Deformation Mechanism Maps for Bulk Materials

Presentation by

Suresh Beera

12ETMM11

M.Tech

Materials Engineering

SEST, UoH.

Contents

Introduction to creep

Deformation mechanism

Deformation Mechanism Maps – Introduction

Construction of Deformation Mechanism Maps

Deformation mechanism maps in FCC metals

Summary

References

Progressive deformation is subjected to constant load at elevated

temperatures → Creep

έs = Aσn e – (Q / RT)

Steady state creep rate έs in the

range 0.4 Tm < T < 0.6 Tm can be

expressed as power-law function

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

Introduction

Creep Deformation Mechanism

Dislocation Creep Diffusional Creep

Nabarro –

Herring

Creep

Coble Creep

Stress ↓

Temperature ↑

Stress ↑

Temperature ↓

Grain boundary precipitates inhibit grain boundary sliding

Grain size ↓ diffusion / mass transport ↑

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.

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.

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

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

Deformation mechanism in FCC metals:

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

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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