Study of non-equilibrium dendrite growth in an undercooled alloy

ABSTRACT

Nonequilibrium thermodynamics and transportation kinetics near the propagating solid–liquid interface dominates the rapid solidification process, which is far from a thermodynamically stable state. Rapid solidification process can be described more precisely using quantitative thermodynamic calculation of phase diagram with nonlinear liquidus and solidus and evaluating the nonequilibrium effect in diffusion kinetics. Based on these basic principles, we have used a current nonequilibrium dendrite growth model to describe the rapid solidification process and the recalescence temperature of deeply undercooled alloys. Evolution of the key fundamental solidification parameters was also evaluated. The experimental data agree well with the model prediction.     

Corrosion of Al—Fe in hydrochloric acid

Abstract

Nanoprocessing can be considered a distinct form of grain boundary engineering by which property enhancements are achieved by deliberately increasing the volume fractions of grain boundaries and triple junctions in a material. Electrodeposition has been shown to be a technologically viable production method to synthesize such materials both in bulk form and as thin films. The mechanical, magnetic, electrical and corrosion properties exhibited by nanocrystalline metals produced by this method make them strong contenders for a number of advanced materials applications.

Résumé

Nous pouvons considérer le nanotraitement comme une forme distincte de génie des joints de grains par lequel nous obtenons des améliorations de propriétés en augmentant délibérément le volume des fractions des joints de grains et des jonctions triples dans les matériaux. On a montré que l'électrodéposition est une méthode technologiquement viable pour synthétiser de tels matériaux, aussi bien en films minces que sous forme massive. Les propriétés mécaniques, magnétiques, électriques et corrosives des métaux nanocristallins obtenus par cette méthode, font d'eux des concurrents sérieux pour nombreuses applications de matériaux avancés.     

Stress induced deformation in the solidification of undercooled Co80Pd20 alloys

Substantial undercooling ΔT up to 415 K was achieved for Co₈₀Pd₂₀ melt applying molten glass denucleation combined with cyclic superheating. The as-solidified structure as function of ΔT was described concisely. On this basis, dense-regular fault (DRF) ribbons were detected provided if a critical undercooling is surpassed. An integrated analysis of the DRF formation with a model calculation [16] for shrinkage-stress developed in the coherent dendrite network upon rapid solidification was then performed. This confirms that the as-formed DRF originates from a stress-induced deformation, which also plays an important role in understanding the grain refinement occurring upon rapid solidification of undercooled melts.     

球墨铸铁缩孔缩松的铁液冶金因素影响探析

梳理了国内外关于冶金因素对球铁缩孔缩松影响的认知,并列述了一些具有共识的看法和一些具有争议的观点。对球铁碳当量的选择、过冷度的控制,炉料选择等给出了自己的看法,同时阐明了需加强通过提高冶金质量及浇注过程中气体排放等措施来改善球铁的缩孔缩松倾向。     

Undercooling associated with slow freezing and its influence on the microstructure and properties of rice starch gels

It is well accepted that high undercooling or supercooling usually produces numerous small ice crystals. This paper shows that if heat transfer is not rapid enough, high undercooling causes non-homogenous sized ice crystals. Three freezing regimes (i.e. fast, slow and slow with undercooling) were used in this study. Fast freezing produced numerous homogeneously small ice crystals embedded in a thin rice starch gel matrix. This microstructure caused low % syneresis and hardness versus slow freezing’s rather homogenous distribution of fewer large ice crystals embedded in a thicker gel matrix resulting in high % syneresis and hardness. However, slow freezing with undercooling produced non-homogenous clusters involving small and large ice crystals embedded in a very thick gel matrix. Starch retrogradation before ice formation played an important role in this frozen structure. The information gained from this study enhances understanding of the behavior of starch-based food during freezing and storage.     

