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This study focuses on the transient liquid phase (TLP) bonding of DD5 single-crystal superalloy to CrCoNi-based medium-entropy alloy (MEA) using a BNi-2 filler alloy. The microstructure and mechanical properties of the TLP-bonded DD5/MEA joint were evaluated, and the microstructural evolution mechanism was investigated. The formation of the isothermal solidification zone (ISZ) depended on the diffusion of the melting-point depressants (Si and B elements) from the liquid filler into the DD5 and MEA substrates, as well as the dissolution of the substrates. Boron diffused along the γ channel of DD5 and reacted to form M5B3 boride, herein referred to as the diffusion-affected zone (DAZ I). Similarly, the Cr5B3 boride precipitated in the Ni-rich MEA matrix adjacent to the MEA substrate (i.e., DAZ II). Additionally, a coherent orientation of [0]BCT // [0 1 1]FCC and (0 0 2)BCT // (2 0 0)FCC was detected between M5B3 boride with a body-centered tetragonal (BCT) structure and the face-centered cubic (FCC) matrix. The performance of the joint was dominated by the properties of the bonding seam. As the bonding time increased from 20 to 80 min, the athermal solidification zone (including eutectic microstructure) was gradually replaced by the ISZ exhibiting excellent plastic deformation capability, and the shear strength of the joint was improved. The maximum shear strength (752 MPa) was achieved when the eutectic-free joint was bonded at 1050 °C for 80 min. The fracture morphology revealed a mixture mode, indicating the initiation of cracks in the DAZ II, mainly propagating in the ISZ, and passing through the DAZ I.

In this study, the effects of titanium addition on microstructure, hardness, and wear rate of Fe(13-x)TixB7 (x = 0, 1, 2, 3 and 5) based hard surface alloy layers formed by gas tungsten arc (GTA) welding method were investigated. As a result of the microstructure studies and phase analysis, it was determined that the structures of the coating layers consisted of alpha-Fe, alpha Fe+Fe2B eutectic, alpha-Fe+Fe2Ti eutectic and hard TiB2 phases. In the hard surface alloy layer, as the amount of titanium was increased, the TiB2 phase density formed in the system increased and it was observed that rod-like and long sharp edged phases formed from the equiaxed structure. As a result of wear tests performed at different loads, it was determined that the addition of titanium reduces the wear rates in the coating layers. In addition, scanning electron microscopy (SEM) images of the worn surfaces showed that the wear mechanisms were adhesive and oxidative. Sakarya University Scientific Research Projects Unit [2014-50-02-013] Authors are grateful to the financial support provided by Sakarya University Scientific Research Projects Unit with Project No. 2014-50-02-013.

In this work, a novel Mo(Co,Fe)B cermet was prepared using Mo, Fe, Co and B powders and the reaction boronizing sintering route. The phase transformation, thermal behavior, densification behavior, microstructure and mechanical properties of cermets are systematically investigated. After sintering, the microstructure of cermets was composed of Mo(Co,Fe)B hard and CoFe binder phases. The formation of Mo(Co,Fe)B phase during solid-state sintering occurred by the reactions of 2Fe + B → Fe2B, 2Co + B → Co2B and 2Mo + Fe2B + Co2B → 2Mo(Co,Fe)B. During liquid-phase sintering, two liquid phases, namely L1 and L2, were sequentially generated by the reactions of CoFe + Fe2B + Co2B → L1 and CoFe + L1 + Mo(Co,Fe)B → L2, respectively. The as-obtained cermet exhibited excellent comprehensive mechanical properties with hardness of 89.5 ± 0.2 HRA, transverse rupture strength of 1927 ± 31 MPa and fracture toughness of 17.5 ± 0.4 MPa•m1/2. In summary, these findings look promising for future preparation and application of low-cost and high-performance ternary boride cermets.

This study reports the densification behavior and novel design of core-rim microstructure for different ZrB2–MoSi2-SiCw (ZMSw) composites. Four ZrB2–MoSi2 composites containing various amounts of SiCw (0, 5, 10, and 20 vol%) were densified by multistage spark plasma sintering. Optimal combinations of sintering schedule, temperature, and composition are recognized for producing dense ZrB2-based ceramic-matrix composite with core-rim grain structure in the boride matrix, where ZrB2 core is surrounded by a solid solution of (Mo,W)Si2. Fragmentation, rearrangement, plastic deformation, and grain boundary diffusion mechanisms are responsible for densification of composites sintered by multistage SPS conditions at 1700 °C. Addition of SiCw significantly improves the density and mechanical properties of ZrB2–MoSi2–SiCw composites due to the formation of a core-rim structure.

Transition metal impurities such as chromium (Cr) and vanadium (V) in solution deteriorate electrical conductivity of smelter grade aluminium (Al). These impurities can be removed from solution via boron treatment in which borides form upon their in-situ reaction with boron (B)-bearing substances. However, Cr removal from smelter grade Al solution is not well understood. A disagreement related to chromium boride (CrB2) formation in molten Al in the presence of other transition metals (V, Ti, Zr, Fe) by adding Al–B master alloy has been reported in literatures. This study presents an effort to understand the mechanism of Cr removal from Al–0.50%Cr–0.50%V (mass fraction) alloy by adding Al–B (AlB12) master alloy at 1023 K in the Al alloy solution. Results indicate that Cr removal from molten Al–0.50%Cr–0.50%V alloy by forming stable borides cannot be achieved at 1023 K; whereas excess of B in the solution preferentially forms aluminium boride (AlB2) over CrB2 during boron treatment of molten Al. The underlying kinetics of V removal from molten Al–0.50%Cr–0.50%V alloy revealed that early reaction stage is controlled by [B]/[V] mass transfer through liquid phase and mass transfer coefficient (km) was measured to be 9.6×10−4 m/s. The later reaction stage was controlled by [B]/[V] diffusion through boride (VB2) ring. This study, therefore, advocates to investigate alternative ways to remove Cr from molten Al.

The transient liquid phase (TLP) bonding of CoCuFeMnNi high entropy alloy (HEA) was studied. The TLP bonding was performed using AWS BNi-2 interlayer at 1050 °C with the TLP bonding time of 20, 60, 180 and 240 min. The effect of bonding time on the joint microstructure was characterized by SEM and EDS. Microstructural results confirmed that complete isothermal solidification occurred approximately at 240 min of bonding time. For samples bonded at 20, 60 and 180 min, athermal solidification zone was formed in the bonding area which included Cr-rich boride and Mn3Si intermetallic compound. For all samples, the γ solid solution was formed in the isothermal solidification zone of the bonding zone. To evaluate the effect of TLP bonding time on mechanical properties of joints, the shear strength and micro-hardness of joints were measured. The results indicated a decrement of micro-hardness in the bonding zone and an increment of micro-hardness in the adjacent zone of joints. The minimum and maximum values of shear strength were 100 and 180 MPa for joints with the bonding time of 20 and 240 min, respectively.

