PHASE-DIAGRAMS OF THE MN-AL-O SYSTEM

A high temperature phase equilibrium diagram of the Mn-Al-O system in air has been refined and phase diagrams of unstable states occurring u pon cooling the system from an equilibrium state to room temperature i n air by different methods (quenching in water, quenching in air, cool ing at the rate of 25 K/h) have been constructed with the use of radio graphy methods. It is shown that cooling at any rate does not preserve the high temperature phase composition of the system and is accompani ed by one (or a combination) of the following phenomena: (1) a tetrago nal distortion of the spinel lattice; (2) the coalescence of a tetrago nal and cubic spinel phase into one phase with the hausmannite structu re; (3) decomposition of a homogeneous spinel with the formation of tw o spinel-type phases. Projections of the phase equilibrium diagram of the Mn-Al-O system on the composition triangle and the 'oxygen pressur e-composition' plane have been constructed. Correlating the data obtai ned within the framework of a single P-T-x diagram has revealed a numb er of phase equilibria in the system which have not been determined ex perimentally.