CREEP OF METALS AND ALLOYS
COURSE DESCRIPTION:
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Total credits: 2
Instructor: Dr. S.M.K. Hosseini
Time and location: Every Sunday, 13-15 pm, Faculty of Engineering,
Room # 301
INTRODUCTION:
The slow deformation of metals, known as creep, first came clearly into focus
about half a century ago. As a problem in metal use it has grown steadily in
importance because engineers have persistently raised their operating
temperatures in many fields, until now it is one of the half-dozen most
important of these problems. Besides the abundance of technical data that
has necessarily been accumulated, there have been many studies in the last
fifteen years of the physics of creep, and it is with these that this article is
concerned. These physical studies have shown that there are several different
creep régimes depending mainly on the temperature. If TM is the melting point
of the metal in question, the different régimes roughly cover the temperature
ranges 0-0·3 TM, 0·3-0·5 TM, 0·5-0·9 TM and 0·9-1·0 TM. The bottom range
includes the so-called logarithmic creep and the top range creep by
diffusion, which is somewhat similar to flow in liquids. Both are quite well
understood but neither is particularly important and they are dealt with briefly.
It is the middle two temperature ranges in which creep worries engineers;
they have therefore received much more attention and the creep behaviour
in them is described more fully in this course.
In both of these temperature bands the rate of creep strain is very dependent
on temperature, stress applied and composition of the metal. The
temperature dependence is no surprise, since it is quite clear that creep is a
thermally activated phenomenon and its rate is therefore governed by an
Arrhenius factor. In many pure metals the temperature dependence is
quantitatively close to that of self-diffusion, particularly in the 0·5-0·9 TM band,
and the agreement grows better the more refined the measurements
become. In the last ten years this result has guided most theories, and has
encouraged attempts to be made with partial success to calculate the
absolute rates from first principles. Calculations have been founded on both
the two basic theoretical models. One assumes that the factor controlling
creep rate lies in the deformation process, or glide movement of dislocations,
itself. The other assumes that the rate-controlling factor lies in the continuous
annealing that takes place at the high temperatures involved. The two
models are really two sides of the one penny, since both slip and recovery
take place simultaneously and are unavoidably interconnected. When the
two models are properly joined they explain the influence of composition,
which may affect creep rate by well over a millionfold, and the great
influence of stress as well as the temperature effect. The physics of creep
deformation in the middle temperature ranges is therefore quite well
understood. Indeed, the important parameters like diffusion rate, stress and
stacking fault energy combined in a single equation give the creep rates of
many simple metals with fair accuracy.
A consequence of the growing success in producing alloys that deform very
slowly even at high stress and temperature is that the problem of creep
fracture has loomed larger. There is a fracture mechanism quite distinctive to
creep, in which tiny holes nucleate and grow by some means until they are so
large, or sufficiently linked together, that the metal breaks. The speed of this
fracture process increases with temperature and stress, and evidently also
depends on composition in a complicated way, which has made possible a
certain degree of manufacturing control if not of understanding. In essence,
the formation and growth of the holes is a phase change in which stress
provides the driving free energy since the holes enlarge the overall
dimensions. Both the nucleation and growth rates help to determine the time
to fracture, which is what really matters. There are several obscure points such
as the nature of many of the nucleating sites and the strong influence of
deformation rate during nucleation and growth. Both theoretically and
practically, understanding of the youthful problem of creep fracture is less
mature than that of the older problem of creep deformation.
OUTLINE:
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Introduction and definitions (1 session)
Creep test and creep data illustration (1 session)
Effects of temperature and stress on creep rate (1 session)
Deformation Mechanisms Map: Power-law creep and power-law
creep breakdown, Diffusional creep, Harper-Dorn creep, Grain
boundary sliding (GBS) (4 sessions)
Creep damage mechanisms and fractography of fracture surfaces
(1 seesion)
Creep-life assessment: Larson-Miller parameter, Sherby-Dorn,
Manson-Hafferd, etc. (2 sessions)
Creep Fracture (1 seesion)
Creep/Fatigue/Environmental Interactions (1 session)
High-temperature and creep-resistant materials (2 sessions)
REFERENCES:
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Fundamentals of Creep if Metals and Alloys, M.E. Kassner, 2nd ed.,
Elsevier, 2007
Creep of Metals at High Temperatures, P. Greenfield, Mills & Boon,
1972.
Physics of Creep and Creep-Resistant Alloys, F.R.N. Nabarro, N.R.
Nabarro, 1st ed. Taylor and Francis, 1995.
Creep Mechanics, Josef Betten, 2nd ed., Springer, 2005
Design for Creep, R.K. Penny and D.L. Marriott, 2nd ed. Chapman
and Hall, 1995.
Mechanical Metallurgy, G.E. Dieter, 2nd ed., McGraw-Hill Book
company, 1988.
Creep-Resistant Steels, F. Abe, T-U. Kern and R. Viswanathan, 1st ed.,
Woodhead Publishing Limited, 2008.
D. McLean, “The Physics of High-Temperature Creep in Metals, Rep.
Prog. Phys., 1966, pp:29-133
COURSE ASSESSMENT:
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Midterm (6 points)
Final exam (8 points)
Quiz and Homework (2 points)
Term Project (4 points)