Residual strain scanning of alumina-based ceramic composites by neutron diffraction

Residual strain profiles were measured by neutron diffraction in alumina-aluminum titanate ceramic composites sintered at two different temperatures, namely 1450 and 1550°C. The results show that irrespective of the direction and the sintering temperature, the obtained profiles are almost flat, with very similar results for both temperatures. In addition, the results demonstrate that the alumina is in compression whereas the aluminium titanate is subjected to tensile residual stresses.


Introduction
Ceramic composites have been investigated during decades to obtain improved mechanical performance. Some ceramic composites (Alumina Al 2 O 3 -aluminium titanate Al 2 TiO 5 composites) offer improved flaw tolerance and toughness [1][2][3][4], which is thought to be due to thermally-induced residual stresses. The average crystallographic thermal expansion of aluminium titanate is slightly higher than that of alumina (α A25-1000ºC = 8.7·10 -6 ºC -1 , α AT25-1000ºC = 9.7·10 -6 ºC -1 ) and it is highly anisotropic. As a consequence, in an alumina-aluminium titanate composite, high tensile and/or compressive residual stresses are expected to develop among grains, depending on the particular relative orientations. These stresses might lead to a wide variety of possible toughening mechanisms: crack deflection, crack branching, crack bridging and microcracking. Therefore, the characteristics of the toughness curve and, as a consequence, the flaw tolerance of alumina-aluminium titanate composites may be strongly modified by appropriately altering the microstructure: grain size and aluminium titanate content.
In this work, residual strain profiles were measured by neutron diffraction in 60 vol.% Al 2 O 3 +40 vol.% Al 2 TiO 5 ceramic composites (from now on A-AT40) prepared by slip casting and sintered at two different temperatures, namely 1450 ºC and 1550 ºC.

Results and discussion
The residual strain profiles along the sample thickness are depicted in Fig. 2, for the two sintering temperatures, namely 1450ºC (Fig. 2a) and 1550ºC (Fig. 2b). The strain results for both sintering temperatures are very similar. This suggests that an increase of sintering temperature from 1450ºC to 1550ºC has no clear effect on residual strains for alumina-aluminium titanate monolithic composites.
Linear elasticity was employed to calculate residual stresses from residual strains [10]. The diffraction elastic constants of the alumina phase were calculated by a Kröner average for the (116) reflection (E A,116 = 407 GPa, ν A,116 = 0.21), from the single crystal data published in the literature [11]. Since there are no reliable plane-specific diffraction elastic constants available for Al 2 TiO 5 , bulk elastic constants (E AT = 155 GPa, ν AT = 0.33), were used in the present work [12]. For the residual stress calculation, the (153) reflection of Al 2 TiO 5 was chosen, because it was more intense and narrower than the (062) reflection.
Residual stresses profiles for the sample sintered at 1450ºC are shown in Fig. 3. Stresses are not reported for the sample sintered at 1550ºC because there are not enough measurements in the parallel direction. It can be seen that the stress profiles are almost flat, with tensile residual stresses in Al 2 TiO 5 particles (average around 500 MPa) and compressive ones in the Al 2 O 3 matrix (average around -160 MPa). The differences between normal and parallel stresses lie within experimental error in all cases. This suggests a hydrostatic residual stress state in both phases, which is consistent with the fabrication process.
Consequently, the calculated phase stresses should verify the equilibrium condition, i.e. negligible macro residual stresses [10]: where σ M is the macro residual stress, f A and f T are the volume fractions of Al 2 O 3 and Al 2 TiO 5 , respectively, and σ A and σ AT are the corresponding phase stresses. From Fig. 3 it can be concluded that the calculated residual stresses approximately verify the equilibrium condition (Eq. 2).

Conclusions
Residual strain scanning using neutron diffraction was used to calculate the residual stress profile in the alumina and aluminium titanate phases of Al 2 O 3 +40 vol. % Al 2 TiO 5 ceramic composites. The results show that alumina is under compression (around -160 MPa) and aluminum titanate is under tension (around 500 MPa). The residual stress state is hydrostatic in both phases. A change in the sintering temperature from 1450ºC to 1550ºC does not have a remarkable influence on residual strains. The calculated macro residual stresses are negligible for the sample sintered at 1450ºC.

Acknowledgements
This work was supported by the project BIA2011-26486. The authors wish to thank the EPSRC and the ILL for beamtime in SALSA.