Ji-xue Zhou

Microstructure refinement of AZ91D alloy solidified with pulsed magnetic field

The effects of a pulsed magnetic field on the solidified microstructure of an AZ31 magnesium alloy were investigated. The experimental results show that the remarkable microstructural refinement is achieved when the pulsed magnetic field is applied to the solidification of the AZ31 alloy. The average grain size of the as-cast microstructure of the AZ31 alloy is refined to 107 mu m. By quenching the AZ31 alloy, the different primary alpha-Mg microstructures are preserved during the course of solidification. The microstructure evolution reveals that the primary alpha-Mg generates and grows in globular shape with pulsed magnetic field, contrast with the dendritic shape without pulsed magnetic field. The pulsed magnetic field causes melt convection during solidification, which makes the temperature of the whole melt homogenized, and produces an undercooling zone in front of the liquid/solid interface, which makes the nucleation rate increased and big dendrites prohibited. In addition, the Joule heat effect induced in the melt also strengthens the grain refinement effect and spheroidization of dendrite arms.

Effect of Ag alloying on microstructure, mechanical and electrical properties of NiAl intermetallic compound

In this paper, the effects of Ag on microstructure, mechanical and electrical properties of NiAl intermetallic compound were investigated. The present results show that the NiAl-Ag alloys consisted of two phases: beta-NiAl and Ag-rich solid solution. The amount of the Ag-rich phase increased with increasing Ag content. Ag has very low solubility in NiAl and they form pseudo-binary monotectic systems. The addition of 0.5-1at.% Ag to NiAl increased its strength while the addition of more than 1at.% Ag decreased its strength. The strengthening and weakening effects come from solid solution hardening and second ductile phase softening. In addition, Ag alloying can improve NiAls room temperature compressive ductility. The NiAl-Ag alloy has high hardness and electrical conductivity that make it an attractive candidate for electrical contact material

Morphology modification of Mg2Si by Sr addition in Mg-4%Si alloy

A modification of Mg(2)Si in the hypereutectic Mg-4%Si alloy (mass fraction) with Sr was investigated. Two types of Mg(2)Si in the alloys were found: polygonal primary Mg(2)Si and Chinese script type eutectic Mg(2)Si. Adding Al-10% Sr master alloy to the Mg-4% Si alloy clearly reduced the average size of primary Mg(2)Si and changed the morphology of eutectic Mg(2)Si from Chinese script type to polyhedral or fine fibre shape. The refinement of primary Mg(2)Si is mainly attributed to the heterogeneous nucleation mechanism induced by the Sr-rich particles. The modification of eutectic Mg(2)Si results from the dissolved Sr, which alters the preferred growth manner of the eutectic.

Effect of purification treatment on properties of Mg-Gd-Y-Zr alloy

Mg-Gd-Y-Zr alloy was purified by the method of filtering purification. The type, morphology, size distribution and volume fraction of inclusions in the alloy were analyzed using optical microscope and scanning electron microscope, and the effects of the inclusions on the mechanical and corrosion properties of the alloy were investigated. The results indicate that the filtering purification method is effective to remove inclusions in the alloy. By the filtering purification method, the average size of inclusions in the alloy reduces from 12.7 mu m to 4.3 mu m and the average volume fraction of inclusions in the alloy reduces from 0.26% to 0.06%. The ultimate tensile strength, yield strength and elongation of the alloy are improved from 200 MPa, 156 MPa and 3.4% to 232 MPa, 167 MPa and 7.0%, respectively. The corrosion rate of the alloy decreases dramatically from 38.8g/(m(2.)d) to 2.5 g/(m(2).d) in the salt spray test.

Effect of Bi Addition on Microstructures and Mechanical Properties of AZ80 Magnesium Alloy

The effects of Bi addition on the microstructures and mechanical properties of as-cast AZ80 alloy were investigated. The results show that with the addition of Bi, the coarse eutectic phases are refined and become discontinuous; some flaky and granular Mg(3)Bi(2) phases with a hexagonal structure of D5(2) are observed along the grain boundaries and between dendrites. The tensile strength and elongation increase first, and then decrease with increasing Bi content. AZ80-0.5% Bi alloy has optimum combination mechanical properties. When the content of Bi is above 1.0% ( mass fraction), the amount of flaky Mg(3)Bi(2) phase increases markedly, which splits the matrix and deteriorates the tensile strength and elongation.

Effects of RE and Sr additions on dendrite growth and phase precipitation in AZ91D magnesium alloy

To develop AZ91D alloys with fine microstructure, effects of the addition of rare earth (RE), Sr and RE + Sr on the dendrite growth and phase precipitation in AZ91D magnesium alloy were studied, respectively. The results show that the microstructure is refined and the morphology of beta-Mg(17)A1(12) phase is modified with RE or Sr addition, especially with the RE+Sr composite addition which can reduce the average grain size of AZ91D alloy obviously to 141 mu m. The needle-like or block-like new phases adhering to mu-Mg(17)A1(12) phase form at interdendrites during solidification. The enrichment of RE or/and Sr elements in front of the solidification interface, especially at the tips of alpha-Mg dendrite, which restricts the growth of alpha-Mg dendrite, changes the preferential growth of alpha-Mg and finally results in the grain refinement and the blunting of alpha-Mg dendrite.

Structure refinement of pure Mg under pulsed magnetic field

Abstract A pulsed magnetic field (PMF) was introduced into the solidification of pure Mg. Fine uniform equiaxed grains are acquired in the whole ingot from the PMF treatment, in contrast with the coarse columnar grains observed in conventional casting, and the average grain size is refined to 260 μm with a 200 V PMF treatment. Pulsed magnetic field increases melt convection during solidification, and the violent agitation causes warmer liquid to fracture the tip of columnar dendrites or to break off dendrite branches to promote the formation of an equiaxed structure, with the broken pieces transported into the bulk liquid acting as nuclei. In addition, the uniform temperature field resulting from the stirring increases the likelihood of nuclei survival. The Joule heat effect also participates in the structure refinement. The pure Mg produced with a 200 V PMF treatment exhibits improved mechanical properties, such as the ultimate compressive strength (227 MPa) and fracture strain (33·2%).

The phosphorus effect on the superplastic deformation of NiAl

The P effect on the superplastic deformation of NiAl has been investigated. It is shown that P segregates to grain boundaries and the segregation has profound influence on the high temperature tensile properties of the NiAl alloy. The segregation retards dynamic recovery and recrystallization that leads to a decrease in elongation and enhancement in cavitation compared with the stoichiometric NiAl. Therefore, the P has harmful effect on the superplastic deformation of NiAl.

Microstructure and mechanical properties of as-cast Mg-4Zn-0.5Zr-0.2Cu-0.2Ce alloy

ABSTRACT In this study, an approach is proposed to improve the microstructure and mechanical properties of Mg-4Zn-0.5Zr alloy by combining trace Cu and rare earth Ce addition. The results showed that Cu and Ce additions led to obvious grain refinement and the formation of Mg-Zn-Cu and Mg-Zn-Ce phases. The Mg-Zn-Ce phase was identified to have an orthorhombic structure. The length of the [0001]α rods in the Cu-containing alloys remarkably decreased. The yield strength increased slightly after Cu and Ce co-addition, which was attributed to grain refinement and precipitation strengthening. The coarse Mg-Zn-Ce phase distributed at the grain boundaries would reduce the ductility by promoting crack propagation during tensile processes.