HETEROGENEOUS NUCLEATION AND GROWTH OF DELTA' IN AL-8 AT-PERCENT-LI

The nucleation of delta' at dislocations at small undercooling is investigated. The primary site for heterogeneous nucleation is matrix dislocations with strong edge components. The edge dislocation half-plane and the development of a delta' anti-phase boundary is considered. The morphological development of the delta' precipitate is that of a "kidney" shape with a consistent relationship with the edge dislocation half-plane. A model is developed based on the energetics of loss of coherency of a spherical precipitate and is employed to predict coherency loss of the delta' precipitate and to compare it to the experimentally determined diameter of 400 nm. The loss of coherency is shown to be assisted by the growth of the equilibrium precipitate, delta.