1. “The Structure of Metals” Group # 3 2/06/2006 Keith Dager David Fuller Grant Thomas Austin Weddington Jared Martinez

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

3. The Crystal Structure of Metals
Metals and Crystals
What determines the strength of a specific metal.
Three basic atomic arrangements

4. 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.)

5. 2 of 3 basic atomic arrangements
2. Face-centered cubic (fcc) also known as Cubic close packing

6. 3 of 3 basic atomic arrangements
3. Hexagonal close-packed (hcp)

7. Review of the three basic Atomic Structures
B.C.C.
F.C.C
H.C.P

8. 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.

9. Deformation and Strength of Single Crystals
When a single crystal is subjected to an external force, it goes through Elastic Deformation
If the force on the crystal is increased sufficiently, the crystal goes through Plastic/Permanent Deformation
The amount of stress required for a crystal to permanently deform is the called the Critical Shear Stress

10. Shear Stress Shear Stress is the ratio of the applied
Cross-Section F2
Shear Stress is the ratio of the applied
shearing force to the cross-sectional
area being sheared
When this occurs, one plane of atoms
slips across an adjacent plane of
atoms F1
Tensile Force2
Tensile Force1

11. Deformation
Video here

12. Shear Stress
b/a ratio – Proportional to the amount of shear stress needed to cause slip in single crystals
Atoms
b is inversely proportional to
the atomic density in the atomic plane
a is the spacing
of atomic planes

13. Shear Stress
Anisotropic – The different properties of a single crystal when tested in different directions
Examples : Plywood & Cloth
Twinning – The crystal forms a mirror image of itself across the plane of twinning
Tensile Force2
Tensile Force1

14. Slip Systems
A slip system is the combination of a slip plane and its direction of slip
Body-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)

15. Slip Systems Face-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

16. Slip Systems Hexagonal 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

17. Imperfections in metal crystal structures
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

18. Point Defects Vacancy - a missing atom
Interstitial Atom - an extra atom in the structure
Impurity – a foreign atom that has replaced an atom of metal

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

20. Edge Dislocations

21. 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

22. Grains and Grain Boundaries and 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.

23. What are grains?
Grains- individual, randomly oriented crystals within a metal
Grain Structure of metal alloys

24. 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)

25. 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

26. 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

27. 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
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.
Wear-resistant nanometals could make sporting goods more durable.

28. Controlling grain size
In general:
Rapid cooling produces smaller grains.
Slow cooling produces larger grains.
The smaller the grain size, the stronger the metal

29. How is grain size measured?
Counting the grains in a given area
Counting number of grains that intersect a length of a line (microscopic)
Determined by comparing to a standard chart (ASTM Chart)

30. 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

31. 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

32. 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
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

33. What are grain boundaries?
Surfaces that separate individual grains
1. Influence strain hardening
(boundaries interfere with the movement of dislocations)
2. More reactive than the grains themselves
(atoms along grain boundaries are more disordered and packed less efficiently)
3. Great influence on the strength and ductility of the metal
Cast Iron grain boundaries

34. What can happen at grain boundaries?
Grain boundary sliding
Grain boundary
embrittlement
3. Hot shortness

35. Grain boundary sliding
Process by which grains will begin to slide along one another at the boundary
Possible Effects on Metal:
Plastic deformation
Creep mechanism (elongation under stress over period of time)
Note: These types of deformation are usually accompanied with high temperatures as well

36. Illustration of Creep Mechanism

37. Grain Boundary embrittlement
Generally: weakening of the grain boundaries by embrittling elements
Liquid-metal embrittlement (elements are in liquid state)
Solid-Metal embrittlement (elements are in the solid state)
Temper embrittlement (in alloy steels -caused by segregation (movement) of impurities to grain boundaries)

38. 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

39. Plastic Deformation of Polycrystalline Metals
Equiaxed grains
Plastic deformation
Strain (deformation)
Anisotropy
Preferred Orientation
Mechanical Fibering

40. Polycrystalline metals
What is Plastic Deformation of Polycrystalline Metals and all that goes with it?
polycrystalline materials: metals, alloys, intermetallic compounds,
ceramic materials, compound materials, polymers, semiconductors,
nanocrystals, supraconductors, rocks;
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.

41. Equiaxed grains Equiaxed grains
                                         
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

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

43. 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.
Homologous Temperature Ranges for Various Processes

44. 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.

45. Compressing and stretching (Turbine Disk and Car Body)

46. Types of compressions and strains

47. ANISOTROPY (Texture)
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 surface will change in appearance as it is rotated about its geometric normal, as is the case with velvet.
anisotropic properties: plasticity, elasticity, hardness, strength, cleavability, thermal expansion and conductivity, electric conductivity, magnetization, corrosion resistance, optical properties.

48. 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.

49. Anisotropy
(b)
Figure (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.

50. Anisotropy
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. .

51. 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.

52. 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.

53. Preferred Orientation
1.12
(equiaxed) grains in a specimen subjected to compression (such as occurs Figure 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 all
Directions, as shown in the in Fig.1.12a)

54. 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.

55. Mechanical Fibering impurity effects on metals (graph explanation)
The first model proposed that elements that draw
charge from the neighbouring metal–metal bonds weaken them and
embrittle the materials1,2.
The second model states that the tendency to
embrittlement is determined by whether the impurity is more likely
to segregate to a grain boundary or a surface3,4.The model suggests that
this parameter predicts the relative likelihood of forming a sharp crack
(brittle fracture) over a blunt crack (as in a tough material).Using this
criterion, the segregation data of Miolinari et al.5 would predict that Bi
would not embrittle Cu.
The third model states that embrittlement
occurs when impurities segregate to the grain boundaries and make
the normally non-directional metallic bonds between grains more
directional (and less flexible)1,6.

56. Effects of impurities (graphs)1 2 3
Perfect analogy:

57.                                                              
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.

58. Recovery, Recrystallization, Grain Growth

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

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

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

62. Cold, Hot and Warm Working

63. 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

64. Methods of cold working
Processes include rolling, drawing, pressing, spinning, and peening

65. Effects of cold work Increased Strength Increased Hardness
Decreased Malleability
Decreased Ductility

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

67. Method of Hot work
Rolling, Pressing, Forging
Quenching

68. Effects of Hot Work Undesirable surface finish
Less force required than cold work
Decreased yield strength

69. 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.

70. Annealing Annealing consists of: (2) recrystallization,
(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

71. A Real World Application

72. 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.

73. 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.

74. Glossary
Ductility
the property of a metal which allows it to be permanently deformed, in tension, before final rupture.
Elasticity
The property which enables a material to return to its original shape and dimension.
Fatigue Strength
the unit stress that ruptures a bar after an enormous (around 40 million) number of
repetitions of a load covering a range of values.
Rolling
The process of shaping metals by passing it between rolls revolving at the same peripheral speed and in opposite directions.

75. References http://www.nd.edu/~manufact/figures.html
info.lu.farmingdale.edu/.../ met205/coldwork.html

76. Work cited

77. Questions