Pre-alloyed Powder Research Articles

Workability and corrosion behavior studies on sintered iron‐based hybrid powder metallurgy alloys

AbstractSintered low carbon steels are developed using prealloyed and elemental powders to improve the mechanical properties of powder metallurgy and powder forged parts. The research focuses on the mechanism of workability and corrosion studies on sintered preforms of Alloy 1 (ATOMET4601 + 0.35%C) and Alloy 2 (ATOMET4601‐0.35%C‐0.25%Mn‐0.1%Si‐0.9%Cr). Sintered preforms of relative densities of 81%, 84%, and 90% were used for the present work. The preforms with 84% relative density have been used to study the formability parameters. It is observed from the experimental study that the Alloy 2 preforms with the addition of alloying elements have undergone lesser densification and deformation due to the work hardening mechanism. Corrosion studies have been carried out by conducting aqueous immersion and electrochemical corrosion tests on these two alloys using 18% HCl solution at different timings. It is found that the Alloy 2 has exhibited a better corrosion resistance than the Alloy 1 due to the addition of various alloying elements. It is also observed that the corrosion rate has decreased with an increase in densification irrespective of the alloys. The microstructures, scanning electron microscopy, and X‐ray diffraction of corroded surfaces have been corroborated with densification and the corrosion behavior of alloys.

Effects of C and Nb on Pore-Grain Boundary Separation Behavior during Sintering of 420 Stainless Steel

This study investigated the evolution of density, grain size, and pore characteristics during the sintering of metal injection molding (MIM) 420, 420 + 0.3C and pre-alloyed 420Nb stainless steel powders. The results show that C promotes the reduction of oxides on the surface of stainless steel, thereby accelerating sintering at 1330 °C, which is the initial sintering stage of MIM 420. MIM 420Nb showed the slowest sintering rate due to the strong binding force between Nb and C. At 1350 °C, the sintering densities of MIM 420 and 420 + 0.3C slightly improved, whereas their grain sizes grew significantly. Scanning electron microscopy images show grain boundary-pore separation, which significantly retarded the grain boundary diffusion mechanism and hence reduced the densification rate. The addition of C accelerated the pore-grain boundary separation; thus MIM 420 + 0.3C showed the lowest density at this temperature among the materials analyzed in this study. Nb suppressed the grain growth rate; thus, MIM 420Nb exhibited the highest density among the three materials. At 1370 °C, MIM 420 + 0.3C reached the highest density owing to the creation of a liquid phase. Theoretical calculations proved that there is a linear relationship between the grain boundary area per unit volume and the interfacial pore area per unit volume. Furthermore, when the ratio of grain size to pore size is 28, the contact probability between the grain boundaries and pores is significantly reduced to approximately 10%, leading to an extremely slow densification rate and a rapid grain growth rate, which is consistent with the experimental results.

An Al-5Fe-6Cr alloy with outstanding high temperature mechanical behavior by laser powder bed fusion

This research aims to study the laser powder bed fusion ( L -PBF) processability, the microstructure and the tensile mechanical properties of an AlFeCr alloy at a wide range of temperatures. With this goal, Al-5Fe-6Cr (wt%) pre-alloyed powder was produced by casting and gas atomization. The microstructure of the as-atomized powders and the as-built specimens was characterized via scanning electron microscopy, x-ray diffraction, and transmission electron microscopy. Following a parameter optimization study, dense as-built specimens with a high relative density of 99.8% and a Vickers hardness at room temperature of 192 HV were fabricated. In addition, the newly developed AlFeCr alloy exhibits yield strength values of 273 ± 5 MPa and 179 ± 3 MPa at 300ºC and 400ºC, respectively. The exceptional strength of this alloy at high temperature is attributed to the homogeneous precipitation during processing of large density of nanoscale icosahedral (i-phase) and intermetallic phases, which are endowed with high thermal stability. These values are significantly higher than those corresponding to the conventional wrought Al7075 aluminum alloy in the T6 condition and to all Al alloys manufactured by L -PBF to date. The current research sheds guidelines for the design of high strength aluminum alloys containing transition alloying elements that are suitable to be processed by L -PBF for high temperature structural applications in a cost effective way.

