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Abstract: The Fe–6.5wt.%Si alloy powders were prepared by high-pressure gas atomization in which the nucleation ability of the melt varied with the different purities of the raw materials and experimental conditions. The results indicated that the number of heterogeneous cores has a substantial effect on the grain size of the larger atomized powder particles, i.e. the more heterogeneous cores that are contained in the melt, the smaller the grain size of the atomized powder particles will be. However, the above effect became less significant when the powder particle size decreased. When the particle size was less than a certain critical value, single-crystal particles were formed. In addition to this, it was seen that the effect of the heterogeneous core number on the secondary dendrite arm spacing was fairly insignificant. [Copyright &y& Elsevier]

Co–Cr based alloys are recognized as one of the most widely used category of alloys amongst the biomedical materials. This is because these alloys show a high strength, corrosion resistance and good biocompatibility suitable for biomedical applications. Many researchers have been focused on the processing of these alloys using a selective laser melting process. The effects of different processing parameters on the obtained microstructural and mechanical properties of the processed parts have been investigated in these studies. However, the effect of powder distribution size (powder granulometry), as a primary processing parameter, on the microstructural and mechanical properties of Co–Cr alloys is not well investigated and studied so far. This study investigated the influence of this parameter, by using powders with the same composition and morphology but a different particle size of coarse and fine, on the microstructural and mechanical properties of as-built, homogenizing heat treated and homogenizing heat treated followed by subsequent aging treatment SLM fabricated Co28Cr9W1.5Si (wt.%) samples. X-ray diffraction, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy were used to characterize and identify the microstructure, phase composition and crystallographic information of the manufactured samples. Besides, the mechanical properties of samples, processed using powders having different size distributions, were measured using a tensile test machine and results were correlated to the microstructure of the samples. It was concluded that the powder granulometry has a negligible effect on the microstructural and mechanical properties of the as-built SLM Co–Cr–W–Si parts but a remarkable influence on those of the heat-treated ones. [ABSTRACT FROM AUTHOR]

This study investigated the interfacial bond performances between ordinary Portland cement concrete (OPCC) and self-compacting geopolymer concrete (SCGC). The studied factors include nano-SiO 2 , repair material type, curing time, casting direction, and interface pretreatment method. The interfacial bond performances were assessed by Bi-surface shear and drying shrinkage tests. The evolutions in elemental compositions, hydration products, and microstructure in the matrix and interface were verified by BSE/EDS, XRD, and FTIR tests, respectively. ANOVA analysis was conducted to quantitatively analyze the influences of each factor from a statistical approach. The results demonstrated that the bond strength of SCGC increased by 24.5% to 31.7% compared to conventional OPCC. Meanwhile, geopolymers exhibited more compact microstructures but higher drying shrinkage (ε sh,u =891 µε). While nanofillers significantly amplified the cracking potential of geopolymers, which was attributed to the higher early-stage autogenous shrinkage instead of drying shrinkage. The SCGC showed a high enrichment degree of calcium at the interface, indicating that more calcium-rich products were formed. In contrast, the use of nano-SiO 2 in SCGC caused the enrichment of Na, Al, and Si at the interfacial regions, which provided extra nucleation sites for the polymerization of (C, N)-A-S-H gels. The nano-SiO 2 enhanced the intensity of the hydroxide ion and Si-O-T band and promoted the formation of (C, N)-A-S-H gels. The ANOVA analysis proved that the repair material, nano-SiO 2 , and interface treatment were significant factors to affect the interfacial bond strength. In addition, geopolymer-based repair materials were less sensitive to the casting direction, which differed from the conventional self-compacting OPCC. [Display omitted] • Geopolymer concrete at the same strength grade has 25-30% higher bond strength than conventional OPCC. • Nanofillers amplified the cracking potential of geopolymers due to higher early-stage autogenous shrinkage. • nano-SiO 2 in SCGC caused the enrichment of Na, Al, and Si at the interfacial regions. • geopolymer-based repair materials were less sensitive to the casting direction. [ABSTRACT FROM AUTHOR]

In this study, spherical tantalum (Ta) powders, having different oxygen contents, were fabricated by a radio frequency plasma spheroidization technique. A laser powder bed fusion (LPBF) technique was then used to consolidate these powders and the effects of different LPBF processing parameters on the densification, microstructure and mechanical properties of parts were investigated. The results indicated that the oxygen content of the used Ta powder has significant influences on the densification, microstructure and the consequent mechanical properties of printed parts. The relative density of the parts printed from the powders with a low oxygen content of ∼150 ppm was insensitive to the applied laser energy density and was higher than the ones prepared from the powders with higher oxygen contents. The grain refinement was observed and the texture in the side-view of the as-built samples changed from {100}∥BD to {111}∥BD with the increase in the oxygen content of the used powders. The microhardness and ultimate tensile strength of samples increased and the ductility of parts decreased with the increase in the oxygen content of the used raw powders. The main fracture mode of the LPBF consolidated Ta samples changed from a typical ductile rupture to the combination of cleavage and ductile rupture modes with the increase of the oxygen content in the processed parts. • Low-oxygen sphericalTa powder was successfully fabricated. • The oxygen content plays an important role in densification, microstructure and mechanical properties of the LPBF consolidated Ta samples. • Oxygen was enriched at grain and subgrain boundaries in the Ta H sample. [ABSTRACT FROM AUTHOR]

