By invitation only: overview articleTwinning-induced plasticity (TWIP) steels
Introduction
Some iron alloys and steels have impressive plasticity-enhancing potential, which is not yet fully exploited in engineering applications, as it requires a thorough understanding of the underlying mechanisms and their activation during straining. This necessitates an approach to steel design incorporating a selection of composition, microstructure, and processing parameters based on sound theoretical principles. Most formable ferritic steels exhibit uniform engineering elongation less than 25% and relatively low ultimate tensile strength (≪1 GPa). The formability of these steels is based on the control of their crystallographic texture, rather than the strain hardening. As a consequence, higher strength is usually achieved at the cost of ductility. There is a way to resolve this conflict of properties, though: by designing fully austenitic steels or austenite-containing multi-phase steels with an enhanced strain hardening rate, both high strength and good formability can be achieved. Twinning-induced plasticity, or TWIP, steels belong to this category of ferrous alloys. They are characterized by a high strain hardening, large uniform elongation and high ultimate tensile strength levels. These properties make them candidate lightweighting materials for large scale use in the automotive industry, LNG-shipbuilding, oil-and-gas exploration and non-magnetic structural applications. The rapid progress made in the fundamental understanding of the mechanical behavior of TWIP steels is a result of a tremendous global research effort. It is a convincing illustration of what materials research can achieve through consequent use of computational thermodynamics and first principle calculations. The experimental analysis of the properties of TWIP steels has also profited from the use of advanced techniques for microstructural characterization of materials, such as synchrotron X-ray diffraction, electron backscattering diffraction, 3D atom probe tomography, and micromechancial testing methods (nano-hardness, micro-pillar testing). Finally, a more sophisticated analysis of the results of standard macroscopic mechanical tests involving the strain rate and temperature dependence of the mechanical properties has contributed to a better understanding of the mechanisms underlying strength and plasticity of TWIP steels. In this article, a historical overview and an assessment of the current state of our understanding of the mechanical properties of TWIP steels are given.
The ground breaking contributions of Grässel et al. [1], [2], [3], [4] and Frommeyer et al. [5] to the science of Fe-Mn-Si-Al TWIP steels with a very low carbon content (<100 ppm) prompted a global research effort aimed at developing a better understanding of the plasticity-enhancing mechanisms which are activated during the plastic deformation of these steels and similar C-alloyed high Mn face-centered cubic (fcc) γ-Fe. Grässel and Frommeyer observed that large scale deformation twinning occurred in a TWIP steel when its Mn content was larger than 25%, the Al content was in excess of 3%, and the Si content was in the range of 2–3% (by weight). A very favorable strength-ductility balance was attained. An important quantity that characterizes this balance is the product of the ultimate tensile strength and the total elongation (UTS × TE value, or Rm × A value, in European publications). A very large value of this quantity in excess of 50,000 MPa × %, resulting mainly from an extremely large total tensile elongation of 80%, was reported.
Contemporary TWIP steels exhibit high ultimate tensile stresses paired with exceptionally large tensile elongations (60%) over a wide range of strain rates (10−3-103 s−1). The importance of TWIP steels as breakthrough structural materials cannot be underestimated as the TWIP steel concept has also opened up the possibility of creating new steel grades with a very wide range of properties. These developments have already been addressed in the literature [6], [7], [8]. The present review aims at critically evaluating our current understanding of TWIP steels and accounting for the most recent developments in this area. The review focusses on the strain hardening behavior of TWIP steels, as a better understanding of many aspects of this fundamental property has evolved in the recent years, which makes it necessary to review its defining role in the strength and plasticity enhancement in TWIP steels. Other aspects of the mechanical performance of TWIP steels are also discussed in relation to their strain hardening behavior.
Section snippets
The taxonomy of strain hardening stages
Fig. 1, Fig. 2 illustrate an exceptional strength-ductility combination of TWIP steels, as exemplified by 18%Mn-0.6%C-1.5%Al TWIP steel, by comparing its properties with those of Ti-stabilized interstitial-free (Ti-IF) ferritic steel. The latter is considered as one of the most formable industrial steel grades. The formability of these steels is controlled by their crystallographic texture, their grain size and the presence of precipitates. The development of a pronounced ND//〈111〉 fiber
Intrinsic vs. extrinsic stacking faults
There are relatively few publications which have addressed the important experimental verification of the character of stacking faults occurring in TWIP steels. Despite the fact that the intrinsic and extrinsic stacking faults of fcc metals and alloys are believed to be very similar [87] extrinsic stacking faults are not often observed in practice, and a stacking fault in fcc metals and alloys is usually assumed to be intrinsic. Vercammen et al. [88] were the first to report that the stacking
Mechanical properties of TWIP steel
The yield strength of TWIP steels is relatively low, and a large strain is needed to see the strength benefits associated with strain hardening. This implies that additional strengthening mechanisms have to be introduced to attain a well-balanced property profile. An obvious option is pre-straining, but it results in lower tensile elongation under subsequent loading. Alternative strengthening mechanisms involve grain refinement and precipitation hardening by micro-alloying.
Twinning mechanisms in TWIP steels
As deformation twinning is at the core of the mechanical properties of TWIP steels, a detailed exposé on this subject will be given in this section.