Influence of processing parameter on phase transformation and superelastic recovery strain of laser solid forming NiTi alloy

NiTi alloys with superelasticity and shape memory effects are currently widely used because of its excellent deformation recovery ability, mechanical strength, and biocompatibility. The Ni 50.8 Ti 49.2 alloy was manufactured by using laser solid forming (LSF) technology with pre-alloyed powder in this work. The microstructure, phase transformation and shape recovery of the deposited alloy were influenced by the parameters used in depositing process. The as-deposited Ni 50.8 Ti 49.2 alloy formed by LSF is columnar crystals epitaxially grown along the deposition direction, with a small amount of equiaxed crystals appear between the deposition layers. Two-step/multi-step martensitic transformation occurs in the deposited Ni 50.8 Ti 49.2 alloy. The martensitic transformation temperature slightly changes with the deposition height, whereas the R transformation temperature basically does not change with the deposition height. In the test of the mechanical properties, sample with a laser power of 1700 W and a scanning rate of 680 mm/min has a compression superelastic recovery as high as 9.5%. Moreover, Ti 2 Ni precipitated phase and R phase appeared in all samples, but only the precipitated Ti 2 Ni and R phase in this sample had a certain orientation relationship with the matrix. Shape memory characteristics was also tested by compression deformation, and the maximum recovery strain of shape memory effect is 6.1%, which is obtained in the sample deposited with 1500 W laser power and 480 mm/min scanning rate. • Columnar crystals grow along the deposition direction in LSF Ni 50.8 Ti 49.2 alloy. • Superelastic recovery as high as 9.5% was obtained in LSF Ni 50.8 Ti 49.2 alloy. • Orientation relationship of precipitation with matrix leads to high recovery rate.

Operando X-ray diffraction study of thermal and phase evolution during laser powder bed fusion of Al-Sc-Zr elemental powder blends

Elemental powder blends are an emerging alternative to prealloyed powders for high-throughput alloy design via additive manufacturing techniques. Elemental Al+Sc(+Zr) powder blends were processed by laser powder bed fusion into Al-Sc and Al-Sc-Zr alloys, with operando X-ray diffraction at the Swiss Light Source extracting the structural and thermal history of the process. The pure Sc and Zr particles were found to react with the molten Al pool at 550–650 °C, well below their respective melting temperatures. Various scan areas (1 × 1, 2 × 2, 4 × 4, and 8 × 2 mm 2 ) were studied to compare (i) the base plate “preheating” effect caused by prior laser scans, (ii) the return temperature reached after the melting scan and before the following scan, (iii) the initial cooling rate immediately after solidification, and (iv) the time spent in the “intrinsic heat treatment range”, defined as 300–650 °C, where secondary Al 3 (Sc,Zr) precipitation occurs. Microstructural analysis of the as-built samples show 110–140 nm L1 2 -Al 3 (Sc,Zr) primary precipitates at the bottom of the melt pool. The 1 × 1 mm 2 samples exhibit the most elongated grains (long axis of 10 ± 5 µm), which correlates with the highest build plate temperature and the slowest initial cooling rate (3–5 × 10 5 K/s). In comparison, the 4 × 4 mm 2 samples exhibit the smallest equiaxed grains (2 ± 0.6 µm), corresponding to the lowest build plate temperature and the fastest initial cooling rate (6–7 × 10 5 K/s). These results indicate the need for establishing a minimum feature size during part design, or for modifying the laser parameters during processing, to mitigate microstructure and performance differences across features of different sizes.

Investigation of Microstructural, Mechanical and Corrosion Properties of Cu-10Sn Bronze Parts Produced by Selective Laser Melting

In this study, the production of full-density Cu-10Sn bronze parts by selective laser melting (SLM) technique and the examination of microstructural, mechanical and corrosion properties were carried out. Cu-10Sn pre-alloyed powders produced by gas atomization technique were shaped using SLM technique within selected parameters and then microstructural properties were determined. Depending on the microstructural properties, the mechanical and corrosion behaviors were determined. The obtained results were compared with similar bronze materials produced by conventional methods. Different characterization techniques were used for microstructural characterization. The microstructure of the Cu-10Sn alloy was observed to consist of dendritic primary α and δ-Cu41Sn11 phases. According to the mechanical test results of the samples produced at densities of 8.75 g cm-3 at room temperature, the yield strength was measured as 420 MPa, the tensile strength was 578 MPa, the elongation was 32 % and the hardness value was 160.3 HV0.2. For the electrochemical experiments, the corrosion rate of the samples was found to be 4.38 mpy. As a result of the productions and experiments, it was determined that the samples produced by the SLM method provide very good mechanical and corrosion properties compared to the literature.

