•The oxidation behaviours of Mg alloys from low to high temperatures are reviewed.•Characteristics of MgO during oxidation is discussed.•Mechanisms of Mg oxidation is presented.•Effect of various ...alloying elements is analysed.
This paper reviews (i) the oxidation of Mg alloys at elevated temperatures in air; (ii) the influence of alloying elements on the oxidation of Mg alloys; and (iii) the progress in the development of oxidation-resistant Mg alloys. Low oxidation rates have been shown by Mg alloys containing the alloying elements, Ca, Be and rare earth elements. However, these alloying elements may also decrease mechanical properties, such as ductility.
Understanding the densification behaviours and formation mechanisms of defects are essential to fabricate high quality and high strength aluminium components using selective laser melting (SLM) ...technology. In this work, the effects of laser power and scanning speed on the densification, defects evolution and their formation mechanisms in a SLMed 2024 aluminium (Al) alloy were investigated in consideration of the corresponding laser energy input, melting mode transition and microstructural evolution. The results showed that optimizing the processing parameters effectively reduced the porosity level below 1% by avoiding the lack of fusion and keyhole melting mode, and minimizing the gas pores. However, optimization of the processing parameters could not eliminate the columnar structure associated with the SLMed 2024 Al alloy, which contributed to the hot-tearing cracks in the SLMed parts. It was found that the dependence of porosity formation on SLM processing parameters was contrary to the crack density. Hence, to further improve the SLM-processability of the 2024 Al alloy it is necessary to develop SLM methods in order avoid the hot-cracking within the optimized processing parameter window associated with the minimum porosity formation.
Abstract
Additive manufacturing (AM) creates digitally designed parts by successive addition of material. However, owing to intrinsic thermal cycling, metallic parts produced by AM almost inevitably ...suffer from spatially dependent heterogeneities in phases and mechanical properties, which may cause unpredictable service failures. Here, we demonstrate a synergistic alloy design approach to overcome this issue in titanium alloys manufactured by laser powder bed fusion. The key to our approach is in-situ alloying of Ti−6Al−4V (in weight per cent) with combined additions of pure titanium powders and iron oxide (Fe
2
O
3
) nanoparticles. This not only enables in-situ elimination of phase heterogeneity through diluting V concentration whilst introducing small amounts of Fe, but also compensates for the strength loss via oxygen solute strengthening. Our alloys achieve spatially uniform microstructures and mechanical properties which are superior to those of Ti−6Al−4V. This study may help to guide the design of other alloys, which not only overcomes the challenge inherent to the AM processes, but also takes advantage of the alloy design opportunities offered by AM.
Based on the current pseudo-binary design strategy of eutectic high entropy alloy (EHEA), a novel approach to design eutectic or near-eutectic HEA was proposed. The eutectic composition can be ...experimentally specified through adjusting the atomic content ratio (RI/F) of the intermetallic forming elements to the face-centered cubic (FCC) forming elements in an equiatomic HEA system. This adjustment was done by reducing the contents of intermetallic forming elements (Mo and Cr) and accordingly increasing the content of FCC forming element (Fe). Based on this new design strategy, a cost-effective Fe-based near EHEA (Fe45Ni25Cr5Mo25 alloys) were designed and produced after decreasing the RI/F value from 1 to 3/7. The as-cast alloy was associated with compressive strength of 1661 MPa and ductility of 17%. It was considered that the hard trigonal μ phase together with the relatively ductile lamellar eutectic structure were responsible for the high combined properties. In addition, the effects of solute content, Mo and Cr in particular, on the microstructure and mechanical properties of this type of HEAs were also studied.
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•A novel strategy to design cost-effective eutectic high entropy alloys is proposed and experimentally verified.•A new Fe-based high entropy alloy with high balanced strength and ductility is developed based on this approach.•The mechanical properties of Fe-based high entropy alloys are sensitive to the volume fraction of eutectic structure.
The advanced electron backscatter diffraction (EBSD) technique was used to examine the microstructure of a widely used A517GrQ low-carbon low-alloy steel after different heat treatments. Three ...distinguishable microstructures were studied. Slow cooling in the furnace after austenitization led to the formation of a granular structure that consisted of massive ferrite and randomly distributed M–A constituents. Medium rate cooling in air produced granular bainite that was composed of lath ferrite, and M–A constituents were distributed between the laths. Lath martensite was formed by fast cooling into ice brine. EBSD analysis revealed that, in one austenite grain, the massive ferrite in the granular structure and the lath ferrite in the granular bainite were predominately separated by high-angle boundaries, whilst the ferrite laths in the martensite were separated by low-angle boundaries. The specimens with granular bainite formed by medium rate cooling had higher strength (both yield strength and tensile strength), and also almost 5 times higher Charpy impact energy than that of the specimens containing granular structure obtained at the slow cooling. The strength of the specimens with lath martensite after quenching into ice brine was slightly higher than the granular bainite but were associated with much lower Charpy impact energy. The present work indicates that it is critical to control the cooling rate after austenitization in order to simultaneously achieve high strength and high toughness of low-carbon low-alloy steels.
