M5B3 Borides Research Articles

Surface hardening of high modulus steels through carburizing and nitriding: First insights into microstructure property relationships

High modulus steels are promising materials for future lightweight design solutions, as their embedded boride particles in a ductile steel matrix increase the stiffness/density ratio. One key requirement for maturing them towards industrial application is their suitability for improving the surface hardness. In this study we investigated the effects of low-pressure carburizing and plasma nitriding on the microstructure and mechanical properties of selected Fe-TiB2- and Fe-Cr-M2B-based high modulus steels. Nitriding resulted with the formation of expanded ferrite in the strongest hardness increase to about 1100 HV0.05 from the alloy system's base hardness of 240 and 500 HV0.05, respectively, albeit with different hardness depth profiles. The Fe-Ti-B alloy indicated deformation phenomena in the ferritic matrix after nitriding, whereas nitriding of Fe-Cr-B-C resulted in a diffusion-controlled particle transformation of M2B borides into CrN nitrides of lower stiffness. Carburizing on the other hand led to a slightly lower maximum hardness value of about 800 HV0.05 over an increased depth for Fe-Cr-B, as martensite and additional M23C6 carbides were formed in the surface zone. The surface hardness of the Fe-TiB2-based alloy could not be increased by the deployed carburization parameters, most likely due to excessive Ti dissolved in the matrix. Consequences for the transfer to engineering applications as well as the refinement of both, thermochemical processing parameters and designated alloy concepts, of high modulus steels are outlined and discussed.

Enhancing mechanical properties and corrosion resistance of Fe–Cr–B–Mo alloy via the 'Divide and Conquer' strategy for Ti regulation

The corrosion of molten aluminium on components in the aluminium industry poses a significant bottleneck, hindering the development of aluminium products and equipment. This study focused on the Fe–Cr–B–Mo alloy, addressing challenges related to the susceptibility of the matrix to corrosion, the excessive brittleness of M2B borides (M = Fe, Cr, etc.), and the detachment of corrosion products. A comprehensive study was performed to study the microstructure evolution, mechanical properties, and corrosion behavior of Fe–Cr–B–Mo alloy, considering the 'Divide and Conquer' strategy for Ti regulation. The findings indicate that the heterogeneous nucleation, induced by in situ TiB2 particles, significantly impacts the refinement of M2B borides size and enhances the matrix strength. Notably, the addition of 4.5 wt. % Ti to the T3 alloy significantly enhances its mechanical properties and corrosion resistance. The T3 alloy exhibits an impact toughness of 32.4 kJ/m2 and a compressive fracture strain of 19.5 %, representing a considerable increase of 58 % and 167 % over the Ti-free alloy, respectively. Furthermore, the alloy has a volume loss rate of 11.0 mm3 cm−2 h−1, which is substantially lower, by 73.5 % compared to H13 steel and by 21.4 % compared to the Ti-free alloy. The synergistic presence of TiB2 and M2B borides, along with their corrosion products, functions as an effective diffusion barrier against molten aluminium corrosion.

Effect of Nb and B on the Precipitation Behaviors in Al-Ti-Nb Balanced-Ratio Ni-Based Superalloy: A Phase-Field Study

In this paper, quantitative two-dimensional (2-D) phase-field simulations were performed to gain insight into the effects of B and Nb for Al-Ti-Nb balanced-ratio GH4742 alloys. The microstructure evolution during the precipitation process was simulated using the MICRESS (MICRostructure Evolution Simulation Software) package developed in the formalism of the multi-phase field model. The coupling to CALPHAD (CALculation of PHAse Diagram) thermodynamic databases was realized via the TQ interface. The morphological evolution, concentration distribution, and thermodynamic properties were extensively analyzed. It is indicated that a higher Nb content contributes to a faster precipitation rate and higher amounts and the smaller precipitate size of the γ′ phase, contributing to better mechanical properties. The segregation of the W element in γ′ precipitate due to its sluggish diffusion effect has also been observed. Higher temperatures and lower B contents accelerate the dissolution of boride and reduce the precipitation of borides. With the increased addition of B, the formation of borides may have a pinning effect on the grain boundary to hinder the kinetic process. In addition, borides are prone to precipitate around the interface rather than in the bulk phase. Once the M3B2 borides nucleate, they grow in the consumption of γ′ phases.

