3094 S. Deville et al. /Journal of the European Ceramic Sociery 25(2005)3089-3096 the free surface normal and with a potential junction plane theories requirements and inputs. The first point is the lying in a perpendicular position to the crack propagation di- influence of alloying addition on the potential for trans- rection were preferentially activated(see for example Fig 4). formation toughening. For the 10Ce-TZP samples, a very Less transformed grains are found when their junction plane large propensity for martensitic transformation is found is deviating from the crack direction. The same analysis was The Ms temperature has been reduced close to room tem- performed on the variants in the near surroundings of the perature, so that transformation can be easily stress induced propagated crack, and an average value of 4o was found. The providing a large potential for transformation toughening behavior is thus the same in the secondary bands and in the The toughness measured by double torsion was indeed near crack transformed zones found to be very high, i.e. 18 MPam-1/2. On the other As far as the propensity for transformation is concerned, hand, when the transformation is less easily stress induced two crystallographic features must be considered to explain i.e. 12 mol% CeO samples, the toughness falls down to the observed behavior, i.e. the potential junction plane ori- 8 MPam-I/2. The transformation toughening contribution entation with respect to the stress field and the orientation to toughness becomes negligible, as observed on the aFm relationship of the potential junction plane with the sample micrographs. When the alloying content reaches 16 mol%, surface. If the junction plane is perpendicular to the surface, no transformation at all is observed, and the toughness falls as for example in Fig 3 the large residual shear can potentially down to 4.3/2. The only remaining crack-shielding accommodate all of the strain induced by the transformation mechanism is crack bridging. It is also worth noticing that in the vertical direction. 28 This way of accommodating the since the grain size was the same in all the samples, the mag- strain is much more favorable than when the junction plane nitude of crack bridging can be assumed to be the same for is lying parallel to the surface, which would mean the strain all the samples, so that it does not interfere with the present should be accommodated in lateral directions which is much bservations more difficult, considering the restricting mechanical effect The observed autocatalytic transformed bands ahead of of the matrix. The propensity for transformation is therefore the crack tip can be explained by the PTMC. In fact, the a consequence of the combination of these two parameters transformation induced shear strain can be accommodated though they do not have the same relative importance. Other by the formation of self-accommodating martensitic variant factors such as local residual stress inhomogeneities and mi- pairs. However, residual stress not accommodated by the crostructural defects may also play a role, though of sec- transformation-induced plasticity may be used to trigger ond order; their influence is consequently not discussed here. the transformation of neighboring grains, providing the Comparing this analysis with the observed relief of Fig. 2 orientation relationships of the two grains are energetically it seems that the controlling factor is the relationship of the favorable for the transformation to proceed, as shown junction plane and the applied stress more than the orienta- by previous studies. 10 The formation of these elongated tion to the surface, since among the transformed grains, just transformed zones could thus be explained by the auto- a few present a junction plane perpendicular to the surface. catalytic nature of the transformation of ceria-stabilized a uniaxial stress state seems to be the more favorable zirconia state for the transformation. The possibility of inducing the More important was the observation that transformed transformation by uniaxial compression was first reported grains were always formed by self-accommodating variant by Lankford. This particular point may be understood pairs, implying the presence of large residual local shear af by considering the fact that all of the transformed gra ter transformation completion but very low net shear, if any. present self-accommodating variant pairs. The net shear in- This may actually have major implications as far as the trans duced by the transformation is therefore systematically an- formation toughening theories are concerned. In fact, all the nihilated upon transformation completion. In the case where local investigations of the last twenty years were performed self-accommodation would not be occurring (leading to a by transmission electron microscopy, on thin foils samples large net shear), strain induced transformation should be ac- The microstructural environment of the samples is modified commodated only by volume increase. In this case, applying in particular during the grinding process, and the stress state a compressive stress state would inhibit the transformation. may be modified by the very low thickness of the foils. The When transformation induced strain is accommodated more comparison of such results with macroscopic observations by shear than volume increase, either uniaxial tensile or com- on bulk samples may therefore be questioned. In particular, pressive stress will be favorable and shear stress prevails over all the transformation toughening models developed so far volume increase(opposed to the transformation)in activating demonstrate a net improvement to transformation toughen- the transformation ing when a net shear component is added to the transfor- mation strains 14, 15,30 In the same time. it was believed that only transforming zirconia particles in stable matrices 4. General discussion (e.g. Mg-PSZ, zirconia-toughened alumina, etc . )would exhibit self-accommodating variant pair formation, leading <i The new features brought by this study provide valuable to a reduced net shear, due to the restricting influence of the rmation in regards to the transformation toughening matrix. These twin-related variants were also thought to be a3094 S. Deville