Master of Engineering: Materials Engineering Course Lecture

1. MM-501 Phase
Transformation in Solids
Fall Semester - 2016
Engr. Muhammad Ali Siddiqui
Assistant Professor,
m.siddiqui@neduet.edu.pk
Department of Metallurgical Engineering
NED University of Engineering &
Technology, Pakistan

2. Bainite Formation

3. Upper and Lower Bainite
(Microstructure)
3

4. Upper bainite
4Upper bainite in Fe–0.095C–1.63Si–2Mn–2Cr wt% steel
transformed isothermally at 400◦C.

5. 5
Lower bainite

6. By using Atomic Force microscope
or Scanning Tunneling Microscope
in order to study at higher
Magnification.
6
Surface Relief Shape Change:Surface Relief Shape Change:
Intense dislocation debris at
a bainite/austenite interface.
TEM

7. • Bainite grows at relatively high temperatures
compare to Martensite.
• The large strains associated with the shape change
cannot be sustained by the austenite, the strength of
which decreases as the temperature rises.
• These strains are relaxed by the plastic deformation
of the adjacent austenite.
• The local increase in dislocation density caused by
the yielding of the austenite blocks the further
movement of the glissile transformation interface.
• This localized plastic deformation therefore stops the
growth of the ferrite plate so that each sub-unit only
achieves a limited size which is much less than the
size of an austenite grain.

8. 8
Optical micrograph:
Microstructure of lower bainite. Fe–0.8C
wt% steel transformed
at 300◦C, showing sheaves of lower
bainite.
• Black line is not a single
plate it is actually a
cluster of plate (thousand
of plates).
• Each plate is stopped by
plastic accommodation..
• This produce fine
structure than
martensite.

9. 9

10. Introduction to Bainitic Alloys

11. Carbides in
Lower Bainitic
Ferrite plates
Since long
range diffusion
is not allowed at
lower temp, so
only iron
carbides (like ε,
η, κ, or
cementite)
precipitates.

12. Harmful Effect of Cementite θ :
how can be avoided?
Brittle Fracture = initiates cleavage
cracks.
Ductile Fracture = initiate nucleation of
voids.
As consequence of carbides
there is reduction in
Toughness.

14. Bainitic Alloys: (Carbide Free Alloys)
1 Fe - 0.4C 2Si 3Mn
2 Fe - 0.2C 2Si 3Mn
3 Fe - 0.4C 2Si 4Ni
Wt%
?? Suppress the
θ
Precipitation *
Mn/Ni for
hardenability.
Stop other
transformation product
* Al can do the same job as Si, but presently don’t
have any prove. [2004]
Role of Si, Mn & Ni =?

16. Microstructure of Bainitic Alloy
• Ferrite + Carbon enriched films of
Austenite.
• No carbide particles in that material
• Both strength and toughness are depend
upon the scale of bainitic ferrite & films of
C-enriched-γ
γ

17. Advantage as a Results
1. Can achieve very fine structure just by phase
transformation.
Fine Plates of Bainite 0.2μm thick and 10μm in
length.
2. Got a mixture of ferrite & films of austenite.
3. Each ferrite plate is only about 10μm long b/c of
plasticity associated with shape deformation; stop
it from growing, once it reaches about that length.
So actually finer than martensite.
0.2 μm
10 μm

18. 3. Tougher than all structure; strength is due
to fine structure. (it is considered as an ideal
microstructure; grain refinement is only
mechanism for increase both strength and
toughness).
4. Due to austenite in the microstructure; “H”
embrittlement problem would be solved.
(diffusion rate of hydrogen in austenite is
slow)

19. Now have a look on toughness
Notice that the impact transition temp is more than 100o
C, so that
completely unacceptable for any engineering material that below
100o
C one can get fracture by cleavage.
So, something is very wrong in our science?

20. • As soon as we apply stress over here the austenite is transformed into
untampered martensite which is extremely hard and brittle.
• Why do we have these large region of austenite left in our material; we
have transformned isothermally?
Bainite Sheaf
Untransformed
high carbon
Austenite
Microstructure of that Alloy

21. α γ
γα
T
Ae3Ae1
o
Carbon Concentration
TemperatureFreeEnergy
T
1
T
1

22. How many ways can one increase
volume fraction of bainitic alloys?

23. 1. Reduce the average
carbon concentration;
shift to the Y-axis
(means lowering the
“carbon”)
2. Addition / modify of
substitutional solute Mn
etc. shift/move the To
curve to higher carbon.
3. Lower the transformation
temperature but this is
limited to Ms
temperature.