In situ synthesis and characterization of TiB2/FeMnAl metal matrix neutron absorption composites

A novel in-situ TiB2/FeMnAl metal matrix composites with different Mn content have been prepared and characterized. The effects of the Mn content on the microstructure evolution and performances were discussed. In all fabricated FeMnAl metal matrix composites, the in-situ TiB2 particles were dispersedly distributed in austenite matrix after hot rolling and annealing. Significantly, no eutectic M2B (M = Fe, Mn) boride formed along grain boundaries in all composites that shows the excellent hot workability. The tensile strength and elongation after fracture of the all composite above 20 wt% Mn are more than 750 MPa and 12 % at room temperature, respectively. The ultimate tensile strength of composites with 20 wt% Mn can maintain above 560 MPa at 500 °C. The shielding properties was also investigated by Monte Carlo method, and this designed composites are expected to be well materials for practical application.

Precipitation evolution of M5B3 boride in diffusion affected zone and its effect on mechanical properties of TLP bonded Mar-M247 superalloys

Boron is usually added in the bonding interlayer as the melting point depressant element in the transient liquid phase (TLP) bonding of Ni-based superalloys, which inevitably leads to the precipitation of borides and therefore has an important impact on the mechanical properties of the joints. M5B3-type borides (where M is a mixture of Cr, W and Mo) are the predominant borides in diffusion affected zone (DAZ) of the Mar-M247 bonded joints. In present work, the precipitation and evolution of M5B3 in the joints with different bonding time have been systematically investigated by multi-scale methods. Three kinds of crystallographic orientation relationships between M5B3 and matrix have been observed, and the morphologies and precipitation positions of M5B3 borides are found to have certain characteristics in specific bonding time joints. These crystallographic orientation relationships are consistent with the predictions based on calculations of coincidence of reciprocal lattice points (CRLP) model. The effect of the precipitation of borides on mechanical properties has been evaluated by means of microhardness and micropillar compression testing.

The role of the microstructural changes during induction preheating on the HAZ liquation cracking susceptibility of Ni-based superalloy

This work presents the influence of high-frequency induction preheating (900, 1000, 1100 °C) on liquation crack formation in the René 108 Ni-based superalloy. The investigation was divided into two parts: (1) characterization of the material's microstructure after preheating and (2) determining the influence of preheating on liquation cracking during autogenous gas tungsten arc welding. During preheating, the dissolution of γ′ precipitates showed accelerated progress with increase in temperatures. This dissolution involved the continuous thinning of each precipitate, as well as more intricate mechanisms, such as splitting. The mean size of the secondary γ′ decreased from 0.32 to 0.26 μm. In the heat-affected zone (HAZ) induced by welding, constitutional liquation of mainly γ' precipitates, with a contribution of M23C6 carbides and M5B3 borides, was observed. The formation of a thin non-equilibrium liquid film along high-angle grain boundaries led to the crack initiation and their further propagation during cooling. The eutectic γ–γ' re-solidification products were visible on the crack edges independently of preheat temperature. Preheating at 900 °C decreased the length and amount of liquation cracks, while preheating at 1100 °C allowed to prevent them due to the liquid-healing effect.

Microstructure gradient formation in electron-beam melting powder-bed fusion of a gamma-prime Ni-based superalloy

Microstructure gradient is formed during the additive manufacturing of Ni-based superalloy Haynes® 282® via electron-beam melting powder-bed fusion (EB-PBF) process. The precipitation of secondary phases occurs owing to EB-PBF in-situ heat treatment as consequence of high temperature powder bed pre-heating and multiple melting and re-heating thermal cycles, which can be summarised as a carbide precipitation heat treatment at 1000–1025 °C/8 h, aging between 875 and 975 °C/30 h, followed by a slow cooling step. Relatively large precipitates formed inter- and intragranularly with M3B2 boride and gamma-prime (γ') have sizes of approximately 5 μm and 110 nm, respectively, at the bottom of the build. The size of the precipitates decreases along the build direction. In the top region, the precipitate sizes of boride and γ' are 1 μm and 50 nm, respectively. MC carbide with a size of <1 μm precipitated throughout the build. A good correlation is observed in terms of the γ' size to hardness with average values of 350 HV at the bottom and 400 HV at the top. The ultimate tensile strength (UTS), yield strength (YS), and elongation (EL.) values at the bottom and top regions are 1080 ± 40 MPa, 805 ± 25 MPa, and 20 ± 5%; and 1105 ± 20 MPa, 825 ± 15 MPa, and 25 ± 5%, respectively. Overall, EB-PBF Haynes® 282® exhibits higher UTS and YS but lower EL than those of the as-received wrought alloy (950 ± MPa, 650 ± 15 MPa, and 40 ± 5%, respectively).

