Influence of B4C coating on graphitization for diamond/WC-Fe-Ni composite

Diamond/WC-Fe-Ni composite is a potential composition for impregnated diamond drill bits. It is necessary to avoid the graphitization of the diamond from Fe and Ni under the powder metallurgy process. Boron carbide (B₄C) was coated on diamond, and diamond/WC-Fe-Ni composites were consolidated by hot pressing at different temperatures. The influences of sintering temperature and interfacial structure on bending strength and wear behavior were investigated. The bending strength for diamond/WC-Fe-Ni composite was dependent on matrix densification and interfacial graphitization. Un-coated diamond was eroded by Fe-Ni matrix and partially converted to graphite during the sintering process at all sintering temperatures. In opposite, B₄C coating was beneficial to matrix densification at a lower sintering temperature, and delayed the appearance of graphitization to around 1300 °C. Therefore, the diamond/WC-Fe-Ni composites with B₄C coating exhibited larger bending strength and better wear behavior at a relative low sintering temperature.     

On the properties of the eutectic alloy Al3(Nb,Cr) + Cr(Al,Nb)

The eutectic alloy Al₃(Nb,Cr) + Cr(Al,Nb) forms an in situ composite and the Al₃Nb presents high specific strength and low oxidation rate that may be improved by the combination with other phases. The purpose of this work is to investigate physical, mechanical and oxidation properties of the eutectic alloy. Therefore, Rietveld analysis was carried out for furnace cooled and water quenched samples and oxidation tests were performed on directional solidified samples. Compressive tests were performed for the eutectic alloy and also for the Nb–74.8% Cr–24.6% Al alloy in the as-cast condition. The alloy presents 12.9% Cr(Al,Nb) at room temperature, retained from the transformation Cr(Al,Nb) to Al(Nb)Cr₂. The combination of Al₃Nb with Cr(Al,Nb) and Al(Nb)Cr₂ considerably improves mechanical behaviour, leading the yield strength to 1525 MPa at 800 °C and 925 MPa at 900 °C. The oxidation tests showed the formation of several oxides at all temperatures studied and that from 900 °C on alfa Al₂O₃ is formed both in air and O₂ except under O₂ at 1000 °C. It is believed that the Cr(Al,Nb) phase acts as an Al reservoir for the formation of the various Al₂O₃ scales.     

Mechanical behavior of diamond matrix composites with ceramic Ti3(Si,Ge)C2 bonding phase

Polycrystalline diamond, PCD, compacts are usually produced by high pressure–high temperature (HP–HT) sintering. This technique always introduces strong internal stresses into the compacts, which may result in self-fragmentation or graphitization of diamond. This may be prevented by a bonding phase and Ti₃(Si,Ge)C₂ was so investigated. This layered ceramic was produced by Self Propagating High Temperature Synthesis and the product milled. The Ti₃(Si,Ge)C₂ milled powder was mechanically mixed, in the range 10 to 30 wt.%, with 3–6 μm diamond powder (MDA, De Beers) and compacted into disks 15 mm in diameter and 5 mm high. These were sintered at a pressure of 8.0 GPa and temperature of 2235 K in a Bridgman-type high pressure apparatus. The amount of the bonding phase affected the mechanical properties: Vickers hardness from 20.0 to 60.0 GPa and Young's modulus from 200 to 500 GPa, with their highest values recorded for 10 wt.% Ti₃(Si,Ge)C₂. For this composite fracture toughness was 7.0 MPa m^1/2, tensile strength 402 MPa and friction coefficient 0.08. Scanning and transmission electron microscopy, X-ray and electron diffraction phase analysis were used to examine the composites.     

