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.

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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

Additive manufacturing (AM), widely known as 3D printing, is a method of manufacturing that forms parts from powder, wire or sheets in a process that proceeds layer by layer. Many techniques (using many different names) have been developed to accomplish this via melting or solid-state joining. In this review, these techniques for producing metal parts are explored, with a focus on the science of metal AM: processing defects, heat transfer, solidification, solid-state precipitation, mechanical properties and post-processing metallurgy. The various metal AM techniques are compared, with analysis of the strengths and limitations of each. Only a few alloys have been developed for commercial production, but recent efforts are presented as a path for the ongoing development of new materials for AM processes.

The metallurgy and processing science of metal additive manufacturing

Additive manufacturing (AM) technology has been researched and developed for more than 20 years. Rather than removing materials, AM processes make three-dimensional parts directly from CAD models by adding materials layer by layer, offering the beneficial ability to build parts with geometric and material complexities that could not be produced by subtractive manufacturing processes. Through intensive research over the past two decades, significant progress has been made in the development and commercialization of new and innovative AM processes, as well as numerous practical applications in aerospace, automotive, biomedical, energy and other fields. This paper reviews the main processes, materials and applications of the current AM technology and presents future research needs for this technology.

Additive manufacturing: technology, applications and research needs

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

The microstructural properties of advanced high strength and supra-ductile TRIP and TWIP steels with high-manganese concentrations (15 to 25 mass%) and additions of aluminum and silicon (2 to 4mass%) were investigated as a function of temperature (−196 to 400°C) and strain rate (10−4≤e≤103 s−1). Multiple martensitic γfcc (austenie)→ehcpMs (hcp-martensite)→αbccMs (bcc-martensite)-transformations occurred in the TRIP steel when deformed at higher strain rates and ambient temperatures. This mechanism leads to a pronounced strain hardening and high tensile strength (>1 000 MPa) with improved elongations to failure of >50%. The austenitic TWIP steel reveals extensive twin formation when deformed below 150°C at low and high strain rates. Under these conditions extremely high tensile ductility (>80%) and energy absorption is achieved and no brittle fracture transition temperature occurs. The governing microstructural parameter is the stacking fault energy Γfcc of the fcc austenite and the phase stability determined by the Gibbs free energy ΔGγ→e. These factors are strongly influenced by the manganese content and additions of aluminum and silicon.The stacking fault energy Γfcc and the Gibbs free energy G were calculated using the regular solution model. The results show that aluminum increases Γfcc and suppresses the γfcc→ehcpMs transformation, whereas silicon sustains the γfcc→ehcpMs transformation and decreases the stacking fault energy. At the critical value of Γfcc≈25 mJ/mol and for ΔGγ→e>0, the twinning mechanism is favored. At lower stacking fault energy of (Γfcc 0, martensitic phase transformation will be the governing deformation mechanism.The excellent ductility and the enhanced impact properties enable complex deep drawing or stretch forming operations of sheets and the fabrication of crash absorbing frame structures.

Supra-Ductile and High-Strength Manganese-TRIP/TWIP Steels for High Energy Absorption Purposes

High-strength TRIPLEX light-weight steels of the generic composition Fe-xMn-yAl-zC contain 18 - 28% manganese, 9 - 12% aluminium, and 0.7 - 1.2% C (in mass%). The microstructure is composed of an austenitic {gamma}-Fe(Mn, Al, C) solid solution matrix possessing a fine dispersion of nano size {kappa}-carbides (Fe,Mn){sub 3} AlC{sub 1-x} and {alpha}-Fe(Al, Mn) ferrite of varying volume fractions. The calculated Gibbs free energy of the phase transformation {gamma}{sub fcc} {yields} {epsilon}{sub hcp} amounts to {delta}G{sup {gamma}}{sup {yields}}{sup {epsilon}} = 1757 J/mol and the stacking fault energy was determined to {gamma}{sub SF} =110 mJ/m{sup 2}. This indicates that the austenite is very stable and no strain induced {epsilon}-martensite will be formed. Mechanical twinning is almost inhibited during plastic deformation. The TRIPLEX steels exhibit low density of 6.5 to 7 g/cm{sup 3} and superior mechanical properties, such as high strength of 700 to 1100 MPa and total elongations up to 60% and more. The specific energy absorption achieved at high strain rates of 10{sup 3} s{sup -1} is about 0.43 J/mm{sup 3}. TEM investigations revealed clearly that homogeneous shear band formation accompanied by dislocation glide occurred in deformed tensile samples. The dominant deformation mechanism of these steels is shear band induced plasticity -SIP effect- sustained by the uniform arrangement of nano size {kappa}-carbides coherent to the austenitic matrix. The high flow stresses and tensile strengths are caused by effective solid solution hardening and superimposed dispersion strengthening. (orig.)

Microstructures and Mechanical Properties of High-Strength Fe-Mn-Al-C Light-Weight TRIPLEX Steels

Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe-(15-30)wt%Mn alloys with additions of aluminium and silicon have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α ' - and £-martensite during plastic deformation was analysed by optical microscopy, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The stacking fault energy γ fcc and the free energy ΔG γ-e for the γ→e phase transformation were calculated using the regular solution model. It is known that additions of aluminium increase γ fcc and therefore strongly suppress the γ→e transformation while silicon decrease γ fcc and sustains the γ-e transformation. The γ→e phase transformation takes place in alloys with γ fcc 20 mJ/m 2 . The stacking fault energy of the Fe-25Mn-3Si-3Al alloy was calculated as a function of temperature and related with microstructural changes of the strained sample at different temperatures. These steels with reduced density of about 7,3 g/cm -3 combine high tensile ductility up to 80 % at high strain rates with true tensile strength of about 1000 MPa. The excellent plasticity induced by twinning and additional phase transformation up to extremely high strain rates of about e = 10 3 s -1 results in an extraordinary shock resistence and enables deep drawing and backward extrusion operations of parts with complex shapes and high production rates.

https://hal.archives-ouvertes.fr/jpa-00255657/document

Phase Transformations and Mechanical Properties of Fe-Mn-Si-Al TRIP-Steels

A nickel alloy of a composition similar to that of the nickel based superalloy Inconel alloy 718 (IN718) was produced with the electron beam melting (EBM) process developed by Arcam AB. The microstructures of the as processed and heat treated material are similar to that of conventionally produced IN718, except that the EBM material showed some porosity and the δ phase did not dissolve during the solution heat treatment because the temperature of 1000°C apparently was too low. Mechanical testing of the layer structured material, parallel and perpendicular to the built layers, revealed sufficient strength in both directions. However, it showed only limited elongation when tested perpendicular to the built layers due to local agglomerations of pores. Otherwise, data for the hardness, Young’s modulus, 0·2% yield tensile strength and ultimate tensile strength match those recommended for IN718.

Georg Frommeyer

Papers

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

Supra-Ductile and High-Strength Manganese-TRIP/TWIP Steels for High Energy Absorption Purposes

Microstructures and Mechanical Properties of High-Strength Fe-Mn-Al-C Light-Weight TRIPLEX Steels

Phase Transformations and Mechanical Properties of Fe-Mn-Si-Al TRIP-Steels

Microstructure and mechanical properties of nickel based superalloy IN718 produced by rapid prototyping with electron beam melting (EBM)