Phase Transformations in Metals Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 1

Transcription

1 Ferrite - BCC Martensite - BCT Fe 3 C (cementite)- orthorhombic Austenite - FCC Chapter 10 Phase Transformations in Metals Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 1

2 Why do we study phase transformations? Tensile strength of an Fe-C alloy of eutectoid composition can be varied between MPa depending on HT process adopted. Desirable mechanical properties of a material can be obtained as a result of phase transformations using the right HT process. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 2

3 Why do we study phase transformations? In order to design a HT for some alloy with desired RT properties, time & temperature dependencies of some phase transformations can be represented on modified phase diagrams. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 3

4 Why do we study phase transformations? Based on this, we will learn: A. Phase transformations in metals B. Microstructure & property dependence in Fe-C alloy system C. Precipitation Hardening, Crystallization, Melting, and Glass Transition Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 4

5 Topics to be covered: Transformation rate Kinetics of Phase Transformation Nucleation: homogeneous, heterogeneous Free Energy, Growth Isothermal Transformations (TTT diagrams) Pearlite, Martensite, Spheroidite, Bainite Continuous Cooling Mechanical Behavior Precipitation Hardening Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 5

6 Phase Transformations (PT) Phase transformations: change in number or character of phases Simple diffusion-dependent PT No change in # of phases No change in composition Example: solidification of a pure metal, allotropic transformation, recrystallization, grain growth More complicated diffusion-dependent PT Change in # of phases Change in composition Example: eutectoid reaction Diffusion-less PT Example: meta-stable phase : martensite Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 6

7 Phase Transformations -Stages Most phase transformations begin with the formation of numerous small particles of the new phase that increase in size until the transformation is complete. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 7

8 Phase Transformations -Stages Nucleation: process whereby nuclei (seeds) act as templates for crystal growth 1. Homogeneous nucleation - nuclei form uniformly throughout the parent phase; requires considerable super-cooling (typically C). 2. Heterogeneous nucleation - form at structural inhomogeneities (container surfaces, impurities, grain boundaries, dislocations) in liquid phase much easier since stable nucleating surface is already present; requires slight super-cooling ( C). Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 8

9 Supercooling During the cooling of a liquid, solidification (nucleation) will begin only after temperature has been lowered below the equilibrium solidification (melting) temperature T m. This phenomenon is termed super-cooling or under-cooling. The driving force to nucleate increases as T increases Small super-cooling slow nucleation rate - few nuclei - large crystals Large super-cooling rapid nucleation rate - many nuclei - small crystals Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 9

10 Kinetics of Solid State Reactions Transformations involving diffusion depend on time. Time is necessary for the energy increase associated with phase boundaries between parent & product phases. Nucleation, growth of nuclei, formation of grains & grain boundaries and establishment of equilibrium take time. Transformation rate is a function of time Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 10

11 Kinetics of Solid State Reactions The fraction of reaction completed is measured as a function of time at constant T. Tranformation progress can be measured by microscopic examination or measuring a physical property (conductivity). The obtained data is plotted as fraction of transformation versus logarithm of time. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 11

12 Fraction of Transformation Fraction transformed depends on time. Transformation rate depends on T. r often small: equil not possible 2 Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 12

13 Fe 3 C (cementite) Transformations & Undercooling Eutectoid transformation (Fe-Fe 3 C system): For transformation to occur, must cool to below 727 C T( C) a ferrite 1600 d g g +L (austenite) 1148 C L g +Fe 3 C Eutectoid: Equil. Cooling: T transf. = 727ºC T a +Fe 3 C Undercooling by T transf. < 727C g a + Fe 3 C 0.76 wt% C 6.7 wt% C wt% C L+Fe 3 C 727 C Tuesday, December 24, (Fe) Dr. Mohammad Suliman Abuhaiba, PE C, wt% C 13

14 % transformed Generation of Isothermal Transformation Diagrams The Fe-Fe 3 C system, for C o = 0.76 wt% C A transformation temperature of 675 C T = 675 C T( C) Austenite (unstable) Austenite (stable) Pearlite time (s) T E (727C) isothermal transformation at 675 C Tuesday, December 24, Dr. Mohammad Suliman Abuhaiba, PE time (s) 14

