As-fabricated State Research Articles

Tuning of the mechanical properties of a laser powder bed fused eutectic high entropy alloy Ni30Co30Cr10Fe10Al18W2 through heat treatment

Eutectic high entropy alloys (EHEA) with an exceptional combination of strength and ductility are promising candidates as advanced structural materials. However, achieving an optimal balance between the properties in additively manufactured EHEAs is an outstanding challenge. In this work, Ni30Co30Cr10Fe10Al18W2 EHEA was additively manufactured using the laser powder bed fusion (LPBF) technique. In the as-fabricated state, it exhibits a dual-phase nano-lamellar structure consisting of FCC/L12 and B2 phases. Post-fabrication heat treatments were explored for modulating microstructure and, in turn, enhancing the mechanical properties, so as to achieve an optimum balance between strength and ductility. Upon heat treatment at 750 °C for 1 h, part of the B2 phase transformed into FCC, with the appearance of the tungsten-rich precipitates inside the B2 phase. The average interlayer spacing of the FCC/L12 lamellae increased to 124 nm, while that of B2 lamellae decreased to 86 nm, resulting in an alloy with an ultimate tensile strength (UTS) of 1811 MPa, although the strain to failure (εf) decreased to 2 %. Upon increasing the heat treatment temperature to 1000 °C, the average interlayer spacings of the FCC and B2 phases increased to 210 and 187 nm, respectively, which resulted in a more balanced mechanical behavior with UTS and εf of 1332 MPa and 9.3 %, respectively. This study provides an effective approach for microstructural modulation and enhancement of mechanical properties of LPBF fabricated EHEA via post-fabrication heat treatment, offering insights for developing future high-performance alloys for advanced structural applications.

Role of the γ′′ precipitation at the cell boundaries in enhancing the creep resistance of additively manufactured Inconel 718 alloy using the laser powder bed fusion technique

Laser powder bed fusion (LPBF) manufactured Inconel 718 superalloy (IN718) parts exhibit inferior creep properties compared to their conventionally manufactured (CM) counterparts due to the unique microstructural features associated with them. In this study, two different heat treatment schedules were employed, to critically examine role of distinct microstructural features associated with LPBF on the creep performance of LPBF IN718. Experimental results reveal that the creep performance is controlled by the Laves and γ′′ phases, as well as the cellular structure. Specially, the effective dissolution of Laves phases that are distributed along the grain and cellular boundaries in the as-fabricated state during solution treatment reduces the cavity nucleation sites and hence, extends the creep lifetime. The release of Nb into the matrix contributes to the precipitation of primary strengthening γ′′ phase during subsequent aging, enhancing the creep resistance. More importantly, the formation of a unique cellular structure with densely distributed γ′′ phase along the cellular boundaries, which exhibits high impediment of dislocation movement and inhibition of crack propagation, results in superior creep performance. The findings of the current study provide an effective heat treatment strategy for achieving high creep performance in LPBF nickel-based superalloys.

Corrosion resistant high entropy conventional alloy (HECA) with superior work hardenability

Cu-based alloys are known for their high corrosion resistance but with a lack of processing ability and vice versa. Here we present a novel alloy design concept realizing Cu-rich high entropy conventional alloy (HECA) having exceptional corrosion resistance (Ecorr = - 0.27 V, icorr = 2.83 × 10−6 A/cm2) and work hardenability (strength = 413 MPa, total elongation = 28 %) synergy simultaneously in as-fabricated state. This cocktailing effect in HECA (Cu-12Fe-8Mn-7.5Co-2.5Cr, all in at%) is attributed to the formation of Fe-Mn-Co-Cr containing high entropy phase (HEP) in the Cu-rich matrix. A pronounced two-phase hardening by hetero-deformation induced (HDI) strengthening effect at the HEP/Cu matrix resulted in excellent work hardenability whereas, exceptional corrosion resistance was attributed to delayed corrosion kinetics owing to the preferential corrosion of the Cu-rich phase over HEP in HECA. Thus, Cu-rich HECA overcomes the corrosion-strength ductility dilemma in Cu-rich alloys when compared with classical binary Cu-Fe alloys in an as-fabricated state.

