Browsing by Author "Oliver, EC"

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    Combined in situ neutron diffraction and acoustic emission of twin nucleation & twin growth in extruded ZM20 Mg alloy
    (Trans Tech Publications, 2009-11-10) Muránsky, O; Barnett, MR; Carr, DG; Vogel, SC; Oliver, EC
    In the present work in situ neutron diffraction and acoustic emission were used concurrently to study deformation twinning in two ZM20 Mg alloys with significantly different grain sizes at room temperature. The combination of these techniques allows differentionation between the twin nucleation and the twin growth mechanisms. It is shown, that yielding and immediate post-yielding plasticity in compression is governed primarily by twin nucleation, whereas the plasticity at higher strains is governed by twin growth. The current results further suggest that yielding by twinning happens in a slightly different manner in the fine-grained as compared to the coarse-grained alloy. © Trans Tech Publications Ltd
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    In situ neutron diffraction investigation of deformation twinning and pseudoelastic-like behaviour of extruded AZ31 magnesium alloy
    (Elsevier, 2009-06) Muránsky, O; Carr, DG; Šittner, P; Oliver, EC
    In situ neutron diffraction has been used to investigate the deformation twinning and untwinning during cyclic uniaxial straining of hydrostatically extruded AZ31 magnesium alloy. The development of the internal stresses and microstructure in the polycrystalline alloy when twinning takes place is explained on the basis of the two pairs of parent {10.0}||, {11.0}|| and twin {00.2}||, {10.3}|| grain families. The experimentally observed pseudoelastic-like behaviour in stress–strain cycles is interpreted as being due to the activation of reversal twinning processes during loading–unloading cycles. It is proposed that the driving force for the observed untwinning is the existence of high tensile stresses in favourably oriented grains which result from significant twinning activity prior to unloading from the peak stress. © 2009, Elsevier Ltd.
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    An inert-gas furnace for neutron scattering measurements of internal stresses in engineering materials
    (IOP Publishing Ltd, 2012-04-01) Haynes, R; Paradowska, AM; Chowdhury, MAH; Goodway, CM; Done, R; Kirichek, O; Oliver, EC
    The ENGIN-X beamline is a dedicated engineering science facility at ISIS optimized for the measurement of strain, and thus stress, deep within crystalline materials using the atomic lattice planes as an atomic ‘strain gauge’. Internal stresses in materials have a considerable effect on material properties including fatigue resistance, fracture toughness and strength. The growing interest in properties of materials at high temperatures may be attributed to the dynamic development in technologies where materials are exposed to a high-temperature environment for example in the aerospace industry or fission and fusion nuclear reactors. This article describes in detail the design and construction of a furnace for neutron scattering measurements of internal stress in engineering materials under mechanical load and in elevated temperature environments, designed to permit a range of gases to provide a non-oxidizing atmosphere for hot samples. © 2012 IOP Publishing LTD.
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    Investigation of deformation twinning in a fine-grained and coarse-grained ZM20 Mg alloy: combined in situ neutron diffraction and acoustic emission
    (Elsevier, 2010-03) Muránsky, O; Barnett, MR; Carr, DG; Vogel, SC; Oliver, EC
    Neutron diffraction and acoustic emission were used in a single in situ experiment in order to study the deformation twinning of two ZM20 Mg alloys with significantly different grain sizes at room temperature. The combination of these two techniques facilitates the distinction between twin nucleation and twin growth. It is shown that yielding and immediate post-yielding plasticity in compression along the extrusion direction is governed primarily by twin nucleation, whereas plasticity at higher strains is presumably governed by twin growth and dislocation slip. It is further shown that, in the fine-grained alloy, collaborative twin nucleation in many grains dominates yielding, whereas twin nucleation in the coarse-grained alloy is progressive and occurs over a larger strain range. In addition, it is shown that, despite twin nucleation stresses increasing with decreasing grain size, roughly the same overall volume fraction of twins is formed in both fine and coarse parent grains. This confirms the difficulty of the alternative deformation modes and suggests a negligible suppressive effect of grain size on twinning in the case of the strongly textured fine-grained alloy. The current results also show that twins in the coarse-grained alloy are born less relaxed with respect to surrounding polycrystalline aggregate than those in the fine-grained alloy. This is believed to lead to lower reversal stresses acting on twin grains in the coarse-grained alloy upon unloading and thus to less untwinning and thus to a smaller pseudoelastic-like hysteresis. © 2010, Elsevier Ltd.