THE EFFECT OF UNDERCOOLING ON THE CELLULAR PRECIPITATION REACTION IN CU-3 PCT TI

The effect of undercooling on the morphology of the cellular precipita tion reaction in Cu-3 Pet Ti is examined by scanning electron microsco py (SEM, transmission electron microscopy (TEM), and serial sectioning experiments. The reaction front formed at small undercooling, which e xhibits strong faceting of the precipitate growth interfaces, graduall y changes with increasing undercooling to a smoothly curved reaction f ront with concave precipitate growth interfaces and convex grain bound ary segments. This concealment of the faceted reaction front appears t o be due to the rapid accumulation of growth ledges with increasing un dercooling. This study also indicates that the cellular precipitation reaction, at small undercooling, is initiated by Widmanstatten precipi tation. At larger undercoolings, a second mechanism is responsible for cellular genesis. Finally, contrary to accepted models of colony deve lopment, serial sectioning experiments show that nucleation of additio nal lamellae may occur at the faces of existing lamellae, from where t hey extend laterally to achieve the characteristic interlamellar spaci ng for that temperature.