Materials: Metals and Alloys
Resources, Processes & Materials Engineering
LECTURE M1
Dr Sofia Hazarabedian
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Lecture focus
Reproduced from “Materials and Man’s Needs”, National Academy of Sciences, Washington D.C., 1974.
Lecture Outline
• What are ‘raw’ or ‘bulk’ materials?
• What are engineering materials? Do they matter? Are they matter?
• Gases, Liquids, Solids
• Focus on Solids
• Whatholdsthemtogether?Bonding,structure,order,crystalline,amorphous. • Classifications–Metals,Polymers,Ceramics,Composites
• Crystal Structures and Crystallography
• SimpleCubic,BCC,FCC,HCPandMillerIndices
• Polycrystallinemetalsandcrystalimperfections-vacanciesanddislocations
• Strengthening Mechanisms in Metallic Materials
• ElasticandPlasticDeformation-dislocationsandslipsystems
• Mechanical properties in relation to structure
• Crystal (grain) size, work hardening
• Alloying
(Reference and extract sources in this section – Materials: Engineering, Science, Processing and Design 3E; M Ashby, H Shercliff, and D Cebon. [Ashby] Materials Science and Engineering-An Introduction 10E; WD Callister and DG Rethwisch. [Callister])
States of Matter
Matter is commonly found on Earth in the following states
Solid Liquid Gas
Solid Materials
Material Bonding
• As two atoms come close together they experience attractive forces between the (negative) electron clouds and the (positive) nuclei.
• The two atoms also experience the repulsive force between the two nuclei as well as the repulsive force between the two electron clouds.
• At a separation (ro) the sum of both the attractive forces and the repulsive forces equals zero.
• Significantotherparameter-Temperature
Solid Materials
Force against distance for approaching atoms
Solid Materials
Force and energy in relation to interatomic spacing ( . 30)
• Where there is no net force (∑F = 0) or minimum net energy, a state of equilibrium exists and the centres of the two atoms will remain separated by the equilibrium spacing (r0) or bond-length.
• The bonding energy (E0) between two atoms corresponds to the energy at r0, and represents the energy required to separate them.
Solid Material – Bonding
Types of interatomic and intermolecular bonds
Primary (strongest bonds) Ionic
Covalent Metallic
Secondary (Weaker) Van der Waals Hydrogen
Primary Bonds
Ionic Bonding
•Found in compounds that are composed of both metallic and non-metallic ions; eg. Na+Cl-
•The attractive forces are Coulombic, i.e., positive and negative ions attract one another.
•Ionic bonding is non-directional, i.e. the magnitude of the bond is equal in all directions around an ion. •Bonding energies between 600 – 1500 kJ/mol.
•Ionic-bonded materials have high melting points, hard, brittle, stiff, poor electrical and thermal conductors.
Primary Bonds
Covalent Bonding
•Covalent bonding involves the sharing of electrons between adjacent atoms.
•Found in non-metallic molecules (H2, Cl2, F2, H2O, HF) and solids (Si, Ge, GaAs, SiC, diamond) as well as polymeric materials (e.g., plastics, rubbers).
•Covalent-bonded solids may be
– very strong, hard, stiff, brittle and high melting point (e.g., diamond) – very weak with low melting point (e.g. Bismuth melts at 270oC).
Primary Bonds
Metallic Bonding
•Ions in a ‘sea ‘ of electrons
•Metallic bonding involves the attraction between ion cores and valence electrons
•Common in metals and their alloys
•The free valence electrons act as a “glue” to hold the ion cores together.
•Bonding may be weak with low melting point (e.g., Hg) or strong with high melting point (e.g., W).
•Presence of mobile electrons means good heat and electrical conductivity, and in terms of mechanical properties enables the feature of ductility in many metallic materials .
Secondary Bonds
Van der Waals and Hydrogen bonding
• Very weak when compared to the primary bonding.
• Bonding arises from attraction between atomic or molecular dipoles.
Structure and order
Crystalline and amorphous solids
Crystalline • Dense, ordered packing
typical neighbour bond energy
typical neighbour bond length
Dense, ordered packed structures tend to have
lower energies.
Amorphous • Non dense, random packing
typical neighbour bond energy
typical neighbour bond length
Structure and order
Crystalline and amorphous
• Crystallinematerials–atomspackinregularperiodic3Darrays
• Typicalof
• Many ceramics
• Some polymers
• Amorphous(Non–Crystalline)materials–atomicarrangementispurelyrandom
• Typicalof
• Many polymers
• Complex structures
• Rapidly solidified solids
Materials Classifications
Families of Materials
Steels, Aluminium, Nickel, etc.
