Vacuum Pressureless Sintering Research Articles

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.

High-Performance Ti-6554 Alloy Manufactured Using Irregular Powder via Vacuum Pressureless Sintering Followed by Forging

High-Performance Ti-6554 Alloy Manufactured Using Irregular Powder via Vacuum Pressureless Sintering Followed by Forging

Structure and Luminescence Properties of Ho:(Y1−xScx)2O3 Ceramics

Ho‐doped sesquioxide is one of the most important laser gain media for 2 micron wavelength band laser applications. It is hard to be transparent by sintering because of its high melting point. Herein, the structural and spectral properties of 0.5 at% Ho:(Y1−xScx)2O3 (x = 0, 0.3, 0.5, 0.7, 1.0) ceramics fabricated by vacuum pressureless sintering are studied. Then, 0.5 at% Ho:(Y0.7Sc0.3)2O3 transparent ceramics with 1.0 at% ZrO2 as a sintering aid are prepared by vacuum pressureless presintering (1680 °C for 4 h) combined with hot isostatic pressing sintering treatment (1600 °C/190 MPa for 3 h). The transmittance of Ho:(Y0.7Sc0.3)2O3 ceramic reaches 80.8% at 1100 nm (6 mm in thickness), which is close to its theoretic value. The addition of ZrO2 has no significant influence on spectroscopic properties of 0.5 at% Ho:(Y0.7Sc0.3)2O3 ceramics.

Effect of Al<sub>2</sub>O<sub>3</sub> nanofibers on compaction, phase composition, and mechanical properties of ZrO<sub>2</sub>-based composites obtained by vacuum pressureless sintering

This work studies the effect of the relative content of Al2O3 nanofibers on the compaction, phase composition, and physicomechanical properties of composites based on ZrO2 obtained by free vacuum sintering. It was found that in the process of manufacturing composites, nanofibers are sintered into Al2O3 grains of complex, elongated shape, which form a solid, frame-reinforcing structure. The relative density of composites with 5 wt. % and 10 wt. % of nanofibers, decreases up to 95%. It is shown that in all sintered samples the tetragonal modification of ZrO2 acts as the main phase, and the different content of nanofibers affects the amount of cubic and monoclinic modifications of ZrO2. It was found that addition of 5 wt. % and 10 wt. % of Al2O3 nanofibers increases the microhardness of the composite by 11% and crack resistance by 46%.

Structure, phase composition and properties of vacuum pressureless sintered ZrO2-based composites reinforced with carbon nanotubes

In this work, composites based on 3 mol. % yttria-stabilized zirconia (3YSZ) with additives of 1, 5 and 10 wt. % multi-walled carbon nanotubes (MWCNT) were investigated. Samples were obtained by cold pressing followed by vacuum pressureless sintering at a temperature of 1500 °C. It was found that MWCNTs retain their structure after high-temperature sintering, they are located along the zirconia grain boundaries and they lead a grain refining effect for zirconia matrix. Nanotubes can significantly affect the phase composition of composites. Additives 5 wt. % and 10 wt. % MWCNT partially limits the monoclinic-tetragonal phase transition of ZrO2 and leads to the formation of a new phase of zirconium carbide. An increase of MWCNTs content in composites leads to a smooth decrease of the relative density from 99.2 % to 81.2 %. To obtain fully dense 3YSZ/MWCNT composites, longer sintering at higher temperatures is required compared to the conditions for sintering 3YSZ ceramics without additives.

High-temperature plastic deformation behaviour of powder metallurgy Ti6Al4V alloy prepared from HDH powder

ABSTRACT Powder metallurgy (PM) Ti6Al4V alloy was prepared from HDH alloy powder by vacuum pressureless sintering. To study its high-temperature deformation behaviour, isothermal compression testing was carried out with temperature range of 900–1050°C, strain rate range of 0.01–10 s−1, and compression ratio of 60%. Based on the hot deformation data, the corresponding constitutive equation and hot processing map were clarified. The deformation activation energy for PM Ti6Al4V alloy was 606.5 kJ mol−1. Its deformation instability zone was very small, locating in the range of 960∼990°C with strain rate >1 s−1. PM Ti6Al4V alloy had large hot working window and good hot workability. The ideal hot processing domain was at 970–990°C and 0.05–0.01 s−1. Besides, the domain at 1000–1040°C and 1–10 s−1 was also suitable for hot processing. This study proposed an effective guidance for the hot deformation (e.g. forging, extrusion, rolling) of PM Ti6Al4V alloy.

