MT 610Advanced Physical Metallurgy   Session : Phase Transformations             in Solids II                         Mate...
Contents Diffusional transformations   Long-range    diffusion      Precipitate reaction      Eutectoid transformation...
Eutectoid transformation S 1 → S2 + S 3 Fe-C binary system     γ → α + θ (Fe3C)   cF4 cI2 oP16                         E...
Eutectoid transformation S1 →   S2 + S 3 Fe-C binary system    γ → α + Fe3C Two types of eutectoid transformation    P...
Eutectoid structure Lamellarstructure Non-lamellar structure   Upper bainite   Lower bainite                          ...
Pearlite reaction in Fe-C alloys         Pearlitic     nodules             Create  at grain boundaries,              tri...
Nucleation of pearlite Heterogeneous    nucleation  generally at γ grain boundaries Cooling below A1,   Small    underc...
Nucleation of pearlite Eitherα or Fe3C plates nucleate and grow which promotes the growth of the other phase. At the   r...
Nucleation of pearlite Hypoeutectoid steel  γ’ → αproeutectoid + γ     → αproeutectoid + α + Fe3C Formation  of proeutec...
Nucleation of pearlite To minimize the activation   free energy barrier to nucleation   Epitaxial relationships exist be...
Orientation relationshipsα -   γ1   Kurdjumov-Sachs (KS)            ( 111) γ ( 110) α                   1                ...
Orientation relationships Fe3C -   γ1     Pitch                  ( 100) Fe C                           3                 ...
Orientation relationshipsα - Fe3C Pitch/Patch (eutectoid composition)   ( 001) Fe C ( 521)   [ 110] Fe C 2.6 from 131α...
Growth of pearlite Edgewise   growth occurs by the motion  of the incoherent boundary Sidewise growth occurs by         ...
Cellular growth Composition  and orientation of α’ phase changes discontinuously from Cα’ to Cα for the α phase colony S...
Pearlite transformation Fora given temperature and γ grain size, transformation rate occurs in 3 stages.   1st,low trans...
Pearlite transformation Volume   fraction of γ transformed to pearlite                   π NG 3t 4       f = 1 − exp  ...
Pearlite transformation Temperature at   which the austenite is  transformed also affects the pearlite  growth rate Lowe...
Pearlite transformation Maximum   rate of transformation occurs at about 550°C Above bainite grows faster than pearlite ...
Pearlite transformation Decreasingγ grain size will increase the number of nucleation sites (more heterogeneous nucleatio...
Pearlite transformation Interlamellar spacing is also a strong function of transformation temperature Lower  temperature...
Finer pearlite structure Lower  temperatures will result in a finer lamellar structure                              22
Finer pearlite structureα -   Fe3C   Pitch/Patch or Bagaryatski                        Cementite ledges                  ...
Bainite transformation Decomposition    of γ in  steels at temperatures  below pearlite reaction,  but above martensitic ...
Bainite Influence of carbon content in Fe-C alloys to bainitic transformation temperature                                ...
Bainite Ferrous bainite  consists of   Non-lamellar aggregate    of lath- or plate- shaped     α grains   Carbide preci...
Bainite In steels containing high Si content,   Carbide precipitation can be    suppressed completely     Result in car...
Bainite Important characteristic of bainite in ferrous and nonferrous alloys   Formation of bainitic α plates     Resul...
Bainite Surface relief from formation  of bainitic plates                                  29
Bainite transformation Dependence   of transformation temperature Bainitic microstructural differences are  presented in...
Upper bainite Upper bainitic microstructure   forms at  temperatures of 350-500 °C Needles/laths of α with Fe3C precipit...
Upper bainite Ferritelaths grow into γ in a similar way to Widmanstätten side-plates  Ferrite nucleates on grain   bound...
Upper bainite As ferrite           laths thicken,   Carbon content of austenite    increases till reaching a level    of...
Upper bainiteIftemperature of formation upper bainite increases,   Upper   bainitic structure is more similar      to Wi...
Upper bainite As temperature of formation increases,   It is difficult to distinguish the pearlite    colonies and the u...
