Tài liệu physical metallurgy 4e volume2 pptx

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1

IC

Robert W. Cahn and Peter Haasen (-I), editors

FOURTH, REVISED AND ENHANCED EDITION

[NORTH-HOLLAND

PHYSICAL METALLURGY

VOLUME I1

LIST OF CONTRIBUTORS

A.S. Argon

E. Arzt

H. K. D. H. Bhadeshia

H. Biloni

J. L Bocquet

W. J. Boettinger

G. Brebec

R.W. Cahn

G.Y. Chin?

T. W. Clyne

R.D. Doherty

H.E. Exner

R. Ferro

D.R. Gaskell

H. Gleiter

A.L. Greer

P. Haasen?

J. P. Hirth

S. Hofmann

E.D. Hondros

E. Hornbogen

G. Kostorz

C. Laird

I? LejEek

W.C. Leslie

Y. Limoge

J. D. Livingston

E E. Luborsky

T.B. Massalski

J. R. Nicholls

AD. Pelton

D.G. Pettifor

D.P. Pope

M. Riihle

A. Saccone

S. R. J. Saunders

M.P. Seah

W. Steurer

J.-L. Strudel

R.M. Thomson

C.M. Wayman

M. Wilkens

A. H. Windle

H. J. Wollenberger

PHYSICAL

METALLURGY

Fourth, revised and enhanced edition

Edited by

Robert W. CAHN Peter HAASEN?

University of Cambridge University of Gottingen

VOLUME II

1996

NORTH-HOLLAIW

AMSTERDAM-LAUSANNJ%-NEW YORK-OXFORD-SHKYO

ELSEVIER SCIENCE B.V.

Sara Burgerhartstraat 25

PO. Box 211, lo00 AE Amsterdam, The Netherlands

ISBN 0 444 89875 1

0 19% Elsevier Science B.V. All rights reserved.

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This book is printed on acid-free paper.

Printed in The Netherlands

SYNOPSIS OF CONTENTS

Volume 1

1. Crystal structure of the metallic elements

2. Electron theory of metals

3. Structure and stability of alloys

4. Structure of intermetallic compounds and phases

Appendh: Quasicrystals

5. Metallurgical thermodynamics

6. Phase diagrams

7. Diffusion in metals and alloys

8. Solidification

9. Microstructure

Volume 2

10. Surface microscopy, qualitative and quantitative

11. Transmission electron microscopy

12. X-ray and neutron scattering

13. Interfacial and surface microchemistry

14. Oxidation, hot corrosion and protection of metallic materials

15. Diffusive phase transformations in the solid state

16. Nondiffisive phase transformations

17. Physical metallurgy of steels

18. Point defects

19. Metastable states of alloys

Volume 3

20. Dislocations

21. Mechanical properties of single-phase crystalline media:

deformation at low temperatures

22. Mechanical properties of single-phase crystalline media:

deformation in the presence of diffusion

23. Mechanical properties of solid solutions

24. Mechanical properties of intermetallic compounds

25. Mechanical properties of multiphase alloys

26. Fracture

27. Fatigue

28. Recovery and recrystallization

29. Magnetic properties of metals and alloys

30. Metallic composite materials

3 1. Sintering processes

32. A metallurgist’s guide to polymers

V

Steurer

Pettifor

Massalski

Ferro, Saccone

Steurer

Gaskell

Pelton

Bocquet, Limge, Brebec

Biloni, Boettinger

Gleiter

Exner

Riihle, Wlkens

Kostorz

Hondms, Seah, Homnn,

LejCek

Saunders, Nicholls

Doherty

Waymn, Bhudeshia

Leslie, Hornbogen

Wollenberger

Cahn, Greer

Hirth

Argon

Argon

Haaseni

Pope

Srmdel

Thomson

Laird

Ch

Livingston, Luborsly, Chin?

Clyne

her; Am

windle

CHAPTER 10

QUALITATIVE AND QUANTITATIVE

SURFACE MICROSCOPY

H.E. EXNER

Technical University Darmstadt

Department of Materials Science

0-64287 Darmstadt, Germany

R. W Cahn and P: Haasen?, eds.

Physical Metallursy; fourth, revised and enhanced edition

0 Elsevier Science BY 19%

944 H. E. Exner Ch. 10, 0 1

1. Introduction

In technical materials, the microstructure develops during processing. Apart from rare

cases where microstructural features persist unchanged in nature and geometry from the

raw material to the final product (e.g., hard nonmetallic refractory particles), each

individual processing step influences the amount, the composition and the geometric

appearance of the constituents and of defects in a material. This is due to the fact that

microstructures usually are far from the ideal thermodynamic and geometric equilibrium

predicted by thermodynamic (or rather thermostatic) considerations (see ch. 5 on

metallurgical thermodynamics and ch. 6 on phase diagrams). The usual route - casting,

plastic forming, heat-treating - leads to microstructures vastly different from those

obtained after powder-metallurgical production, for example. Vice versa, the mechanics

of the individual processing techniques can be best studied by monitoring the microstruc￾tural changes as a function of processing conditions (compare, for example, ch. 8 on

solidification; ch. 9, $2; chs. 15 and 16 on diffusive and nondiffisive phase transfor￾mation, respectively, or ch. 28 on recrystallization). Knowledge of the details of the

formation of microstructures is essential in order to understand the relationships between

processing parameters and the behaviour of materials in practical application. Since the

most important technological properties are strongly influenced by the microstructure

