micro structures & properties
Post on 08-Apr-2018
222 Views
Preview:
TRANSCRIPT
-
8/7/2019 Micro Structures & Properties
1/30
Microstructure
&
Mechanical Properties
V Arunkumar
Roll No 2010413002
M.Tech (II Year) - Nanoscience & Nanotechnology
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
-
8/7/2019 Micro Structures & Properties
2/30
Contents
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
S.No TOPICS
1. Introduction to Plastic Deformation
2. Grains & Dislocation
3. Strengthening Mechanisms
4. Reference
Microstructure & Mechanical Properties
-
8/7/2019 Micro Structures & Properties
3/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Material undergoes Elastic Deformation if Stress < Yield Stress
Material undergoes Plastic Deformation if Stress > Yield Stress
Plastic deformation the force to break all bonds in the slip plane is much
higher than the force needed to cause the deformation.
Theoretical yield strength predicted for perfect crystals is much greater than
the measured strength.The large discrepancy puzzled many scientists until
Orowan, Polanyi, and Taylor (1934).
The existence of defects (specifically, dislocations) explains the discrepancy.
The reason was proposed by in 1934 by Taylor, Orowan and Polyani: Plastic
deformation is due to the motion of a large number of dislocations.
-
8/7/2019 Micro Structures & Properties
4/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
DefectsPoint defects: vacancies, interstitial atoms, substitional atoms, etc.
Line defects: dislocations(edge, screw, mixed)
Most important for plastic deformation
Surface defects: grain boundaries, phase boundaries, free surfaces,etc.
-
8/7/2019 Micro Structures & Properties
5/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Edge dislocations
Burgers vector: characterizes the strength of dislocations
Edge dislocations: b B dislocation line
-
8/7/2019 Micro Structures & Properties
6/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Screw dislocations
Burgers vector b parallel to dislocation line
Mixed dislocationHave both edge and screw
components.
-
8/7/2019 Micro Structures & Properties
7/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Movement of an edge dislocation
Break one bond at a time, much easier than breaking all the bondsalong the slip plane simultaneously, and thus lower yield stress.
-
8/7/2019 Micro Structures & Properties
8/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Dislocations allow deformation at much lower stress than in a perfect crystal
If the top half of the crystal is slipping one plane at a time then only a small fraction of
the bonds are broken at any given time and this would require a much smaller force. The
propagation of one dislocation across the plane causes the top half of the crystal tomove (toslip) with respect to the bottom half but we do not have to break all the bonds
across the middle plane simultaneously (which would require a very large force).The slip
planethe crystallographic plane of dislocation motion.
-
8/7/2019 Micro Structures & Properties
9/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Motion of dislocations
-
8/7/2019 Micro Structures & Properties
10/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Interactions of dislocationsTwo dislocations may repel or attract each other, depending on their
directions.
Repulsion Attraction
-
8/7/2019 Micro Structures & Properties
11/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Line tension of a dislocation
Atoms near the core of a dislocation have a higher energy due to
distortion.
Dislocation line tends to shorten to minimize energy, as if it had a line
tension.
Line tension = strain energy per unit length
T
T
2
2
1GbT }
-
8/7/2019 Micro Structures & Properties
12/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Dislocation multiplicationSome dislocations form during the process of crystallization.
More dislocations are created during plastic deformation.
Frank-Read Sources: a dislocation breeding mechanism.
-
8/7/2019 Micro Structures & Properties
13/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Strengthening mechanisms
Pure metals have low resistance to dislocation motion, thus low
yield strength.
Increase the resistance by strengthening:
Solution strengthening
Precipitate strengthening
Work hardening
-
8/7/2019 Micro Structures & Properties
14/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Solution strengthening
Add impurities to form solid solution (alloy)
Example: add Zn in Cu to form brass, strengthincreased by up to 10 times.
Cu Cu Cu Cu Cu Cu
Cu Cu Cu
Cu Cu Cu Cu
Zn Zn
B
igger Zn atoms make the slipplane rougher, thus increase the
resistance to dislocation motion.
-
8/7/2019 Micro Structures & Properties
15/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Solid solution hardening
Dissolve other elements (solute) into the parent material (solvent) to form solid solution.
Interstitial solution Substitutional solution
Steel (C in Fe) Brass (Zn in Cu)
-
8/7/2019 Micro Structures & Properties
16/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Solid solution hardening
Large distortion due to mismatch makes
it hard for dislocation to move.
Large population of solute atoms
obstruct dislocation motion.
-
8/7/2019 Micro Structures & Properties
17/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Precipitate strengthening
Dispersion strengthening: mix small particles(dispersoids) into a powdered metal, then compact and
sinter.
Proper heat treatments can control the formation of
precipitates (more later).
Precipitates: compound particles precipitates out from
the solution as it is cooled.
Solid solution: single-phase, random mixture of atoms
(substitutionalor interstitutional)
-
8/7/2019 Micro Structures & Properties
18/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Precipitate strengthening
Precipitates (small particles) can promote
strengthening by impeding dislocation
motion.
Dislocation bowing and looping.
Critical condition at semicircularconfiguration:
TbL 2!X
L
Gb
bL
T}!
2X
M.F. Ashby and D.R.H. Jones, Engineering Materials 1, 2nd
ed. (2002)
-
8/7/2019 Micro Structures & Properties
19/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Work-hardeningDislocations interact and obstruct each other.
