Forming mechánism of refined granular crystalline of Cu₇₀Ni₃₀ alloy by undercooled solidification

Both glass fluxing and cyclic superheating techniques were adopted to effectively undercool the Cu₇₀Ni₃₀ alloy in vacuum. Within the undercooling range of 21 K to 270 K, the microstructure evolution of the alloy was investigated. When the melt was undercooled to ΔT > ΔT″₃ (210 K), the grain refinement took place abruptly. Based on the observation of the solidified microstructure, the microchemical-analysis and the calculated results with BCT model, it is found that the secondary grain refinement mechanism consists of two stages. 'The dendrite is, firstly, broken into fragments owing to the stress caused by uneven shrinking during rapid solidification, then the fragments, under the driving force of surface and strain energies, merge through the migration of boundaries, i. e. recrystallization, thus leading to the formation of secondary granular-crystalline.