Abnormal Grain Growth: A Spontaneous Activation of Competing Grain Rotation
Advanced Engineering Materials
Unconventional white-beam Laue synchrotron X-ray diffraction is used on fine-grained, as-rolled magnesium alloy during an in situ heating experiment. At high temperatures, reflections of single grains are superimposed on the halo stemming from matrix grains. Some unique grain reflections spontaneously move, indicating grain rotations in response to torque expedited at grain boundaries. When a grain boundary spontaneously activates, it can begin to rotate, allowing diffusive mass transport and activating the boundaries of its other neighbors. Now the given grain can freely rotate toward coalescence; however, the multitude of grain boundaries compete in torque orientation and magnitude, resulting in zigzag rotations. After coalescence, the larger grain is still active and continues this scenario of growth, while the majority of the matrix grains remain inactive. The first-time experimental observation of such erratic grain behavior supplies the missing puzzlestone leading to anomalous grain growth, long postulated in literature. The method of white beam Laue diffraction on fine-grained polycrystalline materials delivers a novel experimental method to study the erratic behavior of grain reorientation, as requested long ago by the scientific community. Such findings apply to wide ranges of materials undergoing grain growth, creep, and superplasticity, including those in metal engineering, ceramics, and geophysical disciplines.
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