Accounts for higher strength of cold rolled steels.
W
I
WYU
WYL
Strain hardening
WUTS
If
-
8/7/2019 Micro Structures & Properties
20/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Dislocation yield strength
Combine the resistance due to lattice, solid solution,
precipitates, and dislocation tangles in an additive way:
-
8/7/2019 Micro Structures & Properties
21/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Polycrystalline materialsDifferent crystal orientations in different grains.
Crystal structure is disturbed at grain boundaries.
-
8/7/2019 Micro Structures & Properties
22/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Plastic deformation in Polycrystals
Slip in each grain is constrained
Dislocations pile up at grain boundaries
Gross yield-strength is higher than single crystals (Taylor factor)
Strength depends on grain size
(Hall-Petch Relation
YYXW 3!
2/1
0
! KdY
WW
-
8/7/2019 Micro Structures & Properties
23/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Important features of plasticity :Stress-strain curves provide a straightforward way to measure yield stress, ultimate tensile
stress and ductility.
The maximum load and maximum uniform elongation are predictable from the stress-strain
curve (e.g. power law).
Single crystal behavior reflects the anisotropy of the crystal for both elastic and plastic
behavior.
Single crystal plastic behavior is controlled by dislocation movement; deformation twinning
can supplement dislocation glide
The presence of dislocations that can glide at low (critical resolved) shear stresses means that
metals yield plastically at stresses far below the theoretical strength.
-
8/7/2019 Micro Structures & Properties
24/30
-
8/7/2019 Micro Structures & Properties
25/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Strain Rate - Dependence Temperature & Grain size
The variation of mechanical behavior with temperature and strain rate depends on the kind of
obstacle that dislocations have to move past.
In fcc metals, yield is dominated by other dislocations (the "forest hardening model") such that
the strain rate/temperature variation is dominated by the (weak) variation in shear
modulus(with temperature) through the "Taylor equation", =MGb.
In bcc metals, yield at low temperatures is dominated by lattice friction (i.e. the Peierls stress)
and large strain rate/temperature sensitivities are observed. Most ceramics follow the bcc
model because they too have high lattice frictions at low temperatures (but become plastic
and ductile at elevated temperatures).
Single crystals are important because many high temperature applications require singlecrystal or coarse poly-crystals in order to maximize creep resistance, i.e. by minimizing grain
boundary area.
Microelectronic applications use single crystals ofSi where the absence of grain boundaries is
not important unless MEMS devices are being designed.
-
8/7/2019 Micro Structures & Properties
26/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Strain Rate - Dependence Temperature & Grain size
The work hardening behavior of single crystals is summarized by fourstages: stage I is known
as "easy glide"; stage II as "linear, athermalhardening"; stage III as "dynamic recovery"; and
stage IV as "linearhardening".
For a polycrystal to exhibit ductility, it must be possible for every grain to deform plastically in
an arbitrary manner. This is summarized as vonMises criterion which states that a minimum of
five independent systems are required for ductility. This can be understood most easily by
considering that an arbitrary strain has five independent components there is an equation
(linear) that links the slip on an individual slip system (or twinning system) to the macroscopic
shape change (i.e. strain); therefore five independent systems are needed in order to satisfy
the five independent strain components
-
8/7/2019 Micro Structures & Properties
27/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Strain Rate - Dependence Temperature & Grain size
Dislocation flow in a polycrystal is quite heterogeneous. Dislocations get entangled in one
another as they expand over their slip planes. The major consequence of this is that any
dislocation motion (over a distance larger than the mean spacing) leaves behind a certain
amount of dislocation; this is called dislocation storage and hardens the crystal. By a
combination of collapse of tangles and cross-slip (switching of slip planes by screw-
configuration segments), however, dislocations of opposite sign can meet and annihilate; this
is called dynamic recovery (because it only happens during continuing straining)and decreases
the hardening rate (i.e. the net storage rate of dislocations decreases because of dynamic
recovery). Eventually dynamic recovery balances storage and the flow stress saturates, or
nearly so
-
8/7/2019 Micro Structures & Properties
28/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
Strain Rate - Dependence Temperature & Grain size
At high temperatures, dynamic recovery occurs early on in straining and, withthe ease of non-
conservative motion (climb), the work hardening becomesnegligible. With rapid dynamic (and
static) recovery, the dislocation structurebecomes a sub-grain structure with well defined, low
angle boundaries. If single crystal is bent, then the dislocations left behind after the
deformation tendto re-arrange themselves into walls of edge dislocations of the same type
andsign. Such a recovered or polygonized structure is a clear example ofgeometrically
necessary dislocations.
-
8/7/2019 Micro Structures & Properties
29/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
REFERENCE
1. Material Science and Engineering by William Callister
2. Material Science for Engineers by Ashby & Miller
3. Study Materials on Strengthening of Materials by University of Tennessee, Dept. of
Materials Science and Engineering
4. Study Materials on Microstructues by Carnegie Mellon University
5. Lecture Materials on Materials Science by University of Texas - Austin
6. Notes by University of Illinois on Material Science - Thermal & Mechanical Properties
of Materials (TOO GOOD HENCE ENCLOSED HEREWITH)
-
8/7/2019 Micro Structures & Properties
30/30
V Arunkumar - Roll No 2010413002 M.Tech (II Sem) - Nanoscience & Nanotechnology
Microstructure & Mechanical Properties
top related