15 % pearlite Transformation of austenite to pearlite: Austenite (g) grain boundary Coarse pearlite formed at higher temperatures relatively soft Fine pearlite Eutectoid Transformation Rate ~ T a a a a a a For this transformation, rate increases with ( T) [T eutectoid T ]. g g cementite (Fe 3 C) Ferrite (a) pearlite growth direction C (T larger) C 675 C (T smaller) Diffusion of C during transformation formed at lower temperatures relatively hard g a a a g Carbon diffusion Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 15

16 Eutectoid Transformation Rate At T just below 727 C, very long times (on order of 10 5 s) are required for 50% transformation and therefore transformation rate is slow. Transformation rate increases as T decreases Example: at 540 C, 3 s is required for 50% completion. This observation is in clear contradiction with the equation of Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 16

17 Eutectoid Transformation Rate This is because in T range of 540 C-727 C, transformation rate is mainly controlled by the rate of pearlite nucleation and nucleation rate decreases with T increase. Q in this equation is the activation energy for nucleation and it increases with T increase. At lower T, the austenite decomposition is diffusion controlled and the rate behavior can be calculated using Q for diffusion which is independent of T. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 17

18 Nucleation and Growth Reaction rate is a result of nucleation and growth of crystals Examples: Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 18

19 Isothermal Transformation Diagrams Solid curves are plotted: one represents time required at each temperature for start of transformation the other is for transformation completion Dashed curve corresponds to 50% completion Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 19

20 Isothermal Transformation Diagrams Austenite to pearlite transformation will occur only if alloy is supercooled to below eutectoid temperature (727 C). Time for process to complete depends on temperature. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 20

21 Isothermal Transformation Diagram Eutectoid iron-carbon alloy; C o = 0.76 wt% C Begin at T > 727 C Rapidly cool to 625 C and hold isothermally. Austenite-to-Pearlite Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 21

22 Fe 3 C (cementite) Transformations Involving Noneutectoid Compositions Consider C 0 = 1.13 wt% C 1600 d T( C) g (austenite) T g +L L g +Fe 3 C a +Fe 3 C L+Fe 3 C 727 C (Fe) C, wt%c Hypereutectoid composition proeutectoid cementite Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 22

23 Strength Ductility Possible Transformations Tuesday, December 24, 2013 Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends Dr. Mohammad Suliman Abuhaiba, PE 23

24 Coarse pearlite (high diffusion rate) and (b) fine pearlite Tuesday, December 24, Smaller T: colonies are larger Dr. Mohammad Suliman Abuhaiba, PE - Larger T: colonies are smaller 24

25 Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE Bainite: Non-Equilibrium Transformation Products elongated Fe 3 C particles in a-ferrite matrix diffusion controlled a lathes (strips) with long rods of Fe 3 C 800 T( C) 600 Austenite (stable) A P 100% pearlite T E Martensite 400 A B 100% bainite Cementite Ferrite time (s) 25

26 Bainite Microstructure Bainite: formed as a result of transformation of austenite Bainite consists of ferrite & cementite and diffusion processes take place as a result. This structure looks like needles or plates. There is no proeutectoid phase in bainite. Bainite consists of acicular (needle-like) ferrite with very small cementite particles dispersed throughout. Carbon content is typically greater than 0.1%. Bainite transforms to iron & cementite with sufficient time and temperature. 26 Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE

27 Spheroidite: Nonequilibrium Transformation Fe 3 C particles within an a-ferrite matrix diffusion dependent heat bainite or pearlite at temperature just below eutectoid for long times driving force reduction of a-ferrite/fe 3 C interfacial area 10 Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 27

28 Pearlitic Steel partially transformed to Spheroidite Dr. Mohammad Suliman Abuhaiba, PE Tuesday, December 24,

29 Martensite Formation 800 T( C) 600 Austenite (stable) A P T E 400 A B 200 single phase body centered tetragonal (BCT) crystal structure BCT if C 0 > 0.15 wt% C Diffusion-less transformation BCT 10-1 M + A M + A M + A 0% 50% 90% time (s) few slip planes hard, brittle % transformation depends only on T of rapid cooling Martensite needles Austenite Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 29

30 A micrograph of austenite that was polished flat and then allowed to transform into martensite. The different colors indicate the displacements caused when martensite forms. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 30

31 Isothermal Transformation Diagram Iron-carbon alloy with eutectoid composition. A: Austenite P: Pearlite B: Bainite M: Martensite Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 31