The effects of joining temperature (TJ) and time (tJ) on microstructure of the transient liquid phase (TLP) bonding of GTD-111 superalloy were investigated. The bonding process was applied using BNi-3 filler at temperatures of 1080, 1120, and 1160 °C for isothermal solidification time of 195, 135, and 90 min, respectively. Homogenization heat treatment was also applied to all of the joints. The results show that intermetallic and eutectic compounds such as Ni-rich borides, Ni−B−Si ternary compound and eutectic-γ continuously are formed in the joint region during cooling. By increasing tJ, intermetallic phases are firstly reduced and eventually eliminated and isothermal solidification is completed as well. With the increase of the holding time at all of the three bonding temperatures, the thickness of the athermally solidified zone (ASZ) and the volume fraction of precipitates in the bonding area decrease and the width of the diffusion affected zone (DAZ) increases. Similar results are also obtained by increasing TJ from 1080 to 1160 °C at tJ=90 min. Furthermore, increasing the TJ from 1080 to 1160 °C leads to the faster elimination of intermetallic phases from the ASZ. However, these phases are again observed in the joint region at 1180 °C. It is observed that by increasing the bonding temperature, the bonding width and the rate of dissolution of the base metal increase. Based on these results, increasing the homogenization time from 180 to 300 min leads to the elimination of boride precipitates in the DAZ and a high uniformity of the concentration of alloying elements in the joint region and the base metal.

This paper delineates a single-step production method of nanostructured diiron boride (Fe2B) and its structural, magnetic and magnetothermal properties. Structurally Fe2B resembles the tetragonal copper aluminide, CuAl2. The samples of nanostructured Fe2B were synthesized by milling Fe and B powders without any pre-treatment. Single phase Fe2B nanoparticles were successfully produced with the crystallite sizes of 68 and 46 nm after milling the powders for 10 h and 20 h, respectively. The saturation magnetization of the samples was found to decrease with increased milling time indicating that the surface spin disorder plays a crucial role in the magnetic properties. The highest saturation magnetization (Ms) of 141 emu/g with low coercivity (Hc) of 48 Oe was obtained for the 10 h milled sample of Fe2B, whereas the 20 h milled sample exhibited Ms and Hc as 129 emu/g and 149 Oe. This paper also presents a detailed information on the total and atom projected densities of state functions as well as the magnetic moment contribution of the individual atoms of Fe and B in Fe2B explaining the strong room temperature ferromagnetic properties contributed by the large number of unpaired 3 d electrons in Fe. The magnetothermal properties of the as-made Fe2B nanocrystals of high magnetic moment were investigated by measuring the rise in temperature as a function of time in the presence of AC magnetic fields. The magnetic Fe2B nanocrystals show significant thermal response with the high specific absorption rate of 172 W/g, demonstrating the advantages of using Fe2B nanocrystals for the application in magnetic fluid therapy for hyperthermia.

Hardox steels are widely used in the industry as they have many excellent mechanical properties besides good wear resistance. In this study, Hardox-450 steel was boronized at different temperatures (1123, 1173, 1223 K) and holding times (2, 4, 6 h). Following the boronizing process, the morphology of the boride layers formed on the steel surfaces was examined by SEM microscope. Hardness values were measured with microhardness device and phases formed on the layer were analyzed by X-ray diffraction technique. XRD analysis showed that a layer of boride consisting of the Fe2B phase was formed as a single-phase structure. It was also observed that the thickness of the boride layer obtained in boronized Hardox-450 steel increased with the increase in the boronizing time and temperature. While the original hardness values of Hardox-450 steel were 430 HV0.05, as a result of the boriding process, it reached up to 1880 HV0.05. The thickness of the boride layer on Hardox-450 steel and growth kinetics of boride layer were also examined. The activation energy (Q) of boronized Hardox-450 steel was determined as 157.990 kJ/mol. Wear tests were carried out using a ball-disc wear method at a sliding speed of 0.3 m/s under a load of 10 N in a dry environment and at a sliding distance of 500 m. Adhesion properties of the boride layer were examined by Daimler-Benz Rockwell-C indentation test. Wear resistance increased with boriding process. It has been determined that the wear and adhesion resistance decrease with the increase of boronizing temperature and time.

Powder metallurgy (P/M) technology, which is a method of mixing, pressing and sintering stages, has many advantages (complex designs, low cost) as well as disadvantages (porosity, low surface hardness). Nowadays, surface coating methods are preferred to eliminate the disadvantage of powder metallurgy methods. In this study, samples of 96.4Fe–Mn, 89.25Fe–7.24Cr–Mn, 82.12Fe–14.39Cr–Mn, 75.08Fe–21.45Cr–Mn and 68.13Fe–28.42Cr–Mn were produced in volumetrically varying compositions. In order to increase the lifetime of ceramic-metal composites produced by the powder metallurgy method, pack boriding process was applied in 950°C for 4 h. The morphology of the boride layers formed on the surfaces of the produced samples was examined with SEM microscope. Hardness values of uncoated samples ranged from 47.5 to 240.6 HV0.05 according to their composition, and hardness values of borided samples increased from 1641 to 1950 HV0.05. Wear tests were carried out using a ball-disc wear method at a slip speed of 0.3 m/s under a load of 5 N under a load of 250 m in dry environment. Adhesion properties of the boride layer were examined by the Daimler-Benz Rockwell-C indentation test. With the boriding process, the friction coefficient has decreased and the low wear resistance has increased.

Post-bond heat treatment (PBHT) applied to a transient liquid phase (TLP) bonding joint is an effective approach to remove the brittle borides and improve its properties. Herein, we proposed two types of PBHT strategies to obtain a TLP bonded γ′-strengthened Co-based single crystal superalloy, and the microstructural characteristics and tensile properties of the two heat treated joints were compared to identify the optimal PBHT strategy. The evolution of the brittle boride in the joint after the PBHT was studied by using in-situ microscopy. The experimental results allowed to provide a theoretic model to quantitatively evaluate the distribution of the brittle phase after the optimal PBHT and analyze the joint fractures to understand the failure mechanisms. The obtained results revealed that a post-bond solid solution treatment performed to the joint at a high temperature (over 1275 °C) could decrease the area fraction of the boride from 7.2 % to 1.4 % and increase the elongation from 1.9 % to 7.8 %. This work emphasizes the relevance of solid solution temperature when a PBHT strategy is applied.

