A novel medium Mn steel of composition Fe-12Mn-4.8Al-2Si-0.32C-0.3V was manufactured with 1.09 GPa yield strength, 1.26 GPa tensile strength and 54% elongation. The thermomechanical process route was designed to be industrially translatable and consists of hot and then warm rolling before a 30 min intercritical anneal. The resulting microstructure comprised of coarse elongated austenite grains in the rolling direction surrounded by necklace layers of fine austenite and ferrite grains. The tensile behaviour was investigated by in-situ neutron diffraction and the evolution of microstructure studied with Electron Backscattered Diffraction (EBSD). It was found that the coarse austenite grains contributed to the first stage of strain hardening by transforming into martensite and the fine austenite necklace grains contributed to the second stage of strain hardening by a mixture of twinning and transformation induced plasticity (TWIP and TRIP) mechanisms. This hierarchical deformation behaviour contributed to the exceptional ductility of this steel.
A metastable austenitic Fe-Cr-Ni steel was processed by selective laser melting with low power, resulting in nearly random crystallographic texture. In-situ tensile loading and neutron diffraction experiments were undertaken and the results indicate high ductility, despite the presence of porosity, and pronounced strain-induced martensitic transformation. A secondary hardening is observed in the mechanical data due to the martensite carrying significant load, upon its formation. The pronounced martensitic transformation is discussed with respect to the initial and the evolving deformation text…
A novel medium manganese steel named Novalloy with composition Fe-8.3Mn-3.8Al-1.8Si-0.5C-0.06V-0.05Sn was developed and thermomechanically processed through hot rolling and intercritical annealing. The steel possessed a yield strength of 1 GPa, tensile strength of 1.13 GPa and ductility of 41\%. In order to study the effect of cold rolling after intercritical annealing on subsequent tensile properties, the steel was further cold rolled up to 20\% reduction. After cold rolling, it was observed that the strain hardening rate increased continuously with increasing cold rolling reduction but witho…
Effectiveness of galaxite (Al2O3MnO) and manganese oxide (MnO2) in nucleating acicular ferrite was demonstrated in a steel with composition of Fe-13.92Mn-4.53Al-1.28Si-0.11C. The hot rolled and quenched steel had a duplex microstructure of delta-ferrite and partially transformed austenite. Thermal treatments were used to manipulate the concentration and type of oxides and the ferrite plate density was found to correlate with inclusions of low misfit. Both bainitic and acicular ferrite were observed and the prior austenite had a grain diameter of 16.5 micrometers. Upon tensile testing the retai…
The outstanding mechanical properties of high-manganese steels (HMnS) are a result of their high strain-hardenability. That is facilitated by strong suppression of dynamic recovery, predominant planar glide, and the activation of additional deformation mechanisms, such as transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP). However, depending on the final application, strongly differing requirements on the mechanical properties of HMnS are of relevance. In order to design HMnS for specific applications, multi-scale material simulation under consideration of the proc…
Metallic materials, especially steel, underpin transportation technologies. High-manganese twinning induced plasticity (TWIP) austenitic steels exhibit exceptional strength and ductility from twins, low-energy microstructural defects that form during plastic loading. Their high-strength could help light-weighting vehicles, and hence cut carbon emissions. TWIP steels are however very sensitive to hydrogen embrittlement that causes dramatic losses of ductility and toughness leading to catastrophic failure of engineering parts. Here, we examine the atomic-scale chemistry and interaction of hydrog…
A compositionally-graded steel composed of martensite with 0.4%C on the centre and bainite with 0.1%C on the surface was manufactured by partial decarburization. It is reported that the as quenched material can be cold rolled up to an equivalent strain of 1.5 without cracks. The mechanical properties of the cold-rolled material exhibits up to 2.5 GPa strength and ductility. A simple mechanical model is developed to predict the stress state after rolling of the graded structure explaining the good ductility of the present high strength materials.
