“The Structure of Metals” Group # 3

2/06/2006 Keith DagerDavid Fuller

Grant ThomasAustin Weddington

Jared Martinez

Introduction

Many questions about metal can be answered by knowing their ATOMIC STRUCTURE (the arrangement of the atoms within the metals).

The Crystal Structure of Metals

Metals and Crystals What determines the strength of a

specific metal.

Three basic atomic arrangements

1of 3 basic atomic arrangements

1. Body-centered cubic (bcc)

1. A portion of the structure of a body-centered cubic metal (b.c.c.)

2 of 3 basic atomic arrangements2. Face-centered cubic (fcc) also known as Cubic close packing

3 of 3 basic atomic arrangements3. Hexagonal close-packed (hcp)

Review of the three basic Atomic Structures

B.C.C.

F.C.C

H.C.P

Basic terminology

Hard-ball / Hard-sphere - the small spheres used to display the unit cell and show the individual atomic arrangement.

Basal planes- This is the orientation/layout of the atomic arrangement specifically in the h.c.p. layout of the ABAB pattern.

Alloying- this is formed by adding atoms of one metal/metals to some other metal/metals.

Allotropism/Polymorphism (meaning many shapes)- the appearance of more than one type of crystals.

When a single crystal is subjected to an external force, it goes through Elastic DeformationElastic Deformation

If the force on the crystal is increased sufficiently, the crystal goes through Plastic/Permanent DeformationPlastic/Permanent Deformation

The amount of stress required for a crystal to permanently deform is the called the Critical Shear StressCritical Shear Stress

Shear StressShear Stress is the ratio of the applied

shearing force to the cross-sectional

area being sheared

When this occurs, one plane of atoms

slipsslips across an adjacent plane of

atoms

Tensile Force1

Tensile Force2

Cross-Section

F2

F1

Video here

b/a ratiob/a ratio – Proportional to the amount of shear stress needed to cause slip in single crystals

aa is the spacing of atomic planes

bb is inversely proportional to the atomic density in the atomic plane

Atoms

AnisotropicAnisotropic – The different properties of a single crystal when tested in different directions

Examples : Plywood & Cloth

TwinningTwinning – The crystal forms a mirror image of itself across the plane of twinningplane of twinning

Tensile Force1Tensile Force2

A slip systemslip system is the combination of a slip plane and its direction of slip

Body-Centered CubicBody-Centered Cubic structure 48 slip systems Highly probable for any shear stress to act on one of these

systems, but because of a high b/a ratio, the shear stress required must be high

Metals with these structures have good strength and moderate ductility (flexibility)

Face-Centered CubicFace-Centered Cubic structure 12 slip systems Moderate probability for a shear stress to act on one

of these systems Low b/a ratio, the shear stress required is low Metals with these structures have moderate strength

and good ductility

Hexagonal Close-PackedHexagonal Close-Packed structure 3 slip systems Low probability for a shear stress to act on one of

these systems More systems become active at elevated

temperatures Metals with these structures are usually brittle at

room temperature

The actual strengths of metals are lower than theoretical calculations because of defects and imperfections in crystal structures

This includes: Grain/Phase Boundaries (Next Section) Volume/Bulk Imperfections

Voids, Cracks, Inclusions ( nonmetallic elements) Point Defect Dislocations

Vacancy - a missing atom Interstitial Atom - an extra atom in the structure Impurity – a foreign atom that has replaced an atom of

metal

Dislocations are defects in the orderly arrangement of the atomic structure including:Edge Dislocations

Screw Dislocations

Dislocations can lower the required shear stress to cause slip

They can also interfere with each other and be impeded by barriers ( grain boundaries, impurities, inclusions ) which can cause the required shear stress for slip to go up.

This is referred to as Work/Strain Hardening Increases strength of metal Increases hardness of metal

Grains and Grain Boundariesand their effects on a metal

Grain size has a significant effect on the

strength of metals.Grain boundaries have a

major influence on metal behavior.

What are grains?

Grains- individual, randomly oriented crystals within a metal

Grain Structure of metal alloys

How are grains formed? Molten metal begins to solidify Crystals begin to form independently of

each other Each crystal has random unrelated

orientation to the other Each of these crystals grows into a

crystalline structure (a grain)

What difference does Grain Size make?

Significantly influences mechanical properties of the metal

Large grain size is generally associated with- Low strength

- Low hardness- Low ductility (extent of deformation before fracture)

Small grain size is generally the opposite

What determines Grain size?

Three factors that influence the median size of developed grains

• Rate of nucleation (initial rate of formation of the individual crystals)

• Number of sites where crystals begin to form

• Rate crystals grow

Calcite crystal (~50 µm) grown at T = 800 °C and p = 300 MPa within 10 hours

What determines Grain number?

