Solidification and Strengthening Chapter 9

Solidification Occurs by: Nucleation Growth Formation of tiny stable solid in liquid Growth Nucleus or tiny stable solid increases in size

Cooling Curves ***These will become important again when we talk about phase diagrams***

Nucleation

Important terms Embryo – A tiny particle of solid that forms from the liquid as the atoms cluster together. The embryo may grow into a stable solid or may remelt . Nuclei – Tiny particles of solid that form from the liquid as atoms cluster together. Because these atoms are large enough to be stable , nucleation has occurred and growth of the stable solid can begin

Types of Nucleation Homogeneous – formation of critically sized solid from the liquid by the clustering together of a large number of atoms at a high undercooling (without an external surface) ; rarely occurs except under extreme laboratory conditions Heterogeneous- Formation of a critically sized solid on an impurity surface; most common

Some important terms Innoculation – The addition of heterogeneous nuclei (“impurities”) in a controlled matter to increase the number of sites for heterogeneous nucleation, hence the number of grains…now why would we want to do that????? Any ideas?

Brainstorm with another student – 3 minutes Given the following cooling curve what can we say for sure about the type of nucleation?

We can’t say for sure what type of nucleation occurred here We can’t say for sure what type of nucleation occurred here. Homogeneous requires significant undercooling, but heterogeneous nucleation, in many situations, may also require undercooling. How about this cooling curve?

Since there is no undercooling, then nucleation had to occur by heterogeneous nucleation. Homogeneous nucleation requires undercooling.

Growth Mechanisms Planar Growth

Planar Growth Well innoculated (many sites for heterogeneous nucleation) Cooled under equilibrium conditions (very, very slowly) Thermal arrest on cooling curve

Growth Mechanisms Dendritic Growth

Dendritic Growth Poor nucleation Requires undercooling (recalescence part of cooling curve) Dendritic growth continues until the undercooled liquid is warmed to the freezing temp, any remaining liquid solidifies by planar growth.

Have you ever seen a dendrite? http://www.tc.gov.yk.ca/digitization/images_web/007332.jpg

Casting Structures