Upper bainite Bainitic microstructure in hypo-eutectoid steel  Aggregate of ferrite laths are   usually formed in parall...
Upper bainite Bainitic microstructure in hypo-eutectoid steel   Orientation relationship    between bainitic α and paren...
Upper bainite Bainitic microstructure in hypo-eutectoid steel   Decreasing  transformation    temperature or   Increasi...
Upper bainite Bainitic microstructure in hypo-eutectoid steel   Orientation relationship    between Fe3C and bainitic α ...
Upper bainite Bainitic microstructure in hypo-eutectoid steel   Orientation relationship    between Fe3C and parent γ   ...
Upper bainite Bainitic microstructure in hypo-eutectoid steel   High carbide contents can form as    stringers     Poor...
Lower bainite Lower bainitic microstructure forms at lower portion of bainitic transformation curves                     ...
Lower bainite Bainitic microstructure   changes from  laths to plates Carbide precipitates become much finer Lower bain...
Lower bainite Most characteristic metallographic difference is the distribution of carbides   Carbide   precipitates are...
Lower bainite Orientation relationship between lower bainite α plates and parent austenite γ   Close to Kurdjumov-Sachs ...
Lower bainiteC rejection is slow and C cannot move away fast   Precipitates  occur and move to the next    level with th...
Other bainite Inverse bainitic structure in hyper-eutectoid steels   Carbide phase    nucleate first   Precipitates as ...
Other bainite Nonferrous bainite   Ti   – 4 Ni     Nonlamellar α     Retained β phase     Precipitates of Ti Ni      ...
Effect of alloying elements     Alloying     elements added to Fe-C system          can alter eutectoid transformation.  ...
Effect of alloying elements     Alloying     elements added to Fe-C system          can alter eutectoid transformation.  ...
Effect of alloying elements     Alloying     elements added to Fe-C system          can alter eutectoid transformation.  ...
Effect of alloying elements     Pearlite                              growth rate of Fe-C-X           X         is subst...
Effect of alloying elements Carbide   former                          53
Contents Diffusional transformations   Long-range    diffusion      Precipitate reaction      Eutectoid transformation...
Ordering reaction α’→α Ordered structures, or called  superlattices, result from the ability of  atoms to arrange themse...
Ordered structure B2, CsCl prototype      L12,   AuCu3 prototype   Cl atomic position       Au atomic position    ½½½ ...
Ordered structure D03, BiF3 prototype   Bi atomic positions    000,½½0,½0½,0½½   F atomic positions    ½00,¼¼¼,¾¾¾,    ...
Ordered structure   C15, Cu2Mg prototype       Cu atomic positions        1/8 1/8 5/8 , 3/8 3/8 5/8 , 5/8 5/8 5/8 ,     ...
Ordering reaction During cooling, ordering  occurs independently  in various portions of crystal Long-range    order par...
Ordering reaction If   L = 1, the lowest internal energy.   Entropy   becomes more important factor as    temperatures i...
Ordering reaction Most ordering   reaction occurs in what is called “1st – order transformation”   At equilibrium transf...
Ordering reaction 2nd –   order transformation   ∂G/∂T   and ∂G/∂P are continuous.   ∂2G/∂T2 and ∂2G/∂P2 are discontinu...
Ordering reaction 2 mechanisms for creating ordered phase from disordered phase on cooling  1. Continuous increase in sho...
Ordering reaction 2. Energy barrier to form ordered domains for a process    of nucleation and growth      Generally more...
Antiphase boundary AuCu3alloy form structure AlFe alloy no any meet point or any vertical and horizontal lines          ...
Antiphase boundary Antiphase boundaries can also be generated by the motion of dislocations.                         APB ...
Antiphase boundary Antiphase boundariescan also be generated by deformation.                           APB generated by  ...
Contents Diffusional transformations   Long-range    diffusion      Precipitate reaction      Eutectoid transformation...
Massive transformation 2 different crystal structures                               must be  simple and stable/metastable...
Massive transformation An  alloy must be cooled  fast enough to  temperature below T2   So, no time for precipitation M...
