Chapter 11: Applications and Processing of Metal Alloys

Transcription

1 Chapter 11: Applications and Processing of Metal Alloys ISSUES TO ADDRESS... How are metal alloys classified and what are their common applications? What are some of the common fabrication techniques for metals? What heat treatment procedures are used to improve the mechanical properties of both ferrous and nonferrous alloys? Chapter 11-1

2 Classification of Metal Alloys Metal Alloys Ferrous Steels <1.4wt%C C Cast Irons wt%c C Nonferrous Adapted from Fig. 11.1, Callister & Rethwisch 8e δ α ferrite T(ºC) γ+l γ austenite 727ºC Eutectoid: ºC L γ+fe 3 C α+fe 3 C 4.30 L+Fe 3 C Eutectic: microstructure: ferrite, graphite/cementite Adapted from Fig. 9.24, Callister & Rethwisch 8e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.) Fe3C cementite (Fe) C o, wt% C Chapter 11-2

3 Steels low carbon <0.25wt%C Low Alloy Med carbon wt% C high carbon wt% C High Alloy heat Name plain HSLA plain treatable Additions none Cr,V Ni, Mo none Cr, Ni Mo Example , 409 Hardenability varies TS varies EL Uses auto struc. sheet bridges towers press. vessels crank shafts bolts hammers blades pistons gears wear applic. none wear applic. drills saws dies increasing strength, cost, decreasing ductility Based on data provided in Tables 11.1(b), 11.2(b), 11.3, and 11.4, Callister & Rethwisch 8e. plain tool stainless Cr, V, Mo, W Cr, Ni, Mo high T applic. turbines furnaces Very corros. resistant Chapter 11-3

4 Iron-based alloys Steels Cast Irons Ferrous Alloys Nomenclature for steels (AISI/SAE) 10xx Plain Carbon Steels 11xx Plain Carbon Steels (resulfurized for machinability) 15xx Mn ( %) 40xx Mo (0.20 ~ 0.30%) 43xx Ni ( %), Cr ( %), Mo ( %) 44xx Mo (0.5%) where xx is wt% C x 100 example: 1060 steel plain carbon steel with 0.60 wt% C Stainless Steel >11% Cr Chapter 11-4

5 Cast Irons Ferrous alloys with > 2.1 wt% C more commonly wt% C Low melting relatively easy to cast Generally brittle Cementite decomposes to ferrite + graphite Fe 3 C 3 Fe (α) + C (graphite) generally a slow process Chapter 11-5

6 Limitations of Ferrous Alloys 1) Relatively high densities 2) Relatively low electrical conductivities 3) Generally poor corrosion resistance Chapter 11-6

7 Cu Alloys Brass: Zn is subst. impurity (costume jewelry, coins, corrosion resistant) Bronze : Sn, Al, Si, Ni are subst. impurities (bushings, landing gear) Cu-Be: precip. hardened for strength Ti Alloys -relatively low ρ: 4.5 g/cm 3 vs 7.9 for steel -reactive at high T s -space applic. Nonferrous Alloys NonFerrous Alloys Noble metals -Ag, Au, Pt -oxid./corr. resistant Al Alloys -low ρ: 2.7 g/cm 3 -Cu, Mg, Si, Mn, Zn additions -solid sol. or precip. strengthened (struct. aircraft parts & packaging) Mg Alloys -very low ρ: 1.7g/cm 3 -ignites easily -aircraft, missiles Refractory metals -high melting T s -Nb, Mo, W, Ta Based on discussion and data provided in Section 11.3, Callister & Rethwisch 3e. Chapter 11-7

8 Metal Fabrication How do we fabricate metals? Blacksmith - hammer (forged) Cast molten metal into mold Forming Operations Rough stock formed to final shape Hot working vs. Cold working Deformation temperature high enough for recrystallization Large deformations Deformation below recrystallization temperature Strain hardening occurs Small deformations Chapter 11-8

9 Metal Fabrication Methods (i) FORMING Forging (Hammering; Stamping) (wrenches, crankshafts) Ao die blank force force Drawing (rods, wire, tubing) Ao die die Ad Ad tensile force often at elev. T die must be well lubricated & clean CASTING Rolling (Hot or Cold Rolling) (I-beams, rails, sheet & plate) Ao force Ao MISCELLANEOUS ram roll roll Extrusion (rods, tubing) container billet container Ad Adapted from Fig. 11.8, Callister & Rethwisch 8e. die holder extrusion die ductile metals, e.g. Cu, Al (hot) Chapter 11-9 Ad

