Microstructural evolution of the supersaturated ZA27 alloy and its damping capacities

The microstructural evolution of the supersaturated ZA27 alloy and its damping capacities were investigated during the natural aging. The investigation showed that the microstructure of the alloy transformed from single phase into a fine mixture of equiaxed , and grains during the process. The maximum of the interface area in the alloy existed during the aging. Microstructure changes were accompanied with the variance of damping capacity. It was found that the damping capacities increased quickly and greatly at the beginning of aging and then it decreased gradually and finally reached a constant during the following aging. It is believed that two mechanisms are responsible for the occurrence of the damping behavior in supersaturated ZA27 alloy, i.e., interface damping and dislocation damping.     

Dimensional shrinkage of supersaturated ZA27Cu1 and ZA27Cu2 alloys

Abstract

The dimensional changes of supersaturated ZA27Cu1 and ZA27Cu2 alloys based on Zn-27 Al-Cu, quenched from 360°C, and the mechanisms influencing the dimensional changes were investigated with a universal tool measuring microscope and X-ray diffraction. It wasfound that the dimensions of the supersaturated alloys shrunk rapidly and considerably after aging at 80°C, and that Cu enhanced shrinkage. The dimensional shrinkage which occurred after aging was a result of rapid precipitation of η (Zn base solid solution) and e (CuZn₄) phases from the supersaturated β phase (Al base solid solution).     

Microstructure and mechanical properties of ZA62 Mg alloy by equal-channel angular pressing

The Mg-6Zn-2Al alloy was processed by ECAP and microstructure and mechanical properties of the alloy before and after ECAP were studied. The results revealed that the microstructure of the ZA62 alloy was successfully refined after two-step ECAP (2 passes at 473 K and 2-8 passes at 423 K). The course bulk interphase of Mg₅₁Zn₂₀ was crushed into fine particles and mixed with fine matrix grains forming “stripes” in the microstructure after the second step of ECAP extrusion. A bimodal microstructure of small grains of the matrix with size of ∼0.5 μm in the stripes and large grains of the matrix with size of ∼2 μm out of stripes was observed in the microstructure of samples after 4-8 passes of ECAP extrusion at the second step. The mechanical properties of the alloy studied were significantly improved after ECAP and the highest yield strength and elongation at room temperature were obtained at the samples after 4 and 8 ECAP passes at the second step, respectively. Tensile tests carried out at temperature of 473 K to 573 K and strain rate of 1 × 10⁻³ s⁻¹ to 3 × 10⁻² s⁻¹ revealed that the alloy after 8 ECAP passes at the second step showed superplasticity and the highest elongation and strain rate sensitivity (m-value) reached 520% and 0.45, respectively.     

热挤压铸态ZA27合金阻尼性能研究

对铸态ZA27合金在250℃进行了挤压比为7的热挤压,利用多功能内耗仪测试合金的阻尼。结果表明,挤压ZA27合金的阻尼随着温度升高而升高,随频率增大而减小,与应变振幅无关。挤压可提高合金阻尼,温度越高,提高幅度越明显。如25℃时挤压可使合金阻尼提高17%,而145℃时则提高了44%。热挤压使ZA27合金晶粒细化,位错增多,从而提高了合金阻尼。     

Effects of quench aging treatment on microstructure and tensile properties of thixoformed ZA27 alloy

Abstract

The effects of quench aging heat treatment on microstructure and tensile properties of thixoformed ZA27 alloy were investigated. The results indicated that the microstructure of the alloy became into polygonal β phase particles after solid solution treatment at 350°C for 48 h. The β particles then decomposed into α and η phases which would coarsen during the subsequent aging at 150°C. It was observed that the Zn concentration in the β phase near the polygonal boundaries was higher than that within the β particles. As a consequence, both the decomposition speed of the former β phase and the subsequent coarsening speed were faster than those of the latter β phase. Thus, α and η phases near the boundaries were always coarser than those within the particles during aging. Owing to the coarsening, the ultimate tensile strength continuously decreased with increasing aging time. The percentage elongation increased up to 10 h, but decreased with aging time owing to bad deformation accommodation and low bonding strength between particles. Cracks initiated from some defects (e.g. inclusions and porosities) during tensile test, and the path for the cracks to propagation changed with the aging time.     

