Magnetism and structure of Fe-Cu binary solid solutions obtained by high-energy ball milling.

Magnetism and structure of Fe-Cu binary solid solutions obtained by high-energy ball milling.

Martensitic phase transformation is found in various metals, alloys, ceramics and even biological systems. This paper reports on the Fe segregated anomalous alpha-gamma martensitic transformation observed in Fe(x)Cu(100-x) binary solid solutions (x = 25 and 50) after subsequent heating-cooling processes. A characteristic observable feature of a martensitic transformation is the microstructure it produces, thermal hysteresis, evolution and metastability. We have analysed the microstructure using neutron thermo-diffraction experiments up to 1100 K. The structural changes of the Fe segregated phase, from bcc to fcc crystal structures, have been followed in detail. The most relevant feature is that the onset of martensite transformation is observed, on heating, more than 100 K below the expected temperature for pure bec-Fe (1183 K), while the reverse transformation occurs below 900 K on cooling. This anomalous behaviour for the alpha-gamma transformation depends on sample composition, being more important for low Fe contents. Besides that, magnetisation vs. temperature measurements show a clear correlation with structure changes, thus exhibiting thermal hysteresis on heating-cooling cycles. Moreover, a magnetisation enhancement above 550K is observed on heating, this fact can be attributed to thermal-induced ferromagnetism on isolated gamma-Fe precipitates via strong magheto-volume coupling. (c) 2006 Elsevier B.V. All rights reserved.