A novel solid-solution MXene (Ti0.5V0.5)3C2 is successfully synthesized by exfoliating a solid-solution MAX phase (Ti0.5V0.5)3AlC2, and its catalytic effect on the hydrogen storage reaction of Mg is ...systemically evaluated for the first time. Typical layer morphology is observed for the prepared (Ti0.5V0.5)3C2, which exhibits a better catalytic activity than that of Ti3C2. The addition of 10 wt% (Ti0.5V0.5)3C2 remarkably reduces the dehydrogenation onset temperature of MgH2 by 70 °C, from 266 to 196 °C. At 250 °C, approximately 5.0 wt% H2 is released from the 10 wt% (Ti0.5V0.5)3C2-containing MgH2 within 20 min. The dehydrogenated sample rapidly absorbs 4.8 wt% H2 within 5 s at 120 °C; these hydrogenation kinetics are much more superior even to the well-studied Nb2O5 catalyst. The apparent activation energy is calculated to be 77.3 kJ/mol for the MgH2-10 wt% (Ti0.5V0.5)3C2 sample, which is only around half of that of the pristine MgH2 (153.8 kJ/mol). This is responsible for the remarkably reduced dehydrogenation operating temperature. Moreover, the chemical states of (Ti0.5V0.5)3C2 during dehydrogenation are also analysed and discussed.
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This paper reports the microstructural evolution and phase stability in a newly developed low-density Al10Nb15Ta5Ti30Zr40 refractory high entropy alloy (RHEA) at different temperatures. This alloy ...composition was adapted from the composition of the B2 phase in a two-phase B2+BCC mixture at 1000°C in the refractory high entropy superalloy Al0.25NbTaTiZr. After homogenizing at a high-temperature, followed by fast or slow cooling to room temperature, this alloy exhibited a nano-scale mixture of co-continuous BCC and B2 phases, resembling a spinodally decomposed microstructure with concurrent ordering. Interestingly, this novel nano-scale BCC+B2 microstructure exhibits excellent room temperature compressive yield strength (~1075MPa) and ductility (true strain at failure ~0.55). Annealing at 600°C and 750°C resulted in the formation of additional ordered omega type AlZr2 phase in this alloy. The experimentally observed phase evolution is in fair agreement with CALPHAD predictions.
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•Analytic model of the Orowan bypass stress (τOrowan) required for a dislocation to bypass an array of precipitates.•Examine the influence of precipitate size, shape, density, and orientation on ...τOrowan.•Prediction of the scaling of τOrowan with orientation and diameter of plate-shaped θ″ precipitates within an Al–Cu molecular statics materials system.
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The interaction between glissile dislocations and precipitates within a continuum is responsible for marked increases in material strength. Due to their desirable engineering features, dislocation-precipitate interactions have been the subject of study for decades. Towards enhancing our mechanistic understanding of the Orowan dislocation-precipitate bypass process, we present an analytic model of the Orowan bypass stress (τOrowan) required for a dislocation to bypass a planar array of precipitates. We initially consider spherical precipitates described by a diameter (D) and inner precipitate spacing (L). Our model suggests a τOrowan scaling logarithmically with the precipitate diameter, τOrowan∼lnDe−D/L, which we validate against a well established, yet empirical model. We also examine the influence of precipitate aspect ratio on τOrowan. We find that the precipitate width along the dislocation line is the dominant length scale governing τOrowan. Finally, we demonstrate the application of our model towards predicting the scaling of τOrowan for an array of plate-shaped θ″ precipitates within an Al-Cu molecular statics materials system. Our analyses provide insight into relationships between precipitate size, shape, density, orientation and metallic strengthening mechanisms.
