Fundamentals of Aluminium Metallurgy

Recent Advances

Roger Lumley

• Publisher: Woodhead Publishing
• Year: 2018
• Print ISBN: 9780081020630
• eBook ISBN: 9780081020647
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Fundamentals of Aluminium Metallurgy: Recent Advances updates the very successful book Fundamentals of Aluminium Metallurgy. As the technologies related to casting and forming of aluminum components are rapidly improving, with new technologies generating alternative manufacturing methods that improve competitiveness, this book is a timely resource. Sections provide an overview of recent research breakthroughs, methods and techniques of advanced manufacture, including additive manufacturing and 3D printing, a comprehensive discussion of the status of metalcasting technologies, including sand casting, permanent mold casting, pressure diecastings and investment casting, and recent information on advanced wrought alloy development, including automotive bodysheet materials, amorphous glassy materials, and more. Target readership for the book includes PhD students and academics, the casting industry, and those interested in new industrial opportunities and advanced products. - Includes detailed and specific information on the processing of aluminum alloys, including additive manufacturing and advanced casting techniques - Written for a broad ranging readership, from academics, to those in the industry who need to know about the latest techniques for working with aluminum - Comprehensive, up-to-date coverage, with the most recent advances in the industry

Chapter 1

New Research Techniques in Aluminium Alloy Development

Ross K.W. Marceau; Thomas Dorin Institute for Frontier Materials, Deakin University, Geelong, VIC, Australia

Abstract

This chapter reviews new research techniques and recent progress in the development and understanding of aluminium alloys by the application of advanced characterization for composition and structure determination. In particular, this chapter will focus on high-resolution transmission electron microscopy, atom probe tomography, and small-angle X-ray scattering, which all allow atomic-scale characterization. Furthermore, the complementary use of these techniques as well as correlation with other microscopy and microanalysis techniques, provide opportunities to overcome the inherent limitations of the individual methods and to capitalize on their unique advantages. This multitechnique approach will also be discussed in the context of both combinatorial studies and direct correlation with atomic-scale, first-principles modelling, namely density functional theory simulations, in the process of aluminium alloy development by investigating structure–property relationships.

Keywords

Aluminium alloys; Structure–property relationships; Atomic-scale microstructure; Transmission electron microscopy; Atom probe tomography; Small-angle X-ray scattering; In situ characterization; Correlative microscopy; Combinatorial studies

1.1 Introduction

A significant fraction of aluminium alloy research and development involves characterization of structure and chemical composition at the nanometre scale using advanced microscopy and microanalysis techniques in order to understand and further progress structure–property relationships for industrial application. History has taught us that every time a new form of microscopy or a new type of microscope has been introduced, major advances have been made in the understanding of materials [1]. This chapter reviews new research techniques with respect to both cutting edge development in transmission electron microscopy (TEM), atom probe tomography (APT), and small-angle X-ray scattering (SAXS), together with correlative use of the aforementioned, their respective ancillary techniques, and also complementary and direct correlative modelling at the same length scale, namely via density functional theory (DFT) simulations.

1.2 Transmission Electron Microscopy

1.2.1 Aberration-Corrected Scanning Transmission Electron Microscopy

Advances in aberration correctors have been instrumental in producing electron beams with a spot size below 1 Å (< 0.1 nm) for high-resolution scanning transmission electron microscopy (STEM) with the ability to resolve atomic columns [2]. Readers are referred to a recent review of aberration-corrected STEM for further details [3]. The major advantage of this technique is that when aligned to a specific crystallographic zone axis, the contrast in a typical aberration-corrected annular dark-field (ADF) STEM micrograph allows a relatively straightforward interpretation of the image in terms of atomic positions, in which a bright spot is an atomic column. This also allows highly localized elemental analysis at atomic spatial resolution, through the techniques of electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX or EDS).