Austenite reversion during tempering of a Fe-13.6Cr-0.44C (wt.%) martensite results in an ultrahigh strength ferritic stainless steel with excellent ductility. The austenite reversion mechanism is coupled to the kinetic freezing of carbon during low-temperature partitioning at the interfaces between martensite and retained austenite and to carbon segregation at martensite-martensite grain boundaries. An advantage of austenite reversion is its scalability, i.e., changing tempering time and temperature tailors the desired strength-ductility profiles (e.g. tempering at 400°C for 1 min. produces a…
My scientific activities aim at understand and predict the mechanical behaviour of steels, from fundamental mechanisms to macroscopic deformation. This manuscript is dedicated to the TWIP effect (TWinning Induced Plasticity) of high manganese austenitic steels and DP effect (Dual-Phase) in more conventional Ferrite-Martensite steels.
Temperature and misorientation-dependent austenite nucleation at ferrite grain boundaries in a medium manganese steel: role of misorientation-dependent grain boundary segregation
Second Generation advanced high strength steel is the material of choice for modern automotive structural parts because of its outstanding maximal elongation and tensile strength. Nonetheless there is still a lack of corrosion protection for this material due to the fact that cost efficient hot dip galvanizing can not be applied. The reason for the insufficient coatability with zinc is found in the segregation of manganese to the surface during annealing and the formation of manganese oxides prior coating. This work analyses the structure and chemical composition of the surface oxides on so ca…
In situ synchrotron X-ray diffraction study of flash austenitization and process design insights in medium-Manganese steels for energy applications
University of Kassel（卡塞尔大学）；Technische Universität Bergakademie Freiberg（弗莱贝格工业大学）；Karlsruhe University of Applied Sciences (HKA)（卡尔斯鲁厄应用科学大学）；Karlsruhe Institute of Technology (KIT)（卡尔斯鲁厄理工学院）
During the last years, the scientific and industrial community has focused on the astonishing properties of Fe-Mn-Al-C steels. These high advanced steels allow high-density reductions about ~15% lighter than conventional steels, high corrosion resistance, high strength (ultimate tensile strength (UTS) ~1 Gpa) and at the same time ductilities above 60%. The increase of the tensile or yield strength and the ductility at the same time is almost a special feature of this kind of new steels, which makes them so interesting for many applications such as in the automotive, armor and mining industry. …
A medium Mn steel of composition Fe-4.8Mn-2.8Al-1.5Si-0.51C (wt.\%) was processed to obtain two different microstructures representing two different approaches in the hot rolling mill, resulting in equiaxed vs. a mixed equiaxed and lamellar microstructures. Both were found to exhibit a simultaneous TWIP$+$TRIP plasticity enhancing mechanism where deformation twins and $α'$-martensite formed independently of twinning with strain. Interrupted tensile tests were conducted in order to investigate the differences in deformation structures between the two microstructures. A constitutive model was us…
Atomic scale understanding of the role of hydrogen and oxygen segregation in the embrittlement of grain boundaries in a twinning induced plasticity steel
Recent advances in additive manufacturing methods make it possible, for the first time, to manufacture complex micro-architectured solids that achieve desired stress versus strain responses. Here, we report experimental measurements and associated finite element (FE) calculations on the effect of strut shape upon the tensile response of two-dimensional (2D) lattices made from low-carbon steel sheets. Two lattice topologies are considered: (i) a stretching-dominated triangular lattice and (ii) a bending-dominated hexagonal lattice. It is found that strut waviness can enhance the ductility of ea…
In the drive towards higher strength alloys, a diverse range of alloying elements is employed to enhance their strength and ductility. Limited solid solubility of these elements in steel leads to segregation during casting which affects the entire down-stream processing and eventually the mechanical properties of the finished product. Although it is thought that the presence of continuous bands lead to premature failure, it has not been possible to verify this link. This poses as increasingly greater risk for higher alloyed, higher strength steels which are prone to centre-line segregation: it…