Alumina, SiC, PS Zirconia, etc.
Polyethylene, Teflon, Rubber, etc.
GFRP, CFRP, Concrete, Wood, etc.
….focus on Metals…..
Learning Outcome Check 1
Explain the following terms and give an example of each: – metal
– composite
Which type of bonding is predominant in each type of material above? What is the difference between a crystalline and amorphous structure?
Crystal Structures and Crystallography
Crystals and Grains in Metallic Materials
• Theadjacentfigureshowsasectionthroughaningotof cast aluminium.
• Thishasbeenobtainedafterthealuminiumhasbeen extracted from its ore, refined at high temperature in the molten condition and then allowed to solidify.
• Ineverydaylifemetallicmaterialsgenerallyappearas shiny, machined or perhaps painted or coated surfaces.
• However,deeperanalysisshowsthatthebulksolid material consists of crystals, or grains.
• Howandwherethecrystalsformonsolidificationhasa major bearing on the properties of these materials, and thus their applications.
• Thecrystalandgrainstructuresneedtobeunderstoodso that they can be modified to develop the useful engineering materials we have today and for the future.
Crystal Structures and Crystallography ( L1-13)
Crystal Systems
Unit cell: smallest repetitive volume which contains the complete lattice pattern of a crystal.
Callister (Pg.59).
7 crystal systems 14 crystal lattices
a, b, and c are the lattice constants
Crystal Structures and Crystallography ( L1-13
How can we stack metal atoms to minimize empty space?
2-dimensions
Now stack these 2-D layers to make 3-D structures
Crystal Structures and Crystallography ( L1-13)
Metallic Crystal Structures
• SimpleCubic(SC)
Reasons for metals’ dense packing:
Typically, only one element is present, so all atomic radii are the same.
Metallic bonding is not directional.
Nearest neighbor distances tend to be small in order to lower bond energy. Electron cloud shields cores from each other
Have the simplest crystal structures.
Coordination Number (CN) = Number of nearest neighbours
Atomic Packing Factor (APF) = Volume of atoms in unit cell Volume of unit cell
• BodyCentredCubic(BCC)
• Chromium
• Manganese
• Molybdenum
Crystal Structures and Crystallography ( L1-13)
Metallic Crystal Structures
• FaceCentredCubic(FCC)
• HexagonalClosePacked(HCP)
• Aluminium
• Titanium,
• Magnesium • Zinc
• Zirconium
Crystal Structures and Crystallography ( L1-13)
Miller Indices
A pseudo-quantitative method of describing crystallographic orientations, i.e., atomic locations in terms of coordination, planes and directions.
Reciprocals of the (three) axial intercepts for a plane, cleared of fractions and common multiples.
All parallel planes have same Miller indices.
1. Read off intercepts of plane with axes in terms of a, b, c 2. Take reciprocals of intercepts
3. Reduce to smallest integer values
4. Enclose in parentheses, no commas i.e., (hkl)
Crystallographic Directions
1. Vector repositioned (if necessary) to pass through origin.
2. Read off projections in terms of unit cell dimensions a, b, and c
3. Adjust to smallest integer values
4. Enclose in square brackets, no commas
[uvw] e.g.1,0,1⁄2 => 2,0,1 => [201]
-1,1,1 => [111]
Families of directions represented by
where overbar represents a negative index
Crystallographic Planes
1. Intercepts 1 1 ∞
2. Reciprocals 1/1 1/1 1/∞ 110
3. Reduction 1 1 0
4. Miller Indices (110)
1. Intercepts 1/2 ∞ ∞
2. Reciprocals 1/1⁄2 1/∞ 1/∞ 200
3. Reduction 1 0 0
4. Miller Indices (100)
Crystallographic Planes (cont.)
example Intercepts
Reciprocals
1/2 1 3/4 1/1⁄2 1/1 1/3⁄4
2 1 4/3 6 3 4
Miller Indices (634)
Family of Planes represented by
E.g. {100} = (100),(010), (001), (100), (010), (001)
Densities of Material Classes (Callister)
In general
Metals/ Alloys
Graphite/ Composites/ Ceramics/ Polymers fibers
Based on data from Callister (Pg. 8)
*GFRE, CFRE, & AFRE are Glass, Carbon, & Aramid Fiber-Reinforced Epoxy composites (values based on
60% volume fraction of aligned fibers in an epoxy matrix).