Massive transformation Controlled  by interface diffusion Growth of the product phase at speeds  up to 10 to 20 mm/s No...
Massive transformation Fe -   0.002 C alloy   Quenched   in iced brine from 1000 °C   Microstructure shows ferrite grai...
Massive transformation Cu-37.8   at.% Zn alloy   Aftera partial massive transformation   Massive α phase (dark, mottled...
Massive transformation Cu-21.5   at.% Ga alloy   Quenched   from β structure (above 775°C)   Twinned feathery grains fo...
Contents Diffusional transformations   Long-range    diffusion      Precipitate reaction      Eutectoid transformation...
Polymorphic transformation Polymorphic   transformation involves alteration of structure but not of composition, and the ...
Upcoming SlideShare
Loading in …5
×

Mt 610 phasetransformationsinsolids_ii

969 views

Published on

1 Comment
1 Like
Statistics
Notes
  • i need this file.. why not download?
       Reply 
    Are you sure you want to  Yes  No
    Your message goes here
No Downloads
Views
Total views
969
On SlideShare
0
From Embeds
0
Number of Embeds
2
Actions
Shares
0
Downloads
0
Comments
1
Likes
1
Embeds 0
No embeds

No notes for slide

Mt 610 phasetransformationsinsolids_ii

  1. 1. MT 610Advanced Physical Metallurgy Session : Phase Transformations in Solids II Materials Technology School of Energy and Materials
  2. 2. Contents Diffusional transformations  Long-range diffusion  Precipitate reaction  Eutectoid transformation  Short-range diffusion  Ordering reaction  Massive transformation  Polymorphic transformation Diffusionless transformations  Martensitic transformation 2
  3. 3. Eutectoid transformation S 1 → S2 + S 3 Fe-C binary system γ → α + θ (Fe3C) cF4 cI2 oP16 Eutectoid composition 3
  4. 4. Eutectoid transformation S1 → S2 + S 3 Fe-C binary system γ → α + Fe3C Two types of eutectoid transformation Pearlitic transformation Bainite transformation 4
  5. 5. Eutectoid structure Lamellarstructure Non-lamellar structure  Upper bainite  Lower bainite Nonferrous alloy Cu-27 Sn 5
  6. 6. Pearlite reaction in Fe-C alloys  Pearlitic nodules  Create at grain boundaries, triple points, grain corners, and surfaces  Form alternating parallel lamellae of two product phases (α + Fe3C)  Grow with constant radial velocity into adjoining austenite grains 6 ← Partially transformed eutectoid steel
  7. 7. Nucleation of pearlite Heterogeneous nucleation generally at γ grain boundaries Cooling below A1,  Small undercooling  Small nucleation rate – nodules grow as spheres hemispheres without interfering with each other  High undercooling  Higher nucleation rate – nodules quickly cover all boundaries 20-25 % of total transformation time 7
  8. 8. Nucleation of pearlite Eitherα or Fe3C plates nucleate and grow which promotes the growth of the other phase. At the region ahead of α C is depleting and to promote α formation At the region ahead of Fe3C C is increasing and to promote Fe3C formation 8
  9. 9. Nucleation of pearlite Hypoeutectoid steel γ’ → αproeutectoid + γ → αproeutectoid + α + Fe3C Formation of proeutectoid α leads to the rejection of C to the surrounding γ phase When supersaturation of γ phase with respect to both α and Fe3C is reached,  Pearlite begins to form 9
  10. 10. Nucleation of pearlite To minimize the activation free energy barrier to nucleation  Epitaxial relationships exist between γ, α, and Fe3C 10
  11. 11. Orientation relationshipsα - γ1 Kurdjumov-Sachs (KS) ( 111) γ ( 110) α 1 110 γ1 111 αα - γ2 Incoherent interface 11
  12. 12. Orientation relationships Fe3C - γ1 Pitch ( 100) Fe C 3 ( 554 ) γ1 ( 001) Fe C 3 ( 225) γ1 ( 010) Fe Cγ ( 110) 3 1 Fe3C - γ2 Incoherent interface 12
