Martensite: Morphology and Modes | Steel | Metallurgy

In this article we will discuss about the morphology and modes of martensite.

Morphology of Martensite:

Martensitic transformation takes place by a combination of two shears-one of which is homogeneous lattice (shape) deformation (called pure strain). In steels, it may be taken as Bain strain. It is assumed to occur to achieve the new lattice of martensite from austenite.

The second shear is the inhomogeneous lattice deformation (lattice invariant deformation), which makes the interface to be undistorted. The lattice invariant deformation is as a result of dislocation movement, which could be in the form of deformation by slip, or, by twinning.

Any of these two modes, occurring on atomic scale, allows the interface between martensite and austenite to remain planar and invariant during deformation, at least macroscopically. Fig. 3.56 (a) and (b) illustrate schematic representation of slip, or twinning occurring within martensite plates. The surface has a tilt and, the habit plane is macroscopically planar interface, unrotated and undistorted, although there are irregularities on microscopic finer scale.

The lattice invariant deformation can occur either by slip, or by twinning, depending on the factors, such as, composition of austenite (carbon as well as other substitutional alloying elements), temperature of transformation, and strain rate.

The critical resolved shear stress for twinning is almost always higher than for slip. Also, any factor which strengthens the austenite raises the yield stress of austenite and martensite, increasing thereby the probability of twinning deformation than by the slip. Now, the increase of carbon of steel increases the yield strength both of austenite and martensite, and thereby, the twinning is more likely to take place. Alloying elements too, increase the strength and thus, should favour twinning.

Fig. 3.57 illustrates that the critical resolved shear stress for twinning is not much affected by the decrease of the temperature of transformation, but the resistance to slip increases rapidly, and is higher from a definite temperature, T₁, than twinning. Increasing carbon and the alloying elements also lower M_s temperature to help in the formation of twins.

As the martensite forms over a range of temperatures, M_s – M_f, it is possible that, in some steels, the first formed martensite (in higher temperature ranges) is free of twins, but forms by slip, whereas the plates formed later near M_f would be twinned.

Quite often, the plates have a mid-rib Fig. 3.45 along-which twinning occurs, the outer regions of the plate being twin-free, but with slip dislocations. This is possible, when the M_s is below room temperature. In some systems like Fe-Cr, Fe- Mo, Fe-W, the M_s temperature is so high that twinning does not occur at all. High strain rates also favour twinning.

The term ‘morphology’ in martensitic transformation, defines the shape of a martensite particle.

In steels, two distinct morphologies of martensite are seen:

1. Lath Martensite

2. Plate Martensite

A lath has the shape of a strip, the thickness of which being of the smallest dimension of about 0.1 – 0.2 µm (barely resolvable under optical microscope), the breadth of intermediate dimension of ≈ 1 – 2 µm, and the length, being of largest dimension, is limited by the grain boundary of austenite (Fig. 3.58 a), but is about 30-40 µm for medium grain size. Laths are grouped together in packets in a parallel fashion with low angle boundaries between them. A minority of laths is separated by high angle boundaries.

Individual laths are too fine to be resolved under optical microscope, and any retained austenite present is too fine to be resolved. The units of a packet being non-parallel to adjacent groups, several packets can form within a austenite grain. The laths do not cross the austenite grain boundaries. Laths have a network of tangled dislocations with high density of around 10¹⁵-10¹⁶ m^-2 as in a heavily cold worked metal. It is presumed that high dislocation density is caused by the slip between stacks of laths to produce lattice-invariant deformation.

Lath martensite forms in low and medium carbon steels with low content of alloying elements, i.e. basically when the temperature of transformations (M_s) is high. The packets of laths are delineated because of different etching characteristics of different orientations of laths in the various packets. Lath martensites have over whelming industrial importance as most hardenable steels have them in hardened state. Some heat treatment application, like carburising, does have plate martensites.

The well-known classical type of martensite-the plate martensite-which in a micro-section at low magnification give a false picture of needle-like-shape-also called acicular martensite-has the shape like a disc, plate, or biconvex lens-thus commonly called lenticular martensite.

The shape is similar to shape of mechanical twins as it corresponds to minimum of energy of elastic distortions when forms in austenite. Martensite plates in steel are frequently not parallel sided, instead they are often lenticular (Fig. 3.58 b). This is due to the constraints in the matrix which oppose the shape change resulting from the transformation.

Individual neighbouring plates of martensite in an austenite grain are non-parallel to one another. The first formed plate usually extend up to the austenite grain boundaries (except in very large grains), and thus, divide the grain into portions. The plates that form subsequently are limited in size by the volume of the portion of austenite in which they can form (Fig. 3.58).

Even with several sub-partitioning of the austenite, the grain does not get completely filled with martensite, retaining some retained austenite. The transformation continues by the nucleation of new plates rather than by the growth of existing plates. A cluster of plates may form more or less simultaneously in a grain.

