As-sintered Alloy Research Articles

Effect of Ti/Ta ratio on the microstructure and mechanical properties of Hf20Mo15Nb25Ta20-xTi20+x refractory high-entropy alloys

The limited deformation capacity at room temperature remains a significant challenge in refractory high-entropy alloys (RHEAs). In this study, the fine-grained Hf20Mo15Nb25Ta20-xTi20+x (x = 0, 5, 10, 15) alloys were prepared by mechanical alloying and subsequent spark plasma sintering (SPS), based on precise component regulation. The impact of altering the Ti/Ta ratio (R, R = (20+x)/(20-x)) on the pre-alloyed powders and the as-sintered alloys were investigated. An increase in R-value led to larger powder particle sizes and lower powder yields, which was attributed to the improvement in the plasticity of the pre-alloyed powders. After SPS sintering, the as-sintered alloys were comprised of two BCC solid solution matrices and nanoscale precipitated phases. As the R-value increases, the stability of the microstructure decreases slightly, while the yield strength shows a slight improvement, and the plastic strain experiences a relatively significant enhancement. The as-sintered Hf20Mo15Nb25Ta20-xTi20+x alloys all exhibited an excellent balance of strength and plasticity. Theoretical calculations revealed that the yield strength of the alloys was the result of the combined effect of several strengthening mechanisms, predominantly substitutional solid solution strengthening. Furthermore, a systematic discussion was conducted on the reasons for the improvement in the plasticity of the as-sintered alloys. This study presents an effective approach to enhancing the room temperature deformation capacity of RHEAs, thereby facilitating their broader application.

Enhanced mechanical properties of dispersed carbide-strengthened CrFeNi-based medium entropy alloys prepared via powder metallurgy

In this study, a set of CrFeNi -based medium-entropy alloys (MEAs) with varying carbon contents were prepared by spark plasm sintering (SPS) using atomized alloy powders as raw material. The microstructures of powders and as-sintered alloys were characterized using ECCI and EBSD. The mechanical properties of as-sintered alloys were tested and the strengthening mechanical were discussed. The results showed that the microstructures of the CrFeNi gas atomized powder and sintered alloy were both single FCC phase in an equiaxed state. However, the matrix grains of powder with carbon addition were mostly dendritic, and few eutectic carbides could be observed between matrix grains. Compared with the CrFeNi MEA, the addition of 8 at. % C led to an increase in the yield strength and tensile strength from 395 MPa to 630 MPa–590 MPa and 990 MPa, respectively. Orowan strengthening and grain refinement resulting micro/nano carbides are responsible for the improvement in mechanical properties.

Microstructural design to Al-6Mg-5Gd alloy for the unification of structural and neutron shielding properties

Design of composition and microstructure can effective improve the structural and neutron shielding properties of neutron shielding materials. Based on the shielding standard of 30%B4C/Al, an Al-6Mg-5Gd (wt%) alloy was designed by Shmakov-Yamamoto model and successfully fabricated by Powder Metallurgy-based routines (e.g., powder fabrication, spark plasma sintering and hot extrusion techniques). By the rapid solidification in the atomization process, the submicron cell structures were formed in powders. Due to the difference in the original powder size, the as-sintered alloys had heterogeneous microstructures, leading to the bimodal grain structures of as-extruded alloys. During these processes, the formation of nano-sized τ(C15) (Al4MgGd) phase in combination with its firstly observed transformation behavior showed the great impacts on microstructure evolutions. Therefore, the as-extruded alloy exhibited the superior yield strength (335±7 MPa), ultimate tensile strength (462±5 MPa) and elongation (10.2±0.3%). Through the neutron absorption test, the as-extruded alloy showed the comparable of macroscopic transmission cross section with 30%B4C/Al. In comparison with the available Al-based neutron shielding materials, our report exhibited the best unification of mechanical and functional properties. The underlying mechanisms for synergic strength-ductility behavior were discussed on the nano-sized phase strengthening, α-Al/τ interface modulation, and bimodal grain features. Critically, this work designed and studied a high-performance neutron shielding material, shedding light on the advanced structural-functional integrated materials.

