The mechanism for spontaneous grain refinement in undercooled pure Cu melts

A melt encasement (fluxing) technique has been used to study the undercooling-property relationship and microstructural development in pure Cu samples. The maximum undercooling achieved was 352 K. Grain refinement was observed at an undercooling of 352 K. Microstructural analysis using light microscopy and TEM, X-ray diffraction and microhardness measurements reveal that recrystallisation accompanies grain refinement at these extremely high levels of melt undercooling. Furthermore, at undercoolings between 310–320 K, a high density of subgrains are seen within the microstructure which is indicative for the occurrence of recovery, a phenomenon previously unseen in pure Cu samples solidified from highly undercooled melts.