Heterogeneous nucleation of Pb Particles Embedded in a Zn Matrix

Zinc-10 and 20 wt pct Pb alloys have been rapidly solidified by melt spinning to obtain a very fine scale dispersion of nanometer-sized Pb particles embedded in Zn matrix. The microstructure and crystallography of the Pb particles have been studied using transmission electron microscopy (TEM). Each embedded Pb particle is a single crystal, with a truncated hexagonal biprism shape with the 6/mmm Zn matrix point group symmetry surrounded by {0001},\(\left\{ {10\bar 10} \right\}\) and\(\left\{ {10\bar 11} \right\}\) facets. The Pb particles solidify with a well-defined orientation relationship with the Zn matrix of (0001)_zn‖(111)_pb and\([11\bar 20]_{Zn} ||[1\bar 10]_{Pb} \). The melting and solidification behavior of the Pb par- ticle have been studied using differential scanning calorimetry (DSC). The Pb particles solidify with an undercooling of approximately 30 K, by heterogeneous nucleation on the {0001} facets of the surrounding Zn matrix, with an apparent contact angle of 23 deg.     

Solidification behaviour of Al particles embedded in a Zr Aluminide matrix

A hyperperitectic Al-50 wt% Zr alloy has been manufactured by melt spinning, and the resulting microstructure has been examined by transmission electron microscopy. As-melt spun and annealed hyperperitectic Al-50 wt% Zr consist of a Zr aluminide matrix and an Al rich phase distributed in the form of small and large particles with sizes ∼ 15 and ∼ 100 nm, and as an irregular layer at the cell and grain boundaries. Diffraction analysis of the Zr aluminide matrix is consistent with the aluminide having a tetragonal unit cell with a = 4.014 Å and c = 17.32 Å, similar to equilibrium D0₂₃ tetragonal ZrAl₃ but with a different stoichiometry and different atomic ordering on alternate (004) planes. The Al rich particles show a (001)_Al (001)_Matrix; [100]_Al [100]_Matrix orientation relationship with the Zr aluminide matrix. The solidification nucleation kinetics of the Al rich particles have been examined by heating and cooling experiments in a differential scanning calorimeter over a range of heating and cooling rates. Solidification of Al rich particles is nucleated catalytically on the Zr aluminide matrix at an undercooling in the range 0–5 K. Analysis of the solidification nucleation kinetics of the Al rich particles supports the hypothesis that the classical spherical cap model of heterogeneous nucleation breaks down at low undercoolings and small contact angles.     

In situ stress-strain response of small metal particles embedded in a ceramic matrix

The in situ stress-strain response of metal particles embedded in a ceramic matrix was obtained by combining the measurement and the modeling of the crack opening displacement field of a crack in a brittle material bridged by metal particles. The experiments were done on a composite made from platinum particles with a volume fraction of 10% in a magnesium aluminate spinel matrix. The size of the platinum particles was varied from 1 to 12 μm to study the influence of scale on the deformation behavior. Large strain to failure (85%) and ultimate tensile strength of 550 MPa were obtained for the 1 μm particles. But the larger particles failed at a strain of less than 25%; the ultimate tensile strength was also lower. This difference in ductility is explained in terms of debonding at the metal ceramic interface. It is argued that the debonding depends on the length of the dislocation pile up at the interface, and, therefore, on the particle size. The results and the metallographic observations are consistent with a model presented here; in this model the failure condition is given by a combination of the intrinsic yield stress of platinum, and the hydrostatic constraining stress in the metal particle.     

Heterogeneous nucleation in solidifying metals

Heterogeneous nucleation experiments have been carried out using alloy droplets contained in a nucleant matrix. These specimens were made using a simple casting and annealing treatment of selected alloys. Thermal analysis was used to detect the nucleation in the alloy droplets. The results obtained show that this method was capable of giving reproducible undercooling results which in most cases were much larger than had been achieved by the use of other techniques. Some results were, in fact, greater than those previously accepted as being close to homogeneous nucleation. Analysis of these results gave no support to the non-reciprocal nucleation theory or the electrostatic effect put forward by other workers. Nucleation at small undercoolings was found only in those couples whose crystal structure and bonding were similar, but these factors were considered to be insufficient in themselves for efficacious heterogeneous nucleation. A relationship between the melting point of a substrate and its potency as a nucleant was also found for some materials.     