Influence of Biaxial Strain and Interfacial Layer Growth on Ferroelectric Wake-Up and Phase Transition Fields in ZrO2.

Investigations on fluorite-structured ferroelectric HfO2/ZrO2 thin films are aiming to achieve high-performance films required for memory and computing technologies. These films exhibit excellent scalability and compatibility with the complementary metal-oxide semiconductor process used by semiconductor foundries, but stabilizing ferroelectric properties with a low operation voltage in the as-fabricated state of these films is a critical component for technology advancement. After removing the influence of interfacial layers, a linear correlation is observed between the biaxial strain and the electric field for transforming the nonferroelectric tetragonal to the ferroelectric orthorhombic phase in ZrO2 thin films. This observation is supported by applying the principle of energy conservation in combination with ab initio and molecular dynamics simulations. According to the simulations, a rarely reported Pnm21 orthorhombic phase may be stabilized by tuning biaxial strain in the ZrO2 films. This study demonstrates the significant influence of interfacial layers and biaxial strain on the phase transition fields and shows how strain engineering can be used to improve ferroelectric wake-up in ZrO2.

Adaptation of a heat-treatment condition to a precipitation-hardened nickel-based superalloy produced by laser powder bed fusion

This study evaluated the microstructural evolution and mechanical properties of a novel printable nickel-based superalloy subjected to a solution aging treatment. The microstructures of the alloys in as-fabricated and solution-treated states were characterized using a combination of electron backscatter diffraction analysis and scanning electron microscopy. The findings indicate that the cellular crystals gradually transitioned into equiaxed grains with increasing solution temperature. The grain orientation shifts from the <001> orientation perpendicular to the building direction to a random orientation. The average grain size increased, the proportion of low-angle grain boundaries (LAGBs) decreased, and the percentage of annealing twin boundaries (TBs) increased from 0.18% in the as-fabricated state to 65.68% at 1230 °C solution treatment (ST). After lower-temperature ST, a large amount of primary γ′ and η phases precipitated in the as-fabricated alloy, with spherical and needle-like shapes, respectively. As the solution temperature increased, the quantity of the primary γ′ phase gradually decreased, and the η phase became relatively more elongated. At 1130 °C ST, both phases dissolved; following aging treatment, γ′ phase precipitated fully. High strength (1493 MPa) was achieved after direct aging treatment, attributed to the combination of cellular structures and numerous LAGBs produced during deposition with the complete precipitation of γ′ phase during aging. This study demonstrated that laser powder bed fusion alloys can achieve excellent mechanical performance through direct aging without needing ST.

Creep deformation and cavitation in an additively manufactured Al-8.6Cu-0.4Mn-0.9Zr (wt%) alloy

Creep deformation and cavitation were investigated at 300 ºC in both tension and compression for an additively manufactured Al-8.6Cu-0.5Mn-0.9Zr (wt%) alloy in the as-fabricated state and after various aging treatments (aging at 300 °C/200 h or 350 °C/24 h and overaging at 400 °C/200 h). Creep mechanisms at 300 °C were determined by relating the measured creep response to corresponding microstructural and X-ray computed tomography observations. In compression, alloys in the as-fabricated and two aging conditions exhibited similarly high creep resistance. Overaging (400 °C/200 h) led to substantial coarsening of intragranular θ-Al2Cu precipitates and an expected drop in their Orowan strengthening contribution. In tension, minimum strain rates comparable to those in compression were obtained at any given stress; however, upon accumulation of some plastic strain in the matrix, creep cavities started to form, leading to accelerated tertiary stage creep deformation and rupture. Cavitation occurred exclusively along melt pool boundaries due to locally enhanced diffusion enabled by (ⅰ) large grain-boundary area in adjacent fine-grained zones and (ⅱ) localization of creep strain in nearby heat-affected zones. Although cavity growth was initially diffusion-controlled, its rate was determined by matrix creep rate, consistent with constrained cavity growth mechanisms. This study reveals how microstructural complexities induced by the additive manufacturing process affect the creep and cavitation behavior of Al-Cu-Mn-Zr alloys. The underlying creep and cavitation mechanisms uncovered in this study point to pathways that improve the high-temperature properties of additively manufactured alloys.

