High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m(1/2). Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening.

/pdf/a-fracture-resistant-high-entropy-alloy-for-cryogenic-4v6ic2t1x4.pdf

A fracture-resistant high-entropy alloy for cryogenic applications

Metals have been mankind's most essential materials for thousands of years; however, their use is affected by ecological and economical concerns Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase) This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys

Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off

https://zenodo.org/record/1258680/files/article.pdf

The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy

Shear-banding is a ubiquitous plastic-deformation mode in materials. In metallic glasses, shear bands are particularly important as they play the decisive role in controlling plasticity and failure at room temperature. While there have been several reviews on the general mechanical properties of metallic glasses, a pressing need remains for an overview focused exclusively on shear bands, which have received tremendous attention in the past several years. This article attempts to provide a comprehensive and up-to-date review on the rapid progress achieved very recently on this subject. We describe the shear bands from the inside out, and treat key materials-science issues of general interest, including the initiation of shear localization starting from shear transformations, the temperature and velocity reached in the propagating or sliding band, the structural evolution inside the shear-band material, and the parameters that strongly influence shear-banding. Several new discoveries and concepts, such as stick-slip cold shear-banding and strength/plasticity enhancement at sub-micrometer sample sizes, will also be highlighted. The understanding built-up from these accounts will be used to explain the successful control of shear bands achieved so far in the laboratory. The review also identifies a number of key remaining questions to be answered, and presents an outlook for the field.

Shear bands in metallic glasses

A significant increase in the research activity dedicated to high manganese TWIP steels has occurred during the past five years, motivated by the breakthrough combination of strength and ductility possessed by these alloys. Here a review of the relations between microstructure and mechanical properties is presented focusing on plasticity mechanisms, strain-hardening, yield stress, texture, fracture and fatigue. This summarized knowledge explains why TWIP steel metallurgy is currently a topic of great practical interest and fundamental importance. Finally, this publication indicates some of the main avenues for future investigations required in order to sustain the quality and the dynamism in this field.

High manganese austenitic twinning induced plasticity steels: A review of the microstructure properties relationships

High strength Fe–Mn–(Al, Si) TRIP/TWIP steels development — properties — application

Iron manganese steels with Mn mass contents of 15 to 30 % exhibit microstructural related superior ductility and extraordinary strengthening behaviour during plastic deformation, which strongly depends on the Mn content. This influences the austenite stability and stacking fault energy γ fcc and shows a great impact on the microstructure to be developed under certain stress state or during severe plastic deformation. At medium Mn mass contents (15 to 20 %) the martensitic γ-e-α' phase transformation plays an important role in the deformation mechanisms of the TRIP effect in addition to dislocation glide. With increasing Mn mass content large elongation is favoured by intensive twinning formation. The mechanical properties of plain iron manganese alloys are strongly influenced by the alloying elements, Al and Si. Alloying with Al increases the stacking fault energy and therefore strongly suppresses the martensitic γ-e transformation, while Si sustains the γ-e transformation by decreasing the stacking fault energy γ fcc . The γ-e phase transformation takes place in Fe-Mn-X alloys with γ fcc ≤ 20 mJm -2 . The developed light weight high manganese TRIP and TWIP (twinning induced plasticity) steels exhibit high ultimate tensile strength (600 to 1100 MPa) and extremely large elongation of 60 to 95 % even at high strain rates of e = 10 3 s -1 . Particularly due to the advanced specific energy absorption of TRIP and TWIP steels compared to conventional deep drawing steels high dynamic tensile and compression tests were carried out in order to investigate the change in the microstructure under near crash conditions. Tensile and compression tests of iron manganese alloys with varying Mn content were performed at different temperatures and strain rates. The resulting formation of y twins, a'- and e martensite by plastic deformation was analysed by optical microscopy and X-ray diffraction. The deep drawing and stretch forming behaviour at varying deformation rates were determined by performing cupping tests and digitalised stress-strain-analysis.

Development and characterization of high strength impact resistant Fe–Mn–(Al, Si) TRIP/TWIP steels

The invention relates to an easily cold-workable, in particular easily deep-drawable ultra high strength austenitic lightweight construction steel with an ultimate tensile strength of up to 1100 MPa and with TRIP and TWIP characteristics, as well as its use for motor vehicle body sheetmetal components, structural components used for stiffening, as well as cryogenic containers and pipelines.

Light constructional steel and the use thereof

Microstructure and mechanical properties of high manganese (Al, Si) TRIP (transformation induced plasticity) and TWIP (twinning induced plasticity) steels were investigated at quasistatic and dynamic tensile loading in a wide range of temperature. The mechanical properties are strongly dependent on the stacking fault energy and are influenced by chemical composition, temperature and loading rates. A gradual formation of deformation twins and/or martensitic phase transformation are essential to achieve excellent formability at high strength levels.

Stress-deformation behaviour of high manganese (AI, Si) TRIP and TWIP steels

Die Erfindung betrifft einen gut kaltumformbaren, insbesondere gut tiefziehfahigen, ultrahochfesten austenitischen Leichtbaustahl mit einer Zugfestigkeit bis 1100 MPa und mit TRIP- und TWIP-Eigenschaften sowie seine Verwendung fur Karosserieblechteile, versteifende Strukturkomponenten und Cryogen-Behalter und Rohrleitungen. Der Stahl hat die chemische Zuzammensetzung (in Masse-%): 1 bis 6% Si, 1 bis 8% Al mit (Al + Si) <= 12%, 10 bis 30% Mn, Rest Eisen, einschlieslich ublicher Stahlbegleitelemente.

O. Grässel

Papers

High strength Fe–Mn–(Al, Si) TRIP/TWIP steels development — properties — application

Development and characterization of high strength impact resistant Fe–Mn–(Al, Si) TRIP/TWIP steels

Light constructional steel and the use thereof

Stress-deformation behaviour of high manganese (AI, Si) TRIP and TWIP steels

Leichtbaustahl und seine verwendung