Pekguleryuz / Kainer / Kaya Fundamentals of Magnesium Alloy Metallurgy

1. Auflage 2013
ISBN: 978-0-85709-729-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 376 Seiten

Reihe: Woodhead Publishing Series in Metals and Surface Engineering

Magnesium and magnesium alloys offer a wealth of valuable properties, making them of great interest for use across a wide range of fields. This has led to extensive research focused on understanding the properties of magnesium and how these can be controlled during processing. Fundamentals of magnesium alloy metallurgy presents an authoritative overview of all aspects of magnesium alloy metallurgy, including physical metallurgy, deformation, corrosion and applications.Beginning with an introduction to the primary production of magnesium, the book goes on to discuss physical metallurgy of magnesium and thermodynamic properties of magnesium alloys. Further chapters focus on understanding precipitation processes of magnesium alloys, alloying behaviour of magnesium, and alloy design. The formation, corrosion and surface finishing of magnesium and its alloys are reviewed, before Fundamentals of magnesium alloy metallurgy concludes by exploring applications across a range of fields. Aerospace, automotive and other structural applications of magnesium are considered, followed by magnesium-based metal matrix composites and the use of magnesium in medical applications.With its distinguished editors and international team of expert contributors, Fundamentals of magnesium alloy metallurgy is a comprehensive tool for all those involved in the production and application of magnesium and its alloys, including manufacturers, welders, heat-treatment and coating companies, engineers, metallurgists, researchers, designers and scientists working with these important materials. - Overviews all aspects of magnesium alloy metallurgy - Discusses physical metallurgy of magnesium and thermodynamic properties of magnesium alloys - Reviews the formation, corrosion and surface finishing of magnesium and its alloys

Autoren/Hrsg.