  13. 13. Orientation relationshipsα - Fe3C Pitch/Patch (eutectoid composition) ( 001) Fe C ( 521) [ 110] Fe C 2.6 from 131α   3 α 3 [ 010] Fe Cα2.6 from [ 113] 3 or Bagaryatski (off-eutectoid) ( 001) Fe C 3 ( 211) α [ 100] Fe C 011α 3   [ 010] Fe Cα [ 111] 3 13
  14. 14. Growth of pearlite Edgewise growth occurs by the motion of the incoherent boundary Sidewise growth occurs by Nucleation  Repeated nucleation (Mehl)  Branching (Hillert) Growth rate is as a function of  Time  Transformation temperature Nucleation  Prior-austenite grain size 14
  15. 15. Cellular growth Composition and orientation of α’ phase changes discontinuously from Cα’ to Cα for the α phase colony Solutes diffuse to form β phase colony from neighboring α colonies with a distance of d = So/2 15
  16. 16. Pearlite transformation Fora given temperature and γ grain size, transformation rate occurs in 3 stages.  1st,low transformation rate, site-saturation dependent  2nd, more nodules develop, increase transformation rate  3rd, nodules impinge, the rate slows as microstructure gradually approaches complete transformation 16
  17. 17. Pearlite transformation Volume fraction of γ transformed to pearlite  π NG 3t 4  f = 1 − exp  −  3   t – a given temperature N – nucleation rate of pearlite colonies G – rate at which the colonies grow into γ 17
  18. 18. Pearlite transformation Temperature at which the austenite is transformed also affects the pearlite growth rate Lowering temperature increases driving force for nucleation, which increases transformation rate 18
  19. 19. Pearlite transformation Maximum rate of transformation occurs at about 550°C Above bainite grows faster than pearlite and results in bainitic transformation 19
  20. 20. Pearlite transformation Decreasingγ grain size will increase the number of nucleation sites (more heterogeneous nucleation sites) More nuclei growing into γ  Decrease transformation time  Increase transformation rate 20
  21. 21. Pearlite transformation Interlamellar spacing is also a strong function of transformation temperature Lower temperatures will result in a finer lamellar structure 21
  22. 22. Finer pearlite structure Lower temperatures will result in a finer lamellar structure 22
  23. 23. Finer pearlite structureα - Fe3C Pitch/Patch or Bagaryatski Cementite ledges stop advancing at a boundary Bending of lamellar because of series of growth steps 23
  24. 24. Bainite transformation Decomposition of γ in steels at temperatures below pearlite reaction, but above martensitic transformation Two types of eutectoid transformation Pearlitic transformation Bainite transformation 24
  25. 25. Bainite Influence of carbon content in Fe-C alloys to bainitic transformation temperature 25
  26. 26. Bainite Ferrous bainite consists of  Non-lamellar aggregate of lath- or plate- shaped α grains  Carbide precipitation within the α grains or in the inter-laths (between thin strip) 26
  27. 27. Bainite In steels containing high Si content,  Carbide precipitation can be suppressed completely Result in carbide-free structures Still referred to as bainitic structures. 27
  28. 28. Bainite Important characteristic of bainite in ferrous and nonferrous alloys  Formation of bainitic α plates Results in surface relief  Indication : shape change accompanied by shear component similar to that found in martensite plates 28
  29. 29. Bainite Surface relief from formation of bainitic plates 29
  30. 30. Bainite transformation Dependence of transformation temperature Bainitic microstructural differences are presented in the distribution of carbides formed in  Upper portion  Lower portion of temperature range. 30
  31. 31. Upper bainite Upper bainitic microstructure forms at temperatures of 350-500 °C Needles/laths of α with Fe3C precipitates between the α laths 31
