Rapid synthesis of Ti alloy with B addition by spark plasma sintering

The in situ TiB/Ti–4.0Fe–7.3Mo composites were fabricated by plasma spark sintering (SPS) technique using mechanical alloying of Ti, Fe65Mo and B powder. The effects of sintering temperature on densification of the composites and microstructure of the in situ synthesized TiB were investigated using scanning electron microscope (SEM) and transmission electron microscope (TEM). A dense composite was obtained after sintering at 1000 °C for 5 min. Under proper ball milling and SPS conditions in situ TiB reinforced Ti–4.0Fe–7.3Mo composites have been prepared. The in situ TiB showed a typical needle shape grow along [0 1 0] direction. The transverse cross-section of TiB grain is a hexagonal shape with the planes of (1 0 0), (1 0 1) and . The stacking faults in the (1 0 0) plane were observed in the TiB needles.     

Microstructure and mechanical properties of nanocrystalline high strength Al–Mg–Si (AA6061) alloy by high energy ball milling and spark plasma sintering

In the present paper, the microstructure and mechanical properties of nanostructured Al–Mg–Si based AA6061 alloy obtained by high energy ball milling and spark plasma sintering were reported. Gas atomized microcrystalline powder of AA6061 alloy was ball milled under wet condition at room temperature to obtain nanocrystalline powder with grain size of 30 nm. The nanocrystalline powder was consolidated to fully dense compacts by spark plasma sintering (SPS) at 500 °C. The grain size after SPS consolidation was found to be 85 nm. The resultant SPS compacts exhibited microhardness of 190–200 HV_100 g, compressive strength of 800 MPa and strain to fracture of 15%.     

Joining of AlN and graphite disks using interlayer tapes by spark plasma sintering

AlN and graphite disks were successfully joined using a polymer plasticized ceramic tape as the interlayer by spark plasma sintering (SPS). The tape contains either composite powders of AlN and graphite or AlN powders without graphite. Both tapes contained 5 mass% Y₂O₃ as the sintering aid of AlN. The joining was carried out at 1700–1900 °C and 30 MPa for 5 min. No other reaction phase except for Al₂Y₄O₉ was identified in the joints. The maximum tensile strength of the joints was obtained when the AlN–graphite composite interlayer tape was used. The joining mechanism is attributed not to the chemical bonding, but to the physical bonding of the Al₂Y₄O₉ phase, which is solidified from the molten Al–Y–O squeezing into the porous graphite under pressure during SPS.     

Preparation of titanium diboride TiB2 by spark plasma sintering at slow heating rate

A systematic study of the spark plasma sintering of TiB₂ starting from mixture of elemental powders was performed to investigate the temperature distribution between pressing tool and sinter body. The reaction mechanism and time-dependant evolution of the sintering behavior are established. The simultaneous application of pulsed high dc current and load leads to a microstructure with needle-shaped grains. Electron backscattering diffraction analyses show the preferred orientation of small crystallites parallel to the pressing direction.     

A novel high-entropy alloy with multi-scale precipitates and excellent mechanical properties fabricated by spark plasma sintering

Body-centered-cubic (BCC) high entropy alloys (HEAs) usually exhibit high strength but poor ductility. To overcome such strength-ductility trade-off, a novel (FeCr)₄₅(AlNi)₅₀Co₅ HEA was presented in this paper, which was designed and fabricated with mechanical alloying (MA) followed by spark plasma sintering (SPS), and has a heterogeneous microstructure with multi-scale precipitates. Electron microscopy characterization revealed that the sizes of the precipitates range from nano (<300 nm), sub-micron (300～800 nm) to micron (>1 μm). The bulk HEA exhibits excellent mechanical properties, of which the compressive yield strength, fracture strength, and plasticity at room temperature can reach 1508 MPa, 3106 MPa and 30.4 %, respectively, which are much higher than that of most HEAs prepared by Powder Metallurgy reported in the literatures, suggesting that the HEA developed is highly promising for engineering applications. The excellent mechanical properties of the bulk HEA can be attributed to that the multi-scale precipitates are fully coherent with the matrix, which could reduce the misfit strain at the interface, and relieve the stress concentration during deformation.     

