Mt 610 phasetransformationsinsolids_iii


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Mt 610 phasetransformationsinsolids_iii

  1. 1. MT 610Advanced Physical Metallurgy Session : Phase Transformations in Solids III Materials Technology School of Energy and Materials
  2. 2. Contents Diffusional transformations  Long-range diffusion  Short-range diffusion Diffusionless transformations  Martensitictransformation  Geometric observation  Mechanism 2
  3. 3. Martensitic transformation Rapidly quenched γ from temp. above A1 to temp. below Ms  Very slow diffusion rate at temp. below Ms  Rapid cooling prevents decomposition of γ by diffusional process.  γ → α + Fe C transformation cannot 3 occur by any diffusion transformation 3
  4. 4. Martensitic transformationγ will minimize its free energy by  Transforming to a metastable phase γ → α’ with a lower energy by a shear process  The energy decreases with displacement of atoms and causes change in microstructure.  This mode of transformation kinetics is referred to as athermal, meaning without thermal activation. 4
  5. 5. Martensitic transformation Martensite A general term for microstructure formed by diffusionless phase transformation γ → α’  Parent γ and product α’ phases have a specific crystallographic relationship  No change in composition 5
  6. 6. Martensitic transformation Atomic movements during transformation are cooperative in a regimented fashion with distance less than one interatomic spacing  Cause shape change in the transformed region  Surface tilt if the product α’ phase intersects a free surface of the parent γ phase 6
  7. 7. Martensitic transformation Shear mechanism  Shear is on habit planes, on which the transformation was initiated.  Ideally, martensite crystal has planar interfaces with the parent austenite  Preferred crystal planes on interfaces  Midrib is considered to be the starting plane for the formation of a martensite plate. 7
  8. 8. Martensitic transformationA martensite unit  Flatand Lies in a habit plane parallel to a crystallographic plane in γ  Once nucleated inside a γ grain at M , s α’ propagates ~ 1 km/s   In a direction lying in the plane of α’ plate  α’ plate shape is similar to a lens.  Grow until reaching grain boundary, twin boundary, or another martensite  Can form again inside retained γ but 8 smaller
  9. 9. Martensitic transformation Thermodynamic driving force  Temperature Ms reflects the amount of thermodynamic driving force required to initiate shear transformation.  Smaller martensite is still required further cooling rate for next α’ to form.  Each α’ platelet will introduce its own strain. 9
  10. 10. Martensitic transformation Thermodynamic driving force  Next α’ platelet will require higher energy (∆T) to overcome extra stress from previous α’ platelets and to make the next martensitic transformation to occur.  The martensitic transformation will end at Mf.  Density of α’ plates is not a function of the γ grain size. 10
  11. 11. Martensitic transformation Surface tilting  Martensite crystal is displaced by the shear partly above and partly below the surface of the austenite.  Shear transformation tilts the originally horizontal surface of the parent phase into a new orientation.  An important feature of shear-type or martensitic transformation 11
  12. 12. Martensitic transformation Surface tilting  Large change in shape would cause large strain.  Minimized by deformation  Twinning  Slip with no crystal structure change 12
  13. 13. Martensitic transformation Martensite crystalline structure  In steel  Parent phase usually has fcc  Product phase can be bcc, bct, or hcp  Iron-based martensite  Fe-Ni and Fe-Ni-Cr usually have bcc  Fe- >15% Mn alloys can form martensite with hcp crystal structure. 13
  14. 14. Martensitic transformation Martensite crystalline structure  fcc → bcc transformation  Kurdjumov-Sachs orientation relationship for high-carbon steels {111}γ || {101}α’ and <110>γ || <111>α’ with {225}γ habit planes  Kurdjumov-Sachs orientation relationship for low-carbon steels {111}γ || {101}α’ and <110>γ || <111>α’ with {557}γ habit planes 14
  15. 15. Martensitic transformation Martensite crystalline structure  fcc → bcc transformation  Nishiyama orientation relationship for high-carbon steels {111}γ || {011}α’ and <112>γ || <011>α’ with {259}γ habit planes  fcc → hcp transformation  orientation relationship {111}γ || {001}α’ and <110>γ || <110>α’ 15
  16. 16. Martensitic transformation  During cooling to Ms DC = 1.54 Å 2 types of interstitial site in fcc  Tetrahedral site DFe = 2.52 Å surrounded by 4 atoms  d fcc = 0.225D w/o matrix distortion, max. space 4 0.568 Å  Octahedral site surrounded by 6 atoms  d fcc = 0.414D w/o matrix distortion, max. space 6∴ cause distortion of γ lattice 16 1.044 Å