Melting behaviour of In and Pb particles embedded in an Al matrix

Microstructures of melt spun hypomonotectic Al−7wt%In, hypermonotectic Al−5wt%Pb and near monotectic Al−2wt%Pb alloys have been examined by transmission electron microscopy and consist of 10–150 nm diameter faceted In particles and 5–150 nm faceted Pb particles homogeneously distributed in an Al matrix. As-melt spun In and Pb particles exhibit near cube-cube and cube-cube orientation relationships with the Al matrix respectively, and truncated octahedral shapes bounded by {111} and {100} facets. The melting behaviour of In and Pb particles in as-melt spun Al−7wt%In, Al−5wt%Pb and Al−2wt%Pb alloys has been investigated by heating and cooling experiments in a differential scanning calorimeter and in situ heating experiments in a transmission electron microscope. In and Pb particles embedded within the Al matrix grains melt at superheatings in the range 0–40 K above the bulk equilibrium In and Pb melting points. Superheating of In and Pb particle melting within the Al matrix grains is caused by a kinetic difficulty of nucleating melting which increases with decreasing In and Pb particle size. In and Pb particles along the grain boundaries of the Al matrix melt at undercoolings in the range 0–7 K below the bulk equilibrium In and Pb melting points.     

Heterogeneous nucleation in entrained Sn droplets

Differential thermal analysis has been used in conjunction with entrained droplet specimens to measure the nucleation rates of uncontaminated Sn-rich liquids in contact with solid Bi, Zn and Al. The Bi(Sn) system gave a linear nucleation rate plot and an analysis of the data in terms of the classical theory of heterogeneous nucleation at variable composition, yielded a pre-exponential frequency factor, J^s₀, that was in good agreement with the predicted value for pure Sn. On the other hand, the nucleation rates plots for Zn(Sn) and Al(Sn) were irregular and could only be analysed to give approximate estimates of J^s₀ for these two systems. The origin of the irregularity was discussed in terms of the surface structure of entrained droplets. In the case of Bi(Sn), the nucleation rate data yielded a catalyst surface energy difference of − 50 ± 26 mJm⁻² which agreed with a theoretical estimate for pure Sn nucleating in contact with pure Bi.     

Heterogeneous nucleation and grain refinement in cast Mg(AZ91)/SiCp metal matrix composites

Thermal analysis and microstructural investigation have been carried out to study the heterogeneous nucleation, crystal growth and grain refinement in the solidification of magnesium alloy AZ91 and AZ91/SiC_P composites with two different volume fractions of the reinforcement. The results show the evidence of heterogeneous nucleation of primary magnesium on SiC particles in the two composite samples, which is attributed to the small (4%) lattice disregistry between SiC and magnesium with an orientation relationship of (111)_Sic//(0001)_Mg. Analysis of nucleation/growth competition during the initial stage of grain growth suggests that the particle/solidification front interaction during the solidification of composites results in a reduced growth rate for the primary phase, which is especially significant when the volume fraction of reinforcement is high. Grain refinement in AZ91/SiC_P composites is the result of two separate and competing processes: nucleation and growth. In the composite with 10 vol.% SiC, the significant grain refinement (67%) is the combined result of heterogeneous nucleation and restricted grain growth. In the other composite with 0.5 vol.% SiC, the moderate grain refinement (37%) is mainly attributed to a heterogeneous nucleation mechanism.     

Heterogeneous nucleation of σ′ on dislocations in a dilute aluminum-lithium alloy

The nucleation of S on dislocations with small undercooling in binary aluminum-lithium alloys has been examined. The study of related microstructures was performed using transmission electron microscopy (TEM), which demonstrates that σ′ preferentially nucleates on dislocations with a strong edge character and locates at the side where the stress field is compressive without destroying the dislocation core structure. This qualitatively justifies the theoretical prediction by Larché on coherent heterogeneous nucleation on edge dislocations. Following the evaluation of the volume free energy change for the binary system by the ideal solution model and the mean-field model by Khachaturyan, the nucleation barrier and the nucleation rate were calculated and compared with experimentally determined data based on Larché's model. Specifically, the back-calculated interfacial energies from the experimentally determined nucleation rate data are in good agreement with the interfacial energy temperature dependence predicted by the related interfacial energy model. The effects of misfit strain, volume diffusion, interfacial energy, and nucleation sites are discussed. Formerly Graduate Student This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5. 1994 in Rosemont, Illinois.     

