ZA84 Alloy Research Articles

Aperiodic structures of rod-shaped precipitates in a Mg-Zn-Al alloy

Rod-shaped precipitates with their long axis parallel to [0001]α commonly form in Mg-Zn-Al alloys, providing strengthening effects. These precipitates are metastable and are generally accepted to be the MgZn2 and/or Mg4Zn7 phases. In this paper, the cross-sectional structure of precipitate rods in an aged ZA84 alloy is examined at the atomic scale using high-angle annular dark-field scanning transmission electron microscopy. It is found that these precipitates are neither pure MgZn2 nor Mg4Zn7. The structures of the rod-shaped precipitates do not have long-range translational symmetry nor long-range 5-fold rotational symmetry in the two-dimensional projection plane perpendicular to the rod long axis. However, these structures can be described by the tiling of different shapes, with four such shapes being most frequently observed. The structural details revealed in this work may help rationalize the many complex precipitate structures in Mg-Zn-Al and Mg-Zn alloys.

HOT COMPRESSION DEFORMATION BEHAVIOR OF Mg- Zn-Al-(Y) ALLOYS REINFORCED WITH QUASICRYSTAL

Quasicrystal phase offers a good combination of strength and ductility due to the strong interface between the quasicrystal phase and the Mg-matrix. Hot compression tests of Mg-Zn-Al-(Y) based alloys reinforced with quasicrystal were performed on Gleeble-1500 thermal simulation machine at a constant deformation temperature of 230 and strain rates ranged from 0.0015 s �1 to 1.5 s �1 . Microstructure evolution of hot-compressed Mg-Zn-Al-(Y) alloys and the relationship between flow stress and strain rate were studied. XRD and SAED results show that the microstructures of as-cast Mg-8Zn-4Al (ZA84) and Mg-8Zn-4Al-0.5Y (ZAY8405) are composed of icosahedral quasicrystal phase and α-Mg matrix. The quasicrystals in ZA84 and ZAY8405 alloys have a stoichiometric composition of Mg38Zn43Al19 and Mg51Zn30Al19 respectively. Dynamic recrystalization (DRX) take place during hot compression and the flow stress increases when increasing the strain rate at a constant compression temperature, which can be represented by the Power Exponential Equation. Deformation twinning and dynamic recrystalization are easier to take place in ZAY8405 alloys due to the refined and dispersed quasicrystal phase with Y addition.

Effect of Direct Current on the Semi-Solid Isothermal Heat-Treated Microstructure of ZA84 Magnesium Alloy

The effect of direct current on the semi-solid isothermal heat-treated microstructure of ZA84 alloy is investigated. The results indicate that it is possible to produce ZA84 alloy with non-dendritic microstructure by semi-solid isothermal treatment. Furthermore, imposing direct current during the semi-solid isothermal treatment of ZA84 alloy can accelerate the non-dendritic structure evolution of the alloy. Under the experimental conditions of this work, higher the current density, quicker is the rate of non-dendritic structure evolution for the semi-solid isothermal heat-treated ZA84 alloy. Further investigations need to be considered in order to optimize the current density, isothermal temperature and holding time.

Effect of silicon on phase selection of ternary compounds during solidification of ZA84 magnesium alloy

Effect of silicon on the phase selection between τ phase (Mg 32(Al, Zn) 49) and ϕ phase (Al 2Mg 5Zn 2) in ZA84 (Mg-8Zn-4Al-0.3Mn) magnesium alloy produced by steel mold cast was studied using X-ray diffractometer, scanning electron microscope and differential scanning calorimeter. The results show that with increasing Si addition in ZA84 alloy, the liquidus temperatures of the alloys and the solidification temperature ranges decrease. The ternary compound in ZA84 alloy is mainly τ phase and a little ϕ phase. When adding Si to ZA84 alloy, the preferential precipitation sequence of the ternary compounds changes, ϕ phase preferentially forms, whereas τ phase is suppressed. The solidification kinetics study of phase selection indicates that there is a critical degree of undercooling of the melt. If the undercooling exceeds the critical degree, τ phase preferentially forms while ϕ phase is suppressed; otherwise, ϕ phase preferentially forms while τ phase is suppressed.

