Thermal stability of ultrafine-grained copper during high speed micro-extrusion

The stored energy and thermal stability of micro-extruded ultrafine-grained copper were compared to micro-extruded coarse-grained copper. Each starting material was micro-extruded at room temperature at different speeds to a strain of 2.77 and then studied by differential scanning calorimetry, X-ray diffraction, nano-indentation and electron backscattered diffraction. The stored energy of the ultrafine-grained copper decreased with extrusion speed from a value of 0.95 ± 0.05 J/g for the starting material to 0.71 ± 0.03 J/g for material extruded at 25 mm/min. The peak temperature decreased from 202 °C for material extruded at 0.1 mm/min to 177 °C for material extruded at 25 mm/min. Stored energy and peak temperature values for the conventional copper were approximately constant with extrusion speed at around 0.75 ± 0.03 J/g and 230 °C. The lower stored energy and lower peak temperature of the ultrafine-grained copper compared to conventional copper are attributed to recrystallization commencing during micro-extrusion.