AZ91C magnesium alloy modified by Cd

In the present work, the effect of Cd on the microstructure, mechanical properties and general corrosion behaviour of AZ91C alloys was investigated. Addition of Cd was found not to be efficient in modifying/refining the microstructure or β-phase. A morphology change in β-phase from fine continuous precipitates to discontinuous β-phase upon the addition of Cd was observed. A marginal increment in mechanical properties was observed. General corrosion behaviour was followed with weight loss measurements, potentiostatic polarisation studies and surface studies in 3.5% sodium chloride solution and 3.5% sodium chloride with 2% potassium dichromate solution. Cd addition deteriorated the corrosion behaviour of AZ91C. This behaviour was attributed to the formation of chunks of β-phase upon the addition of Cd. AZ91C with refined β-phase distribution, performed rather better in the NaCl solutions.     

High-frequency induction soldering of magnesium alloy AZ31B using a Zn−Al filler metal

Magnesium alloy AZ31B plates were soldered by means of high-frequency induction soldering using a Zn−Al filler metal in argon gas shield condition. The interfacial microstructure and phase constitution of the soldered joint were investigated. The experimental results show that α-Mg solid solution and α-Mg + MgZn eutectoid structure were formed in soldering region. Moreover, the zinc solid solution and the aluminum solid solution in the original filler metal disappeared completely after the soldering process. Test results indicate that the shear strength of the soldered joint is 19 MPa. The fracture morphology of the soldered joint exhibits intergranular fracture mode and the crack originates from α-Mg + MgZn eutectoid structure.     

Degradation and cytotoxicity of lotus-type porous pure magnesium as potential tissue engineering scaffold material

The lotus-type porous pure magnesium was prepared using a metal/gas eutectic unidirectional solidification method (GASAR process). The corrosion behavior, decay of mechanical property and the cytocompatibility were evaluated with the compact pure Mg as control. The porous pure Mg indicates better corrosion resistance than that of compact pure Mg in SBF at 37 °C. The compressive yield strength of compact and porous pure Mg is (110.3 ± 8.5) MPa and (23.9 ± 4.9) MPa before immersion test, and porous pure Mg exhibits slower decay in compressive yield strength with the extension of immersion period than that of compact pure Mg. With larger exposed surface area, porous pure Mg shows higher Mg concentration in the extract than that of compact pure Mg, which leads to a higher osmotic pressure to cells and might affect its indirect cytotoxicity assay result, but is still within the Grade I RGR value (no toxicity), implying the feasibility as potential tissue engineering scaffold.     

Nanoporous surface fabricated on metal sheets by alloying/dealloying technique

An alloying/dealloying technique including stacking rolling was used to fabricate nanoporous architecture on Au sheet surface. Under appropriate conditions, a nanoporous surface layer with a thickness of 150-250 nm was generated. The extended immersion of the sample in the electrolyte (HNO₃) increased the ligament size of the nanoporous architecture up to 41 nm. Cross-sectional observations suggested that the nanoporous surface layer can be seamlessly bonded to the bulk substrate Au. True surface area, which was measured by an electrochemical method, decreased as ligament size increased by the extended immersion. The relationship among surface area, ligament size and volume shrinkage was investigated.     

Improving the strength and biocompatibility of porous titanium scaffolds by creating elongated pores coated with a bioactive, nanoporous TiO2 layer

This paper reports a novel way of improving the mechanical properties and biocompatibility of porous Ti scaffolds using a combination of the modified sponge replication method and anodization process. The use of a stretched polymeric sponge as a novel template allowed the creation of elongated pores in a porous Ti scaffold, which, accordingly, led to a high compressive strength of 24.2 ± 2.08 MPa at a porosity of approximately 70 vol%. Furthermore, the surfaces of the Ti walls were coated successfully with a bioactive nanoporous TiO₂ layer using the anodization process, which enhanced the biocompatibility remarkably, as assessed by the attachment of MC3T3-E1 cells.     

