1222 Journal of the American Ceramic Society-Leoni et al. Vol 91. No 4 600 400 AM40 AM20 AZO AZ AZ40 20 mm Fig 4. Comparison of the length of different strips after sintering. The tarting size of the green sample (It The presence of residual porosity strongly influences the irconia content(wo‰%) macroscopic elastic properties of the AM set of homogeneous composites(cf Table ID). As a consequence, as shown in Fig. 5 elastic moduli for alumina-mullite composites are far from the theoretical limits(Voigt-Reuss and Hashin-Shtrikman 2.37)for an equivalent fully dense homogeneous specimen. Nevertheless porosity should not influence the strain measurements carried 0 out in the present work as they involve the interaction stresses among crystalline grains, keeping in mind that diffraction is in -200·9 sensitive to voids. This does not mean that porosity plays no role in determining the actual stress level state in the grains Figure 5 also reports the elastic modulus trend for the al- mullite content (vols s250 nia and or alumina, cont ues. This aspect is not consid nor by viable analytical alter ues for zirconia are approached when a AT of ca 700 K is used An analogous reasoning is valid for the AM 100 composites. Again, the trend is matched for the guest phase and mullite content(vol%) the absolute values are lower than the expectations (cf. Fig. 3) Similitude between Az and am composites can also be evi- denced king at the sintering behavior. a good sintering behavior of homogeneous laminates has already been reported orks 13,15 but some further information can be useful here. Figure 4 shows the final length of several sintered tapes of homogeneous composition compared with the starting length(100 mm)of a green sample Linear shrinkage values after sintering are included in the range between 12% and 20%, the exact values being reported in Table Il. Although differences 200 among various compositions do not appear to be very large, the corresponding final densities can be very dissimilar because the volume reduction is approximately three times the linear con- traction: this is confirmed by the density results proposed in Table ii where elastic modulus and poissons ratio are also from independent measurements previously performed One can observe that in alumina-mullite composites a resid- ual porosity is always present, its amount increasing with mullite zirconia content(vol%) content. Such behavior can be accounted for by the relatively Fig. 5. Avert 'expermental values are represented by points.The lower sintering temperature used in the present work(1600C)in macroscopic Youngs modulus(E) for(a) AM and(b mparison with higher temperature schedules usually used for Voigt and Reuss limits (dotted lines)and upper and lower Hashin mullite consolidation .3 bounds(continuous lines) for a fully dense composite are also shown.zirconia and/or alumina, contributing to lower actual stress val￾ues. This aspect is not considered by the Kreher–Pompe model nor by viable analytical alternatives to it. The experimental val￾ues for zirconia are approached when a DT of ca. 700 K is used in Eqs. (2) and (3). An analogous reasoning is valid for the AM homogeneous composites. Again, the trend is matched for the guest phase and the absolute values are lower than the expectations (cf. Fig. 3). Similitude between AZ and AM composites can also be evi￾denced by looking at the sintering behavior. A good sintering behavior of homogeneous laminates has already been reported in previous works,13,15 but some further information can be useful here. Figure 4 shows the final length of several sintered tapes of homogeneous composition compared with the starting length (100 mm) of a green sample. Linear shrinkage values after sintering are included in the range between 12% and 20%, the exact values being reported in Table II. Although differences among various compositions do not appear to be very large, the corresponding final densities can be very dissimilar because the volume reduction is approximately three times the linear con￾traction: this is confirmed by the density results proposed in Table II where elastic modulus and Poisson’s ratio are also reported as from independent measurements previously performed.13–17 One can observe that in alumina–mullite composites a resid￾ual porosity is always present, its amount increasing with mullite content. Such behavior can be accounted for by the relatively lower sintering temperature used in the present work (16001C) in comparison with higher temperature schedules usually used for mullite consolidation.35,36 The presence of residual porosity strongly influences the macroscopic elastic properties of the AM set of homogeneous composites (cf. Table II). As a consequence, as shown in Fig. 5, elastic moduli for alumina–mullite composites are far from the theoretical limits (Voigt–Reuss and Hashin–Shtrikman32,37) for an equivalent fully dense homogeneous specimen. Nevertheless, porosity should not influence the strain measurements carried out in the present work as they involve the interaction stresses among crystalline grains, keeping in mind that diffraction is in￾sensitive to voids. This does not mean that porosity plays no role in determining the actual stress level state in the grains. Figure 5 also reports the elastic modulus trend for the al￾umina–zirconia homogeneous laminates. Data are well within 20 mm AZ40 AZ20 AZ0 AM20 AM40 green Fig. 4. Comparison of the length of different strips after sintering. The starting size of the green sample (100 mm) is shown on the top. 0 10 20 30 40 0 10 20 30 40 −800 −600 −400 −200 0 200 400 600 σ (MPa) zirconia content (vol%) (a) −800 −600 −400 −200 0 200 400 σ (MPa) mullite content (vol%) (b) Fig. 3. Residual stress in the phases as a function of composition in (a) alumina–zirconia and (b) alumina–mullite composites. Limiting stress values for ideal composites have been calculated according to the Kreher–Pompe model (see text for details). 0 20 40 60 80 100 0 20 40 60 80 100 0 50 100 150 200 250 300 350 400 450 E (GPa) mullite content (vol%) (a) 0 50 100 150 200 250 300 350 400 450 E (GPa) zirconia content (vol%) (b) Fig. 5. Average macroscopic Young’s modulus (E) for (a) AM and (b) AZ composites. Experimental values are represented by points. The Voigt and Reuss limits (dotted lines) and upper and lower Hashin bounds (continuous lines) for a fully dense composite are also shown. 1222 Journal of the American Ceramic Society—Leoni et al. Vol. 91, No. 4