The article focuses on the effect of alloying and microstructure on formability of advanced high strength steels (AHSSs) processed via quenching and partitioning (Q&P). Three different Q&P steels with different combination of alloying elements and volume fraction of retained austenite are subjected to uniaxial tensile and Nakajima testing. Tensile mechanical properties are determined, and the forming limit diagrams (FLDs) are plotted. Microstructure of the tested samples is analyzed, and dramatic reduction of retained austenite fraction is detected. It is demonstrated that all steels are able …
Carbon is a well known austenite stabiliser and can be used to alter the stacking fault energy and stability against martensitic transformation in medium Mn steels, producing a range of deformation mechanisms such as the Transformation Induced Plasticity (TRIP) or combined Twinning and Transformation Induced Plasticity (TWIP $+$ TRIP) effects. However, the effect of C beyond quasi-static tensile behaviour is less well known. Therefore, two medium Mn steels with 0.2 wt\% and 0.5 wt\% C were designed to produce similar austenite fractions and stability and therefore tensile behaviour. These were…
Metastable alloys with transformation/twinning-induced plasticity (TRIP/TWIP) can overcome the strength-ductility trade-off in structural materials. Originated from the development of traditional alloys, the intrinsic stacking fault energy (ISFE) has been relied to tailor TRIP/TWIP in high-entropy alloys (HEA), but with limited quantitative success. Herein, we demonstrate a new strategy for designing metastable HEAs and validate its effectiveness by discovering seven new alloys with experimentally observed metastability for TRIP/TWIP. We propose unstable fault energies as the more effective de…
This study presents experimental and computational analyses of the phase composition and mechanical behavior of a functionally graded material (FGM) grading from 100 vol% 304L stainless steel (SS304L) to 100 vol% Ni-20Cr (NiCr) in 10 vol% increments. The mechanical behavior of the FGM was evaluated by extracting tensile specimens from SS304L-rich and NiCr-rich regions of the FGM, with the former behaving in a ductile manner and the latter being relatively brittle. The brittle response of the NiCr-rich sample was explained by the presence of the eutectic FCC+BCC phase in this sample, as identif…
Understanding the mechanism of martensitic transformation is of great importance in developing advanced high strength steels, especially TRansformation-Induced Plasticity (TRIP) steels. The TRIP effect leads to enhanced work-hardening rate, postponed onset of necking and excellent formability. In-situ transmission electron microscopy has been performed to systematically investigate the dynamic interactions between dislocations and alpha martensite at microscale. Local stress concentrations, e.g. from notches or dislocation pile-ups, render free edges and grain boundaries favorable nucleation s…
Hydrogen embrittlement in 304L (18wt.% Cr, 8-10wt.% Ni) austenitic stainless steel (ASS) fabricated by laser powder-bed-fusion (LPBF) was investigated by tensile testing after electrochemical hydrogen pre-charging and compared to conventionally available 304L ASSs with two different processing histories, (i) casting plus annealing (CA) and (ii) CA plus thermomechanical treatment (TMT). It was revealed that hydrogen-charging led to a significant reduction in ductility for the CA sample, but only a small effect of hydrogen was observed for the LPBF and CA-TMT samples. Hydrogen-assisted cracking …
Nanolamellar Hybrid High-Entropy Alloys with Superior Micromechanical Properties
WPI, International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University（九州大学 碳中性能源研究国际研究所）；Department of Automotive Science, Graduate School of Integrated Frontier Sciences, Kyushu University（九州大学 前沿融合科学研究生院 汽车科学系）；Department of Materials Science and Engineering, Kumamoto University（熊本大学 材料科学与工程系）；Department of Mechanical Engineering, Kyushu University（九州大学 机械工程系）；Univ Rouen Normandie, INSA Rouen Normandie, CNRS, Groupe de Physique des Matériaux, UMR6634（鲁昂诺曼底大学 鲁昂诺曼底国立应用科学学院 法国国家科学研究中心 材料物理小组）；Interdisciplinary Research Center, Liaoning Academy of Materials（辽宁材料研究院 交叉研究中心）
Robust nanoporous polymer films with approximate 100-nm thicknesses would be broadly applicable in technological areas such as flexible sensors, artificial skins, separators, antireflection and self-cleaning films. However, the creation of these films has been extremely challenging. To date, all reported ultrathin films are of insufficient mechanical strength for use without substrate supports. We describe here the fabrication of a new ultrastrong and highly flexible ultrahigh molecular weight polyethylene (UHMWPE) 100-nm porous membrane. Scanning and transmission electron microscopy evaluatio…