24. Changed
Original

26. Fe-0.2C -3Mn-2Si
Fe-0.4C -4Ni-2Si

28. Product of these alloy
Fig: Section of railway line
What is the normal structure of Sections?

29. Microstructure of Pearlite

33. Tunnel b/w Britain and France ;under the sea

35. Talk about
World first Bulk Nanostructured steel
ever created
Bulk Nanocrystalline Steel

36. • Imagine, a steel
1.Exceptionally strong, = GPa
2.Be made in large chunks = bulk crystalline
3.Easy to manufacture
4.Low cost which is affordable = cheap
How ?

37. Problem: to design a bulk
nanocrystalline steel which is
very strong, tough, cheap ….

38. Before describing this novel
material, it is important to
review the meaning of strength,

39. • Put an apple on 1 m2
= 1 pa
• 100 MPa = I00 million apples on 1 m2
• 1GPa = billion apples on 1 m2
• 1TPa = 1000 billion apples on 1 m2
Understanding unit

40. Brenner, 1956
10 GPa
Theoretical Strength• Brenner achieved
tensile strength =
greater than 13 GPa
in an iron whisker
about 1.5 mm in
length.
• Theoretically =
possible to achieve a
tensile strength of 21
or 22 GPa in ideal
crystals of iron.

41. • The strength of a crystal increases sharply
as it is made smaller because the
probability of avoiding defects increases.
• Note these are the crystals only.
• Strength collapses as we make bigger in
size because of defects increases.
• Now remember Aim ~ 22 Gpa, if we
eliminating the defects in the materials.

42. 1. Strengthening by Deformation
• It has been possible for some time to obtain
commercially, steel wire which has an ultimate tensile
strength of 5.5 GPa and yet is very ductile in fracture.
• made by Kobe Steel Japan.
Scifer, Scientific Iron

43. • See strength 5.5 GPa and ductility (tie knot)
• We can not make a knot with Carbon fiber which has 3.3GPa
strength & virtually zero ductility.
• Scifer, as the wire is known is made by drawing a dual-
phase microstructure of martensite and ferrite in Fe–0.2C–
0.8Si–1Mn (wt-%) steel.
• So can we make a cable bridge from this = ?

44. 1 Denier: weight in grams, of 9 km of
fibre or yarn.
50-10 Denier
Scifer is 9 Denier
So we can use it for cutting semi conductors
not for making bridge cables

45. Figure: Comparison of size-sensitivity of single-crystals
whiskers of iron and Scifer

46. 2. Strengthening of Carbon Nanotubes
Carbon nanotube to catalyze to grow

47. Morinobu Endo, 2004

48. Claimed strength of carbon
nanotube is 130 GPa
Edwards, Acta Astronautica, 2000
Claimed modulus is 1.2 TPa
(1000 GPa) 6X greater than
Steel
Terrones et al., Phil. Trans. Roy. Soc., 2004

49. Space-elevator concept (originally due to Arthur C.
Clark), requiring a cable 120 000 km in length.2 Cable
would be launched in both directions from
geosynchronous orbit at a height of 36 000 km
People starting research to built an Space elevator
(Russian Concept)
What is wrong with this ?

50. as soon as make it big the strength
collapses due to increase in the
defects as we scale up
[as we know that about Fe in 1956.
(22 GPa) ]
Equilibrium number of defects (1020
)
Strength of a nanotube rope 2 mm
long is less than 2000 MPa.

51. Limit of Nanotube

52. •Strength produced by deformation
limits shape: wires, sheets...
•Strength in small particles relies on
perfection. Doomed as size increases.
Summary
So far; we are unsuccessful to produce Bulk Nanocrystalline Steel

53. 3. Thermomechanical
processing
• Smallest size possible in polycrystalline substance?
• Back in 1960 (Micro alloying = dramatic change in
grainsize improves the quality of steel)
• 10 billion tons of steel are in service today by micro
alloying only. (HSLA steels)

55. Yokota & Bhadeshia, 2004

56. Limit of Thermomechanical Treatment

57. Thermomechanical processing
limited by recalescence
Summary
Need to store the heat
Reduce rate
Transform at low temperature
Heating up the steel by itself

58. Courtesy of Tsuji,
Ito, Saito,
Minamino, Scripta
Mater. 47 (2002)
893.
Howe, Materials
Science and
Technology 16
(2000) 1264.
Another
problem = ?