Improved Ductility by Reducing Powder Size in Laser Powder Bed Fusion of AlSi10Mg

Strict control of powder properties, especially particle size distribution (PSD), is critical in the laser powder bed fusion (LPBF) process to ensure the quality of the fabricated parts. This work shows that reducing the powder size could improve the ductility of LPBF fabricated AlSi10Mg. The tensile elongation increased from 4.1% to 8.8% when AlSi10Mg powder sizes (D50 value) decreased from 76 µm to 16 µm in the as-fabricated state. Our results showed that fine powder size stimulates epitaxial grain growth due to excessive remelting. The presence of epitaxial columnar grains makes the crack path across inner melt pools more frequent and travels from the coarse melt pool zones to the fine melt pool zones. This work suggests that powder size is a crucial factor to be considered in maintaining repeatable mechanical properties in LPBF processes, particularly when ductility and ductility-dominated properties are critical.

Phase transformation and thermal stability of the laser powder bed fused high-strength and heat-resistant Al–Ce–Mg alloy

The coarsening of strengthening phases, phase transformations, and interface stability between these phases and the matrix play a crucial role in determining the strength, ductility, and heat resistance of aluminum alloys fabricated through additive manufacturing (AM). In this study, a heat exposure experiment at 400 °C for 1 h was conducted on the laser powder bed fused Al–Ce–Mg alloy. A comprehensive analysis of the microstructural evolution, phase composition, interfacial bonding strength, and mechanical properties before and after heat exposure was performed using synchrotron X-ray diffraction techniques, first-principles calculations, transmission electron microscopy, and mechanical testing. For the first time, a phase transformation from Al11Ce3 to Al4Ce was discovered. Some semi-coherent relationships of [301]Al11Ce3//[011]Al, (060)Al11Ce3//(00)Al were transformed into coherent relationships of [001]Al4Ce//[001]Al, (200)Al4Ce//(00)Al, and the interfacial energy was reduced by 4.385 J m−2. In-situ Al11Ce3 nano-networks after heat exposure were retained and Al4Ce nanoparticles contribute to the thermal stability of the alloy by hindering dislocation motion and grain coarsening. The strength of the heat exposure alloy reached up to 416 MPa, which is about 95 % of the strength of the as-fabricated state and the elongation increased from 9.3 % to 13.8 %. These results provide a new approach for the design of high strength-ductility synergy and heat-resistant aluminum alloys via AM.

Evaluation of Ca-alloyed and HAp-reinforced magnesium matrix surface composite properties developed by friction stir processing

ABSTRACT The objective of this study is to fabricate AZ31 surface composites (MMSC) through friction stir processing (FSP) by introducing calcium (Ca) as an alloying element and hydroxyapatite (HAp) as reinforcement in different proportions. The analysis reveals that the β-Mg17Al12 phase remains more stable in its as-fabricated state, while the Mg2Ca phase exhibits greater stability during the aging period. The addition of Ca and HAp leads to higher Mg2Ca and Ca3Mg4Al5 formation during aging process. The incorporation of Ca and HAp results in an increased elastic modulus (E) of the as-fabricated MMSC in comparison to AZ31, with no significant degradation observed following the aging process. The hardness of MMSCs increases with the increase in β-Mg17Al12, and Mg2Ca phases. Moreover, the incorporation of Ca and HAp results in enhanced corrosion resistance of the as-fabricated MMSCs in comparison to AZ31.