Hybrid parts of nickel aluminum bronze (NAB) and 316L stainless steel were fabricated using a commercially available wire-arc additive manufacturing (WAAM) technology to evaluate the feasibility and ...cracking tendency. Focused Ion beam (FIB) based Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Electron Backscatter Diffraction (EBSD), and Transmission Electron Microscopy (TEM) were used to characterize the built (NAB)-substrate (SS) interfacial characteristics. FIB extracted a selected region of the interface, and the spatial distribution of the interface across several sections was characterized by using the state-of-the-art technique for 3D EBSD mapping. A metallurgically bonded interface without any pores and cracks, with the inter-diffusion region in a thickness of 2 μm, was formed, which was further confirmed by a video with the results of 3D reconstructed EBSD maps. The interface did not exhibit any strong texture orientation owing to the control of the thermal gradient as NAB is more conductive than 316L. EDS elemental mapping confirmed that Fe3Al intermetallic was formed at the NAB/SS bimetallic-joint interface. Occasional liquation cracks on the grain boundaries in the heat-affected zone (HAZ) of 316L substrate were observed. Fe-Al based intermetallic formation, along with the penetration of copper along the HAZ cracks, was noticed. The problems associated were highlighted, and remedial measures were suggested to open up the possibilities of additive manufacturing to fabricate NAB-Stainless steel hybrid parts for industrial repair and maintenance applications.
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This paper presents a discussion on the phase-transformation aspects of additively manufactured Alloy 718 during the additive manufacturing (AM) process and subsequent commonly used post-heat ...treatments. To this end, fundamental theoretical principles, thermodynamic and kinetics modeling, and existing literature data are employed. Two different AM processes, namely, laser-directed energy deposition and electron-beam powder-bed fusion are considered. The general aspects of phase formation during solidification and solid state in Alloy 718 are first examined, followed by a detailed discussion on phase transformations during the two processes and subsequent standard post heat-treatments. The effect of cooling rates, thermal gradients, and thermal cycling on the phase transformation in Alloy 718 during the AM processes are considered. Special attention is given to illustrate how the segregated composition during the solidification could affect the phase transformations in the Alloy 718. The information provided in this study will contribute to a better understanding of the overall process–structure–property relationship in the AM of Alloy 718 718.
The γ′ phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser ...powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ′ precipitated with various size and fraction depending on heat treatment process. For room-temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built sample while dislocations bypassing the precipitates via Orowan looping in the γ′ strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ′ strengthened samples have higher work hardening rate than as-built sample but encounter premature failure. Experimental evidence shows that the embrittlement mechanism in the γ′ strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strength and ductility of Nickel-base superalloy is discussed in detail.
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Graphene, the thinnest two-dimensional atomic material, has immerged as a revolutionary material and sparked a flurry of research and innovation owing to its outstanding mechanical, electrical, ...optical and thermal properties as well as high specific surface area. Graphene-based materials and their composites possess promising applications in a wide range of fields such as sensors, actuators, electronics, biomedical aids and membranes. In this review paper, a critical and comprehensive review has been carried out on the synthesis process and mechanical properties of graphene and graphene-based nanocomposites. Firstly, the concept and structure of graphene materials are discussed then different synthesis techniques and their advantages and limitations have been reviewed. The addition of graphene and its derivatives in producing different polymer and metal-based nanocomposite as well as fabricating hybrid nanocomposite has been thoroughly reviewed. Almost all the papers show that the presence of graphene even at very low loadings can provide significant improvement to the final material. Besides, other parameters that affect the nanocomposite are thoroughly reviewed. Furthermore, the perspective application of graphene materials and its nanocomposite in different promising fields has been discussed.