Mechanical properties of Mo-Nb-Si-B quaternary alloy fabricated by powder metallurgical method

Mo-Si-B alloys composed of two intermetallic compound phases (Mo₅SiB₂ and Mo₃Si) and a molybdenum solid solution matrix phase have been investigated for use as high-temperature structural materials due to their high melting point and good creep resistance. However, despite these advantages, Mo-Si-B alloys are difficult to use in practical applications because they have insufficient fracture toughness at room temperature. So, in many researches, microstructure control and the addition of other elements in the α-Mo matrix phase are conducted as an effective way to improve the fracture toughness.In this study, niobium (Nb) was added to a Mo-Si-B alloy by a powder metallurgical method to improve the mechanical properties. First, the Mo and Nb powders were pulverized by high-energy ball milling. Then, the synthesized intermetallic compound powders, which were fabricated by continuous heat treatment under a H₂ atmosphere, were mixed with ball-milled Mo and Nb powder. Pressureless sintering was conducted at 1400 °C for 3 h under a H₂ atmosphere. The Vickers hardness and fracture toughness were measured to investigate the mechanical properties of the sintered Mo-Si-B and Mo-Nb-Si-B alloy. The Vickers hardness was about 425 Hv for a Mo-Nb-Si-B alloy, which was lower value of 165 Hv compared to Mo-Si-B alloy (590 Hv). On the other hand, the fracture toughness of the Mo-Nb-Si-B alloy (14.5 MPa·√m) greatly increased compared to that of the Mo-Si-B alloy (12.6 MPa·√m).     

On the use of TiO2 in Ti(C,N)-WC/Mo2C-(Ta,Nb)C-Co/Ni cermets

In this study, Ti(C,N)-WC/Mo₂C-(Ta,Nb)C-Co/Ni cermets with varying [Mo]/([W] + [Mo]) mole ratios and constant [W] + [Mo] content were prepared by using TiO₂ and carbon as substitutes for a part of Ti(C,N). In situ carbothermal reaction took place with subsequent liquid phase sintering. The thermal behavior and outgassing behavior were conducted using differential thermal analysis/thermal gravity analysis (DTA-TG) and mass-spectrometric evolving gas analysis (MS-EGA). The microstructure was investigated by scanning electron microscopy (SEM). After being sintered in N₂ atmosphere, cermets without TiO₂ additions generally showed rimless microstructures with black grains independently distributed from other phases, whereas the cermets with TiO₂ addition showed complete core-rim microstructures. The average grain size generally decreased with increasing Mo content, and increased with increasing TiO₂ substitution. Moreover, the average grain size increased with decreased N₂ pressure from 50 mbar to 10 mbar at any [Mo]/([Mo] + [W]) ratio. The most interesting result was for the Mo-free cermets with 5 mol% TiO₂ and 10 mol% TiO₂ addition sintered in 50 mbar N₂: both the hardness and fracture toughness increased compared with the cermets without TiO₂ addition. The HV-K_IC behavior of the majority of the cermets was superior to the industrial grades.     

VC- and Cr3C2-doped WC–NbC–Co hardmetals

This study compares the microstructure and mechanical properties of plain and 0.9 or 3.6 wt% VC- or Cr₃C₂-doped WC–12 wt% Co hardmetals with 40 wt% NbC, prepared by pulsed electric current sintering (PECS) in the solid state for 4 min at 1240 °C and conventional pressureless liquid phase sintering (CS) for 1 h at 1420 °C. The addition of VC or Cr₃C₂ was found to inhibit grain growth of the residual WC grains, whereas the size of the solid solution (Nb,W,V/Cr)C grains was hardly influenced. The type of grain growth inhibitor and densification temperature however, strongly influenced the composition of the NbC solid solution formed, which was thermodynamically and experimentally assessed.     

