M.O. Lai

Influence of SiC particles on mechanical properties of Mg based composite

AZ91 magnesium alloy reinforced with different sizes of SiC particulates has been fabricated using powder metallurgy route. Mechanical properties of the specimens have been studied. Yield and ultimate tensile stresses show a decrease with the increase in the size of SiC particulates. The influence of thermal shock between 400°C and 30°C on the mechanical properties was also investigated. The results show a decrease in yield stress and elongation to fracture with the number of thermal shock cycles.

On the prediction of tensile properties from hardness tests

The possibility of correlating the hardness to the tensile properties of a material has been investigated using Assab 760 steel, mild steel and API Std 5LX grade X60 pipeline steel that have been heat-treated for different times at various tempering temperatures and 6063-T1 aluminium that has been solution heat-treated. It is found that the strain hardening coefficient and the strength coefficient of all materials tested were linearly related to the hardness, irrespective of the type of hardness measurement used. Using these relationships, equations were defined to estimate the yield and ultimate tensile stresses of the materials. Good agreement between experimental results and estimated values was obtained for all materials studied. The feasibility of using the present findings in non-destructive field testing is discussed.

Improvement of microstructure and mechanical properties of AZ91/SiC composite by mechanical alloying

AZ91 magnesium alloy reinforced with SiC particulates was fabricated via powder metallurgy technique as well as mechanical alloying process where a planetary ball mill was employed. Microstructure and mechanical properties of the fabricated AZ91 composites had been evaluated. Microstructural study showed that grain size of the material was refined and SiC particulates were well distributed after mechanical alloying. Mechanical tests of the composite showed an enhanced yield and ultimate tensile strengths for the mechanically alloyed samples compared with those prepared via the powder metallurgical route.

Deformation behaviour of ultrafine and nanosize-grained Mg alloy synthesized via mechanical alloying

Bulk Mg–5Al alloys were consolidated from powders that had been mechanically alloyed over different milling durations. The microstructural evolution, and physical and mechanical properties of the alloys were investigated. Mechanical measurements revealed a change in deformation behaviour after certain milling durations. At short milling duration, high yield strength was obtained through dislocation strengthening mechanisms predominantly by grain refinement and to a lesser extent by solid solution strengthening and particle dispersion strengthening. However, at longer milling durations, low yield strength was observed and the strengthening mechanisms at work in short milling durations appeared to be no longer effective. Enhanced ductility with no work hardening behaviour was observed in samples with a mean grain size of 45u2009nm. It appeared that the significantly large increase in the grain boundary regions played an important role in the room temperature deformation of the alloys. The possibility of a diminishing effect of the dislocation strengthening mechanisms and the onset of grain boundary deformation modes for the softening phenomenon and the absence of work hardening at some nanoscale grain sizes are discussed.

Processing of thermally stable doped perovskite PZT ceramics

Abstract In this paper, various methods of synthesizing lead zirconium titanate (PZT) ceramics with high Curie temperature (Tc) and thermal stability are discussed. Perovskite PZT variants were synthesized from stoichiometric oxide ratios of Pb, Zr, Ti and dopant element. The oxide powders were mixed mechanically and calcinated, and then sintered for the desired perovskite phase to form. Varied mixing and sintering parameters allowed investigation of the optimum settings required for proper densification with enhanced material properties. The piezoelectric properties and relative permittivity of doped PZT ceramics were analyzed using the resonant frequency method. Pure and Mg-doped PZT samples were subjected to thermal shock and cyclic test and the material properties of these samples were compared to verify the extent of degradation. Results were investigated for PZT ceramics’ viability in high temperature applications. Differential scanning calorimetry (DSC) was used to evaluate the Tc for various dopants, and thus to identify the ideal dopant that yields highest Tc and operational temperature with appreciable piezoelectric properties.

Relationship between fracture topography and fracture toughness of a high strength steel

The relationship between fracture topography and the plane strain fracture toughness of Comsteel En25 tempered at nine different temperatures and with crack planes in the R-L, R-C and L-R orientations has been studied. The results show that fracture toughness is qualitatively relatable to the fracture morphology. Due to the shape and alignment of the elongated MnS inclusions, fracture toughness in the L-R orientation was found to be 1.8 times those in the other two orientations. Terrace-type fracture prevailed in the R-L orientation, but this frequently was observed to change to zigzag type fracture in the R-C orientation. However, both these fracture mechanisms were absent in the L-R orientation.

From waste to high strength alloy - recycling of magnesium chips

Abstract Ultrafine grained magnesium alloy was synthesized via mechanical milling of AZ91 chips. Mechanical property measurement revealed enhanced yield strength of 470 MPa after mechanical milling. The increase in yield strength is associated with reduction in grain size which restricts twining and dislocation gliding. The present investigation demonstrates that magnesium alloys can be cost-effectively recycled through the process of mechanical milling.

Synthesis and Mechanical Properties of Nanostructured Mg-Al-Nd Alloys

In this work, nanostructured Mg-Al-Nd alloys were successfully fabricated using mechanical alloying method. Phase structures of the milled powders and the extruded rods were examined using X-ray diffraction (XRD) and the microstructures were observed using a transmission electron microscope (TEM). The element distributions were also examined by an energy dispersive spectroscopy (EDS) system. The mechanical properties were estimated using tensile and impact tests. The fracture surfaces of the tensile and impact specimens were observed using a scanning electron microscope (SEM). The results showed that the grain size of the Mg-Al-Nd alloys could be reduced to below 100 nm after 20~30 hours milling. The effects of alloy composition and processing conditions on strength and ductility were analyzed. The possible mechanisms of plastic deformation in these alloys were also proposed.

Fabrication and Microstructure of Mechanically Alloyed Mg-Al-Nd Alloys

In this work, nanocrystalline Mg-Al-Nd alloys were fabricated using mechanical alloying method. Phase structure of the extrided rods was examined using X-ray diffraction (XRD) and the microstructures were observed using transmission electronic microscopy (TEM). High yield strength was obtained in the alloys with a high Nd content due to grain refinement and Nd rich precipitate phase.