Term
| What is recovery? How does this change mechanical properties? electrical properties? |
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Definition
| When a cold-worked sample is annealed by heating to .3Tm, its stored internal strain energy is reduced as excess dislocations glide and climb to form networks of sub-grains within the original elongated deformed grains(also, some dislocations of opposite sign may annihilate each other). As the overall dislocation density is not changed much, the strain energy is not reduced and so the mechanical properties don't change. conductivity s recovered and resistivity decreases |
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Term
| What is recrystallization? |
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Definition
- occurs at T> .6~.7 Tm
- favorably oriented sub-grains grow forming new, equiaxed grains |
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Term
| What determines the final recrystallization grain size? |
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Definition
1) the extent of original cold-work
2)alloying additions which will either be present in solid solution leading to solute drag or will form second-phase precipitates leading to Zener drag |
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Term
| What properties must precipitates have in order to pin dislocations? Why? |
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Definition
| They must be finely disperse. Otherwise, if precipitates coarsen, you might get Orowan bowing |
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Term
| How do you form fine precipitates in alloys? Give an example of characteristic temperatures associated with each of the steps. |
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Definition
1) hold at a temperature to attain complete solid solution
2) quench to maintain solid solution
3)age at a slightly elevate temperature which is below the solvus temperature
for Al-4wt%Cu Tage~160C, Tsolvus~550C |
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Term
| Where does precipitation preferentially occur? |
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Definition
| grain boundaries, dislocations, vacancies, near impurities |
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Term
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Definition
| Precipitation Free Zones, these occur adjacent to grain boundaries, since there are lower vacancy concentrations there. |
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Term
| What re some problems associated with PFZs? |
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Definition
- PFZs are softer or weaker, so deformation may be localized along PFZs leading to failure.
-you can get localized corrosion at PFZs due to the formation of an electrochemical cell |
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Term
| How can you minimize the with of PFZs? |
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Definition
- quench so that you have an excess of vacancies
- heat at low ageing temperatures to promote initial metastable precipitates as formation of these is less dependent on the number of vacancies because precipitates are coherent and have a smaller nucleation barrier
- raise ageing temperature slightly to grow more stable precipitates and to optimise strength by cntrol of precipitate size, coherence and spacing |
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Term
| What is dispersion strengthening based on? |
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Definition
| introducing fine, insoluble particles into an alloy during alloy fabrication in order to avoid problems with pecipitate coarsening. |
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Term
| What favors grey cast irons? |
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Definition
| high C or Si content ad slow cooling rates |
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Term
| If you have a hypereutectoid cast iron, what would you expect? |
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Definition
-graphite flakes to form when the temperature falls below the liquidus line
-a graphite-gamma eutectic to form at eutectic temperature
-upon cooling bellow eutectic; rejection of C from the solid solutin
- transformation to pearlite at eutectoid temperature |
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Term
| What properties do graphite flakes in grey cast irons give rise to? |
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Definition
- good surface definition due to its volume expansion upon solidification
-acts as a lubricant for cutting tools
- act as stress concentrators and interconnected leading to easy crack propagation |
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Term
| What does addition of Mg or Ce to grey cast irons do? What are possible reasons for this? |
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Definition
causes graphite to form as spherical nodules rather than interconnected flakes this rsults in increased toughness.
Possible reasons:
Mg or Ce impurities poison the preferred growth sites by attaching to the edges of the graphite planes
2)alters surface energies |
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Term
| How can you prevent the formation of brittle interconnected Si plates in cast irons? |
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Definition
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Term
| What are white cast irons? |
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Definition
| hypo-eutectoid or low C low Si content cast irons and fast cooling rates. fast cooling rates favors the formation of cementite over graphite |
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Term
| Compare the properties of white and grey cast irons. |
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Definition
-White cast irons are harder and more wear resistant
- white cast irons are brittle while grey cast irons are more ductile |
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Term
| What are steels? cast irons? |
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Definition
| steels have carbon contents of up to 1.5 wt% while cast irons have 2-4wt% C content |
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Term
| What are the 3 types of stainless steels? What is the addition added to stainless steels? Why? |
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Definition
Cr is added to stainless steels to confer corrosion resistance.
