High entropy alloys (HEAs) based on Co-Cr-Fe-Mn-Ni transition metals with Al and Ti additions constitute a versatile system to generate complex microstructures. In this study, microstructures consisting of a ductile FCC matrix with hard BCC, a, and fine precipitates are obtained after thermo-mechanical treatments by varying the amounts of Al and Ti, specifically (CoCrFeMnNi)100-x(Al2Ti)x, x = 6, 8, 10. Ageing is performed in the range of 650 to 850 degrees C up to 160 h. Additionally, the effect of ageing is compared between as-cold-rolled state and after solution treatment. Bulk hardness values ranges from 300 to 600 HV due to the precipitation of a (Cr, Fe, Mn)-rich a phase and a (Co, Ni, Al, Ti)-rich BCC phase, as characterized by nanoindentation, EBSD, and STEM. A higher content of Al/Ti increases the volume fraction of secondary phases, increasing hardness. During ageing, precipitation occurs in three stages with first the precipitation of a BCC secondary phase, second the concurrent precipitation of a and BCC phases, and third the formation of needle-like BCC precipitates. L12 precipitates have not been observed in the range of compositions and heat treatments investigated.

Biodegradable intravascular stents offer a promising alternative to permanent stents for treating atherosclerosis-related artery narrowing by potentially avoiding long-term complications. Identifying materials that degrade harmlessly and uniformly at a suitable rate is crucial. This study evaluated an advanced zinc alloy (Zn-Ag-Cu-Mn-Zr) alongside pure iron and pure zinc, using a simplified stent model of metallic wires implanted in the rat aorta. Assessments were made at 7, 24, and 84 days post-implantation using X-ray microfocus computed tomography (microCT) and contrast-enhanced microCT (CECT). For CECT, a contrast agent was chosen to provide optimal soft tissue contrast and minimal interaction with the wires. This combination of imaging techniques allowed us to evaluate degradation behavior, compare volume loss in various locations (outside the arterial lumen, inside the lumen, and encapsulated by neointima), compute degradation rates, and evaluate neointima tissue formation. Results showed that zinc and its alloy degrade less uniformly than iron, which demonstrates uniform surface degradation. The zinc alloy had a higher initial volume loss than the other materials but showed little increase over time. Neointima formation was similar for zinc and the zinc alloy, while iron provoked less tissue formation than both zinc and the reference cobalt-chromium alloy. Additionally, unlike cobalt-chromium and zinc, iron wires did not achieve consistent tissue encapsulation along their entire length, which may impair their performance. Mild inflammation was noted around zinc-based implants. Combining microCT and CECT provided 3D information on degradation uniformity, degradation products, and neointima morphometrics, highlighting the power of these imaging techniques to evaluate implant materials in a highly accurate way compared to previous 2D methods. Statement of Significance Biodegradable intravascular stents offer a promising solution to long-term complications associated with permanent stents by gradually dissolving in the body. To evaluate a novel zinc alloy (Zn-Ag-Cu-Mn-Zr) with improved mechanical properties, microstructure, and biocompatibility, we compared it to pure iron and zinc. We used advanced 3D imaging techniques, i.e., microCT and contrast-enhanced microCT, to assess the degradation behavior and the tissue response in a rat aorta model. The zinc alloy demonstrated promising properties despite less uniform degradation and mild inflammation compared to iron. Our findings highlight the superiority of 3D imaging over previously used 2D techniques in evaluating stent materials, offering critical insights into degradation processes and biocompatibility. These highly accurate measurements provide crucial information for developing improved biodegradable implants.

Medium manganese steel sheets exhibit an unusual alternating failure mode transition involving an arrowhead fracture pattern under various loading conditions. Tests were performed using an Arcan setup with different shear-to-tension ratios and specimen orientations to explore the failure mechanisms and unravel the root causes of the transition. The fracture surfaces, characterized by optical and electron microscopy, show a periodic switchover from ductile damage to quasi-cleavage, organized into repeating arrowhead zones pointing towards the crack propagation direction. The step-by-step crack propagation leaves a signature on the load-displacement curve matching the discontinuous cracking events found on the fracture surfaces. A reduction of the stress-intensity factor due to sudden crack advance and associated load drop causes the brittle crack to arrest. Periodic porosity clusters under the fracture surface indicate re-blunting after each cycle of stable/unstable fracture.