Impurity levels of titanium (Ti) and vanadium (V) depreciate the electrical conductivity of aluminum (Al) when present in solution. Industrially, a boron (B) based alloy is introduced to remove these impurities from the Al melt as borides. Subsequently, these boride particles are separated from the Al melt by gravity settling or filtration, rendering much improved electrical conductivity to the solidified Al material. However, the kinetics of V removal from molten Al in the presence of Ti is not well delineated yet. Additionally, the mechanism for the formation of TiB2 and VB2 in molten Al remains unclear, especially for the formation of VB2 in the presence of Ti. In this study, the kinetics of Ti and V removal from molten Al is investigated in detail by inoculating an Al–0.50 pct Ti–0.50 pct V alloy melt with an Al–5 pct B master alloy at 750 ± 10 °C. Samples are taken at 5, 10, 15, 30, 45 and 60 minutes during the boron treatment. Then, the collected samples are characterized for both the reaction products (TiB2/VB2) and the change in Ti and V concentrations in the molten Al alloy. Reaction products of TiB2 and VB2 in the form of boride cluster rings are observed. The TiB2/VB2 cluster rings are less dense than those observed in the Al–1 pct V–0.45 pct B alloy. The reaction rate between Ti and V with B/AlB2 is rapid, which leads to the formation of cluster rings in the first 5-minute reaction period. The mass transfer coefficients (km) of Ti and V in the molten Al alloy at 750 ± 10 °C are determined to be 5.13 × 10−4 and 3.00 × 10−4 m/s, respectively. These calculated values of km are within the range of 10−3 to 10−4 m/s for typical solid–liquid metallurgical reactions. Therefore, the reaction between Ti/V with B/AlB2 can be assumed to be predominately controlled by the mass transfer of Ti/V through the Al melt. The removal rates of Ti and V are almost the same in the molten Al–0.50 pct Ti–0.50 pct V–0.235 pct B alloy at 750 °C, and they exhibited a strong linear relationship with each other, although thermodynamic predictions suggest that in the presence of Ti, there should be no removal of V in the form of VB2. The reasons are discussed in detail based on kinetic analyses. The findings of this study provide an important scientific basis for boron treatment of molten Al alloys for improved electrical conductivity.

In this study, titanium matrix composites (TMCs) were prepared by plasma activated sintering. The microstructure and mechanical properties of the composites were studied, and the strengthening mechanism was determined. The results showed that the main phases of Ti-rich samples are TiB, α-Ti and β-Ti, while the primary phases of TiB2-rich samples are TiB2 and TiB. The composites containing 70 wt%TC4-30 wt%TiB2 demonstrated the best performance which achieved a balance of high strength and ductility, with a yield strength of 1800 ± 28 MPa and compressive strength of 2215 ± 39 MPa. The key reinforcing and toughening mechanisms were due to the dislocation defects and stacking faults in the samples, which consume more fracture energy during crack propagation and comprehensively improve the strength and toughness of the composites. Furthermore, crack deflection and whisker pull-out caused by TiB can toughen the composites.

In the current work, fine-grained dual-phase, high-entropy ceramics (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2-(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C with different phase ratios were prepared from powders synthesized via a boro/carbothermal reduction approach, by adjusting the content of B4C and C in the precursor powders. Phase compositions, densification, microstructure, and mechanical properties were investigated and correlated. Due to the combination of pinning effect and the boro/carbothermal reduction approach, the average grain size (∼0.5−1.5 μm) of the dual-phase high-entropy ceramics was roughly one order of magnitude smaller than previously reported literature. The dual-phase high-entropy ceramics had residual porosity ranging from 0.3 to 3.2 % upon sintering by SPS and the material with about 18 vol% boride phase exhibited the highest Vickers hardness (24.2±0.3 GPa) and fracture toughness (3.19±0.24 MPam 1/2).

A powder mixture of eutectic composition in the LaB6–NbB2 system has been prepared by carboborothermic reduction of a La(OH)3 + NbO0.5–2(OH)4–1 hydroxide mixture in vacuum at an isothermal holding temperature of 1650°C. The hydroxide mixture was prepared by reverse coprecipitation in a suspension of amorphous boron and carbon from aqueous solutions of lanthanum nitrate, niobium fluoride, and niobium oxyfluoride. We have examined the effect of the boron-to-carbon ratio on the elemental and phase compositions of the boride mixture. The average particle diameter of the synthesized borides has been determined to be 200–250 nm. The powders have been sintered at 1700°C to a relative density of 87% and arc-remelted. The resultant rodlike eutectic structures had a NbB2 rod diameter near 600 nm. The structure of the solidified materials has been studied using scanning electron microscopy, X-ray microanalysis, and X-ray diffraction, and the eutectic composition has been accurately determined (58 mol % LaB6).

The removal of boron from silicon is a crucial step for silicon purification by the metallurgical route. The mechanism of boron removal was investigated by a combined method of using calcium silicate slag containing CaCl2 and O2 gas blowing. It is proved both experimentally and theoretically that the combined refining method is favorable for boron removal from silicon and shows a significantly enhanced boron removal. Thermodynamic calculations show that in addition to borate, a gaseous BOCl species is generated during the processing by the combined refining technique. Ab initio molecular dynamics simulations of BOCl gaseous species formation show that the Si-B bond breaks and a new Si–O–B structure is formed when the oxygen is introduced into the Si–B lattice plane. The existence of chloride further leads to the breaking of the Si–O bond and ultimately generates the O–B–Cl structure that escapes from the Si lattice plane in the form of free boride BOCl.

The hot pressing process was employed to produce dense ultrahigh-temperature ZrB2–15 vol.% SiC–5 vol.% WC ceramics. Refractory (Zr, W)C and WB phases emerged in hot pressing at 2050°C and 30 MPa with a holding time of 15 min. The hot pressing process was peculiar in that a (Zr, W)B2 solid solution formed at the zirconium boride grain boundaries. Annealing in vacuum at 1600°C decreased the amount of oxygen in the ceramics from 0.3 to 0.1 wt.% through interaction of B2O3, SiO2, and ZrO2 with tungsten carbide. The ZrB2–15 vol.% SiC–5 vol.% WC ceramics had 505 ± 60 MPa strength at room temperature and 802 ± 94 MPa strength at 1800°C. The high strength at 1800°C was reached through transcrystalline fracture of zirconium boride grains. High-temperature oxidation resulted in scale consisting of three layers: borosilicate glass as the upper layer, zirconium oxide with other oxide phases (WO3 and SiO2) as the middle layer, and the base material depleted of boron and silicon as the lower layer. At an oxidation temperature of 1500°C and a holding time of 50 h, the scale was 85 μm thick, including a SiO2–B2O3 layer 64 μm thick and a layer of ZrO2 + SiO2 + MexOy and base material depleted of boron and silicon. At an oxidation temperature of 1600°C and a holding time of 2 h, the scale was 84 μm thick, including a SiO2–B2O3 layer 10 μm thick and a layer of ZrO2 + SiO2 + MexOy and base material depleted of boron and silicon. The dense scale developed on the material allowed 70% of its initial strength to be retained after oxidation at 1500°C with a holding time of 50 h and 50% strength after oxidation at 1600°C for 2 h, which was higher than for the base ZrB2–15 vol.% SiC–5 vol.% ceramics.

