1. Lecture Notes:
The Microscopic Nature of Pure Metals and Alloys and How that Determines a Metal’s Hardness

2. Ideas Included: Crystal Defects Point Defects and Dislocations
Microscopic Nature of Hardening
Diffusion and Case Hardening
Ways to Strengthen and Harden Metals
Alloying
Different Solid Phase Changes in Metals (Austenite and Martensite) Nitinol versus Steel
Thermal Treatment (Heat Treating)

3. Crystal Defects
All crystals have some defects. These “defects” are not necessarily bad.
They’re often made on purpose to make a metal harder and tougher
Adding alloying elements to a metal is one way of introducing a
crystal defect
Basic Kinds of Crystal Defects: (See Demo of BB’s in Clear Tray.)
1. Point Defects - places where an atom is missing or irregularly
wedged in the crystal lattice
2. Linear Defects (aka Dislocations) - Linear groups of atoms in
irregular positions ; When metal is deformed, these move
like a bump in a rug until encountering a point defect where
they get locked up.

4. 1. Point Defects
Self-Interstitial Atom- atom that has crowded into a void.
Vacancy- empty space where an atom should be, but is missing
Interstitial Impurity Atom – A much smaller than the regular atom fits in between the bigger atoms of the lattice structure like BB’s between marbles.
Substituted Impurity Atom - a different type but similar size atom which has replaced one of atoms in the lattice.

5. Summary of the Kinds of Point Defects Illustrated on Previous Slide (Do Not Memorize these.) Think BB’s in a Tray
A self-interstitial atom - an extra regular atom that has crowded its way into an interstitial void in the crystal structure. Occur only in low concentrations in metals because they distort and highly stress the tightly packed lattice structure.
A substituted impurity atom - a different type of atom which has replaced one of the bulk atoms in the lattice. Usually close in size (within approximately 15%) to the bulk atom. An example= impurity atoms is the zinc atoms in brass.
Interstitial impurity atoms - much smaller than the regular atoms. Interstitial impurity atoms fit into the open space between the bulk atoms of the lattice structure like BB’s between marbles. An example = carbon atoms added to iron to make steel.
Vacancies - empty spaces where an atom should be, but is missing. They are common, especially at high temperatures when atoms are frequently and randomly change their positions leaving behind empty lattice sites.

6. 2. Dislocations (Line Defects)
Dislocation motion is analogous to movement of a caterpillar. The caterpillar would have to exert a large force to move its entire body at once. Instead, it moves the rear portion of its body forward a small amount and creates a hump. The hump then moves forward and eventual moves all of the body forward by a small amount.
See Illustration of this on Next Page.

7. Caterpillar Movement Analogy

8. Model of Moving Dislocations

9. Dislocations (Line Defects)

10. Illustration of a Moving Dislocation (Encircled) Getting Locked up by a Point Defect (Red Dot)

11. Why Metals can be Hardened
Any defect in the crystal lattice can interfere with the moving dislocation, preventing atoms from sliding past one another.
A moving Dislocation can cause additional dislocations; when these dislocations run into each other, they tangle up and are less able to move.
This “lock-up” of the dislocations drives up the force needed to move the dislocation, thereby strengthening (hardening) the material.

12. A Second Example of a Moving Dislocation getting Locked up by a Defect

13. A Third Example

14. Elastic/Plastic Deformation
Elastic Deformation = A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape. Involves stretching of the bonds, but the atoms do not slip past each other.
Plastic Deformation = When the stress is sufficient to permanently deform the metal.

15. Diffusion
Atom diffusion can occur by the motion of atoms into any gap available. This is more easily happens at high temperatures when the atoms are vibrating strongly.
Example: Case Hardening (Carburizing) is a heat treatment process in which iron or steel absorbs carbon by diffusion when the iron is heated in the presence of charcoal or carbon monoxide thereby making the material harder.

16. Ways to Strength/Harden Metals
Controlling the Grain Size
Work-Hardening
Alloying

17. First way to lock up or stop dislocations from moving:
Controlling the Grain Size -The boundary between grains acts as a barrier which stops further dislocation movement and resulting slip because adjacent grains have different orientations. The smaller the grains, the shorter the distance atoms can move along a particular slip plane. Therefore, smaller grains improve the strength of a material.
Heat Treatments can control Grain size. See later slides.

18. Grain Size (continued)
Hardness, yield strength, tensile strength, fatigue strength and impact strength all increase with decreasing grain size.
Machinability is also affected; rough machining favors coarse grain size while finish machining favors fine grain size.
Fine-grain steels do not harden quite as deeply and have less tendency to crack than coarse-grain steels of similar analysis. Also, fine-grain steels have greater fatigue resistance, and a fine grain size promotes a somewhat greater toughness and shock resistance.

19. Yield Strength Increases when Grains are Smaller

20. Second way to stop dislocations from moving: Work Hardening of Metal

21. Work-Hardening - or strain hardening - creating and tangling of dislocations by “working” the metal (example: hammering or bending).
When a metal is “worked”, dislocations move and additional dislocations are generated. The more dislocations within a material, the more chance they will become tangled and locked up.
(Example: Bending a coat hanger back and forth “work hardens” the bend and makes it brittle, crack fatigue occurs, and it snaps.) .