The treatment of early onset scoliosis using surgical growing rods suffers from high failure rate. Fatigue resistance can be improved by inducing compressive residual stresses within the near surface region. An in-depth investigation of the residual stresses profile evolution is performed through the sequence of material processing steps followed by surgeons handling operations, in connection to material properties. The final goal is to guide further improvements of growing rod lifetime. Residual stress evaluation was carried out on Ti-6Al-4V rods using digital image correlation applied to microbeam ring-core milling by focused ion beam. This provided experimental stress profiles in shot-peened rods before and after bending and demonstrated that compressive residual stresses are maintained at both concave and convex rod sides. A finite element model using different core and skin conditions was validated by comparison to experiments. The combination of an initial shot peening profile associated with a significant level of backstress was found to primarily control the generation of compressive stresses at the rod surface after bending. Guidelines to promote larger compressive stresses at the surface were formulated based on a parametric analysis. The analysis revealed the first order impact of the initial yield strength, kinematic hardening parameters and intensity of the shot peening operation, while the bending angle and the depth of shot peening stresses were found to be of minor importance. Materials exhibiting large kinematic hardening and low yield strength should be selected in order to induce compressive residual stresses at key fatigue initiation site.

Press hardenable steels (PHS) coated with Al-Si alloy are widely used in the automotive industry owing to their good mechanical properties (Yield Strength >1200 MPa and Tensile Strength >1500 MPa). The presence of Al-Si coating prevents oxidation and decarburization of steel during austenitization. However, aluminized PHS are sensitive to hydrogen absorption during austenitization, while Al-Si coating prevents hydrogen degassing from the specimen at room temperature so that diffusible hydrogen could lead to hydrogen embrittlement in specific conditions. In this study, H trapping in aluminized steels was critically assessed using deuterium sources instead of hydrogen sources during austenitization and thermal desorption analysis (TDA). 22MnB5 steel coated with Al and Al-Si alloys were studied. D is mainly trapped in the dislocations strain field in the steel substrate of aluminized steels and it is responsible for the three main D2 desorption peaks in Al and Al-Si coated steels. Multiple peaks observed on D2 desorption profiles of Al and Al-Si coated steels are explained by the preferential desorption path of D depending on the sample temperature.

Sustainable energy production, inherently transient and non-uniformly distributed around the world, requires the rapid development of sustainable energy storage technologies. Recently, pure iron powder was proposed as a high-energy density carrier. While promising, challenges are faced, such as nanoparticle emissions, micro-explosions or cavitation. In this work, a screening of the impact of the most common impurities in iron sources on these mechanisms was conducted through purely thermodynamic simulations. Two idealized models were considered to obtain a range of plausible flame temperatures and emitted gases when considering a purely diffusive regime in standard conditions and stoichiometric air-fuel mixture. The flame temperature and iron evaporation are increasing with the specific energy. A strong evaporation of C, S, Mo, Cu and P is also expected. Most impurities are predicted to decrease cavitation, except for Mn and MnO. The regeneration process by hydrogen-based direct reduction in fluidized bed reactors is also discussed. MgO and CaO are the most promising additions in terms of reducing nanoparticles and porosities, as well as to improve the fluidization and reduction kinetics of the combusted products. The potential of Fe powder as sustainable fuel, already very promising, could be further improved by the addition of selectively chosen impurities.This article is part of the discussion meeting issue 'Sustainable metals: science and systems'.

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In vitro testing for evaluating degradation mode and rate of candidate biodegradable metals to be used as intravascular stents is crucial before going to in vivo animal models. In this study, we show that X-ray microfocus computed tomography (microCT) presents a key added value to visualize degradation mode and to evaluate degradation rate and material surface properties in 3D and at high resolution of large regions of interest. The in vitro degradation behavior of three candidate biodegradable stent materials was evaluated: pure iron (Fe), pure zinc (Zn), and a quinary Zn alloy (Zn-Ag-Cu-Mn-Zr). These metals were compared to a reference biostable cobalt chromium (Co Cr) alloy. To compare the degradation mode and degradation rate evaluated with microCT, scanning electron microscopy (SEM) and inductively-coupled plasma (ICP) were included. We confirmed that Fe degrades very slowly but with desirable uniform surface corrosion. Zn degrades faster but exhibits localized deep pitting corrosion. The Zn alloy degrades at a similar rate as the pure Zn, but more homogeneously. However, the formation of deep internal dendrites was observed. Our study provides a detailed microCT-based comparison of essential surface and corrosion properties, with a structural characterization of the corrosion behavior, of different candidate stent materials in 3D in a non-destructive way.

