Mechanical Behavior of Materials
Chapter 2: Structure and Deformation in Materials
Jincheng Lei （!"#）
Shien-Ming Wu School of Intelligent Engineering
South China University of Technology
Email: leijincheng@scut.edu.cn
Fall 2022
What we are going to learn
Mechanical Behavior of Materials
Mechanical properties:
Strength, ductility, toughness,
elastic modulus, shear modulus
Chapter 2 & 3
Material structures:
Chemical bonding
Crystal structure
Microstructure
Processing
Structures → Properties
Chapter 4
Failure of materials:
• Permanent deformation
• fracture
Chapter 5, 12, & 13
Chapter 7
Mechanical testing:
Tension test
Compression test
Stress-strain curve
Stress-strain relationships
and behavior:
Models for Elastic, plastic,
creep deformation
The criteria of failure
(4 strength theories)
Fracture criterion
Yielding criterion
Evaluate mechanical
properties
The study of
deformation behavior
The conditions of failure
Chapter 6
Chapter 8
Chapter 9, 10, & 11
Chapter 15
Analysis of
stress and strain:
Fracture of
Cracked Members:
Fracture mechanics
Fatigue of materials:
Cyclic loading
The physical nature of fatigue
Fatigue crack growth
Creep and damping:
Time-dependent
deformation behavior
Fracture due to crack
Fracture due to fatigue
Time-dependent failure
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Objectives
•
Overview of chemical bonding and crystal structures
•
How are they related to mechanical behavior?
•
Basic mechanism of elastic and inelastic deformation
•
Why actual strengths of materials normally smaller than
the theoretical strength to break chemical bonds？
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Introduction of Engineering Materials
—
Engineering materials: used where resistance to mechanical
loading is necessary
◦ That’s why we care about their mechanical behavior
◦ Classification: metals and alloys, polymers, ceramics and
glasses, and composites
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Introduction of Engineering Materials
Metals and alloys
Polymers
Ceramics
Glasses
—
Composite materials: a material which is produced from two or more
constituent materials.
—
The constituent materials have dissimilar chemical or physical properties
—
The combined material has properties unlike the individual elements
Fiber-reinforced polymer composite
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Introduction of Engineering Materials
—
What properties are related to the mechanical behavior of
materials?
◦ Chemical bonding and microstructure
median
weak
strong
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Introduction of Engineering Materials
—
How to study the mechanical behavior of materials?
◦ An understanding of the behavior can be sought by looking at what
happens at a smaller scale
◦ The macroscopic behavior of the material is explained by the
microstructural features (10−3 – 10−9 m)
◦ Microstructural features: chemical bonding, crystal structures, defects in
crystals
micro features
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Bonding in Solids
—
Chemical bonding: the bonds that hold atoms and molecules
together in solids
◦ Primary bonds: ionic, covalent, metallic
– Strong, stiff, do not easily melt with increasing temperature
– The bonding of metals and ceramics
– High elastic modulus (E)
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Bonding in Solids
—
Ionic bonds
◦ Many common salts, oxides, and other solids have bonds that are mostly or
partially ionic
◦ Mechanical properties: hard and brittle
Example: MgCl2
Mg atom
2 Cl atom
Mg2+ ion
exceeds 2 e-
needs 1 e- for each
donate 2 e-
2 Cl- ion
accept 1 e- for each
More examples – metal oxides: MgO, FeO, Fe2O3
Mechanical Behavior of Materials
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Bonding in Solids
—
Covalent bonds
◦ Share the electrons with other atom to form a stable outer shell of 8 (or 2) e◦ Occur where the outer shells are half full or more than half full (≥ 4)
◦ Inorganic simple molecules – H2O, O2, Cl2
◦ Very tight – simple molecules are relatively independent of one another
◦ In the forms of liquids or gases at ambient temperature
Example: H2O
H atom
O atom
H atom
H2O molecule
Sharing of e-
needs 1 e-
needs 2 e-
Mechanical Behavior of Materials
needs 1 e9
Bonding in Solids
—
Covalent bonds
◦ Bonding: share e- with specific neighboring atoms – strongly directional
◦ Make a solid? Yes, a continuous arrangement of covalent bonds can form a
3D network
◦ Mechanical properties: very hard and stiff (inorganic crystals)
Example: Diamond (carbon crystal) – one of the hardest materials in nature
C
