2. characteristics of diffusionless transformations the solid solution of carbon in steels...
TRANSCRIPT
1
Characteristics of Diffusionless Transformations
•The Solid Solution of Carbon in Steels
Martensite Crystallography
The Bain Model of FCC to BCT Transformation
Comparison of Crystallographic Theory With Experimental Results
Theories of Martensite Nucleation
•Formation of Coherent Nuclei of Martensite
•Role of Dislocation in Martensite Nucleation
•Dislocation Strain Energy Assisted Transformation
Characteristics of Diffusionless Transformations
•The Solid Solution of Carbon in Steels
Martensite Crystallography
The Bain Model of FCC to BCT Transformation
Comparison of Crystallographic Theory With Experimental Results
Theories of Martensite Nucleation
•Formation of Coherent Nuclei of Martensite
•Role of Dislocation in Martensite Nucleation
•Dislocation Strain Energy Assisted Transformation
TopicsTopics
INTRODUCTION
• Maraging steels
• Trip Steels
• Ausforming Steels
• Dual phase steels
• Military transformation: Individual atom
movements are less than one interatomic spacing.
Characteristics of Difusionless Transformations
• Lens/Plate shape
• Elastic stress on the surface: Surface relief
• Continuity across the lens on the surface
• Speed of transformation: speed of the sound in solid,
completion in 10-7s.
• No thermal activation needed except for a Fe-Ni alloy.
• Ms: 500°C in low carbon steel, decreases with carbon content.
• Mf: Retained austenite due to elastic stress in between the
transformed plates.
• Driving force for the nucleation of martensite:
• Ordered alloys need small ΔG.
• Solid solution of carbon in Iron
Characteristics of Difusionless Transformations
Fig. 6.1 (a), (b) Growth of martensite with increasing cooling below Ms. (c)-(e)
Different martensite morphologies in iron alloys: (c) low C (lath), (d) medium C
(plate), (e) Fe-Ni (plate).
Characteristics of Difusionless Transformations
Fig. 6.2 Illustrating how a martensite plate remains (macroscopically coherent with the surrounding austenite and even the surface it intersects.
Characteristics of Difusionless Transformations
Fig. 6.3 Various ways of showing the
martensite transformation. (a) Free
energy temperature diagram for
austenite and martensite of fixed
carbon concentration (C0 in (b)). (b)
Free energy-composition diagram for
the austenite and martensite phases
at the Ms temperature. (c) Iron-
carbon phase diagram with T0 as
defined in (a), Ms and Mf
superimposed. (d) Ms and Mf in
relation to the TTT diagram for alloy
C0 in (c).
Characteristics of Difusionless Transformations
Fig. 6.5 Illustrating (a) possible sites for interstitial atoms in bcc lattice, and (b)
the large distortion necessary to accommodate a carbon atom (1.54 A diameter)
compared with the space available (0.346 A). (c) Variation of a and c as a function
of carbon content.
Characteristics of Difusionless Transformations
• Interface of α/γ: Sound speed, but not always associated with
dislocations
• Habit plane: Undistorted (direction and angular separation unchanged)
• No rotation. • Transformation strains: Homogeneous, shear parallel to
the habit plane. Dilatation (4%, γ to α′) normal to the habit plane. Analogy
to twinning, Fig 6.6.
• Invariant plane strain: Homogeneous shear parallel to the habit plane
(or the twinning planes) and the dilatation normal to the habit plane do
not change the positions or the magnitudes of the vectors on the habit
plane (or twinning plane).
Martensite Crystallography
Fig. 6.6 (a) Showing the
twinning of an fcc structure.
Black and white circles
represent atoms on different
levels (b) Graphical
representation of a twinning
shear occurring on a plane K1
in a direction d
The Bain Model of the fcc-bcc transformation
• 1924, Bain
• Two fcc unit cells to one bcc unit cell, see F. 6.7
• Contraction along z-direction: 20%
• Expansion along the x- and y-directions: 12%
• Carbon atoms: <100>/2 position (z-axes) expands the lattice.
