In this article we will discuss about:- 1. Introduction to Martensitic Transformation in Steels 2. Characteristics of Martensitic Transformation in Steels 3. Crystal Structure of Martensite 4. Crystallography 5. Thermodynamics 6. Reversibility.
Contents:
- Introduction to Martensitic Transformation in Steels
- Characteristics of Martensitic Transformation in Steels
- Crystal Structure of Martensite
- Crystallography of Martensitic Transformation in Steels
- Thermodynamics of Martensitic Transformation in Steels
- Reversibility of Martensitic Transformation in Steels
1. Introduction to Martensitic Transformation in Steels:
A steel, when rapidly cooled from austenitic state, usually transforms to martensite—a very hard structure—which is the basis of hardening of steels. A cooling rate faster than its critical cooling rate avoids the transformation of austenite by diffusion processes (to pearlite and/or bainite), but instead transforms to martensite—a diffusion less shear transformation product. Martensite is a supersaturated solid solution of carbon in iron—named after the German metallurgist-Adolf Marten.
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In carbon steels, as the amount of martensite increases, the hardness and the strength increase, but toughness decreases. The magnitude of these effects is strongly dependent on the carbon content of the steel. Martensitic transformation occurs in many other systems like Cu-Al, Au-Cd, Fe-Ni, some ceramics. This generic name describes transformations occurring by shear without change in chemical composition. Martensite need not always be hard and brittle. Carbon free iron-nickel alloys yield soft and ductile martensite.
2. Characteristics of Martensitic Transformation in Steels:
Some important characteristics of martensitic transformation in steels are:
1. Diffusionless Transformation:
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Martensite has exactly the same compositions as its parent austenite phase—carbon in solid solution state in former austenite remains in solid solution state in martensite. The carbon atoms occupy precisely the same octahedral sites in martensite as in octahedral sites in face- centred cubic austenite matrix without diffusion. Diffusion less behaviour is further confirmed by the fact that in other alloy systems, the ordered solid solution remains ordered after the martensitic transformation.
2. Martensite Forms by Shear (or Displacive) Mechanism Indicated by Surface Relief of Polished Surface (Which Indicates Highly Crystallographic Nature of Transformation):
Fig. 3.45 (b) indicates that the just formed martensite crystal is displaced partly above and partly below the surface of the parent austenite by the shear. The original horizontal surface of austenite is tilted into new orientation by shear transformation and is easily seen as surface relief that occurs. Surface tilting, or relief is an important characteristic of martensitic transformation.
Even the straight scratch EGHF (Fig. 3.45 a), remains unrotated and rectilinear, even in inclined portion of the surface, OPQR, and remains continuous along its whole length (Fig. 3.45 b) and does not show any sharp break. It is clear that the martensitic reaction is accompanied by a ‘shape change’. The figure also illustrates that large plastic deformation of the parent austenite accompanies the formation of the martensite crystal. Thus, the constraints of plastic deformation of parent phase limit the width of martensite plate.
Further transformation of martensite takes place only by the formation of new plates (and not by the growth of these already formed plates). The shear character of martensitic transformation is also the reason of the high rate of formation of martensite crystal, as it has been shown that plates form within 10-7 seconds, even at 40°K. If the parent phase is unable to accommodate the change in shape due to martensite shears, the nucleation of crack at the martensite/parent phase interface would take place. Austenite in steels has good ductility and could accommodate this strain.
The line OP (Fig. 3.45 b) remains unrotated and also, that the habit plane OPTS of the martensite crystal is macroscopically approximately invariant during the martensitic transformation, i.e., this plane is neither distorted, nor rotated (as any section on this plane remains invariant in both length and direction), i.e. martensite crystals ideally have planar interfaces with the parent phase austenite, at least macroscopically. Habit plane is the preferred crystal plane of austenite on which martensite crystal forms.