Conditions for the in-situ formation of carbide coatings on diamond grains during their sintering with CuWC binders

In present work, self-assembled WC/W coatings on diamond grains, sintered with Cu matrix were firstly obtained by gas phase transport mechanism. Conditions for spontaneous coating formation were also determined. The coatings can improve adhesion strength between diamond grains and a binder. Discovered method can be used for production of heat sink and heat spreader devices, as well as cutting tools. As functional additives (precursors) for coating formation, WC and WO₃ were used. Morphology and composition of the coatings were identified by XPS, SEM, and TEM techniques. Coating consisting of mixture of metallic tungsten and tungsten carbide is formed on diamond when the Cu + WC binder is used. No coating is formed when copper is modified with WO₃ nanoparticles; introduction of 20% Fe to the binder leads to formation of a coating containing 100% tungsten in metallic form. Hence, conditions for formation of the tungsten (tungsten carbide) coating onto the diamond surface during sintering with a metal binder include the presence of a nucleating agent WC and a metal that catalyzes graphitization. It was found that the driving force for spontaneous coating formation is a chemically activated gas-phase mass transfer of WO₃ to the diamond surface, its chemisorption and subsequent reduction.     

Investigation on strengthening and toughening mechanisms of Nb-Ti-ZrB2 metal matrix ceramic composites reinforced with in situ niobium and titanium boride

Nb-Ti-ZrB₂ metal matrix ceramic composites with a fixed atomic ratio Nb/Ti = 2/1 and ZrB₂ volume fraction changing from 0, 11 vol%, 23 vol% to 36 vol% were hot pressed at 1600 °C under 30 MPa. The influence of ZrB₂ content and Ti addition on the phase constitution, microstructure evolution, toughening mechanisms and strengthening mechanisms were investigated. It was shown that the formation of in situ Nb-rich (Ti,Nb)B and Ti-rich (Nb,Ti)B was attributed to a high mutual solubility of monoborides and the amount of niobium and titanium borides increased with increasing ZrB₂ content. The needle-shaped (Ti,Nb)B phase weakened the damage to fracture toughness caused by ZrB₂ particle fracture due to crack bridging, crack defection and the pull-out toughening mechanisms. The highest fracture toughness of the Nb-Ti-ZrB₂ composites was 12.0 MPa·m^1/2. The stiff (Nb,Ti)B phase acted as a strong obstacle to the dislocation motion, leading to dislocation pile-up and enhancing the strength of the Nb-Ti-ZrB₂ composites during compression tests. However, stress concentration around the needle-shaped (Ti,Nb)B phase easily leads to crack initiation and extension, resulting in decreased strength. The yield strength of Nb-Ti-ZrB₂ composites ranged from 657.3 MPa to 1783.0 MPa owing to the combined influence of the strenghening mechanism caused by (Nb,Ti)B and the weakening mechanism caused by (Ti,Nb)B. The compressive deformation and failure process were also discussed in detail in this study.     

Research on polycrystalline diamond compact (PDC) with low residual stress prepared using nickel-based additive

Polycrystalline diamond compact (PDC) with low residual stress was synthesized using nickel-based additive (Ni₇₀Mn₂₅Co₅) as sintering solvent at high temperature/pressure (HTHP). A systematic study of the residual stress in a polycrystalline diamond (PCD) layer was performed using micro-Raman spectroscopy. The stress was measured by determining the Raman shift and was approximated as being biaxial to calculate the magnitudes of stress. Sintering process parameters such as temperature, the diamond size and content of binder additive were all found to affect residual stress levels. The sample with low stress measured on the surface of PCD is compressive and has an average value of 0.10 GPa. Scanning electron microscopy (SEM) analysis of morphology shows that the dense microstructure with diamond-diamond (D-D) direct bonding has formed in the PCD layer. X-ray diffraction (XRD) performed in the cross-section of PCD confirmed the presence of diamond, nickel-based alloy, WC and Co_xW_xC.     