(see, for example, ch. 25 on the mechanical properties of multiphase alloys) this under￾standing is important for the development of metallic (as well as non-metallic) materials.

Several definitions of the term microstructure have been proposed in the literature

(see, for example HORNBOGEN and PETZOW [1970.1991], SCHATT [1991], HOUGARDY

[1981], HORNBOGEN [1981, l984,1986a,b], LOCKE [1984], METALS HANDBOOK [1985],

HEROLD-SCHMIDT [1988] or JEGLITSCH [1989]. For the purpose of this book, the

following seems appropriate: The microstructure of crystalline materials is defined by the

type, the structure and the number of phases, by the number, the geometric appearance

(size, shape etc.) and the topological arrangement of the individual phase regions and

their interfaces, and by the type, structure and geometry of lattice defects (which are in

most cases not part of the thermodynamic equilibrium stsucture). The experimental study

of metallic microstructures, and their qualitative and quantitative description is termed

metallography. (Sometimes, this term has also been used for the preparation of ceramic

materials and polymers for microscopic inspection. Eventually, this improper use will be

substituted by muteriulogruphy or a similar term to be newly introduced to include

metallography, ceramography and plastography.)

Metallography dates back to the 17th century when English, French and German

scientists first studied metallic objects by means of simple optical devices (see, for

example, SMITH [19601, TENSI 119681 or PuSCH [1979]). The birth of modem metallo￾graphy took place 200 years later and is dated to 1863 when H. C. Sorby developed an

incident-light microscope, or to 1865 when he first observed and described some

microstructural elements of technical iron. Today, a large arsenal of devices and techniques

for microstructural investigations has become available (see, for example, METALS HAND￾BOOK, Vol. 9 119851 and Vol. 10 119861, LIFSHIN 11992, 1994a1, CAJXN and LIPSHIN [1993],

and the books and journals listed under Further Reading at the end of this chapter).

Ch. 10, $2 Sur$ace microscopy 945

This chapter deals with the techniques of microscopy and metallography as means for

microstructural investigation. It focuses on qualitative and quantitative methods of

optical, scanning-electron and scanning tunneling surface microscopies with short reviews

of the other imaging and compositional analyzing techniques. A special chapter (chapter

11) is devoted to transmission electron microscopy, including analytical TEM.

2. Optical microscopy

Metallic materials are usually opaque; therefore investigations of plane cross-sections

by incident light prevail in metallography. However, the transparency of some metals and

silicon to infrared light in thin sections has been effectively exploited. Optically, the

individual components of a metallic alloy differ in their amplitude and phase characteris￾tics. While amplitude objects become visible owing to differences in light absorption and

thus appear in different grey shades or even colours, phase objects only differ in the

refractive indices which cannot be recognized without additional provision. The preparation

of cross-sections, the enhancement of contrast by etching and other methods, as well as

the microscopic set-up must be carefully optimized for the material under investigation

and adjusted to the purpose of the investigation in order to get maximum information

from a microscopic study.

2.1. Metallographic specimen preparation

The essential steps and techniques of metallographic sample preparation are shown

in table 1. This large variety of methods has been described in handbooks, monographs

and review articles, e.g., by PETZOW and EXNER [1968], SAMUELS [1971], in METALS

HANDBOOK [1985, 19861, by ELSSNER and KOPP [1984], VANDER VOORT [1984a],

SCHUMANN [1990], LLWTHAN [1992], nLLE and PETZOW [1992], ASH HANDBOOK

[1992], or F’ETZOW [1994]. Details are discussed in a multitude of original papers in a

variety of journals and conference proceedings (see also Further Reading at the end of

this chapter). Though some systematic studies of the construction of metallographic

devices (e.g., WASCHULL [1985], KOPP and MULLER [1987] or FUNDAL and GROSS

[1993]) and of the consecutive steps of sample preparation (e.g., NELSON [1989],

MULLER and KOPP [1989], TELLE and PETZOW [I9921 or WASCHULL [1993]) have been

published, successful preparation of metallographic laboratories samples is still a matter of

skill. Since accreditation of metallographic became a major issue, systematic evaluations of

procedures and standard documentation are necessary additions to the empirically developed

recipes (see, for example, RUCKER and BJERREGARD [1993] or WIELAND [1993]). In the

following, a few of the more basic aspects of the present state of the art are reviewed.

