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Additive manufacturing, often known as three-dimensional (3D) printing, is a process in which a part is built layer-by-layer and is a promising approach for creating components close to their final (net) shape. This process is challenging the dominance of conventional manufacturing processes for products with high complexity and low material waste 1 . Titanium alloys made by additive manufacturing have been used in applications in various industries. However, the intrinsic high cooling rates and high thermal gradient of the fusion-based metal additive manufacturing process often leads to a very fine microstructure and a tendency towards almost exclusively columnar grains, particularly in titanium-based alloys 1 . (Columnar grains in additively manufactured titanium components can result in anisotropic mechanical properties and are therefore undesirable 2 .) Attempts to optimize the processing parameters of additive manufacturing have shown that it is difficult to alter the conditions to promote equiaxed growth of titanium grains 3 . In contrast with other common engineering alloys such as aluminium, there is no commercial grain refiner for titanium that is able to effectively refine the microstructure. To address this challenge, here we report on the development of titanium–copper alloys that have a high constitutional supercooling capacity as a result of partitioning of the alloying element during solidification, which can override the negative effect of a high thermal gradient in the laser-melted region during additive manufacturing. Without any special process control or additional treatment, our as-printed titanium–copper alloy specimens have a fully equiaxed fine-grained microstructure. They also display promising mechanical properties, such as high yield strength and uniform elongation, compared to conventional alloys under similar processing conditions, owing to the formation of an ultrafine eutectoid microstructure that appears as a result of exploiting the high cooling rates and multiple thermal cycles of the manufacturing process. We anticipate that this approach will be applicable to other eutectoid-forming alloy systems, and that it will have applications in the aerospace and biomedical industries. Titanium–copper alloys with fully equiaxed grains and a fine microstructure are realized via an additive manufacturing process that exploits high cooling rates and multiple thermal cycles.

Aluminum alloys have been extensively used as heatproof and heat-dissipation components in automotive and communication industries, and the demand for aluminum alloys with higher thermal conductivity is increasing. Therefore, this review focuses on the thermal conductivity of aluminum alloys. First, we formulate the theory of thermal conduction of metals and effective medium theory, and then analyze the effect of alloying elements, secondary phases, and temperature on the thermal conductivity of aluminum alloys. Alloying elements are the most crucial factor, whose species, existing states, and mutual interactions significantly affect the thermal conductivity of aluminum. Alloying elements in a solid solution weaken the thermal conductivity of aluminum more dramatically than those in the precipitated state. The characteristics and morphology of secondary phases also affect thermal conductivity. Temperature also affects thermal conductivity by influencing the thermal conduction of electrons and phonons in aluminum alloys. Furthermore, recent studies on the effects of casting, heat treatment, and AM processes on the thermal conductivity of aluminum alloys are summarized, in which processes mainly affect thermal conductivity by varying existing states of alloying elements and the morphology of secondary phases. These analyses and summaries will further promote the industrial design and development of aluminum alloys with high thermal conductivity.

Structural alloys are often strengthened through the addition of solute atoms. However, given that solute atoms interact weakly with the elastic fields of screw dislocations, it has long been accepted that solution hardening is only marginally effective in materials with mobile screw dislocations. By using transmission electron microscopy and nanomechanical characterization, we report that the intense hardening effect of dilute oxygen solutes in pure α-Ti is due to the interaction between oxygen and the core of screw dislocations that mainly glide on prismatic planes. First-principles calculations reveal that distortion of the interstitial sites at the screw dislocation core creates a very strong but short-range repulsion for oxygen that is consistent with experimental observations. These results establish a highly effective mechanism for strengthening by interstitial solutes.

Sn-based lead-free solders such as Sn-Ag-Cu, Sn-Cu, and Sn-Bi have been used extensively for a long time in the electronic packaging field. Recently, low-temperature Sn-Bi solder alloys attract much attention from industries for flexible printed circuit board (FPCB) applications. Low melting temperatures of Sn-Bi solders avoid warpage wherein printed circuit board and electronic parts deform or deviate from the initial state due to their thermal mismatch during soldering. However, the addition of alloying elements and nanoparticles Sn-Bi solders improves the melting temperature, wettability, microstructure, and mechanical properties. Improving the brittleness of the eutectic Sn-58wt%Bi solder alloy by grain refinement of the Bi-phase becomes a hot topic. In this paper, literature studies about melting temperature, microstructure, inter-metallic thickness, and mechanical properties of Sn-Bi solder alloys upon alloying and nanoparticle addition are reviewed.

