Interface of AL-B4C composites

INTERFACE OF AL-B4C COMPOSITES

DEFINITION OF THE PROBLEM

In recent years, AI-B4C composite materials have been increasingly used as neutron absorber components in the nuclear industry. This is due to their special capability of capturing neutrons, and then lightweight, superior thermal conductivity and mechanical properties[M] Generally, processing techniques for Al MMCs can be classified into: 1)liquid mixing process, 2) semi-solid process and 3) powder metallurgy process. The liquid mixing process is an effective method to economically produce large quantities of Al-based metal-matrix composites.This process has been employed to produce most of the commercial Al-based metal-matrix composites However, during this process, B4C usually is unstable in liquid aluminum and reacts with the Al melt to form reaction products, AI3BC, AIB2 and AI4C3 . In order to limit the decomposition of B4C particle and improve their wettability in the liquid metal casting process, Ti is added to AI-B4C composites forming a TiB2 barrier layer around the B4C particle surface.The fluidity of Al-based alloys and composites has been studied by several researcher. In an alloy with rich solute, the flow arrest is due to a choking of the flow at the leading tip of the stream. When a critical concentration of solid is reached at the leading tip, the viscosity then rises rapidly and the flow ceases abruptly. The reinforcement characteristics such as size, shape and volume fraction of ceramic particles have an effect on the fluidity of Al-based composites’. It is reported that the volume fraction of the reaction-induced particles increases and its fluidity deteriorates with the increase of melt holding time. Moreover, the particle agglomerates, particle settling and pushing, presence of oxide films, and the appearance of reaction-induced particles influence the flow behavior of the composite melt Furthermore, during remelting, the fluidity of the composites can be influenced by different working processes due to their effect on the distribution of reinforcement particles ^l6\ However, the literature related to the recycling of metal matrix composites and the fluidity evolution of the scrap materials during remelting is very limited. Quantitative characterization of the material microstructure is one of the means of investigating of the influence of the microstructure on the fluidity and mechanical properties. Image analysis techniques may be used to estimate the volume fraction evolution of solid particles. Besides, various methods have been developed for characterizing the spatial distribution of discrete secondary phase bodies on twodimensional sections, including field methods, inter-particle spacing methods and tessellation methods ^l9\Moreover, numerous approaches for assessment of clustering have been proposed and applied in composite materials, such as the Euclidean distance; nearest neighbor distance; radial distribution function; and some others P°~2l\

PROBLEMS

During manufacturing processes, the process scrap from the casting ad transformation processes (extrusion and rolling) can reach 50 to 60% of the total materials produced. The need to recycle AI-B4C composites thus becomes urgent to meet environmental goals and to reduce production costs. Compared to standard aluminum alloys, the fluidity of metal matrix composites is already limited due to the presence of a large quantity of ceramic particles. For the liquid metal casting process, the fluidity of the composites can greatly influence the ability of the composites to be recycled for reuse. Remelting is a promising method to recycle process scrap materials because of its simplicity.

OBJECTIVES

1. Investigate the fluidity evolution as a function of the holding time for two process scrap materials, namely AA6063-10 vol.% B4C (cast billets and extruded plates) and AA1100-16 vol.% B4C (cast ingots and rolled sheets).
2. Study the influence of casting, extrusion and rolling processes on micro structural features, such as particle distribution, particle agglomeration and interfacial reaction products.
3. Study the impact of particle amount, particle segregation and agglomeration, as well as particle distribution on the fluidity evolution.
4. Attempt to propose widely applied approaches for micro structure characterization of metal matrix composites.

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Table des matières

RESUME
ABSTRACT
ACKNOWLEDGMENTS
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
CHAPTER 1
DEFINITION OF THE PROBLEM
1.1 INTRODUCTION
1.2 PROBLEMS
1.3 OBJECTIVES
1.4 METHODOLOGY
CHAPTER 2
LITERATURE REVIEW
2.1 INTERFACE OF AL-B4C COMPOSITES
2.1.1 Interfacial reactions of A1-B4C composites
2.1.2 Interfacial reaction thermodynamics
2.2 FLUIDITY
2.3 FLUIDITY OF COMPOSITE
2.4 INFLUENCING FACTORS OF REINFORCEMENT ON FLUIDITY
2.4.1 Particle volume fraction
2.4.2 Particle surface area
2.4.3 Particle size
2.4.4 Particle shape
2.4.5 Particle agglomeration
2.4.6 Interfacial reaction
2.5 INFLUENCE OF MECHANICAL DEFORMATION ON FLUIDITY
2.6 CHARACTERIZATION OF COMPOSITE 2.6.1 Particle volume fraction
2.6.2 Particle distribution
2.6.3 Particle agglomeration
CHAPTER 3
EXPERIMENTAL PROCEDURES
3.1 FLUIDITYTEST
3.3.1 Material preparation
3.3.2 Vacuum fluidity test procedures
3.2 MICROSTRUCTURE ANALYSIS
3.2.1 Sample preparation
3.2.2 Quantitative analysis of microstructure
3.2.2.1 Particle volume fraction
3.2.2.2 Particle distribution
3.2.2.3 Particle agglomeration
3.2.2.4 Particle effective volume fraction
3.3 ELECTRON MICROSCOPY
CHAPTER 4
RESULTS AND DISCUSSION
4.1 CAST AND EXTRUDED RECYCLED MATERIALS (AA6063-10 VOL.% B4C)
4.1.1 Fluidity evolution
4.1.2 Original scrap materials
4.1.2.1 Microstructure of B4C and reaction-induced particles
4.1.2.2 Microstructure of particle agglomerates
4.1.3 Crucible samples
4.1.3.1 Microstructure of B4C and reaction-induced particles
4.1.3.2 Microstructure of particle agglomerates
4.1.3.3 Quantitative analysis of B4C and reaction-induced parties
4.1.4 Fluidity samples
4.1.4.1 Quantitative analysis of particle agglomerates
4.1.4.2 Quantitative analysis of B4C and reaction-induced particls
4.1.5 Mechanism of flow arrest and explanation of fluidity evolution
4.2 CAST AND ROLLED RECYCLED MATERIALS (AAl 100-16 VOL.% B4C)
4.2.1 Fluidity evolution
4.2.2 Original scrap materials
4.2.2.1 Microstructure of B4C and reaction-induced particles
4.2.2.2 Microstructure of particle agglomerates
4.2.3.1 Microstructure of B4C and reaction-induced particles
4.2.3.2 Quantitative analysis of B4C and reaction-induced particles
4.2.4 Fluidity samples
4.2.4.1 Quantitative analysis of particle agglomerates
4.2.4.2 Quantitative analysis of B4C and reaction-induced particles
CHAPTER 5
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
5.1 CONCLUSIONS
5.2 SUGGESTIONS FOR FUTURE WORK
REFERENCES
APPENDICES