three categories:
ferritic (Cr with low Ni)
austenitic (Cr with higher Ni e.g 18wt%)
martensitic ( as ferritic but higher C content) |
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Term
| What alloying additions stabilize ferrite in steel? austenite? |
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Definition
| Cr and Si stabilize ferrite; C, Ni, and Mn stabilize austenite |
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Term
| Precipitation from a supersaturated solid-solution is generally associated with strengthening. Why? |
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Definition
| As in Al- Cu systems you get an increased interaction of dislocations with precipitates. dislocation pinning |
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Term
| What happens to the strength when you temper a martensite. Why? What does this allow for? |
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Definition
| The strength goes down because even though you form precipitates there is a significant reduction in the internal strain energy. Tempering, therefore, allows you to control the yield strength ductility, and toughness |
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Term
| What is secondary hardening? How does it affect properties? |
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Definition
| Secondary hardening occurs in steels that are tempered around 600C and is due to formation of stable carbides of Cr, Mo, W, Nb, or V. This gives rise to an increase in yield strength, wear resistance, and creep resistance. |
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Term
| How does upper bainite form? lower bainite? |
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Definition
upper bainite:
- ferrite forms at austenite grain boundaries on cooling and grows into the austenite by a shear mechanism
- ferrite is a late-like form; fine scale
- C diffuses to the ferrite/ austenite interface where fine cementite crystals nucleate and grow
Lower bainite
-insufficient time for C to diffuse to ferrite/ austenite interface > fine cementite precipitates nucleat and grow in the ferrite |
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Term
| What are the 3 principal effects of alloying additions on the TTT diagrams of steels? |
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Definition
- temperatures at which phases nucleate
- rates of nucleation and growth
- relative stabilties of various phases |
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Term
| What are some common Al alloys used in cast products? |
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Definition
| Al- 12wt% Si (eutectic) and Al-Zn |
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Term
| Why is adding Si to Al useful? |
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Definition
| Si has diamond structure and therefore compensates fr shrinkage |
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Term
| In Al alloys how can you achieve a microstructure with fine Si needles instead of Si plates? |
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Definition
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Term
| What are 1xxx Al alloys? 2xx? 3xxx? 4xxx? 5xxx? 6xxx? 7xxx? 8xxx? |
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Definition
| 1xxx- pure Al; 2xxx- Cu; 3xxx- Mn; 4xxx- Si; 5xxx-Mg; 6xxx- Mg&Si; 7xxx- Mg&Zn; 8xxx- Li |
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Term
| What are possible strengthening mechanisms? |
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Definition
| work-hardening; solid-solution hardening; grain size control; two-phase alloys; precipitates of optimized size and coherency; oxide-dispersion strengthening |
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Term
| What are GP zones? GP1? GP2? |
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Definition
| metastable precipitates that form in Al-Cu alloys during the initial ageing. GP1- the first metastable precipitate to form in the Al matrix. it takes thr form of a Cu plate fully coherent with the Al matrix (~10nm in diameter ~.5nm thick). GP2- a tetragonal precipitate (~15nm diameter plate .8nm thick) |
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Term
| What is the problem with precipitate dispersions that are too fine? too coarse? |
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Definition
| too fine- they can easily be cut by dislocations; too coarse- dislocations can bow between precipitates |
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Term
| What are the advantageous properties of Ti alloys? What applications does this lend Ti alloys too? |
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Definition
- extremely high strength to weight ratios
- excellent mechanical properties
- very good corrosion resistance
applications:
- aerospace:compressor blades other parts in gas turbine engines, airframes
- energy generation- nuclear fuel plants; heat exchangers
- chemical: food processing plants
- medical: prostheses such as hip implants |
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Term
| What are the phases that Ti can exist as? |
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Definition
T<882 C: alpha phase
T>882C beta phase |
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Term
| What are Widmanstatten alpha laths? What important Titantanium alloy are they present in? |
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Definition
| THis is a "basket-weave" structure that results from slow cooling of the beta phase; it has good strength, toughness, and fatigue resistance. This is important in Ti-6 Al -4V. This allow shows superplasticity. |
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Term
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Definition
| The ability to deform up to ~800% at temperatures f ~.6T/Tm |
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