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.     

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.     

Influence of low-gravity solidification on heterogeneous nucleation in stable iron-carbon alloys

The processes of nucleation and growth of alloys during solidification are linked to the level of gravitational force. In a low-gravity environment, buoyancy-induced convection becomes negligible, resulting in lower convection as compared to normal or high gravity. In this paper, heterogeneous nucleation and grain multiplication during solidification of gray cast iron, and the effect of gravitational level on them, have been studied by means of directional solidification on ground and under low-gravity (low-g) and high-gravity (high-g) conditions obtained by aircraft parabolic flights. It has been assumed that the final number of eutectic grains results from the contribution of heterogeneous nucleation,N _h , heterogeneous nucleation induced by inoculation,N _i , and heterogeneous nucleation induced by convection,N _c . In turn,N _c has two components, a grain multiplication component,N _c ^m , and a kinetics of chemical reactions component,N _c ^k . In all cases, it was found that a higher number of grains are obtained when solidifying in highg as compared with lowg. This was attributed to higher convection in highg. It was demonstrated that grain multiplication due to convection can contribute 20 to 23 pct from the total number of grains resulting from heterogeneous nucleation of uninoculated samples. For the case of inoculated samples, it was shown that the contribution to the convection-induced nucleation of the kinetics of chemical reactions can be as high as 30 pct but can be zero at very low or very high grain numbers. A possible mechanism and an explanation have been given to those findings. The silicon distribution, graphite morphology, and the influence of soak time on experimental results have also been discussed.     

非均质形核细化16Mn钢凝固组织

通过感应炉对比实验,考察了基于不同非均质形核理论计算所得的三类有效形核质点Ce₂O₃、ZrO₂和MgO对细化16Mn钢凝固组织的影响.结果表明:只符合点阵错配度理论判据的MgO形核质点将铸锭等轴晶率从32%提高至37%;只符合静电作用理论判据的ZrO₂形核质点将等轴晶率提高至40%;同时满足点阵错配度理论和静电作用理论判据的Ce₂O₃非均质形核质点将等轴晶率提高至44%.在上述三个非均相成核的粒子中,Ce₂O₃能最大程度细化16Mn钢凝固组织.钢中形成以上三类形核核心后,钢的原始奥氏体晶粒尺寸以及显微组织均得到不同程度细化.     

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 and facets. The Pb particles solidify with a well-defined orientation relationship with the Zn matrix of . The melting and solidification behavior of the Pb particle 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.     

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.     

The superheating and the crystallography of embedded Pb particles in f.c.c. Al,Cu and Ni matrices

Al-1.4 at.%Pb, Cu-3.2 at.%Pb and Ni-3 at.%Pb have been rapidly solidified to obtain a nanoscale dispersion of Pb particles embedded within the higher melting point Al, Cu and Ni matrices. Each Pb particle in Al and Cu matrices is a single crystal and faceted. The shape of the Pb particles in Al and Cu is a truncated octahedron bounded by 111 and 100 facets. The second phase Pb particles in Ni are not well faceted and the shape is a roughened truncated octahedron. A well defined orientation relationship is observed between the second phase Pb particles and the matrices. The melting behaviour of these particles is studied using a differential scanning calorimeter. The majority of the second phase particles in all the matrices start melting below the equilibrium melting point of the Pb with a broad endothermic peak. In Al and Cu matrices, a few Pb particles are seen to be superheated while in the Ni matrix superheating of the Pb particles is not observed. It is shown that the observed difference in the melting behaviour is not due to the size dependent behaviour of melting of the second phase. Our results suggest that the shape of the second phase particle strongly influences the superheating.     

The nucleation and solidification of Al-Ti alloys

A series of hyperperitectic Al-Ti alloys at 0.35, 0.5, 0.7 and 0.8 wt pct Ti has been frozen at rates varying from less than 1°C/s to in excess of 100°C/s. Cooling-curve analyses, metallographic and microprobe examinations, taken altogether, allow identification both of the nucleants and the solidification modes acting in this important alloy system. Two sets of conclusions are drawn, one in general about low concentration peritectic systems like Al-Ti, and the other about particular interactions in Al-Ti. For example, it is revealed that Al_xTi compounds exist; Al₃Ti, Al_xTi and Al are nucleated by TiC; and Al_xTi and TiC are both nucleants for aluminum. Formerly of the Ford Scientific Staff, is now Senior Engineer, Société National d’Etude et de Construction de Moteurs d’Aviation, Gennevilliers, France.     