Effects of minor Sr, Sn and Sc addition on as-cast microstructure and mechanical properties of ZA84 magnesium alloy

The effects of minor Sr, Sn and Sc addition on the as-cast microstructure and mechanical properties of the ZA84 magnesium alloy were compared. The results indicate that addition of 0.1%Sr, 0.5%Sn or 0.3%Sc (mass fraction) to the ZA84 alloy can refine the grains of the alloy. Furthermore, addition of 0.1%Sr to the ZA84 alloy does not obviously change the morphology and distribution of Mg 32(Al,Zn) 49 phase. However, addition of 0.5%Sn or 0.3%Sc not only refines and modifies the Mg 32(Al,Zn) 49 phase but also suppresses the formation of Mg 32(Al,Zn) 49 phase, especially with the addition of 0.3%Sc. Furthermore, addition of 0.1%Sr, 0.5%Sn or 0.3%Sc to the ZA84 alloy improves the tensile properties at room temperature and 150 °C, especially with the addition of 0.1%Sr and 0.3%Sc. However, addition of 0.1%Sr is not beneficial to the creep properties, and addition of 0.5%Sn has no obvious influence on the creep properties. Oppositely, addition of 0.3%Sc to the ZA84 alloy greatly improves the creep properties.

Effects of Minor Sr on As-Cast Microstructure and Mechanical Properties of ZA84 Magnesium Alloy

The effects of minor Sr additions on the as-cast microstructure and mechanical properties of the ZA84 magnesium alloy were investigated. The results indicate that adding 0.05-0.15 wt.% Sr to the ZA84 alloy does not cause an obvious change in the morphology and distribution of the Mg32(Al,Zn)49 phase. However, the grains of the Sr-containing ZA84 alloys are effectively refined. Among the Sr-containing ZA84 alloys, the grains of the alloy added 0.10 wt.% Sr are relatively finer than other alloys. Furthermore, adding 0.05-0.15 wt.% Sr to the ZA84 alloy improves the tensile properties at room temperature and 150 °C but decreases the creep properties. Among the Sr-containing ZA84 alloys, the alloy added 0.10 wt.% Sr obtains the optimum tensile properties at room temperature and 150 °C.

RETRACTED: Effects of Minor Scandium on As-cast Microstructure, Mechanical Properties and Casting Fluidity of ZA84 Magnesium Alloy

The effects of minor Sc on the as-cast microstructure, mechanical properties and casting fluidity of the ZA84 magnesium alloy were investigated. The results indicate that the Mg 32 (Al,Zn) 49 phase in the ZA84 alloy is refined with the addition of 0.12-0.35 wt.% Sc, and the formation of the Mg 32 (Al,Zn) 49 phase is suppressed. An increase in Sc amount from 0.12 wt.% to 0.35 wt.% causes the morphology of the Mg 32 (Al,Zn) 49 phase to gradually change from coarse continuous and/or quasi-continuous net to relatively fine quasi-continuous and/or disconnected shapes. In addition, it is shown that the tensile and creep properties of the ZA84 alloy are improved, but the casting fluidity of the alloy is decreased with the addition of 0.12-0.35 wt.% Sc.

Effects of Sn addition on as-cast microstructure, mechanical properties and casting fluidity of ZA84 magnesium alloy

Abstract The effects of Sn addition on the as-cast microstructure, mechanical properties and casting fluidity of the ZA84 magnesium alloy are investigated. The results indicate that adding 0.5–2.0 wt.%Sn to the ZA84 alloy not only can result in the formation of Mg2Sn phase but also can refine the Mg32(Al, Zn)49 phase and suppress the formation of Mg32(Al, Zn)49 phase, and with the increase of Sn amount from 0.5 wt.% to 2.0 wt.%, the morphology of Mg32(Al, Zn)49 phase gradually changes from coarse continuous and/or quasi-continuous net to relatively fine quasi-continuous and/or disconnected shapes. In addition, adding 0.5–2.0 wt.%Sn to the ZA84 alloy can improve the tensile and creep properties, and casting fluidity of the alloy. Among the Sn-containing ZA84 alloys, the ZA84 alloy added 1.0 wt.%Sn exhibits the best ultimate tensile strength, elongation and casting fluidity while the ZA84 alloy added 2.0 wt.%Sn has the best yield strength and creep properties.