3. Ms Temperature:
For each steel, the austenite-to- martensite transformation begins (after a considerable undercooling) at a definite temperature, called Ms (where ‘S’ stands for the start of martensite transformation) temperatures, which is uniquely determined by the chemical composition. Ms temperature can vary over a wide temperature range from as high as 500°C (≈ 539°C almost pure iron) to well below room temperature, depending on the amount of γ-stabilising alloying elements in the steel. Cobalt and aluminium are exception to increase the Ms temperature.
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The following is the best fit equation due to Andrews:
Ms (°C) = 539 – 423 (% C) – 30.4 (% Mn) – 12.1 (% Cr) – 17.7 (% Ni) – 7.5 (% Mo) …(3.32)
Carbon has the most profound effect in lowering Ms temperature. Ms temperature is independent of the cooling rate over a wide range. Ms temperature has been reported to increase somewhat at very high cooling rates. For example, the Ms temperature of 0.5%C steel is virtually constant at 370°C, if cooling rates are below 6000°C/s, but rises to become constant at 460°C for cooling rate of 16500°C/s and higher. At low cooling rates up to 6000 C/s, carbon atoms are able to segregate at defects in austenite.
This strengthens the austenite to result in the minimum Ms temperature, which is independent of cooling rates up to 6000°C. At cooling rates of 16500°C/s, or higher, austenite remains above Ms temperature for so small a time (few hundredths of a second), that the segregation of carbon to defects in austenite is inhibited almost fully, and are not able to strengthen the austenite thus, Ms temperature rises and is stable at that temperature.
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Martensitic transformation cannot be suppressed even at the highest cooling rate attained. (Ms temperature is raised by coarse grains of austenite).
4. Athermal Transformation:
The martensitic transformation in carbon steels has no incubation period. The TTT diagram, say of an eutectoid Steel, illustrates two horizontal lines one for Ms temperature and other for Mf temperature (martensite finish temperature), both starting from y-axis, i.e., with no incubation period (Fig. 3.46). The horizontal line, Ms, simply indicates the temperature below which a certain amount of martensite forms instantaneously almost.
The martensitic reaction in steels normally occurs athermally, i.e. during cooling in the temperature range between Ms and Mf, i.e. first crystals of martensite start forming at Ms temperature and, for more martensite to be formed further, the steel must be cooled continuously within Ms – Mf Arrange, i.e., the amount of transformation is a function of temperature only (see Fig. 3.47), until the reaction ceases at Mf temperature. At this temperature, all the austenite should have transformed to martensite, but frequently, in practice, a small proportion of austenite may not transform.
Actually the transformation is very sluggish near Mf, which can also be inferred from large gap between M 99% to Mf. This untransformed austenite is called retained austenite. Apart from the chemical composition of steel, Mf temperature is dependent on the cooling rate of the steel (Fig. 3.47). The sluggish transformation also does not allow determination of Mf temperature with great accuracy. Mf temperature is normally indicated as dotted line in ‘S’ curves of steels.
If cooling process is stopped in between Ms – Mf and the steel is kept at a constant temperature in this range, the formation of martensite ceases almost instantly, and no more transformation occurs with time at this temperature. It is an important characteristic of martensitic transformation. It can lead to large amount of retained austenite depending on the temperature of holding the steel in between Ms – Mf.
The plate of martensite forms at a high speed (≈ 1 km/sec) even at sub-zero temperatures in medium and high carbon steels. As the temperature decreases in Ms – Mf range, the amount of martensite increases due to the instantaneous formation of new plates and not by the growth of already existing plates. This is also unique characteristic of martensitic transformation.
The effect of carbon on Ms and Mf is illustrated in Fig. 3.48. Around 1 wt. % of carbon lowers Mf by over 300°C. The important aspect is that Mf temperature becomes below even 0°C, when carbon content of steel becomes higher than 0.7%, and thus, higher carbon steels quenched in water may contain substantial amounts of retained austenite. Steels containing 1.2 to 1.4% carbon may contain as high as 30-40% retained austenite. Fig. 3.49 illustrates the % volume of retained austenite with weight % carbon in steel.
5. Crystal lattice of martensite has a definite crystallographic orientation relationship to the lattice of austenite in which it forms.
