Nature of Metals and Alloys

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Nature of Metals and Alloys
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Objectives

• Understand how metal/alloy structures relate to
  theirs properties
• Know basic classification of alloys
• Understand a simple phase diagram of a binary
  alloy
• Know metal/alloy strengthening methods

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Outline

• Structure of metals
• Crystallization
• Phase diagrams
• Alloy microstructure
• Alloy strengthening

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Atomic Structure

• Cloud of electrons
• The metal ions are held together by their mutual
  attraction to the electron cloud. ? Metallic
  Bond
• Excellent electrical and thermal conductivity

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• Metals exist in one of the 14 crystal structures
  at room temperature.
• Examples
• Body-centered cubic (BCC) e.g. Cr
• Face-centered cubic (FCC) e.g. Ag, Au, Pd, Co,
  Cu, Ni
• Hexagonal closed-pack (HCP) e.g. Ti

a unit cell
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A space/crystal lattice

• combination of unit cells

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Physical Properties of Metals

• All properties of metals result from the metallic
  crystal structure and metallic bonds.
• High density ? the efficient packing of atomic
  centers in the crystal lattice
• Good electrical and thermal conductivity ?the
  mobility of the valence electrons in the crystal
  lattice

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• Opacity and reflective ? the ability of the
  valence electrons to absorb and re-emit light
• Melting point ? metallic bond energies are
  overcome by the applied heat

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• Physical properties change along different
  crystal directions but they are not usually
  observed in pieces of metal or other solids large
  enough for practical use.
• Most solids are polycrystalline (being made up of
  a large numbers of single crystals, called
  grains).
• Each grain is oriented more or less at random
  with respect to its neighbors, therefore, the
  variation in properties with crystal direction
  averages out.

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Formation of Grains

• from a molten state
• The growth starts from the nuclei of
  crystallization, and the crystals grow toward
  each other (A-E).
• When two or more crystals collide, their growth
  is stopped.
• Finally, the entire space is filled with crystals
  (F).
• Each growth crystal is called a grain. Grains
  contact each other at grain boundaries.

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Grain Size

• In general, the smaller the grain size of the
  metal, the better its physical properties.
• Control of Grain Size
• Number of nuclei of crystallization
• The more rapidly the liquid state can be changed
  to the solid state, the smaller or finer the
  grains will be.
• Rate of crystallization
• If the crystals form faster than do the nuclei of
  crystallization, the grains will be larger.
• Slow cooling results in large grains.

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• The shape of the grains may be influenced by the
  shape of the mold in which the metal solidifies.

Square mold
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Mechanical Properties of Metals

• Also a result of the metallic crystal structure
  and metallic bonds
• Good ductility and malleability, relative to
  polymers and ceramics ? the ability of the atomic
  centers to slide against each other into new
  positions within the same crystal lattice
• Ductility ability to be drawn into a wire
• Malleability ability to be pounded into a thin
  sheet

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No.if they were really perfect.
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Lattice imperfections

• Several types exist on various atomic levels
• Point defects
• Line defects (Dislocations)
• Grain Boundaries
• Macroscopic Defects

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Point Defects

• These types of defects are controlled by the size
  of the foreign atom.
• Introduction of point defects alters the lattice
  dimensions and changes the composition of the
  parent metal but does not change the overall
  crystal structure of the parent atom.

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Line Defects (Dislocations)

• An extra plane or line of atoms exists in the
  parent structure.
• The dislocations act as areas of stress
  concentration and allow atomic planes to slip
  over one another. They provide a mechanism for
  metals to deform at much lower stress levels than
  theory would predict.

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Dislocation Movements
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Edge/Screw Dislocations
Edge dislocation
Screw dislocation
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• Whenever the dislocation motions are impeded, the
  material becomes more resistant to slip, making
  it stronger.
• The presence of other defects such as point and
  other line defects helps to immobilize the
  movement of these dislocations during stress.

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Grain Boundaries

• Grain boundaries are defects which have higher
  energy than the grains and are more active with
  chemicals.
• Help to stop the dislocation.

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Macroscopic Defects

• Holes, bubbles, surface imperfections, cracks,
  and macroscopic impurities

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Alloys and Principles of Metallurgy

• Metallurgy is the study of metals and alloys.