32 Effect of Adding Other Elements Other elements (Cr, Ni, Mo, Si and W) may cause significant changes in positions and shapes of TTT curves: Change transition temperature Shift nose of austenite-to-pearlite transformation to longer times Shift pearlite & bainite noses to longer times (decrease critical cooling rate) Form a separate bainite nose 4340 Steel nose plain carbon steel Plain carbon steel: primary alloying element is carbon. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 32

33 Example 1: Iron-carbon alloy with eutectoid composition. Specify nature of final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time temperature treatments: Alloy begins at 760 C and has been held long enough to achieve a complete and homogeneous austenitic structure. Bainite, 100% Treatment (a) Rapidly cool to 350 C Hold for 10 4 seconds Quench to room temperature Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 33

34 Example 2: Iron-carbon alloy with eutectoid composition. Specify nature of final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time temperature treatments: Alloy begins at 760 C and has been held long enough to achieve a complete and homogeneous austenitic structure. Treatment (b) Rapidly cool to 250 C Hold for 100 seconds Quench to room temperature Austenite, 100% Martensite, 100% Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 34

35 Example 3: Iron-carbon alloy with eutectoid composition. Specify nature of final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time temperature treatments: Alloy begins at 760 C and has been held long enough to achieve a complete and homogeneous austenitic structure. Treatment (c) Rapidly cool to 650 C Hold for 20 seconds Rapidly cool to 400 C Hold for 10 3 seconds Quench to room temperature Austenite, 100% Almost 50% Pearlite, 50% Austenite Final: 50% Bainite, 50% Pearlite Bainite, 50% Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 35

36 Continuous Cooling Transformation Diagrams Isothermal heat treatments are not the most practical due to rapidly cooling and constant maintenance at an elevated temperature. Most heat treatments for steels involve continuous cooling of a specimen RT. TTT diagram (dotted curve) is modified for a CCT diagram (solid curve). For continuous cooling, time required for a reaction to begin & end is delayed. Isothermal curves are shifted to longer times & lower temperatures. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 36

37 CCT Diagrams Moderately rapid & slow cooling curves are superimposed on a CCT diagram of a eutectoid iron-carbon alloy. Transformation starts after a time period corresponding to intersection of cooling curve with the beginning reaction curve and ends upon crossing completion transformation curve. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 37

38 CCT Diagrams Normally bainite does not form when an alloy is continuously cooled to RT; austenite transforms to pearlite before bainite has become possible. Austenite-pearlite region (A---B) terminates just below the nose. Continued cooling (below M start ) of austenite will form martensite. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 38

39 CCT Diagrams For continuous cooling of a steel alloy there exists a critical quenching rate that represents minimum rate of quenching that will produce a totally martensitic structure. This curve will just miss the nose where pearlite transformation begins Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 39

40 CCT Diagrams CCT diagram for a 4340 steel alloy & several cooling curves superimposed. This demonstrates the dependence of final microstructure on transformations that occur during cooling. Alloying elements used to modify critical cooling rate for martensite are chromium, nickel, molybdenum, manganese, silicon and tungsten. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 40

41 Mechanical Properties Hardness: Brinell, Rockwell Yield Strength, Tensile Strength Ductility: % Elongation Effect of Carbon Content Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 41

42 Mechanical Properties: Influence of Carbon Content Pearlite (med) ferrite (soft) C 0 < 0.76 wt% C Hypoeutectoid C 0 > 0.76 wt% C Hypereutectoid Pearlite (med) Cementite (hard) Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 42

43 Mechanical Properties: Fe-C System Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 43

44 Tempered Martensite Martensite is hard but also very brittle so that it can not be used in most of the applications. Any internal stress that has been introduced during quenching has a weakening effect. Ductility and toughness of the material can be enhanced by heat treatment called tempering. This also helps to release any internal stress. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 44

45 Tempered Martensite Tempering is performed by heating martensite to a T below eutectoid temperature (250 C-650 C) and keeping at that T for specified period of time. The formation of tempered martensite is by diffusion. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 45

46 Tempered Martensite Tempered martensite is less brittle than martensite; tempered at 594 C. Tempering reduces internal stresses caused by quenching. The small particles are cementite; the matrix is a-ferrite. US Steel Corp steel Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 46

47 Tempered Martensite Tempered martensite may be nearly as hard and strong as martensite, but with substantially enhanced ductility and toughness. Hardness & strength may be due to large area of phase boundary per unit volume of the material. Phase boundary acts like a barrier for dislocaitons. The continuous ferrite phase in tempered martensite adds ductility and toughness to the material. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 47

