Precipitation Of Borides Research Articles

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.

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.

Microstructural Evolution and Mechanical Properties of a Ni-Based Alloy with High Boron Content for the Pre-Sintered Preform (PSP) Application

The pre-sintered preform (PSP) is an advanced technology for repairing the Ni-based superalloy blade in a turbine. In general, boron is added to the Ni-based superalloys in small quantities (<0.1 wt.%) to increase boundary strength and cohesivity. Despite this, the effect of high B content (>1.0 wt.%) on the microstructure evolution and mechanical properties in Ni-based superalloys for the PSP application is rarely studied. The variety, composition and evolution of the precipitates during solution heat treatment in the alloy with high B content were determined by EBSD, EPMA and SEM. The results indicate that Cr, W and Mo-rich M5B3 type borides precipitate from the matrix and its area fraction reaches up to about 8%. The area fraction of boride decreases with the prolonging of solution time and the increase of temperature higher than 1120 °C. The borides nearly disappear after solution treatment at 1160 °C for 2 h. The redissolution of boride and eutectic results in the formation of B-rich area with low incipient melting (about 1189 °C). It can bond metallurgically with the blade under the melting point of the blade, which decreases the precipitation of harmful phases of the blade after PSP repairing. The microhardness within the grain in the PSP work-blank first decreases (lower than 1160 °C) and then increases (higher than 1185 °C) with the increase of solution heat treatment temperature due to the dissolving and precipitation of borides. The tensile strength of the combination of PSP work-blank and Mar-M247 matrix at room temperature after solution treatment is related to the area fraction of boride, incipient melting and the cohesion between PSP work-blank and Mar-M247 matrix.

In-situ observation of the incipient melting of borides and its effect on the hot-workability of Ni-based superalloys

The excellent creep properties of advanced wrought Ni-based superalloys are achieved by adding grain boundary strengthening elements such as B and C. Their low solubility in the γ matrix may promote precipitation of micrometre-sized borides and carbides which potentially deteriorate hot-workability, e.g. through incipient melting. However, their incipient melting has not yet been directly linked to a decrease in hot-workability in Ni-based superalloys. Here, we study the susceptibility of two Ni-based superalloy variants with main differences in B and C contents (high B & C variant with>triple B and C contents) to incipient melting and its effect on hot-workability. We show that incipient melting causes severe intergranular cracking at low strains during compression at 1200 °C in the high B & C variant only. Compression to higher strains separates grain boundaries fully, partially pulverising the macroscopic specimen. Metallographic specimens already show incipient melting at heating rates they experience when put into a pre-heated furnace. During subsequent cooling, the melt re-solidifies and forms lamellar structures consisting of various phases rich in Mo-Cr-B, Zr, and Ti, the constituents of M3B2 and MC. In-situ observations using a customised high temperature confocal laser scanning microscope set-up unambiguously confirmed the origin of these lamellar structures. We demonstrate for the first time directly the constituent liquation of M3B2. This liquation is assumed to be aided by the partial dissolution of adjacent MC carbides into the melt. These results will help to improve thermo-mechanical simulations and microstructural control during industrial processing of advanced wrought superalloy parts.

Microstructure and Mechanical Properties of Intergranular Boride Precipitation-Toughened HfMoNbTaTiZr Refractory High-Entropy Alloy.

To develop strong refractory high-entropy alloys for use at elevated temperatures as well as to overcome grain-boundary brittleness, an equimolar HfMoNbTaTiZr alloy was prepared, and a minor amount of boron (0.1 at.%) was added into the alloy. The microstructures of the alloys were characterized, and their macro-to-microscale mechanical properties were measured. The microstructural observations indicated that the matrices of both the alloys were composed of a body-centered cubic solid-solution structure, and the added boron induced the precipitation of hexagonal close-packed borides (most likely the (Hf, Zr)B2) at the grain boundaries. The modulus and hardness of differently oriented grains were about equivalent, suggesting a diminished anisotropy, and many small slips occurred on multiple {110} planes. While the hardness of the matrix was not increased, the intergranular precipitation of the borides markedly raised the hardness of the grain boundaries. Owing to the enhanced grain boundary cohesion, the work hardenability and ductility were effectively improved with the addition of boron.

Laser Boronizing of Additively Manufactured 18Ni-300 Maraging Steel Part Surface.

