Mechanical Alloying Powders Research Articles

Deposition behaviour of FeCrMnNiCo coatings deposited using mechanically alloyed powder: Comparing Cold Spray, HVOF, HVAF, and Laser Cladding processes

This study examined the characteristics of mechanically alloyed (MA) Cantor alloy powder and the coatings produced from it using various deposition techniques, including cold spray (CS), high-velocity oxy-fuel, high-velocity air-fuel, and laser cladding (LC). Microstructure analysis of the MA powder revealed an irregular morphology and incomplete elemental mixing. The microstructure of the CS coating displayed an FCC crystal structure, with some XRD peaks corresponding to BCC phases due to the presence of unmixed elements. In contrast, all other coatings also exhibited oxides alongside FCC and BCC phases, with the LC coating containing a higher concentration of oxides. These coatings demonstrated high density and diverse microstructures, with CS coatings demonstrating effective transfer of powder microstructure. The CS coating had the highest hardness (679 ± 17 HV0.1) due to the retention of deformed microstructure from the powder, whilst the LC coating had the lowest hardness (215 ± 10 HV0.1). CALPHAD calculations using Thermo-Calc suggest that the presence of oxides in the coatings could be thermodynamically feasible, depending on the conditions. Deposition efficiency varied significantly among the methods, with LC achieving the highest efficiency (63 ± 6 %) and CS the lowest (14 ± 1 %).

Optimizing Cold Spray Parameters for High Entropy Alloy Coatings Using Taguchi and Box–Behnken Design Approaches for Mechanically Alloyed Powder

The present study focused on optimizing the cold spray (CS) process parameters for depositing Fe20Cr20Mn20Ni20Co20 (Cantor alloy) coatings using mechanically alloyed (MA) powder. A two-step design of experiments approach was employed, beginning with the initial screening of input variables using the L8 Taguchi method, followed by the refinement of process parameters through the Box–Behnken design of experiments. Key performance indicators included deposition efficiency (DE), coating thickness per pass, and microstructural parameters including porosity, cracks, and interfacial defects/delamination. The study identified process gas temperature as the primary factor influencing both DE and thickness per pass. Higher gas temperature and pressure, combined with increased scanning speed, resulted in higher DE. The DE of the MA Cantor alloy powder peaked at around 14-15%, with a deposit density greater than 99% achieved at the highest process gas temperature and pressure (1000 °C and 60 bar, respectively). The average hardness of the optimal CS coating deposited using MA powder was found to be 679 ± 17 HV0.1, which is approximately 90% greater than the average hardness reported for CS coatings deposited using atomized powder.

Heterogeneous amorphous/crystalline ZnZrCu biodegradable alloys with synergistic strength–plasticity prepared by mechanical alloying and laser powder bed fusion

ABSTRACT Amorphous Zn-based alloys have shown considerable potential for implant applications owing to the impressive strength. However, they inevitably fall into strength–plasticity trade-off dilemma due to shear bands softening. In this work, a novel and feasible route of mechanical alloying (MA) and laser powder bed fusion (LPBF) was first developed for manufacturing heterostructured ZnZrCu alloys. The results showed that MA destabilised atomic periodicity of Zn/Zr powders and induced amorphization. Furthermore, increasing Cu content enhanced amorphization ability and in-situ synthesised ϵ-phases, endowing ZnZrCu powders with heterostructure. During LPBF, the fast-melting kinetics and highly localised nature restricted devitrification and promoted the uniform distribution of ϵ-phases, constructing heterogeneous amorphous/crystalline ZnZrCu alloys. Consequently, ZnZrCu alloys exhibited good strength–plasticity synergy attributed to extra strain-hardening and hampering premature failure of shear bands. This study not only provides a feasible route to prepare biodegradable Zn-based alloys with strength–plasticity synergy but also sheds light on the design and manufacture of cutting-edge amorphous alloys.

