Mohd Hasbullah Idris

Effect of calcium content on the microstructure, hardness and in-vitro corrosion behavior of biodegradable Mg-Ca binary alloy

Effect of calcium addition on microstructure, hardness value and corrosion behavior of five different Mg-xCa binary alloys (x = 0.7, 1, 2, 3, 4 wt. (%)) was investigated. Notable refinement in microstructure of the alloy occurred with increasing calcium content. In addition, more uniform distribution of Mg 2 Ca phase was observed in α-Mg matrix resulted in an increase in hardness value. The in-vitro corrosion examination using Kokubo simulated body fluid showed that the addition of calcium shifted the fluid pH value to a higher level similar to those found in pure commercial Mg. The high pH value amplified the formation and growth of bone-like apatite. Higher percentage of Ca resulted in needle-shaped growth of the apatite. Electrochemical measurements in the same solution revealed that increasing Ca content led to higher corrosion rates due to the formation of more cathodic Mg 2 Ca precipitate in the microstructure. The results therefore suggested that Mg-0.7Ca with the minimum amount of Mg 2 Ca is a good candidate for bio-implant applications.

Effect of fluoride treatment on corrosion behavior of MG-CA binary alloy for implant application

Abstract The influence of hydrofluoric acid (HF) treatment on the corrosion behavior of the Mg-0.5Ca alloys was investigated by immersion specimen in sodium hydroxide and HF solutions with various concentrations and durations at room temperature. Microstructural evolutions of the specimens were characterized by atomic force microscopy, X-ray diffraction, field-emission scanning electron microscopy. The corrosion resistance was examined through potentiodynamic polarization and immersion test in Kokubo solution. The results revealed that the fluoride treated Mg-0.5Ca alloys produced by immersion in 40% HF provided more uniform, dense and thicker coating layer (12.6 μm) compared with the 35% HF treated specimen. The electrochemical test showed that the corrosion resistance of fluoride treated specimen was 35 times higher compared with the untreated Mg-0.5Ca alloy specimen in Kokubo solution. In vitro degradation rate of the fluoride treated specimens was much lower than untreated Mg-0.5Ca alloy in Kokubo solution. After immersion test the surface of 40% HF treated sample showed a few corrosion dots, while untreated specimens were fully covered by corrosion products and delamination. Fluoride treated Mg-0.5Ca alloy with 40% HF is a promising candidate as biodegradable implants due to its low degradation kinetics and good biocompatibility.

Effect of bismuth on microstructure of unmodified and Sr-modified Al-7Si-0.4Mg alloys

The effects of bismuth and the combination of bismuth and strontium on the eutectic silicon structure in Al-7Si-0.4Mg alloys were investigated under different solidification conditions. The results show that bismuth has a refining effect on the eutectic silicon and its refinement behavior increases with increasing Bi content up to 0.5% (mass fraction). When bismuth is added into the molten alloy modified with strontium, a higher Sr/Bi mass ratio of at least 0.45 is required to attain full modification of the eutectic silicon.

Characterization and Corrosion Behavior of Biodegradable Mg-Ca and Mg-Ca-Zn Implant Alloys

In the present study binary Mg-xCa (x=0.5 and 1.25wt.%) and ternary Mg-1Ca-xZn (x=0.5 and 1.5wt.%) alloys are produced by casting the molten metal in a metal die at a temperature of 740°C. The microstructure analysis of the Mg-Ca and Mg-Ca-Zn alloys were studied by OM, SEM and EDX. The corrosion behavior of alloys was evaluated via potentiodynamic polarization test in Kokubo solution. The result exhibited that the grain size decrease with rising Ca content in Mg-Ca alloys and degree of grain size reduction further decreased by adding Zn to Mg-1Ca-Zn alloys. The microstructure of Mg-Ca alloys were constituted of primary Mg and lamellar eutectic (α-Mg+Mg2Ca) phase, Whilst Mg–1Ca-Zn alloys were composed of primary Mg and eutectic (α-Mg+Mg2Ca+Ca2Mg6Zn3) phases. In addition with increasing Ca and Zn the amount of Mg2Ca and Ca2Mg6Zn3 increased respectively in grain boundaries. Electrochemical test shows that the addition of Zn leads to improve corrosion resistance of the Mg–1Ca-Zn alloys as a result of the formation of Ca2Mg6Zn3 phase, whilst the addition of more than 0.5 wt% Ca to Mg-Ca alloys result in decrease corrosion resistance due to the formation Mg2Ca.

Effect of pattern coating thickness on characteristics of lost foam Al–Si–Cu alloy casting

Abstract An experimental study on lost foam casting of an Al–Si–Cu alloy was conducted. The main objective was to study the effect of pattern coating thickness on casting imperfection and porosity percentage as well as eutectic silicon spacing of the alloy. The results showed that increasing slurry viscosity and flask dipping time influenced the casting integrity and microstructural characteristics. It was found that thinner pattern coating produced improved mould filling, refined microstructure and higher quality castings containing less porosity.