48 Tempered Martensite The size of cementite particles is important factor determining the mechanical behavior. As the cementite particle size increases, material becomes softer and weaker. Temperature of tempering determines the cementite particle size. Since martensite-tempered martensite transformation involves diffusion, Increasing T will accelerate diffusion and rate of cementite particle growth and rate of softening as a result. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 48

49 Hardness as a function of carbon concentration for steels Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 49

50 Rockwell C & Brinell Hardness Hardness versus tempering time for a waterquenched eutectoid plain carbon steel (1080); room temperature. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 50

51 Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 51

52 Precipitation Hardening Strength & hardness of some metal alloys may be improved by the formation of extremely small, uniformly dispersed particles (precipitates) of a second phase within the original phase matrix. Other alloys that can be precipitation hardened or age hardened: Copper-Beryllium (Cu-Be) Copper-Tin (Cu-Sn) Magnesium-Aluminum (Mg-Al) Aluminum-Copper (Al-Cu) High-strength Aluminum alloys Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 52

53 Phase Diagram for Precipitation Hardened Alloy Criteria: Maximum solubility of 1 component in the other (M); Solubility limit that rapidly decreases with decrease in temperature (M N). Process: Solution Heat Treatment first heat treatment where all solute atoms are dissolved to form a single-phase solid solution. Heat to T 0 and dissolve B phase. Rapidly quench to T 1 Nonequilibrium state (a phase solid solution supersaturated with B atoms; alloy is soft, weak-no ppts). Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 53

54 Precipitation Heat Treatment 2 nd stage Supersaturated a solid solution is usually heated to an intermediate temperature T 2 within a+b region (diffusion rates increase). b precipitates (PPT) begin to form as finely dispersed particles. This process is referred to as aging. After aging at T 2, the alloy is cooled to RT Strength & hardness of alloy depend on ppt temperature (T 2 ) and aging time at this temperature. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 54

55 Precipitation Hardening Particles impede dislocation motion Ex: Al-Cu system Procedure: Pt A: solution heat treat (get a solid solution) Pt B: quench to RT (retain a solid solution) Pt C: reheat to nucleate small q particles within a phase. 700 T( C) a A 500 a+l B (Al) Temp. Pt A (solution heat treat) Pt B C Pt C (precipitate q) L a+q q+l CuAl 2 q wt% Cu composition range available for precipitation hardening At RT the stable state of an Al- Cu alloy is AL-rich solid solution (α) and an intermetallic phase with a tetragonal crystal structure having nominal composition CuAl 2 (θ). Time Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 55

56 Precipitation Heat Treatment the 2 nd stage PPT behavior is represented in the diagram: With increasing time, hardness increases, reaching a maximum, then decreasing in strength. Reduction in strength & hardness after long periods is overaging (continued particle growth) Small solute-enriched regions in a solid solution where the lattice is identical or somewhat perturbed from that of the solid solution are called Guinier-Preston zones. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 56

57 Precipitation Strengthening Hard precipitates are difficult to shear. Ex: Ceramics in metals (SiC in Iron or Aluminum). Result: y ~ 1 S 24 Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 57

58 Several stages in the formation of equilibrium PPT (q) phase. (a) supersaturated a solid solution; (b) transition (q ) PPT phase; (c) equilibrium q phase within the a matrix phase. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 58

59 tensile strength (MPa) %EL (2 in sample) Influence of Precipitation Heat Treatment on Tensile Strength (TS), %EL 2014 Al Alloy: TS peak with precipitation time. Increasing T accelerates process. %EL reaches minimum with precipitation time C 204 C 1min 1h 1day 1mo 1yr precipitation heat treat time C 149 C 1min 1h 1day 1mo 1yr precipitation heat treat time Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 59

60 Effects of Temperature Characteristics of a 2014 aluminum alloy (0.9 wt% Si, 4.4 wt% Cu, 0.8 wt% Mn, 0.5 wt% Mg) at 4 different aging temperatures. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 60

61 Aluminum rivets Alloys that experience significant precipitation hardening at room temp and after short periods must be quenched to and stored under refrigerated conditions. Several aluminum alloys that are used for rivets exhibit this behavior. They are driven while still soft, then allowed to age harden at the normal RT. Tuesday, December 24, 2013 Dr. Mohammad Suliman Abuhaiba, PE 61