Design of a low Young’s modulus Ti-Zr-Nb-Sn biocompatible alloy by in situ laser powder bed fusion additive manufacturing process

A new metastable β Ti-22Zr-11 Nb-2Sn (at%) biomedical titanium alloy was elaborated from elemental powders by in situ laser powder bed fusion technique (L-PBF). Iterative design of experiments was used to identify optimized manufacturing parameters to obtain dense and homogeneous parts (>99.3%) with low unmolten niobium fraction (<0.05%). The microstructural and mechanical properties of this alloy were investigated in its as-fabricated state and after two heat-treatments at 400 °C and 700 °C, respectively. The alloy, showing a β-type microstructure, possesses a very low Young’s modulus (∼50 GPa) combined with a high tensile strength reaching about 1100 MPa after heat-treatment. These properties are very suitable for medical devices in osseous site such as hip prostheses or dental implants and the results were compared to the Ti-6Al-4 V ELI (wt%) medical grade fabricated from pre-alloyed powders. Thus, the present work demonstrates the significant potential of the in situ L-PBF technique to elaborate highly biocompatible titanium alloys with tailored chemical compositions.

Characterizing the As-Fabricated State of Additively Fabricated IN718 Using Ultrasonic Nondestructive Evaluation

This article reports on the characterization of the “as-fabricated” state of Inconel 718 samples fabricated using laser directed energy deposition (DED). Laser-DED is known to produce complex metastable microstructures that can significantly influence the baseline ultrasonic response compared to conventional processing methods. The present work uses three parameters to characterize the samples: (a) ultrasonic velocity, (b) an attenuation coefficient, and (c) a backscatter coefficient. The baseline ultrasonic response from the DED sample was compared against the ultrasonic properties of conventional IN718 samples reported in the literature. The results suggest that strong grain boundary scattering from large macrograins can lead to attenuation and backscatter values that are significantly higher than conventional samples. Additionally, the results including velocities, attenuation and backscatter coefficients were found to be dependent on the fabrication direction, with the build direction being different from the transverse directions. Finally, destructive analysis was used to develop conjectures to explain the experimentally observed ultrasonic response.

Aging Property of Halide Solid Electrolyte at the Cathode Interface.

Halide solid electrolytes have recently emerged as a promising option for cathode-compatible catholytes in solid-state batteries (SSBs), owing to their superior oxidation stability at high voltage and their interfacial stability. However, their day- to month-scale aging at the cathode interface has remained unexplored until now, while its elucidation is indispensable for practical deployment. Herein, the stability of halide solid electrolytes (e.g., Li3 InCl6 ) when used with conventional layered oxide cathodes during extended calendar aging is investigated. It is found that, contrary to their well-known oxidation stability, halide solid electrolytes can be vulnerable to reductive side reactions with oxide cathodes (e.g., LiNi0.8 Co0.1 Mn0.1 O2 ) in the long term. More importantly, the calendar aging at a low state of charge or as-fabricated state causes more significant degradation than at a high state of charge, in contrast to typical lithium-ion batteries, which are more susceptible to high-state-of-charge calendar aging. This unique characteristic of halide-based SSBs is related to the reduction propensity of metal ions in halide solid electrolytes and correlated to the formation of an interphase due to the reductive decomposition triggered by the oxide cathode in a lithiated state. This understanding of the long-term aging properties provides new guidelines for the development of cathode-compatible halide solid electrolytes.

Ductility enhancement of additively manufactured CoCrMo alloy via residual stress tailored high stacking fault probability

CoCrMo (CCM) alloys fabricated by laser powder bed fusion (LPBF) normally exhibit low ductility. In the present study, a new plastic deformation mode of residual stress tailored high stacking fault (SF) is proposed for LPBF fabricated CCM alloy, delivering an excellent ductility (∼14.5%) in the as-fabricated state. Elaborate microstructural characterization elucidated that low level of initial residual stress contributes to a high stacking fault probability, generating extensive amounts of SFs to sustain the plastic deformation. Such massive deformation faulting activities mitigated the strain localization between FCC/HCP phase of CCM alloy with high residual stress level. Further low temperature annealing treatment validated the above theory, and increased the ductility to ∼21.4% with a high tensile strength of ∼1181 MPa. The present study demonstrated that the ductility of LPBF fabricated CCM alloy can be significantly enhanced by tuning the internal residual stress and the associated stacking fault probability.

