Differences Between Homogeneous Nucleation and Heterogeneous Nucleation

Newey and Weaver described nucleation as a process that must occur in a system, undergoing a phase transition, before the formation of another phase (Royce). This process is called homogeneous nucleation if it occurs away from any boundaries. On the other hand, heterogeneous nucleation takes place on a surface, interface, dislocation or other defect in the material. In addition, the latter type is favored because it requires a lower free energy change to form the initial stable nucleus where others can adhere resulting an increase in size (cited in Royce).

During nucleation, the atoms are forming nano-sized solid clusters. In homogeneous nucleation, clustering occurs above the melting of the metal (Tm) turns back into the liquid state due to its stability on that phase while clustering below Tm can lead to crystallization-nuclei formation if its size reaches stability against melting (Iqbal 3). High solid-liquid interface surface energy is a thermodynamic hindrance in nucleation. Due to this energy barrier, foreign materials are added to serve as nucleation sites. These nucleation sites have lower surface energy, thus, increases the nucleation rate.

The stable nuclei then grow into an equiaxed and finer grain structure (Iqbal 3). Moreover, nucleation is a kinetic process wherein atoms of the melted metal form into clusters within the liquid medium at solidification temperature (Iqbal 9). These clusters act as crystallization nuclei where other atoms adhere and solidify. The rate of nucleation process is directly affected by the difference between the equilibrium melting temperature (Tm) and the freezing temperature (Tf) or undercooling. As a rule of thumb, a higher undercooling yields higher nucleation rate. Nucleation Mechanism

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Ben Best discussed that mixtures of some metals, like copper and nickel, both in liquid and solid states are highly soluble in all given concentration. Since both copper and nickel have similar crystal structures and atomic radii, in the cooling process the particles formed have properties imparted by both of these metals. This metal mixture type is called isomorphous. In contrast to this, the mixture of lead and tin is eutectic because of partial solubility of these metals in the solid state. Unlike copper and nickel, lead and tin have different crystal structures and atomic radii.

This is the reason why the solid lead-tin alloy can only consist of 2. 5% lead and 19. 2% tin their maximum composition by weight. In addition, a eutectic mixture has composition that completely liquefies at eutectic temperature. For lead-tin mixture, the eutectic composition is 61. 9% that has a eutectic temperature of 183ºC. This property makes lead-tin mixture as a good soldering agent. Metals typically solidify as crystals at a temperature lower than its melting temperature (Best). The difference in melting and solidification temperatures is called as the maximum undercooling.

This undercooling is the effect of pure metal crystallization. During the crystallization process, the nucleation of small particles or crystallization nuclei occurs first then the adherence of other particles on these nuclei follows. As such, other surrounding particles tend to dissolve it back into the liquid phase. Successful fusion into the crystal releases heat which causes other adjacent atoms to dissolve. This means that the high fusion of a metal reflects its tendency for a high solidification temperature and maximum undercooling (Best).

The energy affects the dissolution process with respect to the surface area of the nucleus while energy variation favoring nucleus growth is a factor of volume proportion (Best). Surface area varies with the square of the radius, whereas volume varies with the cube of the radius. Thus, a large crystal is not susceptible to surface dissolution. In addition, a metal at a specific temperature has a critical radius size. Radius bigger than the critical radius tend to increase in size while smaller radius is susceptible to dissolution.

Nonetheless, lower temperature facilitates the attainment of the critical radius (Best). Further, crystallization may occur in less undercooling if a higher melting point metal with similar crystal structure to and insoluble at the melting temperature of the original metal is added (Best). The crystal growth around these insoluble nuclei is referred to as heterogeneous nucleation. In heterogeneous nucleation, specific sites in a material catalyze the nucleation process through the reduction of the critical free energy of nucleation (?Gc) (Balluffi, Allen, and Carter 477).

It is always in kinetic competition with homogeneous nucleation wherein the faster rate mechanism prevails. The lower value of ?Gc supports heterogeneous nucleation while the greater number of potential nucleation sites favors homogeneous nucleation. Moreover, by means of the nucleation rate expressed as J = Z ßc N exp[-?Gc /(kT )], regimes of temperature, supersaturation, relative interfacial energies, and microstructure in which one nucleation mechanism occurs can be predicted.

When a small particle deposits on the grain boundaries, edges or corners of a polycrystalline microstructures such as grain boundaries, edges or corners, these crystal imperfections will be eliminated with an associated free-energy decrease lowering ?Gc (Balluffi, Allen, and Carter 477). Solidification in Metals The solidification of metals and their alloys starts when a welded small portion of metal melts and resolidifies (“Phase Transformation”). Homogeneous nucleation occurs when there are no other chemical species involved in a nucleation process.

