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Featured researches published by A.H.W. Ngan.


Philosophical Magazine | 2009

Deformation of micron-sized aluminium bi-crystal pillars

K.S. Ng; A.H.W. Ngan

The deformation of micron-sized single-crystals is jumpy and stochastic, and this may pose potential formability and reliability problems if components for future micro-machines are to be made from small metal volumes. In this work, micron-sized bi-crystal pillars were fabricated by focussed ion-beam milling from grain-boundary regions in coarse-grained polycrystalline aluminium. Each bi-crystal pillar contained a grain boundary intersecting its top surface, and was subjected to compression using a flat-ended nanoindenter tip. Their deformation was found to have smaller strain bursts, fewer periods of strain hardening at elastic-like rates, as well as greater work-hardening rate and flow stress, than single-crystal pillars of similar sizes. Transmission electron microscopy revealed severe dislocation accumulation in the deformed bi-crystal pillars, whereas the residual dislocation density remained low in single-crystal micro-pillars of similar dimensions after deformation to comparable strains. The results suggest that a grain boundary inside a micro-specimen can trap dislocations inside the specimen, leading to a significant rise in the strain-hardening rate as well as to smoother deformation.


Philosophical Magazine Letters | 2006

Molecular dynamics study on compressive yield strength in Ni3Al micro-pillars

Le Zuo; A.H.W. Ngan

Recent nanoindentation experiments on bulk samples as well as compression tests on micro-pillars indicate that the micron-sized material volumes exhibit a first yield point that strongly depends on the sample size. In this work, molecular dynamics (MD) simulations are carried out to investigate the onset of yielding in Ni3Al nano-sized pillars. The MD results show that dislocation generation is from the free surfaces of the micro-pillars, when thermal vibration induces too large a local interatomic displacement. The statistical distributions of the near-surface thermal atomic displacements gathered from the MD simulations are used in conjunction with a survival probability model to predict the yield conditions of real-sized micro-pillars in real time scales. The predicted results agree fairly well with experimental results in the literature.


Philosophical Magazine Letters | 2007

Creep of micron-sized aluminium columns

K.S. Ng; A.H.W. Ngan

Micron-sized aluminium columns, produced by focused-ion beam milling, were subjected to compressive creep in a nanoindentation machine using a flat-ended tip to study their time-dependent deformation behaviour at room temperature. At constant load, their strain increased almost linearly with time, with sporadic but large strain bursts superimposed. This represents a form of creep behaviour not known beforehand for aluminium. Strain bursts were also observed to occur during reloading at stresses higher than the first yield stress during initial loading.


Philosophical Magazine | 2010

Transition from deterministic to stochastic deformation

A.H.W. Ngan; K.S. Ng

Theory predicts that deformation that occurs by emission of strain bursts falls into two regimes, one in which the burst emission remains a stochastic process as strain increases, and another in which the emission of bursts settles into a deterministic process for large strains. The stochastic regime occurs when the burst emission rate decreases with strain, and in this case, large statistical scatter persists in the stress–strain response on repeated measurements. The deterministic regime occurs when the emission rate increases with strain, and the scatter in the corresponding stress–stress behaviour diminishes at large strains. The strength at the same strain in the stochastic regime is also higher than in the deterministic regime. Factors that affect the burst emission rate include the number of sources as well as the stress dependence of the efficiency of the sources.


Philosophical Magazine Letters | 1995

On generalizing the Peierls-Nabarro model for screw dislocations with non-planar cores

A.H.W. Ngan

Abstract The Peierls-Nabarro model was developed originally to describe core spreading on the slip plane of a dislocation. Screw dislocations have more than one slip plane and are expected to be dissociated into a non-planar fashion conformable to the symmetry about the screw axis. In this work, the Peierls-Nabarro model is generalized to account for non-planar core spreading.


Philosophical Magazine Letters | 1995

A critique on some of the concepts regarding planar faults in crystals

A.H.W. Ngan

Abstract The construction of the γ surface requires the application of mechanical constraints in order that the energy of non-equilibrium fault configurations can be calculated. The choice of mechanical constraints is not unique and it is shown that γ surfaces constructed using different schemes can differ significantly. The measurement of planar fault energies using transmission electron microscopy techniques is also critically assessed. It is discovered that the constraining effects of the bounding partial dislocations can significantly modify the local fault energy. Experimentally measured fault energies should therefore be expected to differ from atomistically calculated values.


Acta Materialia | 2008

Stochastic nature of plasticity of aluminum micro-pillars

K.S. Ng; A.H.W. Ngan


Archive | 2007

Physical metallurgy and advanced materials

R. E. Smallman; A.H.W. Ngan


Scripta Materialia | 2013

Size effect on the deformation behavior of duralumin micropillars

R. Gu; A.H.W. Ngan


International Journal of Plasticity | 2014

Size-dependent creep of duralumin micro-pillars at room temperature

R. Gu; A.H.W. Ngan

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K.S. Ng

University of Hong Kong

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R. Gu

University of Hong Kong

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Jianyu Li

University of Hong Kong

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Le Zuo

University of Hong Kong

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N. Afrin

University of Hong Kong

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