Microstructure and dendrite morphology of Sip/ZA27 composite

Abstract

The effects of pouring temperature, Si content and P modification on the microstructures and dendrite morphologies of Si_p/ZA27 in situ composites have been investigated. Especially, the dendrite morphologies have been visually shown through observation of shrinkage porosity microstructure. The results indicate that the primary α-Al grains change from equiaxed dendrites to parallel distributed columnar-like dendrites as the pouring temperature rises from 575 to 800°C. The change tendency in dendrite morphology with increasing Si content is opposite to that with rising pouring temperature. However, these parallel distributed dendrites with highly anisotropic morphology are not feathery grains. The P modification promotes the formation of feathery grains, and this kind of grains can form when the P modified 4·76 wt-% Si_p/ZA27 composite is poured at 750°C. There are four lines of secondary dendrite arms around one primary trunk of a feathery grain. Two of them are located in the twin plane (111), and they are easier to grow up than the other two. Therefore, the feathery grains are always in a laminar structure that is overlapped by two-dimensional structured laminar dendrites. The formation process and three-dimensional structure of the feathery grains have also been discussed.     

锌基合金钎焊界面区的微观组织形态及相组成

为研究锌基合金钎焊接头界面区的组织、性能及其钎焊接头的结合强度,本文用光学显微镜、扫描电镜、X射线衍射等分析手段,对界面区的微观组织形态及其相组成进行了研究分析;结果表明,用研制的新型高强软钎料钎焊锌基合金,获得的钎焊接头界面区中局部有交互结晶产生;界面区组织构成较复杂,既有Cd、Sn、Zn固溶体,又有少量的细小的Mg2Sn、MgZn等化合物;固溶体可以提高钎焊接头的强度和塑性,少量细小的化合物可强化基体组织,有利于强度的提高;但连续层状的金属间化合物可引起钎焊接头的脆化,使其性能降低.     

Constitutive modeling for hot deformation behavior of ZA27 alloy

The hot deformation behavior of ZA27 alloy was investigated in the temperature range of 473–523 K with the strain rates in the range of 0.01–5 s⁻¹ and the height reduction of 60 % on Gleeble-1500 thermo mechanical simulator. Based on the experimental results, constitutive equations incorporating the effects of temperature, strain rate, and strain have been developed to model the hot deformation behavior of ZA27 alloy. Material constants, α, n, ln A, and activation energy Q in the constitutive equations were calculated as a function of strain. The results showed that the stress–strain curves of ZA27 alloy predicted by the constitutive equations are in good agreement with experimental results, which validates the efficiency of the constitutive equations in describing the hot deformation behavior of the material.     

Microstructural changes inside the lamellar structures of alloy ZA27

The microstructural changes inside the lamellar structure of the cast and aged alloy ZA27 were studied using TEM, XRD and SEM techniques. Using TEM, the network of transitional phase η_m^′ was determined to be of an fcc crystal structure inside the lamellae η phase during ageing at 150 °C. The mechanism of the decomposition of the η phase lamellae can be summarized as follows: η → η_m^′ + η′ →  + η. The adjacent co-existence of the ε phase and the T′ phase inside the phase lamellae confirmed that a four phase transformation,  + ε → T′ + η, had occurred during the prolonged ageing.     

铸态及喷雾沉积态ZA27合金的阻尼性能比较

利用喷雾沉积快速凝固技术制备了ZA27合金,探讨了该工艺对ZA27合金组织和阻尼性能的影响,并与铸态的同类合金作了对比。结果表明,无论是喷雾沉积态还是铸态,随着频率的减小或温度的升高,ZA27合金的阻尼值增大,且在试验范围内阻尼性能不随应变振幅变化。室温下,喷雾沉积态ZA27合金的阻尼性能高出铸态合金的约1倍,如振动频率为1Hz,30℃时喷雾沉积ZA27合金的阻尼可达1.62×10-2。但当温度高于一定值以上时,铸态的阻尼性能高于喷雾沉积态的。阻尼的这种变化是与其组织密切相关的。     