Anisotropic creep property related to non-spherical shape of mechanically alloyed powder of oxide dispersion strengthened F82H

Nanometric oxide particles dispersed within the matrix play an important role in improving the creep property of Oxide Dispersion Strengthened (ODS) steels. Conventional studies have primarily focused on investigating the distribution of the particles to control the creep property of ODS steels. However, achieving complete control over the creep property still poses challenges, as there are potentially other microstructural features like different matrices, micrometric precipitates, and inclusions derived from powder metallurgical processes that can influence it.In our previous research, we examined one such microstructural feature known as prior particle boundary (PPB). PPB refers to the surface of mechanically alloyed (MA) powder before consolidation. We revealed that the ODS steel with fine PPBs produced from smaller MA powder, exhibited shorter creep rupture times, compared to that with coarse PPBs produced from larger MA powder. This result was attributed to the increased formation of creep cavities on the PPBs in the ODS steel produced from smaller MA powder. Therefore, the size of MA powder affected the formation of creep cavity and had an impact on the creep property.In this study, we shifted our focus to the shape of MA powder with non-spherical shapes containing flake-like shape. Such shapes have the potential to induce anisotropy in the formation of creep cavities and/or the creep crack initiation and growth along anisotropic prior particle boundaries (PPBs), ultimately leading to anisotropic creep behavior.We conducted small punch creep tests on specimens with two different orientations to study the possible anisotropy. The results revealed that the creep rupture times varied depending on the orientation of specimen, thus indicating anisotropic creep property. This anisotropic creep property was related to the non-spherical shape of MA powder. This research highlighted the significance of managing the shape of MA powder in controlling the creep property of ODS steels.

Thermoelectric Chalcostibite: Solid-State Synthesis and Thermal Properties

In this study, thermoelectric chalcostibite (CuSbS<sub>2</sub>) compounds were fabricated using mechanical alloying (MA) and hot pressing (HP), and phase identification, microstructural observation, and thermal analysis were conducted. The thermal properties were then measured and compared with those of other Cu–Sb–S ternary compounds synthesized by the same solid-state process, namely, skinnerite (Cu<sub>3</sub>SbS<sub>3</sub>), famatinite (Cu<sub>3</sub>SbS<sub>4</sub>), and tetrahedrite (Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub>). Both the MA powder and HP-sintered samples contained a single-phase chalcostibite with an orthorhombic structure, and relative densities of 94.6–99.7% were obtained based on HP temperature. The full width at half maximum of the X-ray diffraction peak was significantly reduced for the HP specimens compared to that of the MA powder due to stress relaxation and grain growth during HP at elevated temperatures. However, practically no changes were observed in the lattice constants based on HP temperature. Differential scanning calorimetric analysis revealed that one endothermic reaction occurred at 814–815 K for the MA powder and at 818–821 K for the HP specimen, which were interpreted as the melting points of chalcostibite. Densely sintered compacts with densities close to the theoretical density were obtained using HP at temperatures of 623 K or higher. The constituent elements of the chalcostibites were uniformly distributed. As the HP temperature increased, thermal diffusivity and conductivity increased, but they decreased significantly as the measurement temperature increased. For the chalcostibite specimen hot-pressed at 623 K, the thermal diffusivity and conductivity were (0.75–0.36) × 10<sup>-2</sup> cm<sup>2</sup> s<sup>-1</sup> and 1.47–0.72 W m<sup>-1</sup> K<sup>-1</sup> at 323–623 K, respectively. Compared with other Cu–Sb–S ternary compounds, the thermal diffusivity was higher at low temperatures but similar at high temperatures, and the thermal conductivity above 500 K was lower than 1 W m<sup>-1</sup> K<sup>-1</sup>.

