The effect of undercooling on the cellular precipitation reaction in Cu-3 Pct Ti

Abstract

The effect of undercooling on the morphology of the cellular precipitation reaction in Cu-3 Pct Ti is examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and serial sectioning experiments. The reaction front formed at small undercooling, which exhibits strong faceting of the precipitate growth interfaces, gradually changes with increasing undercooling to a smoothly curved reaction front with concave precipitate growth interfaces and convex grain boundary segments. This concealment of the faceted reaction front appears to be due to the rapid accumulation of growth ledges with increasing undercooling. This study also indicates that the cellular precipitation reaction, at small undercooling, is initiated by Widmanstatten precipitation. At larger undercoolings, a second mechanism is responsible for cellular genesis. Finally, contrary to accepted models of colony development, serial sectioning experiments show that nucleation of additional lamellae may occur at the faces of existing lamellae, from where they extend laterally to achieve the characteristic interlamellar spacing for that temperature.