Eutectic modification through minor addition of boron was firstly proposed to improve the malleability of brittle eutectic high-entropy alloys (EHEAs). The proposed approach involves adding small ...amounts of eutectic modifier, boron (0.2–1.6 at.%), to a brittle dual-phase face centred cubic (FCC) and σ phase Fe
35
Ni
25
Cr
25
Mo
15
EHEA. Here, we report the role of boron additions in improvement of the mechanical properties of a Fe
35
Ni
25
Cr
25
Mo
15
EHEA. It was found that with the increase in boron additions up to 1 at.%, the lamellar eutectic structure consisting of FCC and σ intermetallic phase gradually transited to a dendrite-like eutectic structure. As a result, the compressive fracture strain of the alloy was increased about 3 times with a slight reduction in strength. The mechanisms of eutectic morphology transition induced by boron additions in the Fe
35
Ni
25
Cr
25
Mo
15
EHEA and their effects on mechanical properties were studied through microstructural characterization and thermodynamic analyses. The transition of eutectic morphology from lamellar eutectic to dendrite eutectic is believed to be resulted from the increased constitutional undercooling caused by B additions. The solute redistribution of B in the liquid ahead of the solid/liquid interface resulted in a B concentration gradient, which led to the formation of a constitutionally undercooled zone at the front of the σ phase, which destabilized the coupled growth conditions necessary for the formation of the lamellar eutectic structure. As a result, the eutectic morphology was transformed from lamellar eutectic to a dendritic-like eutectic structure. This type of structure exhibits a lower fraction of phase boundaries leading to improved malleability of the brittle EHEA. This insight can be used to design new advanced EHEAs through adjustment of the eutectic morphology.
Data-mining based machine learning (ML) method is emerging as a strategy to predict aluminium (Al) alloy properties with the promise of less intensive experimental work. However, ML models for ...wrought Al alloys are limited due to the difficulty in feature digitalization of the variety of manufacturing processes. Hence, most previous studies were constrained to specific alloy designations, which impeded the applicability of those ML models to broader wrought Al alloys. In the present work, we propose a novel feature engineering, called procedure-oriented decomposition (POD), assisting prediction framework to address the complexity introduced by manufacturing processes for wrought Al alloys. In this model, both chemical compositions and manufacturing processes are integrated as features. Correlation mapping of these features to the wrought Al alloys mechanical properties is established using the support vector regressor (SVR) model. The prediction framework demonstrates a high prediction accuracy and potential to design new alloys.
Abstract Ceramic oxides, renowned for their exceptional combination of mechanical, thermal, and chemical properties, are indispensable in numerous crucial applications across diverse engineering ...fields. However, conventional manufacturing methods frequently grapple with limitations, such as challenges in shaping intricate geometries, extended processing durations, elevated porosity, and substantial shrinkage deformations. Direct additive manufacturing (dAM) technology stands out as a state-of-the-art solution for ceramic oxides production. It facilitates the one-step fabrication of high-performance, intricately designed components characterized by dense structures. Importantly, dAM eliminates the necessity for post-heat treatments, streamlining the manufacturing process and enhancing overall efficiency. This study undertakes a comprehensive review of recent developments in dAM for ceramic oxides, with a specific emphasis on the laser powder bed fusion and laser directed energy deposition techniques. A thorough investigation is conducted into the shaping quality, microstructure, and properties of diverse ceramic oxides produced through dAM. Critical examination is given to key aspects including feedstock preparation, laser-material coupling, formation and control of defects, in-situ monitoring and simulation. This paper concludes by outlining future trends and potential breakthrough directions, taking into account current gaps in this rapidly evolving field.
Highlights Recent advances in shaping quality, microstructure, and mechanical performance of dAM processed ceramic oxides are reviewed. The research progress on critical issues within dAM of ceramic oxides is thoroughly examined. Future trends and potential breakthrough directions for dAM of ceramic oxides are outlined.
Additive manufacturing (AM), known as 3D printing, enables rapid fabrication of geometrically complex copper (Cu) components for electrical conduction and heat management applications. However, pure ...Cu or Cu alloys produced by 3D printing often suffer from either low strength or low conductivity at room and elevated temperatures. Here, we demonstrate a design strategy for 3D printing of high strength, high conductivity Cu by uniformly dispersing a minor portion of lanthanum hexaboride (LaB
) nanoparticles in pure Cu through laser powder bed fusion (L-PBF). We show that trace additions of LaB
to pure Cu results in an improved L-PBF processability, an enhanced strength, an improved thermal stability, all whilst maintaining a high conductivity. The presented strategy could expand the applicability of 3D printed Cu components to more demanding conditions where high strength, high conductivity and thermal stability are required.
The present work aims to boost our understanding of factors governing the grain-refining efficiency of inoculation treatments by comparing the grain-refining efficiencies of two inoculators: Ti ...nanoparticles and LaB6 nanoparticles, in a 2024 Al alloy during additive manufacturing (AM). Experimental results obtained by scanning electron microscopy show that the LaB6 nanoparticle possessed almost no refining effect on the alloy, with the addition content ranging from 0.5 wt.% to 2 wt.%. Conversely, the Ti nanoparticle resulted in a more pronounced refinement and a fine, fully equiaxed microstructure at 1 wt.% Ti addition. Based on transmission electron microscopy analysis, the higher refining efficiency of Ti inoculation was ascribed to the incorporation of both Ti solute and the in situ-formed L12-Al3Ti nucleation particles. The former significantly increased the overall undercooling ahead of the growing Al grain, which ensured the activation of heterogeneous nucleation on the L12-Al3Ti nanoparticles, leading to grain refinement. This work highlights that despite the addition of nucleation particles, the incorporation of appropriate solutes to generate sufficient undercooling is the prerequisite for the activation of heterogenous nucleation in AM.
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