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In this study, a near equiatomic NiTi alloy was fabricated by selective laser melting method (SLM). The effects of laser power and scanning speed on the tensile properties, shape memory properties ...and microstructure were comparatively investigated. Since the shape memory NiTi alloy can be characterized by its martensitic start phase transformation (Ms temperature) and by its critical stress inducing martensitic transformation (σc), the evolution of these two parameters was investigated as the function of the delivered laser beam energy by differential scanning calorimetry and tensile tests. It was observed that the increase of the scanning speed under a certain laser power and the increase of the laser power under a certain laser beam energy density promote an increase of the critical stress (σc) and a decrease of the Ms temperature. Consequently, high laser beam energy suppresses the formation of the martensitic B19’ phase and therefore stabilizes the austenitic B2 phase. XRD and TEM observations confirm the dependence of the B2 and B19’ phase formation with the processing parameters. On the other hand, the relationship between the Ms temperature and the critical stress σc was also plotted in good accordance with the Clausius–Clapeyron equation. Two new coefficients called “Energy Dependence Coefficient of martensitic transformation Temperature (EDCT)” and “Energy Dependence Coefficient of critical Stress (EDCS)” were defined and calculated to describe the laser beam energy dependence of the martensitic phase transformation. These two new thermodynamic coefficients are thus very suitable to establish a link between the machine parameters (through the delivered laser beam energy) and the nature of the material (through the martensitic transformation) in the NiTi alloy fabricated by SLM.
Eutectic high entropy alloys (EHEAs) composed of soft face centered cubic (FCC) phase and hard body centered cubic (BCC) phases usually exhibit perfect combination of good tensile ductility and high ...fracture strength. Surprisingly, we reveal in this study that a new FCC(L12)/BCC(B2)-structured Al0.9CoCrNi2.1 EHEA shows inferior tensile properties with yield strength of 645 MPa, fracture strength of 1033 MPa and total elongation of merely 6.9%. A careful study of the deformation behavior reveals that the imperfect orientation relationship lead to extensive cleavage fracture and the consequent poor ductility. After thermomechanical treatment, a balanced and greatly enhanced tensile properties (yield strength of 820 MPa, fracture strength of 1400 MPa and total elongation of 11.6%) were achieved. The abnormal simultaneous enhancement for strength and ductility is resulted from the synchronous deformation of L12 and B2 phases. The current work presents a successfully demonstration of using thermomechanical processing to improve inferior mechanical properties of some lamellar eutectic alloys.
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Strain hardening for ductility remains challenging especially at high yield strength when dislocation plasticity is usually invalid. The hetero-deformation provides an effective route to induce extra ...back stress hardening specifically in hetero-structures inherently with large mismatch of mechanical responses, e.g. flow stress and strain hardening etc., upon applied loading. In this paper, both strengthening and strain hardening were investigated in a dual- phase steel, consisting of ductile γ-austenite and almost non-deformable B2 intermetallic phase as the second phase of volume fraction of 23%. The chemical composition was 0.86C, 16Mn, 10Al, 5Ni, balance Fe (wt.%). Of special note is two distinct hetero-deformation responses during both tensile and interrupted unload-reload testing. One is the yield-drop, while the other is hysteresis loop. Both unceasingly appear even from the elasto-plastic yield stage up to whole uniform deformation. The measured back stress and resultant back stress hardening account for a large proportion of global flow stress and strain hardening. Further, both Schmid factor and Kernel average misorientation (KAM) values were measured after tensile deformation. Un-expected, only γ-grains bordering on B2-phase show a significant decrease in the average Schmid factor, relative to almost unchanged in left γ-grains still next to γ-ones as well as B2-phase. This indicates that γ-grains adjacent to B2-phase bear the vast majority of plastic strains, not simple strain partitioning between γ and B2. Because of this, from the onset of yielding to end of tensile deformation, those γ-grains, in contrast to left γ-grains and B2 phase, exhibit a maximal increment in KAM values. This serves as a solid evidence of the generation of geometrically necessary dislocations to accommodate strain gradient near γ/B2 phase boundaries. It turns out that hetero-deformation due to plastic incompatibility induces the operation of back stresses, leading to both strengthening and strain hardening as well. Finally, a microstructure-based model was developed to calculate back stress which was well consistent with experimentally measured back stress.
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