2. Microstructure and Mechanical Properties Flashcards
what is meant by an equiaxed grain structure
when every crystal is approximately the same length in every direction
what is meant by heterogeneous nucleation?
where atoms of a crystal nucleate on a solid surface
state the main property of grain boundaries
high energy regions
what is porosity
holes in metal formed in grain development due to insufficient molten metal in casting (as the volume shrinks during solidifcation)
describe dendrite formation and explain where it happens
when a columnar grain splits off and grows in all directions, occurs in casting and only when a crystal goes from liquid to solid or from gas to solid.
what is each crystal called in a polycrystalline structure?
a grain
what is metallography and list the steps
process used to reveal the “grain structure”. steps: grind to flat surface, polish and then etching (with acid or alkali). Then use a metallographic microscope.
grain boundaries stop what?
dislocation movement (in most cases)
what is the effect of grain size on strength?
smaller grain size = higher strength/higher yield stress (for same metal) which levels off
what is rolling texture?
elongated grains due to rolling of the equiaxed grain structure
how does plastic deformation cause work hardening?
increases dislocations which cause a pile up, dislocations get in each others way which make it harder to move
what is %cold work?
%change in area
what happens to the yield stress and ductility after cold work?
work hardening, yield stress increases (until a point, leveling off), ductility decreases
Define hardness
resistance to localised deformation
Describe the difference(s) in grain structure typically produced when molten PURE metal is cast compared to when an ALLOY is cast
Pure metal - Columnar grains meet in middle Alloy - Columnar grains form but small equiaxed grains form in middle
define recrystallisation temperature
temp at which 50% cold worked metal just fully crystallises in 1 hour
what is hot work?
plastic deformation above recrystallisation temperature
what atoms move by vacancy diffusion? describe this process
lattice atoms or substitutional atoms move into a vacancy, creating another vacancy for the next diffusion
for vacancy diffusion, besides the vacancy, what is needed for this process?
activation energy, as the atom moves to a higher energy level in between moving back down into the vacancy
what does the arrhenius equation state?
reactions occur faster at a higher temperature, increasing exponentially
state the arrhenius equation and state what each symbol means
rate=Ae^(-Q/RT) A is constant; Q is activation energy, R is universal gas constant, T is temperature in Kelvin
describe the arrhenius plot/graph
rearrange equation: ln(RATE) = lnA - Q/R • 1/T plot ln(RATE) against 1/T, negative gradient straight line
define diffusivity
measure of how easily atoms move
equation for diffusivity
D=Do • e^(-Q/RT), Do is the diffusivity constant
what type of diffusion is carbon atoms diffusing through iron?
interstitial diffusion
What is the driving force for the recrystallisation step in annealing?
stored strain energy