  17. 17. Martensitic transformation Temp. below Ms  Interstitialatoms cause more distortion to the bcc lattice.  Distortion causes the martensitic Fe-C structure to form a bct structure.  c/a ratio varies with %Carbon c/a = 1.005 + 0.045 (wt.% C)  Distortion in z direction causes contraction in x and y directions. 17
  18. 18. Martensitic transformation Exp. Ms decreases significantly with increasing %C in Fe-C alloys and carbon steels. Carbon in solid solution increases strength or shear resistance of austenite  Higher carbon alloys require larger undercooling or driving force to begin the shear for martensite formation. 18
  19. 19. Martensitic transformation Martensite plates  Form in the shape of laths that are grouped into larger sheaves, or packets  Generally called lath martensite, massive martensite or packet martensite ASM Handbook 19
  20. 20. Martensitic transformation Martensite plates  Form as individual, lenticular, or plate- shape units  Generally called plate martensite  Forme as thin sheet  Called sheet martensite 20 ASM Handbook
  21. 21. Martensitic transformation Cryogenic treatment  Toincrease an amount of martensite formed during cooling below Ms 21
  22. 22. Martensitic transformation After martensitic transformation  Large interaction energy btw carbon and strain fields  Carbon tends to lower its chemical potential by diffusing to sites close to dislocations and to form carbon-rich clusters.  In low-carbon low-alloy steels  Plenty of time during quenching for carbon to segregate or precipitate as ε-carbide (Fe2.4C) or Fe3C 22
  23. 23. Carbide ε-carbide (Fe2.4C)  Similar to Fe3C  Having the same form of precipitate Cementite (Fe3C)  Lath-likeprecipitate  Coarsens into a spheroidal forms at high T 23
  24. 24. Martempering Interrupted quench from the A1 Delay cooling just above martensitic transformation for a length of time to equalize T throughout the piece  Reduce thermal gradient btw surface & center Minimize distortion, cracking, and residual stress. 24
  25. 25. Geometric observation When martensite is formed  Observe shape deformation (macroscopic deformation)  Well-defined displacement of scratches due to a martensite plate  Straight lines/vectors are transformed into other straight lines/vectors.  Planes are transformed into other planes  Maintain interfacial coherency btw martensite & parent at the habit plane 25
  26. 26. Geometric observation When martensite is formed  Martensite plate leaves the scratches uninterrupted as they cross the interface. Ifthe shape deformation causes any significant rotation of the habit plane Ifthere is any distortion existing in the habit plane due to shape deformation 26
  27. 27. Geometric observation If any significant rotation of the habit plane due to shape deformation  Plastic deformation of the matrix material next to the martensite plate would reveal itself additional plastic deformation of the scratches in the parent phase. 27
  28. 28. Geometric observation If any distortion existing in the habit plane due to shape deform.  Scratches across the interface would appear discontinuous. 28
  29. 29. Geometric observation Additional plastic deformation is not observed. Scratches across the interface appear to be continuous. ∴ Unrotated and undistorted habit plane due to shape deformation 29
  30. 30. Mechanism Bain distortion Bain correspondence Shear transformation Double shear transformation  Invariant plane  Lattice-invariant shear Rigid body rotation 30
  31. 31. Bain correspondence Bain (1924) purposed a simple mechanism for the deformation of austenite lattice  Incomplete model for entire martensitic transformation Martensitic transformationis taken place by a cooperative movement of atoms  Soatomic neighbors are maintained before and after the transformation. 31
  32. 32. Bain correspondence α lattice with bcc can be generated from an fcc γ lattice by  Compression about 20% along one principle axis and a simultaneous uniform expansion about 12% along the other two axes perpendicular to the first principle axis This homogeneous distortion making one lattice change to another is termed a lattice deformation. 32
  33. 33. Bain correspondence Bain distortion  Lattice deformation of fcc to bcc/bct transformation Bain correspondence  Method of determining a correspondence between lattice points in the → initial and ao → c final lattices 33 0.7ao → a
  34. 34. Bain correspondence  Bain distortion of fcc to bcc/bct  [001] becomes [001] f b  [110] becomes [100]b f  Consider corresponding vectors are  x1b = x1f − x2f  x1b = x1f + x2f  x3 = x3f b → x1  1 −1 0  x1  x  =  1 1 0  x  ao → c 2    2 x3  b 0 0 1  x3  f     Bain correspondence matrix 34 0.7ao → a
  35. 35. Bain correspondence Does not provide actual crystal orientation relationships between initial and final lattices. The most feasible deformation  Involves the smallest relative atomic displacements  Results in the smallest strain energy. Unrotated habit plane  Satisfy Undistorted habit plane  Not yet satisfy 35