Why is martensite metastable

Franke and C. Wittkamp and E. Hornbogen, Prakt. Hornbogen, Acta Metall. Reed, Acta Metall. Singh, J. Luther and T. Williams, Mater. Maxwell, A. Goldberg, and J. Shyne, Metall. Bhandarkar, V. Zackay, and E. Parker, Metall. Download references. You can also search for this author in PubMed Google Scholar. Reprints and Permissions. Khan, Z. Stress-induced martensitic transformation in metastable austenitic stainless steels: Effect on fatigue crack growth rate.

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Sorry, a shareable link is not currently available for this article. The recovery of the dislocation structure and the migration of dislocation-cell and martensite boundaries leads not only to a coarsening of the plates, but also an increase in the crystallographic misorientation between adjacent plates, as illustrated in the adjacent figure.

The data are from Suresh et al. However, all of these carbides require the long-range diffusion of substitutional atoms. They can only precipitate when the combination of time and temperature is sufficient to allow this diffusion.

The figure on the left shows the calculated diffusion distance in ferrite for a tempering time of 1 h. It is evident that the precipitation of alloy carbides is impossible below about o C for a typical tempering time of 1 h; the diffusion distance is then just perceptible at about 10 nm. The alloy carbides grow at the expense of the less stable cementite. If the concentration of strong carbide forming elements such as Mo, Cr, Ti, V, Nb is large then all of the carbon can be accommodated in the alloy carbide, thereby completely eliminating the cementite.

The bright field transmission electron micrograph is of a sample tempered for h, whereas the dark-field image shows a sample tempered for h. The precipitates are needles of Mo 2 C particles. This transmission electron micrograph shows large cementite particles and a recovered dislocation substructure. There are sub-grain boundaries due to polygonisation and otherwise clean ferrite almost free from dislocations.

The plate microstructure is coarsened but nevertheless retained because the carbides are located at plate boundaries. An alloy such as this, containing a large fraction of carbides is extremely resistant to tempering. The original microstructure was bainitic, but similar results would be expected for martensite.

The optical micrograph shows some very large spherodised cementite particles. The ferrite has completely recrystallised into equiaxed grains. Whereas the plain carbon steel shows a monotonic decrease in hardness as a function of tempering temperature, molybdenum in this case leads to an increase in hardness once there is sufficient atomic mobility to precipitate Mo 2 C. Secondary hardening is usually identified with the tempering of martensite in steels containing strong carbide forming elements like Cr, V, Mo and Nb.

The formation of these alloy carbides necessitates the long--range diffusion of substitutional atoms and their precipitation is consequently sluggish. Carbides like cementite therefore have a kinetic advantage even though they may be metastable. Tempering at first causes a decrease in hardness as cementite precipitates at the expense of carbon in solid solution, but the hardness begins to increase again as the alloy carbides form. Hence the term secondary hardening.

Coarsening eventually causes a decrease in hardness at high tempering temperatures or long times, so that the net hardness versus time curve shows a secondary hardening peak. Typical time scales associated with the variety of processes that occur during tempering. The actual rates depend on the alloy composition.

Elements such as silicon and aluminium have a very low solubility in cementite. They greatly retard the precipitation of cemenite, thus allowing transition iron-carbides to persist to longer times. AerMet is a martensitic steel which is used in the secondary-hardened condition; its typical chemical composition is as follows:. The cobalt plays a key role in retarding the recovery of martensite during tempering, thereby retaining the defect structure on which M 2 C needles can precipitate as a fine dispersion.

By increasing the stability of body-centred cubic iron, it also reduces the tendency of martensite to revert to austenite during tempering. The carbon concentration is balanced such that all the cementite is replaced by the much finer alloy carbides during secondary hardening. Impurity concentrations and inclusions are kept to a minimum by vacuum induction melting and vacuum arc refining.

Unlike conventional steels, the manganese and silicon concentrations are also kept close to zero because both of these elements reduce the austenite grain boundary cohesion. The as-received steel is usually "homogenised" at o C for 8 hours.

This is because the cast and forged alloy contains banding due to chemical segregation. The sample is then tempered in the range o C, depending on the properties required. Since the Ae 1 temperature is about o C, thin films of nickel-rich austenite grow during tempering.

The films are apparently beneficial to the mechanical properties. The optimum combination of strength and toughness is obtained by tempering at o C. The as-quenched steel has a martensitic microstructure with a few undissolved MC nm and M 23 C 6 -type carbides nm. It has been suggested that the toughness in this state can be further improved by refining the M 23 C 6 particle size; since the steel is not used in the as-quenched condition, the significance of this result is in emphasising the need for cleanliness.

Any inclusions must clearly be smaller than the M 23 C 6 particle size-range. An increase in the tempering temperature to o C leads to the coherent precipitation of needle--shaped molybdenum--rich zones, and a peak in the strength; the precipitation occurs at the expense of the cementite particles, so the increase in strength is also accompanied by a large increase in toughness. The formation of austenite films may also contribute to the toughness. Further tempering leads to the precipitation of M 2 C carbides, recovery of the dislocation substructure, and a greater quantity of less stable reverted-austenite.

The austenite that forms at higher temperatures has a lower nickel concentration and its instability is believed to be responsible for the decrease in toughness beyond about o C tempering, in spite of the decrease in strength. The following are pictures of the landing gears for the Airbus Industrie A and A passenger aircraft. This is the largest landing gear assembly in commercial service, presumably to be superceded by the A The critical components are made from tempered martensite.

Creep resistant steels must perform over long periods of time in severe environments. They are therefore required to resist both creep and oxidation.

Their microstructures must clearly be stable in both the wrought and welded states. To resist thermal fatigue, the steel must have a small thermal expansion coefficient and an high thermal conductivity; ferritic steels are much better than austenitic steels with respect to both of these criteria. The conditions described above correspond to low strain rates and relatively low temperatures. The mechanism of creep then involves the glide of slip dislocations.

Diffusion-assisted dislocation climb in necessary for continued deformation when the glide process is obstructed, for example by the presence of precipitates in the glide plane. Nano Hybrids and Composites.

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Bioscience and Medicine. Civil Engineering. Information Technologies. Industrial Engineering. Environmental Engineering. Foundations of Materials Science and Engineering. Specialized Collections. Retrospective Collection. Home Metastable Austenite. Papers by Keyword: Metastable Austenite. Paper Title Page. Authors: L. Malinov, I. Malysheva, Eduard S. Klimov, Volodymyr V. Kukhar, E. It was confirmed that an increase in the manganese reduces the abrasive wear resistance and increases the impact-abrasive wear resistance.

The expediency of carburization of low-carbon manganese steels is shown in order to obtain the residual austenite in the structure which amount and stability must be optimized in relation to specific abrasive impact characterized by the dynamic ratio with taking into account the chemical composition. Authors: N. Ozerets, Valentina A. Sharapova, A. Levina, T.