The effect of composition and microstructure on the creep behaviour of 14 Cr ODS steels consolidated by SPS

There is a general need for alternative structural materials to improve power plants' efficiency and reduce CO2 emissions. Within this framework, two new compositions of temperature-resistant sintered ODS ferritic steels (14Cr–5Al–3W), strengthened by a fine dispersion of precipitates (5·1022 ox./m3), have been developed. This work focuses on creep properties and microstructure evolution. The creep resistance (at 650 °C) could be improved by prior microstructural optimisation, thanks to the consolidation by spark plasma sintering and the tailoring of precipitates' nature when a single compound introduces the oxide-forming elements (Y–Ti–Zr–O) synthesised for this purpose. To this end, the initial pre-alloyed ferritic powder was mechanically alloyed with the synthesised compound and sintered by spark plasma sintering (SPS). Afterwards, EBSD and TEM characterisation were employed to study the microstructures. Small punch creep tests (SPCT) were performed on the steels to analyse their creep performance. These showed an exceptional enhancement of the creep resistance in the steels containing the Y–Ti–Zr–O additions.

Formation of Cu2ZnSnS4 thin films and optimization of the optical bandgap using the Taguchi approach

Cu2ZnSnS4 thin films were synthesized by deposition of Cu–Zn–Sn alloy films on soda-lime glass using flash evaporation PVD of pre-alloyed powders followed by sulfurization anneal. The influence of process parameters on the structure, morphology and optical response of as-deposited and annealed films was investigated by X-ray diffractometry, Raman spectroscopy, scanning electron microscopy, and UV–visible spectroscopy. Since the preparation process, which influences the structure and chemical composition of the films, determines the optical bandgap of CZTS absorber layers, the Taguchi method was utilized to better predict the effect of process variables to achieve the optimal bandgap. According to the outcomes, the significant parameters, in sequence, were sulfurization time, the composition of PVD source powder, sulfurization temperature, and sulfur content. The prepared sample according to the optimal condition suggested by the Taguchi method showed an optical bandgap of 1.56 eV.

Effect of Carbon Nanotube Content and Mechanical Milling Conditions on the Manufacture of AA7075/MWCNT Composites

Aluminium matrix composites (AlMCs) of AA7075 aluminium alloy reinforced with 0.5 and 1 wt.% multiwall carbon nanotubes (MWCNTs) were fabricated with powder metallurgy techniques using three different mechanical milling strategies, varying the milling energy and the stage in which the reinforcements were added to the pre-alloyed matrix powders. In this paper, we focus on the influence of these parameters on the dispersion of MWCNTs. Characterization of the obtained composite powders by X-ray diffraction and scanning electron microscopy showed that the evolution of the particle size and morphology of the composite powders is influenced by milling conditions and MWCNT content; however, under the conditions tested in this study, there were no significant differences in crystallite size and lattice strain. The best distribution of the reinforcements was obtained after milling 7075 powders and MWCNTs in a high-energy cycle (HEBM), varying the rotation speed between 1200 and 1300 rpm. Raman spectroscopy was used to assess the damage induced by the milling process in the nanotubes, and no reaction products were detected under any of the tested conditions. Nanoindentation tests were performed to measure the elastic modulus and hardness of the composite powders, revealing that the best mechanical behaviour was achieved by the 7075-0.5 wt.% MWCNT composites obtained by the HEBM route.

Effects of weight on bit and rotational speed on the friction and wear properties of Fe based diamond bit matrix

All-hydraulic drill has been widely employed in core drilling recently, which results in larger Weight on Bit (WOB) and higher rotational speed to diamond bit. Thus, new requirements of diamond bit abrasiveness are needed to satisfy the severe challenges. To investigate the effects of WOB and rotational speed on the friction and wear properties of diamond bit matrix, an experimental scheme with 2 factors and 4 levels was designed to carry on the friction and wear test, and two kinds of Fe based diamond bit matrix formula systems, i.e. elemental metal powders and pre-alloyed powders were considered. The samples were manufactured by hot-pressing. Test results indicate that the abrasion loss of the Fe based samples increase with the raise of WOB and rotational speed, while the WOB has a bigger significance. The wear mechanism of the bit matrix is dominated by adhesive wear under smaller WOB and lower rotational speed, while abrasive were will become more and more important with the increase of WOB. Besides, samples from the pre-alloyed powders formula system have better performance than the ones from elemental system, the abrasion can be reduced by up to 15.52%, when under the same test conditions.