Microstructure and mechanical properties of high relative density Ti-48Al-1Fe alloy using irregular blend powder

Conventional pressing and sintering methods provide difficulties in achieving high density for PM TiAl alloys. The Ti-48Al-1Fe alloy was prepared via vacuum pressure-less sintering, and the microstructure evolution and mechanical properties during the sintering densification process of the mixed powder system were investigated. The results shows that the density of the mixed powder reached 98.3 ± 0.14% after pressure-less sintering for 3 hours under high vacuum conditions at 1300°C. The microstructure of the as-sintered alloy is a fine duplex structure composed of α2/γ lamellar colonies and the composite structure. The ultimate tensile strength (UTS) of the as-sintered alloy at room temperature is 315 MPa. The density of the as-sintered alloy reach more than 99.9% after non-capsule hot isostatic pressing (HIP). The as-HIPed alloy exhibits an UTS of 332 MPa at room temperature. At high temperatures, it maintains high strength, with an UTS of 408 MPa and yield strength of 386 MPa at 900°C, which is comparable to that of forged TiAl alloy. Furthermore, the elongation of the sample at 900°C is 20.0%. This process offers a novel method for sintering PM TiAl with high density and mechanical properties at a low cost and near-net shape.

Simulating porcelain firing effect on the structure, corrosion and mechanical properties of Co–Cr–Mo dental alloy fabricated by soft milling

This study aims at evaluating the effect of simulating porcelain firing on the microstructure, corrosion behavior and mechanical properties of a Co–Cr–Mo alloy fabricated by Metal Soft Milling (MSM). Two groups of Co-28Cr-5Mo specimens (25 × 20 × 3 mm) were prepared by MSM: The as-sintered (AS) specimens and the post-fired (PF) specimens that were subjected to 5 simulating porcelain firing cycles without applying the ceramic mass onto their surface. Phase identification by X-ray Diffraction (XRD), microstructure examination by optical microscopy and Scanning Electron Microscopy combined with Energy-Dispersive X-ray Spectroscopy (SEM/EDX), corrosion testing by cyclic polarization and chronoamperometry in simulated body fluid (SBF), the latter test accompanied by Cr3+ and Cr6+ detection in the electrolyte through the 1.5-diphenylcarbazide (DPC) method and UV/visible spectrophotometry, and mechanical testing by micro-/nano-indentation were conducted to evaluate the effect of the post-firing cycles on the properties of Co–Cr–Mo. The results were statistically analyzed by the t test (p < 0.05: statistically significant). All specimens had a mixed γ-fcc and ε-hcp cobalt-based microstructure with a dispersion of pores filled with SiO2 and a fine M23C6 intergranular presence. PF led to an increase in the ε-Co content and slight grain coarsening. Both AS and PF alloys showed high resistance to general and localized corrosion, whereas neither Cr6+ nor Cr3+ were detected during the passivity-breakdown stage. PF improved the mechanical properties of the AS-alloy, especially the indentation modulus and true hardness (statistically significant differences: p = 0.0009 and 0.006, respectively). MSM and MSM/simulating-porcelain firing have been proven trustworthy fabrication methods of Co–Cr–Mo substrates for metal-ceramic prostheses. Moreover, the post-firing cycles improve the mechanical behavior of Co–Cr–Mo, which is vital under the dynamically changing loads in the oral cavity, whereas they do not degrade the corrosion performance.

Mechanical and corrosion properties of Ti–29Nb–13Ta-4.6Zr alloy prepared by cryomilling and spark plasma sintering

In the present study, Ti–29Nb–13Ta-4.6Zr alloys were prepared by cryomilling and spark plasma sintering. Experimental results show that the as-sintered alloys have a fine-grain structured β phase and a small amount of α phase. By optimizing the sintering temperature to 1050 °C, Ti–29Nb–13Ta-4.6Zr alloy with a compression yield strength of 972 ± 18 MPa and an ultimate compression stress of 2071 ± 27 MPa, as well as an ultimate tensile strength of 1034 ± 19 MPa and a plastic strain of 8.7 ± 0.3%, together with a low Young's modulus of 74 ± 2 GPa can be obtained. Additionally, the as-sintered alloys also show preferable corrosion resistance with a low corrosion rate of ∼0.71 μm/year in Ringer's solution.