In this study, SAE 1020 steel samples were boronized at 850 °C for 4 h with different boronizing mixtures by powder-pack boronizing method. H3BO3 or Na2B8O13 was used as the boron source, KBF4 or NaBF4 was used as the activator and SiC was used as the diluent. Thus, 4 different boronizing mixtures (M1-M4) were prepared. Thickness, morphology and phase structure of the formed boride layers were studied by X-ray diffraction and microstructural analyzes. The single phase (Fe2B) boride layer with saw-tooth morphology was formed on surface of the samples. The thickest boride layers were obtained by using M1 and M2 boronizing mixtures. Microhardness measurements were performed to determine hardness of the 3 zones (core, transition zone and boride layer). The means of boride layer hardness of the samples were compared according to 95% Confidence Intervals analysis. It was determined that the average hardness value of the boride layers was about 1475 HV. Wear tests were fulfilled by ball on disc type wear test method under dry sliding condition and at 25 °C. According to the friction coefficient and specific wear rate values of boronized samples, it was identified that the sample SM4 boronized with M4 boronizing mixture has the lowest wear rate (1.64 mm3/N.m) and friction coefficient value (0.63).

In this study, powder metallurgy was applied in a furnace atmosphere to form titanium boride layers on a commercially pure Ti surface. Experiments were carried out using the solid-state boriding method at 900 °C and 1000°C for 12 h and 24 h. Samples were produced by pressing the commercially pure Ti powders under 870 MPa. The sintering process required by the powder metallurgy method was carried out simultaneously with the boriding process. Thus, the sintering and boriding were performed in one stage. The formation of the boride layer was investigated by field emission scanning electron microscopy, optical-light microscopy, X-ray diffraction, and elemental dispersion spectrometry analyses. In addition, microhardness measurements were performed to examine the effect of the boriding process on hardness. The Vickers microhardness of the boronized surface reached 1773 HV, which was much higher than the 150 HV hardness of the commercially pure Ti substrate. The X-ray diffraction analysis showed that the boriding process had enabled the formation of TiB and TiB2 on the powder metallurgy Ti substrate surface. Consequently, the production of Ti via powder metallurgy is a potentially cost-effective alternative to the conventional method, and the boriding process supplies TiB and TiB2 that provide super-high hardness and excellent wear and corrosion resistance.

Tungsten borides are special compounds with potential applications in superhard materials, anti-friction materials, and catalytic materials. In this study, a new synthetic method for tungsten boride is proposed using tungsten (W) and boron carbide (B4C) as raw materials. First, the boronization reaction between W and B4C was carried out at a high temperature; then, the boronization products were reacted with calcium to generate CaC2 for decarburization. After acidic leaching of the CaC2 and Ca, pure tungsten boride could be prepared. By investigating the reaction between different ratios of W and B4C, it was found that only pure W2B and WB were synthesized in the current method, and W2B5 could react rapidly with Ca to form WB and CaB6. However, in the absence of a decarburization treatment after the boronization process, carbon-containing WB (C content = 1.2 wt%) and W2B5 (C content = 3.4 wt%) could also be prepared. In addition, it was concluded that the prepared tungsten boride roughly maintained the morphology and particle size of raw W, and ultra-fine tungsten boride could be synthesized with ultra-fine W powder at a low boronization reaction temperature.

A method of casting surface alloying during casting using gasified models with the formation of a wear-resistant surface layer is considered. The increase in wear resistance is due to formation of boride and carbide components by means of self-propagating high-temperature synthesis between alloyed coating components that occurs during casting manufacture. Results of comparative tests for materials under abrasive wear are presented. Data are given for material composition, structure, and hardness.

The two-stage gas boriding in N2−H2−BCl3 atmosphere was applied to producing a two-zoned borided layer on Nisil-alloy. The process was carried out at 910 °C for 2 h. The microstructure consisted of two zones differing in their phase composition. The outer layer contained only a mixture of nickel borides (Ni2B, Ni3B) only. The inner zone contained additionally nickel silicides (Ni2Si, Ni3Si) occurring together with nickel borides. The aim of this study was to determine the presence of nickel silicides on the mechanical properties of the borided layer produced on Ni-based alloy. The hardness and elastic modulus were measured using the nanoindenter with a Berkovich diamond tip under a load of 50 mN. The average values of indentation hardness (HI) and indentation elastic modulus (EI) obtained in the outer zone were respectively (16.32±1.03) GPa and (232±16.15) GPa. The presence of nickel silicides in the inner zone reduced the indentation hardness (6.8−12.54 GPa) and elastic modulus (111.79−153.99 GPa). The fracture toughness of the boride layers was investigated using a Vickers microindentation under a load of 0.981 N. It was confirmed that the presence of nickel silicides caused an increase in brittleness (by about 40%) of the gas-borided layer.

The nanostructured powder of multicomponent oxides, borides, and borates is widely used due to its high hardness, corrosion resistance, high melting point, insulating, and abrasive. In the present work, the effect of boron content on the morphology of electrospun multicomponent oxide, boride, and borate nanostructures in a quaternary Ti–Al–O–B system was investigated. Different molar ratios of B/(Ti + Al) (0.8, 1.6, and 2.4) were employed and evaluated. Imaging with the field emission scanning electron microscope (FESEM) and the transmission electron microscope (TEM) revealed that after one hour of thermal treatment at 1100 °C, the hybrid electrospun nanofibers (NFs) in the fibrous platform transformed into nanoparticles (NPs), nano-needles, and nano-whiskers at B/(Ti + Al) molar ratios of 0.8, 1.6, and 2.4, respectively. The binding energies were investigated by X-ray photoelectron spectroscopy (XPS), whereas the phase study was conducted via the X-ray diffraction (XRD) technique. The results confirmed the formation of nanostructured ceramic powder platforms composed of multiple components, namely oxides (e.g., B-doped TiO2; Al2O3), borides (TiB, TiB2, Ti2B5, TiB12, and AlB2), and borates (TiBO3; Al18B4O33). Simultaneous thermal analysis (STA) of the Ti–Al–O–B mats indicated that the borides and borates formed consecutively at temperatures above 800 °C through reactions involving molten B2O3. We found that the obtained NPs were well arranged and sintered together throughout the fibers.

