Deformation Mechanisms in Austenitic TRIP/TWIP Steel as a Function of Temperature
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AUSTENITIC stainless steels are widely used in engineering applications because of their excellent formability in combination with high strength and corrosion resistance. In high-alloy austenitic steels, the addition of chromium, nickel, and manganese stabilizes the hightemperature austenitic phase of iron down to room temperature. Depending on the concentration of the alloying elements, the as-produced microstructure consists mainly of fcc—c-austenite, although the bcc ferrite or a¢-martensite and hcp—e-martensite may occur.[1] Furthermore, the austenite often remains in a metastable state and transforms martensitically under mechanical loading into the bcc or hcp state. Consequently, the mechanical properties are altered tremendously if a phase transformation occurs during plastic deformation. During the c fi a¢ or the c fi e fi a¢ martensitic transformation, the transformation-induced plasticity (TRIP) effect is triggered, yielding higher elongation during tensile testing.[2] By the formation of new interfaces STEFAN MARTIN, Postdoc, and DAVID RAFAJA, Full Professor, are with the Institute of Materials Science, TU Bergakademie Freiberg, 09599 Freiberg, Germany. Contact e-mail: stefan.martin@ iww.tu-freiberg.de STEFFEN WOLF, formerly Postdoc, ULRICH MARTIN, Professor Emeritus, and LUTZ KRU¨GER, Full Professor, are with the Institute of Materials Engineering, TU Bergakademie Freiberg, 09599 Freiberg, Germany. Manuscript submitted August 8, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
through the phase transformation, a pronounced workhardening effect occurs.[3] The thermodynamic driving force for the a¢-martensite formation is related to the difference of the Gibbs free energy between austenite and a¢-martensite (DGcfia¢). Since the austenite represents the high-temperature phase, which is artificially stabilized by alloying, the driving force increases with decreasing temperature. For different Ni concentrations, DGcfia¢ is plotted as a function of temperature in Figure 1(a).[4] The graphs exhibit a linear form in which the alloy with the highest Ni concentration shows a negative DGcfia¢ below approx. 313 K (40 C). All other alloys with a lower Ni content exhibit a higher driving force for the a¢-martensite formation. Furthermore, the martensitic transformation can be enhanced with the aid of mechanical stresses acting under plastic deformation, although the level of DGcfia¢ is not high enough to compensate for the interfacial and strain energy during a¢-martensite nucleation at the specific testing temperature.[5] The value of the Md temperature indicates the temperature at which enough activation energy is provided by the deformation—in addition to the chemical driving force—to trigger the martensitic transformation. Through the addition of the alloying elements, the difference between the Gibbs energy (DGcfie) of the fcc crystal lattice and the hcp crystal lattice becomes rather low as well.[6,7] This implies that the formation of stacking faults (SFs) in the fcc austenite lattice, which
(a)
(b)
where n is the number
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