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• Pure metals are apt to be soft and many of them
  tend to corrode rapidly.
• To optimize properties, most of the metals
  commonly used are mixtures of two or more
  metallic elements (metalmetal or metal
  nonmetal).
• A solid mixture of a metal with one or more other
  metals or with one or more nonmetals is called an
  alloy.
• Binary system, ternary system
• Homogeneous (one-phase), heterogeneous (distinct
  phases)

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• Whenever two metals are not completely miscible
  in the liquid state, they cannot form any type of
  alloy.
• e.g. Copper Lead, Zinc Lead
• When a combination of two metals is completely
  miscible in the liquid state, the two metals are
  capable of forming an alloy.
• When the combination is cooled, one of three
  possibilities may take place a solid solution,
  an intermetallic compound formation, or an
  eutectic formation.

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Hume-Rothery Rules for Alloying

• The lattice parameters of the two metals must be
  similar.
• Same type of crystal lattice (FCC,..etc)
• The relative size of the atoms must not exceed
  15-20.
• (gt15 ? multiple phases)
• Large differences in valence state preclude
  solubility.
• The chemical affinity of the atoms should be
  similar.
• A high degree of chemical affinity ? form an
  intermetallic compound on solidification

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Cooling Curves and Phase Diagrams
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• A phase is a state of matter that is distinct in
  some way from the matter around it.
• Eg. A mixture of ice and water 2 phases
• The distinction between single- and multiple
  phase alloys is important to the strength,
  corrosion, biocompatibility, and other alloy
  properties.

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Cooling Curves

• Pure Metals

• Alloys

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Cooling Curves and Phase Diagram
100
100
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Phase Diagram

• Phase of a family of alloys of a general metal
  composition are defined by the Temperature-Composi
  tion (Phase) diagram for that family of alloys.

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Classification of Alloy Systems

• 1. Solid Solutions
• 2. Intermetallic Compound
• 3. Eutectic Alloy

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Solid solutions

• Two metals are completely miscible in the liquid
  state, and they remain completely mixed on
  solidification.
• L ? S
• A single -phase system
• Always have a range of possible compositions
• e.g. the solid phase in the copper-gold (Cu-Au)
  system has a wide range of compositions between
  100 Cu and 100 Au

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Phase Diagram of a Solid Solution

• All compositions above the liquidus line are
  liquid, and those below the solidus line are
  solid.
• Solid and liquid exist in the area between both
  line.
• The solid has only one phase.

LIQUID
liquidus
Temperature
solidus
SOLID
Metal A (100)
Metal B (100)
composition
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Intermetallic Compounds

• The resulting phase has a fixed chemical
  composition or a narrow range of compositions.
• e.g. in an amalgam alloy,
• 73.2 Ag and 26.8 Sn ? Ag3Sn (one phase)
• Silver and tin atoms occupying definite positions
  in the space lattice.

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Phase Diagram of an Intermetallic Compound
Ag3Sn, 73.2 Ag and 26.8 Sn
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Eutectic Alloys

• The metals are soluble in the liquid state, but
  separate into two phases in the solid state.
• L ? S1 S2 ( 2 solid solutions)

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Phase Diagram of a Eutectic Alloy
Silver-copper system

• L ? a-solid solution b-solid solution
• The lowest temperature at which any alloy
  composition is entirely liquid Eutectic Temp
  (779.4C, E)
• The eutectic temperature is lower than the fusion
  temperature of either Ag and Cu.
• At eutectic point, there is no solidification
  range. (pure metal)
• At eutectic composition (72Ag 28 Cu), the two
  phases often precipitate as very fine layers of
  one phase over the other one.

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Silver-copper system
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Ex. Lead-Tin Alloy
Eutectic composition
High tin content
b eutectic
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How to read a simple phase diagram?

• (1) Composition of Liquid and Solid Phases at
  Various Temp.
• (2) Amount of Liquid and Solid Phases at Various
  Temp.

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Composition of Liquid and Solid Phases at Various
Temp.
Alloy (80A 20B)
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Amount of Liquid and Solid Phases at Various Temp.

• The relative amounts of the two phases in the
  liquid-solid region can be determined at a given
  temperature by the inverse lever rule.
• At 560C for 60A and 40B composition
• Liquid XY/XZ
• Solid YZ/XZ

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Ex. Silver-Palladium System
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Alloy Microstructure

• Cast Microstructure
• Wrought Microstructure
• Recrystallization and Grain Growth

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Cast Microstructure

• Grains are usually visible.
• Size of grains ? cooling rate (fast rate ? small
  grains)
• Fine-grained (equiaxed uniform in size and
  shape) alloys are generally more desirable for
  dental applications. ? more uniform properties

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Wrought Microstructure

• Metal ingots ? hot/cold working (rolling,
  swaging, or wire-drawing) ? produce severe
  mechanical deformation of the metal
• E.g. orthodontic wires and bands
• Grains are broken down, entangled in each other,
  and elongated to develop a fibrous structure.
• In general, mechanical properties are superior to
  those of the same cast alloys.

