PRECURSOR FLUCTUATIONS OF MARTENSITIC PHASE TRANSITION ON METALLIC SODIUM. H. Abe¹, R. J. Matsuo¹, K. Ohshima¹, M. Imai² and K. Kakurai², Department of Materials Science and Engineering, National Defense Academy, Yokosuka 239, ¹Institute of Applied Physics, University of Tsukuba, Tsukuba 305, ²Institute for Solid State Physics, University of Tokyo, Tokyo 106, Japan

Nucleation and growth process of the first-order phase transition has been studied for a long time. In particular, the nucleation problem of the martensitic phase transition has also been discussed recently. In fact, critical size was estimated as about 15 Å by Kakeshita model: once local fluctuation is over critical size, nucleation occurs. On the other hand, we have already found the incubation time for metallic sodium and In-l1 alloys, where incubation time is a waiting time until the phase transition starts at fixed temperature. Moreover, it is found that this incubation time is originated from the dynamic nucleation process during the non-equilibrium state. Therefore, small angle neutron scattering (SANS-U at JRR-3M) was performed with using metallic sodium single crystals in order to determine its critical size. Metallic sodium is regarded as the best material to study nucleation process, since it has no composition fluctuations which complicates nucleation problem. In addition, Si single crystal windows attached on Al sample container enable it to suppress the background. As the result, we obtain local fluctuation size during 12 hours incubation time. Precursor local fluctuation size found to be about 30 Å. Peak around large q region shows some correlation between each fluctuation due to elastic anisotropy.