10 Thermal Processing of Metals Annealing: Heat to Tanneal, then cool slowly. Stress Relief: Reduce stresses resulting from: - plastic deformation - nonuniform cooling - phase transform. Spheroidize (steels): Make very soft steels for good machining. Heat just below T eutectoid & hold for h. Process Anneal: Negate effects of cold working by (recovery/ recrystallization) Types of Annealing Full Anneal (steels): Make soft steels for good forming. Heat to get γ, then furnace-cool to obtain coarse pearlite. Normalize (steels): Deform steel with large grains. Then heat treat to allow recrystallization and formation of smaller grains. Based on discussion in Section 11.7, Callister & Rethwisch 8e. Chapter 11-10

11 Heat Treatment Temperature-Time Paths a) Full Annealing b) Quenching A P c) Tempering (Tempered Martensite) A B Fig , Callister & Rethwisch 8e. b) a) c) Chapter 11-11

12 Hardenability -- Steels Hardenability measure of the ability to form martensite Jominy end quench test used to measure hardenability. specimen (heated to γ phase field) 24ºC water flat ground Rockwell C hardness tests Adapted from Fig , Callister & Rethwisch 8e. (Fig adapted from A.G. Guy, Essentials of Materials Science, McGraw-Hill Book Company, New York, 1978.) Plot hardness versus distance from the quenched end. Hardness, HRC Distance from quenched end Adapted from Fig , Callister & Rethwisch 8e. Chapter 11-12

13 Reason Why Hardness Changes with Distance The cooling rate decreases with distance from quenched end. T(ºC) Hardness, HRC M(start) 200 A M 0 M(finish) distance from quenched end (in) 0% 100% Adapted from Fig , Callister & Rethwisch 8e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 376.) Time (s) Chapter 11-13

14 Summary Ferrous alloys: steels and cast irons Non-ferrous alloys: -- Cu, Al, Ti, and Mg alloys; refractory alloys; and noble metals. Metal fabrication techniques: -- forming, casting, miscellaneous. Hardenability of metals -- measure of ability of a steel to be heat treated. -- increases with alloy content. Precipitation hardening --hardening, strengthening due to formation of precipitate particles. --Al, Mg alloys precipitation hardenable. Chapter 11-14

15 Lecture 08 Structures & Properties of Ceramics Chapter 12-1

16 Structures & Properties of Ceramics ISSUES TO ADDRESS... How do the crystal structures of ceramic materials differ from those for metals? How do point defects in ceramics differ from those defects found in metals? How are impurities accommodated in the ceramic lattice? In what ways are ceramic phase diagrams different from phase diagrams for metals? How are the mechanical properties of ceramics measured, and how do they differ from those for metals? Chapter 12-2

17 Atomic Bonding in Ceramics Bonding: -- Can be ionic and/or covalent in character. -- % ionic character increases with difference in electronegativity of atoms. Degree of ionic character may be large or small: CaF 2 : large SiC: small Adapted from Fig. 2.7, Callister & Rethwisch 8e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.) Chapter 12-3

18 Ceramic Crystal Structures Oxide structures oxygen anions larger than metal cations close packed oxygen in a lattice (usually FCC) cations fit into interstitial sites among oxygen ions Eletctrostatic Charge Neutrality has to be preserved at all times! Chapter 12-4

19 Factors that Determine Crystal Structure 1. Relative sizes of ions Formation of stable structures: --maximize the # of oppositely charged ion neighbors unstable stable stable 2. Maintenance of Charge Neutrality : --Net charge in ceramic CaF2: Ca 2+ cation should be zero. --Reflected in chemical formula: AmXp + Adapted from Fig. 12.1, Callister & Rethwisch 8e. F - anions F - m, p values to achieve charge neutrality Chapter 12-5

20 Coordination # and Ionic Radii Coordination # increases with r cation r anion To form a stable structure, how many anions can surround around a cation? r cation Coord ZnS r anion # (zinc blende) < linear ti triangular tetrahedral NaCl (sodium chloride) cubic 6 Adapted from Table 12.2, Callister & Rethwisch 8e. octahedral CsCl (cesium chloride) Chapter 12-6

21 Computation of Minimum Cation-Anion Radius Ratio Determine minimum r cation /r anion for an octahedral site (C N = 6) (C.N. 6) 2 anion cation r 2r 2a a 2r anion 2r anion 2r cation 2 2r anion r anion r cation 2r anion r cation ( 2 1)r anion r cation r anion Chapter 12-7

