S. Deville et al. Acta Materialia 52(2004)5709-5721 2um pop-out(arrows). 20 h later(right, height image). The a s between three grains(denoted a, b and c), 3D and height image(left and middle)and grain Fig 8. Formation of microcracks at the grain boundar ows on the first micrograph represent the approximate orientation of the a, and b, axis of the two adjacent grains. Arrows on the other micrographs indicate the microcracks location. Grains a and b have both the cr axis close the free surface normal, while grain c is differently oriented the fourfold symmetry of surface relief, is represented by tion induced relief features. By using the analysis of the arrows on the first micrograph of Fig. 8 the influence of grain boundaries, the presence of micro- At the various interfaces of theses zones, microcracks cracks and the approximate orientation derived from the are revealed by AFM observations. This is illustrated fourfold symmetry, it possible describing the grain the middle micrograph of Fig. 8, in height mode; the boundaries paths(dashed lines ). Three grains(1, 4 and contrast steps reflect the path of microcracks(arrows). 5) presenting a close orientation(c, axis close to free sur If further aging treatment is performed, microcracking face normal) are observed in the middle part. Other is so extensive that grain pop-out occurs, and some part grains (2, 3 and 6) with different orientations are ob- of the surface are taken away, as shown on the right- served in the surroundings. Microcracks are observed hand side micrograph of the figure(arrows), where the at the locations of orientation misfit. It is also worth remaining holes are easily observed. Holes are located noticing grain pop-out occurred where the misfit was where important microcracking was previously the greatest, i.e. between grains 3 and 4 or 4 and 6 observed, and between grains having the largest diori- Grains 3 and 6 do not show any evidence of a fourfold entation relationships symmetry, so that their cr axis must be away from the These observations allow explaining the whole proc free surface normal, and therefore very different of that ess of microcracks formation: (a) transformation occurs in two adjacent grains, but having different crystallo- graphic orientations (b) variants grow until the grain boundary is reached(c) microcracks are formed due to ransformation accommodation misfit at the grain boundary( d)grain pop-out occurs as a consequence of extensive microcracking Finally, if the two adjacent grains present a crystallo- graphic orientation nearly identical, it is possible observ ing transgranular variants running through the grai boundary. This has already been demonstrated in 3Y TZP [16], and is also probably the case here in Fig. 9 for Ce-TZP. The orientation misfit leads to an interrup- tion of the symmetry in the middle of the transformed parts. The estimated grain orientations are given in the figure, along with the grain boundary location(dashed line). The occurrence of low-disorientation grain bound aries is fairly common in ceramics, so that it is a plausi 2 ble explanation for the relief features observed in Fig. 9 All these observations can be used to interpret the Fig. 9. Probable grain boundary of low disorientation, leading to the ransformation behavior at a larger scale, as shown in orientation of the a, and b, axis of the two adjacent grains is plotted on Fig. 10. A partially transformed zone of larger size the micrograph, along with the position of the grain boundary(dashed hown in the figure, with various different transformathe fourfold symmetry of surface relief, is represented by the arrows on the first micrograph of Fig. 8. At the various interfaces of theses zones, microcracks are revealed by AFM observations. This is illustrated in the middle micrograph of Fig. 8, in height mode; the contrast steps reflect the path of microcracks (arrows). If further aging treatment is performed, microcracking is so extensive that grain pop-out occurs, and some part of the surface are taken away, as shown on the right￾hand side micrograph of the figure (arrows), where the remaining holes are easily observed. Holes are located where important microcracking was previously observed, and between grains having the largest disori￾entation relationships. These observations allow explaining the whole proc￾ess of microcracks formation: (a) transformation occurs in two adjacent grains, but having different crystallo￾graphic orientations (b) variants grow until the grain boundary is reached (c) microcracks are formed due to transformation accommodation misfit at the grain boundary (d) grain pop-out occurs as a consequence of extensive microcracking. Finally, if the two adjacent grains present a crystallo￾graphic orientation nearly identical, it is possible observ￾ing transgranular variants running through the grain boundary. This has already been demonstrated in 3Y– TZP [16], and is also probably the case here in Fig. 9 for Ce–TZP. The orientation misfit leads to an interrup￾tion of the symmetry in the middle of the transformed parts. The estimated grain orientations are given in the figure, along with the grain boundary location (dashed line). The occurrence of low-disorientation grain bound￾aries is fairly common in ceramics, so that it is a plausi￾ble explanation for the relief features observed in Fig. 9. All these observations can be used to interpret the transformation behavior at a larger scale, as shown in Fig. 10. A partially transformed zone of larger size is shown in the figure, with various different transforma￾tion induced relief features. By using the analysis of the influence of grain boundaries, the presence of micro￾cracks and the approximate orientation derived from the fourfold symmetry, it possible describing the grain boundaries paths (dashed lines). Three grains (1, 4 and 5) presenting a close orientation (ct axis close to free sur￾face normal) are observed in the middle part. Other grains (2, 3 and 6) with different orientations are ob￾served in the surroundings. Microcracks are observed at the locations of orientation misfit. It is also worth noticing grain pop-out occurred where the misfit was the greatest, i.e. between grains 3 and 4 or 4 and 6. Grains 3 and 6 do not show any evidence of a fourfold symmetry, so that their ct axis must be away from the free surface normal, and therefore very different of that Fig. 8. Formation of microcracks at the grain boundaries between three grains (denoted a, b and c), 3D and height image (left and middle) and grain pop-out (arrows), 20 h later (right, height image). The arrows on the first micrograph represent the approximate orientation of the at and bt axis of the two adjacent grains. Arrows on the other micrographs indicate the microcracks location. Grains a and b have both the ct axis close the free surface normal, while grain c is differently oriented. Fig. 9. Probable grain boundary of low disorientation, leading to the observed disrupted relief of transgranular variants. The approximate orientation of the at and bt axis of the two adjacent grains is plotted on the micrograph, along with the position of the grain boundary (dashed line). 5716 S. Deville et al. / Acta Materialia 52 (2004) 5709–5721