Additive manufacturing (AM) constitutes the new paradigm in materials processing and its use on metals and alloys opens new unforeseen possibilities, but is facing several challenges regarding the design of the microstructure, which is particularly awkward in the case of functional materials, like shape memory alloys (SMA), as they require a robust microstructure to withstand the constraints appearing during their shape change. In the present work, the attention is focused on the AM of the important Fe-Mn-Si-based SMA family, which is attracting a great technological interest in many industrial sectors. Initially, an overview on the design concepts of this SMA family is offered, with special emphasis to the problems arising during AM. Then, such concepts are considered in order to experimentally develop the AM production of the Fe-20Mn-6Si-9Cr-5Ni (wt%) SMA through laser powder bed fusion (LPBF). The complete methodology is approached, from the gas atomization of powders to the LPBF production and the final thermal treatments to functionalize the SMA. The microstructure is characterized by scanning and transmission electron microscopy after each step of the processing route. The reversibility of the epsilon martensitic transformation and its evolution on cycling are studied by internal friction and electron microscopy. An outstanding 14% of fully reversible thermal transformation of epsilon martensite is obtained. The present results show that, in spite of the still remaining challenges, AM by LPBF offers a good approach to produce this family of Fe-Mn-Si-based SMA, opening new opportunities for its applications.

This work presents a detailed study of hydrogen decrepitation (HD) to obtain mono-crystalline Nd-Fe-B powder. The effect of decrepitation temperature has been investigated to optimize both particle size and shape. Differential scanning calorimetry was applied to analyze the hydrogenation kinetics of Nd2Fe14B and Nd-rich phases in the range of 25 to 300 degrees C. Thermogravimetry and X-ray diffraction allowed determining the hydrogen ab-sorption of the whole alloy and the matrix phase, respectively. While scanning electron microscopy (SEM) was used to visualize particle shape and size, dynamic image analysis was applied to evaluate quantitatively these properties. The high monocrystallinity of the powder was confirmed by electron backscattering diffraction. The partial pressure of hydrogen required to initiate the hydrogenation reactions decreases when the temperature increases. The hydrogen absorbed by the whole alloy and, in particular, by the Nd2Fe14B phase decreases with temperature. Below 150 degrees C, the hydrogen absorbed by the Nd2Fe14B phase produces a significant transgranular cracking that is undesirable for particle shape. At 300 degrees C, the fast and limited absorption of hydrogen by the Nd-rich phase causes insufficient intergranular fracture and, hence, polycrystallinity. Between 150 and 300 degrees C, the controlled fragmentation resulted in monocrystalline particles with a more equiaxial shape, which is a suitable precursor to develop anisotropic ultrafine powders by the hy-drogenation, disproportionation, desorption, recombination (HDDR) process.(c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

New compositions of Nd-Fe-B powders were produced via gas atomization. Helium and argon were used as atomizing gases. For the same process parameters, helium resulted in a powder that is 6 times finer than the powder produced with argon. In both cases, the atomization conditions were set to achieve fine powders and, therefore, fine microstructures, since this is critical to obtain a suitable coercivity. The powders were sieved to separate the fraction below 20 mu m(similar to 90 % of powder atomized with He). The microstructure is formed by a mix of elongated and equiaxial grains with random crystallographic orientation and an average grain size of similar to 1 mu m, which was measured by Electron Backscattered Diffraction. This kind of measurements and images were taken for the first time for fine helium atomized Nd-Fe-B powders. The embedding of the powder in copper was found to be useful, as the conventional mounting in metallographic conductive resin produced charging of the sample. Annealing up to 1000 degrees C did not cause any significant grain growth that could deteriorate the magnetic properties. Annealing at 700 degrees C for 15 min improved the magnetic properties due to the crystallization of residual amorphous phase. Warm compaction was used to produce magnets with a high volume fraction of magnetic powder (>70 %) and low real porosity (similar to 5 %). These magnets exhibit a high remanence (similar to 0.5 T) and maximum energy product (similar to 35 kJ/m(3)), combined with a suitable intrinsic coercivity (>500 kA/m). The new compositions and processing route could cover a market niche. The thoroughly analyzed powders could be also suitable for some additive manufacturing technologies that require spherical powders with very specific particle size distributions. (c) 2023 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Shape memory alloys (SMAs) are functional materials that are being applied in practically all industries, from aerospace to biomedical sectors, and at present the scientific and technologic communities are looking to gain the advantages offered by the new processing technologies of additive manufacturing (AM). However, the use of AM to produce functional materials, like SMAs, constitutes a real challenge due to the particularly well controlled microstructure required to exhibit the functional property of shape memory. In the present work, the design of the complete AM processing route, from powder atomization to laser powder bed fusion for AM and hot isostatic pressing (HIP), is approached for Cu-Al-Ni SMAs. The microstructure of the different processing states is characterized in relationship with the processing parameters. The thermal martensitic transformation, responsible for the functional properties, is analyzed in a comparative way for each one of the different processed samples. The present results demonstrate that a final post-processing thermal treatment to control the microstructure is crucial to obtain the expected functional properties. Finally, it is demonstrated that using the designed processing route of laser powder bed fusion followed by a post-processing HIP and a final specific thermal treatment, a satisfactory shape memory behavior can be obtained in Cu-Al-Ni SMAs, paving the road for further applications.