锰对ZA27合金压缩蠕变的影响

为了解质量分数为0.5%Mn对铸态ZA27合金压缩蠕变行为的影响,采用自制的试验装置研究了ZA27-Mn合金和ZA27合金第一阶段的压蠕变量和稳态蠕变速率.结果表明:ZA27-Mn合金第一阶段的压蠕变量和稳态蠕变速率低于ZA27合金,而两种合金的压蠕变均符合于lnt=C-nlnσ Q/RT,ZA27-Mn合金的应力指数(n)和蠕变激活能(Q)分别为3.83和82.76 kJ·mol-1,而ZA27合金的应力指数和蠕变激活能分别为3.46和81.09kJ·mol-1,材料结构常数C不同,合金压蠕变不同,ZA27-Mn合金的压蠕变抗力高于ZA27合金;加入0.5%Mn对ZA27合金蠕变机制并无影响,均是锌的点阵自扩散和位错的攀移机制.     

Microstructures and properties of laser welding joint of super-eutectic ZA alloy

In this paper, we describe the experimental laser welding of ZA alloy. The formation of the weld demonstrated that pulsed laser welding for ZA alloy was more effective than continuous laser welding. The characteristics of microstructures and properties of the ZA alloy’s bond area welded by pulsed laser were investigated using an optical microscope, X-Ray Diffractometer (referred to as XRD) and other methodologies. The applicable technological parameters include welding speed of 0.9 m/min, pulsed laser power of 1.8 kW, impulse period of 3 ms and duty cycle of 3:4. The average micro-hardness of the laser welding seam zone is higher than that of the ZA alloy substrate, which is as high as 183HV. The microstructures in the weld consist of primary η phase (the white dendritic structure), β phase (the black block distribution around η phase), and eutectic phases (β + η) (platy layer). The average strength of the joints is 119 MPa, 70% of that of the base metal.     

ZA27合金部分重熔过程中组织的扫描电镜观察

用扫描电镜背散射电子成像技术 ,对经Zr变质处理的ZA2 7合金在半固态温度 46 0℃部分重熔过程中的微观组织进行了观察 ,并对有关组织的成分进行了测定。结果表明 ,随着保温时间的延长 ,晶界处的共晶组织向初生晶内部扩散 ,趋于消失 ,晶粒由铸态时的等轴晶变为粒状晶。当保温 10min时 ,未来得及溶入的共晶组织开始熔化 ,使组织分离为近粒状。到 2 0min时 ,初生晶粒演变为粒径为 40 μm左右的球状颗粒 ,同时 ,晶粒中细小、弥散分布的 η相由外到里逐渐长大 ,并伴有熔化现象     

凝固参数对ZA27合金次枝晶间距及抗拉强度的影响

ZA27合金是近20年来发展起来的一种新型铸造有色合金,具有较发孤力学性能,但是,由于其很宽的结晶温区、组织多为的树枝晶、导致力学性能下降,本文要用差热分析方法研究DTA曲线特征参数、凝固开始温度和冷却速率对二次枝晶间距λDAS的影响,同时使用IAS－4图像分析系统分析了合金的微观组织,结果表明：DTA曲线峰高与二次枝晶间距线性关系;二次枝晶间距与冷却速率符合指数关系。     

凝固参数对ZA27合金二次枝晶间距及抗拉强度的影响

ZA27合金是近20年来发展起来的一种新型铸造有色合金,具有较好的力学性能,但是,由于其很宽的结晶温区,组织多为的树枝晶,导致力学性能下降.本文采用差热分析方法研究DTA曲线特征参数、凝固开始温度和冷却速率对二次枝晶间距ADAs的影响,同时使用IAS-4图像分析系统分析了合金的微观组织.结果表明:DTA曲线峰高与二次枝晶间距呈线性关系;二次枝晶间距与冷却速率符合指数关系.     

