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This is a survey of the published works of Prof. Bingbo Wei of the Department of Applied Physics at Northwestern Polytechnical University, Xian P.R. China. Transformations among solid - liquid - and vapor are fundamental to the foundations of life and culture on Earth. The development and understanding of materials has lead the evolution and advancement of the human race since antiquity. Materials and fluids research is continuing today, with us standing on the shoulders of those that have gone before us. Technological and scientific breakthroughs continue due to studies of greater and greater complexity, that include for example, research done at high pressures, in high magnetic fields, at temperatures near absolute zero, and in the low gravity environment of low Earth orbit. Of particular technological importance is the liquid to solid transformation of metals and alloys. Solidification processing is generally the most important factor in the final properties of objects made of metal; and undercooling is the fundamental driving force for all solidification. The interest and resources dedicated to the study of solidification and undercooling are great and World wide. For many years B. Wei and his coworkers have been studying undercooling and rapid solidification and have amassed a significant body of published research in this important field, contributing to the leading edge of the state-of-the-art. It is the goal of this memorandum to provide a review of the research of B. Wei et al.; publications in Chinese are included in the reference list but are not discussed. The bulk of Wei's work has been in the area of undercooling and rapid solidification [1-11, 13-16, 24-36] with papers dating back to 1989, the same year he earned his Ph.D. Below, discussions of Wei's undercooling and rapid solidification research have been grouped together mostly on the basis of alloy type, such as eutectic, intermetallic, or monotectic.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized. Significant accomplishments during the period were achieved in the following areas: Extended Solid Solubilities -- for Production of Alloys with New Compositions and Phases; Interface Stability -- for Production of More Homogeneous Alloys; Interface Kinetics -- for Production of Alloys with Finer Segregation; Prevention of Cracking Caused by Residual Stress -- for Production of Alloys with Improved Surface Structures.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

Solidification theory can be used to predict and control processes occurring during rapid solidification. Two papers prepared for publication provide results showing such predictions. (1) The effects of rapid solidification velocity on microstructure and phase solubility extension in nickel aluminum-chromium quasibinary alloys are reported. (2) Segregation behavior and microstructural spacings in silver-15 wt% copper alloys are reported as a function of solidification velocity around and below the transition velocity from cellular to banded microstructures.

Solidification theory can be used to predict and control processes occurring during rapid solidification. Two papers prepared for publication provide results showing such predictions. (1) The effects of rapid solidification velocity on microstructure and phase solubility extension in nickel aluminum-chromium quasibinary alloys are reported. (2) Segregation behavior and microstructural spacings in silver-15 wt% copper alloys are reported as a function of solidification velocity around and below the transition velocity from cellular to banded microstructures.

Solidification theory can be used to predict and control processes occurring during rapid solidification. Two papers prepared for publication provide results showing such predictions. (1) The effects of rapid solidification velocity on microstructure and phase solubility extension in nickel aluminum-chromium quasibinary alloys are reported. (2) Segregation behavior and microstructural spacings in silver-15 wt% copper alloys are reported as a function of solidification velocity around and below the transition velocity from cellular to banded microstructures.

Effects of 0.01-2.0 wt% yttrium and erbium additions on rapid solidification processing, superplastic forming, welding, and strengthening of titanium alloys were determined. Ti-8Al-Y and Ti-8Al-Er alloys were rapidly solidified by laser surface melting and electron beam melting and splat-quenching with the objective of simultaneously producing coherent Ti3Al precipitates and incoherent dispersoids. Rapid solidification at cooling rates 10,000 K/s resulted in 10-100 nm incoherent dispersoids in Ti-8Al-1.5Y and in a nonequilibrium, supersaturated, solid solution Ti-8Al-1.5Si. Dispersoid formation and stability at 700-900 C were studied, and the annealing conditions for the production of a large volume-fraction of fine incoherent dispersoids and of coherent Ti3Al precipitates were determined. Metallic yttrium additions to Ti-6Al-2Nb-1Mo-0.8Ta improved the superplastic formability and increased the strength and ductility of laser weldments. The superposition of solid-solution strengthening, precipitation strengthening, and grain-size strengthening in ingot processed Ti-xAl (x = 4, 5.5, 7.0, 8.5, and 10.0) alloys was studied.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

This report covers the progress made in the second year of a three study on the application of rapid solidification processing (RSP) to A1 and Mg alloys. The main emphasis of this study has been on the understanding of the microstructures produced by rapid solidification. The alloy systems studies include: 1) A1 alloys for elevated temperature applications, namely A1-8Fe-2Mo with additions of Si, and 2) Mg alloys for elevated temperature applications, namely, Mg-Gd,Mg-Li,Mg-Li-Si. Keywords: Physical metallurgy; Magnesium; Lithium; Silicon; Gadolinium; Hardness; Tensile strength.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

