Constitution and Magnetism of Iron and its Alloys

229 Pages·2001·8.11 MB·English

by  Prof. Dr. Werner Pepperhoff

Engineering Materials
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Werner Pepperhoff . Mehmet Acet
Constitution
and Magnetism
of Iron and its Alloys
With l32 Figures and 14 Tables
, Springer
Prof. Dr. Werner Pepperhoff
An Hacksteinskuhlen 35
47509 Rheurdt
Germany
Prof. Dr. Mehmet Acet
Tieftemperaturphysik
Gerhard-Mercator Universitat Duisburg
47048 Duisburg
Germany
Library ofC ongress Cataloging-in-Publication Data
Pepperhoff, Werner.
[Konstitution und Magnetismus des Eisens und seiner Legierungen.Englishl
Constitution and magnetism of iron and its alloys I Werner Pepperhoff, Mehmet Acet.
p. cm.--(Engineering materials)
1. Iron. 2. Iron alloys. 3. Ferromagnetism, I. Acet, Mehmet -II. Title. III. Series.
TN705 .P4413 2001
669'.I--dc21 2001049293
ISBN 978-3-642-07630-5 ISBN 978-3-662-04345-5 (eBook)
DOll 0.1 007/978-3-662-04345-5
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Preface to the English edition
Iron has a special place among metallic materials. It has been of so utmost
importance to mankind, that it has given its name to a prehistoric period, the Iron
age. Iron played an important role in the development ofthe industrial society, and
even today, it has not lost any ofi ts significance. It remains as the most functional
material, and it appears that it will maintain this attribute well into any predictable
future. Iron reserves are plentiful, and it is the most encountered element in the
earth crust with 5.6% abundance. It can be separated from its oxides with
relatively little energy. Iron is recyclable, i.e., resource and environment friendly.
The indisputable special function that iron takes up in technology is due to its
versatility. No other material provides so many useful properties that can be
exploited under tolerant economic conditions. This exceptional role relies on its
polymorphism, i.e., on the property that it can be found in different crystal
structures. The very different solubility of substitutional and interstitial elements
in iron provides the possibility of the occurrence of a variety alloys and
compounds with very different microstructure and a wide spectrum of most
different technological and physical properties. Polymorphism is not an unusual
happening in the world of crystals. However, the polymorphism of iron has a
speciality; its polymorphism is based on its magnetism. Understanding the
physical nature of iron means understanding its magnetism.
The early discovery of the ferromagnetism of iron and the important technical
consequences that followed give the impression that the magnetism of iron has
long been understood. This is exactly not the case. Attempts to give a quantitative
and consistent description of the magnetism of iron brought along with it new
complexities. Besides ferromagnetism, anti ferromagnetism - a magnetically
ordered state with no macroscopic net magnetic moment - occurs in numerous
technically important iron alloys. The importance of anti ferromagnetism was
noticed at a later stage in the development of the technology of steels; possibly
because it was not as attractive as the ferromagnetic properties at the time. Both
ferromagnetism and antiferromagnetism are found in the so called austenitic
steels. These feature a further important property, namely the existence oftwo or
more different magnetic states, within which the system can transform. Such
magnetic transitions occurring without an accompanied structural transition were
"I Preface
less known and, therefore, received little attention. They are, however, responsible
for the polymorphism of iron and, thereby, are of decisive significance for the
constitution and physical properties of the alloy systems. The alloys and
compounds of iron exhibit a multitude of magnetic states that strongly influence
the physical properties. They provide the possibility to prepare material with
particular properties, which can then be used for a particular application. In the
last two decades, there has been appreciable development in the understanding of
metallic magnetism, and it has been possible to give, at least, a good qualitative
description of the magnetic phenomena.
The origins of the physical properties of metals can be traced back to their
crystal structure. On the other hand, the technologically important qualities, such
as tensile properties, are governed by the microstructure, i.e., by crystal
imperfections. Properties related to crystal imperfections lie beyond the scope of
this book. It deals, rather, with the inherent properties of the iron crystal,
especially with those related to magnetic phenomena. The title, Constitution and
Magnetism of Iron and its Alloys, is meant to link an association to the better
understanding of the particular stand-point of iron among other metals.