This study proposes a machine-learning model combining ab initio calculations and an experimental dataset of 201 alloys (in addition to pure Ti) to predict the activated plasticity mechanisms in β-Ti alloys. This methodology is shown to be more efficient than the so-called Bo¯-Md¯ approach, achieving 82% prediction accuracy while the Bo-Md approach leads to 52% correct predictions on the same dataset. In addition, four new alloy compositions were produced to verify the model validity. Specific cases where the present model disagreed with the Bo-Md predictions were chosen to increase the benefits of the produced results. The plasticity mechanisms of the four alloys experimentally confirmed the validity of the ML model. This approach particularly helps the design of specific Ti alloys exhibiting a high work hardening rate owing to the simultaneous activations of the Transformation-Induced Plasticity (TRIP) and mechanical twinning (TWIP) effects. Indeed, the class corresponding to the combination of TRIP and TWIP effects reach a prediction accuracy of 88%.

High-entropy alloys have exhibited unusual materials properties. The stability of equimolar single-phase solid solution of five or more elements is supposedly rare and identifying the existence of such alloys has been challenging because of the vast chemical space of possible combinations. Herein, based on high-throughput density-functional theory calculations, we construct a chemical map of single-phase equimolar high-entropy alloys by investigating over 658,000 equimolar quinary alloys through a binary regular solid-solution model. We identify 30,201 potential single-phase equimolar alloys (5% of the possible combinations) forming mainly in body-centered cubic structures. We unveil the chemistries that are likely to form high-entropy alloys, and identify the complex interplay among mixing enthalpy, intermetallics formation, and melting point that drives the formation of these solid solutions. We demonstrate the power of our method by predicting the existence of two new high-entropy alloys, i.e. the body-centered cubic AlCoMnNiV and the face-centered cubic CoFeMnNiZn, which are successfully synthesized.

This study proposes a thorough investigation of the mechanical properties of two β metastable titanium alloys (Ti–12Mo and Ti-8.5Cr-1.5Sn (wt %)) processed by laser powder bed fusion. Particular emphasis is put on the influence of chemical homogenization and defects formation as a function of the processing parameters. It is demonstrated that the Transformation/Twinning – induced plasticity (TRIP/TWIP) mechanisms already revealed as beneficial in the case of wrought alloys, still bring a larger work hardening level and a larger ductility in the case of laser powder bed fusion (L-PBF) processing. Improved mechanical properties in terms of yield and ultimate tensile strengths with similar uniform deformation compared to the wrought counterparts are thus achieved after L-PBF and a very short annealing.

The need for sustainable use of resources requires continuous improvement in the energy efficiency and development of new approaches to the design and processing of suitable materials. The concept of high entropy alloys (HEAs) has recently been extended to more general compositional complex alloys (CCAs) and multi-principal element alloys (MPEAs). One of the major challenges on the way to application of these alloys is the extensive design and selection efforts due to the great variety of possible compositions and its consequences for workability and resulting material properties. The favorable high-temperature strength of Ni-based and Co-based superalloys is ascribed to a defined γ/γ’ structure consisting of a disordered FCC A1 matrix and ordered L12 γ’ precipitates. In the current work we extended this design concept to CCAs, allowing disordered BCC A2 and ordered B2 phases in additions or in substitution of the original γ/γ’ structure. We used a high-throughput screening approach combining CALPHAD-based computational tools with in situ alloying by means of laser cladding. Wall-type specimens with gradient composition in the system Al-Co-Cr-Fe-Ni-Ti with varying Al, Ti and Cr content were analyzed. The combined modelling and experimental screening approach was demonstrated to be a powerful tool for designing new high performance AM-ready feedstock.

Remarkable mechanical properties have been reported for CoCrFeMnNi-based high entropy alloys (HEAs) in recent literature, making these HEAs potentially attractive candidates for future cryogenic applications. However, the damage and fracture behaviour of HEAs is not fully understood yet, especially at low temperature. Here, the mechanical behaviour and fracture resistance of CoCrFeMnNi and CoCrNi sheets are investigated at room and cryogenic temperatures and compared to more conventional alloys. Very good properties were confirmed for HEAs, but outperformed by stainless steels under similar conditions, in terms of ductility, strength, and fracture toughness combination. Exceptional low temperature fracture energy up to 2500 kJ/m2 was found for stainless steels compared to 700 kJ/m2 for HEAs. A predictive model was developed and validated experimentally in order to connect the thin sheet fracture toughness to the strain hardening capacity through separating the necking and damage work spent in the fracture process zone, providing guidelines for further optimization.