Microstructure and mechanical properties of ultra-fine grained MoNbTaTiV refractory high-entropy alloy fabricated by spark plasma sintering

The MoNbTaTiV refractory high-entropy alloy(RHEA) with ultra-fine grains and homogeneous microstructure was successfully fabricated by mechanical alloying(MA) and spark plasma sintering(SPS).The microstructural evolutions,mechanical properties and strengthening mechanisms of the alloys were systematically investigated.The nanocrystalline mechanically alloyed powders with simple bodycentered cubic(BCC) phase were obtained after 40 h MA process.Afterward,the powders were sintered using SPS in the temperature range from 1500 ℃ to 1700 ℃.The bulk alloys were consisted of submicron scale BCC matrix and face-centered cubic(FCC) precipitation phases.The bulk alloy sintered at 1600℃ had an average grain size of 0.58 μm and an FCC precipitation phase of 0.18 μm,exhibiting outstanding micro-hardness of 542 HV,compressive yield strength of 2208 MPa,fracture strength of 3238 MPa and acceptable plastic strain of 24.9% at room temperature.The enhanced mechanical properties of the MoNbTaTiV RHEA fabricated by MA and SPS were mainly attributed to the grain boundary strengthening and the interstitial solid solution strengthening.It is expectable that the MA and SPS processes are the promising methods to synthesize ultra-fine grains and homogenous microstructural RHEA with excellent mechanical properties.     

放电等离子烧结Ti-Al系金属间化合物

用机械合金化法(MA)制备了Ti-45% Al纳米晶合金粉末,并对其进行放电等离子烧结(SPS),烧结时间仅为5min.用D-maxIIA型X射线衍射仪、JEM-2000EX型透射电子显微镜对粉末和烧结块体的微观组织及机械性能进行了研究.研究表明:Ti和Al的粉末随着球磨时间的延长,粉末有明显的细化趋势,球磨5h即有非晶产生,球磨20h后得到接近完全非晶相;采用SPS烧结技术,在1200℃下能够制备出较高硬度的TiAl金属间化合物块体材料.     

A comparative study of nickel and alumina sintering using spark plasma sintering (SPS)

Nickel and alumina powders with particle sizes of 4.3 μm and 108 nm, respectively, have been sintered in moulds of different sizes. Only the diameter (inner and/or outer) of moulds has been varied. The dimension modification is responsible for sample microstructure variation because set temperature, heating rate, dwell time and pressure were identical in all experiments. The influence of the die (or sample) dimension on the sample microstructure appears to depend strongly on the electric characteristics of the powder. The present paper is an attempt to correlate the bulk microstructure evolution with the die and sample size and the electrical current distribution within the system.     

Microstructure and mechanical properties of Al-Si-Ni-Ce alloys prepared by gas-atomization spark plasma sintering and hot-extrusion

Al-Si-Ni-Ce alloys with the composition of Al_78.5Si₁₉Ni₂Ce_0.5, Al₇₆Si₁₉Ni₄Ce₁ and Al₇₃Si₁₉Ni₇Ce₁ were atomized and then sintered by using spark plasma method. The microstructure of the as-atomized powders, sintered and hot-extruded samples was analyzed. The influences of granularity and sintering parameters including time and temperature on the density of sintered alloy were also discussed. It is shown that the atomized powders are composed of Si, Al₁₁Ce₃, Al₃Ni and alpha Al. Tiny Al₃Ni particles precipitate from supersaturated matrix near the powder boundaries during SPS. Hot-extrusion process leads to the layer structure and more homogeneous distribution of precipitates. These alloys exhibit high comprehensive mechanical properties with combination of high Vicker's micro-hardness, moderate tensile properties and elongation, which provide a novel kind of promising engineering materials.     

Fe-based metallic glass particles reinforced Al-7075 matrix composites prepared by spark plasma sintering

Metallic glass (MG) reinforced aluminum matrix composites (AMCs) have attracted the interest of many researchers in the past few years. In this study, Fe₅₀Cr₂₅Mo₉B₁₃C₃ metallic glass (FMG) particles reinforced 7075 aluminum matrix (Al-7075) composites were prepared by spark plasma sintering (SPS) technique. The microstructure of the composites showed good interface bonding between the FMG particles and the matrix. The micro-hardness of the composite with 30 vol% FMG particles reached 160.63 HV, which was increased by 30% compared with that of Al-7075 (119.3 HV). The ultimate compression strength (UCS) of the composite was also improved significantly from 596 MPa for Al-7075 matrix to 749 MPa for the composite reinforced with 30 vol% FMG particles, and the compression strain of the composite reached 22%. These results indicate that the mechanical properties of the composites can be enhanced by adding high volume fraction FMG particles. The enhancement of the strength is resulted from multiple strengthening mechanisms, and the main contributions come from the thermal mismatch and grain refinement.     

Low-temperature spark plasma sintering of NiO nanoparticles

NiO nanoparticles of 20 nm in diameter were spark plasma sintered between 400 °C and 600 °C for 5 and 10 min durations. Application of 100 MPa pressure from room temperature resulted in densities between 75% and 92%. The final grain size was between 26 nm and 68 nm. Lower densities were recorded when 100 MPa was applied at the SPS temperature. Two shrinkage rate maxima of ∼3.4 × 10⁻³ s⁻¹ and ∼2 × 10⁻³ s⁻¹ were observed around 390 ± 10 °C and at the SPS temperature. The two shrinkage rate maxima were related to densification by particle sliding followed by diffusional grain boundary sliding during the heating. The strong effects of the surface and interfacial processes which are active during the SPS were highlighted.     

