Because magnets fabricated using Nd-Fe-B exhibit excellent magnetic properties, this novel material is used in various high-tech industries. However, because of the brittleness and low formability of Nd-Fe-B magnets, the design freedom of shapes for improving the performance is limited based on conventional tooling and postprocessing. Laserpowder bed fusion (L-PBF), the most famous additive manufacturing (AM) technique, has recently emerged as a novel process for producing geometrically complex shapes of Nd-Fe-B parts owing to its high precision and good spatial resolution. However, because of the repeated thermal shock applied to the materials during L-PBF, it is difficult to fabricate a dense Nd-Fe-B magnet. In this study, a high-density (>96%) Nd-Fe-B magnet is successfully fabricated by minimizing the thermal residual stress caused by substrate heating during L-PBF.

Renewed interest in the reinforced carbon graphite composites has intrigued the community in the advanced materials fields. In this work, we present a simple carbon nanofibers reinforced carbon graphite composites synthetic method by incorporating mixture of coal tar pitch, synthetic graphite, pitch coke and the dispersion liquid of carbon nanofibers via liquid-phase mixing process. The impact of carbon nanofiber utilization on the microstructures and mechanical properties of carbon graphite composites are studied systematically. The covalent surface modification of carbon nanofibers effectively improves its microstructure and thereby promotes the carbon graphite composites’ dispersion behavior. We propose that a small amount of carbon nanofibers could promote the carbonization process of carbon graphite composites, facilitating the densification of carbon graphite composites and reducing the undesired open porosity. The amount of 0.7 wt % of carbon nanofiber concentration allows the enhancement of bend and compressive strength of carbon graphite composites up to 36.50 MPa and 60.46 MPa, increased by 167.9% and 146.9% compared with the pure carbon graphite composite, respectively. Our findings can be rationalized due to the improvement in the mechanical strength of carbon graphite composites could be attributed due to pull-out of carbon nanofibers from the matrix and bridging effect across the crack pores within the matrix.

In additive manufacturing, the flowability of feedstock particles determines the quality of the parts that are affected by different parameters, including the chemistry and morphology of the powders and particle size distribution. In this study, the microstructures and flowabilities of gas-atomized heat-resistant alloys for additive manufacturing applications are investigated. A KHR45A alloy powder with a composition of Fe-30Cr-40Mn-1.8Nb (wt.%) is fabricated using gas atomization process. The microstructure and effect of powder chemistry and morphology on the flow behavior are investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and revolution powder analysis. The results reveal the formation of spherical particles composed of single-phase FCC dendritic structures after gas atomization. SEM observations show variations in the microstructures of the powder particles with different size distributions. Elemental distribution maps, line scans, and high-resolution XPS results indicate the presence of a Si-rich oxide accompanied by Fe, Cr, and Nb metal oxides in the outer layer of the powders. The flowability behavior is found to be induced by the particle size distribution, which can be attributed to the interparticle interactions and friction of particles with different sizes.

The evolvement in the microstructure and electrical properties of PAN-based carbon fibers during high-temperature carbonization were investigated. The study showed that as the heat treatment temperature increases, the change of carbon fiber resistivity around 1100 °C can be divided into two stages. In the first stage, the carbon content of the fiber increased rapidly, and small molecules such as nitrogen were gradually released to form a turbostratic of carbon crystal structure. The resistivity dropped rapidly from 3.19 × 10− 5 Ω·m to 2.12 × 10− 5 Ω·m. In the second stage, the carbon microcrystalline structure gradually became regular, and the electron movement area gradually became larger. At this time, the resistivity further decreases, from 2.12 × 10− 5 Ω·m to 1.59 × 10− 5 Ω·m. During carbonization, the tensile strength of carbon fiber first increased and then decreased. This is because the irregular and disordered graphite structure is formed first. As the temperature rose, the graphite layer spacing decreased and the grain thickness gradually increases. The modulus also gradually increased.

Yttria-stabilized zirconia (YSZ) has a low thermal conductivity, high thermal expansion coefficient, and excellent mechanical properties; thus, it is used as a thermal barrier coating material for gas turbines. However, during long-time exposure of YSZ to temperatures of 1200oC or higher, a phase transformation accompanied by a volume change occurs, causing the YSZ coating layer to peel off. To solve this problem, YSZ has been doped with trivalent and tetravalent oxides to obtain coating materials with low thermal conductivity and suppressed phase transformation of zirconia. In this study, YSZ is doped with trivalent oxides, Nd2O3, Yb2O3, Al2O3, and tetravalent oxide, TiO2, and the thermal conductivity of the obtained materials is analyzed according to the composition; furthermore, the relative density change, microstructure change, and m-phase formation behavior are analyzed during long-time heat treatment at high temperatures.

