THERMAL-DECOMPOSITION OF MECHANICALLY ALLOYED NANOCRYSTALLINE FCC FE60CU40

A ferromagnetic and supersaturated fee Fe60Cu40 solid solution was pre pared by mechanical alloying (MA). The phase transformations of the as -milled Fe60Cu40 powder upon heating to 1400 degrees C and subsequentl y cooling to room temperature were characterized by differential therm al analysis (DTA) and thermal magnetic measurement. The fee Fe60Cu40 s olid solution decomposes into alpha-Fe(Cu) + gamma-Fe(Cu) + Cu(Fe) upo n heating from 300 to 460 degrees C, and on further heating, alpha-Fe( Cu) transforms to gamma-Fe(Cu) at 640 --> 760 degrees C; during coolin g, the reverse transformation occurs from 800 --> 640 degrees C (obtai ned from thermomagnetic measurement) or from 700 --> 622 degrees C (ob tained from DTA). The gamma reversible arrow alpha transformation in m echanically alloyed Fe60Cu40 nanocrystalline occurs in a wide temperat ure range; the transformation temperature is higher than that of the m artensite transformation in as-cast Fe-Cu alloys, but is much lower th an that of the allotropic transformation of pure Fe. These differences may be caused by the different fabrication process, the nonequilibriu m microstructure of MA, as well as the inhomogeneous grain size in alp ha-Fe(Cu). High resolution transmission electron microscope (HRTEM) ob servations carried out in the specimen after the DTA run show that N-W or K-S orientation relationships exist between alpha-Fe(Cu) and Cu(Fe ), which also represent the orientation relationship between alpha-Fe( Cu) and gamma-Fe(Cu) due to excellent coherency between gamma-Fe(Cu) a nd Cu(Fe). The grain size of the alpha-Fe(Cu) is inhomogeneous and var ies from 50-600 mm. Energy dispersive x-ray spectroscopy (EDXS) result shows that the Cu content in these alpha-Fe(Cu) grains reaches as hig h as 9.5 at. % even after DTA heating to 1400 degrees C, which is even higher than the maximum solubility of Cu in gamma-Fe above 1094 degre es C. This may be caused by the small grain size of alpha-Fe(Cu).