Materials- Metals: Strengthening Mechanisms Flashcards
What makes a material harder and stronger?
Restricting or hindering dislocation motion
Why do grain boundaries act as a barrier to dislocation motion?
Because the two grains are of different orientations, a dislocation passing into a new grain will have to change its direction of motion (becomes more difficult as crystallographic misorientation increases). The atomic disorder within a grain boundary region will result in a discontinuity of slip planes from one grain into the other.
Hall-Petch equation
σy=σ0+kyd^-0.5
σy is yield strength
σ0 and ky are constants
d is average diameter of grains
All y are subscript
Does yield strength increase or decrease with grain size?
Decrease. Like decay curve
How does solid solution strengthening work.
Alloy with impurity atoms that either go into substitutional or interstitial solid solution. These produce either a tensile or compressive strain. The impurity atoms diffuse to and segregate around dislocations to reduce overall strain energy. The resistance to slip is therefore greater because the overall lattice strain must increase if a dislocation is torn away from them.
How can dislocations transfer between grains for high angle grain boundaries?
For high angle grain boundaries, the dislocations can pile up at the grain boundary introducing stress concentrations ahead of their slip planes, which generate new dislocations in adjacent grains.
What is strain hardening?
The phenomenon whereby a ductile material becomes harder and stronger as it is plastically deformed
What is strain hardening also known as?
Work hardening or cold working
Formula for percentage cold work
100x(A0-Ad)/A0
Where A0 is original CSA
Ad is CSA after deformation
What does cold work decrease?
Ductility
How does strain hardening work?
Dislocation density increases with cold work. Average separation between them decreases. On average, dislocation-dislocation strain interactions are repulsive. Net result is that the motion of a dislocation is hindered by the presence of other dislocations. As dislocation density increases, this type of resistance to dislocation motion becomes more pronounced and imposed stress necessary to deform metal increases.
Describe a typical stress-strain curve for a plain carbon steel
Steep straight line up to yield strength when dislocations become unpinned from C atoms. Goes down a bit to horizontal wiggly line when C atoms are catching up with dislocations by diffusion and re-pin them (repeated several times). Then normal dome shape until fracture. At start of curve to dome is when level of dislocations present increases to such an extent that work hardening starts to dominate.
What is an annealing treatment?
A heat treatment that can revert the properties of a metal to the precold-worked states. Has two steps of recovery and recrystallisation sometimes followed by grain growth.
How does recovery work?
Metal is heated so atomic diffusion is enhanced. This enhances dislocation motion so some of the stored internal strain energy can be relieved by dislocation motion. This reduces number of dislocations and dislocation configurations are produced having low strain energies. Electrical and thermal conductivities are restored to precold-worked states.
What is recrystallisation?
The formation of a new set of strain-free and equiaxed grains that have low dislocation densities and are characteristic of precold-worked conditions
What is recrystallisation temperature?
The temperature at which recrystallisation just reaches completion in one hour.
What affects the recrystallisation temperature?
The amount of prior cold work. Increasing percentage cold work enhances rate of recrystallisation so lowers recrystallisation temperature. Purity of metal. Purer metal has lower recrystallisation temperature.
Typical values of recrystallisation temperature compared to melting point
Pure metal: 0.4Tm
Alloy: put to 0.7Tm
What is the driving force for recrystallisation?
The difference in internal energy between strained and unstrained material
What is critical deformation?
The minimum percentage cold work required for recrystallisation to be able to occur. Normally between 2 and 20%.
What happens during recrystallisation?
Metal heated. New grains form as very small nuclei and grow Sputnik they completely consume the parent material. The process involves short range diffusion.
Influence of annealing temperature (over same time) on tensile strength
Graph of tensile strength against annealing temperature curves down from top left, then steeper down, then less steep to nearly horizontal
Influence of annealing temperature (over same time) on ductility
Graph of ductility against annealing temperature curves up from bottom left, then up more, then less steep to horizontal.
Variation of grain size with annealing temperature
Increase together like exponentially on a graph
Describe how precipitation hardening works in ODS steels
Put small amounts of Y2O3 or TiO2 (refractory ceramics) in as artificial pinning sites. They are stable at very high temperatures so dislocations can still be pinned at these conditions and the material is still strong.
