Thursday, April 8, 2010

Martensite Formation

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.

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 Ms 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 Ms is reached, further transformation takes place during cooling until the reaction ceases at the Mf temperature. Larger volume fractions of austenite are retained in some highly alloyed steels, where the Mf temperature is well below room temperature.
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.

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.

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. 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.

Thus it might be expected that those factors, which raise the yield stress of the austenite, and martensite, will increase the likelihood of twinning. The important variables are: 1. carbon concentration; 2. alloying element concentration; 3. temperature of transformation; 4. strain rate. The yield stress of both austenite and martensite increases with carbon content, so it would normally be expected that twinning would, therefore, be encouraged. Likewise, an increase in the substitutional solute concentration raises the strength and should also increase the incidence of twinning, even in the absence of carbon, which would account for the twins observed in martensite in high concentration binary alloys such as Fe-32%Ni. A decrease in transformation temperature, i.e. reduction in Ms, 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 Ms. 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 Ms 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 Ms is below room temperature leading to twinned plates which might then grow further on resting at room temperature.

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