The problem of insufficient wear resistance of maraging steels (MSt) has so far been solved mainly by the use of the thermochemical nitriding process, which has a number of limitations and disadvantages. In the present work, for MSt parts manufactured by laser powder bed fusion (LPBF), a more flexible laser alloying process was suggested as an alternative surface hardening process. The purpose of the present work is to give a better understanding on the possible hardening effect obtainable when amorphous boron is used as an alloying additive in relation with microstructural evolution and specific process parameters and to promote further development of this technology. For the alloying, a one kilowatt CO2 laser was applied at 0.5–4.0 mm laser spot and 250–1500 mm/min laser operating speed, providing 50,955–796 W∙cm−2 power density and 24.0–4.0 J∙mm−1 heat input. Before laser processing, surfaces were covered with amorphous boron. The appropriate melt pool geometry was obtained at 0.5 mm laser spot, for which XPS analysis revealed an increase in boron concentration from ~3.1 to ~5.7 wt.% with a laser speed increase from 500 to 1500 mm/min. XRD analysis revealed domination of Fe3B type borides along with the presence of FeB, Fe2B, Ni4B3 borides, austenitic and martensitic phases. The microstructure of modified layers exhibited evolution from hypoeutectic microstructure, having ~630–780 HK0.5 hardness, to superfine lamellar nanoeutectic (~1000–1030 HK0.2) and further to submicron-sized dendritic boride structure (~1770 HK0.2). Aging of laser-boronized layers resulted in the change of phase composition and microstructure, which is mainly expressed in a plenty precipitation of Mo2B5 borides and leads to a reduction in hardness—more significant (by ~200–300 HK0.2) for hypoeutectic and hypereutectic layers and insignificant (by ~50 HK0.2) for near-eutectic. With the application of the laser boronizing technique, the hardness of MSt parts surface was increased up to ~three times before aging and up to ~2.3 times after aging, as compared with the hardness of aged MST part.

Microstructure evolution and mechanical properties of a Fe, Cr-rich multiphase Ni3Al-based superalloy during transient liquid phase bonding process

In this paper, a Fe, Cr-rich multiphase Ni3Al-based alloy is joined through transient liquid phase bonding (TLP) technique. Different from typical γ+γ′ dual phase Ni3Al-based alloys, the joint can be divided into three distinct zones: isothermal solidification zone (ISZ), gamma prime zone (GPZ) and diffusion affected zone (DAZ). Interestingly, the massive fine grains with γ+γ′ dual phase are observed inside the ISZ, and the GPZ consists of blocky γ′ phase. The precipitation of Cr-rich borides in DAZ provides an advantageous compositional condition for the local coarsening of γ′ phase. In addition, hardness value of the bonded samples fluctuates from the centerline to the BM and reaches the peak at the DAZ. The bonded specimens share equivalent room-temperature tensile strength and elongation to failure with the parent metal after the weld thermal cycle. Moreover, the fracture path propagates along the base metal (BM), which indicates excellent tensile properties of the joint.

Preparation and characterization of novel TiB2-12(Fe-Co-Cr-Ni) cermets and their corrosion resistance in molten aluminum

Nine novel TiB2-12(Fe-Co-Cr-Ni) cermets were prepared using spark plasma sintering at 1400 °C, 1500 °C, and 1600 °C, and their corrosion behaviors were investigated in molten aluminum at 700 °C. The corrosion kinetics of these cermets exhibited a linear relation with the immersion time. A comparison of their corrosion rates suggested that their corrosion resistance could be improved by lowering the size of TiB2 powder, increasing the proportion of Fe-Co-Cr-Ni solid solution, and controlling the porosity of the cermet and the precipitation of secondary borides. The corrosion mechanism of the TiB2-12(Fe-Co-Cr-Ni) cermet in molten aluminum was also discussed.

Titanium Carbide Effects on the Fracture of Inconel 100—A Case Study

Carbides and borides appear as minor phases in Inconel 100, although the carbon content (0.18%) is relatively high in comparison to other nickel-base superalloys. The material properties of this alloy depend on a number of interrelated microstructural parameters, including the volume fraction of γ' to γ, grain size, elemental distribution, and precipitation of carbides and borides. This study presents a characterization and a failure analysis of Inconel 100 samples loaded to fracture at 760℃ and an examination of their fractography. Chemical analysis, optical metallography, SEM and EDS were used for the characterization of the unusual areas of fracture that were found on the samples. The thermodynamic stability of TiC led to the conclusion that the possibility of creating very large, stable mono carbides, especially TiC carbide, during the production process is the reason for the short time to failure obtained in this work.