Gas atomized Nd-Fe-B powders of several compositions were separated in different size fractions by sieving. These fractions were annealed between 1100 degrees C and 1150 degrees C for 24 and 96 h. The oxygen content of the powders was measured before and after annealing for the different size fractions. The oxygen concentration of the powders depends strongly on the particle size and increases significantly during annealing, particularly in the case of small particle sizes. The effect of particle size on the microstructural changes was analyzed in detail, particularly on grain growth, using high resolution scanning electron microscopy and transmission electron microscopy. Electron back scattering diffraction was used to measure grain size. When the particle size rises, the degree of sintering decreases and the higher solid/vapor surface area reduces the mobility of grain boundaries. Oxidation also reduces grain growth rate and its effect is more evident for particles sizes below 45-63 mu m and high Nd concentrations. Nb addition leads to the formation of intra- and intergranular precipitates. The size of these Nb-Fe-containing precipitates increases with the particle size for equivalent annealing conditions. At 1150 degrees C, Nb loses its effect as an inhibitor of grain growth in the particle size fractions larger than 45-63 mu m.

Among the different families of shape memory alloys (SMA), the Fe-Mn-Si-Cr-Ni alloys have attracted a renewed interest because of its low cost, high corrosion resistance and high recovery strength during the shape memory effect, and the new technologies of additive manufacturing offer unforeseen possibilities for this family of SMA. In the present work, the reversible gamma - epsilon martensitic transformation (MT), responsible for the shape memory effect, is studied in two Fe-Mn-Si-Cr-Ni alloys with high (20.2 wt%) and low (15.8 wt%) Mn content, produced by the conventional route of casting and rolling, in comparison with the MT in another similar alloy, with intermediate Mn content (19.4 wt%), which was produced by gas atomization and additive manufacturing through laser metal deposition. The forward and reverse gamma - epsilon MT is studied by mechanical spectroscopy through the internal friction spectra and the dynamic modulus variation, together with a parallel microstructural characterization including in-situ observation of the gamma - epsilon MT during cooling and heating at the scanning electron microscope. The evolution of the transformed fraction of epsilon martensite, evaluated through the integral area of the internal friction peak, was followed along thermal cycling in all three alloys. Both, the internal friction and the electron microscopy studies show that the epsilon martensite amount increases very fast during the first few cycles, and then decreases with a tendency towards its stabilization for many tens of cycles. The results show that the gamma - epsilon MT is more stable on cycling in the additive manufactured sample than in the conventionally processed samples, opening new avenues for designing shape memory steels to be specifically processed through additive manufacturing.