An attempt is made to cover the whole of the topic of biodegradable magnesium (Mg) alloys with a focus on the biocompatibility of the individual alloying elements, as well as shed light on the degradation characteristics, microstructure, and mechanical properties of most binary alloys. Some of the various work processes carried out by researchers to achieve the alloys and their surface modifications have been highlighted. Additionally, a brief look into the literature on magnesium composites as also been included towards the end, to provide a more complete picture of the topic. In most cases, the chronological order of events has not been particularly followed, and instead, this work is concentrated on compiling and presenting an update of the work carried out on the topic of biodegradable magnesium alloys from the recent literature available to us.

The effect of ternary alloying elements (Mo and Ta) on the mechanical and superelastic properties of binary Ti-14Nb alloy fabricated by the mechanical alloying and spark plasma sintering was investigated. The materials were prepared in two ways: (i) by substituting Nb in base Ti-14Nb alloy by 2 at.% of the ternary addition, giving the following compositions: Ti-8Nb-2Mo and Ti-12Nb-2Ta and (ii) by adding 2 at.% of the ternary element to the base alloy. The microstructures of the materials consisted of the equiaxed β-grains and fine precipitations of TiC. The substitution of Nb by both Mo and Ta did not significantly affect the mechanical properties of the base Ti-14Nb alloy, however, their addition resulted in a decrease of yield strength and increase of plasticity. This was associated with the occurrence of the 332 twinning that was found during the in-situ observations. The elevated concentration of interstitial elements (oxygen and carbon) lead to the occurrence of stress-induced martensitic transformation and twinning mechanisms at lower concentration of β-stabilizers in comparison to the conventionally fabricated materials. The substitution of Nb by Mo, and Ta caused the slight improvement of the superelastic properties of the base Ti-14Nb alloy, whereas their addition deteriorated the superelasticity.

The utilization of Ti–Mo alloys in biomedical applications has gained attention for use in biomedical applications owing to their non-toxicity, reasonable cost, and favorable properties. In the present study, Ti–12Mo–6Zr and Ti–15Mo–6Zr alloys were prepared using elemental blend and mechanical alloying techniques. The effect of alloying elements Mo and Zr of Ti–Mo alloy, as well as the effect of fabrication techniques of Ti–Mo–Zr trinary alloys, were investigated. Thermodynamic calculations supported by CALPHAD analysis revealed that the addition of Zr increases lattice distortion, which contributes to enhancing the strength. Conversely, adding Mo decreases the enthalpy, facilitating improved mixing and solid solution formation. The as-sintered samples were characterized by X-ray diffraction, optical microscope, and scanning electron microscopy, and their microhardness, compressive, and corrosion behavior were investigated. Among all the investigated alloys, Ti–15Mo–6Zr alloy prepared by the mechanical alloying technique, milled for six hours at 300 rpm, compacted at 600 MPa, and sintered at 1250 ℃, shows good comprehensive mechanical properties with a preferable compressive strength (− 1710 MPa) and hardness (396 HV5), as well as the lowest wear rate (0.69%) and corrosion rate (0.557 × 10 –3  mm/year). This can be related to the solid solution strengthening and relative density, together with dispersion and precipitation strengthening of the α phase. Remarkably, the combination of high mechanical and corrosion properties can be achieved by tailoring the content of the α phase, controlling the density, and providing new fabricating techniques for β Ti alloys. Graphical Abstract

