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

Both glass fluxing and cyclic superheating techniques were adopted to effec tively undercool the Cu-70-Ni-30 alloy in vacuum. Within the undercooling r ange of 21 K to 270 K, the microstructure evolution of the alloy was invest igated. When the melt was undercooled to DeltaT > DeltaT(3)* (210 K), the g rain refinement took place abruptly. Based on the observation of the solidi fied microstructure, the microchemical-analysis and the calculated results with BCT model, it is found that the secondary gr ain refinement mechanism consists of two stages, The dendrite is, firstly, broken into fragments owi ng to the stress caused by uneven shrinking during rapid solidification, th en the fragments, under the driving force of surface and strain energies, m erge through the migration of boundaries, i.e. recrystallization, thus lead ing to the formation of secondary granular-crystalline.