The effect of dopants on the heterogeneous nucleation of solidification of Cd and Pb particles embedded in an Al Matrix

The effect of dopant ternary additions of Zn, Ga, Ge, Cu, Mg and Si on the heterogenous nucleation of solidification of Cd and Pb particles embedded in an Al matrix has been investigated in rapidly solidified Al-Cd-X and Al-Pb-X alloys using a combination of differential scanning calorimetry and transmission electron microscopy. Except for Si, the dopant additions have no effect on the alloy microstructures and remain in solution in the Al matrix and the Cd or Pb particles after rapid solidification and during heat treatment in the calorimeter. In general, the catalytic efficiency of Al as a heterogeneous nucleant for Cd and Pb solidification does not change inversely with changes in lattice disregistry across the nucleating interface in disagreement with Turnbull and Vonnegut's prediction. These results demonstrate the importance of chemical factors in determining the catalytic efficiency of heterogeneous nucleants.     

Solidification behaviour of Pb droplets embedded in a Cu matrix

A hypomonotectic alloy of Cu-5wt%Pb has been manufactured by melt spinning and the resulting microstructure examined by transmission electron microscopy. As-melt spun hypomonotectic Cu-5wt%Pb consists of a homogeneous distribution of faceted 20–100 nm diameter Pb particles embedded in a matrix of Cu, formed during the monotectic solidification reaction. The Pb particles show a cube-cube orientation relationship with the surrounding Cu matrix and a truncated octahedral shape bounded by {111} and {100} facets. The kinetics of Pb particle solidification have been examined by heating and cooling experiments in a differential scanning calorimeter over a range of heating and cooling rates. Pb particle solidification is nucleated catalytically by the surrounding Cu matrix, with an undercooling of 0.5 K and a contact angle of 4°. Analysis of the nucleation kinetics of Pb particle solidification seems to indicate a breakdown of the classical spherical cap model of heterogeneous nucleation.     

Solidification of Pb particles embedded in Al

Hypermonotectic alloys of Al-5 wt% Pb and Al-5 wt% Pb-0.5 wt% X where X = Mn, Cu, Zn, Fe and Si have been manufactured by chill-casting and melt-spinning. The resulting microstructures have been examined by a combination of optical microscopy, scanning and transmission electron microscopy, and electron probe microanalysis. The as-solidified hypermonotectic alloys exhibit a homogeneous bimodal distribution of faceted Pb particles embedded in a matrix of Al, with chill-cast Pb particle sizes of 1–2 μm and 5–50 μm, and melt-spun Pb particle sizes of 5–10 nm and 50–100 nm. The larger Pb particles are formed during cooling through the region of liquid immiscibility while the smaller Pb particles are formed during monotectic solidification of the Al matrix. The Pb particles exhibit a cube-cube orientation relationship with the Al matrix, and a truncated octahedral shape with {111} and {100} facets. The as-solidified Pb particle distributions are resistant to coarsening during post-solidification heat treatment. The equilibrium Pb particle shape and therefore the anisotropy of solid Al-solid Pb and solid Al-liquid Pb surface energies have been monitored by in situ heating in the transmission electron microscope over the temperature range between room temperature and 550°C. The anisotropy of solid Al-solid Pb surface energy is constant between room temperature and the Pb melting point, with the {100} surface energy 14% greater than the {111} surface energy, in good agreement with geometric near-neighbour bond energy calculations. The {100} facets disappear when the Pb particles melt, and the anisotropy of solid Al-liquid Pb surface energy decreases gradually with increasing temperature above the Pb melting point, until the Pb particles become spherical at about 550°C. The kinetics of Pb particle solidification have been examined by heating and cooling experiments in a differential scanning calorimeter. Pb particle solidification is nucleated catalytically by the Al matrix on the {111} facet surfaces, with an undercooling of 22K and a contact angle of 21°C. Ternary additions of Mn, Cu, Zn and Fe do not influence the Pb particle solidification behaviour, but Si is a potent catalyst and stimulates the Pb particles to solidify close to the equilibrium Pb melting point.     