At high magnifications, the plates are seen to be composed of stacks of very fine twins. This means, twinning is the shear mode to obtain the lattice invariant shear. In some cases, plates are partly twinned around midrib, the out rim of the plate contains density of slip dislocations around 10⁹ – 10¹⁰ cm^-2, where slip is the lattice invariant shear. The austenite- martensite interface is not always smooth. This is so in cases where there is partial twinning.

The martensite plates that form at very low temperatures tend to be thin and fully twinned with smooth boundaries. Plate martensite forms in steels which, have low M_s and also, which have higher carbon content as well as alloying elements. It has been seen that lower transformation temperature, i.e., lower M_s is mainly the controlling factor to decide between the lath and plate type of martensites. Hardened high carbon steels have plate form of martensites (Fig. 3.59).

Based on habit planes, steels show three types of martensitic structure:

1. Low Carbon Martensite:

For steels with carbon up to 0.5%, Habit plane {111} Lath martensite.

2. Medium Carbon Martensite:

For steels with carbon 0.5% to 1.4%, Habit plane {225}. Steels with carbon content between 0.5% to 1.0% have mixed martensite-lath as well as plate martensites-a complex structure. Acicular name given to this martensite is basically referring the individual lenticular plates, which may show micro-twinning in steels particularly having higher carbon, and lower M_s temperature.

3. High Carbon Martensite:

It is found in steels having carbon more than 1.4% and show a habit plane of {259}. The morphology is lenticular plates, which form individually and are heavily twinned. These plates are formed by the burst mechanism. The martensite can also form if alloy additions are large even with lower carbon, or in carbon free alloys (Fe-with more than 30% Ni).

Micro cracks may be present in martensite plates of different habit plane variants, Micro cracks may form in largest martensite plates in large austenite grain due to large strain associated with them. These may be absent if plate size is fine, i.e., austenitic grain size is fine, or in steels with lath martensite. Thus, high carbon plate martensite is quite brittle and sensitive to micro cracking.

Modes of Martensite:

Martensite forms by three different modes:

1. Athermal (without thermal activation)

2. Burst

3. Isothermal (thermally activated diffusion-controlled)

1. Athermal Martensite:

We have so for described characteristics of athermal type of martensite, as it is the most common in carbon, and alloy steels with M_s temperature normally above 100°C.

Athermal reaction starts at M_s and the amount of martensite increases with decreasing temperature below M_s (Fig. 3.61 a), i.e., the amount of martensite is a function of only the temperature to which the alloy is cooled, and is accomplished virtually instantly as soon as the temperature is reached, should the cooling be stopped at that temperature, no further transformation to martensite occurs.

Additional transformation occurs only by formation of new plates of martensite and is accomplished only by cooling to lower temperature. This kinetics of martensite formation is the dominant mode of transformation in heat treatable carbon steels in industrial practice-as steels are quenched to room temperature normally, and this kinetics proceeds above room temperature.

The fraction of a thermal martensite formed is given by:

f = 1 -exp (-1.10 x 10^-2 ΔT) …(3.37)

where, f is the fraction of martensite, and Δ T is the degree of undercooling below M_s temperature. It has been seen that except for first few percent of transformation below M_s, when the martensite formed exceeds 60-70%, the martensite fraction varies linearly with temperature (Fig. 3.61 a).

2. Burst Kinetics (Jump Like Kinetics):

During athermal martensite formation in Fe-Ni and Fe-Ni-C alloys with subzero M_s temperatures, the burst phenomenon occurs. On reaching a characteristic temperature, M_B which is usually below subzero, martensite forms abruptly in the form of burst (Fig. 3.61 c).

It is related to the ability of plates of martensite to nucleate other plates of martensite, a process called auto catalysis. The stimulus to new nucleation is the stress, generated at the tips of the plates, that helps to initiate the shear transformation process on other favourably oriented habit planes.

Commonly, Zig-zag arrays of plates are observed to form in bursting. All the plates in a ‘burst’ form in a very small fraction of a second and is often, accompanied by an ‘audible click’.

The burst formation may cause considerable adiabatic heating of the steel (sometimes 30°C), which may inhibit further transformation. Subsequent to burst, the mode of transformation can be either athermal, or, isothermal. The amount of martensite formed in a burst varies from a few percent to even 70% of austenite.

3. Isothermal Kinetics:

In some alloys like Fe-Ni-Mn and Fe-Ni-Cr, transformation to martensite at sub-zero temperatures takes place isothermally i.e., as a function of time at a constant temperature (Fig. 3.61 b). It is seen that martensite reaction starts slowly, then accelerates due to autocatalysis, and finally decays. The transformation kinetics show a typical ‘C’-curve as a function of temperature, say for an alloy (Fig. 3.62) the nose of the curve is at -140°C.

The presence of induction period helps to suppress the transformation by quick cooling to say 78°K, but again transforms on heating isothermally. Isothermal martensitic transformation does not go to complete transformation (Fig. 3.61 b). The amount of such martensite may vary from fraction of a percent to even 50% depending on temperature of holding, and composition.

In contrast to this fully isothermal change, some alloys transform isothermally, only after some martensite has formed on cooling (athermally).