Influence of the nitrogen/nickel-ratio cyclic behaviour of materials with TWIP and TRIP effects

Sedat Gueler, Universität Duisburg-Essen, ITM, Lehrstuhl für Werkstofftechnik, Duisburg, Germany
Alfons Fischer, Universität Duisburg-Essen, ITM, Lehrstuhl für Werkstofftechnik, Duisburg, Germany

The growing significance of increasing toughness and ductility for metallic materials promotes the development of materials with TWIP and TRIP effects. These materials were used in many different applications due to both effects with the automotive industry as one of the most important branches. Steels with TWIP and TRIP effects were used for crash absorber and safety parts. These types of steel strengthen and protect the vehicle interior during an accident. Since the early 2000´s the number of patent applications increased for TWIP and TRIP materials.

Low stacking fault energy is the main reason for TWIP and TRIP effects. Materials with SFE range from 20 to 60 mJ/m2 show mechanical twinning. If the SFE is less than 20 mJ/m2, the TRIP effect can be expected. One option to decrease the stacking fault energy in steels is to substitute nickel by manganese. Cobalt base alloys have naturally low SFE. The TWIP and TRIP effects do not lead to cyclic strengthening. For example, CoCrMo-alloys are typical materials with TRIP effect. These alloys show a phase transformation from austenite into ε martensite during cyclic loading while cyclic softening prevails.

In this study, the fatigue behaviour of six TRIP and seven TWIP materials were investigated. After total strain controlled uniaxial fatigue tests at room temperature, the cyclic stress-strain hysteresis were analysed by means of the Ramberg-Osgood method. Thus, the hystereses were subdivided into elastic and plastic strain amplitude. The resulting cyclic stress-strain diagram in combination with the monotonic stress-strain curve provides information on cyclic softening or strengthening for such materials. From these, one can derive that the TRIP effect my lead to cyclic softening or strengthening depending on the type of martensite that the austenite transforms into: If α’ martensite is most abundant, cyclic strengthening is found while for ε martensite cyclic softening dominates. In contrast to this, TWIP materials show softening and strengthening de-pending on the total strain amplitude and the N/Ni-ratio: For very low N/Ni-ratios, cyclic strengthening can be detected at small total strain amplitudes while for higher N/Ni-ratios strengthening begins at bigger total strain amplitudes. After mechanical testing the samples were investigated by means of SEM -EBSD in order to analyse the local distribution or concentration of cyclic plastics strains.

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