Effect of TiB2 content on microstructure and mechanical properties of GH3536 superalloy formed by laser solid forming

In this paper, GH3536 and TiB2/GH3536 alloys were prepared by laser solid forming(LSF). The effects of TiB2 content on the microstructure and mechanical properties of as-deposited samples were investigated. The results show that, with the increase of TiB2 content in the as-deposited samples, the molten pool boundary becomes shallower, the dendrite structure grows coarser, and the grain size gets larger. Meanwhile, phase segregation intensifies at the interdendritic and grain boundaries. The shape and types of the precipitated phases change significantly: the strip M23C6 carbide phase progressively evolves into a network phase of M23C6 carbide, M2B boride, and Mo-rich phase. Besides, the grain size of the solid-solution samples becomes smaller, and the columnar grains in both sample groups tend to be equiaxed. Interdendritic precipitates undergo a morphological change from a continuous network to a granular one as they gradually dissolve into the matrix. The microhardness of both sample groups also rises, and their elongation gradually declines as the amount of TiB2 increases. However, the tensile strength trend exhibits the opposite fluctuation, as the tensile strength of as-deposited samples drops and that of solid-solution samples increases with the increase of TiB2 content.

Thermodynamic and Ab Initio Design of Multicomponent Alloys Based on (Fe50Mn30Co10Cr10)-xBx (x = 0, 5, 7, 10, and 15 at.%).

Multicomponent alloys have attained general interest in recent years due to their remarkable performance. Non-equiatomic alloys with boron addition as an interstitial element are being studied, exhibiting outstanding mechanical properties. In order to estimate the mechanical behavior of potential alloys, thermodynamic and ab initio calculations were utilized in this work to investigate phase stability and stacking fault energy (SFE) for (Fe50Mn30Co10Cr10)-xBx (x = 0, 5, 7, 10, and 15 at.%) systems. Thermodynamic experiments revealed two structural variations of borides, M2B(C16) with a tetragonal structure and M2B(CB) with an orthorhombic structure. Borides precipitate when boron content increases, and the FCC matrix becomes deficient in Mn and Cr. According to ab initio calculations, the presence of boron in the FCC and HCP structures primarily disrupts the surroundings of the Fe and Mn atoms, resulting in an increased distortion of the crystal lattice. This is related to the antiferromagnetic condition of the alloys. Furthermore, for alloys with a low boron concentration, the stacking fault energy was found to be near 20 mJ/m2 and greater than 50 mJ/m2 when 10 and 15 at.% boron was added. As boron concentrations increase, M2B borides are formed, generating changes in the matrix composition prone to fault-induced phase transitions that could modify and potentially impair mechanical properties.

Analysis of the As-Cast Microstructure and Properties of the Ni-Based Superalloy MAR-M247® Produced Via Directional Solidification

The presented research investigates MAR-M247® Ni-based superalloy castings produced via directional solidification at various mold preheating temperatures (1510, 1566 °C) and withdrawal rates (3.4, 5.0 mm/min). Casting analyses were carried out via thermodynamic simulations, differential scanning calorimetry (DSC), X-ray diffraction (XRD), light microscopy (LM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), scanning transmission electron microscopy (STEM), and tensile testing. On DSC curve, four effects have been registered during cooling: liquidus (1337 °C), formation of eutectic γ − γ′ (1315 °C), precipitation of Ni7(Hf, Zr)2 (1244 °C), and M5B3 borides (1201 °C). The castings’ primary and secondary dendrite arm spacing decreases with increasing withdrawal rates for both shell mold temperatures. The dendritic regions of the castings are characterized by a relatively homogenous microstructure, consisting of γ′ precipitates surrounded by the matrix, with a mean size in the range of 0.437 to 0.481 μm, depending on the casting parameters. In the interdendritic spaces, γ − γ′ eutectic, MC carbides, M5B3, Ni7(Hf, Zr)2, and M3C2 phases were identified. The ultimate tensile strength of the produced castings was in the range of 970 to 1088 MPa.

