Identification of ageing behavior of Mg-Al-Zn and AZ91 reinforced carbon nanotube at nlevated temperature

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.