Melt Overheating Research Articles

Primary Solidification in Aluminum Alloys under Melt Overheating

We examined the effect of melt overheating on primary solidification in aluminum alloys of various systems. The alloys were poured from the temperature of overheating, and no special treatment of the melt was performed. We assessed the sizes of equiaxed and columnar grains or primary crystals and the size of dendritic cells. The experimental results show that the size of main primary crystals depends nonlinearly on the overheating temperature. One can observe the growth of primary crystals and their following refinement, the latter occurring at considerably high pouring temperatures depending on the alloy composition and the cooling rate. The observed results were interpreted in terms of the formation of solidification nuclei upon decomposition of a supersaturated during undercooling liquid solution (melt).

Structural perfection and mechanical properties of ZnSe 1− xTe x crystals grown by the Bridgman technique

Preparation conditions of ZnSe 1− x Te x crystals should guarantee stability of their scintillation parameters (light output, after glow, luminescence maximum wavelength), optical uniformity, high mechanical strength and resistance to cracking. These crystals were grown by the Bridgman technique under different degrees of melt overheating and related evaporation of the more volatile components. Annealing was carried out at 1170–1270 K under zinc vapor and at 1270 K in argon atmosphere. In this paper studies are reported on the influence of the growth parameters on the distribution of Te along the crystal length, the structural perfection of the grown crystals, and their optical and mechanical properties. Wurtzite-sphalerite transformations occurring during isothermal annealing of the crystals and their effects upon resistance to cracking are studied. It can be concluded that the type and concentration of defects, distribution of tellurium, luminescent and optical characteristics of ZnSe 1− x Te x grown using the Bridgman method strongly depend on the degree of melt overheating. Melt overheating by 50 K ensured crystal uniformity of optical absorption. Melt overheating over 100 K led to twin interlayers. With concentration of Te (C Te) >0.3%, the sphalerite structure is stabilized and the packing defects due to twinning disappear. The data obtained provide evidence of a positive effect of Te isovalent dopant upon perfection of zinc selenide crystals, which is presumable due to stabilization of the sphalerite structure leading to optical uniformity. The thermal processing of the grown crystals results in almost complete disappearance of cracking of the material. During this process, the distribution of an optical transmission on a section of crystals becomes more homogeneous, the magnitude of mechanical strength of a central part essentially increases and is levelled in all the volume of the material. Possible chemical reactions are considered which lead to the contamination of crystals with an impurity of carbon, resulting in a decrease of transparency of scintillator ZnSe 1− x Te x in the region of intrinsic radiation.

Dependence of supercooling of a liquid on its overheating

The degree of overheating of a melt often plays a significant role in determining its supercooling behavior and thus the nucleation and growth of crystals and their qualities. However, the relationship between the level of melt overheating and the resulting degree of supercooling is far from being understood. It is demonstrated that the dependence of the degree of supercooling on the level of melt overheating for bismuth is a function of the duration of melt overheating above its melting point. Depending on the duration, the degree of supercooling can increase either discontinuously or continuously with the level of melt overheating. These observations, which can be linked to the transient evolution of short-range structures during melting and solidification, suggest that the similar observations should also be observable for substances exhibiting substantially different short-range structures in the solid and liquid state.

Abrupt discontinuous relationships between supercooling and melt overheating

The dependence of the degree of supercooling (ΔT−) of melts preceding the onset of solidification on the level of melt overheating (ΔT+)above the equilibrium liquidus or melting temperature, is investigated on Sn and SnPb. We demonstrate the dependence of ΔT− on ΔT+ can be either abrupt or continuous, depending on the length of holding time. In particular, the dependence is bounded by two discontinuous limits, and the known continuous linear relationship between ΔT− and ΔT+ is only a special possibility. Our observations can be tentatively linked to the transient structural evolution of melts, and are probably general, occurring also for element Bi and SnSb. Our results may have important consequences for any thermal modeling of electronics manufacturing involving soldering operations.

Effect of Melt Overheating on Primary Solidification in Aluminium Alloys

We examined the effect of melt overheating on the primary solidification in aluminium alloys of various systems. The as-cast structure was shown to be refined on reaching a specific overheating temperature. A mechanism explaining the observed effects was suggested. The decomposition of a supersaturated liquid solution precipitating solidification nuclei is the base of the discussed mechanism.

Sm 2O 3Ga 2O 3 and Gd 2O 3Ga 2O 3 phase diagrams

Four definite compounds exist in the Sm 2O 3Ga 2O 3 binary phase diagram, namely: Sm 3GaO 6, Sm 4Ga 2O 9, SmGaO 3, and Sm 3Ga 5O 12. The 3 1 compound is orthorhombic (space group Pnna - Z.4) with the cell parameters: a = 11.40 0Å, b = 5.51 5Å, c = 9.07Å and belongs to the oxysel family. Sm 3GaO 6 and SmGaO 3 melt incongruently at 1715 and 1565°C; Sm 4Ga 2O 9 and Sm 3Ga 5O 12 have a congruent melting point at 1710 and 1655°C. With regard to the Gd 2O 3Ga 2O 3 system three definite compounds have been identified: Gd 3GaO 6, Gd 4Ga 2O 9, and Gd 3Ga 5O 12. Only the garnet melts congruently at 1740°C with the following composition: Gd 3.12Ga 4.88O 12. Gd 3GaO 6, and Gd 4Ga 2O 9 melt incongruently at 1760 and 1700°C. GdGaO 3 is only obtained by melt overheating which may yield an equilibrium or a metastable phase diagram.

Crystallization of a germanium melt in an external electric field

AbstractData are presented on experimental studies of the influence of an external electric field on crystallization of a germanium melt under the layer of a B2O3 flux. It has been found out that with the field supercoolings of the melt sharply change. This effect is due to the change of the number of active nucleation centres at the germanium – B2O3 flux interface. The maximum supercoolings of the germanium melt ΔT− = 190 K were obtained when a negative potential was connected to germanium. The dependences of supercooling on preliminary melt overheating were measured.

Effect of Composition and the Processing Parameters on the Electrochemical Corrosion of Iron‐Boron Metallic Glasses

The electrochemical corrosion of quenched metallic glasses was investigated by the potentiostatic polarization method. Room temperature Mòssbauer spectra were also measured to obtain information about the state of the samples. The potentiostatic measurements were used to obtain current‐voltage curves over a range −800 to +200 mV (vs. SHE) in order to study the effects of boron content, melt overheating, quenching rate, relaxation, and crystallization processes on the corrosion current. It was found that the corrosion current varied with the quenching rate and was influenced by the boron content in the range investigated. The rate of corrosion was also different at the opposite sides of the as‐quenched ribbons. The corrosion current for eutectic Fe‐B ribbons was found to increase in the course of the structural relaxation and during the crystallization to , but slightly decreased again when the equilibrium structure was reached.

Influence of the melt overheating and the cooling rate on the magnetic properties of Fe 83.4B 16.6 amorphous alloys

The correlation between some technological parameters and magnetic properties of amorphous rapid quenched Fe 83,4B 16,6 ribbons has been investigated. The results show an important role of the cooling rate and the melt overheating on the coercive force, magnetization and microstructure as well as on the amorphous-crystalline transformation of the alloys.