This research investigated how adding Sb (0.75, 1.0, 2.0 and 5.0 wt%) to as-extruded aluminum alloys affected their microstructure, mechanical properties, electric and thermal conductivity. The addition of Sb resulted in the formation of AlSb intermetallic compounds. It was observed that intermetallic compounds in the alloys were distributed homogenously in the Al matrix. As the content of Sb increased, the area fraction of intermetallic compounds increased. It can be clearly seen that the intermetallic compounds were crushed into fine particles and homogenously arrayed during the extrusion process. As the Sb content increased, the average grain size decreased remarkably from 282.6 μm (0.75 wt%) to 109.2 μm (5.0 wt%) due to dynamic recrystallization by the dispersed intermetallic compounds in the aluminum matrix during the hot extrusion. As the Sb content increased from 0.75 to 2.0 wt%, the electrical conductivity decreased from 61.0 to 59.8 % of the International Annealed Copper Standard. Also, as the Sb content increased from 0.75 to 2.0 wt%, the ultimate tensile strength did not significantly change, from 67.3 to 67.8 MPa.

This research investigated the effect of Si addition on the microstructure, mechanical properties, electric and thermal conductivity of as-extruded Al 6013 alloys. As the content of Si increased, the area fraction of the second phase increased. As the Si content increased, the average grain size decreased remarkably, from 182 (no Si addition) to 142 (1.5Si), 78 (3.0Si) and 77 μm (4.5Si) due to dynamic recrystallization by the dispersed second particles in the aluminum matrix during the hot extrusion. As the Si content increased, the yield strength and ultimate tensile strength increased. The maximum values of yield strength and ultimate tensile strength were 224 MPa and 103 MPa for the 6013-4.5Si alloy. As the amount of Si added increased, the electrical and thermal conductivity decreased. The electrical and thermal conductivity of the Al6013-4.5Si alloy were 44.0% IACS and 165.0 W/mK, respectively. The addition of Si to Al 6013 alloy had a significant effect on its thermal conductivity and mechanical properties.

Changes in the mechanical properties and microstructure of an IN 939 W alloy according to the sintering heating rate were evaluated. IN 939 W alloy samples were fabricated by spark plasma sintering. The phase fraction, number density, and mean radius of the IN 939W alloy were calculated using a thermodynamic calculation. A universal testing machine and micro-Vickers hardness tester were employed to confirm the mechanical properties of the IN 939W alloy. X-ray diffraction, optical microscopy, field-emission scanning electron microscopy, Cs-corrected-field emission transmission electron microscopy, and energy dispersive X-ray spectrometry were used to evaluate the microstructure of the alloy. The rapid sintering heating rate resulted in a slightly dispersed γ' phase and chromium oxide. It also suppressed the precipitation of the η phase. These helped to reinforce the mechanical properties.

A typical trade-off relationship exists between strength and elongation in face-centered cubic metals. Studies have recently been conducted to enhance strength without ductility reduction through surface-treatment-based ultrasonic nanocrystalline surface modification (UNSM), which creates a gradient microstructure in which grains become smaller from the inside to the surface. The transformation-induced plasticity effect in Fe-Mn alloys results in excellent strength and ductility due to their high work-hardening rate. This rate is achieved through strain-induced martensitic transformation when an alloy is plastically deformed. In this study, Fe-6%Mn powders with different sizes were prepared by high-energy ball milling and sintered through spark plasma sintering to produce Fe-6%Mn samples. A gradient microstructure was obtained by stacking the different-sized powders to achieve similar effects as those derived from UNSM. A compressive test was performed to investigate the mechanical properties, including the yielding behavior. The deformed microstructure was observed through electron backscatter diffraction to determine the effects of gradient plastic deformation.

Beta-titanium alloys are used in many industries due to their increased elongation resulting from their BCC structure and low modulus of elasticity. However, there are many limitations to their use due to the high cost of betastabilizer elements. In this study, biocompatible Ti-Mo-Fe beta titanium alloys are designed by replacing costly betastabilizer elements (e.g., Nb, Zr, or Ta) with inexpensive Mo and Fe elements. Additionally, Ti-Mo-Fe alloys designed with different Fe contents are fabricated using powder metallurgy. Fe is a strong, biocompatible beta-stabilizer element and a low-cost alloying element. The mechanical properties of the Ti-Mo-Fe metastable beta titanium alloys are analyzed in relation to the microstructural changes. When the Fe content increases, the tensile strength and elongation decrease due to brittle fracture despite a decreasing pore fraction. It is confirmed that the hardness and tensile strength of Ti-5Mo-2Fe P/M improve to more than 360 Hv and 900 MPa, respectively.

The effect of the laser beam diameter on the microstructure and hardness of 17-4 PH stainless steel manufactured via the directed energy deposition process is investigated. The pore size and area fraction are much lower using a laser beam diameter of 1.0 mm compared with those observed using a laser beam diameter of 1.8 mm. Additionally, using a relatively larger beam diameter results in pores in the form of incomplete melting. Martensite and retained austenite are observed under both conditions. A smaller width of the weld track and overlapping area are observed in the sample fabricated with a 1.0 mm beam diameter. This difference appears to be mainly caused by the energy density based on the variation in the beam diameter. The sample prepared with a beam diameter of 1.0 mm had a higher hardness near the substrate than that prepared with a 1.8 mm beam diameter, which may be influenced by the degree of melt mixing between the 17-4 PH metal powder and carbon steel substrate.