ρmetals >ρceramics >ρpolymers Why? 30
Metals have…
• close-packing
(metallic bonding) 10 • often large atomic masses
Ceramics have…
• less dense packing
• often lighter elements
Polymers have…
• low packing density
(often amorphous) 1 • lighter elements (C,H,O)
Composites have…
• intermediate values 0.4
ρ: Density
Data from Gold, W Tantalum
Silver, Mo
Cu,Ni Steels Tin, Zinc
Aluminum Magnesium
Al oxide Diamond Si nitride
Glass -soda Concrete Silicon
Silicone PVC
PC HDPE, PS PP, LDPE
Glass fibers
GFRE* Carbon fibers CFRE* Aramid fibers AFRE *
Imperfections and Defects in Crystals
Crystal Defects
Substitutional Solute
Grain Boundary
Dislocations
Phase Boundary
Interstitial Solute
Imperfections and Defects in Crystals
Dislocations
Edge Dislocation
The edge of an extra portion of a plane of atoms, or a half-plane, terminates within the crystal
It is a linear defect, that centers on the line defined along the end of the extra half-plane of atoms (the dislocation line)
Screw Dislocation
Formed by a shear stress that is applied to produce the distortion shown in the Figure
The atomic distortion is also linear and along the dislocation line
Imperfections and Defects in Crystals
Dislocations
A transmission electron microscopy (TEM) image of a titanium alloy in which the dark lines are dislocations.
Learning Outcome Check 2
What are Millers Indices used to represent?
What is the difference between FCC, BCC and HCP crystal structures? Name three types of crystalline defects
What are the two main types of dislocation
Strengthening Mechanisms in Metallic Materials ( . 128-163)
Elastic and Plastic Deformation
Fundamental Mechanical Properties
• Strength
Usually considered as the tensile strength, which is defined as the maximum force required to fracture per unit cross sectional area in tension. In most cases however, the yield strength, the force at which the material begins to permanently deform, is the limiting factor
• Ductility
This is considered to be the capacity to undergo deformation (generally under tension) without rupture. This is
distinct from malleability
• Toughness
This is the ability to withstand bending or deflection, or absorb energy, without fracture. Effectively, it is the
resistance to fracture.
• Hardness
This is ability to resist plastic deformation, indentation or abrasion. This property is very important in an engineering application where resistance to wear is a requirement.
Strengthening Mechanisms in Metallic Materials
Elastic Deformation
A result of an extension of the interatomic bonds, retractable after the load is removed (reversible)
It occurs when a material is loaded within its elastic limit and stress and strain are proportional
Hooke’s Law: σ =Eε
σ: Stress [MPa]
E: Young’s modulus or modulus of elasticity [GPa] ε: Strain (no units)
F: Force [N]
A: Cross-sectional area [m2]
L: Length [cm]
Where σ and ε are defined as:
Initial rod
Rod pulled L
Strengthening Mechanisms in Metallic Materials
Plastic Deformation
A result of interatomic bonds being broken and atoms moving to different positions relative to each other (irreversible or permanent deformation). This occurs when the material is loaded beyond its elastic limit.
Simple Tensile Test curve
Stress (MPa)
Yield point: plastic deformation starts
Plastic strain
‘Non-linear’ plastic deformation
Atoms remain in same position relative to each other
Slip has now occurred
Elastic (reversible) deformation
Plastic (permanent) deformation after load is removed
Strengthening Mechanisms in Metallic Materials
Slip Systems and Dislocation Theory
• Incrystallinesolids,theplasticdeformationprocessisalsoknownasSLIP,asplanesofatomstendto slide over each other into new stable positions
• Slipoccursonclosepackedplanesandinclosepackeddirectionswithinthecrystal–sothatthe atoms can follow the shortest path to their new positions under the most favourable energetic conditions
• Thecombinationofslipplaneandslipdirectionisknownasaslipsystem.
Slip plane – plane on which easiest slippage occurs – highest planar densities (and large interplanar spacing)
Slip direction – directions of movement – highest linear densities
12 x {111}(110) systems in a FCC unit cell
Strengthening Mechanisms in Metallic Materials
Slip Systems and Dislocation Theory
Strengthening Mechanisms in Metallic Materials
Dislocation Movement
• Whenslipoccursincrystalsithasbeenshownthattheenergymeasuredtoproducethe deformation is approximately 1000 times less than that expected by theoretical calculation.
• Reasonfordiscrepancy:theoryassumesaperfectcrystalstructureandallatomsonslip plane move simultaneously.
• Inpractice,slipoccursbyincrementalmovementofdiscreteslipeventsorhalfplanesi.e., dislocations, which are also known as line defects or lines of (potential) energy
• Insimpleterms,itisthemovementofthesedislocations,individuallyorincombination, that produces slip in crystals, and therefore, permanent deformation in metals and alloys (and some ceramics).