High strength-ductility synergy of MgAlB4 whisker reinforced aluminum matrix composites achieved by in situ synthesis

For ceramic reinforced metal matrix composites, it is a critical issue to improve the limited ductility to achieve a combination of high strength and high toughness. Herein, in-situ synthesized MgAlB4 whisker reinforced Al (MgAlB4w/Al) composites with rational distribution of the reinforcement were successfully prepared by an efficient process combining vacuum pressureless sintering and hot-extrusion. The results demonstrate that the MgAlB4 whiskers noticeably coarsen with increasing sintering temperature, while the sintering duration has little effect on the size of the whiskers. The as-extruded composite with 15 wt% MgAlB4w has excellent comprehensive properties with the ultimate tensile strength of 380 MPa and the fracture elongation of ~9.6%. Fracture analysis reveals that the whiskers bear most loads and fracture firstly to form crack sources with the load increasing during drawing. In addition, the alignment distribution of MgAlB4w along extrusion direction is the main reason for the improved ductility of the MgAlB4w/Al composite.

Microstructure and mechanical properties of WC–Ni multiphase ceramic materials with NiCl2·6H2O as a binder

Abstract High-density WC–Ni composite ceramics were prepared by cold isostatic pressing–vacuum pressureless sintering–hot isostatic pressing with tungsten carbide (WC) powder and NiCl2·6H2O as a binder. Results show that with an increase in the contents of Ni in the metal binder phase, the relative density of WC–Ni composite ceramics is improved, and the formation of the carbon-deficient W2C phase is reduced. There is no W2C generated in the WC–1 wt% Ni material. At high temperatures, the Ni phase changes into the liquid phase and enters between the WC particles, thereby promoting the close alignment of the WC particles. Moreover, the WC particles will be more closely aligned under their own surface tension and capillary action, thereby promoting the densification of WC–Ni composite ceramics. The WC–0.5 wt% Ni composite ceramics are fully dense and show the best comprehensive performance with a microhardness of 23.0 GPa, a fracture toughness of 5.28 MPa m1/2, and a flexural strength of 1,396.58 MPa. WC–Ni composite ceramics are mainly composed of elongated triangular prism WC particles and Ni phase. Transgranular fracture was the main fracture mode of WC–Ni multiphase ceramic materials with a small amount of intergranular fracture due to the existence of the Ni phase. Such a fracture mode can increase the flexural strength of the composite material.

Preparation of ZrO2-doped Nd3+: Y2O3 transparent ceramic and the corresponding characteristic of luminescence

Highly transparent ZrO2-doped Nd3+: Y2O3 ceramics were prepared by a simple shaping approach followed by a low-cost sintering and simple sintering approach: Slip casting and vacuum pressureless sintering (1840 × 8 h), with the Nd3+ doping concentration of 0.5 at%, 1.0 at%, 1.5 at%, and 2.0 at% respectively. The in-line transmission (IT) of Nd3+ doped sample at 0.5 at% is the highest at 1074 nm, which is 80.7%, equivalent to 99.6% of the theoretical IT. The 1.0 at% Nd3+-doped sample has strong absorption peaks at 746, 806, 821 and 893 nm. The strongest absorption peak is 821 nm, and the two absorption peaks at 806 and 821 nm correspond to the transition of 4I9/2-[4F5/2+2H (2)9/2]. Under the excitation of 808 nm LD, there are three main emission bands, located at 870–950 nm, 1047–1137 nm and 1310–1460 nm, corresponding to the energy level transitions of 4F3/2-4I9/2, 4F3/2-4I11/2 and 4F3/2-4I13/2, respectively. The main emission peak at 1074 nm corresponds to the 4F3/2-4I11/2 energy level transition. The fluorescence lifetime can be tuned with Nd3+ doping concentration.