Mechanical and magnetic properties of phase separated Cu50Fe50 (at%) alloys synthesized by mechanical alloying and spark plasma sintering

A bulk Cu50Fe50 (at%) alloy was prepared by mechanical alloying (MA) of elemental Cu and Fe powders for 20 h, followed by the spark plasma sintering (SPS). All the powder and bulk samples were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical and magnetic properties of the bulk samples were also evaluated. From 1 h to about 10 h of milling, the formation of nanocrystalline Cu-rich face-centered cubic solid solution (FCC γ phase) and Fe-rich body-centered cubic solid solution (BCC α phase) was observed. Above 10 h of milling, the γ and α phases merged to form a single-phase face-centered cubic (γS) solid solution till 20 h. Subsequently, the 20 h MA powder was consolidated using spark plasma sintering (SPS) at three different temperatures: 519 ℃ (0.5 Tm), 677 ℃(0.6 Tm), and 835 ℃ (0.7 Tm), where Tm is the average melting point of the alloy (1310 ℃). In all three samples, the metastable γS phase decomposed into the nanocrystalline Cu-rich γ and Fe-rich α phases during SPS. Notably, the 0.7 Tm sample exhibited a better balance between strength and ductility by demonstrating a compressive yield strength of 886 MPa and an ultimate compressive strength of 1166 MPa with a ductility of 15%. The high strength and ductility are attributed to a bimodal grain size distribution, i.e., a few microns thick and continuous Cu-rich γ grain boundary phase and an intragranular nanocrystalline mixture of γ and α phases. In addition to its excellent mechanical properties, the 0.7 Tm sample displayed favorable soft ferromagnetic characteristics exhibiting a saturation magnetization (Ms) of 66 emu/g and a coercivity (Hc) of 16 Oe.

Heterogeneous grain structure in biodegradable Zn prepared via mechanical alloying and laser powder bed fusion for strength-plasticity synergy

ABSTRACT This study presented a unique process of mechanical alloying (MA) and laser powder bed fusion (LPBF) to prepare heterogeneous grain structure (HGS) Zn with strength-plasticity synergy. The results showed that the MA-treatment significantly refined the grain sizes of Zn to nano-scale and contributed to the formation of HGS powders. Moreover, the ultra-fast heating and cooling of LPBF process inhibited the grain growth and resulted in a specific core/shell HGS, wherein the cores (micron-grains) were surrounded by interconnected shells (nano-grains). Notably, the prepared HGS Zn demonstrated synergistically improved compressive yield strength (2.1 times) and plasticity (58%) compared with the homogeneous coarse-grained counterparts. This impressive strength-plasticity synergy was primarily attributed to high back stress and dislocation pile-ups generated by the deformation incompatibility between the nano-grained shells and micron-grained cores. These findings open up new fields for MA and LPBF in the preparation of HGS metals and their applications in solving strength-plasticity tradeoff.

Mechanical alloying and direct powder forging of Ni–20Cr–20Fe-0.08C alloy

In the present work Ni–20Cr–20Fe-0.08C nickel-based alloy was prepared by mechanical alloying followed by direct powder forging. Mechanical alloying (MA) of the elemental powders corresponding to a starting composition of Ni–20Cr–20Fe-0.08C (without process control agent) was carried out in a high energy attritor mill (Simoloyer). X-ray diffraction studies indicated that a single phase (γ) solid solution was obtained at 8 h of milling. The crystallite size of the milled powder decreased significantly while lattice strain, lattice parameter, and dislocation density increased with milling time. Dynamic recrystallization was evident after 4 h of milling; as a result, a slight increase in crystallite size and a slight decrease in lattice strain and dislocation density were observed. The 8 h MA powders were further filled into a mild steel can and placed in a tubular furnace to carry out the loose sintering at 1150 °C after which they were hot forged. The forged slab exhibited almost full density, near isotropic properties, ultrafine equiaxed grained structure and minimised prior particles boundaries (PPB). This suggests that powder forging has a potential to be an alternative processing method to hot isostatic pressing (HIP), hot powder extrusion and additive manufacturing (AM) for the processing of powder metallurgy nickel based alloys.

Effect of Starting Powder Particle Size on the Thermoelectric Properties of Hot-Pressed Bi0.3Sb1.7Te3 Alloys.