Dynamic strain aging and strain rate sensitivity in carbon-alloyed TWIP steel
Plasticity controlled by thermally activated dislocation motion discussed in Section 4.4 is normally associated with a positive strain rate sensitivity of the flow stress defined as The strain rate sensitivity parameter, , is related to the activation volume of the underlying mechanism governing dislocation glide, which may be associated with thermally activated overcoming of dislocation forest junctions, stationary solute atoms or their clusters, or the Peierls
Recovery annealing
Recovery annealing is an unusual microstructure optimization process, consisting in a partial recrystallization annealing treatment in the temperature range of 575–625 °C. It is effective for the production of TWIP steels with a wide range of strength-ductility combinations. The retention of the deformation twins in cold deformed TWIP steel during recovery annealing is utilized to obtain a microstructure that provides a high yield strength and an appreciable tensile elongation. Through a
TWIP effect during cyclic deformation
The fatigue performance of TWIP steels is generally considered to be excellent, and superior to the fatigue performance of austenitic stainless steels such as AISI 301 and 316 [320]. Three specific characteristics of TWIP steel, viz. a low magnitude of γisf, which inhibits cross slip, a pronounced glide planarity, and the occurrence of deformation twinning, are known to influence the fatigue properties in both high cycle and low cycle fatigue of fcc alloys in general. A low γisf generally
High strain rate behavior of TWIP steels
High strain rate properties are of considerable technical interest, especially for automotive applications for passenger safety-related parts where high impact energy absorption is at a premium. Fig. 62 illustrates the results of axial crush tests carried out on various advanced high strength steel grades. The TWIP steel evidently stands out as the material with a superior crashworthiness.
Using a split Hopkinson pressure bar, Ha et al. studied the microstructure of Fe-22%Mn-0.4%C TWIP steel
Fracture of TWIP steels
Due to their low yield strength and large ductility, the failure mode of TWIP steel is usually ductile. The negative strain rate sensitivity makes diffuse necking strain small, and fast fracture is observed once necking is initiated. Although not much work has been done in this area, fracture of TWIP steel has been given considerable attention in connection to three phenomena (a) edge fracture in hole expansion, (b) liquid metal-induced embrittlement and (c) the hydrogen-related delayed
Fracture in hole expansion
Cut edge stretchability is an important aspect of sheet formability. Whereas the press forming performance of TWIP steels is excellent due to its high strain hardening capability, the lower normal anisotropy, represented by the r-value (r0° = 0.79, r45° = 1.13, r90° = 1.28), and the negative strain rate sensitivity of the flow stress of TWIP steels are detrimental to stretchability. Collectively, these factors result in premature ductile fracture during stretch-flanging (i.e. hole expansion)
Hydrogen-delayed fracture
Hydrogen has been reported to reduce the stacking fault energy of austenitic steels [332], [333]. This effect promotes the deformation-induced ε-martensitic transformation [334], [335] and deformation twinning [336], [337], resulting in a marked change in stress-strain response. When captured by a dislocation in an fcc metal or alloy, a hydrogen atom affects the dislocation mobility by decreasing the activation barrier to dislocation motion. At temperatures slightly below room temperature, H
Liquid metal-induced embrittlement
Liquid metal induced embrittlement (LMIE) is an undesired phenomenon that occurs due to transgranular or intergranular decohesion of a metal or an alloy by rapid penetration of another liquid metal or alloy into its microstructure. The penetration is usually along grain boundaries in a polycrystalline material, or along sub-grain boundaries in a single crystal. When tensile stresses are present, a liquid film appears to cause grain boundary decohesion, as the boundary within a solid is replaced
Hot ductility
Numerous production difficulties have been reported for casting, hot rolling, and cold rolling of high Mn steels. Mn loss occurs due to a high Mn vapour pressure reducing the Mn yield during steelmaking. The casting microstructure of TWIP steels is coarse with well-developed dendrites and coarse equiaxed grains. This is due to the wide solidification range and the low thermal conductivity of TWIP steels. According to Scott et al. [363], the solidification range of Fe-22%Mn-0.6%C is 120 °C, from
Conclusions
The understanding of the mechanical properties of TWIP steels has greatly improved in the recent years, yet some fundamental questions still remain unanswered. There is a general agreement on the most likely strain hardening mechanisms operating in TWIP steel individually or collectively. These include deformation twinning, nano-twin hardening, buildup of back-stress, pronounced planar glide of dislocations, and dynamic strain aging. The relative magnitude of the individual contributions of
Acknowledgements
The authors gratefully acknowledge the support and contributions form the following persons: Dr Gwan Keun Chin (POSLAB), Dr Jin Kyung Kim (POSTECH), Professor Wolfgang Bleck (RWTH Aachen), Professor Young-Kook Lee (Yonsei University), Dr Stephanie Sandlöbes (Max-Planck-Institut für Eisenforschung, Düsseldorf), Professor Dierk Raabe (Max-Planck-Institut für Eisenforschung, Düsseldorf), Professor Se Kyun Kwon (POSTECH), and Mr Hojun Gwon (POSTECH). YE also acknowledges support from the Russian
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