2.1.1. Sampling

The location from which a specimen is taken depends on whether the investigation

is aimed (1) to give data for a specific area (systematic sampling), e.g., if the origin of

a failure is clearly visible, (2) to characterize a larger piece (e.g., a laboratory sample) or

(3) to characterize the quality of a large amount of material (as in quality control). In the

References: p. 1016.

Table 1

Steps of metallographic preparation (after Pmmw and EXNER [1%8]).

SAMPLING

clamping mGpl embedding

sawing

disc cutting

ultrasonic cutting

turning

breaking

powder-jet cutting

sheet electrode

rod electrode

mechanical

--i electro-erosive

acid sawing

acid milling

acid-jet cutting

low-melting alloys

sulphur, glass

cement, plaster

electmhernid

4 glue

inorganic material

Mhual Bin

elastomers

duromers

organic material

galvanic

stamping

vibro-e.ngmving

electro-erosive -Gi writing

co-embedding enpving+ of label

GRINLlING-1 electrolytic

microtome cutting rotating disc

chemicdmmhanical +I combined manual

mechanical

mechanical

chemical

electrolytic

electro-erosive

band

micromilling

vibrational

I electrolytic/mwhanical I alternating

POLISHING

CLEANING-fl gyF

b air jet

DRYING-* Vacuum

heating

a. 10, 92 Sugace microscopy 947

last two cases the statistical fluctuations due to unavoidable inhomogeneities must be

considered, and usually more than one specimen is necessary to get a reliable result

(statistical sampling). Since usually nothing is known as to the degree of homogeneity,

statistical parameters (usually taking the arithmetic mean and the relative standard error,

see, e.g., RTZOW and EXNER [1968]) should be determined from samples which are

taken either at arbitrary or at specially defined locations. Furthermore, damaging the

specimen during cutting it from a larger piece gives rise to erroneous results: Electro￾erosive cutting (“spark-machining”), for example, changes the composition near the cut

faces to an appreciable depth: e.g., 0.9 and 0.3 wt% carbon (stemming from the

electrolyte) and 0.8 and 0.2 wt% copper (from the electrode) were found in pure iron at

50 and 150 pm depth, respectively, below the electro-eroded surface. Careful work (slow

and interrupted cutting) reduces the depth of influence to 10 pm. Mechanical cutting

(usually by water-cooled wheels) does not change the composition but introduces stresses

to a depth of 100 pm and more (WAVER [1973], WELLNER [1980], KIESSLER et al.

[1982], VANDER VOORT [1984a] or TELLS and F’ETZOW [1992]). In spite of the dis￾advantages of these commonly used techniques, others, like chemical cutting by a fast￾moving endless wire wetted by an aggressive liquid, available commercially as “acid

saws”, are only used for special purposes (for single crystals, semiconductors, brittle

intermetallics, etc.) because of the long cutting times needed (hours, instead of the

minutes needed for mechanical cutting). The same considerations apply to ultrasonic

erosion, electrochemical sectioning or laser cutting. In order to avoid artifacts, a careful

choice of the sampling technique adjusted to the specific material and its conditions and

a careful control of the result must be made.

2.12. Mounting

Embedding or clamping are relatively uncritical operations. Some resins reach a

temperature up to 150°C during curing, which may lead to annealing effects in the

specimen; others are cold-setting. Galvanic deposition of a thin copper or nickel layer

reduces edge-rounding during preparation to an acceptable level even for oblique

sectioning (see below). Smearing and edge-rounding of open porosity during polishing

can be avoided by infiltration of a low-viscosity resin under vacuum or by a well-wetting

melt (solder for metals, glass for ceramics).

2.1.3. Grinding

The surface of a cut cross-section usually shows a high degree of irregularity which is

removed in successive steps of grinding with emery paper (paper covered with Sic particles

closely graded from coarse to fine between 80 and 20 pm, see fig. 1). Heating can be limited

to a tolerable degree using water-cooling, but deformation of the surface is unavoidable

(SAMUELS [1971], PETZOW and EXNER [1968], WAVER [1973], KIESSLER et al. [1978],

VANDER VOORT [1984a], TELLE and PETZOW [1992], and F’ETZOW [1994]). It was found

empirically (LIHL and MEYER [1960]) that the deformation depth X, is a square function of

scratch depth Xs (X, =ax,- bX,’, where a and b are material constants). Figure 1 shows the

depth of scratches, the deformation depth and the total depth influenced in grinding of steel.

In an oblique taper section, the deformed layer becomes visible after etching (fig. 2).

References: p. 1016.