  32. 32. Upper bainite Ferritelaths grow into γ in a similar way to Widmanstätten side-plates  Ferrite nucleates on grain boundary with Kurdjumov-Sachs orientation ( 011) α ( 111) γ relationship with austenite 111 101  α   γ   large undercooling, ferrite nucleus grow rapidly into austenite and form ferrite laths with semicoherent interfaces 32
  33. 33. Upper bainite As ferrite laths thicken,  Carbon content of austenite increases till reaching a level of cementite formation  Cementite nucleates and grows from carbon-rich regions in austenite 33
  34. 34. Upper bainiteIftemperature of formation upper bainite increases,  Upper bainitic structure is more similar to Widmanstätten side-plates 34
  35. 35. Upper bainite As temperature of formation increases,  It is difficult to distinguish the pearlite colonies and the upper bainite Both grow competitively Pearlite cementite may form as broken lamellae HW 1 How to distinguish these two structures? 35
  36. 36. Upper bainite Bainitic microstructure in hypo-eutectoid steel  Aggregate of ferrite laths are usually formed in parallel groups, called sheaves. 36
  37. 37. Upper bainite Bainitic microstructure in hypo-eutectoid steel  Orientation relationship between bainitic α and parent γ  Kurdjumov-Sachs ( 011) α ( 111) γ 111 101  α   γ  Nishiyama-Wassermann ( 011) α ( 111) γ 111 α 112  γ     37
  38. 38. Upper bainite Bainitic microstructure in hypo-eutectoid steel  Decreasing transformation temperature or  Increasing carbon content Decreases widths of individual ferrite laths Increases amount of carbide precipitation 38
  39. 39. Upper bainite Bainitic microstructure in hypo-eutectoid steel  Orientation relationship between Fe3C and bainitic α  Bagaryatski ( 001) Fe Cα ( 211) [ 100] Fe C 011α 3 3   Isaichev ( 001) Fe C 3 ( 111) [ 103] α Fe3Cα [ 101] 39
  40. 40. Upper bainite Bainitic microstructure in hypo-eutectoid steel  Orientation relationship between Fe3C and parent γ  Pitsch ( 010) Fe Cγ ( 110) [ 001] Fe C 3 3 225  α 40
  41. 41. Upper bainite Bainitic microstructure in hypo-eutectoid steel  High carbide contents can form as stringers Poor mechanical properties, particularly if a crack is created on the carbides Crack will easily propagate through the carbide 41
  42. 42. Lower bainite Lower bainitic microstructure forms at lower portion of bainitic transformation curves 42
  43. 43. Lower bainite Bainitic microstructure changes from laths to plates Carbide precipitates become much finer Lower bainitic structure consists of heavily dislocated ferrite plates, rather than laths 43
  44. 44. Lower bainite Most characteristic metallographic difference is the distribution of carbides  Carbide precipitates are located within the ferrite plates rather than between plates  Carbide precipitates are oriented at a characteristic angle of ~60° to the long axis of the bainitic plate 44
  45. 45. Lower bainite Orientation relationship between lower bainite α plates and parent austenite γ  Close to Kurdjumov-Sachs ( 011) α ( 111) γ 111 101  α   γ Nishiyama-Wassermann ( 011) α ( 111) γ 111 α 112  γ     45
  46. 46. Lower bainiteC rejection is slow and C cannot move away fast  Precipitates occur and move to the next level with the advance of ferrite plate  Carbide will form exactly about the same size and lattice orientation Orientationrelationships between Fe3C has and α plane  Bagaryatski ( 001) Fe Cα ( 211) [ 100] Fe C 011α 3 3   Isaichev ( 001) Fe C 3 ( 111) [ 103] α Fe3Cα [ 101] 46
  47. 47. Other bainite Inverse bainitic structure in hyper-eutectoid steels  Carbide phase nucleate first  Precipitates as a lath or plate and then become surrounded ferrite 47
  48. 48. Other bainite Nonferrous bainite  Ti – 4 Ni  Nonlamellar α  Retained β phase  Precipitates of Ti Ni 2  Cu – 27 Sn  α laths/plates  Interlath precipitations 48
