Communications of the American Ceramic Societv Vol. 86. No, I consistent with the crystallography, since two perpendicular planes of the tetragonal system are crystallographic Another point of debate was whether a grain would transform all at once. due to high-speed transformation, or if the martensitic plates would progressively invade the grain. Some TEM observa- ions.of partially transformed grain have already been reported However, due to the intrinsic nature of TEM, the observed samples might be considered as being in a modified physical and chemical ing grains, neither on their side nor above or below them. Thus, the extrapolation to massive samples might be questionable. This henomenon is reported here for the first time on the surface, by means of direct observations, as shown in Fig 3, where the first stage of growth of some monoclinic plates is observed initially at 500nm the grain boundary and propagated later on to the adjacent part of the grain. This clearly proves that the transformation of a single grain does not occur instantaneously, completing previous TEM Fig. 2. AFM height image of a grain(vertical scale 80 nm), Martensitic observations. It was also expected from calculations and stress variants crossing at right angles are observed. Arrows indicale the junction analysis , that the transformation would start at a triple junction of grain boundaries. due to higher residual stresses, though no observations have been reported so far. This is observed in Fig. 3 Further statistical analysis will be performed to confirm this tions of the martensitic relief for each grain. The image of Fig 2n Finally, several theoretical studies have shown the propagation observations here clearly show the apparition of different orienta- echanism of martensitic plates from one grain to another one. was acquired on a single grain and at very low scanning speed considering that high local stresses appear at the junction of the re.The very regular martensitic laths due to shear strain accommodation. Thus. the provide maximum definition of the pictu and different variants apparition of a single martensitic variant within a grain might of various orientations and sizes are present. Though the grain is provoke the transformation of another variant of the neighboring not very large, several laths within just a grain appeared during the grain, providing enough stresses-related energy to overcome the aging treatment. Moreover, it is worth noticing that the three main nucleation barrier. Considering this mechanism, the orientation of variant pairs are intersecting at a right angle. This observation is most of the transformed laths is different from that of the initial 300nm Fig. 3. AFM deflection images at four different aging times (20 25 30, and 40) min at 383 K). The growth and progressive invasion (arrows)of martensitic variants within a single grain are clear. DO nm Fig. 4. AFM deflection image. A detailed zone is extracted on the right, showing a transgranular martensitic variant