The non-equilibrium state of an undercooled melt allows the investigation of various phenomena involved in the formation of metastable solid phase. The present work reviews the present state of the art in this field. In detail, the thermodynamics of the undercooled melt, nucleation and growth processes are discussed, with special emphasis on the conditions of the formation of metastable states, such as metastable crystalline metals, quasi-crystals, metallic glasses, supersaturated solutions and grain-refined materials. The techniques needed for the in-situ investigation of non-equilibrium solidification phenomena in undercooled melts are discussed, and their potential for such investigations is evaluated. Aspects of microgravity research in this field are also included.

Non-equilibrium solidification of undercooled metallic metls

The study of metallic melts, their physical properties, and their solidification behaviour is of relevance to understanding of fundamental principles and to metallurgical applications. Many, if not...

Containerless processing in the study of metallic melts and their solidification

An overview of rapid solidification is undertaken and various rapid solidification processes are grouped into three classes. This leads to a better understanding of the similarities and differences between the various methods in terms of solidification parameters. Microstructures (with specific attention to rapid laser treatment) are analysed and various microstructure selection maps are presented.

Rapid solidification processing and microstructure formation

This is the first account of the history of our understanding of, and ability to model, solidification microstructures. Its objective is to retrace the scientific steps made, from the earliest obse...

Progress in modelling solidification microstructures in metals and alloys: dendrites and cells from 1700 to 2000

The status of rapid solidification of highly undercooled bulk liquid superalloys is critically evaluated and future directions of research in this technologically important area are proposed. First, consideration is given to purification mechanisms and preparation of a novel SiO2–ZrO2–B2O3 (i.e. Si–Zr–B) compound mould coating, for multi-component superalloy melts, with an emphasis on DD3 alloy. On this basis, thermodynamics of the undercooled multi-component melt and the nucleation inhibiting effect of the Si–Zr–B coating are discussed. Consideration is also given to evolution of microstructure and sub-structure within a wide undercooling regime, in particular, the mechanism of dendrite growth and the characteristics of rapid solidification. Growth mechanisms for the two types of grain morphology, at low and high undercoolings, are reviewed, as is precipitation of the strengthening phase, γ' (Ni3Al(Ti)), in the as solidified superalloy subjected to different melt undercoolings. Finally, direction...

Rapid solidification of highly undercooled bulk liquid superalloy: recent developments, future directions

A comparison is made between high speed cinematography and optical temperature measurements of the solidification of an undercooled Ni-25 wt pct Sn alloy The first part of this study notes that solidification during the recalescence period at all undercoolings studied occurred in the form of a dendritelike front moving across the sample surface, and that the growth velocities observed agree with calculation results for the dendrite growth model of Lipton et al (1986); it is concluded that the coarse structure observed comprises an array of much finer, solute-controlled dendrites In the second part, attention is given to the solidification of levitated metal samples within a transparent glass medium for the cases of two undercooled Ni-Sn alloys, one of which is eutectic and another hypoeutectic The data obtained suggest a solidification model involving dendrites of very fine structure growing into the melt at temperatures near the bulk undercooling temperature

Dendritic growth of undercooled nickel-tin: Part I

The effects of undercooling on the thermal behavior and structure of Ni-Sn alloys are investigated. Hypoeutectic (Ni-25 wt pct Sn) and eutectic (Ni-32.5 wt pct Sn) compositions of the Ni-Sn alloy were undercooled using a levitation melting with glass encasement technique, and the recalescence of these alloys was measured using a high speed temperature sensing device and a digital oscilloscope. It is observed that in both samples the total solidification and recalescence times decrease with increasing undercooling; the volume fraction of normal lamellar eutectic decreases with increasing undercooling; and in the hypoeutectic sample, the morphology of the primary phase changes from dendritic to spherical with increasing undercooling.

Structure and recalescence behavior of undercooled nickel-tin alloys

An Alloy Undercooling experiment was performed in an electromagnetic levitator during the Columbia STS 61-C mission in January 1986. One eutectic nickel-tin alloy specimen was partially processed before an equipment failure terminated the experiment. Examination of the specimen showed evidence of undercooling and some unusual microstructural features.

The alloy undercooling experiment on the Columbia STA 61-C space shuttle mission

A comparison is made of microstructures of droplets of Ni-Sn alloys rapidly solidified by gas atomization and in a glass emulsifying medium. Cooling rate of the gas atomized particles ranged from 10 to the 3rd to 10 to the 6th K/s depending on droplet diameter (20-230 microns). In the hypoeutectic alloy studied, /Ni-(25 wt pct Sn)/, most particles showed a dendritic structure. These same particles, melted and resolidified in a glass medium using DTA (Differential Thermal Analysis), showed undercoolings up to 280 K: the structures were dendritic at low undercoolings and nondendritic at undercoolings above 220 K. It is concluded that the gas atomized particles exhibited little or no undercooling before nucleation; the solidification time of the undercooled emulsified droplets is substantially less than that of gas atomized droplets, and the undercooling required to achieve nondendritic structure depends on sample size.

Comparison of Structures of Gas Atomized and of Emulsified Highly Undercooled Ni-Sn Alloy Droplets

The Space Shuttle Columbia carried an Alloy Undercooling Experiment on its STS 61-C mission in January, 1986. The experiment was performed in the electromagnetic levitator (EML) designed and produced by the General Electric Company. A sample of Ni-32.5wt% Sn eutectic was melted and solidified under microgravity conditions in the Space Shuttle. The specimen achieved only a fairly small undercooling, probably less than 30 K. The specimen was examined by optical and scanning electron microscopy. The surface and cross-sectional microstructures were primarily composed of normal lamellar eutectic, but showed several interesting features, including an apparent surface nucleation site, curved dendrites with non-orthogonal secondary arms, dendrite fragments with extremely fine arm spacing, submicron precipitates, and faceted crystals. The results of the space experiment are presented and compared with ground-based results obtained with the same alloy.

Yanzhong Wu

Papers

Dendritic growth of undercooled nickel-tin: Part I

Structure and recalescence behavior of undercooled nickel-tin alloys

The alloy undercooling experiment on the Columbia STA 61-C space shuttle mission

Comparison of Structures of Gas Atomized and of Emulsified Highly Undercooled Ni-Sn Alloy Droplets

Solidification of Undercooled Ni-Sn Eutectic Alloy Under Microgravity Conditions in the Space