Nucleation Of Second Phase Research Articles

Effect of variation in physical properties of the metallic matrix on the microstructural characteristics and the ageing behaviour of Al-Cu/SiC metal matrix composites

In this study, aluminium-based metallic matrices with varying amount of copper (1 wt% Cu and 4.5 wt% Cu) were reinforced with SiC particulates using a partial liquid phase casting technique. The results of the present investigation showed smaller sized and higher weight percent of SiC particulates being successfully incorporated with a decrease in the weight percent of copper in the matrix. Microstructural characterisation studies conducted on the composite samples revealed an increase in uniformity of distribution of SiC particulates, improved SiC/Al interfacial integrity and smaller grain size of the metallic matrices with decreasing weight percent of copper. Results of the microstructural characterisation studies also exhibited the presence of solute rich zone in the near vicinity of SiC particulates and the nucleation of secondary phases both at and in near vicinity of SiC particulates. The result of the ageing studies revealed an accelerated ageing kinetics for the Al-1%Cu/SiC composite when compared to the Al-4.5%Cu/SiC composite samples. The results of accelerated ageing kinetics were rationalised in terms of the effect of variation in the physical properties of the metallic matrix and the ensuing microstructural characteristics due to variation in the amount of copper in the matrix.

Calorimetric Control of Aluminium Casting Quality

Differential scanning calorimetry (DSC) combined with an entrained droplet technique [1] has been successfully used on a series of melt spun alloys with deliberate impurity additions to study the nucleation related aspects of secondary phase selection during solidification of dilute Al alloys. This paper illustrates how DSC is a sensitive tool for determining the effect impurities can have on the nucleation of secondary phases, and hence material properties of these alloys. Stepped cooling/isothermal holding profile DSC has also been used in preliminary investigations of the thermodynamic range of formation of the Al—Fe eutectic phases and their nucleation and growth solidification kinetics.

Heterogeneous nucleation of solidification of equilibrium and metastable phases in melt-spun Al-Fe-Si alloys

1xxx séries Al-0.3 wt% Fe-0.1 wt% Si alloys with different impurity contents have been melt spun to produce microstructures of fine scale ~ 0.1–0.2 μm diameter secondary phase particles entrained in a higher melting point Al matrix. The melting and resolidification behaviour of the secondary phase particles while entrained in the solid matrix was monitored using differential scanning calorimetry, to simulate the nucleation of secondary phases in liquid interdendritic regions during the final stages of solidification in casting. The effect of impurities on the nucleation of solidification behaviour, and in particular on secondary phase selection, has been investigated by comparison of commercial and higher purity alloys.

Nucleation behavior during solidification of cast iron at high undercooling

During the solidification, of cast iron, a modest melt undercooling is a common observation, but the identity of the nucleation catalyst limiting the amount of undercooling has been elusive. In order to examine the influence of primary phases on the heterogeneous nucleation behavior of secondary phases in both hypoeutectic and hypereutectic cast irons, solidification catalysis experiments were carried out with differential thermal analysis (DTA) and droplet samples. To prevent decarburization and desiliconizaiton a SiO 2-Al 2O 3-CaO slag containing a droplet dispersion has been developed for cast irons. With the droplet sizes of less than 100 μm, large maximum undercoolings below the liquidus of 200–620 °C were obtained in both hypoeutectic and hypereutectic alloys. The maximum undercooling level (ΔT max/T L) reached up to 0.40 for a Fe-2.85wt.%C-1.91wt.%Si alloy. The undercooling for heterogeneous nucleation of secondary phases below the stable eutectic extended up to 430 °C for hypoeutectic alloys. The undercooling for a hypereutectic alloy was 20 °C. Based on the undercooling results, for hypoeutectic alloys the primary austenite iron (γ-Fe) is a poor nucleant for both graphite and cementite (Fe 3C). For hypereutectic alloys, primary graphite is very catalytic for γ-Fe. These results were examined by a lattice disregistry model and relative interfacial energy between nucleating solid and catalyst with respect to liquid. At large undercoolings, a variety of solidification microstructures have been observed.