Processing of porous Ti and Ti5Mn foams by spark plasma sintering

Titanium and its alloys are one of the best metallic biomaterials to be used for implant application. In this study, porous Ti and Ti5Mn alloy with different porosities were successfully synthesized by powder metallurgy process with the addition of NH₄HCO₃ as space holder and TiH₂ as foaming agent. The consolidation of powder was achieved by spark plasma sintering process (SPS) at 16 MPa and pressureless conditions. The morphology of porous structure was investigated by using scanning electron microscopy (SEM) and X-ray micro-tomography (μ-CT). Nano-indentation tester was used to evaluate Young’s modulus of the porous Ti and Ti5Mn alloy. Experimental results showed that pure Ti sample, which sintered under pressure of 16 MPa, full relative density was achieved even at a relative low sintering temperature 750 °C; however, in the case of pressureless condition at sintering temperature 1000 °C the porosity was 53% and Young’s modulus was 40 GPa. The Ti5Mn alloy indicated a good pore distribution, and the porosity decreased from 56% to 21% by increasing the sintering temperature from 950 °C to 1100 °C. Young’s modulus was increased from 35 GPa to 51.83 GPa with increasing of the sintering temperatures from 950 °C to 1100 °C.     

Green-emitting Yb-doped α-SiAlON phosphors prepared by spark plasma sintering

Divalent ytterbium-doped Ca-α-SiAlON phosphors were synthesized by a spark plasma sintering (SPS) method without a reducing gas. A single phase Ca-α-SiAlON:Yb²⁺ was successfully achieved at relatively low temperature of 1600 °C. Its photoluminescence excitation spectrum showed a broad band peaking at 450 nm due to the 4f¹³5d¹ → 4f¹⁴ transition of Yb²⁺ ions, while a corresponding green emission band was observed at 545 nm. The post-annealing in a reduced atmosphere significantly improved the luminescent properties by increasing the crystallinity, reducing the carbon contamination, and changing the residual Yb³⁺ to Yb²⁺ in as-synthesized samples. The post-annealed powders, compared to commercial YAG:Ce³⁺ and silicate phosphors, showed a low thermal quenching due to the rigid crystal structure.     

Densification of pure magnesium by spark plasma sintering-discussion of sintering mechanism

In this study, high-quantity pure magnesium was prepared by SPS, and the sintering densification mechanism was discussed by using numerical simulation and simultaneous experiment. Results show that a layer of dense magnesium oxide was formed at the surface of Mg particles, and the oxide reduction can be observed owing to the effect of oxide film removal in SPS. The high energy pulsed current flows preferentially through particle contact surfaces, which provides the conditions for the generation of micro-arc between particles and the temperature at the particles contact point can be up to 1979 °C. With the low pressure at the initial sintering stage, local high temperature induced by micro-arc makes the melting (even evaporation) appearance. At the same sintering temperature of 570 °C, the obvious difference in bending strength also demonstrates the significance of spark discharge in SPS. The formation process of sintering neck demonstrates that sintering process is the reflection of melting (even evaporation), diffusion and plastic deformation. In order to realize tiny area and high-quality connection between powder particles, an innovative powder sintering technology using high-frequency pulse electric current (High frequency pulse current assisted sintering) is proposed based on the enlightenment of the skin, proximity and arc discharge effect of high-frequency pulse current. This technology is also instructive for other alloys and engineering materials.     

Densification and alloying of ball milled Silicon-Germanium powder mixture during spark plasma sintering

In this research, the influence of process parameters such as sintering temperature and current during alloying and densification of silicon-germanium (Si₈₀-Ge₂₀) powder mixture using spark plasma sintering (SPS) was reported. Si₈₀-Ge₂₀ powder mixture was consolidated at the temperature range 900–1200 °C with 40 MPa pressure for 5 min. soaking. X-ray diffraction (XRD) study was made on sintered compacts to confirm the Si(Ge) alloy formation. Scanning electron microscope (SEM) was used to understand the morphology, particle size and distribution of un-milled and milled Si₈₀-Ge₂₀ powder mixture. Transmission electron microscope (TEM) study was made on milled Si₈₀-Ge₂₀ powder mixture and bulk SiGe alloy to confirm the nano-crystallinity and alloy formation. Fracture toughness of sintered bulk SiGe alloy was determined from Palmqvist cracks geometry model using Vickers hardness testing. It is understood that, during spark plasma sintering nano-structured Si₈₀-Ge₂₀ powder simultaneously increases the densification and reaction kinetics. It helps to achieve homogenous nanostructured SiGe alloy of near theoretical density. The superior hardness and benchmarked fracture toughness (K_IC) values of 630 VHN and 2.19 MPa√m was achieved for SiGe alloy sintered at 1200 °C, respectively.     