60. Fine crystals by transformation
1. Introduce work-hardening
capacity--- How …
2. Need to store the heat
3. Reduce rate
4. Transform at low temperature
Requirement for Scale up:

61. Design criteria for Bulk
Nanocrystalline Steel
1. It should ideally be possible to
manufacture components which are large
in all dimensions, not simply in the form of
wires or thin sheets.

62. 2. There are commercially available steels
in which the distance between interfaces is of the order
of 250–100 nm. The novelty is in approaching a
structural scale in polycrystalline metals that is an order
of magnitude smaller.
3. The material concerned must be cheap to produce. A
good standard for an affordable material is that its cost
must be similar to that of bottled water when
considering weight or volume.

63. • The following conditions are required to
achieve this:
1.

64. 2.
3.

65. 4.

66. • All of these conditions can in principle be met by
the phase transformation of austenite into
bainite, partly because the reaction is
particularly amenable to control by either
isothermal or continuous cooling heat treatment.
• Furthermore, the transformation is displacive,
i.e., it leads to a shape deformation which is
macroscopically an invariant plane strain with a
large shear component, as illustrated in figure.

67. There is in principle no lower limit to the temperature at
which bainite can be generated.
How the bainite-start BS and martensite-start MS
temperatures vary as a function of the carbon
concentration?

68. 0
200
400
600
800
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Carbon / wt%
Temperature/K
Fe-2Si-3Mn-C wt%
BS
MS
Temperature?

69. 1.E+00
1.E+04
1.E+08
0 0.5 1 1.5
Carbon / wt%
Time/s Fe-2Si-3Mn-C wt%
1 month
1 year
Take 100 year to produce bainite at room temperature

70. • On the other hand, the rate at which bainite
forms slows down drastically as the
transformation temperature is reduced, as
shown by the calculations in the right plot of
Fig.
• It may take hundreds or thousands of years
to generate bainite at room temperature.
• For practical purposes, a transformation time
of tens of days is reasonable.

71. C Si Mn Mo Cr V P
0.98 1.46 1.89 0.26 1.26 0.09 < 0.002
wt%
Low transformation temperature
Bainitic hardenability
Reasonable transformation time
Elimination of cementite
Austenite grain size control
Avoidance of temper embrittlement

72. Temperature
Time
1200 oC
2 days
1000 o
C
15 min
Isothermal
transformation
125 o
C-325 o
C
hours-monthsslow
cooling
Air
cooling
Quench
AustenitisationHomogenisation

74. X-ray diffraction results
0
20
40
60
80
100
200 250 300 325
Temperature/o
C
Percentageofphase
bainitic ferrite
retained austenite

76. 200 Å
γ
γ
α
α
α
Caballero, Mateo,
Bhadeshia
Transformation took 10 days at 200 o
C
C Nano tube
same X

78. Low temperature transformation: 0.25 T/Tm
Fine microstructure: 20-40 nm thick plates
Harder than most martensites (710 HV)
Carbide-free
Designed using theory alone

79. Effect of Elongation due to increase in volume fraction of austenite
Strain is uniform

81. “more serious battlefield threats”

82. ballistic mass efficiency
consider unit area of armour

84. 200 Å
γ
γ
α
α
α
Very strong 2.5GPa, 710HV
Huge uniform ductility
No deformation
No rapid cooling
No residual stresses
Cheap
Uniform in very large sections

85. Chatterjee & Bhadeshia, 2004Fe-1.75C-Si-Mn wt%
2104

86. Further Reading

87. Cobalt (1.5 wt%) and aluminium (1 wt%)
increase the stability of ferrite relative
to austenite
Refine austenite grain size
Faster Transformation
C Si Mn Mo Cr V P
0.98 1.46 1.89 0.26 1.26 0.09 < 0.002

88. Original 5h 3/4d 63 550
Co 4h 11h 77 640
Co + Al 1h 8h 76 640
200o
C
250o
C
300o
C
Steel Beginning End % Bainite HV
Original 4d 9d 69 618
Co 2d 5d 79 690
Co+ Al 16h 3d 78 690
Original 2.5h 1/2d 55 420
Co 1h 5h 66 490
Co + Al 0.5h 4h 66 490

89. original

90. Co

91. Co+Al

96. References
• H.K.D.H Bhadeshia
(Online Lectures)

97. Thanks