The carbon atoms need to be shuffled to become the right
positions in the bct.
• Orientation relationship in the Bain model:
– Bain: (111)γ // (011)α′ [-101]γ // [-1-11]α′
[1-10]γ // [100]α′ [11-2]γ // [01-1]α′
– KS: (111)γ // (011)α′ <-101>γ // <1-11>α′
– NW: (111)γ // (011)α′ <1-10>γ // <101>α′,
about 5° rotation about [111]γ
The Bain Model of the fcc-bcc transformation
Fig. 6.7 Bain correspondence for
the α → α′ transformation. Possible
interstitial sites for carbon are
shown by crosses. To obtain α′
the γ unit cell is contracted about
20% on the C axis and expanded
about 12% on the a axes.
The Bain Model of the fcc-bcc transformation
• Does the Bain model fit the observation of the undistorted (invariant)
habit plane?
• Refer to the sphere/ellipsoid model in F. 6.8
• Two vectors are needed to form a plane:
Vector OA or OA′ is invariant but OY′ is deformed by 12%. Therefore
the Bain model does not provide the condition of the habit plane.
Fig. 6.8 The Bain deformation is
here simulated by the pure
deformation in compressing a
sphere elastically to the shape of
an oblate ellipsoid. As in the Bain
deformation, this “transformation”
involves two expansion axes and
one contraction axis.
Question on the Bain Model
Modification of the Bain Model
• Internally twinned martensite model: The deformation of the OY′ axis
can be made to zero by introducing twinning or slip, see F. 6.9. This can
form by having alternate regions in the austenite undergo the Bain strain
along different contraction axes such that the net distortions are
compensated. Then the habit plane becomes a macroscopically invariant
plane.
• Experimental data on the habit planes: {111} to {225} to {259}
transition with increasing C content or Ni content. Also, a transition
occurs from dislocated martensite to twinned martensite with increasing
C or Ni. Thickness of twins in high carbon {259} martensite: approx.
3nm.
Role of slip and twinning in martensitic transformation
Fig. 6.9 This figure illustrates
schematically how dislocation glide or
twinning of the martensite can
compensate such as a Bain deformation
and thereby reduce the strain of the
surrounding austenite. The
transformation shear (s) is defined. Note
how s can be reduced by slip or twinning
Role of slip and twinning in martensitic transformation
Fig. 12.5 (a) Formation of a martensite platelet in a crystal of austenite, (b) the
inhomogeneous twinning shear within the platelet
Role of slip and twinning in martensitic transformation
Martensite transformation (a) to (d) of a crystal region. Its external shape can be restored approximately by slip (c) or twinning (d).
Scratches on the surface (a) are sheared in the martensitic transformation (b) resulting in surface relief
Role of slip and twinning in martensitic transformation
Fig. 6.11 Martensite habit planes in various types of steel
Role of slip and twinning in martensitic transformation
Fig. 6.12 Transmission electron micrographs of (a) lath martensite and (b) twinned
martensite. Note the midrib in the twinned martensite, which is thought to be the
first part of the plate to grow.
Theories of Martensite Nucleation
• Speed of nucleation: 800-1100m/s
• Initial martensite nucleus is coherent with the parent austenite.
• Gibbs energy change for nucleation of coherent nucleus:
– ΔGs is significantly larger than γ.
Theories of Martensite Nucleation
• A lenticular martensite nucleus with radius a and thickness c:
e.g.) s=0.2, γ=20mJ/m2, ΔGv=174mJ/m3 then, c*/a*=1/40,
ΔG*=20eV (unable to overcome by thermal fluctuation)
• Heterogeneity of Martensite nucleation: Dislocation,
Inclusions but not GB and free surface
Theories of Martensite Nucleation
Fig. 6.14 Schematic representation of a martensite nucleus
Theories of Martensite Nucleation
Fig. 6.14 Schematic representation of a martensite nucleus