Strengthening Mechanisms

Question 1

What is Strength?

Answer 1

The ability of the material to resist deformation under an applied stress

Question 2

How do metallic crystals deform?

Answer 2

-They shear by dislocation motion, therefore anything that prevents dislocations increases strength.

Question 3

What are the four categories for strengthening materials?

Answer 3

-Grain size reduction
-Solid solution strengthening (alloying)
-Precipitation strengthening
-Cold working (strain hardening)

Question 4

How does grain size reduction increase strength?

Answer 4

Grain boundaries are barriers to slip, barrier strength increases with increasing angle of misorientation. Therefore smaller grain size means there are more barriers to slip so the materials strength is increased

Question 5

How does solid solution strengthening (alloying) increase strength?

Answer 5

-High purity metals are softer and weaker than alloys composed of the same base metal
-Impurity atoms distort the lattice
-Stress’ can produce a barrier to dislocation motion

Question 6

Explain the effects of alloying

Answer 6

-Small impurities exert tensile strains on the surrounding crystal lattice
-Large impurities exert compressive strains on the surrounding crystal lattice
This reduced mobility of dislocation which increases strength

Question 7

What does alloying lead to?

Answer 7

An increase in yield strength and tensile strength

Question 8

What is precipitation strengthening?

Answer 8

Hard precipitates are difficult to shear. e.g ceramics in metal.

Question 9

What is cold working (strain hardening)?

Answer 9

-The phenomenon when a ductile metal becomes harder and stronger as it is plastically deformed. The leads to a lower ductility metal.
-Most metals strain harden at room temperature.

Question 10

Explain cold working (strain hardening)

Answer 10

• Metal with yield strength is plastically deformed to point D
• Stress is released
• Stress is then reapplied with a new resultant yield strength
• The metal has now became stronger

Question 11

What are the common forging operations to change the area of the material element?

Answer 11

-Forging
-Rolling
-Drawing
-Extrusion

Question 12

Explain Forging

Answer 12

Involves using force on a die, the blank is then inserted

Question 13

Explain Rolling

Answer 13

Involves rolling a blank with Ao through to rollers to change the diameter to Ad

Question 14

Explain Drawing

Answer 14

Similar to rolling but instead of rolling, the material is in tension (pulled) through the die’

Question 15

Explain Extrusion

Answer 15

The opposite of drawing, the material is forced through the die by use of a ram

Question 16

What happens to the material during cold work?

Answer 16

• dislocations entangle with one another during cold work, therefore dislocation becomes more difficult
• As cold work increases
  + Yield strength increases
  + Tensile strength increases
  
  • Ductility decreases