6. Stabilisation of Austenite:
It is the reduction in the amount of transformation of austenite to martensite due to the processes, which interfere with the formation of martensite. The term stabilisation normally means the reduction in austenite transformation, if cooling of a steel is arrested in the Ms– Mf range.
The transformation, when resumed by lowering the temperature, does not start immediately at lower temperature but only after austenite is undercooled to a certain temperature, Ms‘ (Fig. 3.50) and, the transformation does not result in as complete a transformation to martensite as would have been the case if no isothermal pause has occurred, i.e., the amount of martensite thus formed, may often be less and amount of untransformed austenite is more and this increased untransformed austenite is called stablished austenite (‘x. is the amount of stabilised austenite Fig. 3.50).
At an arrest temperature, the degree of stabilisation with time is a bit complex, but in general, can be said to increase to a maximum with time. The degree of stabilisation increases as the Tarr (arrest temperature) approaches Mf, i.e., the stabilisation is at a minimum when only a small amount of martensite is present in the matrix.
The phenomenon of stabilisation also occurs even when the austenite is cooled slowly during Ms – Mf range, i.e. the amount of stabilised austenite may be more in an oil-quenched sample than in the same steel if water-quenched.
Stabilisation of austenite is a complicated process. Stabilisation process is generally seen in iron-base alloys having interstitial elements like carbon and nitrogen, and thus, segregation of carbon may be playing on important role in the process. As martensite-plate forms, the surrounding austenite matrix has to accommodate the accompanying plastic deformation (Fig. 3.45 b), where thus, the density of dislocations becomes high. Any rest, between Ms – Mf range, or slow cooling through this range, provides time for plastic relaxation.
During this, interaction of dislocations in austenite with the glissile dislocations in the martensite-plate boundary can associate to make them sessile dislocations (to be no longer mobile) and thus, the plate cannot grow further, or the carbon atoms may segregate at the dislocations in austenite, which increases the shear resistance of austenite, i.e. the locking of at least interfacial dislocations by carbon atoms causes the stabilisation of austenite.
7. Effect of Stress:
Martensitic transformation is affected very significantly by the applied stress. If plastic deformation is done between Ms and Mf, the amount of martensite formed is increased relative to that formed without plastic deformation. The effect of stressing above Ms temperature is normally to raise M, temperature, i.e., martensite forms above the so called Ms temperature. Hadfield manganese steel which is austenitic at room temperature, transforms to martensite whenever deformation takes place at room temperature (this steel has low-staking-fault energy. Deformation produces stacking-faults, where martensite nucleates).
The reason is that martensite formation above Ms temperature normally does not occur as there is a large strain energy-barrier associated with the formation of martensite, i.e. large strain energy has to be supplied for its formation. The applied stress provides this necessary strain energy, and thus, martensite can form even above Ms temperature. The martensite which forms only by applying elastic strain from outside is stress-assisted martensite, which can nucleate at the same place of austenite if it had transformed below Ms.
The martensite which forms by applying plastic strain from outside is called strain-induced martensite and this nucleates in sites prepared by plastic deformation. It is possible to define a temperature Md, higher than Ms above which deformation of the austenite does not form any martensite.
It is likely that deformation of austenite above Md, then lowers the Ms. The resultant increased stability of austenite is called mechanical stabilisation. It is also seen that slight preliminary deformation usually activates the martensitic transformation on subsequent cooling, whereas, the heavy deformation may have inhibiting effect due to large distortion of initial phase.
3. Crystal Structure of Martensite:
A very significant aspect of the austenite to martensite transformation is the very large difference in the solid solubility of carbon is γ-iron (up to 2.11%) and in α-iron (0.02% max. which at 20°C is 0.00005%). The transformation on rapid cooling of FCC-austenite to room temperature, when the diffusion of carbon is suppressed, and carbon atoms are trapped in the octahedral sites of the body-centred cubic structure, results in BCC, or BCT ferrite, i.e., martensite.
