Papers by Author: Wing Yiu Yeung

Abstract: An enhanced nucleation heat treatment process was employed to refine the grain structure of 7075Al/ 7wt.% SiCp metal matrix composite material. The grain structure of the metal matrix composite was investigated. The resulting microstructures were compared. It was found that a fine grained microstructure developed after the rapid heat treatment process. An increase in hardness was achieved, with hardness increasing from 129 HV in the cold extruded state to 137 – 153 HV after the heat treatment process.

Abstract: There have been growing demands of high performance metal matrix composites in advanced engineering applications in virtue of their high specific strengths. This paper is to report an assessment of the mechanical properties of LC4/SiCp metal matrix composites using an innovative testing technique, small punch test. The composite materials of this study were produced by stir casting method with particulate reinforcements of 7wt.% and 14wt.% of SiC respectively. Small punch testing was performed on the LC4 base alloy and the two composites materials. The small punch test is a relatively new mechanical testing technique capable of utilizing small disk-shaped samples to determine the mechanical properties of the test materials. In this study, the equivalent fracture strain, εqf of the LC4/SiCp MMCs was characterised and compared with the base alloy. The fracture mechanism of the test samples was examined using scanning electron microscopy.

Abstract: Particulate reinforced 10 vol.% (TiB+TiC)/Ti-6Al-4V metal matrix composites (MMCs) were produced by in-situ synthesis using vacuum arc re-melting process, and the mechanical properties of the metal matrix composites were assessed by the small punch testing (SPT). Mechanical properties of the in-situ synthesized Ti MMCs were studied and compared with the Ti base alloy. From the test data, the fracture properties and equivalent fracture strain of the test materials were characterised. The fracture mechanism of the test samples was examined using scanning electron microscopy.

Abstract: Particulate reinforced Ti based metal matrix composites (MMCs) were made by in-situ synthesis using vacuum arc re-melting process. The microstructure of the Ti-6Al-4V base alloy and 10 vol.% (TiB+TiC)/Ti-6Al-4V metal matrix composites was examined. The particulate reinforcements were analysed and identified TiB and TiC particles. The particle distribution was analysed using the quadrat method over 1620 quadrats. A homogeneous particle distribution was found to establish in the composites. The experimental distribution of the reinforcements agreed well with the theoretical Poisson distribution. A skew factor, which characterizes the degree of asymmetry of a statistical distribution, of 1.108 was determined for the particle distribution in the material.

Abstract: The small punch test (SPT) is a relatively new mechanical testing technique capable of utilising small disk-shaped specimens to determine the mechanical behaviour of the test materials. From the test data, fracture properties in particular the equivalent fracture strain (εqf) can be determined and the elastic-plastic fracture toughness of a material can be characterised. Finite element analyses (FEA) of the small punch tests have been performed for the 304H stainless steel in this study. 304H stainless steel was isothermally aged at 650, 700 and 750oC for durations up to 200 hours. Small punch tests were performed to assess the mechanical properties of the heat treated materials. Using the input of the true stress-strain curves of material constitutive behaviour, the theoretical load vs punch displacement curves calculated from the FEA models were found in good agreement with the experimental results. The fracture thickness of the small punch tested specimens and the equivalent fracture strain were also be determined for the test materials.

Abstract: Equal channel angular extrusion (ECAE) was employed in an attempt to develop nanostructural metal matrix composites with homogenized distribution of reinforcing particles. Zn- Al metal matrix composites reinforced with 5 μm SiC particulates were produced by casting method. A non-uniform distribution of the reinforcing particles was evident in the metal matrix. With repetitive shear deformation imposed via the ECAE process, substantial structural improvement was achieved and the reinforcing particulates were de-clustered into a finely dispersed distribution throughout the metal matrix. The homogeneity of the particle distribution was studied by the Quadrat method and the skew factors were determined. It was found that the skew factors were substantially reduced after 8 extrusion passes, showing the homogeneity of the particle distribution was greatly improved in the composites.

Abstract: Development of complex ternary nitride coatings has attracted significant industrial interest in recent years. In deposition of complex ternary nitride coatings, the nitrogen deposition pressure plays an important role in structural evolution of the coatings leading to development of different mechanical properties. This paper summaries some successful analyses by the authors on the relationships amongst the deposition rate, grain size and hardness of the coatings against the nitrogen deposition pressure. It has been established that as the nitrogen pressure decreases, the deposition rate of the coatings increases and the grain size decreases. Hardness of the coatings increases due to the development of a refined and densified coating structure. Taking into account of the reaction kinetics at the targets, the interactions of the sputtered atoms occurred in their transfer to the substrate, the reaction kinetics at the substrate, the target material characteristics and the geometric arrangement of the sputter magnetron configuration, modelling to the relationships of deposition rate with nitrogen deposition pressure, grain size with deposition rate and hardness with grain size have been successfully established in this study. A limiting grain size of the coatings has also been identified in the grain refinement process.

Abstract: Metal laminates have experienced rapid development in many engineering applications and generally possess enhanced properties with improved service performance. Roll bonding is a major technique used in manufacture of metal laminates. This paper is to investigate the effects of atomic diffusion on the interface development of roll bonded metal laminates in the sintering heat treatment process. Copper/aluminium bi-metal laminates were prepared by roll bonding at 430oC with a 40% rolling reduction in a single pass. Sintering was then applied at 450oC for various periods. It was found that multi-phase interfacial layers developed in the sintering process and shifted towards the aluminium metal with increasing sintering time. Composition profiles of the metallic elements were carefully determined across the interfacial area of the bonded material. Interdiffusion coefficients of the metallic elements were determined using Boltzmann-Matano analysis. The results were correlated with the multi-phase development in the interfacial area.

Abstract: Development of advanced ternary nitride coatings such as titanium aluminium nitride and titanium vanadium nitride has attracted significant industrial interest in recent years. Titanium vanadium nitride is considered one of the advanced ternary nitride coatings of great commercial potential. It is believed with the additional element, the oxidation resistance of the coatings can be greatly improved at elevated temperatures. Furthermore, the type of elements selected can produce unique coating properties that can be beneficial to machining of different materials. This paper is to report a study on the structural stability of nanostructured titanium vanadium nitride coatings in high temperature annealing. Nanostructured titanium vanadium nitride coatings were produced by reactive magnetron co-sputtering on AISI H13 tool steel substrates at 240oC. Heat treatment was applied to the coatings at temperatures up to 1000oC. It was found that an unexpected grain refinement of the coatings occurred in the heat treatment process. Grain size of the coatings was found to decrease from ~200-300 nm to ~150 nm after the heat treatments. A strong TiN/TiVN (200) component was found to exist at temperatures up to 700oC but was depleted at higher annealing temperatures. With a finer and densified grain structure, the hardness of the coatings substantially increased from ~800 HV to ~1700 HV.

Abstract: This study is to investigate the crystallite development in nanostructured aluminium using x-ray line broadening analysis. Nanostructured aluminium was produced by equal channel angular extrusion at room temperature to a total deformation strain of ~17. Samples of the extruded metal were then heat treated at temperatures up to 300oC. High order diffraction peaks were obtained using Mo radiation and the integral breadth was determined. It was found that as the annealing temperature increased, the integral breadth of the peak reflections decreased. By establishing the modified Williamson-Hall plots (integral breadth vs contract factor) after instrumental correction, it was determined that the crystallite size of the metal was maintained ~80 nm at 100oC. As the annealing temperature increased to 200oC, the crystallite size increased to ~118 nm. With increasing annealing temperature, the hardness of the metal decreased from ~60 HV to ~45 HV.