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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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.
G. Frommeyer, U. Brüx, and P. Neumann, ISIJ Int. 43, 438 (2003)
Y. Tomota, Y. Morioka, and W. Nakagawara, Acta Mater. 46, 1419 (1998)