Thermal decomposition of mechanically alloyed nanocrystalline fcc Fe₆₀Cu₄₀

Abstract

A ferromagnetic and supersaturated fcc Fe₆0Cu₄0 solid solution was prepared by mechanical alloying (MA). The phase transformations of the as-milled Fe₆0Cu₄0 powder upon heating to 1400 °C and subsequently cooling to room temperature were characterized by differential thermal analysis (DTA) and thermal magnetic measurement. The fcc Fe60Cu40 solid solution decomposes into a-Fe(Cu) + y-Fe(Cu) + Cu(Fe) upon heating from 300 to 460 °C, and on further heating, a-Fe(Cu) transforms to y-Fe(Cu) at 640 → 760 °C; during cooling, the reverse transformation occurs from 800 → 640 °C (obtained from thermomagnetic measurement) or from 700 → 622 °C (obtained from DTA). The γ ⇄ a transformation in mechanically alloyed Fe₆₀Cu₄₀ nanocrystalline occurs in a wide temperature range; the transformation temperature is higher than that of the martensite transformation in as-cast Fe-Cu alloys, but is much lower than that of the allotropic transformation of pure Fe. These differences may be caused by the different fabrication process, the nonequilibrium microstructure of MA, as well as the inhomogeneous grain size in a-Fe(Cu). High resolution transmission electron microscope (HRTEM) observations carried out in the specimen after the DTA run show that N-W or K-S orientation relationships exist between a-Fe(Cu) and Cu(Fe), which also represent the orientation relationship between a-Fe(Cu) and y-Fe(Cu) due to excellent coherency between y-Fe(Cu) and Cu(Fe). The grain size of the a-Fe(Cu) is inhomogeneous and varies from 50-600 mm. Energy dispersive x-ray spectroscopy (EDXS) result shows that the Cu content in these a-Fe(Cu) grains reaches as high as 9.5 at. % even after DTA heating to 1400 °C, which is even higher than the maximum solubility of Cu in y-Fe above 1094 °C. This may be caused by the small grain size of a-Fe(Cu).