The dynamics of extension twins in a single crystal of pure magnesium are studied through uniaxial compression experiments coupled with direct optical imaging. The experiments covered a seven orders of magnitude strain rate, between , covering all rates of engineering applications of magnesium and its alloys. Under uniaxial compression along a direction perpendicular to prismatic planes, only two variants of extension twins are activated, facilitating in-situ tracking of individual twin boundaries throughout the deformation process. Across the entire strain rate range, the twinning transformation evolves in a self-similar pattern of twin nucleation and thickening. Deformation fronts initiate at the edges of the sample and propagate toward the center, creating parallel twins with characteristic thicknesses and distances from each other. The velocity of the deformation fronts scales linearly with the imposed external strain rate and a nearly uniform resolved shear stress is measured over the entire strain rate range. We show that this unique process is attributed to the constant ratio between the nucleation rate and sideways velocity of individual twin boundaries, where both processes are proportional to the imposed strain rate. These findings indicate that, within the large strain rate range investigated in this study, the evolution mechanism of extension twinning is practically rate-independent.

中文翻译：

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单晶镁变形孪晶的速率无关机制


通过单轴压缩实验结合直接光学成像研究了纯镁单晶中延伸孪晶的动力学。实验覆盖了 7 个数量级的应变率，涵盖了镁及其合金工程应用的所有速率。在沿垂直于棱柱面的方向进行单轴压缩时，仅激活两种延伸孪晶变体，从而有助于在整个变形过程中对各个孪晶边界进行原位跟踪。在整个应变率范围内，孪生转变以孪生成核和增厚的自相似模式演变。变形前沿从样品的边缘开始并向中心传播，形成具有特征厚度和彼此距离的平行孪晶。变形前沿的速度与施加的外部应变率成线性比例，并且在整个应变率范围内测量到几乎均匀的解析剪切应力。我们表明，这种独特的过程归因于单个孪晶边界的成核率和侧向速度之间的恒定比率，其中这两个过程都与施加的应变率成正比。这些发现表明，在本研究研究的大应变率范围内，延伸孪晶的演化机制实际上与速率无关。