High rate of nucleation (comparatively)- Number of grains per unit volume of metal will be high

- Grain size will be small

High growth rate (comparatively) - Fewer grains per unit volume of metal

- Grain size will be large

Wear-resistant nanometals could make sporting goods more durable. 

The grain size of a nanocrystalline metal, right, is about 1,000 times smaller than conventional metal, above. When grain size is cut in half, the company says, hardness quadruples. 

Controlling grain sizeIn general:

Rapid cooling produces smaller grains.

Slow cooling produces larger grains.

The smaller the grain size, the The smaller the grain size, the stronger the metalstronger the metal

How is grain size measured?

1. Counting the grains in a given area

2. Counting number of grains that intersect a length of a line (microscopic)

3. Determined by comparing to a standard chart (ASTM Chart)

ASTM Chart

American Society for Testing and Materials

Grain number is determined by formula:

N = 2^(n-1)N- Number of grains

n- grain size number

Per square inch at 100x magnification

TABLE 1.1ASTM No. Grains/mm2 Grains/mm3

–3–2–10123456789101112

1248163264128256512

1,0242,0484,0968,200

16,40032,800

0.72

5.61645

128360

1,0202,9008,200

23,00065,000185,000520,000

1,500,0004,200,000

Examples of grain size

Grain sizes between 5 and 8 are generally considered fine grains

Grain size of 7 is acceptable for sheet metal of car bodies, kitchen utensils, and appliances

Grain size can also be so large as to be seen with the naked eye as in zinc on the surface of galvanized sheet steels

What are grain boundaries?

Surfaces that separate individual grains

1. Influence strain hardening1. Influence strain hardening (boundaries interfere with (boundaries interfere with

the movement of the movement of dislocations)dislocations)

2. More reactive than the 2. More reactive than the grains themselvesgrains themselves

(atoms along grain (atoms along grain boundaries are more boundaries are more disordered and packed less disordered and packed less efficiently)efficiently)

3. Great influence on the 3. Great influence on the strength and ductility of the strength and ductility of the metalmetal

Cast Iron grain boundaries

What can happen at grain boundaries?

1. Grain boundary sliding

2. Grain boundary

embrittlement

3. Hot shortness

Grain boundary sliding

Process by which grains will begin to slide along one another at the boundary

Possible Effects on Metal:1. Plastic deformation

2. Creep mechanism (elongation under stress over period of time)

Note: These types of deformation Note: These types of deformation are usually accompanied with high are usually accompanied with high temperatures as welltemperatures as well

Illustration of Creep Mechanism

Grain Boundary embrittlement

Generally: weakening of the grain boundaries by embrittling elements

1. Liquid-metal embrittlement (elements are in liquid state)2. Solid-Metal embrittlement (elements are in the solid state)3. Temper embrittlement (in alloy steels -caused by segregation

(movement) of impurities to grain boundaries)

Hot Shortness

Generally: softening and/ or melting of metal along grain boundaries

Cause: local melting of impurity in the boundary at a temperature below the melting point of the metal itself

Effect: when subjected to plastic deformation at elevated temperatures, (hot working) the piece of metal crumbles and disintegrates along the metal boundary

Prevention: metal is worked at a lower temperature

Plastic Deformation of Polycrystalline Metals

Polycrystalline Metals Equiaxed grains Plastic deformation Strain (deformation) Anisotropy Preferred Orientation Mechanical Fibering

Polycrystalline metals

•Metals commonly used in manufacturing various products Consist of many individual, randomly oriented crystals (grains);Thus, metal structures typically are not single crystals but Polycrystals. (“many crystals”)

•The most natural and artificial solids (rocks, ceramics, metal alloys or polymers) are polycrystalline. They contain many crystallites of different size, shape and different orientations.

polycrystalline materials: metals, alloys, intermetallic compounds, ceramic materials, compound materials, polymers, semiconductors, nanocrystals, supraconductors, rocks;

What is Plastic Deformation of Polycrystalline Metals and all that goes with it?What is Plastic Deformation of Polycrystalline Metals and all that goes with it?

Equiaxed grains

•Equiaxed grains •Having equal dimensions in all Directions, as shown in the in Fig.1.12a

                                         

Reduction operation resulting in directionality or anisotropy

Grain SizesTABLE 1.1ASTM No. Grains/mm2 Grains/mm3

–3–2–10123456789101112

1248163264

128256512

1,0242,0484,0968,200

16,40032,800

0.72

5.61645

128360

1,0202,9008,200

23,00065,000185,000520,000

1,500,0004,200,000

Equiaxed grains (having equal dimensions in all Directions, as shown in the in Fig.1.12a)

Cold Working

If a polycrystalline metal with uniform equiaxed grains is subjected to plastic deformation at room temperature(cold working), the grains become deformed and elongated.