et al. / Journal of the European Ceramic Society 25 (2005) 3089–3096 the free surface normal and with a potential junction plane lying in a perpendicular position to the crack propagation di￾rection were preferentially activated (see for example Fig. 4). Less transformed grains are found when their junction plane is deviating from the crack direction. The same analysis was performed on the variants in the near surroundings of the propagated crack, and an average value of 4◦ was found. The behavior is thus the same in the secondary bands and in the near crack transformed zones. As far as the propensity for transformation is concerned, two crystallographic features must be considered to explain the observed behavior, i.e. the potential junction plane ori￾entation with respect to the stress field and the orientation relationship of the potential junction plane with the sample surface. If the junction plane is perpendicular to the surface, as for example in Fig. 3 the large residual shear can potentially accommodate all of the strain induced by the transformation in the vertical direction.28 This way of accommodating the strain is much more favorable than when the junction plane is lying parallel to the surface, which would mean the strain should be accommodated in lateral directions, which is much more difficult, considering the restricting mechanical effect of the matrix. The propensity for transformation is therefore a consequence of the combination of these two parameters, though they do not have the same relative importance. Other factors such as local residual stress inhomogeneities and mi￾crostructural defects may also play a role, though of sec￾ond order; their influence is consequently not discussed here. Comparing this analysis with the observed relief of Fig. 2 it seems that the controlling factor is the relationship of the junction plane and the applied stress more than the orienta￾tion to the surface, since among the transformed grains, just a few present a junction plane perpendicular to the surface. A uniaxial stress state seems to be the more favorable state for the transformation. The possibility of inducing the transformation by uniaxial compression was first reported by Lankford.29 This particular point may be understood by considering the fact that all of the transformed grains present self-accommodating variant pairs. The net shear in￾duced by the transformation is therefore systematically an￾nihilated upon transformation completion. In the case where self-accommodation would not be occurring (leading to a large net shear), strain induced transformation should be ac￾commodated only by volume increase. In this case, applying a compressive stress state would inhibit the transformation. When transformation induced strain is accommodated more by shear than volume increase, either uniaxial tensile or com￾pressive stress will be favorable and shear stress prevails over volume increase (opposed to the transformation) in activating the transformation. 4. General discussion The new features brought by this study provide valuable information in regards to the transformation toughening theories requirements and inputs. The first point is the influence of alloying addition on the potential for trans￾formation toughening. For the 10Ce–TZP samples, a very large propensity for martensitic transformation is found. The Ms temperature has been reduced close to room tem￾perature, so that transformation can be easily stress induced, providing a large potential for transformation toughening. The toughness measured by double torsion was indeed found to be very high, i.e. 18 MPa m−1/2. On the other hand, when the transformation is less easily stress induced, i.e. 12 mol% CeO2 samples, the toughness falls down to 8 MPa m−1/2. The transformation toughening contribution to toughness becomes negligible, as observed on the AFM micrographs. When the alloying content reaches 16 mol%, no transformation at all is observed, and the toughness falls down to 4.3 MPa m−1/2. The only remaining crack-shielding mechanism is crack bridging. It is also worth noticing that since the grain size was the same in all the samples, the mag￾nitude of crack bridging can be assumed to be the same for all the samples, so that it does not interfere with the present observations. The observed autocatalytic transformed bands ahead of the crack tip can be explained by the PTMC. In fact, the transformation induced shear strain can be accommodated by the formation of self-accommodating martensitic variant pairs. However, residual stress not accommodated by the transformation-induced plasticity may be used to trigger the transformation of neighboring grains, providing the orientation relationships of the two grains are energetically favorable for the transformation to proceed, as shown by previous studies.10 The formation of these elongated transformed zones could thus be explained by the auto￾catalytic nature of the transformation of ceria-stabilized zirconia. More important was the observation that transformed grains were always formed by self-accommodating variant pairs, implying the presence of large residual local shear af￾ter transformation completion but very low net shear, if any. This may actually have major implications as far as the trans￾formation toughening theories are concerned. In fact, all the local investigations of the last twenty years were performed by transmission electron microscopy, on thin foils samples. The microstructural environment of the samples is modified, in particular during the grinding process, and the stress state may be modified by the very low thickness of the foils. The comparison of such results with macroscopic observations on bulk samples may therefore be questioned. In particular, all the transformation toughening models developed so far demonstrate a net improvement to transformation toughen￾ing when a net shear component is added to the transfor￾mation strains.14,15,30 In the same time, it was believed31 that only transforming zirconia particles in stable matrices (e.g. Mg–PSZ, zirconia-toughened alumina, etc...) would exhibit self-accommodating variant pair formation, leading to a reduced net shear, due to the restricting influence of the matrix. These twin-related variants were also thought to be a