The absorption and emission properties of highly transparent ZrO2‐doped Yb3+: Y2O3 ceramics

AbstractUltra‐highly transparent ZrO2‐doped Yb3+: Y2O3 ceramics were prepared by slip casting and vacuum pressureless sintering and the transmittance reached the highest value of 80.9% for the sample doped with 8.0 at% Yb3+. There are three main absorption peaks at 905, 950, and 976 nm, corresponding to the transition from the lowest level of field splitting of 2F7/2 crystal to every splitting energy levels of 2F5/2 crystal field. We analyzed the absorption and emission spectra of transparent Yb3+: Y2O3 from the energy level structure of Yb3+, and the transmission, absorption, and emission spectra were systematically studied. There are three main absorption peaks at 905, 950, and 976 nm and four emission peaks at 1076, 1031, 1013, and 977 nm, respectively. The emission peaks at 977 and 1013 nm broaden and vanish for 8.0 and 10.0 at% Yb3+‐doped Y2O3, which may be related to the change of Y2O3 crystal field caused by high concentration.

High-density and low-interstitial Ti-23Al-17Nb prepared by vacuum pressureless sintering from blended elemental powders

High-quality Ti2AlNb alloys were prepared from blended elemental powders via cold isostatic pressing and pressureless vacuum sintering. When sintered at 1200 °C, the Ti-23Al-17Nb alloy attained a very high relative density (98.7%) and low contents of interstitials (0.1 wt% O, 0.03 wt% N and 0.005 wt% H). Three crystalline phases – B2, α2, and orthorhombic (O) – were observed in a fine and homogeneous microstructure in the sintered samples. The fine microstructure and low interstitials account for the good mechanical properties achieved: approximately 941 MPa of ultimate tensile strength, 862 MPa of yield strength and 11.7% of elongation.

Influence of Ni and ZrO2 contents on sintering and mechanical properties of WC-2wt.%ZrO2-1wt.%Ni composites

WC-2wt.%ZrO2-1wt.%Ni composites were prepared by vacuum pressureless sintering (VPS) and post-hot isostatic pressing (post-HIP). The microstructure, phase composition, densification, hardness, fracture toughness, and flexural strength of composites with different Ni and ZrO2 contents were systematically investigated. The results show that WC-Ni and WC-ZrO2-Ni composites prepared by VPS can be densified by the addition of a small amount of Ni as the binder phase. Moreover, the densities of the composites can be further enhanced i.e. the composites are rendered nearly fully dense following HIP while the grains remain fine without obvious growth. The binder Ni and ZrO2 phases are uniformly distributed in the WC matrix and exhibit high bonding strength with it. The hardness, fracture toughness, and flexural strength of the WC-ZrO2-Ni composites following HIP could reach 22.4 GPa, 12.0 MPa m1/2, and 1101.2 MPa, respectively. Based on the influence of the Ni and ZrO2 contents on the microstructures and mechanical properties of the WC-2wt.%ZrO2-1wt.%Ni composites, the fracture mechanism was determined to be governed by the phase transformation of ZrO2 that led to the development of some micro-cracks followed by deflection, bridging, and branching of the cracks to improve fracture toughness. The composites are mainly composed of elongated triangular prismatic WC grains and ZrO2 phases, and hence the fracture mode can change such that transgranular fracture becomes the main fracture mode accompanied by a small amount of intergranular fracture. Thus, the flexural strength of the composites can be improved.

Effects of TiB2 particle size on the microstructure and mechanical properties of TiB2-based composites

TiB2–25 vol% (Fe-Ni) and TiB2–30 vol% (Fe-Ni) composites with different TiB2 particle size were fabricated by vacuum pressureless sintering at 1700 °C for 1 h. The influences of TiB2 particle size on the microstructure and mechanical properties was studied in detail. The phase analyses and microstructure of TiB2-based composites were investigated by X-ray diffraction, SEM, EPMA, and TEM. The experimental results showed that only TiB2 and γ(Fe, Ni) phases in the TiB2-based composites, not any other brittle phases such as M2B, M23B6 can be found. The morphology of raw TiB2 particles was irregular, however, many hexagonal crystals TiB2 grains were observed after sintering. The change of dominant fracture mode was strongly affected by the grain size of TiB2 particles, and the dominant fracture mode of TiB2-based composites transformed from trans-granular fracture to inter-granular fracture as the TiB2 particle size increasing from 1.97 to 4.27 µm. The high-toughness of TiB2-based composites was due to the synergetic action of comprehensive toughening mechanisms, including crack deflection, crack branching, as well as the strong bonding strength between TiB2 grains and γ(Fe, Ni) binder, etc. As TiB2 particle size increasing from 1.97 to 4.27 µm, the relative density, Vickers hardness, flexural strength, and fracture toughness of the TiB2-based composites decreased. The relative density, Vickers hardness, flexural strength and fracture toughness of TiB2–30 vol% (Fe-Ni) composites with the TiB2 particle size of 1.97 µm reached up to 98.20 ± 0.20%, 14.00 ± 0.40 GPa, 1000 ± 30 MPa, and 19.26 ± 0.72 MPa m1/2, respectively.