TABLE 1.2 Process T/Tm Cold working Warm working Hot working

< 0.3 0.3 to 0.5 > 0.6

Homologous Temperature Ranges for Various ProcessesHomologous Temperature Ranges for Various Processes

Deformation Process

Compressing the metal: Compression stresses develop within a material when forces compress or

crush the material. A column that supports an overhead beam is in compression, and the internal stresses that develop within the column are compression.

As is done in forging to make a turbine disk

"Stretching"  Stretching is a process where sheet metal is clamped around its edges and

stretched over a die or form block. This process is mainly used for the manufacture of aircraft wings, automotive door and window panels.

As is done in stretching sheet metal to make a car body.

Compressing and stretching(Turbine Disk and Car Body)

Types of compressions and strains

ANISOTROPY (Texture)

Anisotropic surface will change in appearance as it is rotated about its geometric normal, as is the case with velvet.

As result of Plastic deformation, the grains have elongated in one direction and contracted in the other. Consequently, this piece of metal has become anisotropic.

anisotropic properties: plasticity, elasticity, hardness, strength, cleavability, thermal expansion and conductivity, electric conductivity, magnetization, corrosion resistance, optical properties.

ANISOTROPY Texture and Anisotropy of Crystalline Materials A crystal is characterized by the periodic arrangement of its elements

(atoms, ions) in space. This always generates a dependence of the crystal properties on the chosen direction, which is called anisotropy. Thus, the modulus of elasticity can vary by the factor 22 in a graphite crystal depending on the direction.

Anisotropy

Figure 1.13 (a) Schematic illustration of a crack in sheet metal that has been subjected to bulging (caused by, for example, pushing a steel ball against the sheet). Note the orientation of the crack with respect to the rolling direction of the sheet; this sheet is anisotropic. (b) Aluminum sheet with a crack (vertical dark line at the center) developed in a bulge test; the rolling direction of the sheet was vertical. Source: J.S. Kallend, Illinois Institute of Technology.

(b)

The most important parameter describing the anisotropy of polycrystalline materials is their texture. Via the anisotropy of physical properties due to the lattice structure, a regular texture in which the crystallites of one phase have only a few preferred orientations produces anisotropy of the polycrystalline material as well.

.

AnisotropyAnisotropy

Anisotropy

In a single crystal, the physical and mechanical properties often differ with orientation. It can be seen from looking at our models of crystalline structure that atoms should be able to slip over one another or distort in relation to one another easier in some directions than others. When the properties of a material vary with different crystallographic orientations, the material is said to be anisotropic.

Preferred Orientation Also called crystallographic

anisotropy can best be described by referring to fig. 1.6a.When a metal crystal is subjected to tension, the sliding blocks rotate the direction of the pulling force direction.

Note:for polycrystalline with grains in random orientation, all slip directions tend to align themselves with the direction of the pulling force while slip planes under compression tend to align themselves in a direction perpendicular to the direction of the compressing force.

Kalpakjian • SchmidManufacturing Engineering and Technology © 2001 Prentice-Hall Page 1-6

Slip and Twinning

Figure 1.5 Permanent deformation (also called plastic deformation) of a single crystal subjected to a shear stress: (a) structure before deformation; and (b) permanent deformation by slip. The size of the b/a ratio influences the magnitude of the shear stress required to cause slip.

Figure 1.6 (a) Permanent deformation of a single crystal under a tensile load. Note that the slip planes tend to align themselves in the direction of the pulling force. This behavior can be simulated using a deck of cards with a rubber band around them. (b) Twinning in a single crystal in tension.

Preferred Orientation

(equiaxed) grains in a specimen subjected to compression (such as occurs Figure 1.12 Plastic deformation of idealized in the rolling or forging of metals): (a) before deformation; and (b) after deformation. Note hte alignment of grain boundaries along a horizontal direction; this effect is known as preferred orientation.

Equiaxed grains (having equal dimensions in allDirections, as shown in the in Fig.1.12a)

1.12

Mechanical Fibering

This type of anisotropy results from the alignment of inclusions (stringers), impurities and voids in the metal during deformations.•Inclusions (Stringers)- Usually non-metallic particles

contained in metal. In steel they may consist of simple or complex oxides, sulphides, silicates and sometimes nitrides of iron, maganese, silicon, aluminium and other elements. In general they are detrimental to mechanical properties buy much depends on the number, their size, shape and distribution.

Mechanical Fibering impurity effects on metals (graph

explanation)

The first model proposed that elements that drawcharge from the neighbouring metal–metal bonds weaken them andembrittle the materials1,2. The second model states that the tendency toembrittlement is determined by whether the impurity is more likelyto segregate to a grain boundary or a surface3,4.The model suggests thatthis parameter predicts the relative likelihood of forming a sharp crack(brittle fracture) over a blunt crack (as in a tough material).Using thiscriterion, the segregation data of Miolinari et al.5 would predict that Biwould not embrittle Cu. The third model states that embrittlementoccurs when impurities segregate to the grain boundaries and makethe normally non-directional metallic bonds between grains moredirectional (and less flexible)1,6.

