Mechanical properties of high manganese steels

 

The main focus points of the project C2 are (i) the characterization of the mechanical properties of High Mn-Steels (HMnS), (ii) the development and application of new testing methods and (iii) the simulation of mechanical properties.

The mechanical properties of austenitic HMnS in the tensile test are dependent of strain rate and testing temperature. The excellent combination of ductility and strength over a wide array of testing temperatures and strain rates are a result of the high work-hardening rate. Due to their good combination of material properties, HMnS portray great potential in terms of energy absorption that offer interesting potentials in the automotive industry.

The high work-hardening rate is a result of a systematic utilization of the TRIP- (TRansformation Induced Plasticity) and TWIP-effect (TWinning Induced Plasticity). The deformation mechanism is primarily a result of the stacking fault energy (SFE), a parameter that is heavily influenced by factors such as alloying contents and temperature. Because of the influence of temperature, high Mn steels show different active primary deformation mechanisms at different testing temperatures.

Another important material characteristic is the serrated flow curve that is as a result of local deformation behavior. In order to be able to characterize the localized deformation behavior, local strain measurements and infrared-thermography have been coupled. The combination of these techniques enables the quantification of local deformation temperatures and strain rates.

This dynamic strain aging-like effect is usually correlated to the blocking of dislocation movement due to crystallographic short-range ordering (SRO) of Mn, Al and C atoms. It is assumed that a destruction of SRO via dislocation cutting after a short time of hindered dislocation movement and dislocation pile-ups can be reversed via short-range jumps of carbon atoms from and to different vacant interstitial sites. The resulting succession of increase and drop in strength leads to the serrated flow of the stress-strain curve.

For HMnS with very high SFE, a pronounced planar dislocation glide can be observed. This effect usually is attributed to a glide plane softening as a result of a destruction of SRO. In some cases, the formation of parallel shear bands and dislocation cells during deformation could be found. Those steels previously were labeled as MBIP-steels (Microband Induced Plasticity).