  49. 49. Effect of alloying elements  Alloying elements added to Fe-C system can alter eutectoid transformation.  Austenite stabilizers: Zr, Cu, Ni, Mn, N, C  Expand γ field (Reduce A temperature) 1  Ferrite stabilizers: Cr, Si, Be, Al, Mo, W, Nb, V, P, Sn, Ti  Expand α field (Increase A temperature)H 1 HeLi Be Austenite stabilizers Ferrite stabilizers B C N O F NeNa Mg Al Si P S Cl ArK Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br KrRb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I XeCsFr Ba Ra La Ac Hf Rf Ta Db W Sg Re Bh Os Hs Ir Mt Pt Au Hg Tl Pb Bi Po Uun Uuu Uub Uut Uuq Uup Uuh Uus Uuo At Rn 49
  50. 50. Effect of alloying elements  Alloying elements added to Fe-C system can alter eutectoid transformation.  Effect on A1H HeLi Be Austenite stabilizers Ferrite stabilizers B C N O F NeNa Mg Al Si P S Cl ArK Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br KrRb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I XeCsFr Ba Ra La Ac Hf Rf Ta Db W Sg Re Bh Os Hs Ir Mt Pt Au Hg Tl Pb Bi Po Uun Uuu Uub Uut Uuq Uup Uuh Uus Uuo At Rn 50
  51. 51. Effect of alloying elements  Alloying elements added to Fe-C system can alter eutectoid transformation.  Effect on eutectoid carbon contentH HeLi Be Austenite stabilizers Ferrite stabilizers B C N O F NeNa Mg Al Si P S Cl ArK Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br KrRb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I XeCsFr Ba Ra La Ac Hf Rf Ta Db W Sg Re Bh Os Hs Ir Mt Pt Au Hg Tl Pb Bi Po Uun Uuu Uub Uut Uuq Uup Uuh Uus Uuo At Rn 51
  52. 52. Effect of alloying elements  Pearlite growth rate of Fe-C-X X is substitutional element  If X diffuses more slowly than C, transformation rate decreasesH HeLi Be Austenite stabilizers Ferrite stabilizers B C N O F NeNa Mg Al Si P S Cl ArK Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br KrRb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I XeCsFr Ba Ra La Ac Hf Rf Ta Db W Sg Re Bh Os Hs Ir Mt Pt Au Hg Tl Pb Bi Po Uun Uuu Uub Uut Uuq Uup Uuh Uus Uuo At Rn 52
  53. 53. Effect of alloying elements Carbide former 53
  54. 54. Contents Diffusional transformations  Long-range diffusion  Precipitate reaction  Eutectoid transformation  Short-range diffusion  Ordering reaction  Massive transformation  Polymorphic transformation Diffusionless transformations  Martensitic transformation 54
  55. 55. Ordering reaction α’→α Ordered structures, or called superlattices, result from the ability of atoms to arrange themselves into specific ordered configurations. 55
  56. 56. Ordered structure B2, CsCl prototype  L12, AuCu3 prototype  Cl atomic position  Au atomic position ½½½ 000  Cs atomic position  Cu atomic positions 000 ½½0,½0½, 0½½ 56
  57. 57. Ordered structure D03, BiF3 prototype  Bi atomic positions 000,½½0,½0½,0½½  F atomic positions ½00,¼¼¼,¾¾¾, 0½0,¼¼¾,¾¾¼, 00½,¼¾¼,¾¼¾, ½½½,¾¼¼,¼¾¾ 57
  58. 58. Ordered structure C15, Cu2Mg prototype  Cu atomic positions 1/8 1/8 5/8 , 3/8 3/8 5/8 , 5/8 5/8 5/8 , 7/8 7/8 5/8 , 1/8 7/8 3/8 , 3/8 5/8 3/8 , 5/8 3/8 3/8 , 7/8 1/8 3/8 , 1/8 3/8 7/8 , 3/8 1/8 7/8 , 5/8 7/8 7/8 , 7/8 5/8 7/8 , 1/8 5/8 1/8 , 3/8 7/8 1/8 , 5/8 1/8 1/8 , 7/8 3/8 1/8  Mg atomic positions 000,100,010,001,110,101, 011,111,0½½,½0½,½½0, 1½½,½1½,½½1,¼¼¼, ¾¾¼,¼¾¾,¾¼¾ 58
  59. 59. Ordering reaction During cooling, ordering occurs independently in various portions of crystal Long-range order parameter L is given by rA − X A r − XB L= or B 1 − XA 1 − XB rA and rB : probabilities that an A atom occupies an A site and an B atom occupies an B site, respectively XA and XB : mold fractions of A and B, respectively 59
  60. 60. Ordering reaction If L = 1, the lowest internal energy.  Entropy becomes more important factor as temperatures increase  L continuously decreases until above the critical temperature Tc, which L = 0.  L = 0, it is impossible to distinguish separate sublattices extending over long distance 60
