Grains Ain't Misbehaving or Going Wild? The Initiation of Abnormal Grain Growth!

Unconventional white-beam Laue synchrotron X-ray diffraction has been used on rolled magnesium alloys during in-situ heating experiments. 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 towards 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 puzzle stone leading to anomalous grain growth, long postulated in literature. The sub-second time resolution allows, for the first time, the determination of grain reorientation rates and pathways. This method can be easily adapted for higher time resolutions and faster processes.