The Formation of Martensite

Rapid quenching of austenite to room temperature often results in the formation of martensite, a very hard structure in which the carbon, formerly in solid solution in the austenite, remains in solution in the new phase. Unlike ferrite or pearlite, martensite forms by a sudden shear process in the austenite lattice which is not normally accompanied by atomic diffusion.

Ideally, the martensite reaction is a diffusionless shear transformation, highly crystallographic in character, which leads to a characteristic lath or lenticular microstructure. The martensite reaction in steels is the best known of a large group of transformations in alloys in which the transformation occurs by shear without change in chemical composition. The generic name of martensitic transformation describes all such reactions.

It should however be mentioned that there is a large number of transformations which possess the geometric and crystallographic features of martensitic transformations, but which also involve diffusion. Consequently, the broader term of shear transformation is perhaps best used to describe the whole range of possible transformations.

The martensite reaction in steels normally occurs athermally, i.e. during cooling in a temperature range which can be precisely defined for a particular steel. The reaction begins at a martensitic start temperature M_s which can vary over a wide temperature range from as high as 500°C to well below room temperature, depending on the concentration of γ-stabilizing alloying elements in the steel.

Once the M_s is reached, further transformation takes place during cooling until the reaction ceases at the M_f temperature. At this temperature all the austenite should have transformed to martensite but frequently, in practice, a small proportion of the austenite does not transform. Larger volume fractions of austenite are retained in some highly alloyed steels, where the M_f temperature is well below room temperature.

To obtain the martensitic reaction it is usually necessary for the steel to be rapidly cooled, so that the metastable austenite reaches M_s. The rate of cooling must be sufficient to suppress the higher temperature diffusion-controlled ferrite and pearlite reactions, as well as other intermediate reactions such as the formation of bainite. The critical rate of cooling required is very sensitive to the alloying elements present in the steel and, in general, will be lower the higher total alloy concentration.

Each grain of austenite transforms by the sudden formation of thin plates or laths of martensite of striking crystallographic character. The laths have a well-defined habit plane and they normally occur on several variants of this plane within each grain. The habit plane is not constant, but changes as the carbon content is increased.

Martensite is a supersaturated solid solution of carbon in iron which has a body-centred tetragonal structure, a distorted form of bcc iron. It is interesting to note that carbon in interstitial solid solution expands the fcc iron lattice uniformly, but with bcc iron the expansion is nonsymmetrical giving rise to tetragonal distortion.

Analysis of the distortion produced by carbon atoms in the several types of site available in the fcc and bcc lattices, has shown that in the fcc structure the distortion is completely symmetrical, whereas in the bcc one, interstitial atoms in z positions will give rise to much greater expansion of iron-iron atom distances than in the x and y positions.

Assuming that the fcc-bcc tetragonal transformation occurs in a diffusionless way, there will be no opportunity for carbon atoms to move, so those interstitial sites already occupied by carbon will be favored. Since only the z sites are common to both the fcc and bcc lattices, on transformation there are more carbon atoms at these sites causing the z-axis to expand, and the non-regular the martensite, as well as the shape deformation for a number of martensitic transformations including ferrous martensites. It is, however, necessary to have accurate data, so that the habit planes of individual martensite plates can be directly associated with a specific orientation relationship of the plate with the adjacent matrix.

Martensitic planes in steel are frequently not parallel-sided; instead they are often perpendicular as a result of constraints in the matrix, which oppose the shape change resulting from the transformation. This is one of the reasons why it is difficult to identify precisely habit planes in ferrous martensite. However, it is not responsible for the irrational planes, but rather the scatter obtained in experiments.

Another feature of higher carbon martensites is the burst phenomenon, in which one martensite plate nucleates a sequence of plates presumably as a result of stress concentrations set up when the first plate reaches an obstruction such as a grain boundary or another martensite plate.

Perhaps the most striking advances in the structure of ferrous martensites occurred when thin foil electron microscopy was first used on this problem. The two modes of plastic deformation are needed for the in-homogeneous deformation part of the transformation, i.e. slip and twinning. All ferrous martensites show very high dislocation densities of the order of 10¹¹ to 10¹²cm², which are similar to those of very heavily cold-worked alloys. Thus it is usually impossible to analyze systematically the planes on which the dislocations occur or determine their Burgers vectors.

The lower carbon (<0.5% C) martensites on the whole exhibit only dislocations. At higher carbon levels very fine twins (5-10 nm wide) commonly occur. In favorable circumstances the twins can be observed in the optical microscope, but the electron microscope allows the precise identification of twins by the use of the selected area electron diffraction technique. Thus the twin shears can be analyzed precisely and have provided good evidence for the correctness of the crystallographic theories discussed above. However, twinning is not always fully developed and even within one plate some areas are often untwined. The phenomenon is sensitive to composition.

The evidence suggests that deformation by dislocations and by twinning are alternative methods by which the lattice invariant deformation occurs. From general knowledge of the two deformation processes, the critical resolved shear stress for twinning is always much higher than that for slip on the usual slip plane. This applies to numerous alloys of different crystal structure.

A decrease in transformation temperature, i.e. reduction in M_s, should also help the formation of twins, and one would particularly expect this in alloys transformed, for example, well below room temperature.

It should also be noted that carbon concentration and alloying element concentration should assist by lowering M_s. As martensite forms over a range of temperatures, it might be expected in some steels that the first formed plates would be free of twins whereas the plates formed nearer to M_s would more likely be twinned.

However, often plates have a mid-rib along which twinning occurs, the outer regions of the plate being twin-free. This could possibly take place when the M_s is below room temperature leading to twinned plates which might then grow further on resting at room temperature.

Lath Martensite

Medium Carbon Martensite

These plates first start to form in steels with about 0.5% carbon, and can be concurrent with lath martensite in the range 0.5 %-l.0 % carbon. Unlike the laths, the lenticular plates form in isolation rather than in packets, on planes approximating to {225} and on several variants within one small region of a grain, with the result that the structure is very complex.

The burst phenomenon probably plays an important part in propagating the transformation, and the austenite is thus not as uniformly or as efficiently eliminated as with lath martensites. This physical difference cannot be unconnected with the fact that higher percentages of retained austenite occur as the carbon level is increased, and the martensite is predominantly lenticular. The micro twinning referred to earlier is found predominantly in this type of martensite, which forms at lower M_s temperatures, as the carbon content increases.

High Carbon Martensite

Detailed crystallographic analysis shows that this type of martensite obeys more closely the theoretical predictions than the {225} martensite. The plates are formed by the burst mechanism and often an audible click is obtained.

The {259} martensite only forms at very high carbon levels in plain carbon steels, although the addition of metallic alloying elements causes it to occur at much lower carbon contents, and in the extreme case in a carbon-free alloy such as Fe-Ni when the nickel content exceeds about 29 wt%.

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