The body centred tetragonal (BCT) structure is a distorted form of BCC iron. The extent of tetragonality depends on the amount of carbon in solid solution. The tetragonality is due to the severe distortion produced by the carbon atoms in highly supersaturated BCC unit cell.
The lattice parameters are:
Austenite, a0 = 3.548 + 0.044 x (% C) …(3.33)
Martensite, c = 2.861 + 0.116 x (% C) …(3.34)
a = 2.861 – 0.013 x (% C)…(3.35)
and are affected by carbon content of the steel as illustrated in Fig. 3.51.
The tetragonality measured by the ratio between the axes, a/c increases with the carbon content as:
i.e., at 0% C, the c/a = 1, i.e., the crystal structure is BCC. Actually, it has been seen that martensite is BCC up to carbon 0.2% in steel. The carbon atoms up to this carbon content get segregated to dislocations and are not present in the octahedral holes of BCC-iron.
The FCC-austenite lattice expands uniformly as the carbon content in it increases (Fig. 3.51 a), because the carbon atoms sit in octahedral sites of regular octahedrons. A FCC lattice has symmetrical octahedral sites, in which the site is at equidistance from all its six neighbouring atoms of iron. When a carbon atom sits there, it pushes all its six iron neighbours equally to result in uniform expansion.
The expansion of BCC-iron lattice due to the addition of carbon atoms is non-symmetrical and this causes the tetragonal distortion of BCC lattice to body centred tetragonal. As illustrated in Fig. 3.52 (a), the octahedral sites in BCC are unsymmetrical.
The interstitial carbon atom when present in z position (Fig. 3.52 a) makes the octahedral site symmetrical by pushing each of the two nearest Fe-atoms along z-axis (here c-axis) by 0.053 nm (in one of the <100> directions) as illustrated in Fig. 3.52 (b). There takes place unidirectional distortion in the c-axis but, BCC lattice changes into body centred tetragonal lattice of martensite.
Let us examine the probable reason of carbon occupying the octahedral sites only along Z-axis (c-axis). This correspondence between the lattices of austenite and martensite was first suggested by Bain by ‘Bain distortion’ as illustrated in (Fig. 3.53). A tetragonal unit cell could be outlined within two unit cells of austenite (Fig. 3.53 a).
To change this unit cell (Fig. 3.53 b) into a BCT martensite cell, a deformation (called Bain strain) is essential, which requires a contraction of about 17% along the [001]γ, corresponding to the c-axis of the BCT martensite cell, and a uniform expansion of about 12% in the (001)γ.
This correspondence though does not imply a mechanism for this phase transformation, nor does it predict a habit plane, or the orientation relationship, but it helps to understand the crystal geometry of martensitic transformation in steel.
The octahedral holes are marked with crosses in FCC (γ) lattice (Fig. 3.53 a). Carbon atoms may be assumed to be present at places marked as crosses in FCC lattice in this diagram.
As austenite to martensite transformation is a diffusion less transformation, there is no opportunity for the carbon atoms to move, so that interstitial sites already occupied in austenite by the carbon atoms remain occupied by carbon atoms in the changed martensite lattice.
In the carved out tetragonal lattice, they are located only at the middle of edges along the [001] axis and in the centre of the horizontal faces of the cell (Fig. 3.53 b). As the carbon atoms are positioned between the iron atoms parallel to [001] axis, face-centred austenite becomes after transformation to BCT martensite.
Thus, the tetragonolity of martensite in steels is due to two main reasons, one is due to super saturated interstitial solution of carbon atoms in the BCC lattice and second, due to the preference for a particular type of octahedral sites (along (001) direction) imposed by the diffusion less character of the transformation.
4. Crystallography of Martensitic Transformation in Steels:
As the martensitic transformation has extremely high growth rate, even at low temperatures, thermally activated atomic movements cannot take place. The atoms must move in a co-ordinated manner, and the interface between the austenite and martensite must be coherent, or at the most semi-coherent.