Zirconium boride (ZrB2) with high electrical, thermal conductivity, chemical stability and low coefficient of thermal expansion has been considered promising boride ceramic. In this study, the new type polyimide(PI)-based composites filled with ZrB2 nanopellets were prepared via in-situ polymerization. The results show that the ZrB2 nanopellets are dispersed uniformly in the PI matrix and the ZrB2/PI composites with 0.5 wt.% loading exhibits outstanding dielectric, mechanical properties and thermal stability. A mechanism related to the microstructure is proposed to explain the performance improvement. ZrB2 with strong electron affinity could trigger charges to accumulate on its surface, which promotes the interface polarization and consequently enhance dielectric constant. The strong interactions between ZrB2 and PI beneficial to stress effectively transfer from the PI matrix to the ZrB2. The dense structure of the composites and the intrinsically high thermal conductivity of ZrB2 are conducive to heat transfer in the matrix and reduce heat accumulation. The dielectric constant, tensile strength and elongation at break of the composites are increased by 17.5%, 63% and 59.4% compared to pure PI. Meanwhile, the ZrB2/PI composites possess superior electrical insulation property. The comprehensive performance improvement of the composites share a broader prospect for its application in engineering.

In order to understand the high-temperature deformation behaviour of alloy having hard surface layer, thermo-mechanically treated duplex stainless steel (DSS) is boronised for 0.75–6 h at 1223 K and subsequently deformed under compression mode at the same temperature under strain rate condition of 1 × 10−3, 2 × 10−4 and 6 × 10−5 s−1 until strain of 0.4. The substrate microstructure is almost isotropic with grain size after boronising with layer thickness between 1.61 and 2.74 μm. X-ray diffraction results confirm the formation of boride on DSS surface. The surface hardness of DSS increases from 387 to 1000–2400 HV after boronising. Uniform boronised layer with thickness of 20–40 μm is formed at DSS surface. Compression results show that the flow stress of the deformation increases with the strain rate and boronising time. For the boronised samples, the flow stress range is between 5 and 89 MPa. To determine the actual effect of the boronised layer on the flow stress, the results are also compared with those from un-boronised samples having similar microstructure. The results suggest that at a constant grain size, even with the hardest layer, the effect of hard surface layer on the flow stress almost could be negligible when the deformation rate is slow, but at faster deformation rate, even in the layer with the least hardness, the flow stress shows a significant increase. It is also observed that the hard boride surface disintegration could be avoided at a sufficiently low deformation flow stress that could be attributed to superplasticity.

In this study, in-situ synthesis of tungsten boride-carbide composite powders using a mechanochemical processing (MCP) method was investigated on the WO3–B2O3–Mg–C quaternary powder system. The raw blends of WO3–B2O3–Mg–C were processed in a high-energy ball mill for between 8 and 24 h. In addition to the MCP duration, excess B2O3 or C reactants and ball-to-powder weight ratio (BPR) were tested as important process parameters which affected the resultant phases. After the mechanochemical reaction, the products were purified using 6 M HCl solution to eliminate the MgO by-product. FactSage 7.1 and HSC Chemistry Ver.4.1 thermochemical software was used to predict the thermodynamically possible reactions and products. Microstructural properties of the fabricated powders were inspected through X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM) and energy dispersive spectroscopy (EDS) techniques. Additionally, changes in particle size with duration of the MCP were determined for the synthesized powders. According to the XRD results, tungsten boride and tungsten carbide phases were obtained as the main reaction products for all reaction durations and reactant stoichiometries. Distribution of the total amounts of tungsten boride and tungsten carbide phases were determined respectively as 79.8 and 20.2 wt% for the 8 h-processed stoichiometric powders. However, the total tungsten boride amount decreased with the increasing MCP duration and its respective percentage was calculated as 57.1 wt% for the 24 h-processed stoichiometric powders. In addition, by increasing the processing duration to over 12 h, W2B, W2C and B4C phases were detected. When 200 wt% B2O3 was added, the B:W ratio and total amount of tungsten boride phases in the synthesized powders increased. Similarly, by utilizing 200 wt% C, a significant increase in the B4C peaks together with W2C peaks was detected. Therefore, the properties of tungsten boride-carbide composite powders synthesized by the mechanochemical route are highly affected by the process parameters.

Textured high-entropy boride ceramics, prototypically (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2, with nearly single phase were fabricated for the first time by spark plasma sintering (SPS) of the corresponding powder, which was derived from boron/carbothermal reduction. Lotgering orientation factor calculation and the pole figures revealed that the c-axis of high-entropy boride grains aligned parallel to the SPS pressing direction after sintering and grains grew along the a,b-axis to form platelet-like morphology. The remarkably high Vickers’ hardness of ∼29 GPa was observed on both of the side surface and the top surface of textured ceramics. The surface perpendicular to the SPS pressing direction demonstrated an improved fracture toughness owing to the oriented elongated grains.

Transition metal boride (TMB) as a new type of catalyst has attracted much attention in recent years. To further enhance their catalytic performance, much effort has been made on improving their electrical conductivity and specific surface area. Herein, we have constructed a heterojunction by anchoring 1D nickel boride (NixB) nanoparticles on the surface of 2D N-doped Ti3C2 MXene (N10TC) to overcome these limitations. The resulting heterojunction (NixB/N10TC) electrode exhibited improved catalytic activities and stability on water splitting. In particular, this NixB/N10TC electrode exhibits excellent stability within 50 h continued testing.

In this study, boron carbide-metallic boride (B4C-MeBx, Me = Ti, Zr, Nb, Ta, or W) multiphase ceramics were fabricated via in situ pressureless sintering at 2250 °C for 1 h. The effects of transition metal carbides, namely, TiC, ZrC, NbC, TaC, and WC, on the phase composition, microstructure, and mechanical properties of the ceramics were investigated. The results showed that MeC could facilitate the sintering densification of B4C by distributing second-phase particles uniformly throughout the B4C. Additionally, the main phases observed were B4C and (Me, W)Bx (Me = Ti, Zr, Nb, or Ta) due to the doping of a small amount of WC during the ball milling process. As a result, the mechanical properties of B4C-MeBx showed significant improvements when compared with those of single-phase B4C ceramics. B4C–NbB2 ceramics were found to exhibit the best mechanical properties, with an elastic modulus of 393.0 GPa, a hardness of 28.7 GPa, a flexural strength of 368.0 MPa, and a fracture toughness of 6.94 MPa m1/2.