22 Bond Hybridization Bond Hybridization is possible when there is significant covalent bonding hybrid electron orbitals form For example for SiC X Si = 1.8 and X C = 2.5 % ionic character 100 {1-exp[-0.25(X X C 2 Si C) ]} 11.5% ~ 89% covalent bonding Both Si and C prefer sp 3 hybridization Therefore, for SiC, Si atoms occupy tetrahedral sites Chapter 12-8

23 Example Problem: Predicting the Crystal Structure of FeO On the basis of ionic radii, what crystal structure would you predict for FeO? Cation Al 3+ Fe 2+ Fe 3+ Ca 2+ Anion O 2- Cl - F - Ionic radius (nm) Answer: r cation r anion based on this ratio, -- coord # = 6 because < < crystal structure is NaCl Chapter 12-9

24 Rock Salt Structure Same concepts can be applied to ionic solids in general. Example: NaCl (rock salt) structure r Na = nm r Cl = nm r Na /r Cl = cations (Na + ) prefer octahedral sites Chapter 12-10

25 MgO and FeO MgO and FeO also have the NaCl structure O 2- Mg 2+ r O = nm r Mg = nm g r Mg /r O = cations prefer octahedral sites So each Mg 2+ (or Fe 2+ ) has 6 neighbor oxygen atoms Chapter 12-11

26 Cesium Chloride structure: AX Crystal Structures AX Type Crystal Structures include NaCl, CsCl, and zinc blende r r Cs Cl Since < < 1.0, cubic sites preferred So each Cs + has 8 neighbor Cl - Chapter 12-12

27 AX 2 Crystal Structures 2 Fluorite structure Calcium Fluorite (CaF 2 ) Cations in cubic sites UO 2, ThO 2, ZrO 2, CeO 2 Antifluorite structure positions of cations and anions reversed Chapter 12-13

28 ABX 3 Crystal Structures Perovskite structure 3 Ex: complex oxide BaTiO 3 Chapter 12-14

29 Density Computations for Ceramics Number of formula units/unit cell n ( A C AA ) V C N A Volume of unit cell Avogadro s number A C = sum of atomic weights of all cations in formula unit A A = sum of atomic weights of all anions in formula unit Chapter 12-15

30 Silicate Ceramics Most common elements on earth are Si & O Si 4+ O 2- crystobalite SiO 2 (silica) polymorphic p forms are quartz, crystobalite, & tridymite The strong Si-O bonds lead to a high melting temperature (1710ºC) for this material Chapter 12-16

31 Silicates Bonding of adjacent SiO 4-4 accomplished by the sharing of common corners, edges, or faces Mg 2 SiO 4 Ca 2 MgSi 2 O 7 Presence of cations such as Ca 2+, Mg 2+, & Al maintain charge neutrality, and 2. ionically bond SiO 4-4 to one another Chapter 12-17

32 Basic Unit: Si04 4- tetrahedron Si 4+ O 2- Glass Structure Glass is noncrystalline (amorphous) Fused silica is SiO 2 to which no impurities iti have been added d Other common glasses contain impurity ions such as Na +, Ca 2+, Al 3+, and B 3+ Quartz is crystalline SiO2: Na + Si 4+ O 2- (soda glass) Chapter 12-18

33 Layered Silicates Layered silicates (e.g., clays, mica, talc) SiO 4 tetrahedra connected together to form 2-D plane A net negative charge is associated with each (Si 2 O 5 ) 2- unit Negative charge balanced by adjacent plane rich in positively charged cations Chapter 12-19

34 Layered Silicates (cont.) Kaolinite it clay alternates t (Si 2 O 5 ) 2- layer with Al 2 (OH) 2+ 4 layer Note: Adjacent sheets of this type are loosely bound to one another by van der Waal s forces. Chapter 12-20

35 Diamond Polymorphic Forms of Carbon tetrahedral bonding of carbon hardest material known very high thermal conductivity large single crystals gem stones small crystals used to grind/cut other materials diamond thin films hard surface coatings used for cutting tools, medical devices, etc. Chapter 12-21

36 Polymorphic Forms of Carbon (cont) Graphite layered structure parallel hexagonal arrays of carbon atoms weak van der Waal s forces between layers planes slide easily over one another -- good lubricant Chapter 12-22

