Effects of heat treatment on microstructure and mechanical properties of ZA27 alloy

The effects of heat treatment on the microstructure and mechanical properties of ZA27 alloy were studied by X-ray diffraction（XRD）,scanning electron microscopy（SEM） and mechanical characterization.The results indicated that the as-cast microstructure of the alloy was mainly composed of α,decomposed β,η and ε phases.After solid solution treatment at 365 ℃ for 1 h,α and η phases dissolved,and the microstructure of specimen was mainly composed of the supersaturated β phases.The phase decomposition of supersaturated ZA27 alloy is a two-stage phase transformation：the decomposition of the supersaturated β phase at the early stage of aging,and with the increase of aging time,ε phase decomposition through a four-phase transformation：α＋ε→T ＇＋ η.A good combination of high tensile elongation and reasonable strength can be achieved by suitable heat treatments.

Effect of rare earths on phase transformation in as-cast ZA27 alloy dur-ing compressive creep

The effect of the mixed rare earths of Ce on the phase transformation in ascast ZA 27 alloy during compressive was investigated under 37 MPa and at 160 deg C by X-raydiffraction technique and SEM. The results showed that the as cast microstructure of ZA 27-RE alloyconsisted of a dendritic Al-rich alpha' surrounded by Zn-rich beta' phase, interdendritic epsilonphase and Zn-rich eta phase together with a complex Z phase which was a complex constitute compound,(RE,Cu)Al_5Zn_(16), dispersed in crystal interfaces or branch crystal interfaces and stable duringcompressive creep test at 160 deg C. The phase transformations of ZA 27-RE alloy, decomposition ofbeta' phase arid four transformation, were delayed by the addition of rare earths, also the lamellarstructure and the spheroidized structure in ZA 27-RE alloy were finer than in ZA 27 alloy duringcompressive creep test at 160 deg C at the same creep time, and the compressive creep resistance ofZA 27-RE alloy was higher than that of ZA 27 alloy.

Formation of Liquid Pools Entrapped within Solid Grains of ZA27 Alloys during Partial Remelting

The investigations were performed into the formation processes of liquid pools entrapped within solid grains of three ZA27 alloys, produced respectively by either grain refinement, or traditional permanent casting or mechanical stirring, during partial remelting at semisolid temperature of 460℃. The results show that the rapid coalescence of primary grains due to merging of secondary arms during the initial stage of partial remelting is the main cause for the formation of the entrapped liquid pools. This coalescence resulted in that a high quality of eutectics, especially the 77 phase, was entrapped within the grains, and then remelted to form liquid pools during the subsequent heating. In addition, the growth of the η phase decomposed from the primary α' phase and β phase and the subsequent remelting is another cause for the refined and permanent mould casting alloys. Furthermore, the agglomeration of the solid grains also resulted in the entrapment of liquid in the interior of grains.

Effect of Rectangle Wave Pulse Current on Solidification Structure of ZA27 Alloy

The effect of rectangle wave pulse current on solidification structure of ZA27 alloy was studied. The restdts show that the wave pattern relies on the frequency range of harmonic wave and the energy of pulse current within the frequency range of pulse current. Imposed pulse current could induce the solidification system to oscillate. The frequency range and the relevant energy distribution of pulse current exert an influence on the amount of atoms involved for forming critical nucleus, the surface states of dusters in melt, the oscillating state of melt on the surface of dusters, the active energy of atom diffusion , the frequnce response of the resonance of bulk melt and the absorbability of the solidification system to the external work. Rectangle wave pulse current involves rich harmonic waves ; the amplitudes of high order of harmonic waves are higher and reduce slowly, so it has a better effect on inoculation and modification.