Wear characteristics of Fe-based diamond composites with cerium oxide (CeO2) reinforcements

To obtain better wear resistance for the metal bond diamond grinding tools, cerium oxide (CeO₂) with different contents were introduced into Fe-based diamond composites. A pin-on-disc wear test was performed to assess the wear properties of the fabricated specimens, and the morphological properties of the worn surface and corresponding wear debris were evaluated to examine the wear mechanism. Results show that the Fe-based diamond composites with CeO₂ addition exhibited an improvement in the densification, mechanical properties and wear resistance. The original long rod-shaped CeO₂ particles converted into the spherical particles <1 μm, dispersing in the Sn phase. The cerium oxide acted as a sintering aid, promoting the diffusion of Fe in the Sn phase during the sintering process. The dominant wear mechanism of the specimen with CeO₂ addition was the adhesive wear, compared with the abrasive wear in the specimen without CeO₂. With the increase in CeO₂ addition amount, the wear rate decreased. But an excessive amount of CeO₂ was detrimental to mechanical and wear performances. The optimal amount of cerium oxide to achieve the best wear resistance was investigated to be 0.8 wt%.     

Microstructural characteristics and microwave dielectric properties of Ba[Mg1/3(Nbx/4Ta(4−x)/4)2/3]O3 ceramics

Sintering behaviors and microstructural characteristics in solid solutions of Ba[Mg_1/3(Nb_x/4Ta_(4−x)/4)_2/3]O₃ (BMN_xT_4−x, x = 0, 1, 2, 3 and 4) were investigated by X-ray diffraction, SEM and TEM. Microwave dielectric properties, such as the relative permittivity (ɛ_r), quality factor (Q) value and temperature coefficient of resonator frequency (τ_f), were also measured. The excellent microwave dielectric property of Ba(Mg_1/3Ta_2/3)O₃ (BMT) sample imply the necessity to sinter at higher temperature (1650 °C) and to use longer soaking times (9 h), but not for Ba(Mg_1/3Ta_2/3)O₃ (BMN). The 1:2 B-site ordering was maintained at all Nb substitution contents and the 1:2 B-site ordering existed in the grains with antiphase domain boundaries (APBs). The Ba[Mg_1/3(Nb_1/4Ta_3/4)_2/3]O₃ specimen exhibited excellent microwave dielectric properties, ɛ_r = 25.534, Qf = 140 666 GHz, and τ_f = 4.8 (ppm/°C). The excellent microwave dielectric property is due to the improvement of sintering property by appropriate Nb atoms substitution in the BMT matrix and the maintaining of 1:2 ordering in the BMN_xT_4−x series.     

Influence of Nb content on martensitic transformation and mechanical properties of TiNiCuNb shape memory alloys

In present work, microstructure, martensitic transformation behavior and mechanical properties of (Ti₅₀Ni₄₀Cu₁₀)_100−xNb_x (x = 0, 5, 10, 15 at.%) alloys were investigated as a function of Nb content. The addition of Nb to TiNiCu alloy leads to the presence of β-Nb phase. During cooling and heating, the alloys show one-step B2 ↔ B19 transformation. As the Nb content increases, the transformation temperatures almost linearly decrease and the transformation hysteresis monotonously increases due to the decrease of middle eigenvalue of the phase transformation matrix. The addition of Nb is effective in improving the elongation because of the introduction of β-Nb phase. With the increase of Nb content, both the yield strength and the critical stress to induce martensitic transformation increase, resulting in the improved superelastic strain.     

烧结温度对(Ti, W, Mo, Nb)(C, N)-(Co, Ni)金属陶瓷组织结构和性能的影响

本文以(Ti,W,Mo,Nb)(C,N)-(Co,Ni)基金属陶瓷材料为研究对象,研究烧结温度对金属陶瓷的成分、微观组织和力学性能的影响,初步探讨成分、微观组织与材料强度的关系。研究结果表明：烧结温度对(Ti,W,Mo,Nb)(C,N)-(Co,Ni)基金属陶瓷组织特征有显著的影响;合金的总碳(Ct%)随着烧结温度的提高而降低,当烧结温度达到1490℃时,合金总碳的急剧降低,导致合金组织中出现脱碳相(η相),从而使得合金的硬度(HV₃₀) 、断裂韧性(KIC)和抗弯强度(TRS)降低;1470℃烧结温度下,(Ti,W,Mo,Nb)(C,N)-(Co,Ni)基金属陶瓷合金的硬度(HV₃₀) 、断裂韧性(KIC)和抗弯强度(TRS)的匹配最佳,表现为在实际应用工况下的综合切削性能最优。     

