FE-2: Continuation of part 1 Polymers, phase diagrams, steel

1. FE-2: Continuation of part 1Polymers, phase diagrams, steel • Carbon-based of concern here. • One or more monomers joined to form giant molecules. • The bonding within a molecule is primarily covalent. • Polymer solids held together by: • Entanglement of the polymer chains. • Van der Waals forces. • Cross linking between polymer chains by chemical reactions, often at elevated temperature (thermoset). For rubber, called vulcanization, typically by sulfur. Cross-linked polymers can't be heated and reshaped as can thermoplastics. • May have partial crystallization, with molecule chains folded within small crystals and going between crystals. Crystals have higher density (g/cc) • Crystallization favored by polymer molecules having the same shape, and without cross linking. For example, polyethylene. • Another example: isotactic polyvinyl chloride rather than syndiotactic or atactic chains. Last revised January 11, 2014 by W.R.Wilcox at Clarkson University.

2. Mechanical behavior of polymers • Plastic deformation of polymers usually involves the movement of polymer molecules past one another. • In addition to brittle and plastic behavior, can also be highly elastic (elastomeric). • An amorphous polymer may behave like a brittle glass below a glass transition temperature and a rubbery solid at intermediate temperatures. • For small deformations, the behavior depends on how quickly the stress is applied and released. If this is fast, the material behaves elastically. If very slow, it flows and takes a new permanent shape. (Think silly putty.) • For intermediate rates, the deformation is viscoelastic, so that only part of the strain is recovered when the stress is removed. brittle  ductile elastomeric 

3. From the FE exam handbook • Tg is the glass-transition T, below which it's brittle. • Tm is the melting T, above which it flows when stressed and can be formed into shapes. (But it's not a usual liquid.) • Notice that these are not sharp transitions like the melting point of non-polymers.

4. Conditions favoring solubility in solid metals Substitutional impurities: Hume-Rothery rules • Atomic size: The closer the atomic radii the greater the solubility. • Electronegativity: The closer the electronegativities, the greater the solubility. True when metals are near one another in the periodic table. If not near, formation of an intermetallic compound is favored. • For complete solid solubility, the pure components must have the same crystal structure, i.e. "isomorphous." Uncommon. • The electronegativities must be near and the atomic radii close. • Most often get limited solubility with formation of other phase(s). The solubility usually depends strongly on temperature. • Example of complete solid solubility: Ni-Cu Interstitial impurities • Atomic radius of impurity must be much smaller than host, e.g. C (0.071nm) in Fe (0.1241nm).

5. Nickel-copper binary phase diagram at 1 atm • Only melt above the liquidus. • Only solid  below the solidus. • Both in between • Isotherm shows composition of the liquid and solid in equil. • Called a tie line At B: T = __oC? Solid = __%Ni? Liquid = __%Cu?

6. 1 Melting point pure B Solubility of B Melting point pure A Solubility of A Eutectic point Liquid T 2 3 When two phases are in equilibrium with one another they are at the same temperature. Solution 2 and solid B 4 A + 4 5 Find compositions in equilibrium with one another by drawing an isotherm, called a “tie line.” For example: Solid A and solid B in equilibrium with one another A B Fraction of B Binary phase diagram with no solid solubility – simple eutectic

7. Eutectic with some solubility, e.g.Pb-Sn Greek letters  and  used for solid solutions. Metallurgists call eutectic liquid going to solid the “eutectic reaction” L +

8. Compound formation, e.g. Mg-Pb Two eutectics Intermetallic compound Mg2Pb shown at exact comp’n, but would exist over small comp’n range. Some compounds decompose before melting

9. cool heat Peritectic • At the peritectic point, when heated a solid goes to another solid and a melt. Vice versa when cooled. • Metallurgists call this the “peritectic reaction” and write it: • At 184oC, 27wt%Bi goes from  to  + L. • Where’s the eutectic point? • What phases can be in equilibrium at the peritectic point? • At A? S1 + L S2 A Pb Bi

10. Eutectoid points • A eutectoid point is where a solid dissociates to two solids when cooled. Analogous to a eutectic point, at which a liquid dissociates to two solids when cooled. For example, V-Zr phase diagram: • Eutectoid point: • Zr V2Zr + Zr • What is sequence of phases as A is cooled ? • L • L + Zr • Zr • Zr + Zr • V2Zr + Zr A

11. Liquid immiscibility and monotectic points • Sometimes melts separate into two liquids below a certain temperature,e.g. Pb & Zn: • At the monotectic point, a liquid separates into a solid and the other liquid. • Here liquid A  Zn + liquid B • What happens as we cool from the blue dot? • What do we have at the red oval? Zn Pb

12. Another viewpoint For example: simple eutectic with no solid solubility. Fraction of A equals the distance from the mixture composition to the opposite phase (B) divided by Cmix the total distance between phases A & B Distance to opposite phase Check: the closer the mixture composition is to a phase the more of that phase must be present, in the limit 100%! Total distance

13. Fraction of grains with eutectic structure • Consider the red point. • Rather than asking how much of A and B are present, we can ask what weight fraction of the grains is eutectic and what fraction is primary B. • To do this, treat the eutectic as a compound. • Then use the lever rule in the usual way to calculate the weight fraction of grains that have the eutectic microstructure. • The fraction of eutectic is opposite/total. Liquid T B + L A + L opposite total A B Weight fraction of B

14. Fe – Fe3C (cementite): C steels and cast iron

15. Eutectoid reaction to form pearlite • When slowly cool eutectic or eutectoid compositions get a lamellar structure. • For example, 0.76 wt% C austenite gives pearlite, which consists of alternating layers of ferrite and cementite. To left of eutectoid, get pearlite + ferrite steel. To right, brittle pearlite+Fe3C.