Effects of Si alloying and T6 treatment on mechanical properties and wear resistance of ZA27 alloys

To improve the mechanical properties and wear resistance of ZA27 alloy, Si was introduced to the alloy, and the effect of Si alloying and T6 heat treatment on the microstructure, mechanical properties and wear resistance was investigated. The results show that with 0.55% Si, the microstructure of the alloy can be refined effectively, which leads to the increase of hardness. But the tensile strength and elongation decrease because Si undermines the integrity of the matrix. On the other hand, the dendrites are transformed into a desired α+η+(α+η)mixture with T6 heat treatment, which introduces a remarkable increase to the elongation and hardness of the alloy. The wear resistance of the ZA27 alloy with Si alloying is significantly better than that of the ZA27 alloy without Si. With the increase of Si addition, the wear resistance of the alloy firstly increases and then decreases.In the alloy without Si alloying, severe plastic deformation and large delamination were observed on the worn surface of the alloy. However, with the increase of Si, the main wear mechanism transformed to abrasive wear gradually. In addition, the T6 treatment can further improve the wear resistance of the alloy with Si alloying.

EFFECT OF ANNEALING ON DAMPING CAPACITIES OF AS-CAST ZA27 ALLOY

ZA27 alloy was prepared by casting with permanent mold and then annealed at 250℃ for 1-4h. The damping capaciG of the alloy was measured using a testing apparatus based on the cantilever beam technique. It was found that the as-cast ZA27 alloy possesses high damping capacity with the value of 1.3 × 10^4 at 320Hz. After annealed at 250℃ for lh, the damping capacity decreases to 1.1 × 10^-3 and then remains constant even when the annealing time is increased to 4h. The microstructure of the as-cast ZA27 alloy consists of large dendrites of Al-rich PrimaG （x-phases, eutectoid （α ＋ η） and nonequilibrium eutectic phases （α ＋ η ＋ ε）. After annealing at 250℃ for lh, the e phase disappears due to dissolution into the matrix, and the spacing between the flakes of eutectoid increases. The further increase in the annealing time has little effect on the spacing. The damping mechanism of the alloy was discussed considering the thermoelastic damping and defect damping. The value of thermoelastic damping accounts only for 7%-8% in the overall damping in cantilever beam damping measurements and the damping capacity of the ZA27 alloy came mainly from defect damping.

Corrosion behaviour of thixoformed and heat-treated ZA27 alloys in NaCl solution

The influence of corrosion on the microstructure of thixoformed and heat-treated ZA27 alloys was investigated. The microstructure of ZA27 alloy was affected by heat treatment. The process of electrochemical corrosion occurs in both ZA27 alloys through the area of r/phase. According to the results of immersion test and electrochemical measurements, the corrosion rate of the thixoformed ZA27 alloy is at least 50% lower than that of the thixoformed and thermally processed alloy. This indicates the unfavourable influence of applied heat treatment （T4 regime） on the corrosion resistance of the thixoformed ZA27 alloy.

Microstructural evolution of fine-grained ZA27 alloy during partial remelting

The microstructural evolution process of fined-grained ZA27 alloy during partial remelting has been investigated. The relationship between the as-cast and semi-solid microstructures has been discussed in particular. The results indicate that a semi-solid microstructure with small and spheroidal primary particles can be obtained when the ZA27 alloy is partially remelted. The microstructural evolution can be divided into four stages, the initial coarsening of the dendrites due to coalescence of dendrite arms, structural separation resulted from the melting of residual interdendritic eutectic, spheroidization due to the partial melting of solid particles and final coarsening attributed to the coalescence and Ostwald ripening. An equiaxed dendrite in the as-cast microstructure may evolve into one spheroidal particle in the semi-solid microsturucture after being partially remelted. The more equiaxed the dendrites in an as-cast microstructure are, the more spheroidal the solid particles in the semi-solid microstructure will be. Finer primary particles could be obtained if the alloy with finer as-cast microstructure was partially remelted. However, due to the coalescence effect, their sizes cannot be reduced further if the refined as-cast microstructure reached a certain extent.

