Strengthening mechanisms and annealing

how does friction affect strength?

increases shear yield stress which strengthens the crystal

what is intrinsic strength caused by?

bonds between atoms

why are covalent bonds strong?

they give high intrinsic lattice resistance, making slip difficult

what are the two types of solid solution hardening?

substitutional and interstitial

what is substitutional solid solution hardening?

foreign atoms replace parent atoms

what happens when zinc is added to copper (to create brass)?

Zn atoms are bigger, which generates stresses and roughen the plane, causing resistance to dislocation

how does increasing solute concentration increase strength?

reduces the separation between solute atoms, hence increasing roughness which increases dislocation resistance

what is interstitial solid solution hardening?

solute atoms move into gaps around dislocation cores, reducing strain energy but pinning the dislocations and making them harder to move

precipitation hardening definition

precipitates form in a crystal and block dislocations, causing the dislocation to curve, which creates line tension which strengthens the lattice

how do dislocations obstruct each other in work hardening?

they repel each other or tangle up

why do grain boundaries act as dislocation obstacles at room temperature?

grains have different orientations, atomic disorder at grain boundaries cause a discontinuity of slip planes

what are the advantages of having a finer grain size?

stronger, harder, tougher, more susceptible to strengthening, results in more uniform and isotropic material behaviour

what are the disadvantages of having a finer grain size?

less resistant to corrosion and creep

why are smaller grains good for strengthening?

they reduce the pile up of dislocations on grain boundaries, since these create stress concentrations which can allow slip to continue or initiate a crack

how does recovery and recrystallisation reduce strength?

plastic deformation reduces dislocation energy which increases the energy stored in the grain. the crystal is heated to rearrange dislocations to reduce energy, and this recrystallises to form a new set of dislocation-free grains