In-vitro corrosion and biocompatibility properties of heat treated Mg-4Y-2.25Nd-0.5Zr alloy

In this study, corrosion and biocompatibility behaviour of Mg-4Y-2.25Nd-0.5Zr alloy (Mg-Y-Nd-Zr) was investigated. The pre-alloyed powder of Mg-Y-Nd-Zr was processed using powder metallurgy (PM) method, followed by further heat treatment at 250 °C for 12 h. The microstructural features of as-sintered and heat treated Mg-Y-Nd-Zr alloy showed secondary phases like Mg24Y5 and Mg41Nd5 in α-Mg matrix. In heat treated samples, the alloy showed improved compressive strength and hardness. The hardness and compressive strength was better for the heat treated Mg-Y-Nd-Zr alloy samples (54 ± 2 HV and 243 ± 30 MPa) compared to heat treated pure Mg samples (44 ± 5 HV and 129 ± 11 MPa). This is because of rearrangement of the secondary phases in the matrix. The corrosion current density of Mg-Y-Nd-Zr alloy samples reduced by 45% (82.2 μA/cm2 for heat treated and 149.3 μA/cm2 for as-sintered) after heat treatment indicating its better corrosion resistance than as-sintered samples. Both heat treated and as-sintered alloy exhibited good biocompatibility and therefore the Mg-Y-Nd-Zr alloy is a potential candidate for biodegradable implants.

Nanocrystalline AlCoFeNiTiZn high entropy Alloy: Microstructural, Magnetic, and thermodynamic properties

AlCoFeNiTiZn high entropy alloy was successfully produced in powder form by the mechanical alloying process. The ball-milled alloyed product was characterized by X-ray diffractometry, scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy techniques, which indicated that after 120 h of milling, the solid solution was formed as predicted by thermodynamic calculations. Mechanical alloying began to form the BCC phase almost at 30 h and the FCC phase after about 30 h. Nucleation and growth were the processes involved in the formation of these phases, as shown by the Johnson-Mehl-Avrami kinetic model. Sintering was then used to fabricate the alloy in bulk metallic form. The powders were cold pressed and sintered after 120 h of mechanical alloying using a tube furnace with a controlled atmosphere at 500 °C. A similar FCC + BCC phase mixture was present after sintering. The sintered sample also contained minor amounts of Gahnite (ZnAl2O4) spinel material. DSC analysis revealed that recrystallization occurred at 280 °C. The as-milled and as-sintered alloys exhibit semi-hard magnetic properties measured by vibrating sample magnetometer (VSM), with saturation magnetization values of 39.14 and 65.78 emu/g, respectively.

Effect of Mn and Mo on the microstructure and electrical resistivity of Ti-Al alloy prepared by mechanical alloying and spark plasma sintering

High electrical resistivity titanium alloys can effectively reduce the attenuation of eddy current shielding to magnetic field, and have a good application prospect in magnetic compression devices. The Ti-Al alloys alloyed with Mn and Mo were prepared by mechanical alloying and spark plasma sintering in this study. The effect of alloying elements Mn and Mo on microstructure characteristics, electrical resistivity and micro-hardness of Ti-Al alloy, as well as the effect of annealing on Ti-Al-Mn-Mo quaternary alloy were investigated. The results show that the microstructure of Ti-9Al-xMn (x = 0, 1, 2, 4, 6 wt%) alloys was transformed from a single-phase α of Ti-9Al alloy to multiphase consisting of α + β + α2 phases. With the increase of Mn content, the number of α phase decreased, and (β + α2) dual phase region increased. The electrical resistivity of Ti-9Al-xMn alloys increased with the increase of Mn content and decreased as Mn content was more than 4 wt%. The Ti-9Al-4Mn alloy had the electrical resistivity of 2.176 μΩ‧m. The α phase grains in Ti-8Al-4Mn-1Mo and Ti-7Al-4Mn-2Mo were refined by Mo alloying, the content of α2 phase reduced, and β phase increased compared with Ti-9Al-4Mn alloy. The electrical resistivity of the quaternary alloys increased and the micro-hardness decreased. The Ti-8Al-4Mn-1Mo alloy had the higher electrical resistivity (2.252 μΩ‧m) and micro-hardness (627 HV). Annealing slightly decreased the micro-hardness of Ti-8Al-4Mn-1Mo alloy, but the electrical resistivity remained at the level of as-sintered alloy.