平面絷原法测量均质固体的蓄热系数

本建立了平面热源法实验条件下的非稳态导热的数学模型,应用拉普拉斯变换和热四端网络法,求解了实验测量系统的解析模型和各种相应的简化模型,根据参数估计的基本原理,对模型中的待估计做了灵敏度分析。建立了实验测量条纹,可以满足20－200℃环境下固体材料蓄热系数测量的需要。测量了常温下几种材料的蓄热系数,测量结果与热线法实验结果相吻合。     

Improvement in rollability of AZ91 magnesium alloy by carbon addition

The present study aims to investigate the effect of carbon addition on the hot rolling behavior of as-cast AZ91 alloy. The AZ91 and C-added AZ91 alloys were subjected to hot rolling at 400 °C with a reduction of 30% per one pass. The as-cast C-added AZ91 alloy with very fine equi-axed grains of approximately 75 μm exhibited excellent hot rollability compared to as-cast AZ91 alloy with coarse dendrite structure, although the final grain size of the rolled C-added AZ91 alloy sheet was slightly larger than that of the rolled AZ91 alloy sheet. The side-crack occurrence on the surface during hot-rolling is mainly affected by the existence of twin boundary and the area fraction of grain boundaries. Based on the results, the improvement in rollability of the C-added AZ91 alloy is attributed to fine equi-axed grains and the polygonal Al₈Mn₅ phase located inside grains, which can homogeneously distribute and effectively absorb strain energy and prohibit crack growth.     

Study on the temperature measurement of AZ31B magnesium alloy in gas tungsten arc welding

This paper is based on the Finite Element Analysis (FEA) to study the AZ31B Magnesium Alloy welding temperature filed, using a convenient, non-contact and fast response measured temperature method—Infrared Radiation (IR), the welding temperature field of AZ31B magnesium alloy plate in Gas Tungsten Arc Welding (GTAW) is measured by IR, the isothermal map of magnesium alloy plate is measured using IR device. The cooling curves are measured by thermocouple. Experiments and simulations by FEA are carried out to investigate the welding temperature field. The simulated results showed good agreement with the experiment ones.     

Effect of annealing treatments on the anisotropy of a magnesium alloy sheet processed by severe rolling

The effect of annealing treatments on the normal plastic anisotropy (r-value) of a magnesium alloy, AZ61, processed by severe rolling was investigated. The various annealing treatments produce two effects on microstructure: grain coarsening and slight weakening of the texture. In addition, these treatments produce a noticeable decrease of the anisotropy that was correlated with an increase in strain rate sensitivity and a decrease of work hardening rate. It is concluded that an enhanced contribution of basal slip occurs as a consequence of the annealing treatments.     

消失模铸造AZ91镁合金的研究

研究了ZA91镁合金消失模铸造时铸件的厚度,位置和真空度对铸件质量,组织及力学性能的影响,真空度是决定铸件质量的一个关键的工艺因素,无真空时浇注铸件易产生浇不足缺陷,但真空度过大又会导致形成粘砂和气孔等缺陷,真空浇注射明显细化组织,但真空度进一步增大时细化效果野 不明显,铸件显微组织具有很大的壁厚效应,然 位置对组织的影响与是否采用抽真空措施有关,铸件壁厚较小时,铸件的力学性能总体较差,断裂源自Mg/Mg17Al12界面、且主要是以解决理形式的脆性断裂。     

Optical anisotropy of a color-etched AZ91 magnesium alloy

In this paper, the optical anisotropy of a color-etched AZ91 magnesium alloy grain is studied. In the first part, the rinsing and drying conditions after etching were varied in order to improve the contrast between grains. A rinsing solution was selected and ellipsometric measurements were carried out to characterize the optical anisotropy of the etched surface. The wavelength, grain orientation and angle of incidence were varied. It was found that the reflection intensity at oblique incidence and the phase shift between parallel and perpendicular polarizations depend on the orientation of the etched surface of the grains. The optical contrast under diffuse light is explained by the morphology of the surface film deposited upon etching. The optical contrast under polarized light is attributed to form birefringence induced by the film texture. The birefringence, the fast axis, the slow axis, and the optical axis of the etched surface were also determined with the polarizing microscope.     

Microstructural stability and high-temperature mechanical properties of AZ91 and AZ91 + 2RE magnesium alloys

The effects of 2 wt.% rare earth element addition on the microstructure evolution, thermal stability and shear strength of AZ91 alloy were investigated in the as-cast and annealed conditions. The as-cast structure of AZ91 consists of α-Mg matrix and the β-Mg₁₇Al₁₂ intermetallic phase. Due to the low thermal stability of this phase, the strength of AZ91 significantly decreased as the temperature increased. The addition of rare earth elements refined the microstructure and improved both thermal stability and high-temperature mechanical properties of AZ91. This was documented by the retention of the initial fine microstructure and ultimate shear strength (USS) of the rare earth elements-containing material after long-term annealing at 420 °C. The improved stability and strength are attributed to the reduction in the volume fraction of β-Mg₁₇Al₁₂ and retention of the thermally stable Al₁₁RE₃ intermetallic particles which can hinder grain growth during the annealing process. This behavior is in contrast to that of the base material which developed a coarse grain structure with decreased strength caused by the dissolution of β-Mg₁₇Al₁₂ after exposure to high temperature.     