P-type Bi0.3Sb1.7Te3 polycrystalline pellets were fabricated using different methods: melting and mechanical alloying, followed by hot-press sintering. The effect of starting powder particle size on the thermoelectric properties was investigated in samples prepared using powders of different particle sizes (with micro- and/or nano-scale dimensions). A peak ZT (350 K) of ~1.13 was recorded for hot-pressed samples prepared from mechanical alloyed powder. Moreover, hot-pressed samples prepared from ≤45 μm powder exhibited similar ZT (~1.1). These high ZT values are attributed both to the presence of high-density grain boundaries, which reduced the lattice thermal conductivity, as well as the formation of antisite defects during milling and grinding, which resulted in lower carrier concentrations and higher Seebeck coefficient values. In addition, Bi0.3Sb1.7Te3 bulk nanocomposites were fabricated in an attempt to further reduce the lattice thermal conductivity. Surprisingly, however, the lattice thermal conductivity showed an unexpected increasing trend in nanocomposite samples. This surprising observation can be attributed to a possible overestimation of the lattice thermal conductivity component by using the conventional Wiedemann-Franz law to estimate the electronic thermal conductivity component, which is known to occur in nanocomposite materials with significant grain boundary electrical resistance.

P/M 18Cr ODS steels produced by mechanical alloying and hot powder forging

Mechanical alloying in attritor mill and powder consolidation through hot forging were employed to produce 18Cr ODS steels. Elemental powders were mechanically alloyed with nano-size yttria powder for up to 12 h. The effect of milling time on powder morphology, crystallite size and lattice strain were studied. Optimum milling time of mechanical alloying was determined for dissolution of constituents in the ferrite matrix. Further milling led to peak broadening. TEM confirmed the presence of fine crystallites in milled powders. HRTEM fringes were taken on the oxide particles. Their FFT confirmed that Y2O3 and TiO2 particles were present in the milled powders. The milled powders were consolidated through hot powder forging. Addition of Ti and Y2O3 significantly reduce the grain size in forged alloys. It also leads to formation of fine complex Y-Ti-O oxides. The powder forged alloys exhibited fine grained recrystallized microstructure. Reasons for this are discussed.

Mechanical milling/alloying, characterization and phase formation prediction of Al0.1–0.5(Mn)CoCrCuFeNi-HEA powder feedstocks for cold spray deposition processing

In the present work the synthesis of multicomponent nanocrystalline Al0.1- 0.5(Mn)CoCrFeNi HEA powder feedstocks by mechanical alloying (MA) through high energy ball milling to produce HEA coatings with cold spraying were studied. Pure (>99.5) elemental powders of Al (Mn), Co, Cr, Cu, Fe & Ni were mixed in predetermined ratios and mechanically alloyed under Ar gas in a Planetary Ball Mill. MA HEA powder feedstocks were characterized by optical microscopy, SEM+EDAX, XRD & XPS to determine powder particle size, shape & distribution, particle surface condition, chemical composition, crystal & phase structures & microstructures. MA-processed Al-HEA powder feedstocks exhibited tri-modal particle size distribution (PSD) while Mn-HEA had four distinct PSD modes. Fine size fraction of the MA- HEA powders contained A2/B2 (BCC) harder phases with lattice parameters of 2.8831 and .2.8690 Å, respectively and coarse size fraction contained (ductile) softer A1 phase/Cu-rich regions combined (FCC) having a lattice parameter of 3.6181 Å. The ratio of the phases varied depending on the Al and Mn contents in the powder. For higher aluminium contents, there was more disordered A2/B2 phases and less disordered A1 phase in Al0.1–0.5 HEA powder feedstocks. A1 phase contained Fe, Cr, Co and Ni; A2 phase contained Fe, Cr; and B2 phase contained Al, Ni elements in Al0.1–0.5 HEA powder feedstocks. There were passive surface oxide films on the rough surfaces of MA HEA powder feedstocks consisting of a main mixture of surface oxides such as Al2O3, MnO, Cr2O3, Mn2O3, and minor oxides such as CuO, Fe2O3, NiO, Fe3O4, and a few CoCr2O4 spinel oxide. Dual FCC-dual BCC phase prediction of thermodynamic stable phase formation criteria reported in the literature on MA-HEA powder feedstocks were consistent with the experimental results of the present study.

