Materials science and engineering of hydrogen-induced shear transformation in hydrogen absorbing materials

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The activation of deformation twinning is indispensable to promote the workability of Mg and its alloys, but their activation has not been well understood. The activation of deformation twinning is known to be difficult when the grain size is reduced down to the orders of nano-meters. On the assumption of the existence of critical volume for twin nucleation, we have investigated nucleus sizes for deformation twinning through the determination of the critical volume for the transition in deformation modes from twinning to slip. When single crystals of Mg of the size of micron-meter order are compressed along the a-axis direction, twinning on {11-21} is observed. If the specimen size is further reduced down to sub-micron orders, slip on {11-23} id observed to operate instead of twinning on {11-21}. The similar behavior is observed in LPSO single crystals of the Mg-Zn-Y system. The critical volume for twinning in these alloys is thus determined to be about 1 micrometer.