A.S. Hamada

Deformation Mechanisms in High-Al Bearing High-Mn TWIP Steels in Hot Compression and in Tension at Low Temperatures

The hot deformation behaviour of two high-Mn (23-24 wt-%) TWIP steels containing 6 and 8 wt-% Al with the fully austenitic and duplex microstructures, respectively, has been investigated at temperatures of 900-1100°C. In addition, tensile properties were determined over the temperature range from -80 to 100°C. It was observed that in spite of the lower Al content, the austenitic steel possessed the hot deformation resistance about twice as high as that of the duplex steel. Whereas the flow stress curves of the austenitic steel exhibited work hardening followed by slight softening due to dynamic recrystallisation, the duplex steel showed the absence of work hardening and discontinuous yielding under similar conditions. Tensile tests at low temperatures revealed that the austenitic grade had a lower yield strength than that of the duplex grade, but much better ductility, the elongation increasing with decreasing temperature, contrary to that for the duplex steel. This can be attributed to the intense mechanical twinning in the austenitic steel, while in the duplex steel, twinning occurred in the ferrite only and the austenite showed dislocation glide.

Structure and microstructure evolution during martensitic transformation in wrought Fe–26Mn–0.14C austenitic steel: an effect of cooling rate

Structure and microstructure evolution under various cooling rates of a wrought austenitic steel, Fe-26Mn-0.14C (composition in mass %), were studied by the Rietveld method of X-ray diffraction pattern fitting, grain boundary characterization by electron back-scattered diffraction (EBSD) and optical microscopy. Cooling rate, density of stacking faults, and austenite grain size and grain boundaries influence the observed gamma(fcc) --> epsilon(hcp) transformation and lead to significant anisotropic X-ray line broadening. Depending on the cooling conditions, the grain boundaries are misoriented at both lower and higher angles. In the epsilon-martensites, the dominant planar fault is twins (similar to 10(-3)). The austenite grains were found to contain low to moderate density of stacking faults (similar to 10(-4)-10(-3)), which act as efficient nucleation sites of the epsilon-martensites. Both X-ray and EBSD analyses estimated negligible twins in the austenite. Approximate average dislocation densities have been estimated and correlated with the grain structure.

Deformation and Damage Mechanisms in Ultrafine-Grained Austenitic Stainless Steel During Cyclic Straining

The ultrafine-grained (UFG) structure of an austenitic stainless steel (Type 301LN), processed by controlled phase-reversion annealing, was fatigued to study the deformation and damage mechanisms during cyclic straining. Fatigue cracking along the grain boundaries and the formation of extended persistent slip band-like shear bands (SBs) were observed to be the fatigue-induced microstructural features in the ultrafine-grained structure. Characterization of SBs was performed by electron backscattered diffraction and atomic force microscopy to study the fine features.

Study on Cyclic Strain Localization and Fatigue Fracture Mechanism in High Manganese Twinning-Induced Plasticity Steels

The fatigue cracking mechanisms of two high Mn TWinning-Induced Plasticity (TWIP) steels are investigated in detail using electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Furthermore, the fracture surfaces of the fatigued steels have been studied by employing a field emission gun scanning electron microscope (FEG-SEM). The fine details of the fatigued surface topography are verified using an atomic force microscope (AFM). The results indicate that the fatigue crack embryos nucleate at an early stage of the fatigue life as a result of local straining at grain and annealing twin boundaries at sites, where persistent slip bands create dislocation piled-ups that impinge on boundaries. The EBSD measurements showed that unlike in monotonic straining, the formation of deformation twins is not observed under cyclic straining.

Application of Physical Simulation for Developing New Steel Types

Processing of a large number of novel steel types, such as DP, TRIP, CP and TWIP, and high-strength low-carbon bainitic and martensitic DQ-T steels, have been developed based on physical simulation and modelling studies. Among stainless steels, guidelines for processing of ultra-fine grained austenitic stainless steels have been created. Physical simulation has been used by employing a Gleeble thermo-mechanical simulator to reveal the phenomena occurring in the hot rolling stage (the flow resistance, recrystallization kinetics and microstructure evolution), and in the cooling stage (CCT diagrams) for carbon steels and in short-term annealing of cold rolled metastable austenitic steels. Connecting these data with microstructures examined in optical and electron microscopes and resultant mechanical properties have improved the understanding on complex phenomena occurring in the processing of these steels and the role of numerous process variables in the optimization of enhanced mechanical properties.