Solid-State Phase Transformations Involving (Nb,Mo)2CrB2 Borides and (Nb,Ti)2CS Carbosulfides at the Grain Boundaries of Superalloy Inconel 718

The present paper studied the solid-state phase transformations occurring at the grain boundaries of a conventional cast-and-wrought alloy within the chemical specifications of superalloy Inconel 718. These phase transformations involved the precipitation of (Nb,Mo)2CrB2 borides and (Nb,Ti)2CS carbosulfides, which are usually phases rather associated to solidification phenomena than to solid-state phase precipitation in superalloy Inconel 718. The study started from a material solution-treated above the δ solvus. When this material was heated subsolvus δ from room temperature, (Nb,Mo)2CrB2 borides was found to precipitate at the grain boundaries, in addition to the δ phase. Then, when the material was reheated supersolvus δ after the subsolvus treatment, the complete dissolution of the δ phase and the partial dissolution of the (Nb,Mo)2CrB2 phase released niobium and titanium atoms in the grain boundaries. Both elements have a strong affinity with carbon and sulfur, and this favored the precipitation of secondary (Nb,Ti)C carbides and (Nb,Ti)2CS carbosulfides at grain boundaries. An analysis of the interactions between the segregating species C, B and S evidenced that the precipitation of (Nb,Mo)2CrB2 borides at grain boundaries was very likely made possible by a decreased carbon activity during the subsolvus δ treatment.

Creep induced precipitation of the (Cr,Mo)5B3-type boride in γ/γ′ eutectic of a Ni-based superalloy

Boron segregation and boride formation at grain boundaries are generally accepted in Ni-based superalloys. However, the addition of boron was found to result in the formation of a high density of nanosized M5B3 (where M is a mixture of Cr, Mo) particles in γ/γ′ eutectics of a Ni-based superalloy during high-temperature creep in this work. M5B3 phase precipitated at γ/γ′ eutectics keeps a well-defined orientation relationship with the eutectics, which is [001]γ/γ′//[001]M5B3, (020)γ/γ′//(130)M5B3. And these nanosized borides could block dislocation slip, result in stress concentration surrounding γ/γ′ eutectics and facilitate crack initiation at γ/γ′ eutectic/matrix interface during the creep at high temperatures.

The bonding temperature effect of the diffusion brazing Inconel 625 superalloy on the microstructure changes, corrosion resistance, and mechanical properties

The aim of this survey was an evaluation of the diffusion brazing (DB) bonding properties of the Inconel 625 superalloy using a BNi-3 interlayer. During the inter-diffusion and the dissolution of the alloy elements at the bonding temperature, and also cooling-cycle process, the joining zone (JZ) divided into three section namely isothermal solidification zone (ISZ), diffusion affected zone (DAZ), and athermal solidification zone (ASZ) which included a Ni-rich solid solution phase, Mo and Cr-rich boride precipitation, and Mo-rich boride precipitation and also Ni-Si binary phase, respectively. The cyclic potential-dynamic polarization tests indicated an inverse relationship between the corrosion behavior and the width of the ASZ. Also, the bonding process leads to the loss of resistance to pitting corrosion. The completion of the ISZ, together with the highest corrosion resistance attained at the bonding temperature at 1180 ᵒC. Likewise, with increasing bonding temperature, the eutectic compounds decreased and ISZ completed. The highest hardness belonged to the JZ/DAZ boundary at 1060 ᵒC and ASZ at 1120 ᵒC measured as 422 HV and 390 HV, respectively; which decreased by increasing bonding temperature, but the ISZ hardness increased.