Additive manufacturing of biodegradable Zn-xMg alloys: Effect of Mg content on manufacturability, microstructure and mechanical properties

Additive Manufacturing (AM) exhibits tremendous advantages in producing customized medical implants and has achieved successful clinical applications for non-degradable metals such as titanium, stainless steel and cobalt alloys, particularly in the form of porous scaffolds for bone implants. Biodegradable materials allow the metal to degrade while natural tissue grows thereby reducing implant lifetime within the body to a minimum. The design freedom of AM and especially Laser Powder Bed Fusion (LPBF) overcomes technical difficulties of conventional technologies to manufacture complex structures. Within this work test geometries and porous scaffolds were manufactured via LPBF with pre-alloyed Zinc-Magnesium (Zn-xMg) powder, where x = 0, 1, 2 and 5 wt% to investigate the effect of Mg on manufacturability, microstructure and mechanical properties. Relative material densities of cubic specimens above 99.5% were achieved with all pre-alloyed powders. The addition of Mg reduces the processing window compared to pure Zn. A decreased grain size and an increase in intermetallic precipitates is observed with the addition of Mg. Zn-1Mg samples exhibited 3.2x the maximum ultimate tensile strength (UTS) of pure Zn (i.e., 119 MPa). However, the tensile elongation is only a fifth compared to pure Zn. With increasing Mg content, the amount of Zn+Mg2Zn11 rises and MgZn2 forms. These brittle phases characteristically increase hardness but reduce ductility. Under the chosen processing conditions, all Zn-xMg scaffolds demonstrated good processability. The geometric porosity was designed to be 80.5% with the achieved value of 70.8 % for compression testing. Zn-1Mg scaffold showed the highest compressive strength and Young’s modulus (19.1 MPa and 0.65 GPa respectively). This paper summarizes the technical challenges of manufacturing Zn-xMg alloys via LPBF for tissue engineering applications.

Laser Powder Bed Fusion Additive Manufacturing of Fe3Al-1.5Ta Iron Aluminide with Strengthening Laves Phase

