Martensite

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martensite

[′mär‚ten‚zīt]
(metallurgy)
A metastable transitional structure formed by a shear process during a phase transformation, characterized by an acicular or needlelike pattern; in carbon steel it is a hard, supersaturated solid solution of carbon in a body-centered tetragonal lattice of iron.

Martensite

 

the structure of crystalline solids arising as the result of a slip, diffusionless polymorphic transformation on cooling. It is named for the German specialist in physical metallurgy A. Martens (1850-1914).

As a result of a lattice deformation, known as cooperative displacement, during the transformation, a relief appears on the surface of the metal. Internal strain arises in the bulk and plastic deformation occurs, which limit crystal growth. The rate of growth reaches 103 m/sec and is independent of temperature. Hence, the rate of martensite formation is usually limited by crystal nucleation. The reaction of the internal strain shifts crystal nucleation greatly below the thermodynamic phase equilibrium point and may stop the transformation at constant temperatures. Thus, the amount of martensite formed usually increases with an increase in supercooling.

Since the elastic energy must be minimal, martensite crystals take the shape of plates (needles in cross section) regularly oriented relative to the initial lattice. The internal strain is also reduced by plastic deformation and thus the crystal contains many dislocations (up to 1012 cm-2) or is split into twins with a thickness of 10-100 nanometers (100-1000 angstroms). Intragranular boundaries and dislocations strengthen martensite.

Martensite is a typical product of low-temperature polymorphic transformations in pure metals such as iron, cobalt, titanium, zirconium, and lithium; in solid solutions based on these metals; and in intermetallic compounds such as CuZn, Cu3Al, NiTi, V3Si, and AuCd.

Martensite in steel is a supersaturated Fe-C solution obtained by hardening from austenite. The ordered position of carbon atoms that results from martensite displacement transforms the body-centered lattice of a-iron from cubic to tetragonal. The distortion of the lattice near the interstitial atoms results in hardness. The tetragonality and the hardness increase with the carbon content. Hardness increases up to 1,000 HV (Vickers hardness).

Carbon martensite is the major structural component of most high-strength steels. The carbon content in the solid solution and the martensite subgranular structure change upon tempering, which is used for increasing the ductility of steel.

Carbon is the most important factor of martensite strength in steel. The strength of noncarbon maraging steel results from the separation of intermetallic compounds upon aging.

The physical characteristics of Fe-C martensite as an interstitial solid solution, the source of its high strength, and the kinetics of martensite formation were established by G. V. Kurdiumov.

M. A. SHTREMEL’

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