Ti–6Al–4V alloy was borided by using a two-stage boriding process. In the first stage, boron atoms accumulated on Ti–6Al–4V alloy samples via electron beam evaporation technique. The second stage included annealing of samples after boron deposition at various temperatures in order to achieve boride layer with high hardness and adherence. After the boriding treatment, phases formed on the surface of the Ti–6Al–4V alloy were identified by X-ray diffraction technique. The microstructure and thickness of boride layers were examined by scanning electron microscope. Microstructural analyses indicated that surface structure of borided samples comprised of TiB2 layer on the top and TiB whiskers towards the substrate. It was revealed that boriding by electron beam evaporation technique resulted in formation of a hard and adhered surface structure after annealing especially at 950°C for 24 h. Microhardness measurements showed that boride layer on Ti–6Al–4V has mean hardness value of about 1937 HV. Boriding at 950°C for 24 h brought about an improvement in wear resistance of Ti–6Al–4V alloy. The borided surface exhibited higher wear resistance along with higher coefficient of friction as compared to as-received one.

The NTC20 and NTC40 composite powders were produced by conglomeration from the NiCrBSiC self-fluxing alloy with additions of 20 and 40 wt.% TiCrC, respectively, using an organic binder. Since the organic binder burnt out in the plasma spray process, components of the composite powders segregated and a certain amount (up to 15–20 vol.%) of the TiCrC particles was lost. The plasma-sprayed NTC20 and NTC40 coatings showed a heterophase lamellar structure, consisting of a nickel-base matrix in which fine chromium boride and carboboride grains (1–2 μm) and TiCrC particles (5–8 μm) were evenly distributed. Electron microprobe analysis revealed a higher content of oxides in the NTC20 and NTC40 coatings compared to the NiCrBSiC coating, which was associated with the oxidation of TiCrC particles in the plasma spray process. The introduction of TiCrC additions into the NiCrBSiC self-fluxing alloy increased porosity of the plasma-sprayed NTC20 and NTC40 coatings (up to 8%) versus the NiCrBSiC coating (5%). Wear tests of the plasma-sprayed NTC20 and NTC40 coatings were performed in dry sliding friction conditions using 65G steel as a counterface. For comparison, a plasma-sprayed coating consisting of the serial NiCrBSiC self-fluxing alloy was tested. Additions of 20 and 40 wt.% TiCrC particles to the NiCrBSiC self-fluxing alloy increased the wear resistance of the plasma-sprayed coatings by 2 to 2.3 times. As the sliding speed increased from 4 to 12 m/sec, the NiCrBSiC coating underwent catastrophic wear (I ≈ 60 μm/km), while the wear rate of the NTC20 and NTC40 coatings remained constant (I ≈ 12–22 μm/km).

Using thermochemical treatments, boronized layers were successfully prepared on Al0.25CoCrFeNi high-entropy alloys (HEAs). The thickness of the boronized layers ranged widely from 20 to 50 µm, depending on the heat treatment time. Boronizing remarkably improved the surface hardness from HV 188 to HV 1265 after treating at 900°C for 9 h. Moreover, boronizing enhanced the yield strength of HEAs from 195 to 265 MPa but deteriorated the tensile ductility. Multiple crackings in the boride layers significantly decreased the plasticity. The insufficient work-hardening capacity essentially facilitated the plastic instability of the boronized HEAs. With decreasing substrate thickness, the fracture modes gradually transformed from dimples to quasi-cleavage and eventually to cleavage.

Dense (Hf0.2,Zr0.2,Ti0.2,Ta0.2,Nb0.2)B2 high-entropy ceramics with high phase purity were produced by two-step spark plasma sintering of precursor powders synthesized by boro/carbothermal reduction of oxides. The reacted powders had low oxygen (0.404 wt%) and carbon (0.034 wt%) contents and a sub-micron average particle size (∼0.3 μm). Powders were synthesized by optimizing the excess B4C content of the reaction mixture and densified by a two-step spark plasma sintering process. The relative density increased from 98.9% to 99.9% as the final sintering temperature increased from 2000 °C to 2200 °C. The resulting ceramics were nominally single-phase (Hf,Zr,Ti,Ta,Nb)B2 with oxygen contents as low as 0.004 wt% and carbon as low as 0.018 wt%. The average grain size increased from 2.3 ± 1.2 μm after densification at 2000 °C to 4.7 ± 1.8 μm after densification at 2100 °C, while significant grain growth occurred during sintering at 2200 °C. The high relative densities, low oxygen and carbon contents, and fine grain sizes achieved in the present study were attributed to the use of synthesized precursor powders with high purity and fine particle size, and the two-step synthesis-densification process. These are the first reported results for dense high-entropy boride ceramics with high purity and fine grain size.

Compact ceramic electrode materials based on the Ti–B–Fe system modified with nanosize particles of aluminum nitride (up to 15 wt.%) are prepared by SHS extrusion. The effect of additives on combustion characteristics of the system and also on structure and phase composition of the materials obtained is studied. Addition of aluminum nitride increases the content of boride and nitride phases in the final product. It is established that introduction of modifying nanosize aluminum nitride particles into an initial charge leads to boride and nitride phase grain refinement that in combination increase microhardness by 10% compared with unmodified specimens.

The structurization of coatings produced by electrospark deposition (ESD) from the commercial selffluxing FeNiCrBSiC alloy and FeNiCrBSiC-based composites, such as FTB20 (FeNiCrBSiC–20 wt.% TiB2) and FCB20 (FeNiCrBSiC–20 wt.% CrB2), on a steel 45 substrate was studied. The electrospark FeNiCrBSiC coating about 70 μm thick has a globular surface and the FTB20 and FCB20 coatings form a continuous layer up to 50 μm thick over the entire samples. The microhardness does not change across the deposited coating thickness and is 10–14 GPa. The chemical compositions of the ESD coatings and the electrodes are the same, which indicates that the electrode material does not mix with the steel substrate. The structure of the FeNiCrBSiC, FTB20, and FCB20 electrodes and coatings differs significantly because chromium boride and/or titanium boride inclusions refine from 20–25 μm to 1 μm in the ESD process. The heterophase structure of the ESD coatings represents a nickel–iron-base matrix reinforced with fine boride and carboboride particles. The effect of speeds and loads on the wear rate of ESD coatings in dry friction conditions was examined. Electrospark coatings produced from the WC–6% Co hardmetal were tested as wear resistance reference. The wear rate of the FeNiCrBSiC, FTB20, and FCB20 coatings decreases and that of the WC–6% Co coating increases when the speed rises from 4 to 12 m/sec. The wear rate of the ESD coatings becomes one order of magnitude higher when the load increases from 0.1 to 0.4 MPa. Analysis of the friction surfaces showed that wear of the FeNiCrBSiC coating was caused by failure of the globules and that of the FCB20 coating by brittle fracture of the deposited layer. The ESD FTB20 coating has two to three times higher wear resistance than the FeNiCrBSiС coating because of the oxidative wear mechanism whereby protective oxide films develop on the friction surfaces and act as a solid lubricant.

