Materials Flashcards
(17 cards)
What are the three types of packing in metal structures
BCC-body centered cubic
FCC-face entered cubic
HCP-hexagonal close packed
What is the main difference in elastic deformation vs plastic (non-elastic) deformation?
In plastic deformation, atoms slip past each other which causes permanent plastic deformation.
Atoms in elastic deformation can return to original position
What is yield strength?
The point where ductile materials begin to deform plastically.
What is ultimate tensile strength?
The maximum stress a material can endure.
What are the two types of diffusion mechanisms and which is faster (and why)
-vacancy diffusion, atoms move into a vacant space
-interstitial diffusion, solute moves into free space
-interstitial faster as more vacancies available (i.e grain boundary)
What are the two main types of diffusion
-interdiffusion/impurity diffusion-diffusion between two solids
-steady state diffusion (flux)
What is the effect of temperature on yield stress for different crystalline structures
-HCP is temperature dependant, not many close packed slip systems
-BCC is temperature dependent, no close packed planes + slip is difficult at low temps
-FCC is temperature independent, as slip is easy
Name the two main types of defects and what they are
Dislocations- shifts in crystal structure
Grain boundaries- interface between 2 crystal structures
What are the two types of dislocations
-edge dislocation, compression of atoms above dislocation line and stretching below it
-screw dislocation, atoms displaced that make helical structure around dislocation line
What is mechanical twinning?
-Alternative deformation via homogenous shear of planes of atoms
-occurs in BCC and HCP metals at low temp but rare in FCC
What are the 4 ways to increase yield strength? (SSGPW)
-solid solution strengthening
-grain boundary hardening
-precipitation hardening
-work hardening
What is solid solution strengthening? And how does it work?
-major element present is solvent into which solute is added
-dislocation compressions relieved by smaller solutes, tensions are relieved by large solute atoms
What is work hardening?
-making material become harder as it is deformed by “tangling up” dislocations
What is grain-boundary hardening? How does it work and how can you make it more effective?
-having dislocations pile up at grain boundaries causes local stresses, so by adding more grain boundaries, makes dislocations harder to initiate slip in the next grain
-having more fine grained metals makes it harder for dislocations to initiate slip
What is precipitation hardening and which type of precipitate is more effective at increasing yield strength?
-growing precipitates (small, solid particles) within the metal matrix that will act as a barrier to dislocations
-incoherent precipitates are more effective as they don’t align with the crystals structure, causes dislocation to be locally pinned
What is the point of annealing and what are the process steps? (RRF)
-reduces the brittleness and hardness of material
-recovery, dislocation re-arrangement but no grain structure change
-recrystallisation, growth of new grains
-further grain growth
What is a eutectic reaction?
A liquid that converts into two different solids