The elements of rapid solidification processing (RSP) are highlighted from the standpoints of (a) the fundamentals involved, (b) the prospects for new structure/property relationships, and (c) the interplay between materials processing and the wider field of materials science and engineering. Prototype methods of rapid solidification and consolidation are briefly described. Particular attention is then given to various solidification modes and resulting morphologies as a function of cooling rate from the molten state. These modes span the range of planar, cellular, dendritic, and segregationless (microcrystalline), solidification, the latter being achieved by critical or hypercritical undercooling. The need for careful characterization of RSP microstructures is emphasized, and important scientific challenges for advancing the field of RSP are stressed.

The work performed during the past three years has involved studies of some elevated temperature Aluminum and Magnesium alloys, and in addition a preliminary study of intermetallic compounds based on Al3X, where X is Ti, V, and Ni+Ti. In the case of the AL alloys, this work was in essence a continuation of the work performed during the preceeding three years of study, where rapid solidification processing (RSP) of AL alloys was investigated. Two areas were investigated, one involving alloys which might exhibit high elastic moduli, and the second concerned the improvement in fracture toughness when Si is added to AL-Fe-Mo alloys. It was found that the modulus increases AL-Be alloys processed by rapid solidification were as expected from theory, whereas those for AL-Mn alloys were somewhat below expectation. This latter result was attributed to casting porosity present in our samples. In terms of the AL-8Fe-2Mo-Si alloys (where both 0.5% and 1.0% si have been added), the development of microstructure following RSP has been studied in alloys with, and without, the Si additions. The presence of Si resulted in the information of the compound alpha-ALFeSi, with space group determined to be Im3 by convergent beam electron diffraction. The morphology of the precipitates of this compound were found to be approximately spherical and when compared with the lenticular precipitates in the case of consolidated Al-8Fe-2Mo, this shape is thought to account for the difference in fracture toughness results. Keywords: Powder metallurgy, Powder compaction.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

Solidification theory can be used to predict and control processes occuring during rapid solidification. Two papers prepared for publication provide results showing such predictions. (1) Interface shapes and compositional fields produced during directional solidification, calculated as a function of solidification velocity, are reported. (2) Rapid solidification microstructures and precipitation produced upon annealing in rapidly solidified Al-Mn alloys having compositions in the extended solid solubility range are reported.

This report comprises the second year of a two year program on a Rapid Solidification, extrusion processing and mechanical property measurement of Magnesium alloys. During the second year, samples were cast using a controlled atmosphere (He gas), and vertical wheel melt-spinning apparatus constructed at MIT. Casting conditions were thus perfected, to obtain larger quantities of ribbons of consistent quality. Extrusion procedures were investigated on rapidly solidified magnesium alloys. A gradual lowering of the extrusion temperature from 470 C to 210 C yielded improved mechanical properties on alloys ZK60, EZ33, and Mg-5.3%Zn. Optimal conditions were: a reduction in area ratio of about 1/30, and an extrusion temperature of about 200-250 C. Results on alloy ZK60, extruded under these conditions, displayed interesting mechanical properties, and made it comparable in specific properties to R.S.P. aluminum alloys. Tensile properties in the as extruded condition were: U.S. 53.0 ksi, and U.T.S. 56.3 ksi. The ductility in particular was significantly higher than that of wrought ZK60 alloy: 19.6% (typical mechanical properties of the conventional extruded material are: Y.S. 41 ksi, U.T.S. 51 ksi, elongation, 11%). Originator furnished keywords include: Rapid soldification, Processing, Extrusion, Mechanical properties, Magnesium alloys, Power metallurgy.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

The first section of the progress report is concerned with the development of rapidly solidified Al-8Fe-2Mo-SI alloys for elevated temperature applications. Rapidly solidified Al-8FE-2Mo alloys containing silicon in the range of 0 to 1wt pct were produced by melt spinning. In the second annual report, preliminary results of the effect of Si on the formation of the Sone A microstructure and on subsequent changes of this structure during heat-treatment were described. It was shown that addition of Si has a dramatic effect on the trasnformation behavior and microstructure of heat-treated ribbons. In contrast to the thermally unstable, multiphase precipitate distribution in the ternary Al-8FE-2Mo alloy, quarternary Si-containing alloys showed a uniform dispersion of fine cuboidal/polygonal precipitates, which displayed a remarkable resistance to coarsening. A typical TEM micrograph portraying this effect is shown in FIg 1 for a Al-8Fe-2Mo0-0.5Si alloy heat treated at 450 C for 50 hrs. To identify the nature of the precipitates, x-ray and electron diffraction were used, and these results are reported here. Keywords: Molybedenum alloys; Silicon aluminum alloys; Iron alloys.