An electron metallographic study of pressure die-cast commercial zinc–aluminium-based alloy ZA27

The microstructure of ZA27 pressure die-castings was examined by scanning and transmission electron microscopy after ageing for 5 years at ambient temperatures. Solidification began with the formation of compact aluminium-rich α′ dendrites and tiny rounded α′ particles, followed by the peritectic reaction whereby a zinc-rich β phase formed around the edges of the primary phases. The extremely high cooling rate during solidification reduced the extent of the peritectic reaction so that the liquid became highly enriched with zinc and solidification was completed by eutectic formation of β and η phases, the β joining the peritectic β and the η remaining in the interdendritic regions. On rapid cooling after casting through the eutectoid transformation temperature, the β phase decomposed eutectoidally into well-formed lamellae or semi-particulate irregular particles of α and η, and some lamellar colonies spread into the low-aluminium α′-phase cores of the dendrites to form coarse lamellar products. The bulk of the α′, however, decomposed into a very fine mixture of zinc-rich phases in an aluminium matrix. These structures are consistent with solidification under conditions of high undercooling. Enclosed within the α constituent of the decomposed peritectic and eutectic β phases were small particles of a phase which was identified as the transitional α′m phase containing 30.2%Al or 14.8%Al, with an fcc crystal structure and lattice parameter (at 14.8%Al) of about 0.395 nm. It had a symmetrical cube/cube orientation relationship with the surrounding α phase. This metastable phase was probably stabilized by copper. Copper became concentrated in the eutectic liquid during the first stages of solidification, and was rejected from the liquid in the form of discrete irregular particles, 1–2 μm in diameter, during eutectic solidification. After solidification, copper was also rejected from solid solution in the zinc-rich η phase in the form of a dense precipitation of small particles of 70–120 nm diameter and 2–3 nm thick. Both of these particles were identified as the metastable cph ε-phase (CuZn4) with lattice parameters a = 0.274 nm, c = 0.429 nm, and c/a = 1.566. This revised version was published online in November 2006 with corrections to the Cover Date.     

ZA27合金在半固态等温热处理中的相变研究

将ZA2 7合金铸锭重新加热到固液两相区进行等温热处理 ,研究了等温热处理过程中发生的主要相变。结果发现 :在半固态等温热处理过程中 ,富锌的共晶组织向α相扩散溶解 ,促使α相发生共析反应 ,未熔共晶体及共析体中的 β相长大以及原α相晶粒边缘部分熔化。     

Effects of Mg and RE additions on the semi-solid microstructure of a zinc alloy ZA27

The effects of 2 mass% Mg and 0.8 mass% RE additions on the microstructural evolution of a Zr modified zinc alloy ZA27 during isothermal holding at semi-solid temperature of 460 ?C have been studied by optical microscope and scanning electron microscope. Results show that these two elements all decrease the coarsening rate of solid primary particles, and improve the uniformity of particle size. The addition of Mg greatly decreases the solid fraction, and then increases the distance between particles. However, the element of RE mainly concentrates at the liquid regions between particles, and hinders both the atom diffusion and the welding of contact particles. In addition, the behaviors of particle coarsening of these two alloys, together with that of the Zr modified ZA27 alloy without the other elements, obey the LSW law after the three semi-solid systems reach at their solid–liquid equilibrium states. The additions of these two elements have no visible effect on the shape factor of particles.     

Effects of Mg and RE additions on the semi-solid microstructure of a zinc alloy ZA27

The effects of 2 mass% Mg and 0.8 mass% RE additions on the microstructural evolution of a Zr modified zinc alloy ZA27 during isothermal holding at semi-solid temperature of 460 °C have been studied by optical microscope and scanning electron microscope. Results show that these two elements all decrease the coarsening rate of solid primary particles, and improve the uniformity of particle size. The addition of Mg greatly decreases the solid fraction, and then increases the distance between particles. However, the element of RE mainly concentrates at the liquid regions between particles, and hinders both the atom diffusion and the welding of contact particles. In addition, the behaviors of particle coarsening of these two alloys, together with that of the Zr modified ZA27 alloy without the other elements, obey the LSW law after the three semi-solid systems reach at their solid–liquid equilibrium states. The additions of these two elements have no visible effect on the shape factor of particles.