Effects of impurities (graphs)11 22

33Perfect analogy:Perfect analogy:

Mechanical fibering

Major consequence: If the spherical grains in fig 1.12a were coated with impurities, these impurities would align themselves generally in a horizontal direction after deformation. Because impurities weaken the grain boundaries. This piece of metal will be weak and less ductile when tested in the vertical direction. Such as a plywood.

                                                             

Recovery, Recrystallization, Grain Growth

Recovery:

The removal of residual stresses by localized plastic flow as the result of low-temperature annealing operations.

Recrystallization:

The change from one crystal structure to another, as occurs on heating or cooling through a critical temperature.

Grain Growth:

The enlarging or coarsening of the individual grains within the metal or alloy during heating at a temperature above the recrystalization temperature.

                                                                                         

Cold, Hot and Warm Working

Cold Work

Altering the shape or size of a metal by plastic deformation. it is carried out below the recrystallisation point usually at room temperature

Methods of cold working

Processes include rolling, drawing, pressing, spinning, and peening

Effects of cold work

Increased Strength

Increased Hardness

Decreased Malleability

Decreased Ductility

Hot Working

The rolling, forging or extruding of a metal at a temperature above its recrystallisation point.

                                    

Method of Hot work

Rolling, Pressing, Forging

Quenching

Effects of Hot Work

Undesirable surface finish

Less force required than cold work

Decreased yield strength

Warm Working

warm working: is carried out at intermediate temperatures. It is a compromise between cold and hot working.

Effects of Warm Working

The effects of this type of working depend on how close is the warm process to be a cold or hot process.

The type of process chosen depends on the physical and mechanical properties needed for the product, meaning the product itself and its uses.

Annealing Annealing consists of:

(1) recovery (stress-relief ),

(2) recrystallization,(3) grain growth

Annealing reduces the hardness, yield strength and tensile strength of the steel.

The benefits of annealing are:

 Improved ductility

 Removal of residual stresses that result from cold-working or machining

 Improved machinability

 Grain refinement

A Real World Application

Conclusions

1. The terms cold, warm and hot working are relative. The difference between them is the temperature at which the process is carried out.

2. There is not a “best” process. It all depends on what will be the use of the product, or what is the product going to be.

Definitions

Quenching: Rapid cooling from a high temperature by immersion in a liquid bath of oil or water. Used to harden metal.

Hardness: is the characteristic of a solid material expressing its resistance to permanent deformation

Ductility: is the physical property of being capable of sustaining large plastic deformation without fracture

Tensile strength: measures the force required to pull something such as rope, wire, or a structural beam to the point where it breaks

Peening: is the mechanical working of metals by means of hammer blows or by blasting with shot

Plasticity:  is  the  ability  of  a  material  to  deform permanently without breaking or rupturing

Brittleness: is the opposite of the property of plastic- ity. A brittle metal is one that breaks or shatters before it deforms Brittleness is the opposite of the property of plastic- ity. A brittle metal is one that breaks or shatters before it deforms

Malleability:  is  the property  that  enables  a  material  to  deform   by  compressive forces without developing defects

Annealing: is the process by which the distorted cold worked lattice structure is changed back to one which is strain free through the application of heat.

Glossary

Ductility the property of a metal which allows it to be permanently

deformed, in tension, before final rupture.

•ElasticityElasticity •The property which enables a material to return to its original shape and dimension.The property which enables a material to return to its original shape and dimension.

•Fatigue StrengthFatigue Strength •the unit stress that ruptures a bar after an enormous (around 40 million) number of the unit stress that ruptures a bar after an enormous (around 40 million) number of •repetitions of a load covering a range of valuesrepetitions of a load covering a range of values.

•RollingRolling

•The process of shaping metals by passing it between rolls The process of shaping metals by passing it between rolls revolving at the same peripheral speed and in opposite revolving at the same peripheral speed and in opposite directions. directions.

References

http://www.nd.edu/~manufact/figures.html http://en.wikipedia.org http://info.lu.farmingdale.edu/depts/met/

met205/annealingstages.html http://www.principalmetals.com/glossary/

rdoc.htm info.lu.farmingdale.edu/.../

met205/coldwork.html http://tech.clayton.edu/eddins/hotworki.htm http://www.seas.upenn.edu/

~chem101/sschem/metallicsolids.html

Work cited

http://www.tpub.com/steelworker1/2.htm

•http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/anisotropy.htmStructure/anisotropy.htm

Questions