• Coal gangue can be employed as primary cementitious material after being modified by the proposed activation method; • The activated coal gangue geopolymer displayed high early and uniaxial compressive strength; • An optimum calcination strategy was proposed for the formation of high reactivity amorphous SiO 2 and Al 2 O 3 ; • Microstructure evolutions and hydration productions at different stages were experimentally verified. The feasibility of coal gangue as primary cementitious material was experimentally investigated in this paper. An Orthogonal method was employed to investigate the optimum activation strategy for coal gangue-based geopolymer. Experiment parameters included the modulus of sodium silicate (M), alkali content (A), calcination temperature (T), holding time (H), and water-to-binder ratio (W/B). Meanwhile, the phase transitions, chemical compositions, and microstructure evolution were investigated by supplementary techniques, such as ESEM, XRD, and FTIR. Experimental results indicated that the hydration reactivity of coal gangue was markedly enhanced. And the formations of high contents of amorphous SiO 2 and Al 2 O 3 phases are the critical factors for the high reactivity, which highly depends on the proper calcination temperature and holding time. The hydration productions of alkali-activated coal gangue, mainly N-A-S-H gels, can rapidly precipitate around the unreacted particles and fill the existing cavities. Meanwhile, the average surface area, diameter, and porosity considerably declined with the hydration degree of geopolymer. In summary, coal gangue can be viewed as a new eco-friendly cementitious material via the proposed composite activation strategy. [ABSTRACT FROM AUTHOR]

58Ni39Ti-3Hf alloy system has been proposed as an alternative for superelastic 60NiTi used in load-bearing applications. This research was focused on the development of heat treatment procedures leading to 58Ni39Ti-3Hf parts with maximum possible hardness values. Meanwhile, the relations existing between the heat treatment – microstructure – hardness of 58Ni39Ti-3Hf as compared to baseline 60NiTi were also investigated. These were done by applying the solution and consequent aging treatments on the studied alloy systems at different temperatures and for different times. The microstructure and hardness of the treated samples were, in the next step, analyzed by applying electron probe micro-analyzer (EPMA), energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and macro hardness testing techniques. It was understood that applying solution treatments at temperatures above about 1000 °C resulted in the relatively full dissolution of Ni-rich phases such as Ni 3 Ti and Ni 3 Ti 2 , existing in the microstructure of as-received parts, in the NiTi matrix of 60NiTi. On the other hand, for 58Ni39Ti-3Hf, temperatures above about 900 °C appeared to be adequate to solve these Ni-rich phases in the NiTi matrix. Applying aging treatments had either no significant effect on the hardness of solution treated 60NiTi samples (~58 HRC) or reduced their values; after coarsening of Ni 4 Ti 3 particles formed in NiTi matrix and their transformation to Ni 3 Ti and Ni 3 Ti 2 phases. On the other hand, some of the applied aging treatments could increase the hardness values of the solution treated 58Ni39Ti-3Hf alloys (~54 HRC). The maximum value of hardness (~59 HRC) for the aged 58Ni39Ti-3Hf parts was obtained for the ones treated at 500 °C for just 0.25 h. This level of obtained hardness is comparable to that of solution treated 60NiTi. A delayed precipitation of Ni 3 Ti and/or Ni 3 Ti 2 phases was noted in aged 58Ni39Ti-3Hf alloy system as compared to 60NiTi. • The difference amongst the solvus temperature of 58Ni39Ti-3Hf as compared to 60NiTi was understood. • The peak hardness for 58Ni39Ti-3Hf (~59 HRC) was obtained by aging the solution treated samples at 500 °C for 0.25 h. • Relations existing between the heat treatment-microstructure-hardness of 58Ni39Ti-3Hf as compared to 60NiTi were defined. [ABSTRACT FROM AUTHOR]

In this work, an (α + β) TRIPLEX heat treatment process was designed, as well as its impact on the microstructures and high–temperature mechanical properties of 2.5 vol% (TiB + TiC)/Ti–6Al–4Sn–7Zr–1Nb–1Mo–1W–0.2Si (wt.%) was carefully investigated. The results show that the microstructures of heat–treated TMCs consist of the primary α phase (α p) and the transformed β (β t) with a large number of nano–scale secondary α phases (α s). The TRIPLEX heat treatment encourages the effect of structural heredity, causing the residual α p lamellae to develop adequately and even mature into lamellae clusters with the same orientation, which induces a visible strengthening of the variant selection for the α phase. After the TRIPLEX heat treatment, the increase in content and size of silicides is mainly due to the process providing favorable sites and sufficient time for nucleation and growth. The longitudinal axes of some silicides exhibit an approximate 40° angle or parallel arrangement with the β/α p interfaces, which means that the silicides will grow preferentially along a specific orientation to minimize the strain energy. The (α + β) TRIPLEX heat treatment significantly increases the high–temperature strength of the TMCs with no appreciable elongation loss (as–cast TMCs: UTS = 487.6 MPa, EL = 17.4%; HT–4#: UTS = 558.1 MPa, EL = 11.5%). The strength improvement of HT–4 # is mainly related to the combined effects of α p content reduction, α s precipitation, obstruction of dislocations by the β t /α p interface, hetero–deformation strengthening, silicides precipitation strengthening, and α 2 dispersion strengthening. [ABSTRACT FROM AUTHOR]

