MICROSTRUCTURAL EVOLUTION OF FINEMET TYPE ALLOYS WITH CHROMIUM - AN ELECTRON-MICROSCOPY STUDY

Microstructural changes in Fe73.5-xCrxCu1Nb3Si13.5B9 (0 less than or e qual to X less than or equal to 5) alloys with thermal treatment were studied by electron microscopy. In a first stage, around 800 K, an Fe( Si) nanocrystalline phase is formed in the amorphous residual matrix. Crystallization onset is en ha need with the Cr content of the alloy. In a second stage, arou nd 950 K, full crystallization of the samples leads to the formation of a body centred cubic (b.c.c.) boride-type un known crystal phase with a lattice parameter of a=1.52 nm, and recryst allization of the previous Fe(Si) nanophase also occurs. No qualitativ e differences were found between dynamic and isothermal crystallizatio n. The size effect for thin samples is limited to a lowering of crysta llization temperatures. For isothermal nanocrystallization in the temp erature range 775-900 K, the mean grain size of the nanocrystals incre ases for short annealing times to stabilize at a constant value of abo ut 10-15 nm for long annealing times. The stabilized grain size increa ses with increasing annealing temperature and slightly decreases with the Cr content of the alloy.