Each carbon atom shares
1 e- with 4 adjacent ones
C
C
Mechanical Behavior of Materials
C
C
Equal angles
(directional)
C
10
Bonding in Solids
—
Covalent bonds
◦ Another important continuous arrangement of covalent bonds is the carbon
chain (organic)
Example: ethylene (C2H4)
C2H4 - gas form
(simple molecule formed by covalent bonds)
Polyethylene –
solid form
(a long chain big
molecule formed
by covalent
bonds)
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Bonding in Solids
—
Metallic bonds – metals or alloys
◦ Metal atom donates its outer shell e- – metal ions
◦ The donated electrons are shared by all the metal ions – a “cloud” of e◦ Bonding: the metal ions are held together by the electrons through
electrostatic attraction
◦ Excellent thermal conductivity, electrical conductivity, ductility
-
Metal atom
1
e-
at the outer shell
-
+
+
Metal crystal
-
+
-
+
+
-
-
-
+
-
+
-
+
+
-
+
-
+
+
-
+
+
-
-
+
Metal atom donates the electron and
shares with other metal ions
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-
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Bonding in Solids
—
Mix bonding
◦ Mixed ionic-covalent – more common
– NaCl is pure ionic boding; diamond is pure covalent bonding
– Ceramics: SiO2, ZrO2, Al2O3
◦ Mixed metallic-ionic or -covalent
– Metallic bonding is dominant, intermetallic compounds may form
(ionic or covalent)
– Alloy: TiAl3, Mg2Ni
Crystal (SiO2)
Mechanical Behavior of Materials
Artificial teeth (ZrO2)
Crucible (Al2O3)
Implant (Ti alloys)
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Bonding in Solids
—
Secondary bonds
◦ Permanent dipole bonds: due to the presence of an electrostatic dipole
– Permanent dipole bonds in water
– Electrostatic dipole: A H atom away from the covalent bond to
the O atom has a positive charge (from two different molecules)
– Dipole causes an attraction between every two adjacent H2O
H2O molecule
electrostatic dipole –
hydrogen bond
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Bonding in Solids
—
Secondary bonds
◦ Permanent dipole bonds: due to the presence of an electrostatic dipole
– Hydrogen bonds:
– Relatively weaker than primary bonds
– Sufficient to bind materials into solids – water to ice
– Hydrogen normally forms strong dipole bonds – HCl, HF, H2O
H2O
Mechanical Behavior of Materials
polyvinyl chloride
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Bonding in Solids
—
Secondary bonds
◦ Fluctuating dipole bonds: uneven distribution of electric charge that
causes a weak attraction between atoms or molecules
– Van der Waals bonds: the fluctuating positions of electrons relative to
an atom’s nucleus
– The dipole is not fixed in direction
– Weaker than the hydrogen bonds
graphite
covalent bonds
Van der Waals bonds
between layers
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Structure in Crystalline Materials
—
Structure of materials
◦ Metals & ceramics: aggregation of small grains (each is an individual
crystal)
◦ Glasses: amorphous or non-crystalline structure (no crystals)
◦ Polymers: chainlike molecules, sometimes in a crystalline manner
(partially crystalline)
Bonding
Structures
Crystal structure
Mechanical behaviors
Micro- and macro-structure
Arrangement of atoms/ions
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Structure in Crystalline Materials
Basic crystal structure – crystal system + lattices
The frame to position the atoms/ions – crystal system (unit cell)
—
◦ Unit cell: the smallest repeated group of atoms/ions in crystals –
building block
◦ 7 basic types of unit cell – crystal systems
◦ All unit cells are arranged in one of these 7 system
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Structure in Crystalline Materials
—
Basic crystal structure
◦ Where to position atoms/ions? – lattices
– One type of unit cell can have various arrangements of atoms
– The atoms can be placed at the corners, body centers or face center
PC: rare
BCC: Cr, Fe, Mo, Na, W
FCC: Ag, Al, Pb, Cu, Ni
HCP: Be, Mg, Ti, Zn
Primitive cubic
(PC)
Mechanical Behavior of Materials
Cubic
Body-centered cubic
(BCC)
Hexagonal
Face-centered cubic
(FCC)
Hexagonal close-packed
(HCP)
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Structure in Crystalline Materials
—
Basic crystal structure
◦ Does the crystal structure of materials change? – temperature, pressure,
alloying elements
Example: Fe
– different processing T will produce different iron crystal structures
– addition of about 10% Ni or Mg changes the crystal structure to FCC at room T
– different crystal structures have different mechanical behaviors
910ºC
1390ºC