In this work, we propose a strategy for high-throughput design and development of compositionally complex alloys combining theoretical and experimental alloy screening. This methodology was applied for the exploration of the (Co 2 CrFeNi 2 ) 1- x - y Ti x Al y subsystem of so-called high entropy superalloys in the Al–Co–Cr–Fe–Ni–Ti alloy system. Alloy design was guided by thermodynamic calculations based on the CALPHAD approach. The evolution of the microstructure with increasing Al and Ti content was analyzed in the as-built, homogenized and age-hardened conditions by means of scanning electron microscopy, energy-dispersive X-ray spectroscopy and electron backscattered diffraction. Additionally, the evolution of the sample hardness with increasing Al and Ti contents was determined for all conditions. Based on the experimental results, the reliability of the CALPHAD calculations was assessed. Generally, a good agreement between calculations and experiments is achieved in the homogenized state. In the aged conditions, the CALPHAD predictions of the precipitation processes are partly inaccurate and need improvement. Optimal Al and Ti concentrations are derived for age hardening through L1 2 and combined L1 2 + B2 precipitations.

Cantor-type high entropy alloys form a new family of metallic alloys characterized by a combination of high strength and high fracture toughness. An experimental study on the CoCrNi alloy is first performed to determine the damage and fracture mechanisms under various stress states. A micromechanics-based ductile fracture model is identified and validated using these experimental data. The model corresponds to a hyperelastic finite strain multi-yield surface constitutive description coupled with multiple nonlocal variables. The yield surfaces consist of three distinct nonlocal solutions corresponding to three different modes of void expansion within an elastoplastic matrix: a void growth mode governed by a Gurson-based yield surface corrected for shear effects, an internal necking-driven coalescence mode governed by an extension of the Thomason yield surface based on the maximum principal stress, and a shear-driven coalescence mode governed by the maximum shear stress. This advanced formulation embedded in a large strain finite element setup captures the effects not only of the stress triaxiality but also of the Lode variable. In particular, the analysis shows that a failure model accounting for these two invariants of the stress tensor captures the fracture in high-entropy alloys over a wide range of conditions.

ß-metastable Ti alloys exhibit a very large work hardening rate together with an outstanding resistance to damage nucleation, bringing a very high ductility. Such a behavior could enable to counteract the decrease of mechanical properties caused by solidification cracking|hot tearing, balling or porosity formation during laser powder bed fusion. The binary Ti-12 wt.% Mo grade was chosen as a case study, using powder mixture as a first approach. As-printed microstructures highlight the formation of structures related to the solidification scheme, as well as specific Mo solute partitioning depending on the printing parameters. Such a specific microstructure brings a large increase of the tensile strength compare to the cast reference. Furthermore, when the chemical homogenisation obtained is large enough to reach 95 % of ß-metastable microstructure, ductility comparable to the cast reference is reached after a simple flash heat treatment, as well as an outstanding low sensitivity to defects.

The essential work of fracture (EWF) method is a powerful approach to characterize the fracture resistance of thin ductile sheets based on a principle of separation of energy contributions. A major drawback of the method is an extensive use of material, requiring a series of double-edge notched tensile (DENT) specimens with several ligament lengths to extract the EWF. This can be a serious limitation when the material is difficult to process and/or expensive. Here, we propose an improved methodology to reduce the amount of material as much as possible while keeping the same statistical level of accuracy for the estimated EWF. We show that the width and height of the DENT specimens can be adapted as a function of the ligament length. A statistical model has been developed to determine the distribution of ligament lengths minimizing the total amount of material. This new approach is validated both numerically with Monte Carlo simulations and experimentally. In the experiments, the strain fields in the ligament were quantified by digital image correlation to ensure that the validity criteria were met for each specimen, as well as to provide an in-depth analysis of the plastic zone development. From these results, guidelines are provided to optimally rationalize EWF experimental data.

Dynamic recrystallization (DRX) within adiabatic shear bands forming during the fracture of TRIP-TWIP beta-metastable Ti-12Mo (wt %) alloy was recently reported. The formation of 1-3 mu m thick-adiabatic shear bands, and of dynamic recrystallization, was quite surprising as their occurrence generally requires high temperature and/or high strain rate loading while these samples were loaded in quasi-static conditions at room temperature. To better understand the fracture mechanism and associated microstructural evolution, thin foils representative of different stages of the fracture process were machined from the fracture surface by Focused Ion Beam (FIB) and analyzed by Transmission Electron Microscopy (TEM) and Automated Crystal Orientation mapping (ACOM-TEM). Complex microstructure transformations involving severe plastic deformed nano-structuration, crystalline rotation and local precipitation of the omega at h phase were identified. The spatial and temporal evolution of the microstructure during the propagation of the crack was explained through dynamic recovery and continuous dynamic recrystallization, and linked to the modelled distribution of temperature and strain level where TEM samples were extracted. (c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.