Heterogeneous nucleation alloys

The homogeneous nucleation of martensite in typical solids is too slow by many orders of magnitude to account for observed results. Calculations of typical values of AQc using the classical nucleation model of Section 19.1.4 (see Exercise 19.3) yield values greatly exceeding 76 kT. Furthermore, nearly all martensitic transformations commence at very sparsely distributed sites. Small-particle experiments [14] have yielded typical nucleation densities on the order of one nucleation event per 50 pm diameter Fe-Ni alloy powder particle.3 Thus, nucleation of martensite is believed to occur at a small number of especially potent heterogeneous nucleation sites. 

Hono et al. [64, 65] confirmed that Cu in both amorphous Fev sSiia.sBgNbsCui and Fe89Zr7B3Cui alloys forms clusters prior to primary crystallization and the Cu clusters act as heterogeneous nucleation sites for bcc-Fe(Si) precipitates. They also confirmed orientation relationships between the Cu clusters and bcc-Fe crystals and concluded that the heterogeneous nucleation of the bcc-Fe phase is due to the lower interfacial free energy for nucleation [66]. 

The well-known demonstrations by metallurgists that heterogeneous nucleation in metals and alloys (e.g., nucleation of pearlite) frequently occurs at dislocations (67, 68), were the precursors of the remarkable experiments of Mitchell aZ. (69-75) who decorated dislocations in transparent crystals of silver halides by exposure to light. The photo-lytic silver was precipitated along dislocations which were thereby rendered visible in the optical microscope. An extension of this technique was effected by Dash (76-80), who succeeded in decorating dislocations in opaque silicon by preferential precipitation of copper which could be detected under infrared illumination. Dash strikingly demonstrated... 

In alloys and RPV steels with > 0.07wt%Cu, and irradiation temperatures > 200°C, Cu-enriched solute clusters form. At irradiation temperatures > 325 °C, these can grow to >4nm diameter, and probably transform to the equilibrium fee -Cu phase, but at the temperatures and fluence of interest most CECs in irradiated steels will be bcc." Radiation-induced point defects enhance the substitutional solute diffusion rate and enhance the rate of precipitation. In addition, nucleation of CECs appears to be easier in the presence of matrix defects. The nature of the matrix defects on which CECs nucleate is not clearThe relative importance of homogeneous and heterogeneous nucleation of CECs under irradiation is not agreed, although homogeneous nucleation will, naturally, become more likely as the Cu supersaturation increases. ... 

---