The effect of carbide and nitride additions on the heterogeneous nucleation behavior of liquid iron

A systematic study of carbide and nitride additions on the heterogeneous nucleation behavior of supercooled liquid iron was undertaken. It was found that titanium nitride and titanium carbide were very effective in promoting heterogeneous nucleation. These compounds were followed by silicon carbide, zirconium nitride, zirconium carbide, and tungsten carbide in decreasing order of effectiveness. The degree of potency of the nucleation catalysts is explained on the basis of the disregistry between the lattice parameters of the substrate and the nucleating phase. Through the inclusion of planar terms the Turnbull-Vonnegut “linear” disregistry equation was modified to more accurately describe the crystallographic relationship at the interface during heterogeneous nucleation.     

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.     

Computation of continuous cooling transformation diagrams for the heterogeneous nucleation in Sn-5 mass pct Pb droplets

A method was developed to compute continuous-cooling-transformation (CCT) diagrams for the heterogeneous nucleation of alloy droplets from a few experimental data. The developed model addresses both oxidation-catalyzed surface nucleation and internal nucleation caused by another catalyst. Droplet surface oxidation is regarded as a first-order reaction in order to account for the effects of the gradual increase in surface oxidation on the kinetics of surface nucleation. CCT curves were computed for Sn-5 mass pct Pb droplets cooling in atmospheres with various oxidation potentials using data determined with monosize droplets produced by capillary jet breakup. The developed model may be used to predict droplet nucleation kinetics in industrial thermal spraying processes.     

The heterogeneous nucleation of bainitic plates in metastable β brass

In a transmission electron microscope study, it is shown that the first bainitic plates to form during the isothermal decomposition of metastable β brass do so in the vicinity of spherical, nonmetallic inclusions 0.1 to 0.7 μm in diameter. These are believed to be particles of zinc oxide, the differential contraction of which during quenching produces a shear strain field that assists nucleation. Secondary nucleation occurs by branching from the primary plates to give a complex network on all variants of the ∼l55β habit.     

The influence of buoyant forces and volume fraction of particles on the particle pushing/entrapment transition during directional solidification of Al/SiC and Al/graphite composites

Directional solidification experiments in a Bridgman-type furnace were used to study particle behavior at the liquid/solid interface in aluminum metal matrix composites. Graphite or siliconcarbide particles were first dispersed in aluminum-base alloysvia a mechanically stirred vortex. Then, 100-mm-diameter and 120-mm-long samples were cast in steel dies and used for directional solidification. The processing variables controlled were the direction and velocity of solidification and the temperature gradient at the interface. The material variables monitored were the interface energy, the liquid/particle density difference, the particle/liquid thermal conductivity ratio, and the volume fraction of particles. These properties were changed by selecting combinations of particles (graphite or silicon carbide) and alloys (Al-Cu, Al-Mg, Al-Ni). A model which considers process thermodynamics, process kinetics (including the role of buoyant forces), and thermophysical properties was developed. Based on solidification direction and velocity, and on materials properties, four types of behavior were predicted. Sessile drop experiments were also used to determine some of the interface energies required in calculation with the proposed model. Experimental results compared favorably with model predictions. BRU K. DHINDAW Visiting Scholar with the Solidification Laboratory at the time this work was performed.     

The effect of solidification rate on microsegregation

An X-ray diffraction technique has been utilized to determine the second phase content and average composition of the primary phase in aluminum-copper and aluminum-silicon alloys solidified at a cooling rate in the range of 0.06 to l0⁵ K/s. For the Al-Cu samples, the normalized Θ phase content (ratio of the 6 content to the value predicted by the Scheil model) was found to be 0.71 at 0.1 K/s (solidification rate = 0.001 cm/s) and to increase with increasing cooling rate to 0.96 at about 180 K/s (1 cm/s). Beyond this cooling rate, it decreased with increasing the cooling rate to 0.44 at about 3.7 x lO4 K/s. The same trend was observed in the Al-Si samples, except that the normalized silicon content was much lower. Also, for both systems the normalized average composition of the primary phase was found to decrease progressively with increasing the solidification rate until it reached a minimum at 1 cm/s, beyond which it increased with higher solidification rates. The results are discussed with respect to the prevailing segregation models that include back-diffusion in the solid, dendrite tip undercooling, and the eutectic temperature depression. An equation which combines these effects at all cooling rates is given.     

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.     

The influence of acceleration forces on nucleation,solidification, and deformation processes in tin single crystals

Growth orientations are given for 50 gram and 2 gram single crystals of tin grown at acceleration levels of 1 through 7-g. Acceleration gradients produce a preferred growth orientation effect not previously observed for tin. Convection currents at approximately 5-g encourage multiple nucleation and subsequent random orientation of growth direction. The macromosaic substructure in the 99.95 pct tin does not obey the(RG) ^-1 growth relationship. The shape of the growth interface changes smoothly as the acceleration force increases, with the direction and magnitude of the change dependent on growth orientation. Deformation effects such as recrystallization and twinning are observed at acceleration levels greater than 2-g. A {431} twin plane was obtained for crystals grown at 6-g’s.