The role of the strengthening phases on the HAZ liquation cracking in a cast Ni-based superalloy used in industrial gas turbines

This work presents the influence of microstructural constituents on liquation crack formation in the cast Ni-based superalloy, René 108. The investigation was divided into three parts: characterisation of the material's microstructure in pre-weld condition, hot ductility studies and analysis of liquation cracking induced by the gas tungsten arc welding process. Using advanced electron microscopy techniques it is shown that the base material in pre-weld condition is characterised by a complex microstructure. The phases identified in René 108 include γ matrix, γ' precipitates, MC and M23C6 carbides, and M5B3 borides. Based on Gleeble testing, it was found that René 108 is characterised by high strength at elevated temperatures with a maximum of 1107 MPa at 975 °C. As a result of constitutional liquation, the superalloy’s strength and ductility were significantly reduced. The nil strength temperature was equal to 1292 °C, while the nil ductility temperature was 1225 °C. The low ductility recovery rate (32.1), ratio of ductility recovery (36.2) and hot cracking factor (Rf = 0.05) values confirmed the low weldability of Renѐ 108. In the heat-affected zone (HAZ) induced by welding, constitutional liquation of mainly γ' precipitates, with a contribution of M23C6 carbides and M5B3 borides, was observed. The thin non-equilibrium liquid film, which formed along high-angle grain boundaries, led to crack initiation and their further propagation during cooling. The eutectic γ–γ' re-solidification products are visible on the crack edges.

The effects of chemistry variations on hot cracking susceptibility of Haynes® 282® for aerospace applications

Hot cracking susceptibility of Haynes® 282® with varying amount of C (0.05–0.09 wt%), Mn (0.03–0.12 wt%), Si (0.03–0.16), B (0.005–0.006 wt%) and Zr (0–0.01) are investigated. Synergistic role of C and B is found on solidification and heat affected zone (HAZ) liquation cracking susceptibility. High amount of C and B promote formation of eutectic constituents during final stages of solidification and promote crack healing by backfilling effect. When C and B are added in low amount the crack healing does not occur due to the absence of eutectic consituents therefore cracking extent increases. Thermodynamics simulations indicate C and B tie up to MC carbides and M3B2 borides during solidification. Scanning Electron Microscopy and Nanoscale secondary ion mass spectrometry analysis reveal C and B to be present both in solid solution and in form of precipitates to Ti-Mo rich carbides and Mo rich borides, respectively. In HAZ, these phases promote liquation cracking where cracking extent correlates to the amount carbides and borides. Lower C and B is found to reduce the liquation cracking in the HAZ. Furthermore, a high temperature homogenization heat treatment at 1190 °C excarbates the cracking by dissolving the borides and releasing B to the grain boundaries.

Sliding wear behavior of Co-Cr-Mo alloys with C and B additions for wear applications

The present study analyzes the effect of boron and carbon additions on the microstructure and wear performance of a Co-30Cr-5Mo alloy. This alloy was based on the chemical composition of the ASTM F75 alloy which is frequently used in gas turbines and high temperature applications subject to wear. For this study, a 0.5 wt% B and C additions were used to modify the microstructure and obtain a 37 %vol of reinforcing phases composed by M23C6 carbides and M3B2 borides. The microstructure of this alloy showed a network formed by the reinforcing phases, similar to the microstructure observed in the Co-30Cr-8.5W alloys, exhibiting hardness values between 43 and 48 HRC and excellent wear properties. Characterization was made through optical and electronic microscopy and X ray diffraction for phase identification in the as-cast and after heat treatment at 1200 °C for 3 h. Hardness was measured in both conditions in HRC scale. Sliding wear tests based on G77 standard were undertaken with 50 and 100 N load during 5 km sliding distance. The friction coefficient was monitored during the tests by measuring the normal and tangential forces. Wear depth, surface roughness, and volume wear losses were measured by using a non-contact 3D profiler. Results show an increase on the hardness and the reduction of the wear losses, in the alloy with the simultaneous addition of 0.5% wt of B and 0.5% wt of C. A decrease in the average friction coefficient with the increase in the volume of reinforcing phases was also observed. The base alloy showed a high degree of adhesion and higher roughness values. On the other hand, the predominant wear mechanism in the reinforced alloys was oxidative wear followed by a small degree of adhesion and micro-ploughing caused by the wear products between sliding surfaces. Cross-sectional analysis showed a lower degree of deformation under the surface in the B and C added alloys, attributed to the presence of the reinforcing phases and the strengthening of the matrix promoted the transition from adhesive wear to oxidative wear.