22. Stress-Strain Graph showing where the metal has been “Work Hardened”

23. Work Hardening coarsens the grains in the work stressed areas
Work Hardening coarsens the grains in the work stressed areas. The original grain size characteristics, however, can be restored by a stress-relieving heat treatment called Annealing.

24. Work Hardening a Metal Coat Hanger by Repeated Bending

25. Up to 20-30% Work-Hardening Increases Yield Strength and Decreases Ductility.

26. How to Remove the Bad Effects of Work Hardening by Heat Treatment (Annealing) Three things can occur during heat this treatment:
Recovery: Heating the metal allows atoms in severely strained regions to move to unstrained positions.
Recrystallization: At a higher temperature, new, strain-free grains grow and replace the old distorted grains produced by the work hardening. Here, the mechanical properties return to their original weaker and more ductile states.
Grain Growth: If left at a high temperature beyond the time needed for complete recrystallization, the grains begin to grow too big. This happens because diffusion occurs across the grain boundaries. Larger grains reduce the strength and toughness of the material…Not Good
See Next Slide for Diagram.

27. Heat Treatment can Soften Work-Hardened Metal

28. Third Way to Lock Up dislocations: Alloying
Alloying - introduces different Elements (new point defects) and more grains to pin / lock up any moving dislocations thereby making the sample harder and more useful.
Example: Adding some zinc atoms with copper atoms forms the alloy called brass. Brass is much harder than either pure zinc or copper alone.

29. What does alloying do Microscopically?
Adding larger ions In the alloy, some of the added ions may be larger than most of the ions making up the metal lattice. They disrupt the regular arrangement of ions and make it more difficult for the layers to Slide over each other. This makes the alloy harder and less malleable and ductile than the pure metal (in which the layers slip over each other more easily).
Adding smaller atoms Smaller sized atoms can also have a significant effect on the alloy structure. In steel, for example, atoms of non-metals such as carbon and nitrogen can fit into holes between the iron atoms. This also distorts the metal lattice and makes it more difficult for the layers to move over each other.

30. Alloying at the Microscopic Level

31. Alloying Example: Picking the Right Carbon Steel for the Right Job
The percentage of carbon has a big effect on the properties of steel and therefore on what it can be used for:
Type of steel % of carbon Properties Uses
Low carbon  (mild steel) – Easily worked Car bodies
Medium carbon – Wear resistant gears
High carbon  (carbon tool steel) 0.85 – 1.2 Strong & wear resistant Tools
Cast iron – Easy to cast but brittle Pistons

32. The major goals of the different kinds of
Heat Treatments
The major goals of the different kinds of
Heat Treatments:
Soften the material for improved workability.
Increase the strength or hardness of the material.
Increase the toughness or resistance to fracture of the material.
Relieve undesirable residual stresses induced during part fabrication.

33. Here are Just a Few Common Heat Treating Processes
Age Hardening- low-temperature heat treatment process; strengthens metal by causing the precipitation of phases of alloy from a super-saturated solid solution condition.
Annealing- is a softening process in which metals are heated and then allowed to cool slowly. The purpose of annealing is to soften the material for improved formability.
Normalizing- is much like annealing, but the cooling process is much faster. This results in increased strength but less ductility in the metal. Its purpose is to refine grain structure, produce more uniform mechanical properties, and sometimes to relieve internal and surface stresses.
(Continued on Next Slide)

34. Common Heat Treating Processes (Continued)
Tempering- involves gently heating a hardened metal and allowing it to cool slowly will produce a metal that is still hard but also less brittle.
Quenching- is the rapid cooling of a hot material. The medium used to quench the material can vary from air, oil, water and others. Many steels are hardened by heating and quenching. Quenching results in a metal that is very hard but also brittle.

35. Revisiting Crystal Packing
To form the strongest metallic bonds, metals are packed together as closely as possible. Instead of atoms, imagine marbles packed in a box. The marbles would be placed on the bottom of the box in neat orderly rows and then a second layer begun. The second layer of marbles cannot be placed directly on top of the other marbles and so the rows of marbles in this layer move into the spaces between marbles in the first layer. The first layer of marbles can be designated as A and the second layer as B giving the two layers a designation of AB.

36. Close Packing and Sliding of Atoms Across Each Other

37. One Option: Hexagonal Close Packing (HCP)
Hexagonal Close Packing Packing marbles in the third layer requires a decision. Again rows of atoms will nest in the hollows between atoms in the second layer but two possibilities exist. If the rows of marbles are packed so they are directly over the first layer (A) then the arrangement could be described as ABA. Such a packing arrangement with alternating layers would be designated as ABABAB. This is called hexagonal close packing.
One Option: Hexagonal Close Packing (HCP)

38. Comparison of previous slide’s HCP with Face-Centered Cubic (FCC)
Face-Centered Cubic (FCC): If the rows of atoms are packed in this third layer so that they do not lie over atoms in either the A or B layer, then the third layer is called C. This packing sequence would be designated ABCABC, and is also known as Face-Centered Cubic (FCC).
Comparison of previous slide’s HCP with Face-Centered Cubic (FCC)

39. Another view of FCC Structure

40. Relevance of FCC Strucure in Heat Treating Steel: The orientation of the atoms in the hot, Austenite phase is called Face Centered Cubic (FCC) and the carbon atoms move to the face of the iron cubes in the metal’s matrix. 