Kinetics of glass transition, crystallization and soft magnetic properties of the Fe76.5Nb3B20Cu0.5 glassy alloys

Differential scanning calorimetry was used to investigate the non-isothermal crystallization kinetics of the Fe76.5Nb3B20Cu0.5 glassy alloys. The nanocrystallization products and soft magnetic properties as a function of annealing temperatures were also studied by using X-ray diffractometer and vibrating sample magnetometer. The activation energies for Eg, Ex, and Ep are equal to 499 ± 3, 558 ± 7, and 693 ± 2 kJ mol−1 by Kissinger method, respectively, and 512 ± 3, 571 ± 9, and 715 ± 2 kJ mol−1 estimated by Ozawa method, respectively. The results indicate that the glass transition process is easier than the nucleation and grain growth process, while the grain growth process is much difficult than the nucleation process. The local Avrami exponent n(x) varies with the crystallized volume fraction x, indicating that the crystallization process has different crystallization mechanisms. The n(x) ranging from 1.0 to 1.2 for 0.05 < x < 0.2 represents the growth of particles of appreciable initial volume. The 0.5 < n(x) < 1.0 for x ranging from 0.2 to 0.8 stands for the growth of a large number of the pre-existing nuclei. Isothermal annealing experiment indicates that the initial nanocrystallization product is α-Fe phase, and the final nanocrystallization products are composed of α-Fe, Fe2B and Fe3B phases. With increasing annealing temperature, both the saturation magnetization Ms and coercivity Hc are enhanced. High temperature annealing is found to weaken their soft magnetic properties, which is ascribed to the precipitation of boride.

Crystallization and magnetic hardening behaviour of Fe-rich FeSiBNb(Cu) melt-spun alloys

The sequential, multi-stage crystallization and magnetic hardening behaviour of Fe82B14Si2Nb2, Fe83B13Si2Nb2, Fe83B12Si2Nb2Cu1 and Fe85B13Nb2 melt-spun alloys have been investigated. The microstructure-crystallization-magnetic property relationship was established using X-ray diffractometry (XRD), differential scanning calorimetry (DSC), magnetometry, transmission electron microscopy (TEM) and magneto-optical Kerr effect microscopy (MOKE) techniques. The increase of Fe content (>82 at%) of as-quenched ribbons imparts microstructural heterogeneity across the ribbon cross–section; i.e., textured α-Fe crystals at the free surface to hetero-amorphous microstructure in the bulk matrix. The isochronal annealing of hetero-amorphous alloys depicts simultaneous surface and bulk crystallization process occurring before and after the crystallization onset temperature (Tx1) temperature. The annealing temperature range (Ta < Tx1) coinciding with the paramagnetic region of the thermo-magnetic plot, induces irreversible magnetic hardening due to simultaneous coarsening of pre-existing crystal nuclei and exchange de-coupling between nanocrystal and intergranular matrix. The onset of primary crystallization in the pre-crystallized ribbons results in bimodal nanocrystallite distribution having an average crystallite size exceeding the ferromagnetic exchange length of Fe-based alloys. The minor Cu addition alters the growth morphology of pre-existing nuclei from dendrite-like to equiaxed, assisting heterogeneous nucleation and improving intergranular amorphous stability by delaying boride precipitation. The soft-magnetic property deterioration of partially crystallized ribbons is discussed within the framework of Extended-Random Anisotropy models.

Tribological behavior of autocatalytic Ni–P–B coatings at elevated temperatures

Ni–P–B coatings are produced directly on surface of carbon steel (AISI 1040) using electroless plating technique. The effects of temperature on the friction and wear behavior of Ni–P–B alloy coatings have been investigated. Tribological tests are conducted at four temperatures (30, 100, 300 and 500 °C) under constant load and sliding speed in a pin-on-disk apparatus. Results show that Ni–P–B coatings sustain the elevated temperatures with minimal effect on friction and wear performances. Ni–P–B coating is found to exhibit the maximum hardness among the binary Ni–P and Ni–B coatings with as-deposited hardness reaching around 700HV. Heat treatment increases the hardness as well as wear resistance by precipitation of boride and phosphide phases of nickel. Room temperature test shows mostly adhesive wear pattern but finally, it leads to a mixture of abrasive–adhesive wear mechanism. Friction and wear of Ni–P–B is overall governed by the formation of oxidative layer, mechanically mixed layer with iron (from ferrous counter face), wear mechanism, phase transformation and changes in microstructure during elevated wear test. Phase transformation and corresponding micro-structural changes occurring during elevated test duration help in providing better wear resistance to the coating.

