Hardness, Plastic Deformation, and Strengthening Mechanisms Flashcards
Define true strain
ln(length / original length)
What is elastic strain recovery?
The amount of strain recovered during elastic unloading of plastic deformation.
Define Hardness
A material’s resistance to local deformation
How do we define hardness qualitatively?
Mohr scale
How do we define hardness quantitatively?
Rockwell, Brinell, Knoop Microharndess test, Vickers Microharndess test
Describe a Rockwell Hardness test
A hardened steel ball or diamond cone is pushed into a material with a minor load of 10 kg and a major load of 60, 100, and 150 kg. The hardness is the depth between these two penetrations.
A thin specimen would use a minor load of 3 kg and major loads of 15, 30, and 45 kg.
Describe a Brinell hardness test
A hardened steel ball at loads of 500-3000kg.
Describe a microhardness test
A diamond pyramid makes a microscopic indent.
Why do we use hardness tests?
Inexpensive
Non-destructive
Other properties can be calculated (such as tensile strength is 3.45HB)
Why might measured hardness vary?
test method operator bias fabrication procedure apparatus calibration inhomogenous material
What causes plastic deformation?
Dislocation movement
What factors are required for dislocation movement?
Shear stress
Slip planes
Slip directions
What determines a slip plane or direction?
The plane or direction with the most densly packed atoms.
Define slip system
Combination of slip plane and direction
How does the number of slip systems in a material affect ductility?
The more slip systems, the easier it is to plastically deform, the more ductile the material.
When does slip begin in a material?
When the shear stress in the slip systems reaches the critical resolved shear stress.
Is critical resolved shear stress a material property?
Yes
What is the angle between two directions?
D1 dot D2 / [|D1| |D2|]
How do you strengthen a material?
Make dislocation motion difficult
What are four strengthening processes?
Grain size reduction
Solid Solution Strengthening
Precipitation hardening
Strain Hardening (cold work)
Describe grain size reduction as a strengthening process.
Grain lines trap dislocations by changing slip planes and causing dislocation to change direction. More grain lines = more strength.
Describe solid solution strenghening.
Adding other elements (alloying). This results in points of lattice strain, which attracts and ties down dislocations, which also have strain.
Smaller impurity atoms add tensile strain, while larger impurity atoms cause compressive strain.
Describe precipitation hardening.
Adds particles of different phase to prohibit dislocation movement through them, and these have lattice strains at the borders, trapping dislocations.
Describe strain hardening
Uses plastic deformation to increase the dislocation density, which gives each dislocation less ‘room to move’.
Describe the stages of annealing heat treatment.
Recovery releases internal energy and restores thermal and electrical conductivity, but does not change grains.
Recrystallization forms new strain free grains, and is driven by the amount of internal energy in the material.
Grain growth amalgomates grains into large ones, and is caused by short range diffusion of atoms across grain boundaries.
What is hot working?
Plastic deformation above the recrystallization temperature. Has no associated strain, and allows for large deformations.
