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Properties of high alloyed CrMnNi steels with strain induced α′ martensite formation

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It is well known that austenitic chromium-nickel or chromium-manganese- nickel steels with low stacking fault energy show a stress and strain induced formation of martensite during mechanical loading. The transformation induced plasticity (TRIP) effect occurs in metastable austenitic or austenitic-martensitic steels for wrought and also cast alloys [1]. The strain induced formation of martensite is controlled as well by its chemical composition as by stresses [2], which are higher than the yield stress of the initial microstructure. They activate shear processes accompanied by deformations. For each type of strain induced martensite formation a triggering stress exists which is controlled by the temperature and the chemical composition of the steel. This can be obtained from suitable experiments [3÷5]. Generally three mechanisms of the martensite formation, the γ→ε, ε→α′ and γ→α′ transformation, are discussed in Cr-Ni steels. During the γ→ε transformation the ε phase is observed as thin straight lines in the microstructure of the austenite [6]. The γ→ε and ε→α′ transformation can proceed simultaneously. The result is the γ→ε→α′ sequence [3÷4]. During this process the ε martensite is an instable intermediate phase and usually cannot be detected. In Cr-Mn-Ni steels the ε martensite is an intensively discussed subject. It is open either the ε martensite is a unique phase or a pseudo-phase caused by the concentration of stacking faults in the direction of load. The reasons for these plastic deformations are the commonly observed phenomena as shearing strain, twinning and different slip processes in the initial phase. Thus, some of these mechanisms are linked to special movements of dislocations. The strain induced formation of martensite generates the TRIP effect. It is [...]

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