For instance when a pure liquid metal is slowly cooled below its equilibrium m freezing temperature to a sufficient degree numerous homogeneous nuclei are created by slow-moving atoms bonding together in a crystalline form. While the involvement of other chemical species to favor nucleation results to heterogeneous nucleation. Solidification is a crucial stage in metallurgical processes such as in ingot casting, continuous casting, squeeze casting, pressure casting, atomization (Phanikumar and Chattopadhyay 25).

This is also an important stage in secondary manufacturing processes such as welding, soldering, brazing, cladding and sintering. For the properties of the product largely depend on the mechanical properties and the microstructure of the different phases. The microstructure of the products on the other hand, is affected by thermal and solutal processing conditions and thermodynamic and kinetics factors of the materials (Phanikumar and Chattopadhyay 25). Solidification involves heat extraction through diffusion and convection processes, and solid-liquid interface movement.

In addition, the microstructure solidification is a complex process affected by the rate of solidification (v), temperature gradient (G), composition (C) and kinetics factors such as phase equilibrium reactions, nucleation and growth, and crystallographic constraints (Phanikumar and Chattopadhyay 25). Solidification and Mechanical Properties Industrial treatments such as rolling or forging, alloying and thermal treatment are done to metals to strengthen their mechanical properties.

For instance, pure aluminum has a tensile strength of around 13,000 pounds per square inch (psi), however, by cold-working its strength is approximately doubled. This can also be done by adding alloying metals such as manganese, silicon, copper, magnesium and zinc. Similarly, heat treatment makes the tensile strength of aluminum over 100, 000 psi (“Property Modification” n. p. ). Plastic or permanent deformation of crystalline materials is largely affected by the tendency of dislocation within the material. Thus, restraining the dislocation movement improves its strength.

This is done by controlling the grain size, strain hardening, and alloying (“Strengthening/Hardening Mechanisms”). In the material science engineering, a grain is a crystal with unsmooth faces due to the deferred growth in contact with a boundary (“Solidification”). The grain boundary is the interface between grains. Atoms in this region are disordered, hence, no crystalline structure. The different orientation of adjacent grains within the material, the boundary between grains hinders the dislocation movement and the resulting slip.

The solidification rate controls the size and number of grains. Smaller grains denote shorter distances between atoms that can move in a slip plane, thus, improving the strength of the material (“Strengthening/Hardening Mechanisms”). The improvement of metallic strength is done through strain or work hardening or cold-working. In plastic deformation of metals, the movement of dislocations produces additional dislocations (“Strengthening/Hardening Mechanisms”). These dislocations interact, pin or tangle resulting to decline in dislocations movement and causes material strengthening.

This strengthening is called as cold-working for the occurrence of plastic deformation is at low temperature which impedes atom movements. However, cold-working process reduces the ductility of metals. On the other hand, when the process is done at higher temperature, the atoms rearrange to improve material strength (“Strengthening/Hardening Mechanisms”). Since cold-working process reduces ductility, thermal or heat treatment is used to remove its effect. The strengthening gained through the cold-working will be lost if the strain hardened materials are exposed at higher temperatures.

Recovery, re-crystallization, and grain growth may occur during the heat treatment (“Strengthening/Hardening Mechanisms”). Nucleation and Mechanical Properties The number of nucleation sites for the freezing metal affects the grain structure of the solid metal product. Few number of nucleation sites means smaller number of crystallization nuclei, hence, large-grain or coarse structure results. An increase in nucleation site numbers, on the other hand, yields fine-grain structure because a lot of crystallization nuclei are available for the dissolve phase attach and solidify.

Fine grain structure is the most desired product for strength and uniformity in metal production (Poster and Easterling 125). An ideal crystal has a perfect crystalline structure and characterized by a regular repetitive lattice in any space direction. However, crystalline materials have crystallographic defects. Minor crystal defect may impart significant metallic properties. The conductivity of silicon, for instance, is doubled when it is contaminated with 10-8 percent mass of boron (Tisza 107).

There are several properties that can be identified based on the ideal lattice structure such as thermal and electrical conductivities, and specific heat. These are called as structure-insensitive properties. However, there are structure-sensitive properties such as mechanical properties that are hardly predicted on the basis of ideal crystal structure. The discrepancy between the ideal and real crystal structures result to the large differences in theoretical and experimental computation of properties (Tisza 107).

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