Advanced aluminides strengthened with incoherent Laves phase precipitates are promising lightweight and creep-resistant alternatives for high-alloy steels and superalloys for high-temperature critical components up to 750 °C service temperature. A significant issue with manufacturing these aluminides with conventional casting is the strong coarsening tendency of the Laves phase precipitates at elevated temperatures, leading to a significant strength reduction. In this context, the short lifetime of the melt pool in additive manufacturing and its fast solidification and cooling rates promise to consolidate these aluminides with homogeneously distributed fine Laves phase particles without coarsening. The main scientific objective of this work is to exploit the unique characteristics of the laser powder bed fusion (L-PBF) additive manufacturing (AM) process to print dense and crack-free bulk Fe3Al-1.5Ta samples containing uniformly distributed (Fe, Al)2Ta Laves phase precipitates. The Fe-25Al-2Ta (at.%) alloy was selected for this work since its creep resistance at 650 °C surpasses the one of the P92 martensitic–ferritic steel (one of the most creep-resistant alloys developed for steam turbine applications). Fundamentals on process–microstructure relationships governing the L-PBF-fabricated builds are provided by a detailed microstructural characterization using X-ray diffractometer (XRD) and ultra-high-resolution scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX) and high-resolution electron backscatter diffraction (EBSD) detectors. Orientation imaging microscopy (OIM) and grain reference orientation deviation (GROD) maps were applied to measure texture and visualize substructures within the grains. The mechanism of voids formation, morphology, and volume fraction as a function of the input energy density was identified. The melting and solidification dynamics led to microstructures with large columnar grains, porosity, and periodic cracks during the printing process. Processing samples at the building temperatures below the brittle-to-ductile transition temperature, BDTT (750 °C), often caused severe macrocracking and delamination. Crack-free samples with densities higher than 99%, some approaching 99.5%, were fabricated from pre-alloyed gas-atomized powders with a combination of high laser power (250–300 W), slow-to-medium scanning speed (500–1000 mm/s), and 800 °C build plate preheating using a 67° rotation scanning strategy. The morphology of the pores in the volume of the samples indicated a relatively sharp transition from spherical geometry for scanning speeds up to 1000 mm/s to crack-like pores for higher values. The ultra-fast cooling during the L-PBF process suppressed D03 Fe3Al-ordering. The Fe3Al-1.5Ta builds were characterized by B2 FeAl-type order clusters dispersed within a disordered A2 α-(Fe, Al) matrix. Additionally, the (Fe, Al)2Ta Laves phase (C14–P63/mmc) was predominantly formed at the matrix phase grain boundaries and frequently dispersed within the grains. The quantitative EDX analysis of the matrix gave 77.6–77.9 at.% Fe, 21.4–21.7 at.% Al, and 0.6–0.8 at.% Ta, while the composition of the Laves phase was 66.3–67.8 at.% Fe, 8.7–9.8 at.% Al, and 22.4–24.9 at.% Ta, indicating that the Laves phase is considerably enriched in Ta with respect to the matrix. The L-PBF-fabricated alloys were characterized by coarse, columnar grains which grow epitaxially from the substrate, were several m in width, and extended across several layers along the building direction. The grains exhibited a relatively strong microtexture close to <0 0 1> with respect to the building direction. The L-PBF builds showed a bulk hardness value comparable to the as-cast and spark plasma-sintered counterparts. A negligible variation of the hardness across the build height was observed. Within the framework of this study, we demonstrated that the porosity and cracking issues could be resolved mainly by controlling the process parameters and preheating the build platform above the BDTT. Nevertheless, alloy modifications and/or post-manufacturing processing are required for microstructure refinement.

Synthesis of Refractory High-Entropy Alloy WTaMoNbV by Powder Bed Fusion Process Using Mixed Elemental Alloying Powder.

The growing interest in refractory high-entropy alloys (HEAs) in the last decade is mainly due to their thermal stability, outstanding mechanical properties, and excellent corrosion resistance. However, currently HEAs are still not considered for use as common structural materials due to their inherent drawbacks in terms of processing and machining operations. The recent progress witnessed in additive manufacturing (AM) technologies has raised the option of producing complex components made of HEAs with minimal machining processes. So far, this could be achieved by using pre-alloyed powders of HEAs that were mainly produced by a conventional arc melting furnace (AMF) in the form of small compounds that were transformed into powder via a gas atomization process. To significantly reduce the production cost, the present study aims to analyze the ability to synthesize HEA WTaMoNbV via a laser powder bed fusion (LPBF) process using mixed elemental alloying powder as the raw material. For comparison, a counterpart alloy with the same chemical composition was analyzed and produced by an AMF process. The microstructures of the tested alloys were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses. The physical properties were evaluated in terms of density and mechanical strength, while the electrochemical behavior was assessed by potentiodynamic polarization analysis. The results disclosed similarities in microstructure, physical properties and electrochemical behavior between HEA WTaMoNbV manufactured by the proposed LPBF process and its counterpart alloy produced by an AMF process.

Aluminium Assisted Nickel Alloying in Submerged Arc Welding of Carbon Steel: Application of Unconstrained Metal Powders

Nickel alloying of carbon steel is used to enhance steel strength and toughness. Nickel alloying of the weld metal via solid weld wire presents several difficulties as highlighted previously, such as work hardening of the solid weld wire in manufacturing and feeding through the SAW wire feeding mechanism, and expensive and time consuming manufacturing of multiple weld wire formulations. The application of nickel and aluminium powders in unconstrained format, meaning not as fluxed cored wire or as metal cored wire, is used to simplify weld metal alloying. Al powder is used to control the oxygen potential at the weld pool-molten flux interface. The results presented here show that the addition of Al powder to the weld metal enhances Ni yield to the weld metal, at 85%, compared to pre-alloyed powder Ni yields of 57–78% as applied in previous work. Carbon steel was alloyed to 6.9% Ni and 3.7% Al. Thermodynamic analysis is applied to elucidate the chemical interaction between Ni and Al, and its effects on Ni yield in the weld pool. Overall process productivity gains stem from weld metal alloying from unconstrained metal powders because the expensive and time consuming step of manufacturing alloyed wire and alloyed powder is eliminated.