Influence of Gating System, Sand Grain Size, and Mould Coating on Microstructure and Mechanical Properties of Thin-Wall Ductile Iron

Two gating systems namely stepped and tapered runners were used to cast strip samples with different thicknesses by CO2/silicate process using sand grain sizes of AFS 151 and 171. To assess the effect of mould coating on the properties of thin-wall ductile iron, half of the moulds were coated whilst the rest were not coated. Molten metal with the carbon equivalent of 4. 29% was prepared and poured at 1450 °C. Microstructure of the specimens was analyzed by optical and scanning electron microscopes. Count, area fraction, roundness and diameter of the graphite nodules of the samples were measured by image analyzer. Brinell hardness and tensile tests of all the samples were also conducted. The results show that by using stepped runner gating system with uncoated and coarse sand mould, roundness and count of the graphite nodules decrease whereas diameter and area fraction increase. Although fine sand and coated mould cause longer distance of molten metal travel, hardness and strength of the samples decrease.

Corrosion resistance of EPD nanohydroxyapatite coated 316L stainless steel

Abstract The effect of nanohydroxyapatite (HA) coating on corrosion resistance of 316L stainless steel was investigated. The stainless steel samples were pretreated in different volume concentrations of H2SO4 solution and were electrophoretically coated for various coating time spans and voltages. The coated samples were sintered in a vacuum furnace at different temperatures to modify the coating structure. The quality and morphology of the pretreated samples and the coating were microscopically characterised, and the compounds presented in the coatings were determined. The electrochemical corrosion test was performed in simulated body fluid to evaluate the coating impact on corrosion behaviour of the samples. The results showed that the deposition of nano-HA using moderate voltage and appropriate time span provides a smooth and almost cracked free coating, improving the corrosion resistance of the samples. Increasing the temperature of sintering modified the HA structure, resulting in the formation of denser coating and enhanced corrosion resistance of the samples.

Effect of thermomechanical treatment on microstructure and hardness behavior of AZ63 magnesium alloy

Due to their high specific strength and low density, magnesium alloys are widely used in many weight-saving applications. This research is aimed at investigating the microstructure and hardness of commercial AZ63 alloy specimens subjected to two different thermomechanical treatments (TMTs). For the first TMT, after solution treated at the temperature of 380 °C for 20 h, AZ63 alloy specimens were 5% cold worked by rolling process followed by ageing at the temperatures of 150 °C and 250 °C for 3, 9 and 25 h. In the second TMT, the specimens were solution treated at the temperature of 380 °C for 20 h, underwent 2% cold worked and quenched in water of 0 °C. Half of the specimens were then 2% cold worked whilst the rest were rolled to 8% cold worked. All the specimens were then aged at the temperatures of 150 °C and 250 °C for 3, 9 and 25 h. Optical microscope was used to analyze the microstructures of the specimens. Hardness test was too conducted to measure the effect of the treatments on the specimens. Results show that two-step aging enhances the hardness of the specimens due to the distribution of fine β -phase (Mg 17 Al 12 ) in the alloy matrix. The results also reveal that, the best hardness from the first TMT was produced by specimen that was pre-aged at 150 °C whereas, in the second TMT, aging at 250 °C exhibited the best hardness values.

Role of bismuth on solidification, microstructure and mechanical properties of a near eutectic Al-Si alloys

Computer aided thermal analysis and microstructural observation showed that addition of bismuth (Bi) within the range of 0.25 and 2 wt% produced a greater effect on the Al-Si eutectic phase than on primary aluminium and Al2Cu phases. Results showed that with addition of 1 wt% Bi the eutectic silicon structure was refined from flake-like morphology into lamellar. Bi refines rather than modifies the Si structure and increases the Al-Si eutectic fraction solid and more significantly there was no fading even up to 180 min of melt holding. Transmission electron microscopy study showed that the Si twin spacing decreased from 160 to 75 nm which is likely attributed to the refining effect of Bi. It was also found that addition of 1 wt% Bi increased the tensile strength, elongation and the absorbed energy for fracture due to the refined eutectic silicon structure.

Mechanical properties of stabilized zirconia nanocrystalline EB-PVD coating evaluated by micro and nano indentation

Yttria-stabilized zirconia (YSZ) thin nanocrystalline coatings at different substrate preheating temperatures were deposited via electron beam-physical vapour deposition (EB-PVD). Nanocrystalline ZrO2-Y2O3 was deposited on the bond coat in order to compensate for the coefficient of thermal expansion (CTE), which can be functionalized as a thermal barrier coating (TBC). The aim of this study was to evaluate mechanical properties with respect to adhesion of zirconia nanocrystalline’s top ceramic layer to the interfacial bond coat by utilizing micro and nano indentation tests. In the present paper, the structural studies were carried out using X-ray diffraction (XRD) analysis of coating content (8 mol% of Y2O3). The tetragonal phase of stabilized zirconia was observed. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) were employed to characterize the coatings’ morphology and microstructure. The mechanical behavior of ZrO2-Y2O3 thin films under point loading conditions was studied by nanoindentation using a Berkovich indenter with 130 nm tip radius. Therefore, adhesion of top coat to the interfacial underlying metallic bond coat known as MCrAlY (M = Ni, Co) was estimated according to the highest peak load tests; for a 120 mN peak load, the film manifested tolerable adhesion properties. Moreover, nanoindentation of ZrO2-Y2O3 nanostructure deposited at 1050 °C substrate preheating temperature produced the highest hardness value of about 21.7 GPa. Vickers micro hardness was utilized with the aid of the Tabor equation in order to achieve deeper insight into the correlation between adhesion and deposition process parameters.