F. Xu, S Wen/ Materials Letters 28(1996)401-407 Fig. 5. HREM image showing microtwins in the transformed m-Zro, lattice revealing that each pair of twins was initiated by a dislocation ated from the area with high concentration of stress strain as the growing velocity of the twins is ex polycrystalline materials, usually the interf tremely large, nearly close to the spreading velocity region is the site which accommodates most stress), of a shock wave. Therefore, it is understandable that the critical shear stress required for the formation of the concentration of stress in the sample due to the twins is much greater than that required for the slip irradiation of electron beams is comparatively weak Fig. 6. Lattice image of multiple twins produced under conditions other than electron beam irradiation406 F. Xu, S. Wen/Materials Letters 28 (1996) 401-407 Fig 5. HREM image showing microtwins in the transformed m-ZrO? lattice revealing that each pair of twins was initiated by a disloca Ition. ate d from the area with high concentration of stress (in polycrystalline materials, usually the interface reg ;ion is the site which accommodates most stress), the critical shear stress required for the formation of twi ns is much greater than that required for the slip strain as the growing velocity of the twins is ex￾tremely large, nearly close to the spreading velc xity of a shock wave. Therefore, it is understandable that the concentration of stress in the sample due to I the irradiation of electron beams is comparatively w eak. Fig. 6. Lattice image of multiple twins produced under conditions other than electron beam irradiation