In situ formation of low interstitials Ti-TiC composites by gas-solid reaction

In this work, in situ formation of low interstitials Ti-TiC composites with a high relative density has been fabricated by the combination of TiH2-CH4 reaction and vacuum pressure-less sintering. During the gas-solid reaction, TiH2 reacted with CH4 to form TiC particles on the powder surface. As a consequence, low interstitials contents (O < 0.21 wt%, N < 0.062 wt%, H < 0.011 wt%) were obtained in as-sintered materials. TiC particles uniformly distributed in the Ti matrix with a particle size of 3–12 μm, resulting in the grain refinement from 87.89 μm to 34.76 μm. The as-sintered Ti-35 vol%TiC exhibited high tensile strength of 644 MPa in YS and 736 MPa in UTS. Considering about ductility, the Ti-15 vol%TiC obtained comprehensive mechanical properties, about 628 MPa in YS, 715 MPa in UTS and 12.1% in elongation. The acceptable ductility (elongation > 10%) is important for following thermomechanical processing. In summary, the achievement of high performance Ti-TiC composites was attributed to the grain refinement, in-situ TiC strengthening, as well as a solid solution strengthening of the O, N and C elements in the Ti matrix.

Vacuum Pressureless Sintering of Ti-6Al-4V Alloy with Full Densification and Forged-Like Mechanical Properties

Ti-6Al-4V ingots with a nearly 100% density, fine and homogeneous basket-weave microstructure, and better comprehensive mechanical properties (UTS = 935 MPa, Y.S. = 865 MPa, El. = 15.8%), have been manufactured by vacuum pressureless sintering of blended elemental powders. Coarse TiH2 powder, Al powder (2, 20 μm), V powder, and Al-V master alloy powder were used as raw materials to produce different powder mixtures (D 50 = 10 μm). Then, the compacts made by cold isostatic pressing were consolidated by different sintering curves. A detailed investigation of different as-sintered samples revealed that a higher density can be obtained by generating transient molten Al in the sintering process. Coarse Al powder and a rapid heating rate under the melting point of Al contribute to molten Al formation. The presence of temporary liquid phase changes the sintering mechanism, accelerating the sintering neck formation, improving sinterability of the powder mixtures. Density of 99.5% was achieved at 1150 °C, which is markedly lower than the sintering temperatures reported for conventional blended elemental powder metallurgy routes. In addition, low interstitial content, especially for oxygen (0.17 wt.%), is obtained by strict process control.

Microstructure and mechanical properties of TiB2-based composites with high volume fraction of Fe-Ni additives prepared by vacuum pressureless sintering

TiB2-based composites with high volume fraction of Fe-Ni as additive were fabricated by vacuum pressureless sintering. The effect of Fe-Ni addition (Fe:Ni=6:4) at different volume fractions from 20 to 35vol% on the microstructure evolution and fracture behaviour of the TiB2-(Fe-Ni) composites was investigated. It was found that the TiB2-(Fe-Ni) composites consisted of only TiB2 and γ(Fe, Ni) phases without any brittle phases. The relative density, flexural strength and fracture toughness of the composites increased as the Fe-Ni content increased. The results indicated that the TiB2–35vol% (Fe-Ni) composites exhibited the optimum combination of physical and mechanical properties, i.e., the relative density of 98.32%, the Vickers hardness of 8.56GPa, the flexural strength of 1050.92MPa and the fracture toughness of 17.75MPam1/2. The high fracture toughness of the TiB2–35vol% (Fe-Ni) composite was due to an intensive coupled mechanism of the near-full density, dimple fracture of Fe-Ni phase, crack deflection, crack bridging, grain's pull-out and transgranular fracture of the TiB2 particles.