D S t i r s e l o n c g a t t h i o e n n i Mn g o Mv e e m c h e a n n t i s m s i n M e t a l l i c M a t e r i a l s
Dislocation Movement
In metal crystals, an edge dislocation slides over adjacent plane half-planes of atoms. If dislocations can’t move, plastic deformation doesn’t occur!
Strengthening Mechanisms in Metallic Materials
Dislocation Movement and Interaction
• Duringyieldthereismultipledislocationmovement,whicheventuallyleadstointeraction and entanglement of these lines. Entanglement restricts their movements. This restriction and therefore resistance to further deformation is known as work hardening or strain hardening.
• Anyprocessortreatmentthatwillrestrictorpreventthemovementofdislocationswithin the crystals or grains will result in strengthening (and hardening).
• Strength is increased by making dislocation motion difficult.
• Strengthofmetalsmaybeincreasedby:
decreasing grain size
solid solution strengthening precipitate hardening
cold working
Learning Outcome Check 3
What is the difference between strength and toughness? What is the overall principle behind strengthening metals?
Strengthening Mechanisms in Metallic Materials
Mechanical Properties in relation to Structure
How to define or determine these properties?… By testing…
Primary Tests
• Tensile Test
• One of the most valuable and commonly used of the mechanical tests for materials.
• Data on strength, toughness and ductility
• Expensive, relatively slow (loading rate) and destructive
• Load capacity: a great range available
• Loading method: mechanically or hydraulically
• HardnessTest
• Surface indentation with a known load
• Quick, low cost, semi destructive
• ImpactTest
• Strike standard specimen with calibrated pendulum load
• Quick, intermediate cost, destructive
Strengthening Mechanisms in Metallic Materials
Tensile Testing
Yield Strength
Adapted from Fig. 6.3, Callister & Rethwisch 8e. (Fig.
6.3 is taken from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, p. 2, and Sons, , 1965.)
Ultimate Tensile Strength (UTS)
Nominal Stress (MPa)
extensometer
Elastic deformation
Strengthening Mechanisms in Metallic Materials
Pure Metals
Copper (Electrical)
Aluminium (Electrical/Decorative) Refractories (Mo, W) – High temperature Nickel (Electrical, Electronic)
• Puremetalsforloadbearingapplicationsareoflittleuse.
• Inordertoimprovemechanicalproperties(andmanyotherproperties)various
mechanisms or treatments are employed, and alloying with other elements is performed.
• Alloyingiseithertoneutralisetheeffectofundesirabletraceelements,ortomodifyand improve desired properties for specific applications.
Strengthening Mechanisms in Metallic Materials
Strengthening by Reducing Grain Size
• Mostcommercialmetalsaremadeupofpolycrystallinegrains,ofrandomcrystallographic orientations, with a common grain boundary
• Whenthesemetalsaresubjectedtoloading,thedislocationmotionmusttakeplaceacrossthe common boundary, from grain A to grain B
• Thegrainboundaryactsasabarrierfordislocation,because:
• thetwograinsarerandomlyorientated,adislocationpassingintoBneedstochangeitsdirection
of motion, which becomes more difficult as the misorientation increases
• Theatomicdisorderwithinagrainboundaryregionwillresultinadiscontinuityofslipplanesfrom one grain to another
Check out the Hall-Petch Equation
σy = σ0 +Kd -1/2
σy: Yield Strength [MPa] d: Grain size [mm]
σ0,k: Constants
Metals having small grains – relatively strong and tough at low temperatures
Metals having large grains – good creep resistance at relatively high temperatures
A fine-grained material is harder and stronger due to a greater total grain boundary area to imposed dislocation motion
Strengthening Mechanisms in Metallic Materials
ColdWork- StrainHardeningorWorkHardening
• Coldworkisalsoknownasstrainhardening.Itisthephenomenonwherebyaductile metal becomes harder and stronger as it is plastically deformed.
• In single crystals: dislocation movements -> plastic deformation/slip
• In polycrystalline metals: dislocation movements occur preferentially in grains with slip systems that is most
favourably located relative to the load direction.
• Rotation occurs to bring the grains into more favourable position, so as to keep the grain boundaries in contact
• Most grains will eventually have a
plane in the direction of deformation.
• A considerable amount of distortion will have occurred, and the materials will have gone straining or work hardening.
How a metal becomes harder and stronger as it is plastically deformed, or work hardened. However, this effect can be ‘removed’ by heat treatment
Strengthening Mechanisms in Metallic Materials
A metal alloy is a mixture or series of metallic solid solutions or phases, composed of two or more elements
Alloys can be produced with mixtures of :
– Metals/Metals e.g., Cu-Zn (Brass)
– Metals/Non-metals e.g., Fe-C (Steel)
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