addition and solid solution?+?artificial ageing at 160 °C on the microstructure and mechanical properties of a Mg-5Sn alloy have been studied. The results show that Ag addition has significantly hardened the solution-treated Mg-5Sn alloy. During the ageing process, the hardness increase rate and the strength and ductility of the Mg-5Sn alloy at each state are also improved by Ag addition. The improved strengthening behavior is primarily attributed to the refinement distribution of the Mg₂Sn precipitates, the enhanced precipitation process, and the synergistic strengthening effect of Mg₂Sn and a metastable plate DO₁₉ phase formed at lower ageing temperature. For each solution-treated alloy, the strength and ductility are higher than the corresponding cast ones. Ageing further enhances the yield strength, and the ductility of the Mg-5Sn-1.5Ag alloy is also increased after ageing. The fracture surfaces of the both peak-aged alloys exhibit the characteristic of a mixture of quasi-cleavage and ductile fracture.     

Corrosion behaviour of twin belt cast EN AW 7075 alloy

The corrosion behaviour of the twin belt cast EN AW 7075 alloy is governed by intermetallic phases, namely Al₁₂(Fe,Cr,Mn)₃Si, Mg₂Si and CuAl₂, and by Mg(Zn,Cu,Al)₂ precipitates. The former are responsible for pitting activities while the Mg(Zn,Cu,Al)₂ precipitates play a key role in intergranular corrosion. The very fine dispersion of Mg(Zn,Cu,Al)₂ precipitates in samples aged to peak hardness undergo coarsening, particularly along the grain boundaries, when the hot band samples are overaged. Overageing improves the resistance to intergranular corrosion while the samples in T6 temper suffer heavy attack along grain boundaries. While ageing treatments hardly produce any change in the features of the intermetallic particles, they nevertheless seem to impact the pitting response. This may be accounted for also by the precipitation activities which in turn, change the chemistry of the solid solution matrix. Overageing to the T73 temper implies a higher purity matrix and thus changes the microgalvanic effects when exposed to neutral chloride solutions.     

Novel multilayer Mg–Al intermetallic coating for corrosion protection of magnesium alloy by molten salts treatment

A novel multilayer Mg–Al intermetallic coating on the magnesium alloy was obtained by AlCl₃–NaCl molten salt bath treatment. The molten salt was treated at 400 °C, which is lower than the treatment temperature of solid diffusion Al powder. The thick Al₁₂Mg₁₇, Al_0.58Mg_0.42 and Al₃Mg₂ multilayer Mg–Al intermetallic coating forms on the magnesium alloy. The corrosion resistance of AZ91D alloy with and without coating by multilayer of Mg-Al intermetallic compound was evaluated by electrochemical impedance spectroscopy measurements in 3.5% (mass fraction) NaCl solution. The polarization resistance value of the multilayer coating on the magnesium alloy by molten salt bath treatment is greater than that of the uncoated one, which is attributed to the homogenously distributed intermetallic phases.     

Development of short alumina fibre and in situ Mg2Si particle reinforced magnesium alloy hybrid composite with high temperature strength

Abstract

Short alumina fibre and in situ Mg₂Si particle reinforced hybrid composites were fabricated by infiltration with the molten magnesium alloy into the preforms consisting of the fibres having Si particles attached to their surfaces. P or CaF₂ particles were used as the refiners of the Mg₂Si particles. All of Si particles reacted with the Mg alloy and melt to form Mg₂Si particles. As the melting temperature decreased or the cooling rate after the infiltration increased, the Mg₂Si particles became finer. The introduction of P or CaF₂ further promoted the refinement of the Mg₂Si particles. When the composite was fabricated by squeeze casting in the permanent mould, fine Mg₂ Si particles with a grain size of ～5 μm were formed regardless of introducing the refiners due to the rapid solidification. The strength of the hybrid composites was higher than that of the conventional fibre reinforced composite at both room temperature and high temperature.     

