Low stacking fault energy face centered cubic (FCC) materials contain characteristic defect structures. Stacking fault tetrahedral are one of those rare structures that occur under special experimental conditions. For the first time, stacking fault tetrahedral were observed in Fe-30Mn-3Al-3Si twinning induced plasticity (TWIP) STEEL. Their presence resulted from a quenching heat treatment.

Advanced high strength STEELs (AHSS) have recently attracted great attention because of their superior mechanical properties. A modern group of these STEELs, known as twinning induced plasticity (TWIP) STEELs, shows a unique combination of strength and ductility even at high rates of strain. In order to examine the functionality of such STEELs in dynamic loading conditions, their mechanical behavior should be characterized by high strain rate experiments. Regarding the precision of its results and characteristics of the loading pulse, the torsional Kolsky bar experiment has well-known advantages over similar high rate experiments. In this study, a high strain rate torsional testing machine was designed and constructed. By using high strain rate torsional experiments in the range of strain rates from 500 to 1700 /s, mechanical behavior of two high manganese STEELs with different levels of carbon (0.49 and 0.07 wt%) was surveyed. Through the experiments, it was observed that the phenomenon of adiabatic temperature rise considerably decreased the work hardening rate during the dynamic testing of the high-carbon STEEL, whereas the low-carbon STEEL showed a definitely higher hardening rate. The influence of pre-straining on the dynamic mechanical behavior of this low-carbon STEEL was also studied.

TWIP STEELs are a family of high Mn austenitic STEELs posses both high strength and high ductility among aotumotive body STEELs. In the present paper, the effect of addition of Mo on improving of mechanical properties of a TWIP STEEL (Fe-33Mn-3Si-2Al) is investigated. To do this, different amount of Mo were added to the chemical composition of STEEL and the resulted mechanical properties, microstructure and crystallographic phases were examined after casting, hot rolling and annealing. The results showed that addition of Mo enhances the mechanical properties; however, the optimum strength was obtained by addition of 0.3% Mo. This resulted to a 50% increase to ultimate strength and 40% decrease in grain size.

In the present study, the influence of annealing temperature on mechanical properties and the microstructure of a high manganese austenitic STEEL (Fe-30Mn-4Al-4Si-0.5C) was investigated. X-ray diffractometry, optical and scanning electron microscopy, hardness and tensile tests were used to analyze the relationship between mechanical properties and microstructure after annealing process. The results indicated an optimum combination of strength and elongation under the condition of high cold rolling reduction with subsequent annealing in the partial recrystallization region.

TWinning Induced Plasticity (TWIP) STEELs, with high manganese percent (17-35%), are used in automotive bodies. Their microstructure at room temperature is austenitic and due to low stacking fault energy, major deformation mechanism in them is twinning inside austenite grains which, on the other hand, leads to enhance mechanical strength in STEEL. In order to improve the mechanical properties, by adding carbide forming elements to STEEL, their strength has been increased. To this end, the castings of STEEL, microstructure analysis by optical and scanning electron microscopy (SEM) were studied. The results showed that the cold-rolling this STEEL, will increase the strength and ductility to great exgant.

In recent years, TRIP/TWIP STEELs have been the focus of great attention thanks due to their excellent tensile strength-ductility combination. The compression tests were performed at different temperatures from 25 to 1000oC to study the mechanical behavior of advanced austenitic STEEL with 21% manganese plus bearing Ti. The results indicated that the plastic deformation is controlled by deformation-induced martensite and mechanical twinning from 25 to 100oC. However, at temperatures 200 to 1000°C the deformation twinning was merely observed. The occurrence of mechanical twinning at such high temperatures is a first-time observation in high manganese austenitic STEELs. Such mechanical twins led to grain refinement via grain partitioning.

Twinning Induced Plasticity (TWIP) STEELs, with high manganese percentage (17-35%), are used in automotive bodies. Their microstructure at room temperature is austenitic and due to low stacking fault energy, major deformation mechanism in them is twinning inside austenite grains which on the other hand, leads to enhanced mechanical strength in STEEL. Due to important effect of heat treatment process on mechanical properties, full annealing heat treatment and effect of Molybdenum were investigated in this study. To this aim, the STEEL was casted and hot rolled and then heat treated at different temperatures and the obtained microstructures were analyzed using optical, SEM and TEM microscopes. The results showed that the full annealing temperature for STEEL without Mo was determined to be 1100oC. and addition Mo to the STEEL annealing Temperature by 50oC reduced.

Selection of the proper material in manufacturing lightweight railway vehicles is a complex and essential challenge considering the available engineering materials. In this paper, the appropriate material has been selected for manufacturing railway wagons using a multi-criteria decision-making technique. Six engineering materials including DP600 STEEL, TRIP700 STEEL, TWIP STEEL, 6005-T6 aluminium, 60826-T6 aluminium and porous aluminium with closed cells were considered as engineering alternatives. Also, density, yield strength, ultimate tensile strength, the ratio of yield strength to maximum tensile strength, Young's modulus, cost and corrosion resistance were considered as selection criteria. To select the best material according to the mentioned criteria, the MOORA multi-criteria decision-making method was applied. By weighting the criteria and performing the analysis, it was found that the TWIP STEEL and 6082-T6 aluminium were chosen as the best candidates and aluminium foam and DP600 STEEL were selected as the worst candidates. Due to its maximum yield strength and ultimate tensile strength compared to other candidates, TWIP STEEL was chosen as the best material considering all criteria. Furthermore, the 6082-T6 material was chosen as the second-best material due to the high ratio of yield strength to ultimate tensile strength (0.88) and lower density and higher corrosion resistance than STEEL materials.