Efects of minor amounts of scandium on microstructure and mechanical properties of ZA27 alloy

The influences of minor amounts of scandium on the microstructure and mechanical properties of as-cast ZA27 alloy have been experimentally investigated. The experimental results show that as far as the Sc addition is individually concerned, the refinement result of the as-cast ZA27 alloy with 0.5wt.%Sc addition is comparatively better. After alloying with minor amounts of Sc, the coarse dendrites of the ZA27 alloy are refined and transformed into the uniform and small equiaxed grain microstructure; also the eutectics among the grain boundaries become finer. The test results of mechanical properties show that the tensile strength and hardness of the as-cast ZA27 alloy containing 0.5wt.%Sc approach 495 MPa and 120.2 HB, increase by 28.5% and 33.1%, respectively, compared with those of the ZA27 alloy with no addition of Sc element, while the elongation of the alloy is improved to 7.6% from 2.7%. The SEM, EDAX and XRD analyses show that in the ZA27 alloy, the trace element Sc combines with AI to form the square AI3Sc phase particles, which serve as heterogeneous nuclei, facilitating the refinement of the microstructure and the improvement of the mechanical properties of the alloy.

Effects of Compression Parameters on Deformation Behaviors of Semi-Solid ZA27 Alloys

An investigation was performed on the effects of semi solid compression parameters,such as strain rate,compression temperature and heating time at these temperatures on deformation behaviors of two kinds of ZA27 alloys,one was modified by Zr and the other was unmodified.The results indicate that with the increasing of the strain,the stress of the modified composite first sharply increases to a peak value,then dramatically decreases to a plateau value,and again increases till the end of deformation.But for the unmodified,after being up to a peak value,the stress only decreases slowly.As the compression temperature or the heating time decreases,or the strain rate increases,the stress level and the cracking degree of these two kinds of alloys increase.Under the same deformation conditions,the stress level and the cracking degree of the unmodified alloy are higher than those of the modified one.But there is an exception that the stress level of the unmodified alloy is minimum and smaller than that of the modified one when deformed at the low temperature of 450℃.These phenomena were mainly discussed through analyzing the microstructures under different conditions and the deformation mechanisms at different deformation stages.

Microstructural Evolution and Phase Transformations of Mechanically Stirred Non-dendritic ZA27 Alloy during Partial Remelting

The microstructural evolution and phase transformations of mechanically stirred non-dendritic ZA27 alloy during partial remelting were studied by using scanning electron microscopy and X-ray diffraction technique.The partial remelting temperature was 460℃ and lower than the stirring temperature of 465℃.So the microstructure with globular grains needed for semi-solid forming can not be obtained and the starting primary non-dendritic grains change in turn to connect non-dendritic grains, long chain-like structures and finally to coarsen connect grains.However,the small near-equiaxed grains between the primary non-dendritic grains are evolved into small globular grains gradually,some of which are also attached to the primary non-dendritic grains during the subsequent heating.The X-ray diffraction results show that a series of phase transformations, α+η+ε→β,η+β→L,β→α′+L,α+η+ε→α′ and α′→L, occur successively during this process.The main reason why the starting primary non-dendritic grains do not separate into the needed independent globular grains is that the reactions of η+β→L and α′→L do not occur or occurr incompletely in the layers used to connect the primary non-dendritic grains.

INFLUENCE OF ZIRCONIUM ON MICROSTRUCTURE AND DAMPING CAPACITY OF ZA27 ALLOY

To improve the damping capacities of metals and alloys, the microstructures and damping capacities of ZA27 alloy modified by Zr were studied, and the damping capacities at room tempemture were measured by using cantilever beam techniques. The experiment results show that Zr can refine the Al-rich primary phase and improve the damping Capacities saliently. Compared with the un-modified one, the damping capacity of ZA27 alloy modified by 0.3wt%Zr received 90 percent increment. The high damping capacities are attributed primarily to grain ablement and the increasing of phase interfaces.