Effect of TiC addition on semisolid isothermal heat treated microstructure of ZA84 magnesium alloy

The semisolid metal processing now is becoming an important forming process for the fabrication of magnesium alloys parts. Among the various methods of obtaining semisolid billet with non-dendritic microstructure, semisolid isothermal heat treatment is thought as a potential method. To provide a theoretical guide for the semisolid processing of ZA84 magnesium alloy, the semisolid isothermal heat treated microstructure of ZA84 magnesium alloy with and without TiC modifications is investigated. The results indicate that it is possible to produce a non-dendritic microstructure of ZA84 alloy by semisolid isothermal heat treatment, and that adding 0·5 wt-%TiC to ZA84 alloy benefits the non-dendritic microstructural evolution of the alloy. After treatment at 570°C for 120 min, the ZA84+0·5TiC alloy gives a non-dendritic microstructure with a higher liquid fraction (0·22 to 0·25) and finer unmelted primary solid particles (57 to 48 μm) than the ZA84 alloy. The formation of finer unmelted primary solid particles in the semisolid ZA84+0·5TiC alloy is possibly related to the refinement of the initial as cast microstructure and the higher liquid fraction of semisolid.

As-cast microstructure, mechanical properties and casting fluidity of ZA84 magnesium alloy containing TiC

The as-cast microstructure, mechanical properties and casting fluidity of ZA84 alloy containing TiC were investigated. The experimental results indicate that adding 0.5wt%TiC to ZA84 alloy can refine the as-cast microstructure, and do not cause the formation of any new phase. After 0.5wt%TiC was added to the ZA84 alloy, the morphology of ternary phases on the grain boundaries changed from coarse quasi-continuous net to fine disconnected net, and the distribution of ternary phases became dispersive and homogeneous. At the same time, the tensile properties of ZA84+0.5TiC alloy at room temperature were comparable to those of AZ91D alloy, and were higher than those of ZA84 alloy. At 150 °C, the tensile and creep properties of ZA84+0.5TiC alloy were also higher than those of ZA84 and AZ91D alloys. In addition, compared with the AZ91D alloy, the casting fluidity of ZA84+0.5TiC alloy was slightly poor, but better than that of ZA84 alloy. The reason could be related to the effect of TiC on the solidification temperature range of ZA84 alloy.

Effects of heat treatment on the microstructure and mechanical properties of ZA84 magnesium alloy

The effects of heat treatment on the microstructure and mechanical properties of ZA84 (Mg-8Zn-4Al-0.25Mn) alloy were investigated. The results indicate that the as-cast microstructure of the alloy is mainly composed of α-Mg matrix and two different morphologies of precipitates (continuous and quasi-continuous Mg 32(Al, Zn) 49 phases and isolated Mg 5Al 2Zn 2 phases). After solid solution treatment at 345°C, the Mg 32(Al, Zn) 49 phases change from continuous and quasi-continuous net to disconnected acute angle shape, and parts of second phases sphericize. The optimum heat treatment condition for the alloy is solution treatment at 345°C for 48 h and water quenching, then aging treatment at 200°C for 12 h and atmosphere cooling. Under the optimum condition, the ultimate tensile strength and yield strength of the alloy can be improved, but the elongation is not effected much by heat treatment.

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

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.

Phase selection of ternary intermetallic compounds during solidification of high zinc magnesium alloy

The phase selection of ternary intermetallic compound τ phase (Mg 32(Al, Zn) 49) and Φ phase (Al 2Mg 5Zn 2) in high zinc magnesium alloys was studied by using scanning electron microscope, X-ray diffractometer and differential scanning calorimeter, etc. The results indicate that, when adding element Si in Mg-8Zn-4Al-0.3Mn (ZA84) alloy, Φ phase is promoted, whereas τ phase is inhibited. The Chinese script-type Mg 2Si and matrix microstructure are greatly refined, the formation of τ phase is facilitated and Φ phase is restrained when modifier Al-AlP master alloy is added in ZA84 alloy containing Si. The kinetics study of phase selection indicates that there is a critical degree of undercooling of the melt. If the undercooling exceeds the critical value, τ phase preferentially forms while Φ phase is restrained; otherwise, Φ phase preferentially forms while τ phase is restrained.

