Nucleation-controlled microstructural development in Al-Si alloys

When surface melting is conducted in a scanning mode, rapid resolidification can result in the loss of epitaxial regrowth. A columnar or epitaxial regrowth develops under the condition of low interfacial undercooling. At high beam scan velocities of several m s−1 or more, and/or for concentrated alloys, the lag of the solidification front behind the liquidus isotherm may be substantial and thus may allow for the nucleation of new grains within the undercooled liquid. The transition from constrained growth to nucleation control of microstructural evolution has been revealed in the solidification of laser-remelted Al-26wt.%Si alloys. An analysis of the transition has been formulated based upon the limiting solidification interface temperature at the transition and a heterogeneous volume nucleation model. The nucleation kinetics analysis indicates that the nucleation of primary silicon restricts the domain of the coupled eutectic zone for silicon compositions greater than 20 wt.%.