Pekguleryuz, Mihriban O

Kainer, Karl

Kaya, A. Arslan

Leseproben

2 Physical metallurgy of magnesium
A.A. Kaya,     Mugla University, Turkey Abstract:
Key features in the deformation behaviour of magnesium have been introduced in terms of the consequences of hexagonal crystal structure, dislocation core width and stacking fault (SF) energy concepts. Elastic and plastic deformation behaviour of magnesium has been addressed in relation to critical resolved shear stress, slip and twinning. The anomalies during plastic deformation, fatigue, creep, recrystallization and grain growth in magnesium and its alloys have been pointed out and discussed under individual headings. Future trends in research and use of magnesium alloys have been indicated. Key words magnesium alloys critical resolved shear stress dislocation core width stacking fault energy anomalies in magnesium slip and twinning in magnesium fatigue of magnesium creep of magnesium recrystallization in magnesium 2.1 Introduction
Magnesium, with its atomic number 12 and hexagonal close-packed (HCP) crystal structure, is an interesting example of an element of true metallic character, as well as for its crystal class. Its electron configuration ends with s2 making it an excellent example of a true metallic-bond, possessing a relatively more homogeneous non-localized free electron cloud, at least when in the pure state. No doubt this changes when magnesium makes solute solutions with other metals. However, even at such an early stage of our discussion, solely based on this fact we can infer that the less effective strengthening contribution of the solute alloying elements in magnesium as compared, for example, to the case of its rival light metal, aluminium, may be attributed partly to magnesium’s true metallic-bond character. A relatively recent study revealed that some directionality in the bond structure in pure aluminium possibly exists, explaining its abnormal intrinsic stacking fault energy (SFE).1 Such directionality in bond structure indicates localization in free electron density. The superior strengthening response of aluminium to alloying, even as a dilute solute solution, may thus be due to an additional effect to the already existing directionality in the bond structure. In magnesium, however, solutes are unable to create sufficient perturbation in the truly delocalized bonding among magnesium atoms. SFE, as a related property to free electron density distribution, is thus given special attention below. Plastic deformation of magnesium invokes several puzzling questions. These anomalies are pointed out in this chapter, and an attempt has been made to gather from the existing literature the possible accounts of each phenomenon. The reader is also referred to the other chapter related to deformation, Chapter 6 ‘Forming of magnesium and its alloys’, in this book. Topics like fatigue, creep and grain size related phenomena have hopefully been compressed into a comprehensible size, highlighting both more interesting as well as mainstream concepts without greatly compromising the meaning of these otherwise vast topics. The title of ‘Physical metallurgy’ (arguably ‘elastic’) had to be limited here to a reasonable size for a book chapter. Needless to say, the chapter may fall short of being an exhaustive coverage of the topic. Finally, a few directions for further research have been pointed out in the closing section. 2.2 Crystal structure and its consequences
HCP crystal lattice and major planes of magnesium are shown in Fig. 2.1 With lattice parameters a = 3.18 Å and c = 5.19 Å, slightly less than the ideal c/a ratio of 1.62354 (at 25 °C) of Mg crystal, appears important in explaining some fundamental characteristics of the metal.2 A comparison of c/a ratios as well as critical resolved shear stress (CRSS) (to be discussed later) for basal planes of different HCP metals is given in Table 2.1 The c/a ratio of Mg, in turn, results in a somewhat larger primitive cell volume compared to other HCP metals, leading to smaller SFE levels, for example, 36 ergs/cm2 for basal plane of magnesium. Let us remember that aluminium is considered to have an unusually high SFE as an example of FCC metal. While copper has a lowly SFE of 40 ergs/cm2 that creates a faulted region of only about 35 Å, aluminium, with its ~7 Å wide fault, has 200 ergs/cm2 SFE.3 For the sake of further comparison, it may be pointed out that the SFEs based on the first-principles calculations for Mg and Zr on the high SFE {11–22} pyramidal plane associated with the two < 11–2–3 >/6 partials are 173 and 388 ergs/cm2, respectively.4 These values have very important consequences in terms of dislocation core width, mobility and configurations, and repercussions on the mechanical behaviour of the metals. Table 2.1 A comparison of CRSS levels for basal planes and c/a ratios of different HCP metals (25 ºC)2 2.1 Schematic description of hexagonal close-packed (HCP) crystal lattice and major planes of magnesium. 2.2.1 Stacking fault energy (SFE) of magnesium
There are few morphological studies on the SFs in magnesium. For a transmission electron microscopy (TEM) work on the interaction of dislocations and SFs, the reader is referred to the detailed work by Li et al.5 A TEM image of SFs in magnesium is given in Fig. 2.2 Michiaki et al.6 also conducted an interesting TEM study, showing that alloying elements form atomic layers around SFs in a Mg-Zn-Gd alloy. 2.2 TEM micrograph of stacking faults in magnesium (courtesy of B. Liet al.5) SFs can be generated either during growth processes or deformation. These are planar defects bounded by partial dislocations. The equivalent of the Shockley partials of FCC system lies in the primary slip system, (0001) < 11–20 >, that is, on the basal plane in magnesium:7 SFE, though conventionally defined in terms of surface tension and repulsive forces between the partial dislocations constituting the boundaries of the fault region, is not a simple concept when examined in detail. Essentially, FCC and HCP systems differ from each other only by the choice of position for the third layer when stacking the close-packed layers. Thus, an SF in HCP (… ABABAB … stacking) is creating a local FCC stacking (… ABCABCABC … stacking). A distinction was made between growth and deformation faults, and the former was considered to be more important in HCP metals.8 Depending on what constitutes the out-of-step stacking, the definitions of ‘extrinsic’ and ‘intrinsic’ SFs are introduced - the former being due to an extra layer of atoms (… ABABCABAB … -E stacking) bound by partial dislocations, and the latter due to vacancy condensation. The intrinsic type is further classified into two, known as growth type, which is obtained if a missing A layer is coupled with some shear above it (… ABABCBCB … -I1 stacking), and deformation type (… ABABCACACA … -I2 stacking), which can be directly created by shear on basal plane in magnesium. In both cases, the necessary shear is by 1/3[-1–100]. In addition to these two types, a twin-like sequence is also possible in the basal planes, constituting yet a third type of SF (… ABABCBABABA …-T stacking).9 Thus, as can be seen from a brief evaluation of possible stacking orders, several different SFE values must be expected. Since an SFE implies a local FCC magnesium structure, one has to consider its slightly larger crystal dimensions and energy. Table 2.29–12 gives a list of energies for different types of SFs, and compares crystal energies for HCP and FCC magnesium. The values given are in agreement with those by others in terms of order scale. As can be deduced from Table 2.2 and the thermodynamic evaluation given by the same authors, well separated I1 type SFs are most likely to be seen in magnesium. A comparison of stable SFEs is given in Table 2.39–15 Table 2.2 Calculated energies (meV) for different types of SFs on basal planes, and crystal energies of HCP and FCC magnesium Note: The figures of each reference are based on different approximations, and the last line is based on a supercell model of 144 atoms with the composition of Mg139 Al4 Sn1.. Table 2.3 Calculated stacking fault energies (mJ/m2) for (0001) basal (I2 type fault), (10–10) and (11–20) prismatic planes, and (10–11) and (11–22) pyramidal planes Note:The last column is based on a supercell model of 144 atom with the composition of...