Final Exam - Questions from Exams

which material would have the highest melting point?

ceramics

when does a material no longer elongate uniformly on a stress-strain curve?

only in the necking region

what is the relationship between interstitial diffusion and vacancy diffusion?

interstitial diffusion is faster than vacancy diffusion

why are metals ductile?

a “sea” of electrons keep the positive atoms together so that if you squeeze or bend them they will not break, metallic bonds pool all electrons together to create a “sea”

do polycrystalline materials exhibit anisotropic or isotropic material properties?

the material properties of polycrystalline materials are not dependent on direction, making them isotropic

how does cold work happen?

strain hardening, plastic deformation leads to increase in the number of dislocations, same oriented dislocations meet each other and stop dislocation motion

how does cold work affect anisotropy and isotropy?

new dislocations created stop each other from moving making metals appear stronger, only in the rolling direction

how do you reverse cold work?

high temperatures, for example; annealing

4 interfacial defects

external surface, phase boundaries, grain boundaries, stacking faults

what are the two conditions for diffusion?

enough energy, and empty space

what are the two types of diffusion

interstitial (vacancy) and self-interstitial

what is the key difference between Fick’s first and second law?

time

which of Fick’s laws depend on time?

2nd law

young’s modulus - physical meaning

measures the stiffness of a material

UTS - physical meaning

max load before permanent deformation and min load to cause failure

yield strength - physical meaning

max force/load before plastic deformation, separates elastic and plastic deformation

where does dislocation motion occur?

on the plane which has the densest atomic packing

resilience - physical meaning

the ability of a material to store energy

what is recrystallization temperature determined by?

chemical composition and amount of cold work

what happens to a material’s strength if the size of grains is decreased

strength of material is increased

what is true stress-strain?

considers the shrinking of the cross-sectional area of material after load is removed

3 manufacturing processes that improve fatigue life

decrease stress, polishing, decrease growth of cracks (geometry)

4 material properties obtained from stress-strain diagram that increase/decrease with temperature increase

yield strength increase, UTS decrease, ductility increase, toughness increase

2 material selection criteria to increase fatigue life

choose higher melting temperature, choose higher young’s modulus

fatigue limi - physical meaning

flatline in a ferrous S-N curve where metal alloys slow/stop deformation before failure

fatigue life - physical meaning

number of cycles before failure

fatigue strength

occurs at 10⁷ cycles

toughness - physical meaning

the energy absorbed/used to break one unit volume of material for fracture

elastic recovery - physical meaning

when metal shrinks after load is removed

when 2 pure metals completely mix in each other as a solid solution the phase diagram is called what?

binary isomorphous

what do you construct to determine the composition at any point in a two phase region?

horizontal tie line

2 stages of a phase transformation

nucleation and grain growth

how do you perform full annealing?

heat material and oven/furnace cool it such that we get coarse pearlite

how can you change the grain size of a polycrystalline metal?

by the amount/magnitude of supercooling

small supercooling → slow nucleation, FAST grain growth

large supercooling → rapid nucleation, SLOW grain growth

how can you change the thickness of alternating layers in pearlite?

by the intensity of change in T

greater delta T → more nuclei, less grain growth, FINE pearlite

less delta T → less nuclei, bigger growth, COARSE pearlite

what are the 2 steps in precipitation hardening?

heat the sample and immediately quench, re-heat sample for certain time and cool again

how does precipitation hardening strengthen material?

increases phase boundaries, decreases dislocation motion

which types of metals usually employ precipitation hardening?

non-ferrous metals