Coarsening in Solidification Processing
Merton C. Flemings
Coarsening manifests itself in solidification of metal alloys as 1) growth of larger particles or dendrite arms with simultaneous dissolution of smaller particles or arms (“ripening”), 2) filling in of spaces between particles or dendrite arms (“coalescence”) and 3) breakup of dendrites (“dendrite multiplication”). Ripening of dendrites during solidification results in the well known dependency of dendrite arm spacing on cooling rate or local solidification time. If initial dendritic grain size is sufficiently small, the grains may spheroidize and then grow non-dendritically during isothermal holding in the liquid-solid zone. Dendrite multiplication results from convection combined with rapid cooling during initial stages of dendritic solidification. If the new grains thus formed are sufficiently high in number density, subsequent growth of these grains is non-dendritic. There is engineering interest today in finding the most reliable and economic ways of achieving such a high initial grain density, either by thermo-mechanical means or by grain refinement.