Bytizite Cu3SbSe3: Solid-State Synthesis and Thermoelectric Performance

Bytizite (Cu3SbSe3 ) has attracted interest as a promising thermoelectric material because of its ultralow thermal conductivity; however, there are few experimental studies. This study investigated the optimal processing conditions for the synthesis of Cu3SbSe3 using mechanical alloying (MA) and hot pressing (HP). The MA powder exhibited an orthorhombic Cu3SbSe3 phase, which remained even after HP. However, secondary phases of permingeatite (Cu3SbSe4) and berzelianite (Cu1.78Se) were also identified in the X-ray diffraction patterns. Thermal analysis revealed that the MA powder and HP compacts exhibited a large endothermic peak near 727 K, which corresponds to the melting point of Cu3SbSe3 . Dense compacts with a relative density higher than 99% were obtained at HP temperatures above 573 K. Microstructural and elemental analyses confirmed the presence of the secondary phase Cu3 SbSe4 in the matrix of Cu3SbSe3 . However, the Cu1.78Se phase could not be observed. All specimens exhibited an electrical conductivity of (0.66–1.06) × 10 3 Sm-1, a Seebeck coefficient of 324–376 µVK-1, and a power factor of 0.09–0.11 mWm-1K-2 at 623 K. The thermal conductivity was lower than 0.7 Wm-1K-1 in the measured temperature range, mainly due to the phonon scattering caused by the lone-pair electrons of Sb. A dip in thermal conductivity was observed at 423 K, which was possibly caused by the order-disorder transition of bytizite. The dimensionless figure of merit ZT increased with increasing temperature, and the maximum <i>ZT</i> was 0.16 at 623 K.

Light Weight MnTiAlNiFe High-Entropy Alloy (LWHEA) Fabricated by Powder Metallurgy Process: Mechanical, Microstructure, and Tribological Properties

The present work is focused on the development of lightweight high entropy alloy (LWHEA) using a powder metallurgy process. The primitive powders of manganese (Mn), titanium (Ti), aluminum (Al), nickel (Ni), and iron (Fe) with a purity of ∼ 99.5 % and particle size in the range of 40 to 70 µm were considered as a raw material for the synthesis of present LWHEA. The elemental powders at an equal molar ratio were mechanically alloyed (MA) in a high-energy ball milling for 28 hrs. The MA powders were then cold compacted using 500 MPa pressure, followed by sintering at 900 °C in an inert atmosphere to produce a bulk cylindrical sample of 8 mm diameter and 8 mm height. The average particle size of the milled powders was measured by FESEM image analysis, while the crystal size of the milled powders was predicted from the XRD peak broadening followed by full-width half maximum (FWHM) peak profile calculation using the Hall-Williamson method. The BCC structure, along with intermetallic phases, was identified in the sintered bulk LWHEA of MnTiAlNiFe. Some intermetallic phases act as reinforcement in the alloy. The EDX analysis of MA powder revealed that all the elemental powders were distributed homogeneously to form a solid solution of LWHEA powder. The deformation behavior of the sintered MnTiAlNiFe LWHEA was studied by analyzing the loading–unloading curve of micro indentation. A remarkable improvement in the hardness values (microhardness ∼ 7.382 GPa and Martens hardness ∼ 3.09 GPa at 3 N load) was observed compared to conventional lightweight metallic alloys. The investigation of the wear behavior of the bulk MnTiAlNiFe LWHEA samples carried out in a Universal Tribometer coupled with a 3D optical profilometer predicts the load-dependent coefficient of friction of the sample.