This manuscript resulted from the participation of the authors in the
collaboration project "Sonderforschungsbereich SFB 166" ofthe universities of
Duisburg and Bochum, which was supported by the DFG (German Research
Council): Structural and magnetic phase transitions in transition metal alloys and
compounds. The English edition is essentially a translation ofthe German version,
however it necessarily includes technical revisions and some additions. We thank
E. F . Wassermann, the speaker of SFB 166, for many fruitful discussions and the
availability of the results of numerous dissertations written during the period of
the project, which are used in this book. We also ~xtend our thanks to Prof. Dr. P.
Entel and coworkers for our collaboration and many discussions on the principles
of the electron theory of magnetism.
W. Pepperhoff
M. Acet
Table of Contents
1. The structure of iron ................................. 1
1.1 The polymorphism of iron . . . . . . . . . . . . . . . . . . . . . . . . . .. 1
1.2 Structure models .................................. 4
1.3 Atomic volume. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11
2. The magnetism of iron .............................. 15
2.1 The free iron atom ................................ 15
2.2 The characteristics of magnetic behavior. . . . . . . . . . . . . .. 20
2.3 Physical models of magnetism. . . . . . . . . . . . . . . . . . . . . .. 25
2.3.1 The local moment model ...................... 25
2.3.2 The band model ............................. 26
2.4 The ferromagnetism of a-iron . . . . . . . . . . . . . . . . . . . . . .. 30
2.4.1 The ground state . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 31
2.4.2 The ferromagnetic-paramagnetic
phase transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 36
2.5 The magnetism ofy-iron . . . . . . . . . . . . . . . . . . . . . . . . . .. 41
2.5.1 The antiferromagnetic ground state .............. 41
2.5.2 Moment-volume coupling in y-iron .............. 46
2.6 The magnetism of €- iron .......................... 51
3. Thermal properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 57
3.1 The heat capacity and enthalpy ...................... 57
3.1.1 a-iron ..................................... 58
3.1.2 y-iron ..................................... 63
3.1.3 The heat of transformation of iron ............... 67
3.2 Thermal expansion ................................ 69
3.3 The phase stabilities of iron . . . . . . . . . . . . . . . . . . . . . . . .. 76
4. Substitutional alloys of iron . . . . . . . . . . . . . . . . . . . . . . . . . .. 83
4.1 The 3d-Metals and the Slater-Pauling curve ............ 85
4.2 Dilute alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 89
4.3 Fe-Cr and Fe-V .................................. 94
VIII Table of Contents
4.4 Fe-Ni 104
..... .o.o.o.o ....... .o ••••••• .o •••••••• .o •••••••••
4.5 Fe-Co ......................................... 115
4.6 Fe-Mn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 121
4.7 The ternary systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 127
4.8 Iron alloys with 4d and 5d metals ................... 135
4.8.1 Fe-Pt and Fe-Pd ............................ 136
4.8.2 Fe-Rh .................................... 142
5. Interstitial alloys and compounds. . . . . . . . . . . . . . . . . . . .. 147
5.1 Interstitialpositions .............................. 147
5.2 Fe-C and Fe-N interstitial alloys .................... 149
5.3 The iron nitrides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 155
5.3.1 y'-Fe N ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 155
4
5.3.2 E-FexN ................................... 157
5.3.3 a"-FesN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 158
5.4 The iron carbides ................................ 163
5.4.1 Cementite Fe C . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 163
3
5.4.2 The unstable carbides. . . . . . . . . . . . . . . . . . . . . . .. 167
5.5 The borides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. 168
5.6 Hydrogen in iron and iron alloys. . . . . . . . . . . . . . . . . . .. 172
6. The influence of magnetism on the physical
properties of iron alloys ...... . . . . . . . . . . . . . . . . . . . . . .. 183
6.1 Magnetic states and transitions ..................... 183
6.2 The heat capacity ..... . . . . . . . . . . . . . . . . . . . . . . . . . .. 190
6.3 Elastic properties ................................ 195
6.4 Transport properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 200
6.5 Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 207
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 215
Index ............................................... 223
1. The structure of iron
1.1 The polymorphism of iron
Iron has two different crystal structures at atmospheric pressure: the body
centered cubic (bcc) and the face centered cubic (fcc). In the ground state the bcc
a-phase is stable, and at the temperature T= 1184K (A3 point), a-Fe transforms
into fcc y-Fe, which is stable up to 1665K (A4 point). Above this temperature,
iron transforms back into the bcc phase (O-Fe), which remains stable up to the
melting temperature T = 1809 K. Since a and 0-Fe are isomorphic, the two are
m
usually not distinguished when referred to bcc Fe. Steels with bcc and fcc
structures are usually referred to as "ferrite" and "austenite" respectively. The
boiling point of iron is about 3300 K. A third form of iron, which has the
hexagonal close packed (hcp) structure, can be stabilized under high pressures.
