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mse test 3
| Question | Answer |
|---|---|
| a region that is chemically and structurally homogeneous is | phase |
| what kind of nucleation occurs at specific sites such as grain boundaries? | heterogeneous nucleation |
| the two phase microstructure with containing lamellae shaped FeC precipitates in alpha iron (BCC iron) is? | pearlite |
| the only phase which can produce banite as a result of rapid quench is | austenite produces what as a result of rapid quench? |
| when austenite decomposes below the eutectoid temperature, which microstructure contains only a single phase? | martensite |
| as the undercooling increases, the size of the nucleus | decreases |
| increasing the undercooling, ____ the nucleation barrier. why? | decreases. because gibbs free energy equals 1/T^2. So increasing delta T, decreases gibbs free energy, thereby decreasing the nuclear barrier |
| the solubility of carbon in bcc alpha iron is ___ than in FCC y iron | lower |
| below the melting point, the gibbs free energy of the solid is ____ then the gibbs free energy of the liquid | lower |
| what prevents formation of a solid right below the melting point | solid-liquid interface energy |
| what are the solid solubility rules on a phase diagram controlled by | hume-rothery factors |
| a reaction that occurs in an alloy system in which there is no change in composition as the solid transforms to a liquid is called... | congruent melting |
| the phase boundary separating a single phase liquid region from a two phase liquid plus solid region is referred to as the... what about single solid from liquid and solid phase? | liquidus solidus |
| the phase boundary separating a solid region from a two phase solid region is the... | solvus |
| what is the interfacial energy dependent on? how does it affect the likeliness of nucleation | 1. area of interface (4pi*r^2) TIMES.. 2. interfacial energy per unit (y) the larger interfacial energy, larger free energy |
| what is the equation for the total change in the free energy of the system? (change in G) how do you use it to derive r* and delta G*? what is the relationship between these w/ undercooling temperature/interfacial energy on nucleation barrier? | sum of interfacial energy+bulk free energy (diff in free energy of solid and liquid phases times vol of particle) see notebook more undercooling: smaller radius/free energy, smaller barrier more interfacial energy: higher radus/free energy, high barr |
| what is delta G? What is delta G*? What is r*? what if r<r* or r>r* graph r vs. delta G (see notebook) | delta G is free energy delta G* is size of energy barrier for nucleation process r* critical particle size, stable can grow or dissolve to lower free energy r<r* is not stable, can only dissolve to lower free energy r>r* can only grow to lower free en |
| is bulk free energy (change in Gv) neg or positive as T less than Te? does the solid have lower/higher free bulk energy than the liquid when going from L to S what is the equation? | Change in Gv=(Change in H/Te)Change in T negative when T<Te Lower |
| what types of energies contribute to interfaces coh/inc/partially coherent? which has the highest? lowest? draw it (notebook) | coherent: coherency strain energy(200 mJ/m^2) partially: periodically spaced dislocations and chemical mismatch(500mJ/m^2), strain energy incoherent: diff crystal structure/atomic spacing, strin energy, chemical mismatch, dislocations (1000 mJ/m^2) |
| which is hetero/homo nucleation for coherent, partially coherent, incoherent? | coherent: homo (lower interfacial energy 200) bc Sol-liquid partially/incoherent: hetero (higher interfacial energy 500, 1000) |
| what is the difference between homogeneous nucleation and heterogeneous nucleation? | homogeneous nucleation is new phase nucleates randomly in parent phase (LIQUID/SOLID), y it is coherent interface (solid-liquid: lowest energy) hetero is new phase nucleates at specific sites (SOL/SOL) hetero has lower energy barrier, occur at high te |
| INCREASING undercooling the temperature does what to the 1. radius 2. nucleation barrier 3. critical nucleus size 4. average solid cluster size 5. grain size 6. free energy 6. avg # nucleation sites | 1. decreases 2. decreases 3.decreases 4.decrease 5. decrease 6. decrease |
| undercooling decreases the nucleation barrier for hetero or homo more? y does hetero occur on solid to solid | hetero for solid to solid, as new particles grow along grain boundary, it is assissted by energy release from that grain boundary, homo has no corresponding energy source. |
| draw the iron-carbon phase diagram including the microstructures | see notebook |
| what does austenite turn into? at what carbon percentage and temperatures? | Austenite at .77 wt%C is right below eutectic temp.. 1pearlite (alpha ferrite/cementite) lamellar morphology 2bainite less than 550 C, same n feathery, carbide particles lye btwn) 3martensite quenched to 200 C BTC crystal structure, inter. solid sol) |
| what is austenite? | interstitial solid solution of carbon in y iron (FCC) |
| what is alpha-ferrite | interstitial solid solution of carbon in sigma iron (bcc) |
| what is delta-ferrite? | interstitial solid solution of carbon in delta iron BCC |
| pearlite | eutectoid of alpha ferrite (alpha) and cementite (Fe3C) with lamellar microstructure of alternate alpha Fe and cementite plates |
| bainite | eutectoid of alpha ferrite and cementite (alpha and Fe3C), alpha ferrite has feathery appearance with carbide particles lying between |
| martensite stable? | interstitial solid solution of carbon in body-centered tetragonal BTC, Fe crystal structure diffusionless, dependent on temp, not time, metastable |
| spheroidite | spherical particles of cementite in a matrix of alpha ferrite (alpha) |
| why does undercooling affect microstructure of solid-solid? 1) diffusion rate 2) nucleationrate | 1) solids diffuse slower, so when temp is quenched , ms change 2) nucl. rate depends on temp bc of nucleation barrier/diffuson rate. result: a max rate of nucleation & a corresp min time for nucleation occurin @ an intermed. temp |