Microstructural evolution in nanoporous gold by thermal and acid treatments

Nanoporous gold was fabricated by dealloying a rolled Au_0.3Ag_0.7 sheet and the effects of thermal and acid treatments on the porous structure and (111) texture were investigated. Both of the coarsening treatments increased the pore and ligament sizes to approximately 500 nm, whereas the resulting porous structure was very different between the two treatments. After the thermal treatment, the (111) plane was highly oriented in a specific direction; however, the change in the (111) texture after the acid treatment was less pronounced than that after the thermal treatment. There may be difference in migration routes of Au atoms between the two treatments.     

On the solidification temperature of undercooled liquids

By analogy to Lindemann's theory of melting a formula for the solidification temperature is derived assuming that in liquid at temperatures near the melting point the atoms vibrate about mean positions which, though not fixed, move slowly compared with the velocity with which the atom vibrate. The relative value of the soldering to the melting temperature decreases exponentially with the rate of latent heat to the melting temperature.     

Fabrication of titanium scaffolds with porosity and pore size gradients by sequential freeze casting

This paper reports a novel method for producing porous Ti scaffolds with a gradient in porosity and pore size using the freeze casting method, in which TiH₂/camphene slurries with various TiH₂ contents (40, 25, and 10 vol.%) were cast sequentially into a mold, followed by freeze drying and heat-treatment in a vacuum at 1300 °C for 3 h. This simple sequential freeze casting method produced good bonding between the layers with different porosities of 35, 53, and 75 vol.% obtained using the TiH₂ contents of 40, 25 and 10 vol.%, respectively. In addition, the pore size could be increased significantly by increasing the freezing time. The pore sizes obtained in the regions produced using 40, 25, and 10 vol.% TiH₂ after freezing for 7 days were 96, 166, and 270 μm, respectively.     

Elevated temperature mechanical properties and microstructures of high pressure die cast magnesium AZ91 alloy cast with different section thicknesses

This paper investigates the influence of variations in the microstructure of high pressure die cast AZ91 on the elevated temperature mechanical properties of the alloy. Thinner-walled high pressure die castings show an improvement in elevated temperature strength, ductility and creep resistance. Further improvements to the creep resistance were achieved by ageing the alloy prior to creep testing. It appears that an increased proportion of fine grained ‘skin’ region in the thinner castings contributed to the improved properties. Also, it appeared that the presence of supersaturated solute Al in the eutectic α-Mg contributes to the poor creep properties, probably due to the microstructural instability. Final failure is associated with the growth of voids either from porosity in the alloy or nucleated from discontinuous precipitates.     

Effect of nanoparticle size on the onset temperature of surface melting

In this letter the onset temperature of surface premelting, T_sm, is determined in terms of the mean-field approximation. There are established relationships, describing the dependence of T_sm on a characteristic size of nanoparticles. It is shown that if a radius of a particle, r, exceeds 10 nm approximately, this dependence is weak. On the contrary, it is strong if the radius of a particle is less than 5 nm. The transition of surface layers into the quasi-liquid and pseudo-crystalline states is under consideration, too. A comparison of the proposed model with available experimental data and with molecular dynamics simulations is carried out.     

Algorithm for the calculation of the coefficient of thermal expansion of multicomponent metallic alloys

An algorithm for the calculation of temperature dependent bulk properties of multicomponent systems is introduced and illustrated with the calculation of the coefficient of thermal expansion, equilibrium volume, energy of formation and compressibility of Ni-Ti-Nb shape memory alloys.     

Influence of transverse load on the high cycle fatigue behaviour of low pressure cast AZ91 magnesium alloy

A new testing procedure, employing transverse load was adopted to investigate the high cycle fatigue behaviour of low pressure cast AZ91 magnesium alloy. The tests were conducted with an electro dynamic shaker system by employing specimens fabricated as per ASTM standard. S–N plot was generated from the test results and compared with that of gravity cast AZ91 alloy tested in identical ambience. The influence of transverse load on the fatigue behaviour of these alloys is discussed. As fatigue cracks were found to have initiated in pores in most of the tested samples, pores were assumed as initial cracks as per linear fracture mechanics and the critical stress intensity amplitude (K_cr) was estimated. Structure–fatigue property correlations are discussed using fractographs. Mean stress effect on the fatigue properties and effects of alloying constituents are also discussed.