Multiphase solidification in multicomponent alloys is pertinent to many commercial materials and industrial processes, while also raising challenging questions from a fundamental point of view. Within the past few years, research activities dedicated to multiphase solidification of ternary and multicomponent alloys experienced considerable amplification. This paper gives an overview of our present understanding in this field and the experimental techniques and theoretical methods research relies on. We start with an introduction to thermodynamic databases and computations and emphasize the importance of thermophysical property data. Then, we address pattern formation during coupled growth in ternary alloys and cover microstructure evolution during successive steps of phase formation in solidifying multicomponent alloys. Subsequently, we review advances made in phase field modeling of multiphase solidification in binary and multicomponent alloys, including various approaches to crystal nucleation and growth. Concluding, we address open questions and outline future prospects on the basis of a close interaction among scientists investigating the thermodynamic, thermophysical and microstructural properties of these alloys.

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Multiphase solidification in multicomponent alloys

The objective of this article is to propose new determinations of the diffusion coefficients of various dopants in liquid silicon. The approach of this work is based on an analysis of the effective segregation coefficients obtained by Kodera in Czochralski growth of silicon-based alloys. However, it will be shown that the solidification model used by Kodera presents some serious deficiencies as it does not account for (i) the dependence of the solute boundary layer thickness on the interface velocity and (ii) the effect of density change upon freezing. In addition, the values of the thermophysical parameters used by Kodera can be questioned in view of recently published data. The approach proposed in the present work can be used to provide a much sounder physical basis for the analysis of the data, but it should be stated that the uncertainty on the partition coefficients is such that a measurement technique based on solidification experiments cannot be expected to be very accurate. In this respect, the scatter of the data, hardly avoidable in Czochralski growth experiments, is also discussed. To put things on a quantitative basis, an error analysis is carried out to quantify the error bar attainable by such a measurement technique.

New Determinations of Diffusion Coefficients for Various Dopants in Liquid Silicon

The microgravity condition is one of the ideal conditions for the measurements of diffusion coefficients in high temperature melts because of the absence of convection in the liquid sample. Many kinds of experimental techniques, such as the long capillary method and the shear cell method, have been devised in ground based researches for the measurement of diffusion coefficient in such high temperature melts. Recently, the shear cell technique coupled with the microgravity condition was applied to the measurements of high temperature metallic melts. This technique enables us to measure the diffusion coefficient with high precision. In this paper, a brief discussion is given about the trends of previous diffusion experiments in space performed particularly by Japanese researchers. Recently, the Japanese space agency, JAXA, has developed the shear cell technique for future space experiments in JEM. The current status of the development of our shear cell technique is summarized and new results are presented as typical experiments on the ground by using test samples, for example, liquid silver–gold alloys.

https://iopscience.iop.org/article/10.1088/0957-0233/16/2/002/pdf

Measurements of diffusion coefficients of metallic melt under microgravity—current status of the development of shear cell technique towards JEM on ISS

An understanding of duplex microstructures encountered during high strength aluminium alloy laser beam melting processing

When in situ techniques became available in recent years this led to a breakthrough in accurately determining diffusion coefficients for liquid alloys. Here we discuss how neutron radiography can be used to measure chemical diffusion in a ternary AlCuAg alloy. Neutron radiography hereby gives complementary information to x-ray radiography used for measuring
chemical diffusion and to quasielastic neutron scattering used mainly for determining self-diffusion. A novel Al2 O3 based furnace that enables one to study diffusion processes by means of neutron radiography is discussed. A chemical diffusion coefficient of Ag against Al around the eutectic composition Al68.6 Cu13.8 Ag17.6 at.% was obtained. It is demonstrated that the in situ technique of neutron radiography is a powerful means to study mass transport properties in situ in binary and ternary alloys that show poor x-ray contrast.

https://iopscience.iop.org/article/10.1088/0953-8984/23/25/254201/pdf

In situ studies of mass transport in liquid alloys by means of neutron radiography

This paper presents results of the measurement of solute diffusivities in the Al–Ni and Sn–Bi–In systems. These experiments were conducted in microgravity and may therefore be assigned as reference values. A dedicated facility, which can be used for processing of four identical samples in parallel, was developed for the experiments under consideration. In addition, a rather thorough analysis of possible error sources in shear cell type experiments is carried out. Comparison with work carried out using magnetic fields to control convective flows, and with previous indirect measurements results, is also provided. A coherent picture emerges from the overall agreement between various measurements, and the data from the current work can thus be considered as reference benchmarks for future experiments. Another conclusion of the present study is that the shear cell technique is well suited for the measurement of solute diffusivities.

G. Mathiak

Papers

Reference Microgravity Measurements of Liquid Phase Solute Diffusivities in Tin- and Aluminum-Based Alloys