Nd-Fe-B powders of different compositions were produced by gas atomization. These powders were annealed between 1000 and 1150 degrees C for several times to study the microstructural evolution. Differential scanning calorimetry was used to determine the thermal transitions on the as-atomized powders and after slow solidification. The microstructure was studied by high resolution scanning electron microscopy at each stage. Electron back scattering diffraction was used to measure grain size and confirm that gas atomized powders are isotropic. It was observed the formation of necks between the particles, densification, and grain growth due to liquid phase sintering. Grain growth and densification occur in parallel by a dissolution-reprecipitation mechanism. The effect of Nd content and Nb addition on the microstructural changes was analyzed in detail, particularly on grain growth. The degree of sintering increases with Nd content, as this element enhances the formation of the liquid phase. Nb addition leads to the formation of precipitates that delay densification and grain growth at 1000 degrees C, but promote abnormal grain growth at 1100 degrees C.

We present the evolution of magnetic anisotropy obtained from the magnetization curve of (Fe0.76Si0.09B0.10P0.05)(97.5)Nb2.0Cu0.5 amorphous and nanocrystalline alloy produced by a gas atomization process. The material obtained by this process is a powder exhibiting amorphous character in the as-atomized state. Heat treatment at 480 degrees C provokes structural relaxation, while annealing the powder at 530 degrees C for 30 and 60 min develops a fine nanocrystalline structure. Magnetic anisotropy distribution is explained by considering dipolar effects and the modified random anisotropy model.

The present work demonstrates the high-frequency core loss performance of Fe-based amorphous and nanocrystalline powder cores, initially produced by gas atomised powder, consolidated using sieved particles <= 20 mu m, and isolated by a precise insulating layer of polymer to limit the inter- and intra-particle eddy currents to attain enhanced performance. The large glass forming ability (GFA) of the gas atomised powder, reflected by different glass forming instruments, such as the supercooled region (Delta T-x = 54 degrees C) and the reduced glass transition temperature (T-rg = 0.56), is consistent with the substantial amorphisation capability of the alloy. To the best of our knowledge, this is the first-ever report to reveal a large Delta T-x in the Finemet-type alloy powders, an essential parameter to gas-atomise the amorphous powders with significantly lower cooling rates compared to the melt-spun ribbons. Further, subsequent annealing of the amorphous powders, between the exothermic events guided by differential scanning calorimetry (DSC), lead to the growth of a fine nanocrystalline structure of grains <= 15 nm, thanks to the positive enthalpy of mixing of Cu with the constituents to act as a nucleation agent, to retain the excellent soft magnetic properties. The DC soft magnetic properties of the powders were significantly improved on thermal annealing, confirmed by hysteretic loops, quantified by reduced coercivity H-c < 1 Oe of annealed powders at < 575 degrees C, and attributed to the reduced magnetoelastic contribution due to zero/near-zero magnetostriction anisotropy, attained due to the homogenous nanocrystalline structure. The amorphous and nanocrystalline powder cores, consolidated by compression moulding, show ultra-high loss performance, due to the ultra-low coercivity attained on nanocrystallisation, and negligible eddy currents loss, owning to efficient insulation of small particles, for high-frequency power conversion applications, such as voltage regulator (VR), and resonant converters, in automotive industry and data storage centres.

Compared to pure tungsten, self-passivating tungsten based alloys for the first wall armor of future fusion reactors shall provide a major safety advantage in case of a loss-of-coolant accident with simultaneous air ingress, due to the formation of a stable protective scale at high temperatures in presence of oxygen preventing the formation of volatile and radioactive WO3. Recently developed W-Cr-Y alloys produced by mechanical alloying and hot isostatic pressing (HIP) exhibit a strong reduction of oxidation rate compared to pure W and high mechanical strength. A heat treatment after HIP at 1555°C results in a one-phase material with a high thermal shock resistance. Nevertheless, the microstructure is metastable and its thermal stability under operational conditions has to be assessed. In this work results of thermal stability tests on heat treated W-10Cr-0.5Y alloy subjected to temperatures of 650, 700, 800 and 1000°C for times ranging from 50 to 1000h are presented. After 1000h at 650°C and 100h at 700°C no visible change of the microstructure is detected. After 100h at 1000°C a complete decomposition takes place with the formation of a uniform, fine-scale mixture of W- and Cr-rich phases, typical for spinodal decomposition.