Solidification microstructure in a supercooled binary alloy

The maximum undercooling that has been achieved for Ni-Cu alloy, by using molten glass purification and cyclic super-heating technology, is 270 K. With the help of high-speed photography, the solidification front images of Ni-Cu alloy at various typical undercooling were obtained. Two grain refinements occurred in the range of 60 K< ΔT < 100 K and ΔT > 170 K, the solidification front became smoother, and the solidification position appeared randomly. With the increase of undercooling, the transition from solute diffusion to thermal diffusion leads to the transition from coarse dendrite to directional fine dendrite. At large undercooling, considerable stress is accumulated and some dislocations exist in the microstructure. However, the proportion of high-angle grain boundaries is as high as 89%, with twin boundaries of 13.6% and most strain-free structures, and the microhardness decreases sharply. This indicates that the accumulated stress at large undercooling causes the plastic strains in the microstructure, and in the later stage of recalescence, part of the plastic strains is dissipated by the system and acts as the driving force to promote the recrystallization of the microstructure.

Microstructure refinement mechanism of undercooled Cu55Ni45alloys

Different undercooling degrees of Cu55Ni45 alloy were obtained by the combination of molten glass purification and cyclic superheating, and the maximum undercooling degree reached 284 K. The microstructure of the alloy was observed by metallographic microscope, and the evolution of microstructure was studied systematically. There are two occasions of grain refinement in the solidification structure of the alloy: one occurs in the case of low undercooling, and the other occurs in the case of high undercooling. Electron backscatter diffraction (EBSD) technology was used to analyze the rapid solidification structure under high undercooling. The features of flat polygonal grain boundary, high proportion of twin boundary, and large proportion of large angle grain boundary indicate recrystallization. The change in microhardness of the alloy under different undercooling degrees was studied by microhardness tester. It was found that the average microhardness decreased sharply at high undercooling degrees, which further confirmed the recrystallization of the solidified structure at high undercooling degrees.

Correlation between Solidified Microstructure Evolution and Undercooling of Au-12 Wt.%Ge Eutectic Alloy

Highly undercooled solidification experiments were carried out by melt purification combined with cyclic superheating method on Au-12 wt.%Ge eutectic alloy. The solidification structures of Au-12 wt.%Ge eutectic alloy under different undercoolings were also analyzed by using the scanning electron microscope (SEM). The experimental results revealed that the maximum undercooling could reach up to 102 K. The microstructure analysis showed that the coarse bulk eutectic existed in the solidification structure when the undercooling was less than 34 K. When the undercooling was larger than 34 K and less than 56 K, the solidification structure transformed into cellular eutectic. The coarse primary (α-Au) phase precipitated from the undercooled alloy melt when the undercooling was larger than 56 K. The volume fraction of the primary (α-Au) phase gradually increased with the increase of undercooling. In this paper, a method to regulate the solidification structure of Au-12 wt.%Ge eutectic alloy is proposed, which provides a new way to improve the solidification structure and has important guiding significance for the processing and forming process of Au-12 wt.%Ge eutectic alloy.

Microstructure and Magnetic Properties of Undercooled Fe-50at%Ni Alloys

Bulk Fe-50at%Ni alloy melts were undercooled using cyclic superheating and glass slag purification. As a result, a maximum undercooling up to 217 K could be achieved. As-solidified microstructures were observed by means of optical microscope. Phase identification of Fe-50at%Ni alloys was performed using the Shimadzu X-ray diffractometer (XRD) system. The chemical constitution revealed using a JEOL Model JSM-6700F scanning electron microscope equipped with energy dispersive X-ray spectroscopy (EDX). The magnetic properties of the alloys were measured by vibrating sample magnetometer (VSM) with a DC M-H analyzer. The results indicated that there were twice grain refinements occurred within an undercooling range of 55-217K, where the first could be ascribed to dendrite-remelting, and the second to recrystallization. The phase composition of undercooled Fe-50at%Ni alloys comprised two phases, i.e., solid solution phases withbccandfccstructure. At various undercoolings, the saturation magnetizationMsandHcrelated closely to the measured grain sizeD, and they were in proportion toD-1by the regression analysis.