Large-Scale Tungsten Fibre-Reinforced Tungsten and Its Mechanical Properties

Tungsten-fibre-reinforced tungsten composites (Wf/W) have been in development to overcome the inherent brittleness of tungsten as one of the most promising candidates for the first wall and divertor armour material in a future fusion power plant. As the development of Wf/W continues, the fracture toughness of the composite is one of the main design drivers. In this contribution, the efforts on size upscaling of Wf/W based on Chemical Vapour Deposition (CVD) are shown together with fracture mechanical tests of two different size samples of Wf/W produced by CVD. Three-point bending tests according to American Society for Testing and Materials (ASTM) Norm E399 for brittle materials were used to obtain a first estimation of the toughness. A provisional fracture toughness value of up to 346MPam1/2 was calculated for the as-fabricated material. As the material does not show a brittle fracture in the as-fabricated state, the J-Integral approach based on the ASTM E1820 was additionally applied. A maximum value of the J-integral of 41kJ/m2 (134.8MPam1/2) was determined for the largest samples. Post mortem investigations were employed to detail the active mechanisms and crack propagation.

Quasilinear pseudoelasticity and small hysteresis in SLM-fabricated NiTi

NiTi shape memory alloys with quasilinear and slim pseudoelasticity are desirable for high fatigue life and easy controllability. This paper reports that the selective laser melting (SLM) technique can encourage formation of quasilinear pseudoplasticity with small hysteresis of NiTi. The NiTi parts are found to present a high density of dislocations in the as-fabricated state. Deformation cycling of the defect-containing SLM NiTi parts promotes the formation of a unique microstructure consisting of nanocrystalline and amorphous-like phases. Such microstructure leads to a continuous and quasilinear type pseudoelasticity with small hysteresis. It is found that the scanning speed of laser affects the dislocation density and transformation behaviour, thus influencing the pseudoelastic behaviour of the SLM-fabricated NiTi. The transformation temperatures are shown to decrease while the transformation intervals to increase with increasing the scanning speed due to the increased dislocation density. The mechanically cycled SLM fabricated NiTi with a scanning speed of 850 mm/s exhibits a stable quasilinear pseudoelasticity of 5.8% and a small energy dissipation of 1.4 MJ/m3.

Texture evolution during processing and post-processing of maraging steel fabricated by laser powder bed fusion

In this study, the evolution of solidification texture during LPBF of Ti-free grade 300 maraging steel, and its effect on texture development during subsequent post-fabrication heat treatments was characterized using Electron Backscatter Diffraction (EBSD). It was found that in the as-fabricated state, no texture was observed in the room temperature martensitic phase. However, the reconstructed parent austenite phase displayed a Cube texture with a minor fraction of Rotated Goss texture. During subsequent aging treatments involving two different routes, namely direct aging of the as-fabricated samples, and conventional solution treatment + aging of the as-fabricated samples, significant changes in the texture components of parent austenite were observed, whereas no changes in texture were observed in the room temperature martensitic phase. During direct aging, it was found that with an increase in the aging temperature up to 520 °C, the texture components of the parent austenite changed from Cube/Rotated Goss to Brass, whereas during the conventional solution treatment and aging cycle, interestingly a change in texture component to rotated copper was observed. The transitions in texture components have been discussed using the concepts of recrystallization and twinning in austenite during annealing and/or aging, and strain energy release maximization (SERM) theory. Furthermore, the importance of these preferred orientations on the mechanical properties was quantified using transformation potential diagrams.

