Lecture 3 Flashcards
what reinforcement increases strength at a greater rate, continuous or particulate for each direction?
continuous - gives greater increase in strength for fibre direction but modulus increase in any other direction is lower.
particulate - slower rate of strength increase than continuous fibre.
what reinforcement is more anisotropic continuous or discontinuous & aligned?
continuous
what are specific strength and specific stiffness?
strength & stiffness divided by specific gravity
what exhibit higher strength stiffness and modulus of elasticity out of the particulate and fibre reinforced MMCs
fibre - better modulus of elasticity
particulate - better strength and stiffness
why is a smaller fibre material much stronger than the bulk material?
less probability of a flaw present
is the diameter of a fibre large or small if the interface surface area is large
interface surface area large — fibre diameter small.
fib diameter & interface SA inversely prop.
what are precipitates and what are their effect on metal alloys?
How are they formed?
precipitates - small, uniformly dispersed particles of a second phase.
Improve strength and hardness
formed by heat treatment
how does precipitation hardening work to strengthen a material?
dislocation motion is impeded, achieved by incorporating fine oxide particles/non shareable precipitates within the matrix.
where is dislocation density highest?
near reinforcements
what are dislocations?
where atoms are out of line with the crystal structure
which of precipitation hardened or dispersion strengthened systems show better strength characteristics at high/low temps
room temp: precipitation hardened systems have higher strength as it is possible to achieve finer distributions.
high temps: show advantages due to the high thermal stability due to the oxide particles.
what are they key element in dispersion strengthened materials?
oxygen
how can you improve fracture toughness of a material through changing the fibres and why?
braiding fibres in weaves because of extensive matrix deformation
what defects can act as fatigue crack initiation sites?
non-metallic inclusions,
particle clusters
how can reinforcing an MMC decrease fatigue and why?
the defects are subjected to lower stress when reinforced as the high stiffness reinforcement will take the load.
what does fatigue failure look like for low stresses and high stresses?
splitting of the fibres is the main mode of failure
single crack propagates catastrophically
how can second phase particles affect fatigue crack interaction?
they can deflect the crack, causing it to circumvent the 2nd phase materials
why do whisker or particle reinforced MMCs produce better creep rate than unreinforced alloys?
There is a better load transfer from the matrix to the reinforcement
what does ceramic reinforcement do for the CTE and electrical conductivity?
decreases both CTE and conductivity
How can we REDUCE the CTE and conductivity?
adding reinforcement in the matrix,,
altering the distribution of reinforcement,
what is something to think about in terms of CTE of matrix and reinforcement?
the difference in CTE