Structure investigation of AlFeNiTiZn nanocrystalline high entropy alloy

The AlFeNiTiZn high entropy alloy with a mean crystallite size of 35–45 nm was synthesized within a two-stage process. Firstly, as a powder form using a planetary ball mill in an argon atmosphere and after that compressed the powder samples and sintered them in tube furnace as bulk samples. The X-ray diffraction, scanning electron microscopy and transmission electron microscopy techniques were used to examine the structural and morphological changes during the process. The two-phase solid solution (FCC + BCC) was formed after mechanical alloying. In addition to FCC + BCC, gahnite (ZnAl2O4) was synthesized after sintering. A differential scanning calorimetry analysis has also confirmed that recrystallization took place at 311 °C. Nucleation and growth were adequate explanations for the mechanochemical synthesis of the AlFeNiTiZn high entropy alloy based on Johnson-Mehl-Avrami model. An interface-controlled growth was the conversion mechanism based upon the Avrami exponent. According to a vibrating sample magnetometer, both as-milled and as-sintered alloys have semi-hard magnetic properties with 37.42 and 25.43 emu/g saturation magnetization, respectively. In sintered HEA, the microhardness was measured to be about 724 ± 15 HV.

Magnetocaloric Effect in CoFe-Electroplated Ni50Mn33In16Cr1 Alloy

A high-saturation magnetization CoFe layer was electroplated onto Ni50Mn33In16Cr1 alloy using a magnetic field assistance electroplating bath. This CoFe-coated alloy can function as an active magnetic regenerator owing to its magnetocaloric effect. The CoFe coating layer did not affect the entropy change calculated from the isothermal magnetization of the alloy, but it significantly affected the temperature variation of the alloy by changing the externally applied magnetic field. The temperature change of the CoFe-coated alloy increases with increasing CoFe coating times. By comparing with the as-sintered alloy, a maximum increase of 150% in temperature change can be observed in the alloy coated with CoFe for 2 h.

Microstructure and mechanical behavior of AISI 4340 steel fabricated via spark plasma sintering and post-heat treatment

We investigated the evolution of microstructure and mechanical properties AISI 4340 steel during high-energy ball milling, spark plasma sintering (SPS), and post heat treatments. High-energy ball milling of the powder mixture led to the formation of a nanocrystalline (∼10 nm) bcc Fe matrix containing some segregation of alloying elements and oxide particles. As-sintered alloy consisted of martensite-austenite (MA) constituent and fine pearlite with an average grain size of 2.4 μm along with oxide particles and M23C6 particles. The quenching after austenitization formed a microstructure composed of martensite and MA constituent, thereby increasing the fraction of retained austenite and decreasing the average grain size (∼0.5 μm) compared to the as-sintered alloy. The M23C6 particles were fully dissolved at the high austenitization temperature, leading to the increase in retained austenite fraction after quenching. Tempering induced decomposition of retained austenite and precipitation of cementite particles, while it has negligible effects on the grain size, undissolved M23C6 particles, and oxide particles. As-sintered alloy exhibited a compressive yield strength of ∼2 GPa due to primary strengthening by dislocations and grain boundaries/cementite lamellae and secondary strengthening by oxide particles. Changes in dislocation strengthening played a dominant role in determining the yield strength of quenched and tempered steels.

Tribological Behavior of Annealed Ni-xFe-yCo Alloys: Effect of Co and Fe Additions

Abstract The influence of annealing on the microstructure, mechanical and sliding wear characteristics of Ni-based alloys produced by spark plasma sintering (SPS) was investigated. As-sintered alloys had a lamellar-like microstructure consisting of (γ′)-FeNi3 and γ-(NiFe) phases blended together. Lower Co contents (i.e., 30, 35 wt%) led to the formation of poorly bonded coarse γ precipitate islands. Annealed Ni-5Fe-45Co alloy exhibited the most excellent wear performance with the lowest coefficients of friction (0.142 ± 0.05) and wear-rate (0.3 ± 0.02 × 10−4 mm3/Nm). Annealing resulted in alloys with good strength-ductility combinations due to appreciable γ′ precipitation enhancement.

