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The elastic and mechanical behavior, from room temperature up to 13OO0C, of stoichiometric polycrystalline magnesium aluminum spinel is studied. Elastic modulus, fracture toughness, and modulus of rupture measurements and observations of polished and fracture surfaces have been performed. Two well-differentiated regions of fracture behavior as a function of temperature have been found. In the low-temperature region, this material behaves elastically, whereas in the high-temperature (>80OoC) region, plastic phenomena take place.

The transformation β→α in Mg‐substituted Ca3(PO4)2 was studied. The results obtained showed that, contrary to common belief, there is, in the system Mg3(PO4)2–Ca3(PO4)2, a binary phase field where β+α‐Ca3(PO4)2 solid solutions coexist. This binary field lies between the single‐phase fields of β‐ and α‐Ca3(PO4)2 solid solution in the Ca3(PO4)2‐rich zone of the mentioned system. In the light of the results and the Palatnik–Landau's Contact Rule of Phase Regions, a corrected phase equilibrium diagram has been proposed. The practical implications of these findings with regard to the synthesis of pure α‐ and β‐ Mg‐substituted Ca3(PO4)2 powders and to the sintering of related bioceramics with improved mechanical properties are pointed out.

NMR study of mullite precursors has shown that local arrangement of A1 in samples synthesized by spray pyrolysis differs considerably from the one adopted by samples obtained by polymeric or colloidal routes. Aluminum is tetraand pentahedrally coordinated in the first type of samples but is tetra-and octahedrally coordinated in the second ones. Segregation of S O z and AIz03 is directly produced in colloidal preparation; however, this phenomenon occurs only in polymeric gels when they are heated between 980" and 1100°C. In polymeric samples, thermal treatment at -980°C produces the formation of y-A1203. A similar treatment in spray-pyrolized powders gives directly 3: 2 mullite. From these results, exothermic and expansive effects detected at -980°C were ascribed to changes in coordination of Al produced during the atomic rearrangement that accompanies formation of these two phases (y-A1203 or mullite). Above 1200"C, incorporation of Si in the Al-rich phase induces the formation of 3:2 mullite in polynieric and colloidal samples. [

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