STABLE AND METASTABLE STATES IN THE AL-ALCO BINARY-SYSTEM

A supercooling diagram for Al-Co alloys from 70 to 85 at.% Al is deter mined for cooling rates from 2 to 10 K/min. In addition, a metastable phase diagram for the rapid cooling regime (splat cooling) is construc ted. With the help of these diagrams, the observed non-stable and meta stable states in Al-Co alloys are discussed. Non-stable states were ob served for as-cast alloys with compositions between 75.75 and nearly 7 7 at.% Al after annealing at temperatures from 1070 to 1110 degrees C, these states are composed of three phases, whereas one of these phase s is liquid. The crystallization from the liquid of these samples afte r quenching into water follows the metastable phase diagram. The phase transformations from M-Co4Al13 and O-Co4Al13 into the high-temperatur e phase Co4Al13(h) during heating is described with the aid of microgr aphs. By quenching, the high-temperature phase cannot be stabilized at room temperature; a metastable Co4Al13(h(m)) (=Y-m, m = metastable) p hase is retained by this procedure. At room temperature, only the phas es Al, Co2Al9, O-Co4Al13, M-Co4Al13, CoA(13), Co2Al5 and CoAl exist in equilibrium. For the construction of the equilibrium phase diagram of the Al-AlCo system, the heat-treatment approach is indispensible.