The kinetics of Nb(C,N) precipitation during the isothermal austenite to ferrite transformation in a low-carbon Nb-microalloyed steel

The kinetics of Nb(C,N) precipitation occurring during the isothermal ferritic (α) transformation were quantitatively measured, along with the transformation kinetics at intercritical temperatures ranging from 710 to 790 °C in a Nb-microalloyed steel by means of electrical resistivity and dilatometry. The precipitation occurred most rapidly at 750 °C, which corresponds to a bay temperature on the start curve of the ferritic transformation of the transformation–time–temperature diagram. While interphase precipitation was observed at and above the bay temperature, precipitation in the α matrix was predominantly below the bay temperature. However, precipitation in the untransformed austenite (γ) matrix during the ferritic transformation was also observed, regardless of the intercritical temperatures. It is suggested that the precipitation occurring in the untransformed γ matrix during the ferritic transformation was accelerated owing to carbon enrichment from the α matrix to the γ matrix during the ferritic transformation. The average size of Nb(C,N) particles observed in the α matrix was slightly larger than that of the γ matrix at a given intercritical temperature. This result is proposed to arise primarily from the rapid diffusion of solute Nb atoms in the body-centered cubic α matrix.     

Microstructure,phase transformation and mechanical property of Nb-doped Ni–Mn–Ga alloys

This study investigated the microstructure, phase transformation and mechanical property of (Ni_49.8Mn_28.5Ga_21.7)_100-xNb_x (x = 1, 3, 6, 9) alloys. The Nb1 alloy exhibited a single austenite phase at room temperature. With increasing Nb content for Nb3, Nb6 and Nb9, the alloy changed to a dual phase consisting of austenitic matrix and Nb-rich second phase with a hexagonal structure, and the amount of the second phase increased with the increase of Nb content. The martensitic transformation temperature and Curie temperature were changed and the transformation enthalpy was gradually reduced with increasing Nb content. The change of martensitic transformation temperature and Curie temperature was related to the introduction of Nb in the Ni–Mn–Ga structure that decreased valence electron concentration (e/a), increased unit cell volume and reduced magnetic exchange of the alloys. The decrease of transformation enthalpy was mainly attributed to the formation and increase of the Nb-rich second phase that reduced volume fraction of the matrix taking part in phase transformation. All the alloys presented a similar compression behavior with progressively fracturing characters (occurrence of several stress drops before complete fracturing). The fracture strength was slightly enhanced with increasing Nb content from Nb0 to Nb9, but the ductility has no apparent improvement.     

Effect of hot pressing temperature on microstructure,mechanical properties and grinding performance of vitrified-metal bond diamond wheels

The self-sharpening vitrified-metal bond diamond wheels added with a 3 wt.% brittle Na₂O-B₂O₃-SiO₂-Al₂O₃-Li₂O vitrified bond were fabricated by hot pressed sintering technique. Using the methods of scan-electroscope, energy spectrum analysis, X-diffraction analysis, XPS analysis, Rockwell hardness test and three-point bending test, the effects of hot pressing temperature on the microstructure, hardness and the transverse rupture strength (TRS) of vitrified-metal bond were investigated. Then the grinding performance of cylinder of the diamond wheels was also studied. The results showed that, when the hot pressing temperature was 850 °C, a thin FeAl₂O₄ transition layer formed, which enhanced the interfacial bending strength between metal and glass phase, and the TRS of vitrified-metal bond reached the maximum value 826.54 MPa. Comparing with metal bond diamond wheel's, the average value of the roundness and straightness of the 50 cylinders ground by the vitrified-metal diamond wheel reduced from 3.1 μm and 2.5 μm to 2.7 μm and 2.1 μm.     

