NUCLEATION-CONTROLLED MICROSTRUCTURAL DEVELOPMENT IN AL-SI ALLOYS

When surface melting is conducted in a scanning mode, rapid resolidifi cation can result in the loss of epitaxial regrowth. A columnar or epi taxial regrowth develops under the condition of low interfacial underc ooling. At high beam scan velocities of several m s(-1) or more, and/o r for concentrated alloys, the lag of the solidification front behind the liquidus isotherm may be substantial and thus may allow for the nu cleation of new grains within the undercooled liquid. The transition f rom constrained growth to nucleation control of microstructural evolut ion has been revealed in the solidification of laser-remelted Al-26wt. %Si alloys. An analysis of the transition has been formulated based up on 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 th e domain of the coupled eutectic zone for silicon compositions greater than 20 wt.%. (C) 1997 Elsevier Science S.A.