STRUCTURAL FLUCTUATIONS AND THE KINETICS OF THE MARTENSITIC PHASE TRANSITION. K. Ohshima, Institute of Applied Physics, University of Tsukuba, Tsukuba 305, Japan

The nonequilibrium kinetics of the nucleation and growth in martensitic phase tarnsition have recently been drawing attention. In particular, athermal and isothermal martensitic phase transitions have been investigated from the thermodynamical point of view on nucleation. The amount of athermal martensite has been considered to be a function of temperature only. The isothermal martensite phase transition has been considered not to take place until the temperature is brought down below Ms, the martensitic transition start temperature, which is always below the thermodynamical equilibrium temperature, T0, between the parent and low-temperature phases. On the other hand, the amount of isothermal martensite has been thought to be dependent on both temperature and time, where a waiting time (or an incubation time) is needed until the martensite phase transition starts while the temperature is kept constant. We have already found a characteristic waiting time at fixed temperature above Ms in three different kinds of materials (metallic sodium, In-Tl and Au-Cd alloys) with the use of X-ray and neutron scattering experiments. Strong diffuse scatterings were also observed on the diffractin patterns for the specimens after phase transition. The relationship between the structural fluctuations and the kinetics of martensitic phase transition is presented using the new concept of first order phase transition proposed by Roitburd ( Mater. Sci. Eng., A127, 229, 1990).