Investigation on Producing the Flux Cored Wire Modified with Concentrated Complex from Tomtor Mineral Assets

Multiple use of pan concentrate from placer deposits directly in technical process without prealltoment of pure components could become one of promising directions in the field of receiving wide range of multicomponent metal and ceramic powdered materials. In this paper as the object of comparative examination, we present the hemimethylated plating made of exploited experimentative flux cored wires based on industrial Ni-Cr-B-Si system powder modified with rare-earth elements of concentrated complex from Tomtor mineral assets Republic of Sakha (Yakutia). The structure of received plating is heterogeneous and has oxidation coating as boundaries. According to the data of the phase shift analysis plating predominantly consist of Fe-Ni austenitic alloy phases with the Cr, Si carbide and Cr boride precipitation strengthening, plating generally consist of Al2O3, Cr2O3 и Fe2O3 oxide. The research of the plating ultimate composition testifies the equable allocation of main and alloying elements, local location of some elements in the structure of the plating. Microhardness of flux cored wire plating substantially depends on composition and fabricated structure. It has been established that the introduction of rare earth elements provided the formation of strengthening phase with the high hardness of plating resulting in wear resistance increase.

Mechanical Properties and Structure of Laser Beam and Wide Gap Brazed Joints Produced Using Mar M247–Amdry DF3 Powders

The microstructure and mechanical properties of materials produced by wide gap brazing (WGB) and laser beam (LBW) cladding with different blends of Mar M247 and Amdry DF-3 brazing powders were studied. It was shown that LBW Mar M247-based materials comprised of 0.6 to 1 wt % B were weldable. The weld properties were superior to WGB deposits with the same bulk chemical composition, due to the formation of a dendritic structure typical for welded joints, and the precipitation of cuboidal borides of Cr, Mo, and W in the ductile Ni–Cr based matrix. Both materials were found to have useful properties for three-dimensional (3D) additive manufacturing (AM) and repair components manufactured from high gamma prime precipitation hardened superalloys.

Microstructural evolution mechanism during brazing of Hastelloy X superalloy using Ni–Si–B filler metal

ABSTRACTThe paper aims at understanding solidification phenomena and solid state precipitations during diffusion brazing of Hastelloy X nickel base superalloy using a ternary Ni–4.5Si–3.2 B (in wt-%) filler metal. The joint is featured by the formation of Ni-rich solid solution in an isothermal solidification zone, borides/silicide formation during eutectic-type solidification in an athermal solidification zone, on-cooling precipitation of fine nickel silicides in the joint centerline, in situ precipitation of Mo–Cr-rich borides in the diffusion affected zone and grain growth in the heat affected zone. The implication of the phase transformations on the joint properties is discussed. It is shown that Hastelloy X exhibited very fast isothermal solidification which can be attributed to its high Mo and Cr content that promotes in situ boride precipitation during brazing.

Vacuum brazing of GH99 superalloy using graphene reinforced BNi-2 composite filler

A novel graphene reinforced BNi-2 composite filler was developed for brazing GH99 superalloy. The interfacial microstructure of brazed joints was analyzed by field emission scanning electron microscope and a transmission electron microscope. The effects of graphene addition on the microstructure evolution and mechanical properties of brazed joints were investigated, and the strengthening mechanism of graphene was analyzed. The results revealed that due to the addition of graphene, M23(C,B)6 compounds were synthesized in the γ solid solution and brittle boride precipitates near the brazing seam decreased. Graphene was effective in retarding solute atoms diffusion thus impeding the precipitation of borides. Furthermore, the low coefficient of thermal expansion (CTE) of graphene was conducive to relieve stress concentration of the brazed joints during the cooling process. The shear strengths of brazed joints were significantly improved by exerting the strengthening effect of graphene. The maximum shear strengths of the brazed joints were 410.4 MPa and 329.7 MPa at room temperature and 800 °C, respectively.

Role of base-metal composition in isothermal solidification during diffusion brazing of nickel-based superalloys

ABSTRACTThe key feature of diffusion brazing, also referred to as transient liquid phase bonding, is isothermal solidification which precludes the formation of intermetallic in the joint centreline. Analysing the available data published in the literature showed that the composition of the nickel-based superalloys plays a strong role in determining the required time for obtaining intermetallic-free joint during diffusion brazing. This effect is not predictable by the standard conventional models. It is proposed that increasing the boride-forming elements in the base superalloy which promotes in situ boride precipitation at the diffusion-affected zone can accelerate the diffusion flux of the boron into the base superalloy, leading to faster isothermal solidification. The higher the Cr + Mo + Nb + Ta + W content in base superalloy, the shorter the isothermal solidification time.