With the implementation of legislations on inhibiting the usage of Sn–Pb solder in consumer electronic products, Sn–Ag–Cu series solder has been gotten the most application. However, there are some stimulations from electronic manufacturers to adopt low temperature soldering such as the economic driver from the reduction in manufacturing assembly cost and the reliability driver to avoid the dynamic warpage of area array components caused from Sn–Ag–Cu solder. Sn–Bi series solder is one of the promising candidates, which met the requirements for low melting point, low cost and environment friendly. However, the disadvantage of brittleness characteristic prevented its wide practical application. In order to promote the better application of Sn–Bi based solders, many efforts have been made to improve the wettability, mechanical properties and reliability of Sn–Bi based solders. This paper will summarize the related results about Sn–Bi solder alloys from wettability, interfacial reaction, mechanical properties of Sn–Bi solder and reliabilities of Sn–Bi solder joints. Moreover, in order to improve the properties of Sn–Bi solders, researchers have done lots of works on effect of addition of element dopants. The corresponding works of effect of alloying elements on the properties of Sn–Bi solder were also focused. According to the existing research results, it provides an important basis of understanding the current development of Sn–Bi solders.

The feasibility of developing NiTi-X alloys by laser powder bed fusion is exemplified. Solution treatment was used to enhance the mechanical and functional properties. Ternary NiTiNb alloys show good tensile properties and wide hysteresis. The relationship between microstructure and performance was established. Post-heat treatment is commonly employed to improve the microstructural homogeneity and enhance the mechanical performances of the additively manufactured metallic materials. In this work, a ternary (NiTi) 91 Nb 9 (at.%) shape memory alloy was produced by laser powder bed fusion (L-PBF) using pre-alloyed NiTi and elemental Nb powders. The effect of solution treatment on the microstructure, phase transformation behavior and mechanical/functional performances was investigated. The in-situ alloyed (NiTi) 91 Nb 9 alloy exhibits a submicron cellular-dendritic structure surrounding the supersaturated B2-NiTi matrix. Upon high-temperature (1273 K) solution treatment, Nb-rich precipitates were precipitated from the supersaturated matrix. The fragmentation and spheroidization of the NiTi/Nb eutectics occurred during solution treatment, leading to a morphological transition from mesh-like into rod-like and sphere-like. Coarsening of the β -Nb phases occurred with increasing holding time. The martensite transformation temperature increases after solution treatment, mainly attributed to: ( i ) reduced lattice distortion due to the Nb expulsion from the supersaturated B2-NiTi, and ( ii ) the Ti expulsion from the β -Nb phases that lowers the ratio Ni/Ti in the B2-NiTi matrix, which resulted from the microstructure changes from non-equilibrium to equilibrium state. The thermal hysteresis of the solutionized alloys is around 145 K after 20% pre-deformation, which is comparable to the conventional NiTiNb alloys. A short-term solution treatment (i.e. at 1 273 K for 30 min) enhances the ductility and strength of the as-printed specimen, with the increase of fracture stress from (613 ± 19) MPa to (781 ± 20) MPa and the increase of fracture strain from (7.6 ± 0.1)% to (9.5 ± 0.4)%. Both the as-printed and solutionized samples exhibit good tensile shape memory effects with recovery rates >90%. This work suggests that post-process heat treatment is essential to optimize the microstructure and improve the mechanical performances of the L-PBF in-situ alloyed parts.

In order to explore the effect of alloying on the microstructures and mechanical properties of AlCoCrFeNi2.1 eutectic high-entropy alloys (EHEAs), 0.1, 0.2, and 0.3 at.% V, Mo, and B were added to the AlCoCrFeNi2.1 alloy in this work. The effects of the elements and contents on the phase composition, microstructures, mechanical properties, and fracture mechanism were investigated. The results showed that the crystal structures of the AlCoCrFeNi2.1 EHEAs remained unchanged, and the alloys were still composed of FCC and BCC structures, whose content varied with the addition of alloying elements. After alloying, the aggregation of Co, Cr, Al, and Ni elements remained unchanged, and the V and Mo were distributed in both dendritic and interdendritic phases. The tensile strengths of the alloys all exceeded 1000 MPa when the V and Mo elements were added, and the Mo0.2 alloy had the highest tensile strength, of 1346.3 MPa, and fracture elongation, of 24.6%. The alloys with the addition of V and Mo elements showed a mixed ductile and brittle fracture, while the B-containing alloy presented a cleavage fracture. The fracture mechanism of Mo0.2 alloy is mainly crack propagation in the BCC lamellae, and the FCC dendritic lamellae exhibit the characteristics of plastic deformation.