Effects of ultrasonic treatment on microstructure and tensile strength of AZ91 magnesium alloy

In this study, the effects of ultrasonic treatment on microstructural features and tensile strength of AZ91 magnesium alloy were investigated. AZ91 melts were subjected to ultrasonic waves of different power levels for 5 min using an ultrasonic device with frequency of about 20 kHz and maximum power of 600 W and cast in sand moulds. The results showed that ultrasonic treatment of the melt prior to casting had a significant effect on the size and sphericity of α-Mg dendrites as well as on the size, continuity, sphericity and distribution of intermetallic particles formed during cooling and solidification of the alloy. Increasing the applied ultrasonic power resulted in smaller, more rounded and better distributed grains and particles. The microstructural effects were mainly attributed to the cavitation and streaming phenomena which took place during ultrasonic treatment in the melt. Tensile strength of the alloy was significantly improved by ultrasonic treatment of the melt. Discontinuity and refinement of Mg₁₇Al₁₂ particles in the ultrasonically treated samples is thought to be the main reason for this improvement. The paper also examines different possible mechanisms responsible for microstructural modification of different phases under ultrasonic treatment conditions.     

Corrosion of twin belt and twin roll cast AlMg3Mn alloys

Corrosion of twin belt and twin roll cast AlMg3Mn sheet samples was investigated. The AlMg3Mn sheet samples submitted to immersion tests undergo alkaline pitting around Al–Fe and α_c-Al(Fe,Mn)Si intermetallic particles in the twin roll and twin belt cast samples respectively. The weight loss is higher in the latter and increases with increasing homogenisation temperature for both groups. The twin roll cast AlMg3Mn samples reveal very few and small alkaline corrosion pits and hence much less weight loss in the immersion tests. The pitting activity is governed in the immersion tests by the microgalvanic corrosion activities between the intermetallic particles and the matrix while the anodic particles were inactive.     

Mechanical and Tribological Characterization of Al-Mg2Si Composites After Yttrium Addition and Heat Treatment

In this study, the effect of heat treatment and yttrium additions on the microstructure, mechanical properties, and tribological behavior of Al-15% Mg₂Si cast composites was investigated. The microstructural study revealed the presence of both primary and secondary Mg₂Si phases in all composite specimens and also Y-containing intermetallics (Al₂Y phases) at higher concentrations. It was also found that Y addition does not change the size and morphology of primary Mg₂Si particles considerably, but the pseudo-eutectic Mg₂Si changed from a flake-like morphology to fine fibrous or rod-like one. The results show that proper content of Y additions can reduce the amount of Mg₂Si phase through dissolving it into the matrix, lead to the precipitation of Al₂Y phase and improve the mechanical properties. Modified composites with 0.5% Y exhibited an ultimate tensile strength (UTS) of 290 MPa with an elongation of 4.3%. After exposing the composite to solution treatment at 520 °C for 4 h, the tensile strength of the composite continuously increased with the increase of Y content, and reached the maximum at 1% Y. The maximum UTS and elongation at room temperature for the heat-treated composites are 294 MPa and 7.4%, respectively. In the cast specimen, fracture surfaces are covered by packets with coarse steps, suggesting a brittle mode of failure. Modified composites with 0.5 wt.% Y contain several cracked particles together with a few decohered primary Mg₂Si particles. In solution heat-treated state, dimples present at the fracture surface are rather coarse but homogenous, showing a semi-ductile mode of fracture. Wear test results showed that the wear resistance of all specimens increases with the addition of Y up to 0.3 wt.%. Scanning electron microscopic observations of the worn surfaces revealed that the dominant wear mechanism was abrasive wear accompanied by some delamination wear mode.     

Improvement of microstructure,mechanical properties,and corrosion resistance of WE43 alloy by squeeze casting

The WE43 magnesium alloy was prepared by squeeze casting, and the influence of squeeze casting parameters on mechanical properties and corrosion resistance was studied and compared with gravity casting. The gravity cast WE43 alloy shows uneven grain size distribution, and some grains even greater than 90 µm. While, the grain size of the squeeze cast WE43 alloy is mainly distributed in 20–50 µm. The Mg₁₂Nd₂Y phase morphology changes from large lamellar to strips after squeeze casting, whereas Mg₂₄Y₅ phase exhibits no obvious change. The yield strength, tensile strength, and elongation of the gravity cast WE43 alloy are 127 MPa, 157 MPa, and 6%, respectively, and 145 MPa, 193 MPa, and 9.1% for squeeze cast alloy. For the squeeze cast WE43 alloy, the average corrosion rate is 0.6056 mm·year⁻¹ according to immersion test results, and according to electrochemical measurements, the corrosion current density is 78.13 µA·cm⁻², which is better than that of the gravity cast WE43 alloy. Compared with gravity casting, the grains and second phase of the WE43 alloy by squeeze casting are refined, and the mechanical properties and corrosion resistance are improved. This may expand the applications of the WE43 alloy.