Plasma arc coatings produced from powder-cored wires developed at the Paton Electric Welding Institute that were deposited onto a low-carbon steel substrate were characterized. Interaction between the steel sheath (constituting at least 80 wt.% of the wire) and powder cores, such as B4C, B4C + ZrO2(nano), B4C + (Cr, Fe)7C3, and B4C + (Cr, Fe)7C3 + Al, in the arc plasma spraying process was analyzed. The resultant coatings were free of defects and had low porosity (to 2.5%) and lamellar structure. The core carbide components were found to dope the coating ferritic matrix and strengthen it with carbide, carboboride, and boride precipitates. An addition of 0.5% ZrO2 nanopowder refined the coating structure and participated in the formation of Fe2B and Fe3B precipitates. An aluminum addition within 10 wt.% did not led to iron aluminides but, being easily melted, activated the interaction between components, reduced porosity, and improved adhesion strength of the coatings. The coatings reached a microhardness of 6.25–8.59 GPa, being 4 to 5.5 times higher than the microhardness of the steel wire sheath. The development and use of such powder-cored wires expanded the application of plasma arc spraying, particularly for the protection of equipment against abrasive gas wear in chemical engineering, in the production of parts for pumps, compressors, and other components, and for the recovery of worn parts.

TiB2, ZrB2, and SiC powders with particles measuring several micrometers were first agglomerated and then deposited onto the substrate of a porous carbon block as a barrier to molten cryolite-based salt. Scanning electron microscopy, X-ray diffraction, and transmission electron microscopy were conducted to investigate the fine microstructure of the obtained ceramic coating and to elucidate the evolution of agglomerated powder from sprayed particles in plasma jet to splats on a substrate. Results indicated that a highly dense ceramic coating consisting of a solid solution (TixZr1-x)B2, residual TiB2, and ZrB2 was obtained. The compactness of the coating and the formation of a solid solution phase was mainly caused by a SiC-rich liquid phase, as determined from a boride and silicon carbide pseudodiagram. After being submerged in molten cryolite-based salt for 8 h, the ceramic coating was firmly bonded to a carbon block. No molten slat permeated the ceramic coating.

In this study, a mixture of 93% Ni–7% Mg and 97% Ni–3% Mg powders were produced by the TM method and the diffusion kinetics, surface properties of borided nickel–magnesium alloy at different temperatures and times were investigated. The samples were sintered at 530°C. The boriding process was performed Ekabor II boron powder in atmospheric furnace environment at 900, 950 and 1000°C temperatures for 1.5, 3.0 and 4.5 h. The phases in the boride layer were found by means of CuKα radiation in XRD. XRD analysis of NiB, Ni2B and Ni3B phases were obtained. The hardness of borides layer formed on surface after boriding of the 93% Ni–7% Mg and 97% Ni–3% Mg alloys substrate ranged from 1518 to 1964 HV0.05, whereas value of the unborided nickel was 112–136 HV0.05. The boron activation energy was estimated as equal to 136.506 and 158.843 kJ/mol for the borided 93% Ni–7% Mg and 97% Ni–3% Mg alloys, respectively.

A synthesis route was successfully established to reliably synthesize the high temperature thermoelectric material REB66 (RE = Y, Sm, Ho, Tm, Yb) much more readily than the typical single crystal growth. Thermoelectric properties of HoB66 and TmB66 are also investigated for the first time. The REB66 samples synthesized by spark plasma sintering showed thermoelectric properties equivalent to those measured on single crystals. A figure of merit ZT around 0.1 at 973 K was obtained with a sharply increasing trend toward higher temperatures. Strikingly, the thermoelectric properties of these compounds appear to be almost independent of the microstructure. The nature of the rare earth does not seem to strongly affect the thermoelectric properties, differences originating mainly from different compositions or the presence of secondary phases. Relatively large negative Curie-Weiss temperatures θ were observed, and an increasing coupling indicated from HoB66 to YbB66. This could be indicative of an unusual coupling mechanism.

Decomposition of WB4 into WB2 at temperature higher than 1200 °C makes the preparation of pure bulk WB4 ceramic difficult. Doping of the third element was reported to be an effective way for thermal stability improvement of ceramic materials. In the present work, the nearest element of W, tantalum (Ta), was introduced into tungsten boride synthesized by a combination of high energy ball milling and thermal treatment with (W:Ta):B= (2/3:1/3):4.5 & 5. The phase composition, chemical state of each element, and thermal stability of the as-synthesized samples were investigated. Results show that, with W:Ta molar ratio of 2/3:1/3, ei, 2:1, when the (W + Ta):B = 1: 4.5, about 49.5% mass fraction of WB4 phase was maintained successfully after being thermal treated at 1500 °C for 1 h, compared with pure WB22 phase obatined by the Ta-free sample. Increasing B content in raw materials to (W + Ta):B = 1:5, the mass fraction of WB4 phase was increased to 97%, while the only phase can be detected in the Ta-free samples is also WB2. The significantly improved thermal stability of WB4 can be mainly attributed to the completely dissolve of Ta into the W-B system. It was also found that the oxidation resistance of the W-B system was increased with the introduction of Ta.

The objective of this research is to investigate the effect of heat treatment and pre-heating and pre-cold temperatures on hot cracks and microstructure of GTD-111 superalloy laser weld joints. For this purpose, a Nd:YAG pulsed-laser with a maximum power of 400 w was used. Numerical computation results through the Rosenthal equation showed that the length of the various welding regions including HAZ, PMZ and MZ increased with increasing pre-welding sample from − 25 to 200 °C, which was in agreement with the experimental results of the experiments. By reducing the sample temperature before to welding from 200 to − 25 °C, the heating rate increased from 162,857 to 195,000 °C/s. The high heating rate in pre-cold conditions increased the cooling rate of the weld metal and thereby decreased the segregation of the alloying elements. In the cast condition, the $$\gamma^{\prime }$$ , $$\gamma\text{-} \gamma^{\prime }$$ eutectic, Cr–Mo boride and Ni–Zr intermetallic phases were responsible of liquation cracks in HAZ, which were all dissolved in austenitic matrix by heat treatment at 1230 °C. The results of HAZ microstructural studies showed that by increasing the liquid film Thickness (LFT) due to the liquation of $$\gamma^{\prime }$$ and MC phases, the susceptibility to liquation crack was reduced due to increased stress-relief. However, as the LFT increases due to the liquation of the $$\gamma\text{-} \gamma^{\prime }$$ , Cr–Mo phases, the susceptibility to crack liquation increases due to the decrease in stress-relief for crack formation. This is attributed to the high $$\Delta T$$ of the $$\gamma\text{-} \gamma^{\prime }$$ , Cr–Mo phases and the low $$\Delta T$$ of the $$\gamma^{\prime }$$ and MC phases.

