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ε-martensite

ε-martensite
Definition:ε-martensite is formed by quenching or plastic deformation of austenite and exhibits the hexagonal close-packed (hcp) crystal structure.
Explanation:Martensite is named after its inventor, German scientist Adolf Martens, who investigated the high hardness of martensitic steels during the late 19th century. Typically, α’-martensite with a body centered tetragonal structure is observed in steels. In steels with higher amount of alloying elements, such as Mn, ε-martensite is detected. The structural difference between ε-martensite and austenite is illustrated in the figure below. Crystal lattice of austenite is face-centred cubic (→fcc) which can be perceived as an ABCABC stacked solid. By producing a stacking fault (→SF) induced through rotating the C plane (transforming it into a B plane), an ABAB sequence (hcp) is obtained, which is the crystal structure of ε-martensite. If the stacking fault energy (→SFE) is large, twinning (→TWIP effect) occurs in high manganese steels. On the other hand, if the SFE is small, ε-martensite forms upon mechanical loading (→TRIP effect). This is strongly affected by alloying. The martensitic transformation is a well-known example of displacive transformation, a type of phase change in which atoms move short distances in unison rather than diffusing individually over longer distances.

ε-martensite often represents an intermediate step in the formation of α’-martensite. Depending on the crystal structure, heat treatment, mechanical loading, among the other intrinsic and extrinsic parameters, martensite exhibits lath-like or plate-like microstructure or a combination thereof. This gives rise to exceptionally large hardness values upon martensite formation in steels. Thus, martensite containing steels are used e.g. in automotive industry, for musical instrument strings, etc.
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Figure /
Diagram:

Stacking sequences in (a) austenite (ABCABC...) and (b) ε-martensite (ABABAB...).
SFB-Link:High manganese steels exhibit high strength and outstanding plasticity due to twinning or martensite formation.
References:http://daten.didaktikchemie.uni-bayreuth.de
G. Frommeyer, U. Brüx, and P. Neumann, ISIJ Int. 43, 438 (2003)
Y. Tomota, Y. Morioka, and W. Nakagawara, Acta Mater. 46, 1419 (1998)