The effects of microstructures on the mechanical performances and fracture mechanisms of boron-alloyed ferritic and martensitic stainless steels fabricated by powder metallurgy

Porosity is generally the major cause of the inferior mechanical properties of powder metallurgy (PM) steels. Boron (B) is an effective element for improving the sintering densification of PM steel via liquid phase sintering (LPS). To date, the microstructure and mechanical properties of B-alloyed PM stainless steels remain to be determined. The objective of this research was to study the influences of B on the LPS, microstructures, mechanical performances, and fracture mechanisms of PM 410L ferritic and 410 martensitic stainless steels. The roles of the Fe-based matrix, eutectic areas, and pores in the fracture behavior and mechanical properties were examined.The results showed that after 1250 °C sintering, the 0.6 wt% B additive in 410L and 410 obviously induced LPS and decreased the porosity from 8.9 vol% to less than 4 vol%. In both 410L+0.6B and 410+0.6B, the B-containing compounds were identified by electron backscatter diffraction as M2B boride with a tetragonal structure. The 0.13 wt% C additive in 410L+0.6B increased the ultimate tensile strength from 420 MPa to 843 MPa but decreased the elongation from 10.4% to 2.7% and the impact energy from 21 J to 6 J due to the transformation of the Fe-based matrix from ferrite to martensite. In the combination of fracture analyses and strain distribution as a function of tensile stress, the results indicated that in 410L+0.6B and 410+0.6B, the predominant sites for strain localization and fracture were respectively the ferritic grains and intergranular eutectic areas. Furthermore, the pores played different roles in the deformation and fracture of the two B-alloyed PM stainless steels.

Microstructure and Friction Response of a Novel Eutectic Alloy Based on the Fe-C-Mn-B System.

This paper focuses on the microstructure and tribological properties of novel hardfacing alloy based on Fe-C-Mn-B doped with Ni, Cr, and Si. The 4 mm-thick coating was deposited on the AISI 1045 carbon steel by the MIG-welding method using flux-cored wires in three passes. The transition zone thickness between the weld layers was ~80 μm, and the width of the substrate-coating interface was 5-10 μm. The following coating constituents were detected: coarser elongated M2B borides, finer particles of Cr7C3 carbides, and an Fe-based matrix consisting of ferrite and austenite. The nanohardness of the matrix was ~5-6 GPa, carbides ~16-19 GPa, and borides 22-23 GPa. A high cooling rate during coating fabrication leads to the formation of a fine mesh of M7C3 carbides; borides grow in the direction of heat removal, from the substrate to the friction surface, while in the transition zone, carbides become coarser. The dry sliding friction tests using a tribometer in PoD configuration were carried out at contact pressure 4, 7, 10, and 15 MPa against the AISI 1045 carbon steel (water-quenched and low-tempered, 50-52 HRC). The leading wear phenomenon at 4 and 7 MPa is fatigue, and at 10 and 15 MPa it is oxidation and delamination.

A new Fe-Cr-Mo-B-Al steel with outstanding tribo-corrosion resistance in liquid aluminium

The tribo-corrosion of a metal by liquid aluminium is problematic; thus, a new Fe-Cr-Mo-B-Al steel was constructed, and its tribo-corrosion behaviours in liquid aluminium were investigated. Our results show that this steel displays outstandingtribo-corrosion resistance in liquid aluminium, mainly arising from the coordinated effect of rod-like Cr-rich M2B (M=Fe, Cr, Mo) borides and irregular blocky Mo-rich M2B borides. The interaction between corrosion and wear primarily contributed to the mass loss of this steel via the degradation of the borides coupled with the fracture and spallation of the corrosion products.