37 Polymorphic Forms of Carbon (cont) Fullerenes and Nanotubes Fullerenes spherical cluster of 60 carbon atoms, C 60 Like a soccer ball Carbon nanotubes sheet of graphite rolled into a tube Ends capped with fullerene hemispheres Chapter 12-23

38 Point Defects in Ceramics (i) Vacancies -- vacancies exist in ceramics for both cations and anions Interstitials -- interstitials exist for cations -- interstitials are not normally observed for anions because anions are large relative to the interstitial titi sites Cation Interstitial Cation Vacancy Anion Vacancy Chapter 12-24

39 Point Defects in Ceramics (ii) Frenkel Defect -- a cation vacancy-cation interstitial pair. Shottky Defect -- a paired set of cation and anion vacancies. Shottky Defect: Frenkel Defect Equilibrium concentration of defects e Q D /kt Chapter 12-25

40 Imperfections in Ceramics Electroneutrality (charge balance) must be maintained when impurities are present + Ex: NaCl Na + Cl - Substitutional cation impurity Ca 2+ Na + cation vacancy Na + without impurity Ca 2+ Ca 2+ impurity with impurity Substitutional anion impurity O 2- anion vacancy without impurity Cl - Cl - O 2- impurity with impurity Chapter 12-26

41 Mechanical Properties Ceramic materials are more brittle than metals. Why is this so? Consider mechanism of deformation In crystalline, by dislocation motion In highly ionic solids, dislocation motion is difficult few slip systems resistance to motion of ions of like charge (e.g., anions) past one another. Chapter 12-27

42 Flexural Tests Measurement of Elastic Modulus Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. -- tensile tests t are difficult for brittle materials. cross section b rect. d R circ. F L/2 L/2 Determine elastic modulus according to: F x slope = F E E F F 3 L 4bd 4 linear-elastic behavior 12 R L 3 3 Adapted from Fig , Callister & Rethwisch 8e. = midpoint deflection (rect. cross section) (circ. cross section) Chapter 12-28

43 Flexural Tests Measurement of Flexural Strength 3-point bend test to measure room-t flexural strength. cross section b rect. d R circ. F L/2 L/2 location of max tension = midpoint i deflection Flexural strength: fs fs 3 L F f 2bd F f R L 3 2 (rect. cross section) (circ. cross section) Material Typical values: fs (MPa) E(GPa) Si nitride id Si carbide Al oxide glass (soda-lime) Chapter 12-29

44 SUMMARY Interatomic bonding in ceramics is ionic and/or covalent. Ceramic crystal structures are based on: -- maintaining charge neutrality -- cation-anion radii ratios. Imperfections -- Atomic point: vacancy, interstitial (cation), Frenkel, Schottky -- Impurities: substitutional, interstitial -- Maintenance of charge neutrality Room-temperature mechanical behavior flexural tests -- linear-elastic; elastic; measurement of elastic modulus -- brittle fracture; measurement of flexural modulus Chapter 12-30

45 Reading: Chapter 12 ANNOUNCEMENTS Homework Problems: TBA Chapter 12-31

46 Chapter 13: Applications and Processing of Ceramics ISSUES TO ADDRESS... How do we classify ceramics? What are some applications of ceramics? How is processing of ceramics different than for metals? Chapter 13-1

47 Classification of Ceramics Ceramic Materials Glasses Clay products Refractories Abrasives Cements Advanced ceramics -optical -composite reinforce -containers/ household -whiteware -structural -bricks for high T (furnaces) -sandpaper -cutting -polishing Adapted from Fig and discussion in Section , Callister & Rethwisch 8e. -composites -structural -engine rotors valves bearings -sensors Chapter 13-2

48 Ceramic Fabrication Methods (i) GLASS FORMING Blowing of Glass Bottles: Parison mold Suspended parison Gob PARTICULATE FORMING Pressing operation Compressed air CEMENTATION Pressing: plates, cheap glasses -- glass formed by application of pressure -- mold is steel with graphite lining Fiber drawing: Finishing mold Adapted from Fig. 13.8, Callister & Rethwisch 8e. (Fig is adapted from C.J. Phillips, Glass: The Miracle Maker, Pittman Publishing Ltd., London.) wind up Chapter 13-3

49 Sheet Glass Forming Sheet forming continuous casting sheets are formed by floating the molten glass on a pool of molten tin Adapted from Fig. 13.9, Callister & Rethwisch 8e. Chapter 13-4

