W.J. Ding

Microstructure refinement of Mg–Al–Zn–Si alloys

Magnesium alloys containing Mg2Si particles, as a promising cheap heat-resistant magnesium alloy for automobile engine applications, are attracting more and more attention of both material scientists and design engineers. Refinement of the Chinese script Mg2Si particle is a key for using this alloy in sand casting or permanent mould casting. In the present work, the refinement effect of antimony(Sb) or calcium(Ca) on the microstructure was investigated. The study shows that Sb or Ca addition promotes the formation of fine polygonal type Mg2Si particles by providing the nucleation sites. Moreover, the grain sizes of Sb or Ca modified Mg–Al–Zn–Si alloys are much finer than that of base alloy. The effect of modification and refinement with Sb addition is more effective than that with Ca addition. Such improved microstructure of the modified alloys results in large improvement in tensile properties and toughness as compared to the base alloy.

Fracture behavior of AZ91 magnesium alloy

Abstract Optical microscopy and scanning electron microscopy (SEM) were used to examine the fracture behavior of AZ91 magnesium alloy ruptured by tensile and impact tests. AZ91 alloy generally revealed features of brittle fracture and cleavage is the principal fracture mode. The fracture behavior for tensile test is different from that for impact test due to different load forms. The fracture morphologies are different in different areas of the impact specimen. The Mg/Mg 17 Al 12 interface often acted as the crack initiation source.

Corrosion protection of polypyrrole electrodeposited on AZ91 magnesium alloys in alkaline solutions

The polypyrrole is electrodeposited on AZ91 magnesium alloys in alkaline solutions. The morphology and compositions are characterized by scanning electron microscopy (SEM) and auger electron spectroscopy (AES), respectively. Cauliflower- and rods-like structure is observed, and the ratio of C:N:O is varied at different locations. The polypyrrole coatings deposited on AZ91 magnesium alloys change the corrosion potential of its substrate.

Behavior of Mg–6Al–xSi alloys during solution heat treatment at 420°C

Abstract The effect of solution heat treatment at 420°C on the morphology of Mg 2 Si particles in Mg–Al–Si alloys was investigated. The Mg 2 Si particles tended to be spherodized during the treatment due to the diffusion of Si atoms along the Mg 2 Si/Mg interface, which is benefit the mechanical properties of the alloys.

Microstructural characterisation of as cast and homogenised Mg-Gd-Nd-Zr alloys

Abstract Microstructure of as cast and homogenised Mg–xGd–2Nd–0˙5Zr (x=6, 8, 11, wt-%) alloys has been investigated by using of optical microscopy, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction. It was observed that all the as cast alloys contained primary α-Mg solid solution, eutectic structures, cuboid shaped phases, rodlike precipitates and Zr rich clusters. The eutectic phase in Mg–11Gd–2Nd–0˙5Zr was characterised to be of Mg5Gd prototype with stoichiometry near Mg5(Nd x Gd1–x , x≈0˙43). Microstructural evolution of the alloys during homogenisation can be divided into three coinstantaneous processes as follows: dissolving of eutectic phases, growth of cuboid shaped phases and coarsening of grains. After appropriate homogenisation, the alloys exhibit microstructure with little eutectic phases, small amount of cuboid shaped phases and relatively fine grains. The cuboid shaped phase with a face centred cubic crystal structure (a≈0˙56 nm) was identified to be a GdH2 type compound.

Microstructure and mechanical properties of extruded ZK60 magnesium alloy containing rare earth

Abstract ZK60 – RE alloys were made by melting ZK60 alloy and cerium rich rare earth (RE) in a crucible electric furnace. The content of RE was 0, 0.5, 1, 1.5, 2, and 3 wt-%, respectively. The influence of RE on the microstructure and tensile mechanical properties of ZK60 magnesium alloys was studied. The results showed that cerium rich misch metal (MM) had an obvious effect of reducing the grain size of the as extruded ZK60 alloys. The ultimate and yield strengths of extruded ZK60 alloys were improved by the addition of RE. The ultimate and yield strength increased with increasing RE content in the range investigated. It was also found that the ultimate strength and yield strength decreased with increasing extrusion temperature. The improvement in tensile properties of ZK60 – RE alloys was due to the extrusion texture and the fine microstructure induced by RE additions.

Study on microstructure of squeeze casting AZ91D alloy

Abstract The present paper reports the investigation of the microstructure distribution of squeeze cast AZ91D alloy. The microstructure of squeeze cast AZ91D alloy is not uniform and is composed of four zones, which are chilled layer, segregation zone, pressured crystallisation area and hot spot area respectively. Moreover, in the pressured crystallisation area, the microstructure sequence in the transverse section from the outside to the inside could be divided into four sublayers, such as fine equiaxial dendrite area, dendrite area with a high directivity, confusion dendrite area and disorder dendrite area. The volume fraction of the intermetallic compound Mg17Al12 also varied with the location. The volume fraction of Mg17Al12 in the pressured crystallisation area is the largest except in the segregated zone.

Microstructural evolution of AZ61 magnesium alloy during hot deformation

Abstract The microstructural evolution of AZ61 magnesium alloy during hot compression at various temperatures was investigated. The experimental results show that dynamic recrystallisation occurs over a wide temperature range. Grains can be greatly refined through dynamic recrystallisation. The mean size of the recrystallised grains increases with a decrease of temperature or value of Z (Zener – Hollomon parameter), while the reciprocal of the recrystallised grain size has a good linear relationship with the natural logarithm of the Z value, as well as the hyperbolic term of the flow stress. Basal and non-basal segments have been found in both recrystallised grains and primary grains, whereas dislocation pileups exist only in recrystallised grains when the temperature is lower than 673 K. The occurrence of twins is dependent on temperature and strain. When the strain increases, primary twins evolve into secondary twins. However, secondary twins grow with an increase of temperature; some secondary twins evolve into subgrains.

Preparation of even polyaniline film on magnesium alloy by pulse potentiostatic method

An even polyaniline film that adheres strongly to the magnesium alloy substrate has been prepared by pulse potentiostatic method in an alkaline solution. The film thickness was followed as a function of polymerization time. The film morphology and the bond strength between the polyaniline film and magnesium alloy substrate are deeply depended on the electrical parameters used in the preparation. The growth mechanism and the morphology of polyaniline film were analyzed by SEM. The infrared spectrum of the polyaninline film was also obtained.

Effect of process parameters on structure and macrosegregation upon direct chill casting of Mg–Nd–Zn–Zr alloy

Abstract Direct chill (DC) semicontinuous casting process has been successfully used to produce sound Mg–3·0Nd–0·4Zn–0·4Zr (NZ30K) billets. The influence of process parameters such as casting speed, casting temperature on the microstructure and macrosegregation was studied. The results show that the casting speed affects the macrosegregation greatly while it has a slight influence on the grain size of the billet; the casting temperature has a slight influence on macrosegregation of the billet while the grain size of the billet increases as the casting temperature increases. The optimal process parameters have been experimentally determined as follows: casting temperature 700°C and casting speed 90 mm min−1. The ultimate tensile strength, yield strength and elongation of billets cast at the optimal casting parameters are 196 MPa, 125 MPa and 16·5% respectively.