Pressureless sintering and mechanical properties of hydroxyapatite/functionalized multi-walled carbon nanotube composite

This work aims to study the optimum sintering conditions of hydroxyapatite/functionalized multi-walled carbon nanotube (HA/f-MWCNT) composite with improved mechanical properties for bone implant applications using a pressureless sintering technique. The carboxyl functional group (COOH) introduced by the acid treatment on the MWCNT surface by which HA molecules are grafted onto the surface of functionalized MWCNT with strong interfacial bonding. The composite exhibits a lower hemolytic rate of 1.27%. The flexible nature of f-MWCNT makes them bend and attached to the HA grains, indicates that f-MWCNT bear significant stress by sharing a portion of the load and it leads to improve their mechanical properties. The maximum Vickers hardness of 3.6GPa is obtained for the HA/f-MWCNT composite sintered at 1100°C, whereas the highest compressive strength of 481.7MPa and fracture toughness of 2.38MPa.m1/2 achieved after sintering at 1150°C. This study demonstrated that HA/f-MWCNT composite create suitable structures by vacuum pressureless sintering technique to satisfy the mechanical requirements for bone tissues.

Experimental investigations on the synthesis of W–Cu nanocomposite through spark plasma sintering

Elemental powders of nanosized tungsten and chemically deposited nanosized copper were used for preparing tungsten/copper composites, which are used as electric contact components. A composite of 70wt.%W/30wt.%Cu (52vol%W/48vol%Cu) composition was prepared by three powder metallurgy techniques. Elemental mixing, mechanical milling and electroless Cu coating on tungsten particles were used for the synthesis. The obtained powder blends underwent consolidation by rapid hot pressing using the spark plasma sintering (SPS) route at 950°C under vacuum and by conventional vacuum pressureless sintering for comparison. The elemental powders and the sintered composites were investigated by optical microscopy and SEM. Electrical conductivity, hardness, transverse rupture strength, and wear properties were measured. Results show that the synthesis of the composite by the investigated route yields good performance. Samples prepared by SPS have shown better mechanical properties than those prepared by compaction and sintering due to their fine microstructure.

Wear behavior of Co-free hardmetals doped by zirconia and produced by conventional PM and SPS routines

In this work, the microstructural features and mechanical properties of Co-free hardmetals doped by 3mol% yttria stabilized tetragonal zirconia and densified by means of conventional vacuum pressureless sintering as well as spark plasma sintering were studied. Influence of dispersed zirconia clusters on mechanical behavior and erosive and abrasive wear resistance of hardmetals with 6wt% of stabilized ZrO2 was investigated. It was shown that zirconia transformability affects the wear performance of the composites. For utilization of the mechanism of transformation toughening some essential requirements such as grain size and grain size distribution should be met. Simultaneous increase in fracture toughness due to stress induced transformation toughening of tetragonal zirconia with increase in hardness due to grain growth inhibition during processing allows considering the cemented carbides doped by zirconia as promising tribomaterials.

Dielectric and microwave absorption properties of Ti3SiC2 powders

Ti3SiC2 powders are synthesized via vacuum pressureless sintering method. The dielectric properties of the powders synthesized at different temperature are investigated in the frequency range from 8.2 to 12.4 GHz (X-band), which suggests the remnant TiC has obvious influence on complex permittivity and the pure Ti3SiC2 powders have the highest dielectric loss. The dielectric and microwave absorption properties of Ti3SiC2 powders are studied. Both the real part (ε′) and imaginary part (ε″) increase with Ti3SiC2 concentration but decrease with increasing frequency, which presents the obvious frequency dispersion effect. Furthermore, the effect of thickness on the microwave absorption property of Ti3SiC2/paraffin mixture is analyzed. The mixture with 70 wt% Ti3SiC2 in 2.0 mm thickness presents the optimum microwave absorption property. The effective absorbing bandwidth (below −10 dB) is almost in the whole frequency range and the lowest reflection loss is −33.83 dB. The results show Ti3SiC2 powders are good microwave absorbents in the X-band frequency range.