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The effect of the substitution of silicon by aluminium on the mechanical properties and the microstructure af cold rolled C-Mn-Si TRI P steels was investigated for different continuous annealing cycles,The mechanical properties were evaluated using tensile testing, It wasseen that the Al alloyed steel had very good mechanical properties with an improved formability comparedto the conventional C-Mn-Si TRIP steel. The strain hardening behaviour was studied in detail. All the investigated specimens showed a very high n value but their strain dependence was different.For the conventional C-Mn-Si TRIP steel the maximum n value was reached at low strain, while the Al substituted TRIP steel showed a gradual increase of the n value. The latter effect resulted in a larger uniform elongation for the C-Mn-Al-Si TRIP steel.Investigation of the microstructure using color etching and scanning electron mic!oscopy revealed that the Al alloyed steel composition contained a larger amount of bainite with a finer structure than the C-M n-Si TRIP steel, Furthermore, XRDmeasurementsshowed that the Al evolution of the M~temperature corresponds to an isothermal holding at 400'C, which results in a phase distribution as shown in Fig. 4on

The thermo-mechanical properties of low stacking fault energy austenitic Fe18Mn0.6C steel exhibiting twinning-induced plasticity were investigated during uniaxial tensile deformation using infrared thermography. Over a wide strain range, the plastic deformation was by the movement of very few well-defined localized deformation bands. The formation and propagation of Portevin-LeChatelier (PLC) bands lead to type A and type B serrated stress-strain curves, exhibiting a negative strain rate sensitivity. The PLC band properties were analyzed in detail: strain, strain rate and mobile dislocation density within the bands were determined. The microstructures of the un-deformed and deformed Fe18Mn0.6C TWIP steel were studied by transmission electron microscopy. The possible dynamic strain aging processes causing the localized deformation are reviewed.KEY WORDS: TWIP steel; thermo-mechanical properties; dynamic strain aging.loyed steel. 12) In TWIP steel, twinning may interact with the ordering, as the homogeneous twinning shear also alters the degree of short range order as it results in a different pair correlation and interstitials in different positions. This mechanism is referred to as pseudo-twinning. [13][14][15] It leads to a meta-stable structure which may transform to a true twin structure as a result of thermal activation.The flow curve of high Mn TWIP steels containing interstitial carbon are often characterized by serrations, the Portevin-LeChatelier (PLC) phenomenon, and a negative strain rate sensitivity. Both result from a microscopic Dynamic Strain Aging (DSA) process. DSA results in an increase in flow stress and strain hardening, but a decrease in post-uniform elongation and a reduction of area at fracture. When both DSA and a negative strain rate sensitivity occur simultaneously, Portevin-LeChatelier (PLC) bands will be observed, and the stress-strain curve will have characteristic serrations. 16)DSA occurs when an aging process, related to solute atoms, is fast enough to occur during deformation, i.e. when the dislocation velocity is similar to the solute mobility, as dislocations move by means of thermally activated jumps, with a characteristic waiting time between two jumps. As pointed out by Cuddy et al. 17) no long range diffusion is required and the solutes involved in the aging process may actually remain immobile in the lattice. Dastur et al.6) have suggested C-Mn pairs or point defect clusters which reorient themselves in the stress field at the core of the dislocations are able to pin the dislocation strongly. It may actually be the C-Mn octahedral clusters rather than C-Mn pairs, and the resulting short range ordering effect, which causes dislocation pinning.The DSA phenomenon is not limited to high Mn TWIP steel, as DSA has also been observed in 316L austenitic stainless steel for which it was shown that plastic deformation changed form wavy slip to a pronounced planar slip during DSA.15) Kibey et al. 8) have pointed out that both C and N affect the dislocation structure and the deform...

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