| increasing undercooling.. does what to microstructures' properties? 1) pearlite spacing/coarseness 2) martensites hardness | 1) inc. undercool, dec. spacing, less coarse (high nucleation rate, low growth rate), lower temp: fine carbides 2)transformation is diffusionless, so carbons stay in interstitial positions in y-Fe lattice (austenite), so no dislocation motion, and strong |
| draw the graph for Ms and Ml vs. concentration of carbon what does Ms and Ml mean? | Ms: temperature that martensite starts Ml: temperature that martensite begins (see notebook) |
| which phases from austenite require the diffusion of carbon? | pearlite, banite require diffusion martensite does not, which is why it is hard |
| what is athermal? | diffusionless |
| on an IT diagram, what does M50 M90 MS mean? | Martensite transformation is 50% done, 90% done, starting |
| what is tempering? | heat up martensites,forms cementites (Fe3C), and energy from the carbons trapped in the BCT, is reduced as it diffuses out2 carbide phase. BCT 2 BCC process where carbide precip formed in martens matrx, spheroidites r rough carbide phase inc. DUCTILIT |
| spheroidized microstructures of carbides (caused by tempering)can form in other things too such as? | ferrite and the cementites from pearlitic/bainitic ms. pearlitic: slowest bc coarse, than bainite bc carbides are fine, then fastest in martensitic |
| higher dislocation density means lower/higher strength? tempering is heating a quenched steel to temp below eutectoid for period of time, why do u do it? martensite is str why do u have to balance strength/ductility? | higher Heat too much, grain grows and strength of material decreases, so optimal temperature is somewhere martensite is strong, brittle, so becomes ductile once heated but loses strength, needs to be between |
| what is jominy end-quench test? draw it | standard size specimen is heated to austenite single phase field&placed in a fixture that puts its lower end over a water quenching pipe. specimen cools at a rate dec w/ distance from quenched surface. hardness measure. found from the strength of the rod |
| in a jominy end-quench test, how do you determine 1) which has highest hardenability 2) highest % martensite for given cooling rate 3) highest critical cooling rate | 1) |
| what is precipitation hardening? | high strength comes from precipitate, as size/number density inc. there is inc in strength/hardness |
| process where powder particles are desnified at elevated temperature either with/without pressure to form nearly fully dense solid | sintering |
| plot temperature vs. tensile strength to show relationship between ductility and strength what is annel? | recover some ductility without loss in tensile strength see notebook |
| what is aging? what is it controlled by? | it is the process of preciptation from a supersaturated solid solution (make stronger) Aging kinetics is controlled by temperature, vacancy concentration (possibly non-equilibrium one), interface energy. |
| why is it advantageous to cool down to lower temperature in the two phase region? and why is rapid cooling rate frequently employed? | 1) bc intial phase formed is metastable, so to reduce inter energy the metaphase resembles the equilibrium phase. so process determined by best way to decompose unstable supersaturated solid sol to 2 phase equil.mixture 2)supersatur of vacancies/ preci |
| proeutectoid phase for a hypoeutectoid alloy | ferrite |
| Compositions to the left of a eutectic are referred to as | hypoeutectic |
| When a liquid of one composition transforms to a liquid of another composition and a solid the reaction is said to be a | monotectic |
| Steels whose compositions are to the left of the eutectoid are referred to as _____________________________________ alloys. | hypoeutectoidal |
| A(n) is a newly formed phase, which can reduce it Gibbs free energy by either growing or dissolving. | nucleus |
| Two phases with same crystal structure and atomic spacings meet at a(n) _____________________________________ interface. | coherent |
| When dislocations are present at an interface to relieve strain the interface is said to be __________. | semi-coherent__________ |
| At constant temperature the fraction of the phases in a two-phase field changes/not changes when the over all composition of the alloy is changed, but remains in the two-phase field. | changes |
| The maximum number of phases that can be in equilibrium in a three component system at one atmosphere is three/four. | four |
| On cooling when a two-phase liquid plus solid transforms to a solid phase the transformation is ____ in nature. | peritectic |
| On cooling the peritectoid reaction written symbolically has _ phases going to _. | 2 going to 1 |
| Increasing the interfacial free energy increases/decreases the likelihood of homogeneous nucleation. | decreases |
| Decreasing the amount of undercooling below the eutectoid temperature increases/decreases the pearlite spacing. | increases |
| Iron carbide (Fe3C) is unstable/stable with respect to graphite. | stable |
| The solubility of carbon at a particular temperature in iron is controlled by the ____ | size of the interstitial holes. |
| The ____ process occurs at temperature roughly equal to about 0.5 T resulting in a gradual decrease in strength with time. | recrystallization |
| Coring in metal castings is a direct consequence of equilibrium/non-equilibrium cooling from the melt to room temperature. | non-equilibrium |
| As the undercooling decreases, the barrier to the nucleation event increases/decreases. | increases |
| how do you read IT diagrams? | 1) homogenize above (eutectic temp) to get in austenite phase 2) rapidly quench to certain temp depending on coarsness, more, higher temp, to find (pearlite, banite)&time 3)martensite if proeutectoid, quench to that area(alpha)if spherodite str8 2 mart |
Created by:
nhussein3