Effects of Cu addition on plasticity of Fe-6.5Si (wt.%) alloy at temperatures below 550 °C

The plasticity of Fe-6.5Si (wt.%) alloys with additions of 0, 0.3, 0.5 and 0.7 wt% Cu was investigated by tensile tests at intermediate temperatures below 550 degrees C. It was found that the plasticity of Fe-6.5Si (wt.%) alloy increased with Cu addition to up to 0.3 wt%, but decreased with further increase in Cu addition. When the Cu addition was increased to 0.7 wt%, the plasticity of the alloy became lower than that for the alloy without Cu addition. The reduction of cross-sectional thickness of the tested specimens confirmed that the onset temperatures of brittle to ductile transition were about 90, 262, 304 and 400 degrees C for Fe-6.5Si (wt.%) alloys with Cu additions of 0.3, 0.5, 0 and 0.7 (wt.%), respectively. Microstructure analysis suggested that the solubility of Cu in the alloy was or higher than 0.3 wt%, but lower than 0.5 wt%. The Cu dissolution in the solid solution increased the plasticity of the Fe-6.5Si (wt.%) alloy as a result of the reduced degree of long-range order. Increasing Cu addition beyond the solid solution limit caused the Cu-rich phase precipitation at grain boundaries, which embrittled the grain boundaries and hence severely reduced the plasticity of the alloy.