During the second year of the program, there have been five areas of emphasis. These are: 1) the development of a model to describe the mechanism of formation of rapidly solidified Al alloys (i.e. zones A and B); ii) the processing of particulate whose microstructure consists entirely of zone A; iii) the microstructures of rapidly solidified particulate of alloys based on Ai-8Fe; iv) the identification of metastable phases which form in some of the Al alloys studied; and v) dynamic compaction of particulate. A simple model for the formation of rapidly solidified hyper-eutectic Al alloys has been developed. This model is based on the degree of undercooling achieved prior to nucleation and the extent to which recalescence effects may be inhibited by a rapid rate of heat-extraction. Zone A microstructures form when the temperature is lowered to below that of the extended alpha-Al liquidus where primary Al may be nucleated. The microstructures of rapidly solidified Al-Ni and Al-Co alloys may be interpreted satisfactorily by this model.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

The semi-annual technical report for ARPA Order 3751 covers the period 1 Oct 79 to 31 Mar 80. Work is reported in the following areas of rapid solidification processing: (1) Interface Stability during Rapid Solidification, (2) Eutectic Solidification and the Formation of Metallic Glasses, and (3) Thermodynamics of Metastable Equilibria.

Rapid solidification techniques for producing laminated, particle- reinforced, and fiber-reinforced metal-matrix composites are discussed. Because the time for which the different phases of the composite are in contact at high temperature is limited to milliseconds, combinations of materials can be used which would react or degrade when held at these temperatures for the length of time required to form composites by conventional techniques. Rapid quenching to room temperature also allows the production of matrix alloys with non- equilibrium concentrations of elements. Subsequent segregation may improve matrix properties or improve the integrity of the bond at the matrix/ reinforcement interface. Additionally, RST is a commercially viable method for producing amorphous alloys in large volumes. (JES)

The objective of this work is to develop guidelines based on kinetic and thermodynamics solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

This report covers the progress made in the first year of a three year study on the rapid solidification processing (RSP) of Al-and Mg based alloys. In this study, effort has been applied in three areas: (1) Processing (both particulate production and consolidation), 2) Microstructural effects, and (3) Mechanical Properties. The underlying objective has been to control the microstructure to obtain favorable mechanical properties. This will, perforce, entail a detailed understanding of the microstructures, as affected by factors such as alloy chemistry and processing variables, and, in turn, their effects on the mechanical properties. The microstructural changes which occur in heat-treated, as-rapidly solidified materials, as well as those in materials consolidated, either by extrusion or by the relatively new technique of dynamic compaction, have been investigated. The alloy systems studied include: (1) high modules A1 alloys, namely, Al-Mn and Al-Be, (2) Al alloys for elevated temperature applications, namely, Al-8Fe-2Mo, and (3) Mg alloys for elevated temperature applications, namely, Mg-20Gd and Mg-Li-Si. The report is divided into three major sections: (1) Brief overview of RSP of individual alloys, (2) Results, and (3) Future work. Individual sections are further subdivided as appropriate.

Work is reported on the following areas of rapid solidification processing: (1) Microsegregation and Stability at the Solidification Interface, (2) Glass Formation by Rapid Cooling of Eutectic Systems, (3) Compositional Limits Imposed by Thermodynamics during Rapid Solidification and Relative Stability of Heterogeneous Microstructures. The theory of rapid solidification was investigated to determine whether major aspects of rapid solidification processes can be explained by conventional solidification theory and to examine non-equilibrium effects which can arise. In particular, segregation effects, glass-forming tendencies, and rules governing the formation of equilibrium and non-equilibrium phases, including alloy composition limits, reaction sequences and metastable phase formation, will be investigated.

Rapid solidification allows the production of alloys with new compositions and phases and also allows production of improved alloys by control of microstructures and homogeneity. The effect of rapid solidification velocity on the micro-structure of Ag-Cu alloys is comprehensively described, mechanisms of microsegregation-free solidification are reviewed, and microstructure and phase solubility extension in rapidly solidified NiCr-Al quasibinary eutectic alloys are discussed in papers prepared for publication.

