Verformungsinduzierte Martensitbildung in metastabilen austenitischen Stählen

Abstract

Die Menge an verformungsinduziertem a -Martensit hangt bei mctastabilen austenitischcn Stiihlen von der Stapelfehlerenergie ab. Liegt diese oberhalb von etwa 100 erg/cm2, so findet eine direkte y -+ a-Umwandlung statt, die wegen des komplizierten Schermechanismus bei der Umgitterung durch iiuRere Spannungen bzw. Deformationen quantitativ nicht oder nur geringfugig beeinflufit w-ird. In dem Mafie, wie durch Zugabe bestimmter The volume of strain induced a-martensite in metastable steels depends on the stacking fault energy. If its value makes more than -100 ergs/cm*, a direct y -a transformation occurs, this being not or little quantitatively influenced because of the complicated shearing mechanism of lattice change by external stresses or strains. Tho tendency to form extended stacking faults or hexagonal e-martensite increases to the ciegrcc as by adding of certain alloying elements, as e. g. Cr + Ni, Cr + Mn or Mn + C, the stacking fault energy is lowered. The indirect y -E -a or y -SF -a-transformation occurs, where &-martensite and stacking faults only gradually differ from eachother. Because of the simple shearing mechanism of the y / & and y/SF transformation external stresses are strongly favouring the indirect transformation. Especially in crossing regions o f &-plates or stacking faults, but also in their internal parts, shearings are generated which are prestages or nuclei of a-crystals. If the SFE is below -20 ergs/cm2 a complete strain induced a-martensite formation becomes possible, if this is in accordance with the thermodynamic stability relations.

Deformation Mechanisms in Austenitic TRIP/TWIP Steel as a Function of Temperature

“…[22,39] The hcp lattice provides a suitable nucleation environment for a¢-martensite, as only a single crystallographic shear operation is necessary for the phase transformation into the bcc lattice instead of the two-step shear operation from the fcc austenite. [24] Inside the deformation bands, many small a¢-martensite nuclei are formed and not only at crossings of deformation bands, as assumed by Olson and Cohen [40] and Sinclair and Hoagland. [41] These a¢-martensite crystallites grow until they reach the interface of other nuclei, forming larger transformation areas.…”

“…By the local shearing of the austenitic matrix by the SFs, the two-step transformation process from fcc to bcc is simplified to a single shear operation, lowering the activation energy of the martensitic phase transformation. [24] By means of plastic deformation, the nucleation sites for the martensitic phase transformation are produced. Through the influence of the low stacking fault energy, the c fi e fi a¢ transformation is triggered.…”

“…This is easily explained by considering the involved crystallographic shear operations described in detail by Schumann. [24] Both, the direct c fi a¢ and the indirect c fi e fi a¢ transformations start with shearing of the austenite by 1/6a h112i (a is the lattice parameter of the austenite). Through the formation of the hexagonal stacking sequence, this initial shear sequence is automatically produced by the dislocation movement at low SFE, and only the second shear operation needs to be initiated by elastic strains.…”

“…Through the formation of the hexagonal stacking sequence, this initial shear sequence is automatically produced by the dislocation movement at low SFE, and only the second shear operation needs to be initiated by elastic strains. [24] (a) (b) (c) Fig. 1-Thermodynamic driving forces influencing the deformation behavior of high-alloy austenitic steels as a function of temperature: (a) difference of Gibbs energy for the martensitic transformations from austenite to a¢-martensite for binary iron nickel alloys (data from Ref.…”

Bildung von α‐Martensit im Schnittbereich von ϵ‐Martensitplatten

“…Die My+'"-Temperatur von Fe-Mn-, Fe-Mn-Cr-, Fe-Ni-Cr-und anderen Eisenlegierungen wird mit steigendem Legierungsgehalt zunachst stetig erniedrigt, bis bei einer charakteristischen Legierungskonzentration der Abfall verlangsamt oder sogar verhindert wird (SCHUMANN 1975 Figur 1 a -c zeigt die erhaltenen Gefugeausbildungen in verschiedenen Anstenitkornern. Die angeschnittene Flache liegt jeweils nahe bei (OOl),, erkennbar an den quadratischen Atzgriibchen.…”