A Cu-15Ag-5P filler metal (BCuP-5) is fabricated on a Ag substrate using a high-velocity oxygen fuel (HVOF) thermal spray process, followed by post-heat treatment (300oC for 1 h and 400oC for 1 h) of the HVOF coating layers to control its microstructure and mechanical properties. Additionally, the microstructure and mechanical properties are evaluated according to the post-heat treatment conditions. The porosity of the heat-treated coating layers are significantly reduced to less than half those of the as-sprayed coating layer, and the pore shape changes to a spherical shape. The constituent phases of the coating layers are Cu, Ag, and Cu-Ag-Cu3P eutectic, which is identical to the initial powder feedstock. A more uniform microstructure is obtained as the heat-treatment temperature increases. The hardness of the coating layer is 154.6 Hv (as-sprayed), 161.2 Hv (300oC for 1 h), and 167.0 Hv (400oC for 1 h), which increases with increasing heat-treatment temperature, and is 2.35 times higher than that of the conventional cast alloy. As a result of the pull-out test, loss or separation of the coating layer rarely occurs in the heat-treated coating layer.

This study demonstrates the effect of the compaction pressure on the microstructure and properties of pressureless-sintered W bodies. W powders are synthesized by ultrasonic spray pyrolysis and hydrogen reduction using ammonium metatungstate hydrate as a precursor. Microstructural investigation reveals that a spherical powder in the form of agglomerated nanosized W particles is successfully synthesized. The W powder synthesized by ultrasonic spray pyrolysis exhibits a relative density of approximately 94% regardless of the compaction pressure, whereas the commercial powder exhibits a relative density of 64% under the same sintering conditions. This change in the relative density of the sintered compact can be explained by the difference in the sizes of the raw powder and the densities of the compacted green body. The grain size increases as the compaction pressure increases, and the sintered compact uniaxially pressed to 50 MPa and then isostatically pressed to 300 MPa exhibits a size of 0.71 m. The Vickers hardness of the sintered W exhibits a high value of 4.7 GPa, mainly due to grain refinement.

In this study, a new manufacturing process for a multilayer-clad electrical contact material is suggested. A thin and dense BCuP-5 (Cu-15Ag-5P filler metal) coating layer is fabricated on a Ag plate using a high-velocity oxygen-fuel (HVOF) process. Subsequently, the microstructure and bonding properties of the HVOF BCuP-5 coating layer are evaluated. The thickness of the HVOF BCuP-5 coating layer is determined as 34.8 μm, and the surface fluctuation is measured as approximately 3.2 μm. The microstructure of the coating layer is composed of Cu, Ag, and Cu-Ag-Cu3P ternary eutectic phases, similar to the initial BCuP-5 powder feedstock. The average hardness of the coating layer is 154.6 HV, which is confirmed to be higher than that of the conventional BCuP-5 alloy. The pull-off strength of the Ag/BCup-5 layer is determined as 21.6 MPa. Thus, the possibility of manufacturing a multilayer-clad electrical contact material using the HVOF process is also discussed.

Abstract In the present study, the effect of nickel nitrate addition as a catalytic precursor for the in situ formation of Ni nanoparticles during the heating process has been investigated on the modification of microstructure and graphitization of amorphous carbon resulting from pyrolysis of phenolic resin. For this purpose, the prepared resin samples were cured in carbon substrate with and without additives at temperatures of 800, 1000, and 1250 °C. XRD, FESEM, and TEM studies were performed to investigate the phase and microstructural changes in the samples during the heating process. In addition to phase and microstructural studies, thermodynamic calculations of the reactions performed for the in situ formation of nickel nanoparticles and their effective factors during the curing process were performed. The results indicated that nickel nitrate is transformed to nickel nanoparticles of different sizes during the reduction process in a reduced atmosphere. The in situ formation of nickel nanoparticles and its catalytic effect led to the graphitization of carbon resulting from the pyrolysis of phenolic resin at a temperature of 800 °C and above. By increasing temperature, the morphology of the formed graphite changed and hollow carbon nanotubes, carbon cells, and onion skin carbon were formed in the microstructure. It was also observed that by increasing the temperature and the amount of additive, carbon nanotubes and their size are increased. A noteworthy point from thermodynamic calculations during the formation of nickel nanoparticles was that the nickel nanoparticles themselves acted as accelerators of nickel oxide reduction reactions and the formation of nickel nanoparticles. This increases the amount of amorphous carbon graphitization resulting from the pyrolysis of phenolic resin which leads to the formation of more carbon nanotubes at higher temperatures.