Homogeneous 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. 

Thompson and Spaepen have used classical nucleation theory to predict the homogeneous nucleation temperatures of binary alloys. The surface free energy they use has been given in Eq. (3.9), and some of its possible limitations... 

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. ... 

The precipitation develops in nanomettic volumes, which can be reahzed either in nanomettic isolated particles or in small spherical regions (of radius R) around nucleation sites in case of simultaneous nucleation at many sites such as in a highly imperfect supersaturated alloy or fast multiple homogeneous nucleation in bulk metallic glasses. Even if we assume that the formation of a new phase has a symmetric form, in principle, different possibUities must be considered. They are represented in Figure 13.10 a a + 1, a a" + 2, Q O " + 1 + 2. The detailed analysis of competitive nucleation is presented in [58, 59, 66]. 

A new synthetic approach in which thin (15-50A), amorphous, elemental layers are sequentially deposited to create a uniquely tailorable initial "reactant" is described. This synthetic approach has two key steps. The initially layered composite is diffused at low temperatures to produce a homogeneous amorphous alloy. Nucleation of this amorphous alloy is the rate limiting step in the formation of a crystalline compound. This approach overcomes the limitations of traditional, diffusion limited, solid-state synthetic methods, which offer no control of the reaction pathway and therefore no selectivity over which intermediates are formed. 

Our synthetic approach is based upon controlling the reaction pathway and making use of an amorphous alloy as the key synthetic intermediate. The desired reaction pathway contains two crucial steps the diffusion of the initially layered reactant to a homogeneous amorphous alloy, and the nucleation and growth of the desired crystalline product from the amorphous alloy. The following discussion describes how these two steps can be rationally controlled and presents supporting data. 

Hence, the decision to use a heated substrate with simultaneous evaporation of the component metals as an aid to homogenization requires consideration of whether or not it might have an adverse effect, i.e., causing preferential nucleation of one component, which interdiffusion may not be able to remedy. It was believed (60) that in preparing Pd-Rh alloys by simultaneous deposition on a substrate at 400°C, rhodium nucleated preferentially and that crystallites grew by the addition of palladium (and rhodium) atoms. The diffusion of palladium atoms into this kernel formed a phase with 88 =t 5% Rh (phase II). The outer shell of the crystallite, phase I, was in effect a solid solution deficient in rhodium compared with the overall film composition, and the Rh content of phase I therefore increased as the Rh flux was increased. 

In ordered alloys there is the question of whether disorder-order transition occurs by homogeneous ordering reaction or by nucleation and... 

H.I. Aaronson and F.K. LeGoues. An assessment of studies on homogeneous diffusional nucleation kinetics in binary metallic alloys. Metall. Trans. A, 23(7) 1915—1945, 1992. 

F.K. LeGoues and H.I. Aaronson. Influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f.c.c.-f.c.c. nucleation—IV. Comparisons between theory and experiment in Cu-Co alloys. Acta Metall., 32(10) 1855-1864, 1984. 

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