Microstructure and Phase Selection of Undercooled Single-Phase Fe-Co Alloy

Fe-Co single-phase alloy melts with different Co contents were undercooled using fluxing method. The maximum undercooling DT = 457K (relative undercooling DT/Tm=0.259) was achieved in this work. At low undercooling (DT), single-phased microstructure was observed, but metastable bcc phase emerged in the as-solidified microstructure once DT exceeded a critical value, DTcrit. In the presence of classical nucleation theory, phase selection in the undercooled Fe-Co melt was investigated, and the theoretical calculation was coincided with the experimental result.

The Contribution of Diffusion Coefficient to the Eutectic Instability and Amorphous Phase Formation

The diffusion coefficient D decides the diffusion length of solute boundary and plays a key role in the microstructure selection. This paper examines quantitatively the contribution of diffusion coefficient to the eutectic instability and amorphorization ability. The maximum growth velocity Vmax and the maximum undercooling Tmax as functions of activation energy Q in strong liquids are deduced theoretically based on eutectic growth model by separating Q from D. It reveals that the larger the Q, the smaller the Tmax and Vmax, which shows the same tendency as experimental values in some Al-based alloys and glass formers. This indicates that it is the sluggish movement of atoms that makes the transition from eutectic to others structural morphologies, even to amorphous phase, occur at smaller interface growth velocity or undercooling, which is the main contribution of the diffusion coefficient to the amorphorization ability.

Microstructure and Magnetic Properties of Undercooled Fe-80at%Ni Alloys

Bulk Fe-80at%Ni melts were undercooled by using cyclic superheating and glass slag purification technique, and the maximum undercooling 340 K could be achieved. The microstructures of Fe-80at%Ni alloys were observed by means of optical microscope (OM). The phase composition was identified by X-ray diffraction (XRD) analysis. The magnetic properties of Fe-80at%Ni alloys were measured by vibrating sample magnetometer (VSM) with a DC M-H analyzer. The results showed that there was only single γ-(Fe, Ni) phase existing in undercooled Fe-80at%Ni alloys. Two grain refinements and one grain coarsening were observed in the undercooling range from 28 K to 340 K. The first grain refinement could be ascribed to dendrite-remelting, and the second to recrystallization induced by the stress originating from rapid solidification. The grain coarsening could be considered as a result of solid-state grain coalescence. The measurement of soft magnetic properties showed that the grain size D decreases with an increase of undercooling, the maximum Ms is 109.98emu/g, corresponding to minimum grain size 42.9μm or undercooling 210 K, and the coercive force Hc is in proportion to the reciprocal of grain size D-1.

The Study of Undercooling of Homogeneous Nucleation for Metallic Melting

In this paper, the thermodynamic and kinetic requirements of heterogeneous and homogeneous nucleation of metallic melting were suggested. Based on the kinetic requirements of nucleation, the mathematic model of wetting angle of heterogeneous nucleation was developed, Based on the wetting angel model, it was predicted that the maximum undercooling of homogeneous nucleation for melts is two thirds of melting temperature. With the wetting angel model, the wetting angles of different catalysts in liquid iron were calculated, and calculation results are in agreement with that of other researchers.

Influence of Containerless Solidification on Hardness in Multifunctional Titanium Based Alloys

A novel synthesis procedure of multifunctional Ti based alloy was suggested under containerless processing using an electromagnetic levitation furnace. In this method, necessary condition to synthesize the alloys with ability of a dislocation-free plastic deformation was determined. That was supported by microstructural observation, hardness measurement and X-ray analysis of the alloys solidified from several undercoolings. The maximum undercooling of the alloy melt was up to 120K. Synthesized alloys that met the condition showed refined microstructures, increase of d-value of (110) plane and a tiny deviation of hardness by cold-working Others partially occurred stress-induced transformation.

Undercooling And X-Ray Structural Studies Of Ti-Zr-Ni Liquids

ABSTRACTMaximum undercooling results for the icosahedral phase (i-phase), polytetrahedral C14 Laves phase, and solid solution phases are presented as a function of composition in Ti-Zr-Ni liquids. Containerless processing was achieved using the electrostatic levitation facility located at NASA/Marshall Space Flight Center. The maximum reduced undercooling decreases with increasing icosahedral short-range order in the ordered phase. The first synchrotron x-ray diffraction data from aerodynamically levitated liquids of Ti-Zr-Ni alloys suggest an icosahedral short-range order in the liquids, supporting Frank's hypothesis, correlating icosahedral order in the liquid with the nucleation barrier to the crystal phase. The strong negative heats of mixing between Ti/Zr and Ni and their relative atomic sizes likely favor the formation of this local icosahedral order.