Effect of high undercooling on Co-Ni-Ga ferromagnetic shape memory alloys

Co₄₅Ni₂₅Ga₃₀ ferromagnetic shape memory alloys were treated by glass fluxing combined with superheating cycling. The effect of high undercooling on solidified microstructure and transformation temperatures was investigated. The martensite lath is obviously refined and a mass of sub-grains are produced with the increase of undercooling. Undercooling rapid solidification introduces a number of dislocations and large internal stress, which give rise to the production of sub-grains in recovery. During the following annealing process, these sub-grains can gradually recrystallize and grow up with an amount of γ′ phase precipitated. Meanwhile, transformation temperatures of undercooled alloys are greatly elevated compared with those of as cast, which results from the large internal stress, and gradually reduce with the heating time.     

Phase selection and microstructure formation in undercooled Co–61.8 at.% Si melts under various containerless processing conditions

The hypoeutectic composition Co–61.8 at.% Si was undercooled and solidified using electromagnetic levitation, electromagnetic levitation under a static magnetic field, electrostatic levitation and glass-fluxing. The samples generally showed two thermal events, either separated or continuous depending on undercooling. In situ monitoring of the two thermal events with a high-speed camera revealed a sudden decrease of dendritic growth velocities of primary phases at a critical undercooling of 88 K. Scanning electron microscopy studies of the solidified samples showed that the CoSi compound and the CoSi₂ compound nucleate as the primary phase for low and high undercoolings, respectively. The microstructure of the samples depends not only on undercooling, but also on the onset temperature or delay time of the second thermal event. Melt convection has no effect on the primary phase selection in undercooled melts, but it has a significant effect on the delay time and therefore on microstructure formation of the samples for high undercoolings.     

In situ observation of Cu segregation and phase nucleation at a solid–liquid interface in an Al alloy

This paper presents a systematic study comparing experimental in situ transmission electron microscopy observation of microstructural and compositional evolution with complementary thermodynamic calculations, to better understand the redistribution of solute elements and the nucleation behavior of different phases in a commercial Al-alloy powder (AA390). The results show that Cu segregation to the solid Si–liquid Al interface, as well as the significant undercooling achieved in the liquid under non-equilibrium conditions because the Al phase cannot nucleate homogeneously, play a important roles in nucleating Al₂Cu at the interface prior to the Mg₂Si phase in the alloy. Although Cu segregation can occur at various locations along the interface, the Al₂Cu phase appears to preferentially nucleate at a high-index Si–liquid interface as opposed to a low-index one. The Cu concentration during segregation remains essentially constant with time, indicating that the observed segregation behavior is a thermodynamic and not a kinetic phenomenon. These in situ observations and complementary thermodynamic calculations substantially enhance our understanding of potential crystal nucleation and growth processes.     

The surface tension of liquid silicon at high temperature

The measurement of the surface tension of liquid silicon has a long history with many results but no general agreement between them. Two values at the melting temperature are cited in reviews (749 and 827 mN/m [N. Eustathopoulos, E. Ricci, B. Drevet, Note Technique DEM No. 97/58, CEA, 1997]) but there are few arguments to determine the correct one. In the present study, new data for the surface tension obtained with the analysis of characteristic frequencies of a levitating drop are presented. The effect of oxygen and nitrogen are also considered. These data are compared with former data obtained with contactless techniques. The most recent surface tension values obtained with drop weights ranging on two orders of magnitude and environments of different natures (argon, hydrogen and vacuum) show excellent agreement (within a 1.5% margin) at temperatures between 1350 K and 2400 K. The comparison of these data to others obtained with different techniques, reveal a good agreement, except those obtained with the sessile drop technique on some supports like BN, SiO₂ and MgO. However, these special cases may be connected with the reactivity of silicon with these supports.     

铸造锌铝系合金底缩现象探讨

锌铝系合金铸造时出现反常的底部缩孔,而用一般上置式冒口对此类缩孔的补缩效果甚微。由于重力偏析造成的异常成分过冷导致底缩现象,因此针对成因提出消除或减轻底缩的途径。