Microstructure and mechanical properties of in situ TiB reinforced titanium matrix composites based on Ti–FeMo–B prepared by spark plasma sintering

The in situ synthesized TiB reinforced titanium matrix composites have been prepared by spark plasma sintering at 800–1200 °C under 20 MPa for 5 min. The effects of sintering temperature and reinforcement volume fraction on flexural strength, Young’s modulus and fracture toughness of the composites are investigated. The titanium matrix consists of -Ti and β-Ti phases, and the volume fraction of β-Ti increases with increasing sintering temperatures. The in situ synthesized TiB reinforcements are distributed randomly and uniformly in matrix. The transverse section of TiB has a hexagonal shape aligned along [0 1 0] direction, and the crystallographic planes of the TiB needles are always of the type . The 10 vol% TiB reinforced composite sintered at 1000 °C exhibits excellent mechanical properties. The flexural strength, Young’s modulus and fracture toughness of this composite are 1560 MPa, 137 GPa and 8.64 MPa · m^1/2, respectively.     

Effect of preliminary high-energy ball milling on the structural-phase state and microhardness of Ni3Al samples obtained by spark plasma sintering

The study of the influence of the duration of preliminary high-energy ball milling on the features of the structural-phase state and the level of microhardness of consolidated Ni₃Al samples obtained by the method of spark plasma sintering has been carried out. It was found that the inhomogeneous state of the precursor from the 3Ni-Al powder mixture in the case of preliminary ball milling of a short duration (1 min) is a cause of the formation of an inhomogeneous structural-phase state of the consolidated Ni₃Al sample. An increase in the duration of high-energy ball milling provides a homogeneous phase composition, promotes the refinement of the grain structure and an increase in the microhardness values of the obtained Ni₃Al samples. The main factors determining the processes of structural-phase transformation during the formation of Ni₃Al under the conditions of spark plasma sintering, depending on the preliminary high-energy ball milling, are revealed. It is shown that grain boundary strengthening is the one of the effective mechanisms for increasing the strength of the material under study.     

Densification and microstructural evolution of spark plasma sintered NiTi shape memory alloy

The effect of particle size and sintering temperature on the densification and microstructural characteristics of nickel-titanium shape memory alloy (NiTi-SMA) has been investigated using spark plasma sintering (SPS) process. The Ni and Ti elements in different particle sizes were alloyed in the composition of Ni_50.6Ti_49.4. The milled NiTi powders were consolidated using SPS process in a temperature range of 700–900?°C. The densification was characterized by plotting temperature, current and relative displacement of punch as a function of holding time. The results showed that a maximum relative density of ～98% can be achieved for NiTi-SMA with an average particle size of 10?µm at a sintering temperature of 900?°C. The microstructure of the sintered NiTi-SMA was examined using scanning electron microscope (SEM) and composition of NiTi alloy was analyzed using energy dispersive spectroscopy (EDS) analysis. The effect of sintering temperature on the microstructural evolution and transformation was also studied.     

放电等离子烧结技术制备 Ti合金表面 HA活性涂层

采用放电等离子烧结(SPS)技术，低温、快速地在Ti合金表面制备HA活性涂层。研究了涂层成分和厚度对涂层与基体结合强度的影响，观察了断口形貌。结果表明：随着涂层厚度的减小，结合强度提高；梯度涂层能提高涂层与基体的结合强度；特别是经钝化处理后烧结的试样，涂层与基体的结合强度显著提高，最高达到了64MPa，超过目前使用的生物涂层种植体材料的指标。     

Spark plasma sintering and hot compression behaviour of AZ91 Mg alloy

Abstract

In the present investigation, the microstructural characterisation of the AZ91 Mg alloy produced by spark plasma sintering (SPS), as well as the evaluation of its hot compression behaviour, has been performed. Based on the differential scanning calorimetry analyses of the starting powders, three SPS cycles are investigated, using temperatures of 400 and 450°C, and at 450°C with previous solubilisation soaking at 420°C. Despite different microstructural and hardness characteristics, the three alloys display similar hot compression behaviour. At 200°C, the formation of an unstable crack, which propagates at 45° with respect to the loading axis, is observed after the occurrence of the peak load. At higher testing temperatures, after reaching the peak stress, the flow stress decreases slowly with increased strain of ～0·51. Such behaviour corresponds with the observation of an accelerated cracking due to the propagation of decohesions at the interparticle regions. Ultimately, SPS allowed for attainment of high relative density; however, the sintering degree of the materials was quite low.