Heterogeneous nucleation of solidification of Si in Al-Si and Al-Si-P alloys

Heterogeneous nucleation of solidification in melt spun Al-Si and Al-Si-P has been studied using differential scanning calorimetry, and transmission, scanning transmission and high resolution electron microscopies. The microstructures of the heat treated melt spun alloys all consist of an Al matrix, Al-Si eutectic distributed along the Al grain boundaries, and Si embedded in the Al matrix. The Si microstructure depends on the level of P: coarse faceted Si particles are nucleated by AlP particles in Al-Si containing 2 ppm P and Al-Si-P containing 35 ppm P whereas eutectic droplets of fine Si particles are nucleated by the surrounding Al matrix at a high undercooling in Al-Si containing 0.25 ppm P. The Si nucleation onset temperature remains approximately constant while the peak and end temperatures both decrease with increasing cooling rate, in agreement with classical nucleation theory. Kinetic analysis, using the spherical cap model gives contact angles of 10°, 43° and 10° for Si nucleation in low and high purity Al-Si and Al-Si-P respectively.     

Effects of precursor matrix events on subsequent nucleation

A precursor transformation may actually increase the chemical driving force for subsequent nucleation of certain phases while decreasing the free energy of the system. Spinodal decomposition of Fe-C austenite has been proposed as a means of providing C-poor regions in which subsequent nucleation of ferrite or martensite is easier. However, the C-C interaction potential in austenite precludes such a reaction. Statistical fluctuations have also been proposed as a means of providing C-poor regions in austenite. However, the activity coefficient of C in austenite varies only slightly with C content, so that the scale of these fluctuations is far too small to provide attractive sites for martensite or ferrite nucleation. Statistical concentration fluctuations observed above the τ solvus in Al-Cu alloys bear a striking resemblance to Guinier Preston (GP) zone critical nuclei which will form only at temperatures that are hundreds of degrees lower. The role played by these fluctuations in subsequent critical nucleus formation is as yet unclear. A proposal that solute segregation to dislocations increases the driving force for nucleation is shown to be rendered invalid by the requirement that the chemical potentials in the system be uniform. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

Heterogeneous nucleation of solidification of Si by solid AI in hypoeutectic Al-Si alloy

Melt-spun Al-3 wt pct Si with and without ternary additions of Na and Sr has been heat-treated above the Al-Si eutectic temperature in a differential scanning calorimeter to form a microstructure of Al-Si eutectic liquid droplets embedded in the α-Al matrix. During subsequent cooling in the calorimeter, the heterogeneous nucleation temperature for solidification of Si in contact with the surrounding Al matrix depends sensitively on the alloy purity, with a nucleation undercooling which increases with increasing alloy purity from 9 to 63 K below the Al-Si eutectic temperature. These results are consistent with Southin’s hypothesis that low levels of trace P impurities are effective in catalyzing Si nucleation in contact with the surrounding Al matrix. With a low Al purity alloy, 0.1 wt pct Na addition increases the Si nucleation undercooling from 9 to 50 K, 0.15 wt pct Sr addition does not affect the Si nucleation temperature, and 0.3 wt pct Sr addition decreases the Si nucleation undercooling from 9 to 3 to 4 K. The solidified microstructure of the liquid Al-Si eutectic droplets embedded in the Al matrix depends on the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle; however, with high Si nucleation undercooling, each Al-Si eutectic droplet solidifies to form a large number of nonfaceted Si particles embedded in Al. Formerly with the Oxford Centre for Advanced Materials and Composites, Department of Materials, Oxford University     

A statistical analysis of cavity nucleation at particles in grain boundaries

Cavity nucleation on grain boundary particles during creep has been examined using a classical thermodynamic method. The particle sizes and spacings are assumed to obey a log-normal distribution. It is found that a threshold shear stress is needed for cavity nucleation to occur. When the resolved shear stress on a grain boundary segment reaches the threshold stress, a critical normal stress for cavity nucleation is produced on the panicle-matrix interface by grain boundary sliding and cavity nucleation occurs. The threshold stress is determined mainly by the concentration and distribution of grain boundary particles and falls in the stress range of engineering applications. A program has been developed to calculate the fraction of particles which can serve as nucleation sites. The model is used to predict the onset of cavitation in the oxide dispersion strengthened alloy Inconel MA 754. Implications for avoiding the nucleation of cavities in engineering alloys are discussed.