BCC: harder, higher stiffness, higher strength, but not easy to fabricate
FCC: softer, higher ductility, easier to fabricate
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Structure in Crystalline Materials
—
More complex crystal structure
◦ Metals: one type of atoms in the crystal structure
◦ Ionic salts and ceramics: more than one type of atom and sometimes
with directional properties
NaCl (FCC of Cl− with Na+ at
intermediate positions)
Diamond cubic (two interpenetrating FCC)
tetragonal bonding
due to the directional
property of covalent
bonds
Similarly, MgO, FeO, TiC, ZrC
are also arranged in FCC
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Structure in Crystalline Materials
—
More complex crystal structure
◦ Metals: one type of atoms in the crystal structure
◦ Ionic salts and ceramics: more than one type of atom and sometimes
with directional properties
Diamond cubic (two interpenetrating FCC)
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Structure in Crystalline Materials
—
More complex crystal structure
◦ Metals: one type of atoms in the crystal structure
◦ Ionic salts and ceramics: more than one type of atom and sometimes
with directional properties
Al2O3 ceramic (HCP)
(at 2/3 position)
• Aluminum atoms occur in 2/3 of the spaces
available between the oxygen atoms.
• Many ceramics have even more complex
crystal structures than these examples
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Structure in Crystalline Materials
—
More complex crystal structure
◦ Amorphous structures
Example: polymers
Amorphous
an irregular tangle of chain molecules
Mechanical Behavior of Materials
Crystalline
The chains arranged in a regular manner
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Structure in Crystalline Materials
—
Defects in crystals
◦ Polycrystalline materials: ceramics and metals in engineering
applications are composed of crystalline grains that are separated by
grain boundaries
◦ Grain size: ~1 𝜇m to > 10 𝜇m (depend on materials and its processing)
◦ The structures within the grain is the crystal structures
Crystal structure inside the grain
Grains and grain boundaries
Grain
Metals
Mechanical Behavior of Materials
Ceramics
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Structure in Crystalline Materials
—
Defects in crystals
◦ Within the grains, the crystals are not perfect – defects: point defects,
line defects, surface defects
◦ Grain boundaries and crystal defects – large effects on mechanical
behaviors
Point defects:
• Substitutional impurity – occupy the
normal lattice site
• Vacancy – absence of an atom at the
normal lattice site
• Self interstitial – same type of atom
occupying a position between normal
lattice site
• Interstitial impurity – another kind of
atom occupying a position between
normal lattice site
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Structure in Crystalline Materials
—
Defects in crystals
Point defects: is not a bad thing
– we need to create point defects to improve the mechanical behaviours of materials
Carbon in iron – interstitial impurity – improve strength and fracture resistance*
Cast iron
*Pure iron is quite ductile, or soft and easily formed.
Mechanical Behavior of Materials
Steel
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Structure in Crystalline Materials
—
Defects in crystals
Point defects: is not a bad thing
– we need to create point defects to improve the mechanical behaviours of materials
Add 10-20% Cr or Ni into iron – substitutional impurities
Stainless steel
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Structure in Crystalline Materials
—
Defects in crystals
Line defects: dislocations
• The edges of surfaces where there is a relative displacement of lattice planes
• Edge dislocation – the border
of an extra plane of atoms
• Screw dislocation – a perfect
crystal is cut and displaced
parallel to the cut and finally
reconnected into the
configuration
• Usually occur as combined
edge and screw
an extra plane of atoms
(dislocation line)
Edge dislocation
Mechanical Behavior of Materials
displaced region
(dislocation line)
Screw dislocation
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Structure in Crystalline Materials
—
Defects in crystals
Surface defects:
• Additional types of surface defects
Twin boundary
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Structure in Crystalline Materials
—
Defects in crystals
Surface defects:
• Additional types of surface defects
Twin boundary
Stacking fault
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Thank You!
Q&A
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