Structural transformations in iron occur with a "discontinuous" first order
transformation like in the liquid-solid transformation. The transformation is
accompanied by a sudden change in the entropy and other physical properties and
is accompanied by a latent heat. Finite surface energy at the boundary of the two
phases acts against the nucleation process and leads to hysteresis effects in the
transformation. On cooling, the transformation occurs at a lower temperature than
on heating. The size ofthe hysteresis depends on the purity and the cooling rate
of the sample. The A3 point is lowered by about 5 -10K at a cooling rate of
1 Klmin. The displacement ofthe A3 point on warming and cooling, which is also
observed in iron alloys, is denoted with the indices c (chauffage) and r
(refroidissement) as A~ and Ay3 respectively.
a-Fe is ferromagnetic up to the Curie temperature T c = 1041 K. The
"continuous" second order nature ofthe transformation from the ferromagnetic to
the paramagnetic state was earlier known as the A2 transformation. Between A3
and A4 paramagnetic iron was referred to as p-Fe. Today, the a-phase and
p-phases are no longer distinguished. y -Fe, in its stability range is paramagnetic.
Hypothetically, y-Fe below its stability range would have ordered
antiferromagnetically below about 50K.
W. Pepperhoff et al., Constitution and Magnetism of Iron and its Alloys
© Springer-Verlag Berlin Heidelberg 2001
2 1. The structure of Iron
Table 1.1. The ground state crystal structures of the transition metals
Number of s + d electrons
Period
3 4 5 6 7 8 9 10 11
!
Cr [Mn]I) [Fe]2) [CO]3) Ni
3d,4s Sc Ti V Cu
AF AF FM FM FM
4d,5s Y Zr Nb Mo Tc Ru Rh Pd Ag
5d,6s (La) Hf Ta W Re Os Ir Pt Au
Structure hcp hcp fcc bcc hcp hcp fcc fcc fcc
The elements shown in brackets have the "wrong" crystal structure in the ground state.
I) complex cubic (Al2); 2) bcc; 3) hcp
AF: antiferromagnetic; FM: ferromagnetic
The existence of more than one stable crystallographic state, known as
polymorphism, is not an uncommon case among the elements. More than one
third of the elements exhibit this property at atmospheric pressure. The general
rule ofp olymorphism is that the low temperature crystal structure is closer packed
(e.g., fcc or hcp) than the high temperature structure (e.g., bcc). Iron, however,
does not comply with this rule and shows exactly the opposite behavior. The bcc
phase, which is stable in the ground state, transforms to the closer packed fcc state
at a higher temperature. The reason for this exception lies in the unusual magnetic
properties of iron which will be discussed in detail in Chap. 3.3.
In the periodic system of the elements, the transition metals show distinct
regularity in the evolution of the crystal structure as the d-orbitals are filled. As
the number of s+d electrons increases, the crystal structure progresses as
hcp"" bcc"" hcp"" fcc (Tab. 1). However, iron and both of its neighboring atoms
Mn and Co show a deviation from this regularity and acquire the "wrong" ground
state crystal structure. According to this scheme, iron should have a hcp structure
as Ru and Os do instead of being bcc. The structure ofi ron transforms to hcp only
at high pressures (small atomic volumes) as is seen in the pressure-temperature
(p-T) phase diagram in Fig 1. This diagram also shows that the decrease in the
atomic spacing caused by increasing the pressure leads to the broadening of the
stability range of y-Fe. Conversely, it would be expected that by applying
"negative pressure" the stability range would contract, and at large enough
volumes, it would eventually disappear as shown by the dashed curve. Under these
conditions there would have been no polymorphism in Fe, and a-Fe would have
been stable from the ground state up to the melting temperature. Above about