1.2.1.1 High-Angle Annular Dark-Field-STEM

As mentioned above, high-resolution and aberration-corrected high-angle annular dark-field (HAADF)-STEM provides interpretability of the atomic columns because there is no contrast inversion over a large focus range, and as a result is it a very good technique for investigating microstructural features that diffract strongly due to the strain that they impart on the crystal lattice, for example, defects such as dislocations, and coherent or semicoherent interfaces, such as from particular grain boundaries and nanoscale precipitates. In addition to this, high-angle elastic, ‘Rutherford’ scattering of electrons by atomic nuclei is strongly dependent on atomic number Z[4], whereby high-resolution HAADF-STEM imaging is often referred to as Z-contrast imaging. This is particularly useful for analysis of nanosized precipitates in aluminium alloys in which the solute atoms are relatively heavier (e.g. Cu, Zn, Ge, Ag, Sn, etc.) or lighter (e.g. Li) than the matrix Al atoms. For Mg and Si however, which are on either side of Al in the periodic table, the Z-contrast effect is weak. This reliance on relative atomic number difference becomes more complicated in more complex, higher-order alloy systems with multiple solute atoms of interest. Further complexity may arise due to the effect of specimen thickness and crystal orientation with regard to dynamical electron diffraction (e.g. channelling of electrons along specific crystallographic directions), which may cause a modification of the scattering dependence away from Z² for isolated atoms [3], and thereby make accurate identification of atomic columns more difficult. While determination of a particular atomic column can be based on a priori knowledge of the structure (e.g. interatomic distances) and intensities from HAADF-STEM [5], in order to avoid mistaken interpretations, atomic-resolution observations are often compared with simulated HAADF-STEM images or atomic-scale structural simulations based on DFT. Examples of this kind for precipitate analysis in aluminium alloys are numerous within the archival literature, and include, to name just a few, atomistic investigation of the strengthening β″ phase in 6xxx series Al–Mg–Si alloys [6] (Fig. 1.1) and the effect on precipitate structures by additions of Cu [7,8], Ag [9,10], Ge [10], and Zn [11]; the analysis of GPB zones in 2xxx series Al–Cu–Mg alloys [12–14], as well as the θ′ precipitate phase in Al–Cu and Al–Cu–Ag alloys [15,16]; the T₁ phase in Al–Cu–Li alloys [17]; the Ω phase in Al–Cu–Mg–Ag alloys [18]; and η-related precipitates in 7xxx series Al–Zn–Mg–Cu alloys [19,20]. A more recent example very nicely demonstrates atomic-resolution HAADF-STEM imaging of a new precipitate phase, ζ (zeta), in the Al–Ag system, and coupled with in situ annealing experiments it was found that this is an intermediate precipitate phase between GP zones and γ′ [21].

Fig. 1.1 (A) HAADF-STEM image of a β″ precipitate in an Al–Mg–Si alloy. (B) Enlargement of the area delineated by the white square shown in (A) with atomic site overlays of possible unit cell configurations, for example, (C). (D) Atomistic model of a β″ precipitate embedded in an Al matrix. The Mg and Si sites are shown as blue and red balls, respectively, while the Al matrix atoms are shown as grey balls. The white square that borders the image indicates the supercell size used in DFT calculations. Reprinted from P.H. Ninive, A. Strandlie, S. Gulbrandsen-Dahl, W. Lefebvre, C.D. Marioara, S.J. Andersen, J. Friis, R. Holmestad, O.M. Løvvik, Detailed atomistic insight into the β″ phase in Al–Mg–Si alloys, Acta Mater. 69 (2014) 126–134, with permission from Elsevier.

A final note in this section pertains to the capacity for aberration-correction ADF-STEM to be used for strain mapping. Despite challenges from the need to reduce scan distortions and maximize the signal-to-noise ratio in experimental images, the strain field surrounding a needle-shaped β″ phase precipitate in an Al–Mg–Si alloy has recently been mapped at atomic-resolution [22] and the precipitate-matrix misfit quantified [23] (Fig. 1.2).

Fig. 1.2 (A) ADF-STEM image of a β″ precipitate in an Al–Mg–Si alloy, realigned and nonrigid registered by the Smart Align algorithm [24]. (B) Strain (ɛ_xx) map measured from (A) using Fourier-space geometric phase analysis (GPA). (C) Plot of precipitate-matrix misfit (ratio between β″ and Al lattice parameters) vs precipitate aspect ratio (cross-sectional side lengths) for 24 different β″ precipitates. (D) Displacement field magnitude in Al of a high aspect ratio β″ precipitate, measured using GPA. Adapted from L. Jones, S. Wenner, M. Nord, P.H. Ninive, O.M. Løvvik, R. Holmestad, P.D. Nellist, Optimising multi-frame ADF-STEM for high-precision atomic-resolution strain mapping, Ul...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Introduction: Aluminium, the Strategic Material
• Chapter 1: New Research Techniques in Aluminium Alloy Development
• Chapter 2: Additive Manufacturing of Aluminium-Based Alloys and Composites
• Chapter 3: How to Design and Buy Aluminium Castings
• Chapter 4: Aluminium Investment Casting and Rapid Prototyping for Aerospace Applications
• Chapter 5: Advances in the Sand Casting of Aluminium Alloys
• Chapter 6: New Hypoeutectic/Hypereutectic Die-Casting Alloys and New Permanent Mould Casting Alloys That Rely on Strontium for Their Die Soldering Resistance
• Chapter 7: Thermal Conductivity of Aluminium High-Pressure Die Castings
• Chapter 8: Advanced Casting Technologies Using High Shear Melt Conditioning
• Chapter 9: Treatment by External Fields
• Chapter 10: Automotive Wrought Aluminium Alloys
• Chapter 11: Aluminium Lithium Alloys
• Chapter 12: Aluminium Scandium Alloys
• Chapter 13: Control of Distortion in Aluminium Heat Treatment
• Chapter 14: Recent Insights Into Corrosion Initiation at the Nanoscale
• Index