Assessment of Ferritic ODS Steels Obtained by Laser Additive Manufacturing

This study aims to assess the potential of Laser Additive Manufacturing (LAM) for the elaboration of Ferritic/Martensitic ODS steels. These materials are usually manufactured by mechanical alloying of powders followed by hot consolidation in a solid state. Two Fe-14Cr-1W ODS powders are considered for this study. The first powder was obtained by mechanical alloying, and the second was through soft mixing of an atomized Fe-14Cr steel powder with yttria nanoparticles. They are representative of the different types of powders that can be used for LAM. The results obtained with the Laser Powder Bed Fusion (LPBF) process are compared to a non-ODS powder and to a conventional ODS material obtained by Hot Isostatic Pressing (HIP). The microstructural and mechanical characterizations show that it is possible to obtain nano-oxides in the material, but their density remains low compared to HIP ODS steels, regardless of the initial powders considered. The ODS obtained by LAM have mechanical properties which remain modest compared to conventional ODS. The current study demonstrated that it is very difficult to obtain F/M ODS grades with the expected characteristics by using LAM processes. Indeed, even if significant progress has been made, the powder melting stage strongly limits, for the moment, the possibility of obtaining fine and dense precipitation of nano-oxides in these steels.

High-temperature oxidation and wear resistance of nano-ZrB2 reinforced CoNiCrAlY composite coating via a step-fashion mechanical alloying powder process

A nano-ZrB2 reinforced CoNiCrAlY composite coating with highly dense structures was prepared by HVOF via a step-fashion mechanical alloying (MA) powder process. The microstructure, mechanical properties, high-temperature oxidation and wear resistance were analyzed. Results show that a high-quality CoNiCrAlY-ZrB2 powder with expected powder size and homogenous dispersion of nano-ZrB2 can be obtained by the 35 h step-fashion MA. The addition of nanoscale ZrB2 greatly improves mechanical properties of CoNiCrAlY-ZrB2 coating, while the low content β-NiAl phase hinders its oxidation performance. Due to the formation of “upper enamel layer” during sliding process and the excellent hardness/fracture toughness, the wear resistance of CoNiCrAlY-ZrB2 coating (2.46 ×10−14 m3·N−1·m−1) at 1000 ℃ is much better than that of CoNiCrAlY coating (1.9 ×10−12 m3·N−1·m−1), showing that CoNiCrAlY-ZrB2 coating has a high potential for protecting hot parts.

Revealing strengthening contribution of grain refinement and phase precipitation in CrMnFeCoNi high-entropy alloy prepared from different powders

Equiatomic CrMnFeCoNi high-entropy alloys (HEAs) were prepared by vacuum hot-pressing sintering with single phase precursor powders made from mechanical alloying (MA) or spray-drying (SD), respectively. MA process was found beneficial for the following precipitation and formation of secondary phases during sintering, after which multiphase bulk HEAs were formed consisting of FCC, M23C6, and precipitated nanoscale σ phases. The tensile and compressive yield strengths of bulk HEAs fabricated from MA powders reached 1150 and 1200 MPa, respectively, which were about three times than SD powders. The excellent mechanical properties of the former were found due to the precipitation of nanoscale σ phase and M23C6 intermetallic resulted from the MA treatment and sintering processing. Nanoscale σ and M23C6 phase can effectively strengthen the alloy not only by self-reinforcement but also through the grain boundaries of the refined grains induced by the precipitation.

Microstructure and tensile properties of nano-sized ZrC particle strengthened RAFM steels

Reduced activation ferritic/martensitic (RAFM) steels containing ZrC nano particles (ZrC-RAFM steels) were prepared by a combination of spark plasma sintering (SPS) and hot-rolling of the mechanical alloyed powders. Moreover, the microstructure and tensile properties of the prepared ZrC-RAFM steels were investigated. The results show that the ZrC nano particles are uniformly dispersed in the RAFM steels, resulting in a significant decrease in grain size and the enhanced tensile properties of the steels. With the increase of ZrC content, the hardness and tensile strength of the steels are improved, and the mechanism of the improved properties is analyzed and discussed. The tensile strength of the 0.75ZrC- RAFM steel is 2069 MPa, which is 19.6% higher than that (1730 MPa) of the RAFM steel without ZrC. The conclusions provide researchers with the roles of ZrC-nanoparticle in micromorphology and tensile properties of the RAFM steel as potential cladding materials for fast reactors.