High thermal stability and oxidation behavior of FeCrNiAl-based medium-entropy alloys prepared by powder metallurgy

The thermal stability and oxidation behavior of powder metallurgical FexCrNiAl medium-entropy alloys at 800 °C~1000 °C were investigated in the air. The as-sintered alloys show fine microstructures with the major phases of BCC, disordered BCC(B2), dispersed oxides together with a small volume fraction of carbides. After long-time annealing, the alloys show excellent thermal stability maintaining the phase constitution obtained from the as-sintered alloys. The oxides and carbides introduced by mechanical alloying inhibit grain growth, which plays a significant role in thermal stability. After isothermal oxidation, the oxide scale follows the sequence of outer-layer Al2O3, middle-layer Cr2O3, and inner-layer spinels respectively. The results indicate that decreasing Fe enhances the oxidation resistance of FeCrNiAl-based alloys by slowing down the oxidation kinetics and reducing the oxide stress. The high internal stress leads to the spallation and wrinkling of the oxide scale at 1000 °C. The reasons for spallation are systematically discussed from the aspect of composition, sample geometry, and oxidation conditions. The selective oxidation mechanism of FeCrNiAl-based high-entropy alloys was proposed.

Microstructure and properties of Cu−2Cr−1Nb alloy fabricated by spark plasma sintering

Cu−2Cr−1Nb alloy was fabricated by spark plasma sintering (SPS) using close coupled argon-atomized alloy powder as the raw material. The optimal SPS parameters obtained using the L9(34) orthogonal test were 950 °C, 50 MPa and 15 min, and the relative density of the as-sintered alloy was 99.8%. The rapid densification of SPS effectively inhibited the growth of the Cr2Nb phase, and the atomized powder microstructure was maintained in the grains of the alloy matrix. Uniformly distributed multi-scale Cr2Nb phases with grain sizes of 0.10−0.40 μm and 20−100 nm and fine grains of alloy matrix with an average size of 3.79 μm were obtained. After heat treatment at 500 °C for 2 h, the room temperature tensile strength, electrical conductivity, and thermal conductivity of the sintered Cu−2Cr−1Nb alloy were 332 MPa, 86.7% (IACS), and 323.1 W/(m·K), respectively, and the high temperature tensile strength (700 °C) was 76 MPa.

Thermal oxidation of a porous Ti[sbnd]23Nb alloy for wear related biomedical applications: Effect of oxidation duration

In this study, an attempt has been made for tailoring the surface features of an 18 vol% porosity containing Ti23Nb alloy manufactured via powder metallurgy method. The aim was enhancing the wear resistance without sacrificing the mechanical properties. For this purpose, porous Ti23Nb alloys having hardness of 280 HV0.025 were subjected to thermal oxidation (TO) at 600 °C in air for two different holding times. TO duration of 6 h, which introduced a ~ 0.5 μm thick TiO2-rutile type exterior oxide layer (OL), provided surface hardness of 620 HV0.025. Extension of TO duration to 60 h caused covering of the surfaces with a ~ 3.0 μm thick OL and increase of surface hardness to 980 HV0.025. As the result of the increased surface hardness, TO'ed. alloys showed superior sliding wear resistance coupled with a reduction in friction coefficient (COF) against alumina ball in 1.5× simulated body fluid (SBF) when compared to the as-sintered state. Despite having the highest surface hardness, the alloy TO'ed. for 60 h exhibited slightly higher wear loss and COF than the alloy TO'ed. for 6 h. More important than this, the alloy TO'ed. for 60 h collapsed at a compressive stress well below the yield strength of the alloy TO'ed. for 6 h, which exhibited almost similar compressive stress-strain curve with that of the as-sintered alloy. From this perspective 6 h appeared as a promising TO duration for the examined porous alloy to be used in biomedical applications as it preserves the mechanical properties while providing remarkable enhancement in the wear resistance.

Microstructure and properties of CoCrNiFeMnAl0.5 high-entropy alloys prepared by gas atomization combined with oscillatory pressure sintering

In order to improve the mechanical strength of Cantor alloy and obtain the uniform microstructure, about 9.1 at.% of Al element was introduced and a new CoCrNiFeMnAl0.5 HEAs was prepared by gas atomization combined with oscillatory pressure sintering. The effects of powder sizes on the microstructure and mechanical properties of the as-sintered alloys were investigated and the corresponding strengthening mechanism was discussed. The results show that the CoCrNiFeMnAl0.5 HEAs were composed of FCC phase with good plasticity and BCC phase with high strength. The as-sintered alloys have higher relative density and uniform microstructure without the defects of composition segregation and coarse structure like the as-cast alloy. The yield strength and microhardness of the as-sintered alloys increased to 775 MPa and 369.3 HV, which were 173% and 104% higher than that of the as-cast alloy, respectively. It is indicated that the strength and microhardness of the as-sintered alloys depend on the powder sizes. The strengthening mechanisms of the as-sintered alloys were mainly attributed to the second phase strengthening and the grain boundary strengthening.

