The widely observed microstructural transitions from coarse dendritic to fine equiaxed grains in solidification of undercooled melts have been puzzling for many years. Here we describe recent progress made in solving this puzzle. We review the basic elements of a simple model that assumes that these transitions result from the fragmentation of dendrites by remelting during the period following recalescence when the interdendritic melt solidifies. Furthermore, we review the results of detailed experiments on Ni–Cu alloys specifically designed to test the predictions of this model. A good overall quantitative agreement is obtained between the model predictions and the experimental observations. In particular, the model predicts the existence of four grain refinement transitions, three of which are observed experimentally. It also describes the dependence of the microstructural transition undercoolings on the cooling rate that follows recalescence and on the alloy composition, in agreement with the experimental findings.

Experiments have been carried out for two alloys Fe69Cr1Ni30 and Fe69Cr7Ni24 to investigate the evolution of microstructures of steels grown from undercooled melts. The microstructures undergo several critical transitions as the undercooling at which solidification starts is increased. Firstly, grain size can change by up to an order of magnitude from small to large and back to small at critical undercoolings. Secondly, the segregation pattern changes from dendritic/equiaxed to dendritic and finally to equiaxed with increasing undercooling. Measurements from the segregation patterns indicate that the primarydendrites had undergone considerable Ostwald ripening depending on the processing conditions (heat-extraction rate). Relating the measured trunk sizes to an original trunk size through ripening and then applying this value to a recent model for grain refinement, which is based on dendrite fragmentation, yields a rough approximation to the critical transitions of the grain size.

Fe–Ni droplets (Ni content 0–30at.%) were containerlessly processed by electromagnetic levitation, in a drop-tube or by atomization. The droplet diameter varied from 10μm to 7mm. The competition between the formation of b.c.c. and c.c.p. phases is elucidated by studies of the recalescence behaviour and the dendrite growth velocities as a function of undercooling. For levitation the results are summarized in a phase-selection map. For droplets produced in the spray methods, a selection map was constructed which relates the predominant phases and microstructures (coarse-dendritic or grain-refined) to the composition and the droplet size. Links are established between the final microstructures, the crystal phases and the processing conditions. Thermodynamic modelling (CALPHAD) and an analysis of dendrite growth velocities are used to analyse the nucleation and growth behaviour.

An electromagnetic levitation facility is used to containerlessly process Fe-Ni droplets; undercoolings ΔT of up to 300 K were achieved. Thermal measurements during solidification showed two types of recalescence behaviour for alloys containing between 7.5 and 17 at% Ni: A single recalescence step for ΔT< ΔT* (primary growth of the stable ccp phase) and two recalescence steps for Δ> ΔT* (primary growth of the metastable bcc phase, and the subsequent transformation of the bcc phase and solidification of the stable ccp phase). The critical undercooling ΔT* strongly increases with the Ni-content. The growth velocities at which the primary dendrites propagate through the droplet have been measured for a number of Fe-Ni alloy compositions. The velocities reflect the phase selection, i.e. primary bcc phases grow markedly more slowly than primary ccp phases. Thermodynamic modelling (CALPHAD) and an analysis of the velocity data within current theories of dendrite growth is undertaken to describe nucleation and growth behaviour. The results suggest that the metastable phase is nucleated in preference at high undercoolings because of its lower solid-liquid interface energy and that the kinetics at the bcc-liquid interface is considerably more sluggish than the kinetics at the ccp-liquid interface.

An electromagnetic levitation facility and a drop-tube have been utilized to containerlessly process liquid Ni-C alloy droplets (C content < 5 at%). The microstructures formed upon solidification are found to be either coarse-grained dendritic (at intermediate undercoolings) or fine-grained equiaxed (at low and high undercoolings). The results obtained by levitation are summarized in a microstructure-selection map with composition and undercooling as variables. The map is fairly well described by a dendrite-fragmentation model for grain refinement. The critical undercoolings for the onset of grain refinement increase with increasing carbon content. This behaviour is confirmed by the results obtained from drop-tube processing. The grain size of refined droplets decreases with increasing post-solidification cooling rate. Dendrite growth velocities have been measured as a function of undercooling for pure Ni and for alloys containing 0.6, 1.7, and 3.1 at% C. In contrast to the prediction of current dendrite growth theory, it is found that small additions of carbon to nickel do not enhance the growth velocity at low undercoolings.