A Calorimetric Evaluation of the Role of Impurities in the Nucleation of Secondary Phases in 1xxx Al Alloys

ABSTRACTSuperpurity (> 99.995wt% pure) lxxx series Al-0.3wt%Fe-0.lwt% Si alloys with different impurity additions have been melt spun to produce ribbons with microstructures of fine scale ∼0.1–0.2μm diameter secondary phase particles entrained in a higher melting point Al matrix. The melting and resolidification behaviour at 2Kmin−1of the secondary phase particles while entrained in the solid matrix was monitored using differential scanning calorimetry, to simulate the nucleation of secondary phases in liquid interdendritic regions during the final stages of solidification in casting. The melting and resolidification behaviour of 500ppm V and Zr doped superpurity ribbons is the same as that of a commercial purity ribbon. This indicates that V is responsible for the promotion of the metastable FeAlmphase in the commercial purity ribbon. The presence of FeAlmin direct chill (DC) cast commercial purity alloys indicates that V also influences phase selection in the faster cooled (∼5-10Ks−1) regions of these alloys.

Effect of increase in heterogeneous nucleation sites on the aging behavior of 6061/SiC metal matrix composites

In this study, aging characteristics of an aluminum based metallic matrix reinforced with various volume fraction of silicon carbide was investigated. The results showed an increase in aging kinetics with an increase in volume fraction of SiC particulates. Results of microstructural characterization studies exhibited the presence of solute rich zone in the near vicinity of SiC particulates and the nucleation of secondary phases both at and in near vicinity of SiC particulates. The results of accelerated aging kinetics thus observed were rationalized in terms of an increase in the heterogeneous nucleation volume in the metallic matrix and the constitutional characteristics of the Al/SiC interfacial region.

Transformation studies of M- to W-type hexaferrite in BaFe12−2xCo3xO19

Samples of general formula BaFe12−2xCo3xO19 (x=0.1, 0.3, 0.5) have been synthesized by ceramic as well as citrate gel routes. The X-ray diffraction patterns indicate that M-type phase forms for x=0.1, while W-type phase is favourable for x=0.3 and 0.5. The results for x=0.3 are particularly interesting. While the X-ray diffraction data show evidence of a monophasic W-type hexaferrite, the magnetic, thermal and microstructural investigations indicate a secondary phase presumably at grain boundaries. Transformation of M- to W-type hexaferrite has been explained in terms of different possibilities such as nucleation of secondary phases, formation of vacancies and different site occupation of ions in the structure.

Microdiffraction studies of nanocrystalline nitride films

Abstract The structure of boron nitride and titanium-boron nitride films has been investigated by transmission electron microscopy using microdiffraction techniques. Nearly stoichiometric BN films were produced by RF sputtering and ion beam assisted deposition. They consisted of predominantly cubic nanocrystallites in a quasicrystalline matrix of hexagonal structure. The mean crystallite diameter of the cubic phase was 10–15 nm in the sputtered films and about half this in the ion beam deposited material. Microarea diffraction studies confirmed that the broadening seen in continuous electron diffraction rings originated from discrete crystallites. Recrystallization and secondary phase nucleation occurred in sputter deposited Ti–BN multilayers as a result of thermal annealing and room temperature ageing. Microdiffraction indicated the presence of TIN and a Ti–B–N solid solution but Ti2N, which has been observed in Ti–TiN multilayers, was not found.

The influence of grain boundaries on the nucleation of secondary phases

In calculating the critical energy required for the nucleation of grains at two, three and four grain junctions it is necessary to know 1. (1) the volume of the nucleus, 2. (2) the surface area of the nucleus, and 3. (3) the matrix grain boundary area which has been absorbed by the nucleus. These three quantities have been calculated as a function of 1. (1) the dihedral angle between the surfaces bounding the nucleus and 2. (2) the radius of curvature of the bounding surfaces. To illustrate the use of the calculated equations the critical energy for the nucleation of ferrite from austenite at two, three and four grain junctions has been calculated.