Browsing by Author "Barnett, MR"

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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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    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.
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    On the correlation between deformation twinning and Lüders-like deformation in an extruded Mg alloy: in situ neutron diffraction and EPSC.4 modelling
    (Elsevier, 2010-03-15) Muránsky, O; Barnett, MR; Luzin, V; Vogel, SC
    The current work focuses on the yielding and immediate post-yielding deformation of fine-grained and coarse-grained ZM20 Mg alloys obtained by extrusion. Compressive deformations along the extrusion direction, known to be governed by profuse twinning are examined in detail. It is shown that the fine-grained alloy exhibits Lüders-like plateaux suggesting heterogeneous transition from elastic to plastic deformation. This is due to the cooperative twinning of neighbouring grains which is promoted in the fine-grained alloy by the high internal stresses borne by the parent grain families in the vicinity of yielding, and the auto-catalytic nature of twin nucleation. The elasto-plastic response of tested alloys was also simulated using version 4 of the Elasto-Plastic Self-Consistent (EPSC) model. The finite initial fraction (FIF) assumption is employed to account for the stress relaxation related to the twin nucleation process. It is shown that the new EPSC.4 model is superior to its previous version as it enables realistic predictions of the development of elastic lattice strains in variously oriented grain families and the macroscopic stress–strain response of a polycrystalline aggregate undergoing profuse twinning. © 2009, Elsevier Ltd.