The techniques of electromagnetic levitation, drop-tube processing and atomization are used to study containerless solidification of the Cu-Ni and Fe-Ni systems. Microstructure-selection maps are developed for the case of droplet solidification where droplet diameter and undercooling are the process parameters. The microstructure varies systematically with undercooling and composition, such that grain-refined microstructures are produced at both low and high undercoolings and dendritic microstructures are produced at intermediate undercoolings. The results are analyzed within a recently developed model for grain refinement which is based on the fragmentation of primary dendrites. The role of cooling rate in determining whether the microstructure transforms in the solid state or is retained as a primary structure is explored. Keywords: Cu-Ni; Dendrites; Droplet solidification; Fe-Ni; Grain refinement; Microstructure-selection maps

For Cu-Ni alloy droplets processed by containerless solidification under levitation, the crystallographic texture is strongly correlated with the microstructure seen in optical metallography and with the undercooling at which solidification started. Systematic analysis of the texture is useful in understanding the development of the microstructure, in particular whether it is dendritic or grain-refined. The variation, with undercooling, of orientation spread in dendritic samples enables texture measurements to be used to estimate the maximum undercooling of a droplet when temperature measurement is not possible. The contribution of dendrite break-up to the observed grain refinement is analysed, both for low and high undercoolings. Copyright (C) 1996 Acta Metallurgica Inc.

Containerless electromagnetic (EM) levitation processing was used to undercool pure Ge and Ge-2at%Sn alloy up to 426 and 368 K, respectively. The crystal growth velocities in undercooled melts were directly measured with a photodiode technique. Experiments and calculations with the slightly modified current dendrite growth theory have shown that solidification mode was transformed from lateral growth (LG), to continuous growth (CG), and eventually to rapid growth (RG), as the driving force for crystallization, i.e. undercooling was increased, correspondingly leading to faceted twins, randomly oriented dendrites, and refined equiaxed grains. It was found that a small addition of solute Sn to pure Ge made roughening of solid-liquid (S/L) interface occur at a lower undercooling, and the growth velocities rise markedly.

Using containerless electromagnetic (EM) levitation, pure Ge, Ge-0.39 at% Sn and Ge-10 at% Sn alloys were undercooled by up to 426, 404 and 334 K, respectively. The crystal growth velocities in undercooled melts were measured with a photodiode technique. Crystal growth behavior was found to fall into three categories of lateral growth (LG) at low, continuous growth (CG) at moderate, and rapid growth (RG) at high undercoolings, leading to various microstructures. The measured growth velocities were analyzed in terms of slightly modified, current dendrite growth theory. The presence of solute, Sn, was found to have a strong influence not only on the magnitude of the growth velocity, but also on the growth kinetics. The latter effect indicates that the critical undercoolings corresponding to the transitions between solidification mechanisms monotonically decrease as the Sn content increases.

The microstructures of bulk Ge and Cu samples solidified from undercooled liquid were investigated. High levels of undercooling were attained by containerless electromagnetic processing and melt fluxing. For both faceted material Ge and non-faceted Cu, a twin microstructure was formed in a certain range of undercooling, whereas the twins were completely replaced by fine equiaxed grains at larger undercooling. The structural transformation in the two materials was analyzed with respect to the relation between undercooling and the velocity of free crystal growth, twinning mechanisms, and a dendrite break-up model.

Large undercoolings of up to Delta T=426 K for pure germanium were reproducibly obtained using the containerless electromagnetic (EM) levitation technique. The crystal growth behavior and grain structure developments of Ge were studied as a function of the bulk undercooling, Delta T. Three undercooling regions of different growth behavior and morphologies were revealed: (1) lateral growth (LG) for Delta T < 300 K: the typical lamellar twins (111) [211] were grown at the lower end of this region, whereas, at the higher end, a mixed structure appeared. (2) Continuous growth (CG) beyond a threshold undercooling: the crystal growth velocity, V, increases rapidly at Delta T=300 K, which is an evidence for a transition from LG to CG. (3) Rapid crystal growth: homogeneous grain refinement was observed in bulk pure Ge samples when V greater than or equal to 0.8 m/s for Delta T greater than or equal to 400 K. In the CG range, the measured growth velocities agree well. with slightly modified, current dendrite growth theory.

For Cu-Ni alloy droplets processed by containerless solidification under levitation, the crystallographic texture correlates strongly with the microstructure seen in optical metallography and with the undercooling at which solidification started. The variation of texture is useful in understanding the development of the microstructure, in particular whether it is dendritic or grain-refined. Texture measurements can be used to estimate the maximum undercooling of a droplet when temperature measurement is not possible.

Having realized electromagnetic (EM) levitation melting and solidification of pure germanium, we are able to undercool Ge melts (8–10 mm diameter) 426 K below its melting temperature, Tm, and to determine the crystal growth velocity V as a function of undercooling ΔT by measuring the recalescence times of the levitated droplets. The results are analysed in terms of current dendrite growth theory with a slight modification. The transition from stepwise to continuous growth with increasing undercooling is revealed.