MICROSTRUCTURAL EVOLUTION OF AS-CAST ZA27 ALLOY DURING COMPRESSIVE CREEP

Phase transformation and micro structural change of an as-cast ZA27 alloy were investigated during compressive creep by X-ray diffraction technique, SEM and TEM. Compressive creep induced decomposition of α metastable η' phase and a four-phase transformation, α + ε → T' + η and T' → θ, occurred during the compressive creep testing. The occurrence of negative creep in the alloy resulted from the volume expansion caused by the four-phase transformation. A micro structural change was also observed from a lamellar structure into a spheroidized structure in higher creep strain of tested specimens. It provided evidence of compressive creep induced phase transformation which occurred in ageing process.

微量元素对ZA27合金腐蚀及老化性能的影响

ZA27合金是目前国内外发展迅速的新型材料。它具有优良的机械性能、耐磨性能和机械加工性能。该合金利用其价廉的优势,主要用于代替昂贵的青铜合金制作耐磨件。但ZA27合金与传统的锌合金一样,也有腐蚀和老化现象,这种现象历来是人们关注的问题,但迄今为止,其机理尚不清楚,也没能提出防止老化的根本措施。本文在研究ZA27于不同介质中的耐蚀性和人工时效时机械性能变化的基础上,探讨了微量Sn,Pb和RE元素对该合金腐蚀及老化性能的影响。

Corrosion influence on surface appearance and microstructure of compo cast ZA27/SiC_p composites in sodium chloride solution

The influence of corrosion on the surface appearance and microstructure of particulate ZA27/SiCp composites was examined after 30 d immersion in a sodium chloride solution with the access of atmospheric oxygen. The composites with different contents of SiC micro-particles were synthesized via compo casting. Microstructural studies by means of optical microscopy （OM） and scanning electron microscopy （SEM） showed that corrosion occurred in the composite matrices, preferentially in regions of the η phase, rich in zinc. The corrosion processes did not affect the silicon carbide particles incorporated in the matrix alloy. According to the results of electrochemical polarization measurements, an increase in the content of SiC particles in the composite matrice has led to the lower corrosion resistance in the composites.

Influence of Rare Earths on High-Temperature Mechanical Properties of ZA-27 Alloys

The influence of mixed RE with c Ce 45%(mass fraction) on the high temperature mechanical properties and microstructure of ZA 27 alloys was investigated. The results show that RE elements can react with elements Al, Zn, Cu etc. to form some complex compounds which have high hardness and hot hardness and can disperse in grain boundaries or between dendrite crystals, so that the alloy can be refined and the deformation of matrix and the movement of grain boundaries at high temperature are hindered. The appropriate addition amount of RE can result in increase of tensile strength by about 30% and hardness by 25% and only a little decrease of elongation, but impact toughness has been improved at 150 ℃. It is also found that the strength at room temperature changes little although the plasticity and impact toughness decrease slightly.

Effect of Nano-sized B4C Addition on the Mechanical Properties of ZA27 Composites

In order to understand the influence of nano-sized B4C additive on ZA27 alloy, mechanical and physical properties of ZA27-B4C nanocomposites were investigated in terms of B4C content. While physical properties were determined in terms of microstructural studies, density and porosity tests, mechanical properties were determined in terms of ultimate tensile strength（UTS） and hardness experiments. Morphological and microstructural studies were carried out with scanning electron microscopy（SEM）. The experimental results indicate that nano-sized B4C can be used to enhance the mechanical properties of ZA27 alloy effectively. The highest mechanical performance can be obtained at ZA27-0.5% B4C（in weight） nanocomposite with values of tensile strength（247 MPa） and hardness（141,18 BH） and low partial porosity（0.5%）. After a pick point, increasing B4C ratio may cause the formation of agglomeration in grain boundaries, that＇s why density, tensile strength, and hardness values are declined.