  61. 61. Ordering reaction Most ordering reaction occurs in what is called “1st – order transformation”  At equilibrium transformation temperature, the first derivatives of the Gibbs free energy ∂G/∂T and ∂G/∂P are discontinuous.  ∂G/∂T = – S  H is also discontinuous. 61
  62. 62. Ordering reaction 2nd – order transformation  ∂G/∂T and ∂G/∂P are continuous.  ∂2G/∂T2 and ∂2G/∂P2 are discontinuous.  (∂2G/∂T2) = – (∂S/∂T) = (∂H/∂T) /T = C /T P P P P H is continuous. 62
  63. 63. Ordering reaction 2 mechanisms for creating ordered phase from disordered phase on cooling 1. Continuous increase in short-range order by local arrangements occurring homogeneously throughout the crystal → leading to long- range order in final  Occur by 2nd – order transformation or at very high supercoolings below Tc  Possible homogeneous nucleation by highly coherent interface between ordered and disordered regions 63
  64. 64. Ordering reaction 2. Energy barrier to form ordered domains for a process of nucleation and growth  Generally more common  Atoms may have wrong kind of neighbors creating well-defined boundaries, termed antiphase boundaries (APBs). 64
  65. 65. Antiphase boundary AuCu3alloy form structure AlFe alloy no any meet point or any vertical and horizontal lines 65
  66. 66. Antiphase boundary Antiphase boundaries can also be generated by the motion of dislocations. APB generated by edge- dislocations in ordered MnNi3 alloy 66
  67. 67. Antiphase boundary Antiphase boundariescan also be generated by deformation. APB generated by moving dislocations in ordered AlFe3 alloy 67
  68. 68. Contents Diffusional transformations  Long-range diffusion  Precipitate reaction  Eutectoid transformation  Short-range diffusion  Ordering reaction  Massive transformation  Polymorphic transformation Diffusionless transformations  Martensitic transformation 68
  69. 69. Massive transformation 2 different crystal structures must be simple and stable/metastable at the same composition, but at different temperature 69
  70. 70. Massive transformation An alloy must be cooled fast enough to temperature below T2  So, no time for precipitation Massive transformation appears to proceed primarily by a non-cooperative (random) transfer of atoms across the interfaces between the parent and product phases. 70
  71. 71. Massive transformation Controlled by interface diffusion Growth of the product phase at speeds up to 10 to 20 mm/s No known simple orientation relationships exist between parent and product phases Microstructure often shows massive patches of grains having irregular boundaries 71
  72. 72. Massive transformation Fe - 0.002 C alloy  Quenched in iced brine from 1000 °C  Microstructure shows ferrite grains with irregular boundaries HW 2 Differences between massive transformation and eutectoid transformation? 72
  73. 73. Massive transformation Cu-37.8 at.% Zn alloy  Aftera partial massive transformation  Massive α phase (dark, mottled) has formed at the boundaries of and inside the parent grains of β phase β α 73
  74. 74. Massive transformation Cu-21.5 at.% Ga alloy  Quenched from β structure (above 775°C)  Twinned feathery grains formed by massive transformation, cross prior grain boundaries  Arrows are α precipitation 74
  75. 75. Contents Diffusional transformations  Long-range diffusion  Precipitate reaction  Eutectoid transformation  Short-range diffusion  Ordering reaction  Massive transformation  Polymorphic transformation Diffusionless transformations  Martensitic transformation 75
  76. 76. Polymorphic transformation Polymorphic transformation involves alteration of structure but not of composition, and the transformation occurs by a diffusional process. 76

×