50 Glass Structure Basic Unit: 4- Si04 tetrahedron Si 4+ O 2- Quartz is crystalline SiO2: Glass is noncrystalline (amorphous) Fused silica is SiO 2 to which no impurities have been added Other common glasses contain impurity ions such as Na +, Ca 2+, Al 3+, and B 3+ Na + Si 4+ O 2- (soda glass) Adapted from Fig , Callister & Rethwisch 8e. Chapter 13-5

51 Glass Properties Specific volume (1/ρ) vs Temperature (T): Specific volume Supercooled Liquid Liquid (disordered) Crystalline materials: -- crystallize at melting temp, T m -- have abrupt change in spec. vol. at T m Glass (amorphous solid) Crystalline (i.e., ordered) T g T m Adapted from Fig. 13.6, Callister & Rethwisch 8e. solid T Glasses: -- do not crystallize -- change in slope in spec. vol. curve at glass transition temperature, T g -- transparent - no grain boundaries to scatter light Chapter 13-6

52 Glass Properties: Viscosity Viscosity, η: -- relates shear stress (τ) and velocity gradient (dv/dy): τ τ glass dy dv dv dy velocity gradient η = τ dv / dy η has units of (Pa-s) Chapter 13-7

53 Log Glass Viscosity vs. Temperature Viscosity decreases with T soda-lime glass: 70% SiO 2 balance Na 2 O (soda) & CaO (lime) borosilicate (Pyrex): 13% B 2 O 3, 3.5% Na 2 O, 2.5% Al 2 O 3 Vycor: 96% SiO 2, 4% B 2 O 3 fused silica: > 99.5 wt% SiO 2 Viscosity [Pa-s] T melt T(ºC) strain point annealing point Working range: glass-forming carried out Adapted from Fig. 13.7, Callister & Rethwisch 8e. (Fig is from E.B. Shand, Engineering Glass, Modern Materials, Vol. 6, Academic Press, New York, 1968, p. 262.) Chapter 13-8

54 Ceramic Fabrication Methods (iia) GLASS FORMING Hydroplastic forming: PARTICULATE FORMING CEMENTATION Mill (grind) and screen constituents: desired particle size Extrude this mass (e.g., into a brick) force Ao ram container billet container die holder extrusion die Ad Adapted from Fig. 12.8(c), Callister & Rethwisch 8e. Dry and fire the formed piece Chapter 13-9

55 Ceramic Fabrication Methods (iib) GLASS FORMING PARTICULATE FORMING CEMENTATION Powder Pressing: used for both clay and non-clay compositions. Powder (plus binder) compacted by pressure in a mold -- Uniaxial compression - compacted in single direction -- Isostatic (hydrostatic) compression - pressure applied by fluid - powder in rubber envelope -- Hot pressing - pressure + heat Chapter 13-10

56 Sintering Sintering occurs during firing of a piece that has been powder pressed -- powder particles coalesce and reduction of pore size Aluminum oxide powder: -- sintered at 1700ºC for 6 minutes. Adapted from Fig , Callister & Rethwisch 8e. Adapted from Fig , Callister & Rethwisch 8e. (Fig is from W.D. Kingery, H.K. Bowen, and D.R. Uhlmann, Introduction to Ceramics, 2nd ed., John Wiley and Sons, Inc., 1976, p. 483.) 15 µm Chapter 13-11

57 Tape Casting Thin sheets of green ceramic cast as flexible tape Used for integrated circuits and capacitors Slip = suspended ceramic particles + organic liquid (contains binders, plasticizers) Fig , Callister & Rethwisch 8e. Chapter 13-12

58 Ceramic Fabrication Methods (iii) GLASS FORMING PARTICULATE FORMING CEMENTATION Hardening of a paste paste formed by mixing cement material with water Formation of rigid structures having varied and complex shapes Hardening process hydration (complex chemical reactions involving water and cement particles) Portland cement production of: -- mix clay and lime-bearing minerals -- calcine (heat to 1400ºC) -- grind into fine powder Chapter 13-13

59 Summary Categories of ceramics: -- glasses -- clay products -- refractories -- cements -- advanced ceramics Ceramic Fabrication techniques: -- glass forming (pressing, blowing, fiber drawing). -- particulate forming (hydroplastic forming, slip casting, powder pressing, tape casting) -- cementation Heat treating procedures -- glasses annealing, tempering -- particulate formed pieces drying, firing (sintering) Chapter 13-14