Identification of ageing behavior of Mg-Al-Zn and AZ91 reinforced carbon nanotube at nlevated temperature
Fabrication and properties of Cobalt-Chromium implant composite
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Publication
Magnesium alloys are potential composite materials that can be applied for lightweight structural engineering applications due to its specific strength in mechanical and physical properties. However, these composite materials lose its strength and creep resistance properties when exposing at certain temperature. It is reported that by adding minor alloy AZ91 and composite AZ91 with reinforced carbon nanotubes (CNT) will improve its mechanical and physical properties. Nevertheless, the work on the effects of heat treatment and prediction of ageing behaviour properties and activation energy of AZ91 and composite AZ91 reinforced with CNT still less and potential to be explored. Mg-Al-Zn (AZ91) and composite AZ91 reinforced with carbon nanotube (CNT) were fabricated using powder metallurgy method. The composite samples were varied with the weight percent of CNT with 0, 0.3, 0.6 and 0.9 wt.%. The samples were mixed via planetary mill for 20 hours and compacted at 400 MPa, pallet shape with diameter 12 mm. All samples undergo sintering at 450 ˚C then undergo T4 heat treatment (solution treatment) at 415 °C and T6 (artificial ageing) at 175 °C, 210 °C and 300 °C. Microstructure of AZ91 and AZ91+ CNT composites were observed by using an optical microscope (OM) and Scanning Electron Microscope (SEM). All samples AZ91 and AZ9+CNT composites were undergone phase analysis by using X-Ray Diffraction (XRD). Meanwhile physical properties were characterized using pycnometer instrument to determine the density of the samples. Mechanical properties studies were performing by using Rockwell hardness test and compression test via Universal Testing Machine (UTM). Finally, the activation energy and hardness prediction of AZ91 and AZ9+CNT composites were evaluated by modifying and improving John Mehl-Avrami Semi-Empirical Model. From the analyses, it was found that CNT were homogeneously distributed into the matrix of AZ91 composites due to successfully mechanical alloying using planetary mill. Their densities were 1.98 g/m3 for AZ91 and 1.87 g/m3 for AZ91+ 0.3% CNT. Meanwhile, the compressive strength obtained were 26.8 MPa for AZ91 and 47.1 MPa for AZ91+CNT. The addition of CNT gives softening effect for composite AZ91 + 0.3% CNT and positive effect for composites AZ91 + 0.6% CNT and AZ91 + 0.9% CNT. Composites AZ91 + 0.6% CNT and AZ91 + 0.9% CNT show accelerated ageing and achieve peak aged hardness at 4 hour of ageing time. The positive effect of hardening is expected due to the precipitation of Mg17Al12. The investigation shows the significant of time and temperature are the main role in the precipitation hardening process of the nanocomposite. It is found that the hardness decreases when the temperature is increases and the hardness is increases together with ageing time. Encouraging prediction results are observed when compared with experimental data at a specific time and temperature. Kinetics study show an activation energy of 21kJ/mol of the AZ91 nanocomposite. The purpose of this study is to determine the optimal heat treatment parameter for producing a high-strength AZ91 composite, which is a critical material component for engine blocks used in the automobile sector.
Publication
Cobalt implant composite (CIC) was produced by powder metallurgy technique. Composition of 0% ,5%, 10%, 15% and 20% of hydroxyapatite was mixed with cobaltchromium alloy. The fabrication technique is mixing, blending, pressing and sintering of the final product. Cobalt, chromium and hydroxyapatite powders were mixed in planetary ball mill at 600 rpm for 30 minutes. The consolidation method for CIC was uni-axial compacting using Universal Testing Machine (UTM) Gotech. The pressure used was 500 MPa. The CIC was sintered at 10000C temperature with 200C/min for 3 hours. The composites then were evaluated and tested to evaluate the microstructure and mechanical properties. The microstructure analysis is carried out by using the Scanning Electron Microscope and Image Analyzer attached to the optical microscope. In microstructure analysis, there are several characteristics need to observe i.e., particle sizes, porosities, mode of shapes, corrosion behaviours and bonding between mixed particles and fracture mechanism, which these can describe the composites material in details. The properties such as hardness, density, and particle sizes distribution, purity of raw materials, compressive strength and corrosion behaviours are analyzed by using Vickers Micro Hardness, AccuPyc 1330 Gas Pycnometer, MALVERN MASTERSIZER 2000 particle analyzer, X-Ray Diffraction (XRD), Compression test and Immersion Fluid test in Natrium Chloride (0.9%.NaCl ), respectively. From the microstructure analysis of the composite, the microstructure indicates the homogenous distribution of the chromium particles, and HAP particles are distributed homogenously in the matrix cobalt chromium. From the X-ray diffraction (XRD) the high peak of the x-ray analysis is indicating the purity of each powder such as chromium and cobalt. In general cobalt and chromium peaks occur at the range of 40 to 50 degree and there is no obvious sign of HAP signal in XRD analysis of the composites. Both experimental and theoretical density graphs have shown a similar pattern line which both experienced the density gradually decreased when percentage of HAP increased. The hardness of the composites decreases slightly with the increasing weight percent of HAP. The sonic modulus analysis, indicating that there is a reciprocate relationship between modulus and sound velocity, whereby modulus will be decreased when the sound velocity increases. The microstructure analysis on compression test, indicated the deformation behavior of the composite started to change from the ductile mode to the brittle mode resultant with the added of HAP. Besides, the crack pattern showed non continuously for the ductile mode behavior and had a continuous line for the brittle mode behavior. Corrosion study indicated that composites experienced more corrosion when the HAP was added.