The effect of Ti and B4C on the microstructure and properties of the laser clad FeCoCrNiMn based high entropy alloy coating

FeCoCrNiMnTix(B4C)y high entropy alloys (HEAs) coatings were laser cladded on Q355 steel using coaxial powder feeding laser 3D printing system by adjusting the additive ratio of pure Ti powder and B4C powder into the pre-alloyed spherical equi-atomic FeCoCrNiMn powder. Hardness, X-Ray Diffraction (XRD) phase composition, Scanning Electron Microscope (SEM) morphology, Energy Dispersive X-ray Spectroscopy (EDS) and wear resistance of the HEAs coatings were examined. The FeCoCrNiMnTix HEAs coatings (x = 0.5, 0.6, 0.8, 1.0) composed of fcc, bcc1 and bcc2 phase, The hardness increased as the Ti adding content increased, which may be attributed to the solution strengthening effect and promoting effect to the formation of bcc1 and bcc2 phase. The addition of B4C induced the formation of compound borides and carbides, and the decreasing of Ti solution in fcc and bcc phases. The phase composition of the FeCoCrNiMnTi0.6(B4C) y coating was mainly fcc, bcc2, borides and carbides. The increase of B4C improved the hardness and bcc2 content in FeCoCrNiMnTi0.6(B4C) y HEAs coating, and the wear resistance increased also to some extent. Among four FeCoCrNiMnTi0.6(B4C)y HEAs coatings (y = 0, 0.2, 0.4 and 0.6), FeCoCrNiMnTi0.6(B4C)0.4 HEAs coatings had the best wear resistance.

A new strategy for additive manufacturing ODS steel using Y-containing gas atomized powder

The 3D printing technology is gradually applied on oxide dispersion strengthened (ODS) steels to address its processing issue and prepare complex components for generation IV nuclear reactor. However, the nanoparticles always aggregate and coarsen during the printing process because of the severe repining behavior in the high-temperature melt. This leads to a large degradation of the strengthening effect of nanoparticles. This study proposes a strategy of in situ synthesis of nanoparticles in ODS steel during the printing process by using pre-alloyed powder with controlling the partial pressure of oxygen to overcome the abnormal growth behavior of nanoparticles. After printing, a large number of nanoparticles were in situ formed and distributed uniformly in the ODS steel. The average size and number density of nanoparticles were ~ 21.09 nm and ~ 4.1 ± 0.5 × 10 21 /m 3 , respectively. As compared with the majority of 3D printed ODS steels from mechanical alloyed powder, the in situ formed nanoparticles in this study exhibit the combination of smaller size, higher number density, and better distribution homogeneity and size uniformity. This provides an outstanding secondary strengthening effect and produces ODS steel with the UTS and YS of printed ODS steel are 725 and 673 MPa, respectively. In addition, the microstructural characterization and formation mechanism of nanoparticles in this newly printed ODS steel are also given and discussed in detail. • An ODS alloys was prepared by SLM technology using the original Y and Ti atomized powders without being mechanical alloyed. • The prepared ODS alloy shows high number density and better uniformity of in situ growth dispersed particles. • The fabricated ODS steel has good tensile property at room temperature, high strength with good plasticity.

Effect of Cr, Mo, and V Elements on the Microstructure and Thermal Fatigue Properties of the Chromium Hot-Work Steels Processed by Selective Laser Melting

Thermal fatigue is the main failure mode for chromium hot-work steels. In this study, pre-alloyed chromium hot-work steel powders with three different Cr, Mo, and V addition levels (low content (LH), medium content (MH), and high content (HH)) were used for selective laser melting (SLM). The microstructure and thermal fatigue properties of these SLM-processed materials were investigated. After thermal fatigue tests, LH possessed the lowest hardness (approximately 573 HV5) and longest crack length, MH possessed the highest hardness (approximately 688 HV5) and HH (with the hardness of approximately 675 HV5) possessed the shortest crack length. It can be concluded that the increase of V content in MH is the main reason for the refined grains which result in an enhanced hardness and thermal fatigue resistance compared to LH. The further increase of the Cr and Mo content in HH leads to the grain coarsening and hardness decreasing, which is supposed to degrade the thermal fatigue resistant properties according to the conventional theory. However, HH exhibited an enhanced thermal fatigue resistance compared to MH. That is because the higher stored energy in MH deteriorated its thermal fatigue resistance compared to HH.