Conventional and two-step heating transient liquid phase bonding (TLP) experiments of martensitic precipitation hardening stainless steel 17-4-PH were studied employing AMS 4777 braze filler at temperature of 1050 °C for different times (15 to 90 min) to accomplish the ideal brazing condition. The effect of TLP methods and bonding conditions on the microstructure and shear bond strength was investigated. The critical bonding time for isothermal solidification in conventional and two-step heating TLP was 60 min and 45 min, respectively. The microstructure of isothermal solidification zone consisted of Ni-rich solid solution phase. Eutectic constituents Cr-rich boride and Ni3Si were formed in the joint centerline in the athermally solidified zone. In the conventional method planar interface was formed, however in TLP bonding under temperature gradient non-planar interface was formed, high angel grain boundaries merged at the end of isothermal solidification and homogenous bond was produced. Due to the decrease of athermally solidified zone the shear strength increased by increasing the holding time and when using the two-step heating process because of better metal to metal contact, the shear strength was higher. The optimum condition was a temperature gradient TLP in 60 min bonding time, which led to shear strength of 435 MPa.

With a motivation to discover efficient materials for direct electrolysis of seawater, bimetallic oxy-boride (Co–Fe–O–B) nanostructures were developed using a facile hydrothermal synthesis strategy...

In this study, silicon carbide-lanthanum hexaboride (SiC–LaB6) and silicon carbide–lanthanum hexaboride–zirconium boride (SiC–LaB6–ZrB2) ceramics were fabricated by spark plasma sintering at 1900 °C, and their ablation resistance was tested under plasma flames over 2300 °C. The results indicate that the SiC–LaB6–ZrB2 ceramic exhibits better ablation resistance than the SiC–LaB6 ceramic. After ablation under the plasma flame for 60 s, the mass and linear ablation rates of the SiC–LaB6 ceramic were 15.83 μg/s and 1.08 μm/s, respectively, while those of SiC–LaB6–ZrB2 were -8.42 μg/s and -0.27 μm/s. With the addition of ZrB2, SiC–LaB6–ZrB2 ceramic attained a high density and fewer inner oxygen diffusion channels. Moreover, the ZrO2–La2O3–SiO2 oxide scale with good self-healing ability and excellent stability was formed in the ablation centre, which can retard the further oxidation during ablation.

The boride layer characteristics, wear and corrosion properties of borided commercially pure titanium by pack boriding near the α → β phase transition temperature were investigated using X-ray diffraction, scanning electron microscopy, electron probe microanalysis, dry reciprocating friction tests, and electrochemical experiments in this work. The pack boriding was carried out at the temperatures of 860 °C, 880 °C, 900 °C, and 920 °C for 5, 10, 15 and 20 h. The results indicated that, in both α and β phase temperatures, the boride layer is composed of the outer TiB2 layer and the inner TiB layer. The α→β phase transition temperature of commercially pure Ti in this work is in the range of 882–900 °C, and the growth of TiB layer can be enhanced at this temperature range. Commercially pure Ti borided at 920 °C for 20 h has the best wear resistance and corrosion resistance. Finally, wear and corrosion mechanisms were also discussed.

Iron-based alloy overlays are widely utilized in industry to extend the service life of components subjected to wear and corrosion attack. Welding is an overlay process commonly employed because of its low cost and high efficiency. The microstructure of an as-welded chromium carbide overlay and a new Fe-Cr-C-B overlay containing multiple alloying elements has been characterized by optical microscopy, scanning electron microscopy, X-ray diffraction, and electron backscatter diffraction (EBSD). The microstructure of the chromium carbide overlay consists of large $$\hbox {M}_7\hbox {X}_3$$ primary carbides and austenite and carbide eutectic phases. The microstructure of the new overlay consists of granular MX-type primary carbide (M = Nb, Ti, Mo; X = C and B), dendritic $$\delta $$ -ferrite/austenite, eutectic phases of austenite and $$\hbox {M}_2\hbox {B}$$ boride (M = Fe and Cr). The austenite portion of the microstructure has been subsequently transformed into martensite and retained austenite. The fine MX-type hard particles and refined eutectic and matrix microstructure lead to the high hardness of the overlay. The non-equilibrium solidification process for the complex microstructure is discussed using ThermoCalc.

The powder compact was directly sintered by a pack boriding process to prepare a Fe-based material having a boride layer. The boriding process was carried out at 850 °C, 950 °C and 1050 °C for 3, 5...

The state of the surface and structure of the diffusion boride coating on the substrate of die steel have been studied after thermal cycling under a constant load of 193 N in hydrogen, nitrogen, and air atmospheres. It is shown that the degradation of boride coating in air occurs due to the development of oxidative processes beneath the coating at the boundary with the steel substrate. In air and nitrogen atmospheres, thermal cycling causes cracking of the boride coating with the formation of quasi-periodic cracks that damage only the boride layer and do not penetrate the steel. Thermal cycling in reducing and inert atmospheres leads to an increase in the values of the microhardness and elastic modulus of the coating due to the diffusion of nitrogen and hydrogen into (Fe,Cr)2B borides.

The influence of Cr and W addition on microstructure and mechanical properties of Mo2FeB2-based cermets, prepared by multi-step sintering, is systematically evaluated by XRD, SEM, EDS, and TEM. After liquid-phase sintering, the final microstructure of Cr-doped Mo2FeB2-based cermets is similar to that of traditional Mo2FeB2-based cermets. The cermet, with 2.5% Cr content, exhibits superior comprehensive mechanical properties, i.e., Rockwell hardness, transverse rupture strength (TRS) and fracture toughness of 87.6 HRA, 2179 MPa and 21.7 MPa m1/2, respectively. In contrast, for W-doped Mo2FeB2-based cermets, a large amount of W-enriched (Mo,Fe,W)3B2 solid solutions precipitated around undissolved Mo2FeB2 particles, resulting in the formation of black core/gray rim structure. Particularly, a transition layer, with several atomic layer thickness, is observed between (Mo,Fe,W)3B2 rim phase and Fe-based binder phase, which corresponds to B-deficient (Mo,Fe,W)3B2−x orthorhombic boride phase. Moreover, the lattice mismatch between (Mo,Fe,W)3B2 and (Mo,Fe,W)3B2−x is found to be 1.9%. When doped with 5 wt% W, Mo2FeB2-based cermet exhibit superior comprehensive mechanical properties with Rockwell hardness of 84.6 HRA, TRS of 2088 MPa and fracture toughness of 24.4 MPa m1/2. The increase of fracture toughness (KIC) can be ascribed to the presence of W-rich transition layer.