dislocation-Deformation Mechanism Maps for Bulk Materials

1. Deformation Mechanism Maps for Bulk Materials
Presentation by
Suresh Beera
12ETMM11
M.Tech
Materials Engineering
SEST, UoH.

2. Contents
Introduction to creep
Deformation mechanism
Deformation Mechanism Maps – Introduction
Construction of Deformation Mechanism Maps
Deformation mechanism maps in FCC metals
Summary
References

3. Introduction
Progressive deformation is subjected to constant load at elevated
temperatures → Creep
Steady state creep rate έs in the
range 0.4 Tm < T < 0.6 Tm can be
expressed as power-law function
έs = Aσn e – (Q / RT)
Pure metal shows activation
energy(Q) for creep is equal to self
diffusion
At higher stress levels creep rate will
be more → Power-law break down

4. Creep Deformation Mechanism
Dislocation Creep Diffusional Creep
Nabarro –
Herring
Creep
Coble Creep
Stress ↓
Temperature ↑
Stress ↑
Temperature ↓
Grain size ↓ diffusion / mass transport ↑
Grain boundary precipitates inhibit grain boundary sliding

5. Deformation Mechanism Maps – Introduction
The map displays the
relationship between the three
macroscopic variables : stress
σs , Temperature T and strain rate
έ.
The various regions of the
map indicate the dominant
deformation mechanism for the
combination of stress and
temperature.

6. Construction of Maps:
Step –I : Gathering the data of material properties, (lattice parameter, Molecular
Volume, Burgers Vector , Moduli, and their temperature dependence)
Step – II: Data for the hardness, low temperature yield, creep are gathered
flow strength-function(temperature , strain rate)
strain rate-function(temperature , stress)
Step – III: Initial estimate is made of the material properties describing glide creep by
fitting equation to the data plotted
From the plot it is possible to make an initial estimate of the stress at which the
simple power-law for creep break down
Step – IV: Using the initial values for the material properties construct a trial maps.
This can be done by simple computer programming
All the maps are divided into fields within each of which a given mechanism is
dominant
Step – V: The data plots are laid over trail maps, allowing the data to be divided into
blocks according to the dominant flow mechanism
It is then possible to make a detail comparison between each block of data and the
appreciate rate equation.

7.  The material properties
are now adjusted to
give the best fit
between theory and
experimental data.
 New maps are now
computed and the
comparison repeated.
Final adjustments are
made by constructing
maps of the different
types
The construction of a deformation-mechanism map. The field
boundaries are the loci of points at which two mechanisms (or
combinations of mechanisms) have equal rates

8. Deformation mechanism in FCC metals:
Above about 0.3 TM , the
f.c.c. metals start to creep.
Diffusion (which is
thought to control creep in
these metals) is slower in
the f.c.c. structure than in
the more-open b.c.c.
structure
This is reflected in lower
creep-rates at the same
values of σs / μ and T/TM

9. Summary
Discussed the basic power-law equation and its breakdown
Different deformation Mechanisms were explained
Creep rate can be known with the other two parameters (Temperature and
Stress) are known with these deformation mechanism maps
Construction of maps for a new materials were discussed in detail
For bulk materials (especially in FCC metals) the deformation mechanism
maps were discussed

10. References
 Deformation Mechanism Maps ,The Plasticity and Creep Of Metals and
Ceramics, H.J.Forst and M.F. Ashby.
Dieter.G.E.Mechanical metallurgy 1988,SI Metric edition, McGraw-hill
publication
Ashby, M.F., A first report on deformation-mechanism maps. Acta
Metallurgica (pre 1990), 1972. 20: p. 887.
Frost, H.J. and M.F. Ashby, A Second Report on Deformation-Mechanism
Maps. 1973, Division of Applied Physics, Harvard University.
F.C.Campbell,editor,chapter 15 ,creep, elements of Metallurgy and
engineering alloys

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