Effects of holding temperature and time on semi-solid isothermal heat-treated microstructure of ZA84 magnesium alloy

The feasibility of fabricating ZA84 magnesium alloy with non-dendritic microstructure by a semi-solid isothermal heat treatment process and the effects of holding temperature and time on the semi-solid isothermal heat-treated microstructure of the alloy were investigated. The results indicate that it is possible to produce ZA84 alloy with non-dendritic microstructure by suitable semi-solid isothermal heat treatment. After being treated at 560-575 °C for 120 min, ZA84 magnesium alloy can obtain a non-dendritic microstructure with 14.2%-25.6% liquid fraction and an average size of 56-65 μm of the unmelted primary solid particles. With the increasing holding time from 30 to 120 min or holding temperature from 560 to 575 °C, the average size of unmelted primary solid particles decreases and globular tendency becomes more obvious. Under the experimental condition, the microstructural evolution of ZA84 alloy during semi-solid isothermal treatment is mainly composed of three stages of initial coarsening, structural separation and spheroidization. The subsequent coarsening of spheroidal grains is not observed.

Effect of antimony addition on the microstructure and mechanical properties of ZA84 magnesium alloy

The effect of antimony addition (0.2, 0.5 and 1 wt%) on the microstructure and mechanical properties of Mg–8Zn–4Al–0.2Mn alloy has been studied. Sb-addition to ZA84 alloy refines the ternary Mg 32(Al,Zn) 49 phase besides forming Mg 3Sb 2 precipitates and having a little effect on the grain refinement. Both the room and elevated temperature strength properties (yield and ultimate tensile strengths) are found to increase in Sb added alloys with slight reduction in ductility. However, the maximum strength properties are obtained with 0.2 wt% Sb-addition. Fractograph of the tensile tested ZA84 alloy shows intergranular and brittle type failure, whereas 0.2% Sb-addition facilitates transgranular fracture, relating to high strength.

Effects of RE on the microstructure and mechanical properties of Mg–8Zn–4Al magnesium alloy

Effects of rare earth (RE) alloy element on the microstructure and mechanical properties of Mg–8Zn–4Al (ZA84) alloy have been investigated. The differential scanning calorimetry (DSC) technique was used to study the solidification and aging process of ZA84 and ZA84– xRE alloys. The results show that a new quaternary precipitate Mg 3Al 4Zn 2RE forms during the solidification and its phase transformation temperature point is 413 °C in RE-containing ZA84 alloys. The microstructure of ZA84 alloy can be refined, ɛ phases formed, microhardness of the matrix increased and time for attaining microhardness peak value of aging ZA84– xRE alloys delayed, with increasing RE. Thus, the room temperature and high temperature tensile properties of as-cast and aged ZA84 alloys can be improved by RE addition.

Effect of Inoculation on the Damping Properties of Mg-8Zn-4Al-0.3Mn Alloy

The effects of inoculation on the damping property of Mg-8Zn-4Al-0.3Mn alloys have been investigated. The results show that the damping property of ZA84 alloys reduces at room temperature after the addition of RE, whereas the damping property at high temperature increases obviously. The damping property of ZA84 alloys at room temperature and high temperature improves respectively after the addition of Al5TiB. It has reached Q-1=0.01 at room temperature with 0.25% Al5TiB. There exists a damping peak in all alloys at high temperature because of the viscous sliding of the grain boundaries and the inteneration of φ (Al2Mg5Zn2) phase at high temperature. It was found that the addition of RE could delay the temperature of damping peak occurs. The main damping mechanisms of ZA84 magnesium alloys with RE and Al5TiB additions are the dislocation damping and interface damping. The higher the movable dislocation density is, the finer the grains are and the more the grain boundaries and phase interfaces are, the better damping property of ZA84 alloys is.