Effect of aging treatment on microstructure, mechanical and corrosion properties of 7055 aluminum alloy prepared using powder by-product

To effectively improve the mechanical properties of 7055 aluminum alloy prepared by powder by-products, aging treatment was carried out on the as-sintered 7055 alloy. The effects of different aging treatment parameters on the microstructure, mechanical and corrosion properties of the as-sintered 7055 alloy were investigated. The results show that the microstructure of as-sintered alloy is still a typical equiaxed crystal structure after aging treatment, however, the number of precipitated phases in matrix was significantly reduced and in uniformly fine needle-like distribution. The metastable η′ phase is mainly precipitated. Moreover, the strength of the aging-treated sample is nearly doubled in comparison to that of the as-sintered alloy, and the sequence of the strength of three aging-treated sample is: RRA > DA > T6. The improvement of mechanical properties of the aging-treated sample is mainly attributed to the precipitation strengthening. Furthermore, the corrosion resistance of three aging-treated sample follows: DA > RRA > T6, and the continuous grains boundary precipitations and Fe-containing precipitates increase the corrosion susceptibility.

Microstructures and mechanical properties of AZ31 magnesium alloys fabricated via vacuum hot-press sintering

Magnesium and its alloys have become attractive materials in automotive and aerospace fields because of their low density and high strength. However, their application is greatly limited by their poor deformation capacity. Powder metallurgy technology can reduce the deformation and processing of magnesium alloys during the preparation process to alleviate this problem because of its near net shape machining characteristics. In this paper, a high-performance AZ31 magnesium alloy was prepared via vacuum hot-press sintering. The effects that sintering temperature has on the structure and mechanical properties of the as-sintered alloy were investigated. The results indicate that the sintered microstructure is composed of approximately equiaxed α-Mg crystal grains, and nanoscale Al-Mn phases precipitated in the crystals and particle boundaries. At an optimal sintering temperature of 535 °C, the average tensile strength, compressive strength, elongation, and fracture strain of the sample are 247.10 MPa, 391.35 MPa, 22.27%, and 24.17%, respectively. In addition, the reasons for the high strength and favorable elongation of the AZ31 magnesium alloy that was sintered via vacuum hot pressing were also analyzed. This is mainly because pressure is applied during the sintering process; this eliminates pores and breaks the oxide film on the powder surface, thereby forming a good metallurgical bond between the powders.

Solid-state Synthesis and High Magnetostriction Performances of Heavy Rare Earth–Free Sm0.88Nd0.12Fex Particulate Composites

The magnetostrictive materials are widely applied as modern smart materials converting electrical to mechanical energy. Here, the pseudobinary Sm0.88Nd0.12Fex compounds, free of both (expensive) heavy rare earths and B/Co stabilizer, have been prepared with mechanical alloying and subsequent solid-state sintering process. An amorphous phase can be formed by high-energy ball milling, followed by the formation of single-phase Sm0.88Nd0.12Fe1.80 alloy at an annealing temperature of 690 °C. A high anisotropic magnetostriction λa as large as 1280 ppm for the as-sintered alloy is obtained at an external field of 12 kOe. The oriented pseudo 1-3 type particulate composite is fabricated by embedding and aligning the as-sintered alloy particles in an epoxy matrix under an oriented magnetic field. The composite with a volume faction of 30% shows large magnetostriction (λa ~895 ppm at 12 kOe). In particular, λa and the longitudinal magnetostriction λ|| at a low field (3 kOe) reaches a value as high as 630 and 450 ppm, respectively, 78% and 84% larger than that of its as-sintered polycrystalline alloy although Vp is only 30%. Ultimately, it is anticipated that this work will open a new design paradigm to develop Sm-based materials with negative-magnetostriction performance.