Damage and Fracture in Creep of a Cast B1914 Superalloy

Damage and fracture processes in high temperature creep of an investment cast B1914 Ni-based superalloy with the increased amount of boron to 0.08wt.% for high temperature applications were analysed. Constant load creep tests in tension were conducted at temperatures from 800 to under applied stress ranging from 150 to 700 MPa. The microstructure of fractured specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. Microstructure investigation showed that the microstructure of the B1941 superalloy consists of a gamma (γ) phase with a dendritic structure and gamma prime (γ ́) phase with a cuboidal shape. Precipitates of γ ́and a lamellar eutectic, composed of γ/(Mo,Cr,Ni)3B2, were identified in the interdendritic region. Creep damage and fracture are closely connected with decohesion of the interface between M3B2 boride and matrix.

The microstructure evolution and precipitation behavior of TiB2/Inconel 718 composites manufactured by selective laser melting

Additive Manufacturing is an advanced material processing technology emerging in recent years. However, coarse columnar grain structure of additive manufactured metals reduces the performance of the material. Facilitating the columnar to equiaxed transition (CET) has become a research hotspot. In this paper, the equiaxed grain of additive manufactured Inconel 718 was achieved by adding TiB2 particles. Simultaneously, the microstructure evolution and precipitation behavior with nano-TiB2 particles addition were studied. The results show that the grain morphology of Inconel 718 exhibited coarse columnar grains with epitaxial growth and strong <100> texture. When 3 wt% TiB2 was added, the columnar grains became finer, further increased the content of TiB2 to 5 wt%, the equiaxed grains were obtained and the texture intensity decreased significantly. In addition, TiB2 partially melted under the high laser energy changed the phase precipitation behavior of Inconel 718 from chain-like Laves phase to reticulated (γ + M3B2) eutectics, which lead to the hardness increased from 276 HV to 578 HV when 5 wt% TiB2 was added. After heat treatment, the precipitation of high density γ′ and γ″ strengthening phase were promoted the hardness increased by 87% to 516 HV in Inconel 718. However, the hardness of TiB2 reinforced samples showed a little change, which could be attributed to the partial dissolution of M3B2 boride during the heat treatment.

Development of a novel method for measuring the interfacial creep strength of laser cladding coatings

In this study, the effect of IN713 coating applied by the laser cladding method on the creep behavior of GTD-111 superalloy by a small punch creep test was investigated. For this purpose, the creep behavior of the specimens in different parts in three conditions of casting, solution heat treatment, and aging was compared. The results showed that the specimen selected from the coating-substrate interface shows the highest creep resistance among other parts. This is attributed to the obstacle to the movement of dislocations by the coating-substrate interface from the substrate to coating. Also, in the aged specimens, due to the increase in γ′ phase volume fraction, the highest creep resistance was obtained. The heat-affected zone (HAZ) of the casting specimen had the lowest creep position due to the formation of partial liquation and cracking by γ-γ′ eutectic, M5B3 boride, Ni-Zr interface, and MC carbide. This problem was solved by performing solution heat treatment before coating and aging after coating.

Boronizing of CoCrFeMnNi High-Entropy Alloys Using Spark Plasma Sintering

In this study, we investigated the formation of a protective coating on a face-centered cubic high-entropy alloy (HEA). The coating was formed by a diffusion coating method. In the conventional diffusion coating method, the degradation of the mechanical properties of the base material owing to prolonged high-temperature treatment is a major issue. Therefore, we formed a ceramic layer using spark plasma sintering (SPS), which suppresses grain growth with rapid heating and enables fast, low-temperature processing. The objective of this study was to form borides on the surface of CoCrFeMnNi HEAs using the SPS method and to investigate their properties. A CoCrFeMnNi HEA prepared by the casting method was used as the base material, and a powdered mixture of B4C and KBF4 was used as the boron source. The analysis of the surfaces of the SPS-treated samples revealed the formation of M2B, MB, and Mn3B4-type borides on the HEA surface. The surface hardness was 2000–2500 HV owing to the formation of a ceramic layer on the HEA surface, and elemental analysis showed that certain elements exhibited characteristic diffusion behaviors.