Microstructure and mechanical properties of micro-nano Ti2AlC-reinforced TiAl composites

In order to improve the mechanical properties of TiAl alloys, micro-nano Ti2AlC-reinforced TiAl composites were successfully fabricated by means of spark plasma sintering (SPS) using the ball-milled multilayer graphene oxide and Ti–48Al–2Nb–2Cr pre-alloyed powders. The micro-nano Ti2AlC phase precipitated at the interface between α2-Ti3Al and γ-TiAl phases utilizing the reaction of TiAl and high-activity graphene. Fully lamellar structure was obtained after sintering above 1300 °C, and Ti2AlC phase characteristic were depended on sintering temperature and graphene content. The compressive strength and fracture strain of TiAl-0.5G alloy were improved at room temperature and elevated temperature, which was improved by 23.61% and 5.03% compared to the TiAl-0G alloy at room temperature. The tribological properties of TiAl composites were significantly improved by micro-nano Ti2AlC at the room temperature, and the average friction coefficient of TiAl-0.5G alloy is 0.217 compared with TiAl-0G alloy is 0.312, and the wear mechanism is ploughing wear. The micro-nano Ti2AlC improves the strength and oxidation resistance of TiAl composites at 650 °C, which is detrimental to the wear resistance due to the lower ductility and the third wear by the loose oxide particles at elevated temperature.

Development of powders of Ti-Fe-Sn ultrafine eutectics for laser additive manufacturing

Pre-alloyed powders of near-eutectic Ti66Fe27Nb3Sn4 (at.%) were produced by two standardised gas atomisation methods, named as Crucible-based Gas atomisation (CGA) and Crucible-Free atomisation (CFA), for powder bed fusion (PBF) techniques. PBF seems the ideal (and largely unexplored) processing route for bulk ultrafine eutectics because of their strict solidification criteria requiring fast and uniform cooling rates. CFA produced more desirable physical properties in powders, although the alloy's high oxygen affinity resulted in a dendritic oxide Fe2Ti4O. Electron Microscopy, X-ray Diffraction, image analysis and nanoindentation were used to understand the solidification behaviour in powder particles across different sizes, thus, different crystallisation kinetics. The alloy is quite sensitive to compositional fluctuations and solidified via different pathways in the two methods resulting in different microstructures. Presence of meta-stable phase and ultrafine inter-lamellar spacing (< 190 μm) of the eutectic matrix, which increases with powder particle size, highlight the potential of gas atomisation as a method to study rapid solidification.

Fatigue Behavior of Alloy Steels Sintered from Pre-Alloyed and Diffusion-Bonding Alloyed Powders

Porosity and phases are considered to be two key factors for the fatigue performance of powder metallurgy steels. In this paper, the fatigue strengths of the alloy steels sintered from two typical types of powders, pre-alloyed Fe-Cr-Mo (Astaloy CrM), and diffusion-bonding alloyed Fe-Cu-Mo-Ni (Distaloy AE), were comparatively analyzed in view of the geometry of porosity, the phases constitution, and fractography of fracture. Different modes of fatigue fracture were distinguished between the two materials. Namely, a trans-particle fracture is predominant in the Disitaloy AE steel due to the heterogeneous phases which consist of soft phases in powder interior and hard phases along powder borders. In contrast, the fatigue fracture of the Astaloy CrM steel with a homogeneous mono-phase of martensite is characterized by an inter-particle fracture at the sintering necks. Moreover, the fatigue endurance limit of the Distaloy AE steel was not pronouncedly improved by increasing sintering temperature in comparison with the Astaloy CrM steel. This was attributed to the softening of the network constructed by martensite at sintering necks. A modified Murakami model which considers micro-scale defect and micro-hardness is effective to predict the fatigue performance of the alloy steels sintered from pre-alloyed and diffusion-bonding alloyed powders, respectively.