MECHANICAL ALLOYING OF IMMISCIBLE CU(70)TM(30) ALLOYS (TM=FE,CO)

Four series of copper-based alloys with compositions Cu70Co30, Cu70Fe30Cu70Co15Fe15 and Cu-70(CoFe)(30) were synthesized by mechanical alloying (MA). The interest in these materials is as parents of alloys which will show the giant magnet resistance (GMR) effect. The structural evolution was studied as a function of the MA time by means of Mossbauer spectroscopy, X-ray and neutron diffraction techniques and differential scanning calorimetry (DSC). In spite of the positive heat of mixing of the alloy constituents, the complete dissolution of the minority phases (iron and/or cobalt) into the copper f.c.c. lattice was observed for each specimen after 16 h of MA. This alloying was accompanied by changes in lattice parameter \Delta a(0)/a(0)\ of up to 0.5% with respect to pure copper. Analysis of the crystallization products arising from DSC treatments up to 600 degrees C revealed a separation into copper f.c.c. and iron-based b.c.c, phases. In the case of the ternary Cu-70(FeCo)(30) alloys, the FeCo intermetallic compound was obtained by heating both alloys which originated as ternaries and also as pseudo-binaries based on copper plus the intermetallic phase. Mossbauer spectra of the annealed Cu70Fe30 specimens also revealed a non-magnetic phase of iron with a f.c.c. environment, which was probably stabilized by the presence of copper and small quantities of nitrogen.