Localized Deformation due to Portevin–LeChatelier Effect in 18Mn–0.6C TWIP Austenitic Steel

Abstract

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...

Microstructure evolution and strengthening mechanisms of Fe–23Mn–0.3C–1.5Al TWIP steel during cold rolling

“…It should be emphasized that such high dislocation densities can be attained in conventional metallic materials by severe plastic deformation to large total strains, which are not attainable in tensile tests. Thus, the extensive mechanical twinning greatly facilitates the storage of lattice dislocations [38]. This role of twinning is key to providing the extraordinary combination of strength and ductility.…”

Deformation Twinning Behavior of Twinning-induced Plasticity Steels with Different Carbon Concentrations – Part 2: Proposal of Dynamic-strain-aging-assisted Deformation Twinning

“…Serrations in Fe-Mn-C austenitic steels are widely accepted to arise from DSA. [17][18][19]33,34) Additionally, the circumstantial evidence mentioned below indicates that the deformation twinning and martensitic transformation did not cause the serrations. The serrations in the present steels were not caused by deformation twinning because the serrations disappeared upon decreasing the deformation temperature from ambient temperature to 223 K (Fig.…”

Ti̇cari̇ Twip900 Ve Dp600 Yüksek Mukavemetli̇ Çeli̇kleri̇n Otomoti̇v Endüstri̇si̇ndeki̇ Performanslarinin Karşilaştirilmasi

“…Son yıllarda yapılan araştırmalar neticesinde gecikmeli çatlama sorunu çelik içerisine alüminyum ilave edilerek çözülmüştür [10]. Literatürdeki birçok çalışmada TWIP çeliklerinin yüksek (n >0,4) bir pekleşme katsayısına sahip olduğu belirtilmektedir [6,[11][12][13][14][15]. Çeliğin tipik akma eğrisi incelendiğinde çeliğin davranışını en iyi temsil eden pekleşme modelinin Swift modeli olduğu görülmüştür [12].…”