41. Body-Centered Cubic (BCC) is a third possibility

42. Another View of BCC Structure

43. Relevance of BCC Crystal Structure in Heat Treating Steel
(If you cool the part slowly from the austenitizing temperature, it will go back to the soft, “annealed” state, with a BCC structure.)

44. Body Centered Tetragonal (BCT) Structure
Martensite Crystals in Steel have this BCT structure. This Elongated Martensite BCT structure is formed when excess Carbon atoms are trapped and unable to diffuse out of the Austenite FCC crystals when quenched so quickly. Therefore, Hard and Brittle.

45.  If the steel is cooled rapidly (“quenched”), it converts into a hardened, “Martensite” phase; the atoms are arranged as Body Centered Tetragonal (BCT). Because the iron matrix is now longer on one side, tetragonal in shape and not cubic, the hardened structure is now approximately 4% larger in volume than the unhardened part. 

46. Steel’s Key Temperatures for BCC, FCC, and BCT Solid Crystalline Phases
Melting Point of Steel at 1538 o C
Solid Steel is in BCC phase called Pearlite between 1400 and 1537 o C
Solid Steel is in FCC phase called Austenite between 900 and 1400 o C
If cooled SLOWLY, Steel changes from Austenite FCC to Ferrite BCC phase below 900 o C . Carbon atoms able to diffuse out in time.
If cooled QUICKLY to less than 200 o C, converts to Martensite with long, brittle BCT structure.
1538
1400
900
200
Temp OC
Time (Seconds)

47. Steel’s Time Temperature Transformation (TTT) Diagram A TTT Diagram is the main way to show crystal structures transforming from one phase to another.
If a sample is Quick Cooled (See Green Arrow) fast enough to prevent diffusion of carbon out of Ausenite’s FCC structure, then hard and brittle martensite (BCT) will be formed as shown at the left.
Quick Cooled

48. Steel Heat Treatment Process; How to Interpret a Graph like this ?

49. Interpreting of a Steel Phase -Transformation Cooling Graph (See the “Red Line” on Graph.)
1. (Red Line) The steel is cooled rapidly to 160 oC and left for 10 seconds. The cooling rate is too rapid for pearlite to form; the steel remains in the Austenite phase while cooling until it cools to the Ms temperature,where 
Martensite begins to form. Since 160 oC is the temperature at which half of the austenite transforms to martensite, this direct Quench method converts 50% of the structure to martensite. So the structure can be assumed to be half martensite and half austenite.

50. Explanation of Steel Phase Transformation Cooling Graph (Continued) (See Green Line.)
2. (Green Line) The steel is quickly Quenched to and held at 250 oC for 100 seconds, which is not long enough to form bainite. It is then quenched from 250 oC to room temperature and develops mainly martensite structure.

51. Explanation of Steel Phase-Transformation Cooling Graph (Continued) (See Blue Line.)
3. (Blue Line) The steel is quickly Quenched down to 300 oC and held there at that same temperature for 500 seconds. This produces a half-bainite and half-austenite structure. Cooling quickly from this point would result in a final structure of martensite and bainite.

52. Explanation of Steel Phase-Transformation Cooling Graph (Continued) (See Orange Line.)
4. (Orange Line) The steel is quickly Quenched to 600 oC. The Austenite phase converts completely to fine grained Pearlite after eight seconds at this 600 oC Temperature. This phase is stable and will not be changed even on holding it there for 100,000 seconds at 600 oC . The final structure, when cooled, is fine grained pearlite.

53. Tempering of Martensite Results in Finer Quality Grain Structure (See Blue line.)
Recall that Tempering means gently heating a hardened metal and allowing it to cool slowly at a temperature that will result in a metal that is still hard but also less brittle. When steel is quickly Quenched, the martensite phase is formed when excess carbon is trapped in the austenite. This untempered martensite must be Gently re-heated below the temperature shown in order to allow some of the carbon to diffuse out , creating a more ductile and stable structure.

54. Here, You can see at the microscopic level how well Tempering works.

55. Thinking Back to the Nitinol Demo: Phase Changes & Shape Memory
Nickel Titanium Alloy (Nitinol)

56. Shape Memory Model of How NiTi Memory Metal Works
Basic mechanism- Two phase transitions . Here, the lattice structure either exists as the highly symmetric Austenite phase A, or as a sheared version called a martensite twin phase shown by either M(+) or 
M(-)  depending on the direction of shear. Martensite is stable at low temperature and Austenite at a higher one.