Effect of different Y components on optimization of dispersive Y2O3 particles in ODS-Cu

In order to reflect the superiority of using Cu6Y as the Y2O3 source, three different Y sources addition, direct Y2O3 by conventional mechanical alloying process, pure metal Y and Cu6Y compound by in-situ fabrication method, were compared in the ODS-Cu with 1.5 wt.% Y2O3. It was found that compared to Y2O3 or pure metal Y, the brittle Cu6Y played the roles of both suitable Y precursor to enhance Y dissolution and process control agent (PCA) to suppress the growth of mechanical alloying (MA) powders. The alloyed layer was thickened with the MA time, and the further MA gradually becomes difficult. It is inefficient to fully alloy the MA powders only by prolonging milling time. The samples by in-situ fabrication method (Y and Cu6Y sourced) had more contaminations with Fe, because of the oxidation of milling mediums by Cu2O, which was used to oxide the Y precursors to form the Y2O3. The contaminations contributed to the hard prior particles boundaries where fracture took place during the tensile tests, resulting in the poor tensile performance for the samples by in-situ fabrication method. Considering that the Cu6Y sourced sample by in-situ fabrication method formed fine and high dense dispersive Y2O3 particles with the size of 19 ± 7 nm and a high number density of 1.8 × 1021/m3, it has a larger potential for improvement once the means to reduce Fe impurities and holes are developed.

Microwave versus Conventional Sintering of NiTi Alloys Processed by Mechanical Alloying.

The present study shows a comparison between two sintering processes, microwave and conventional sintering, for the manufacture of NiTi porous specimens starting from powder mixtures of nickel and titanium hydrogenation–dehydrogenation (HDH) milled by mechanical alloying for a short time (25 min). The samples were sintered at 850 °C for 15 min and 120 min, respectively. Both samples exhibited porosity, and the pore size results are within the range of the human bone. The NiTi intermetallic compound (B2, R-phase, and B19′) was detected in both sintered samples through X-ray diffraction (XRD) and electron backscattering diffraction (EBSD) on scanning electron microscopic (SEM). Two-step phase transformation occurred in both sintering processes with cooling and heating, the latter occurring with an overlap of the peaks, according to the differential scanning calorimetry (DSC) results. From scanning electron microscopy/electron backscatter diffraction, the R-phase and B2/B19′ were detected in microwave and conventional sintering, respectively. The instrumented ultramicrohardness results show the highest elastic work values for the conventionally sintered sample. It was observed throughout this investigation that using mechanical alloying (MA) powders enabled, in both sintering processes, good results, such as intermetallic formation and densification in the range for biomedical applications.

Skinnerite Cu3SbS3: Solid-State Synthesis and Thermoelectric Properties

Skinnerite Cu3SbS3, which consists of non-toxic and low-cost elements, is a potential thermoelectric material, however, studies on its thermoelectric properties are limited. In this study, the optimal conditions of mechanical alloying (MA) and hot pressing (HP) for the synthesis of the Cu3SbS3 phase were investigated, and its thermoelectric properties were evaluated. The MA powder had a theoretically predicted cubic skinnerite phase, which remained even after HP. Thermal analyses revealed that the MA powders and HP specimens exhibited large endothermic peaks at approximately 880 K, corresponding to the melting point of Cu3SbS3. Compacts with high relative densities above 99% without cracks and pores were obtained at an HP temperature of 623 K and above. The lattice constants of the MA powder and HP specimens were in the range of 1.0343–1.0394 nm. As the HP temperature increased, the electrical conductivities of the specimens decreased because of changes in their carrier concentrations; however, their Seebeck coefficients increased. As a result, the power factor values of all the specimens were in the range of 0.58–0.61 mW m−1 K−2 at 623 K. The thermal conductivities were lower than 0.78 Wm−1K−1. The maximum dimensionless figure of merit, <i>ZTmax</i> of 0.57 was obtained at 623 K when the skinnerite was prepared under the optimal solid-state synthesis conditions (HP at 673 K for 2 h using MA powder at 350 rpm for 18 h).