Densification and microstructural evolution of a high niobium containing TiAl alloy consolidated by spark plasma sintering

Gas-atomized Ti–45Al–7Nb–0.3W alloy powders were consolidated by the spark plasma sintering (SPS) process. The densification course and the microstructural evolution of the as-atomized powders during SPS were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) techniques. As a result of SPS densification, special (α + γ) precipitation zones are formed in the initial stage of sintering, and the residual β phases in the microstructure of the powders are fragmentated. During the following SPS course, α₂/γ lamellar colonies at the edge of the precipitation zone, α₂ and B2 phase as well as dynamic recrystallized γ grains are found to form. For the as-atomized powders sintered at 1000 °C, the densification is preceded by the early rearrangement of the powder particles and the following formation of sintering necks. For the powders sintered at 1200 °C, plastic deformation plays an important role in densification. Local melting and surface bulging between two adjacent particles can also serve as one of the densification mechanisms. In the later stage of sintering, the growth of sintering necks controlled by diffusion and the pore closure would make important contributions to the densification.     

Ti (C,N)-based cermets sintered under high pressure

In this study, high pressure and high temperature sintering (HPHT) is adopted in the cermet fabrication process, and the microstructure and mechanical properties of cermets with TiC_0.5N_0.5–15WC–10Mo₂C–5TaC–10Ni–10Co (wt%) sintered under 5 GPa and different temperatures (900–1600 °C) using 6 × 14 MN cubic press are investigated. Results show that the densities of samples can reach up to 7.00 g/cm³. Vickers hardness and fracture toughness of the products are over 1727 HV30 and 7.2 MPa m^1/2 respectively. In addition, the sintering results are compared with the data that obtained from commercial samples which produced via conventional sintering technique. The conclusion is that high density and high hardness cermets can be obtained through HPHT sintering.     

Spark plasma sintering of (Ti0.9W0.1) C powders prepared by mechanical alloying

Nanocrystalline (Ti_0.9W_0.1)C powder with a diffraction crystallite size of about 10 nm was synthesized by mechanical alloying. The formation of (Ti_0.9W_0.1)C carbide was detected by XRD measurements and microscopic observation. The sintering of these powders by a spark plasma sintering (SPS) at different temperatures were also studied. The results show that the maximum hardness was obtained for more relative density materials, meanwhile, the grain size is large. The micro-hardness and the relative density of the powder milled for 10 h and sintered at 1200 °C for 5 min under 100 MPa reach, respectively, 2978 HV and 98.35%.     

Structure of Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy prepared by pack cementation techniques

In order to prepare Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy, both a two-stage pack cementation technique and a co-deposition pack cementation technique were employed. The two-stage process included siliconizing a specimen at 1150 °C for 4 h followed by aluminizing it at 800-1000 °C for 4 h. The coating prepared by pack siliconization was composed of a thick (Nb,X)Si₂ (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X)₅Si₃ transitional layer; after the siliconized specimens were aluminized at or above 860 °C, a (Nb,Ti)₃Si₅Al₂ phase developed at the surface of the coating, and furthermore, when aluminizing was carried out at 860 °C, a new (Nb,Ti)₂Al layer formed in the coating between the (Nb,X)₅Si₃ layer and the substrate, but when aluminizing was performed at 900-1000 °C, the new layer formed was (Nb,Ti)Al₃. The co-deposition process was carried out by co-depositing Si and Al on specimens at 1000-1150 °C for 8 h under different pack compositions, and it was found that the structure of co-deposition coatings was more evidently affected by co-deposition temperature than pack composition. An Al-modified silicide coating with an outer layer composed of (Nb,Ti)₃Si₅Al₂, (Nb,X)Si₂ and (Nb,Ti)Al₃ was obtained by co-depositing Si and Al at 1050 °C.