Acta Materialia 54(2006)1289-I 0. 25Y-TZP, indicating uncompleted transformation, temperature aided the reverse transformation from mono- whereas no monoclinic phase is found in 8Ce-050Y-TZP clinic to tetragonal. It is reasonable to believe that as a con- at ambient temperature Under illumination by the electron sequence, the induced monoclinic martensites, such as laths beam, the martensitic transformation can be induced in A and B in Fig 3(c), are generated by local thermal stress both 8Ce-05Y-TZP and 8Ce-025Y-TZP. For 8Ce- The continuous growth of martensite plates starting at 0. 25Y-TZP, thermal stress-induced t/m interfaces were the edge is illustrated in Figs. 3(cHe). When focusing the produced and annihilated reversibly. However, the thermal electron beam, the first monoclinic lath A nucleated at stress-induced transformation in 8Ce-050Y-TZP showed a the grain boundary and grew across the grain. The second burst-like characteristic, which was also observed in other lath b then formed rapidly and grew to the same size as the systems [7,8]. The transformed martensite could not move first. The whole process was too rapid to obtain an image irrespective of the intensity of the electron beam. The in between. A further increase in the beam intensity athermal transformation was found to be burst-like for resulted in the formation of the martensite lath c and the measurement of the M, temperatures by dilatomets mw speed(Fig. 3(d). As the transformation proceeded, thickening of the formed laths A and B at a relatively of thermal stress-induced martensite in an isolated grain of martensite laths with different orientations were induced 8Ce-0.25Y-TZP caused by the illumination with the elec- and grew until the grain was eventually full of martensite tron beam. Figs. 3(a) and (b)show that some thermally ( Fig. 3(e)). Above TEM observation indicated that the induced monoclinic martensite is embedded in the tetrago- induced monoclinic lath usually nucleated at the grain nal matrix at room temperature. During illumination, an boundary, resided with high stress. Similar results were also elevated temperature on the surface of the sample and a observed in binary systems [6-8] thermally induced stress due to the thermal expansion After the illumination had been turned off for severa anisotropy [27, 28] are expected to occur. The elevated minutes, the twin structure in the grain shown in c Fig. 3. In situ TEM images for stress-induced t- m martensitic transformation in &Ce-025Y-TZP.(a) Bright-field and (b) dark-field images for thermally induced martensite; (cHe) the formation and growth of the stress-induced martensite at the expense of the thermally induced martensite under continuous electron beam illumination0.25Y-TZP, indicating uncompleted transformation, whereas no monoclinic phase is found in 8Ce–0.50Y-TZP at ambient temperature. Under illumination by the electron beam, the martensitic transformation can be induced in both 8Ce–0.5Y-TZP and 8Ce–0.25Y-TZP. For 8Ce– 0.25Y-TZP, thermal stress-induced t/m interfaces were produced and annihilated reversibly. However, the thermal stress-induced transformation in 8Ce–0.50Y-TZP showed a burst-like characteristic, which was also observed in other systems [7,8]. The transformed martensite could not move irrespective of the intensity of the electron beam. The athermal transformation was found to be burst-like for the measurement of the Ms temperatures by dilatometry. The sequential images in Fig. 3 demonstrate the growth of thermal stress-induced martensite in an isolated grain of 8Ce–0.25Y-TZP caused by the illumination with the elec￾tron beam. Figs. 3(a) and (b) show that some thermally induced monoclinic martensite is embedded in the tetrago￾nal matrix at room temperature. During illumination, an elevated temperature on the surface of the sample and a thermally induced stress due to the thermal expansion anisotropy [27,28] are expected to occur. The elevated temperature aided the reverse transformation from mono￾clinic to tetragonal. It is reasonable to believe that as a con￾sequence, the induced monoclinic martensites, such as laths A and B in Fig. 3(c), are generated by local thermal stress. The continuous growth of martensite plates starting at the edge is illustrated in Figs. 3(c)–(e). When focusing the electron beam, the first monoclinic lath A nucleated at the grain boundary and grew across the grain. The second lath B then formed rapidly and grew to the same size as the first. The whole process was too rapid to obtain an image in between. A further increase in the beam intensity resulted in the formation of the martensite lath C and thickening of the formed laths A and B at a relatively low speed (Fig. 3(d)). As the transformation proceeded, martensite laths with different orientations were induced and grew until the grain was eventually full of martensite (Fig. 3(e)). Above TEM observation indicated that the induced monoclinic lath usually nucleated at the grain boundary, resided with high stress. Similar results were also observed in binary systems [6–8]. After the illumination had been turned off for several minutes, the twin structure in the grain shown in Fig. 3. In situ TEM images for stress-induced t ! m martensitic transformation in 8Ce–0.25Y-TZP. (a) Bright-field and (b) dark-field images for thermally induced martensite; (c)–(e) the formation and growth of the stress-induced martensite at the expense of the thermally induced martensite under continuous electron beam illumination. 1292 Y.L. Zhang et al. / Acta Materialia 54 (2006) 1289–1295