The martensitic transformation may be visualised as regular rearrangement of the lattice in which atoms merely displace, relative to one another, over distances not exceeding the inter-atomic distances.
In martensitic transformation, the crystal lattice of martensite has a definite orientation relative to the lattice of original austenite.
For iron alloys, three main orientational relationships are known:
Kurdjumov-Sachs’ orientational relationship (valid for carbon steels having carbon 0.5 to 1.4%) is given by-
This is due to the fact that close-packed planes in FCC {111} are similar to close packed plane in BCC, {110}. Also, the close packed directions in FCC <110> are similar to close packed directions in BCC, <111>. Since, in FCC, there are four planes of {111} type and six <110> directions, thus, there may be 24 orientations of the martensitic crystals for a single position of an austenite grain.
Nishiyami’s orientational relationship (For alloys of iron with 27 – 34% Ni)
Greninger-Troiano relationship (Fe – 0.8% C) is an intermediate case between the two relationship discussed above.
To obtain Kurdjumov-Sach’s relationship, more complex paths of atomic displacement are necessary than those possible by the Bain distortion. These can be obtained only if the Bain distortion is accompanied by the rotation of the martensite crystals. The actual paths of atomic displacements in martensitic transformation are not known.
5. Thermodynamics of Martensitic Transformation in Steels:
Martensite is a metastable phase formed under non-equilibrium conditions, where martensite has same composition as of austenite from which it formed. In Fe-Fe3C diagram, a boundary-T0, may be defined where martensite (α’) and austenite (γ) phases of the same composition are in equilibrium (at T0, both phases have same energy), i.e., above T0, super-cooled austenite and below T0, martensite are metastable phases as illustrated in Fig. 3.54, i.e., below T0, martensitic transformation is thermodynamically possible. Ms is also illustrated as a function of carbon of the steel.
Ms is more or less parallel to T0 line, i.e., the temperature difference between T0 and Ms is approximately 200 to 250°C for all content of carbon in the steels. The austenite to martensite transformation, in carbon steels, begins after this large amount of undercooling, i.e. this transformation requires a definite large amount of thermodynamic driving force (Fig. 3.55). Even the Ms – Mf range is large. This is because in steels, the volume change associated with the austenite → martensite change is large to result in large shape change.
The strain energy arising due to martensite plate formation (also the interfacial energy between austenite and martensite) acts as a barrier to nucleation of martensite till austenite is undercooled to large extent, so that the change in the chemical free energy between the phases can balance the strain energy as well as the surface energy needed.
The classical theory of homogeneous nucleation, when applied, requires an impossible value of critical free energy change. Thus, nucleation of martensite takes place heterogeneously on pre-existing embryos. In some cases, like Hadfield Mn steel, the stacking fault acting as embryo, has been identified. Recently it has been suggested that most likely sites for nuclei are grain boundaries, incoherent twin boundaries and surface of inclusions.
The growth rate of plate of martensite is very high (forms in 10 seconds) but IS found to be constant over wide temperature range, meaning thereby that growth is not a thermally activated process. Growth probably, occurs by the movement of an array of parallel dislocations (of same burgers vector) in the interface on appropriate slip planes along-with interface. The habit plane, thus, has to move in a direction perpendicular to itself.
6. Reversibility of Martensitic Transformation in Steels:
The Fig. 3.55 illustrates that martensite-to-austenite diffusion less transformation may take place if rapidly heated to a temperature higher than T0 with a definite amount of superheating, to obtain a definite driving force for this transformation.
This reverse transformation on quick heating starts at the As temperature (analogous to Ms temperature). The reversibility has been seen in Fe-Ni, Al-Cu systems, Ti-alloys. The reverse transformation has all the characteristic features of the martensitic transformations such as surface relief, As and Af temperatures, Ad temperature, etc.
The Fe-Fe3C system is, however, an exception and is not reversible in this sense, since before the martensite can be reverted back to austenite, or rather before As temperature is attained, the tempering reaction sets in due to the high (interstitial) diffusivity of carbon in the supersaturated body centred tetragonal martensite.