Regarding work on the development of microstructure during rapid solidification, three areas have been addressed. The first of these involved a determination of the mechanism of formation of the so-called zones A and B in hyper-eutectic AL-transition metal alloys. The second area of work involving the development of microstructure concerns submerged phase transformations. In a study of AL-Be hyper-eutectic alloys, it was determined that solidification proceeded by a set of phase transformations that may be described by a monotectic reaction. The third area of study concerning microstructural development involves quasi-crystalline AL alloys. In fact, work done in this program has concentrated on the potentially beneficial aspects of quasi-crystalline phases in the microstructure of AL alloys. Work on the consolidation of particulate has concentrated on the use of conventional techniques (.e. extrusion) and novel processes (i.e. dynamic compaction). An estimate of the mechanical properties of rapidly solidified AL alloys has been obtained. As explained above, the effect of extrusion is to cause decomposition of the rapidly solidified microstructure. A comparison has been made, using the alloy AL-8Fe-2Mo, between the tensile properties of the decomposed microstructure (.e. extruded) and sub-scale test specimens produced by laser surface melting, consisting entirely of zone A.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

Quantitative measurements of melt puddle dynamics and the associated characteristics of continuously solidified ribbon products were made as a function of materials combinations and processing conditions. A three-phase approach was taken to separately evaluate the effects of processing parameter variations, of melt chemistry, and of substrate composition. In Phase 1 two processing parameters, the melt delivery rate and the circumferential wheel velocity, were systematically varied for metalloid solute/ nickel-base alloys with Ni-12.8 a/o B-8.3 a/o Si being the primary alloy. In Phase 2, the amount of boron in ternary Ni-metalloid alloys was varied. In Phase 3, attempts to identify alternate substrates other than copper involved the use of Ni-metalloid as well as Ni-Fe-Cr-metalloid alloy melts.

A standardized test procedure for simulating the dynamic wetting and solidification behavior during melt spinning was designed and is being used to generate pheonomenological information for various combinations of melt and substrate that would be desired. Real differences in dynamic wetting/solidification patterns have been found and suggested correlations with melt spinning performance have been proposed. Concurrently a set of melt spinning experiments are being conducted to provide complementary information about the interrelation of melt puddle/substrate dynamics with processability and product characteristics. (Author)

Rapid solidification process

The objective of this work is to develop guidelines based on kinetic and thermodynamic solidification theory for prediction and control of rapid solidification processes. In particular, segregation effects and rules governing the formation of equilibrium and non-equilibrium phases, including metallic glasses, will be investigated. Areas where significant improvements in alloy properties can be produced by rapid solidification will be emphasized.

During the first period of performance, three tasks have been undertaken. These involve first a study of the specific microstructural changes that accompany rapid solidification of a number of Al alloys together with a determination of the thermal stability of these microstructures, second the production of rapidly solidified particulate and third the use of dynamic powder compaction (DPC) for the consolidation of rapidly solidified particulate in the absence of prolonged thermal excursions. A number of alloys have been rapidly solidified using laser surface melting and melt-spinning and in each case the observed microstructure has been interpreted in terms of the undercooling achieved during processing. Rapidly solidified (gas atomized) powders of 7091 have been consolidated using DPC. Is has been shown that bulk samples may be produced that are close to theoretical density.

Advanced composite processing techniques for fiber reinforced metal matrix composites require the flexibility to meet several widespread objectives. The development of uniquely desired matrix microstructures and uniformly arrayed fiber spacing with sufficient bonding between fiber and matrix to transmit load between them without degradation to the fiber or matrix are the minimum requirements necessary of any fabrication process. For most applications these criteria can be met by fabricating composite monotapes which are then consolidated into composite panels or more complicated components such as fiber reinforced turbine blades. Regardless of the end component, composite monotapes are the building blocks from which near net shape composite structures can be formed. The most common methods for forming composite monotapes are the powder cloth, foil/fiber, plasma spray, and arc spray processes. These practices, however, employ rapid solidification techniques in processing of the composite matrix phase. Consequently, rapid solidification processes play a vital and yet generally overlooked role in composite fabrication. The future potential of rapid solidification processing is discussed.

The detailed formulation of and the results obtained from a continuum mechanics-based mathematical model of the planar flow melt spinning (PFMS) rapid solidification system are presented and discussed. The numerical algorithm proposed is capable of computing the cooling and freezing rates as well as the fluid flow and capillary phenomena which take place inside the molten puddle formed in the PFMS process. The FORTRAN listings of some of the most useful computer programs and a collection of appendices describing the basic equations used for the modeling are included.