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Recrystallization and Grain Growth

• The reappearance of the grain or crystalline
  structure when heated or annealed (usually more
  obvious in the wrought mass).
• Degree of recrystallization is related to
• Alloy composition and mechanical treatment
• Temperature and the duration of the heating
  operation

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gross view microstructure crystal view

• A, the fibrous microstructure and arrows indicate
  residual stresses.
• B, Minimal heat leaves the fibrous structure
  intact but relieves the stresses. The lattice
  remains distorted.
• C, Annealing with more heat allows the lattice
  deformation to be relieved.
• D and E, Further heating causes a loss of the
  fibrous structure and growth of the grains, which
  increase in size with increasing application of
  heat.

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Properties of Alloys
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Phase Structure vs. Properties

• The strength of a material existing in a two
  phased structure is normally greater than that of
  a single phased structure.

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Solid Solution Alloys

• Often have higher strength and hardness and lower
  ductility than either pure metal.
• The alloying atoms are absorbed into dislocation,
  thereby preventing dislocation movement.
• Possess melting ranges and always melt below the
  melting point of the highest fusing metal.
• Have higher corrosion resistance than
  multi-phased alloys, and in some cases higher
  than the pure metal (e.g. Cr Fe ? stainless
  steel).

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Eutectic Alloys

• Are usually harder and stronger than the parent
  metals are often quite brittle.
• Posses a melting point at the eutectic
  composition.
• Often have poor corrosion resistance
• Galvanic action between the two phases at a
  microscopic level can accelerate corrosion.

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Intermetallic Compounds

• Are usually very hard and brittle.
• Properties rarely resemble those of parent
  metals.
• E.g. Ag2Hg3 in dental amalgam has properties
  completely different from those of pure silver or
  mercury.

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Strengthening of Metals and Alloys

• Principle Increased interaction of dislocations
  will increase the strength of the materials.
• (1) Grain size alterations
• Small grains ? reduced ductility but increased
  strength, toughness and polishability.
• Can be achieved from
• Quenching (quick cooling)
• Use of nucleating agents
• Use of grain refiners e.g. Ir ? encourage even
  nucleation (without sacrificing ductility)
• Plastic deforming (cold working)

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• (2) Cold-working
• Work-hardening or strain-hardening rolling, wire
  drawing ? mechanically deform the alloy
• The shape of the grain is changed from equiaxed
  to long and thin.
• Increases hardness and yield strength as well as
  chemical reactivity
• Decreases ductility and corrosion resistance
• The harmful effect of cold-working may be removed
  by heat treatment, recrystallization, and grain
  growth.

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• (3) Annealing
• Heating the alloy to temperatures sufficient to
  alter grain size (1/3 - 1/2 melting temperature)
• Recrystallization and grain growth
• The grains convert from long and thin to equiaxed
  (convert the cold working result)
• (4) Solute-hardening
• Adding solute or impurity atoms which will
  interact with dislocations.

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• (5) Precipitation or age hardening
• Relies on the ability of an alloy to be converted
  from a single solid phase structure to one that
  exhibits two phases.
• When heated at temperature lt melting point,
  diffusion of foreign atoms occurs resulting a
  highly strained lattice exhibiting enhanced
  mechanical properties.
• The rate and length of aging (time and
  temperature) can be manipulated to create
  material with the desired combination of
  properties.
• Interactions between dislocations and
  precipitates result in higher strength and
  toughness but moderate ductility.

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Summary

• Structure of metals
• Crystallization
• Phase diagrams
• Solid solution, Eutectic alloys
• Alloy microstructure
• Cast, wrought, recrystallization
• Properties of metals and alloys
• Alloy/Metal strengthening
• Grain size, Cold working, Annealing,

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Phase Diagram of an Alloy with Incomplete
Solubility

• All alloy compositions within the dome shaped
  region consist of two phases.
• The atoms in this region are not totally soluble
  in one another resulting in precipitation of two
  phases.
• C-rich and D-rich phases
• All alloy compositions outside this region are
  soluble in one another and therefore form solid
  solutions.

liquidus
Temperature
Liquid Solid
solidus
Two-Phased Structure
Metal C (100)
Metal D (100)
composition
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Peritectic Alloys

• Occurs at a particular composition and
  temperature
• Lb ? a
• Ex. Platinum-Silver alloy system