Hypereutectic Melt Research Articles

Growth of Eutectic Austenite in Free Graphite Cast Irons

The present work investigates the growth of the eutectic austenite of hypereutectic free graphite cast irons of different graphite morphology. The study was based on the analysis of several samples obtained from a highly hypereutectic cast iron melt. A hypereutectic melt was used in order to ensure the absence of proeutectic austenite. With the aim of obtaining samples of different graphite morphology but nearly identical chemical composition, the melt was nodularized by using a standard procedure, and different samples were cast after various fading periods. Through this procedure, spheroidal, vermicular and flake graphite samples were obtained The results show that the eutectic austenite grows dendritically in all samples investigated. Nevertheless, significant differences were found in the size of the solidification units (grains) and in the microsegregation patterns as the graphite morphology changes from spheroidal to flake.

Origin of the anomalous volume expansion in Al–Si alloys above liquidus

The sessile-drop method and high-temperature small-angle x-ray diffraction technique were used to characterize the density of liquid Al–Si alloys with Si contentC= 0 to 22 wt%. A distinct abnormal volume expansion of the hyper-eutectic melts was identified just above the liquidus (withinTL–Tk) during the heating sequence (h.s.), which is irreversible during the cooling sequence (c.s.). This phenomenon is explained by the segregation of Si atoms from the supersaturated “quasi-eutectic” structure and formation of silicon clusters in the superheated melt. In addition, a pre-peak in the x-ray diffraction spectrum of Al–14% Si melt was observed at 850 °C, which is associated with the silicon clusters. The pre-peak remains when the superheated melt is cooled down to the liquidus (670 °C); this result is in accord with the density measurements.

Investigations on the directional solidification of the eutectic LiF–LiBaF3

Abstract Important physico-chemical properties of the regular, lamellar eutectic LiF–LiBaF3 and its components have been investigated and the phase diagram LiF–BaF2 was redetermined. These investigations have been carried out partially to prepare and develop experimental concepts for the directional solidification of the eutectic LiF–LiBaF3 under microgravity conditions. Eutectic and off-eutectic LiF–LiBaF3, melts have been solidified under different growth conditions (growth velocity ν, temperature gradient G) and the correlation between the G/ν ratio and the eutectic structure was determined. The correlation between the average phase spacings λ and the growth rate ν (quantitative image analysis data) resulted in the calculated relation λ 2.08 ν = constant (regions of exact eutectic compositions) which is in good agreement with the Zener criterion λ 2 ν = constant. The high melt viscosity leads to low Rayleigh numbers, Ra, which is orders of magnitude lower than the critical Rayleigh number, Ra c , and therefore a strong reduction of convective influences during the eutectic growth is expected. It can be concluded that the growth from eutectic LiF–LiBaF3, melts is mainly governed by diffusion and this confirms the growth theories by Jackson and Hunt. During eutectic growth the G/ν ratio plays an important role in respect to the eutectic structure. Low growth rates and high temperature gradients lead locally to a transition of the eutectic structure from lamellae to fibers. We found empirically a correlation between fibers, lamellae and eutectic cells. But at present no quantitative theory of modification in the eutectic structure has been formulated yet that a given temperature gradient with a given growth rate will cause lamellae to fiber transition in a given normal eutectic. Furthermore the reason for the observed deviations of the volume fractions in the eutectic structure from exact eutectic melt compositions was studied. The enrichment of the LiBaF3, component at the growth interface of a seed with eutectic composition in dependence of the soaking time of the eutectic melt could be attributed to the Soret effect (thermodiffusion) and is not primary a consequence of sedimentation effects (influence of gravity). Our experiments under microgravity conditions are expected to give unequivocal informations concerning the influence of gravity and Soret effect on the directional solidification. The directional solidification of hypo- and hyper-eutectic melts indicate that the LiF (phase with higher undercooling) acts as nucleus for LiBaF3, but not vice versa (‘non-reciprocal nucleation’). Hyper-eutectic melts have no detectable negative influence on the quality of the eutectic structure whereas the crystallization of hypo-eutectic melts leads to disturbances of the regularity in the lamellar structure.

Solubility of Li2 S in LiCl ‐ KCl Melts

The solubilities of in hypereutectic melts between 745 and 827 K, in hypoeutectic melts between 732 and 821 K and in eutectic melts between 681 and 862 K have been measured by an optical technique. In this technique the progress of dissolution with increase in temperature was monitored by observing the sample piece with a stereo‐optical microscope. The solubilities in the hypereutectic, hypoeutectic, and eutectic melts are respectively given by the following equationsAt higher temperatures (>862 K), additional measurements were made in these melts using a differential thermal analysis technique (DTA). In each case, the data obtained by the optical and DTA techniques were combined to present a pseudobinary equilibrium phase diagram of the and system. Neither of the three systems form any compounds and can be represented by only a liquidus.

亜共晶，過共晶鋳鉄の凝固過程に及ぼすMn, Sの影響

The solidification process of hypo-and hyper-eutectic Fe-C alloy was investigated by thermal analysis and quenching method. The flake graphite separated from the eutectic austenite in hypo-eutectic iron, while in hyper-eutectic alloy the primary graphite separated directly from the melt and was surrounded by small austenite dendrites followed by the eutectic solidification around them. Addition of manganese to these alloy made plate-like cementite in the melt at eutectic or at primary solidification which neucleates the ledeburite colonies, and the ledeburite structure was normal and the solidification temperature was not so much supercooled. On the contrary, addition of sulphur changed the flake graphite to undercooled structure containing small amount of nodular graphite or cluster-type mesh-like graphite and then produced the inverse-chilled or massive ledeburite structure. The temperature of austenite-graphite eutectic solidification was lowered by increasing the sulphur content and that of the formation of plate-like cementite in eutectic or hyper-eutectic melt was lowered considerably. By these investigations it was considered that manganese stabilized the cementite and made its formation more easy than that of the eutectic austenite or the primary graphite, but sulphur interrupted the formation of eutectic austenite or primary graphite and supercooled the melt resulting in the formation of some abnormal graphites or massive ledeburite.