Equiatomic and near-equiatomic NiTinol alloys, mainly due to the exhibition of adjustable mechanical and thermal shape memory properties, are widely exploited to produce different smart and functional devices. Laser powder bed fusion technique was used in this research to process dense NiTinol parts from a prealloyed near-equiatomic NiTi powder, using three different volumetric energy densities. In the next step, samples, processed using a specific set of printing parameters, were annealed using two different procedures. Effects of different used processing volumetric energy densities and post-processing heat treatments on the microstructural properties, phase composition, crystallographic information, phase transformation behavior and hardness of the processed samples were investigated. As-printed Parts with Ni contents ranging from 55.12 to 55.27 wt%, density higher than at least 96% of theoretical and a similar phase structure as that of used powder, dominated by austenitic NiTi, were obtained. Applying a hot isostatic processing treatment at 1100 °C for 4 h or annealing the samples in a pressureless sintering furnace, working under a vacuum environment at the same temperature and holding time, had significant effects on the properties of the printed parts. These treatments, in addition to increasing the density of the printed samples, their average grain size and reducing the residual stresses, shifted the phase transformations to lower temperatures. These results suggest that post-printing heat treatments affect and, as similar to NiTinol alloys having Ni contents above 55.7 wt%, can be used as a strategy to alter the phase transformation temperatures of printed less Ni-rich near-equiatomic NiTinol alloys and consequently obtain NiTinol parts suitable for different applications. • Post-processing heat treatments, other than affecting the microstructural characteristics, shifted down the phase transformation temperatures of as-printed parts having Ni-contents less than 55.7 wt% Ni. • These consequently changed the hardness values of treated parts as compared to the as-printed ones. • These results suggest that, the same as that for the Ni-rich NiTinol alloys having Ni-contents above 55.7 wt% Ni, applying post-processing heat treatments can be used as a strategy to alter the phase transformation temperature of less Ni-rich NiTinol parts and subsequently change the obtained properties. [ABSTRACT FROM AUTHOR]

The mechanical properties of high-entropy alloys (HEAs) strongly depend on their microstructural features. In this work, the phase formation and morphologies of Fe 26.7 Co 26.7 Ni 26.7 Si 8.9 B 11.0 HEAs fabricated under different undercooling levels by the drop tube technique together with subsequent quenching were investigated in detail. It was found when the droplet size decreases (i.e. the increase of the undercooling level), the grain refinement is easily achieved without any phase transformation for the present hypoeutectic HEA which is composed of the fcc dendrites and eutectic structure. After quenching under large undercooling levels, the liquid should be deeply undercooled into an asymmetric couple zone, leading to the primary formation of eutectic structure and the subsequent precipitation of the fcc dendrites. The direct product of rapid solidification after deep undercooling and subsequent recalescence may induce the decoupled eutectic growth, resulting in a transition from the regular into anomalous eutectic structures. Furthermore, the volume fractions of the eutectic structures and the fcc dendrites increase and decrease with decreasing droplet size, respectively. As a result, the Vickers hardness gradually increases with decreasing droplet size, which exhibits a linear relationship with the volume fraction of eutectic matrix. The present study may provide a simple way to tailor microstructures and mechanical properties of HEAs. • High-entropy droplet were fabricated by the drop tube technique. • Grain refinement is easily achieved during containerless solidification. • Deep undercooling induces a transition from regular into anomalous eutectic matrix. • Mechanical properties can be tailored by deeply undercooled solidification. [ABSTRACT FROM AUTHOR]

• Agglomerate C element will cause stress concentration and crack formation. • Crack-free Fe-based BMGCs was presented by adding 40 vol% Cu. • 40 Vol.% Cu adding promote multiple shear-bands generation, thus improve plasticity. Selective laser melting of ductile Cu particles reinforced Fe-based bulk metallic glass composites (Fe-based BMGCs) were used to break geometric complexity limitations and improve the mechanical properties. In this work, the effects of Cu content on microstructure, mechanical properties and fracture deformation behavior of Fe-based BMCGs were systematically investigated. The appropriate Cu addition can decrease stress concentration by altering elements distribution, especially C element which will increase the brittleness of BMGs and lead to crack formation. The crack-free Fe-based BMGCs with a well bonding interface between the secondary phase and matrix is presented with 40 vol.% Cu addition. The improved plasticity of BMGCs with Cu addition originates from the inhibition of shear bands via formation of multiple shear bands. This work demonstrates that introducing tough secondary phase into BMGs is an effective method to improve their formability and performance. [ABSTRACT FROM AUTHOR]