Internal reverse-biased p–n junctions: A possible origin of the high resistance in chalcogenide superlattice for interfacial phase change memory

Chalcogenide superlattice (CSL) is one of the emerging material technologies for ultralow-power phase change memories. However, the resistance switching mechanism of the CSL-based device is still hotly debated. Early electrical measurements and recent materials characterizations have suggested that the Kooi-phase CSL is very likely to be the as-fabricated low-resistance state. Due to the difficulty in in situ characterization at atomic resolution, the structure of the electrically switched CSL in its high-resistance state is still unknown and mainly investigated by theoretical modelings. So far, there has been no simple model that can unify experimental results obtained from device-level electrical measurements and atomic-level materials characterizations. In this work, we carry out atomistic transport modelings of the CSL-based device and propose a simple mechanism accounting for its high resistance. The modeled high-resistance state is based on the interfacial SbTe bilayer flipped CSL that has previously been mistaken for the low-resistance state. This work advances the understanding of CSL for emerging memory applications.

Irradiation effects in tungsten—From surface effects to bulk mechanical properties

Advanced materials such as tungsten fibre-reinforced composites allow to overcome severe weaknesses of the baseline materials for plasma-facing components — copper and tungsten. The effect of the fusion environment on the mechanical properties of these materials, e.g. the embrittlement by neutron irradiation, plays a key role for the development of future fusion reactors. To simulate this effect, high-energy ions are used as a substitute for the displacement damage by neutrons. We propose the use of very fine tungsten wire as a possibility of studying the influence of irradiation damage on the mechanical properties. This is possible as they allow full-depth irradiation of almost the entire volume despite the limited penetration depth of ions. Geometrical size effects are mitigated due to the nanoscale microstructure of the wire. In addition, similar wire is used in tungsten fibre-reinforced composites. Thus, the investigation of irradiated wire can directly be used for the prediction of the bulk composite properties. For the proof of this concept tungsten wire with a diameter of 16 μm was electrochemically thinned to 5 μm and irradiated with 20.5 MeV W6+ ions. The mechanical properties were subsequently determined by macroscopic tensile testing. Irradiation to 0.3, 1 and 9 dpa did not lead to a change of the mechanical behaviour. Both strength and ductility, the latter indicated by the reduction of area, were similar to the as-fabricated state.

Thermo-Electro-Chemo-Mechanical Modeling of Solid Oxide Fuel Cell for Stress and Failure Evolution during Duty Cycle

A comprehensive three-dimensional model for an assembled button solid oxide fuel cell is developed by coupling thermal-electrochemical and mechanical models. Different mechanical effects including residual strain, thermal strain, accelerated and normal creep, mechanical properties change of anode, as well as chemical expansion are considered. The mechanical response of the button cell subjected to an idealized duty cycle from the as-fabricated state, heating-up stage, reduction stage, to three operation periods of 800 °C, 700 °C, and 600 °C is numerically simulated. Simulations are based on and validated by the experimental polarization curves and residual stress curve. Results show that the sealant is susceptible to fracture at the as-fabricated state, while the cathode is likely to fail during heating-up stage. The accelerated creep effect during reduction significantly eliminates the tensile stress in the anode nevertheless leads to higher stress in the cathode and electrolyte. It indicates that the assumption of zero-stress temperature at the reduction point could cause an underestimation of stress in the cathode and electrolyte in the case of a constrained cell. The chemical expansion effect in the cathode is more prominent at higher operating temperatures. A minimum failure probability of the cell is found at 700 °C with consideration of chemical expansion.

Comparing the deformation mechanism in 316 L stainless steel fabricated by hybrid and additive manufacturing

This study rationalizes the relatively higher elongation with minimal strength drop of 316L stainless steel (316LSS) manufactured using the hybrid approach in comparison to a fully additive approach followed by machining. It was found that twinning was the dominant deformation mechanism in 316LSS produced by both additive and hybrid approaches. The relatively lower fraction of twins in the as-fabricated state of hybrid approach compared to a fully additive approach resulted in delaying the onset of steady-state strain hardening rate to higher strain levels, thereby resulting in an overall increase in elongation to failure.