Grain Boundary Embrittlement Research Articles

Combined effects of metallic dopants and nonmetallic impurities on interface cohesion in tungsten alloys by first-principles

The effect of element segregation on interface cohesion demonstrates significant potential in tailoring the mechanical performances of materials. In this study, we have investigated the impact of co-segregation of transition metal (Re, Zr and Ti) and non-metallic impurity elements (O and C) on the cohesion properties of two typical interfaces, W/HfC phase boundary (PB) and Σ5(310) grain boundary (GB) in W alloys, using first-principles calculations. Our findings reveal that O atom exhibits comparable segregation tendency at both the PB and GB interfaces, but the PB has a stronger resistance to O-embrittlement than the GB. C atom preferentially segregates at the GB and enhances the interface cohesion. In addition, Re atoms tend to segregate at both the interfaces and enhance the interface cohesion. Co-segregation of Zr/Ti and O atoms at the interface leads to a reduction in impurity O concentration within the W matrix, and further decreases the interface cohesion. In contrast, C atom mitigates the GB embrittlement induced by Zr/Ti atom owing to the formation of W-C bonds. This work deepens the understanding of how the co-segregation of alloying and non-metallic impurity elements affects the interface properties, offering theoretical guidance for optimizing the mechanical performance of W-based materials.

Laser powder bed fusion additive manufacturing of molybdenum using a nitrogen build atmosphere

A method was developed to suppress the crack formation during laser powder bed fusion (LPBF) of molybdenum (Mo). The method uses a nitrogen (N2) atmosphere to fabricate crack-free samples without any additional processing. To compare with the effect of the N2 atmosphere, samples were prepared under an argon (Ar) atmosphere as well with the same process parameters. The microstructure analyses by optical and high-resolution electron microscopy revealed that crack-free samples were produced under this condition, whereas grain boundary cracks were observed throughout the sample fabricated in Ar atmosphere. To understand the mechanism of crack prevention by nitrogen (N), the energetics of solute components in the BCC-Mo structure were studied by Density Functional Theory (DFT). The theoretical calculations showed that the introduction of N into the BCC-Mo hinders oxygen (O) diffusion, causing O entrapment in the lattice at interstitial positions. As a result, reduced oxides within grains and at grain boundaries (GB) were observed. The results are consistent with the experimental observation made from the fracture surface investigation. The reduced embrittlement of GB from the reduced GB oxide formation is believed to suppress GB cracking.

First-principles study of oxygen segregation and its effect on the embrittlement of molybdenum symmetrical tilt grain boundaries

Elemental segregation can pronouncedly affect the cohesion of grain boundary (GB), which shows tremendous potential in altering the thermal stability and mechanical properties of metals. In this work, we systematically investigate the segregation behavior of oxygen (O) atoms at a series of GBs in molybdenum (Mo), using first principles calculations, to reveal the effect of O segregation on the embrittlement of Mo GBs. It is found that O atoms energetically prefer to segregate at sites in both the GB core and the vicinity of the GB plane. The strengthening energy is strongly associated with the GB structure, and it linearly decreases with the continuous deformation of the GB. O segregation can lead to an increase in the volume of polyhedron sites and a decrease in the charge density among adjacent Mo atoms across the GB plane. The weakened Mo-Mo bonds are believed to disrupt the GB cohesion, thereby increasing the intergranular cleavage tendency of Mo. Interestingly, O segregation does not always trigger GB embrittlement. The Σ5(310)[001] symmetric tilt grain boundary (STGB) with an O atom doped at the cap trigonal prism (CTP) site possesses a strength as strong as the clean GB. O segregation promotes the generation of Mo-O bonds across the GB plane, which counteracts the adverse effect of weakened Mo-Mo bonds. This work deepens the understanding of the possible effect of O segregation on GB cohesion of Mo from the atomic and electronic scales, providing guidance for enhancing the mechanical properties of Mo via GB structure optimization.

First-principles study on the corrosion resistance of iron oxide surface and grain boundary in austenitic steel to lead–bismuth eutectic

In this paper, the corrosion properties of austenitic steel with and without protection of Fe3O4 scale was investigated based on first-principles calculations. The results show that lead–bismuth eutectic (LBE) is only adsorbed at some local positions on Fe3O4 scale, but is stably adsorbed at various sites on the Fe surface. The adsorption of LBE reduces the escape energy of Fe on Fe3O4 scale and pure Fe surface, and pure Fe surface is more susceptible to LBE corrosion. Moreover, the interaction between common alloying elements (Ni, Cu, Mo, Mn, Cr, Nb, V, Si, and Al) and oxygen atoms are investigated. Among them, Ni and Cu are weakly bound to O and most prone to escape from the Fe3O4 scale. Mo, Mn, and Cr have a significant escape tendency, while Nb, V, Si, and Al are not susceptible to escape. LBE will embrittle the grain boundary (GB) under failure of oxide layer. Ti, Cr and Ni all improve the strength of the GB structure, but the strengthening effect is limited, and GB embrittlement is dominated by Pb and Bi.

Atomic scale understanding of the role of hydrogen and oxygen segregation in the embrittlement of grain boundaries in a twinning induced plasticity steel

High strength twinning induced plasticity (TWIP) steels have potential for applications in the automotive industry. However, they are prone to hydrogen embrittlement (HE) and galvanic corrosion. Here, we report on the susceptibility towards HE and oxidation of a model Fe 27Mn 0.3C (wt%) TWIP steel by atom probe tomography. We measured the segregation of hydrogen and oxygen at grain boundaries, which appears correlated with manganese depletion. Our study suggests a correlation between HE and oxidation mechanisms in TWIP steels, which can combine to favor the previously reported hydrogen enhanced decohesion of grain boundaries.

Effects of transition metal segregation on the thermodynamic stability and strength of Ni Σ11 [110](113) symmetrical tilt grain boundary

Precisely controlling grain boundary (GB) thermodynamic stability and strength is important for processing nanograined alloys. The segregation-induced stabilization and strengthening GB mechanisms of 20 transition metal elements in Ni Σ11 [110](113) STGB with low interfacial excess energy were clarified from first-principles total-energy calculations in this work. The results indicate that the segregation energy and GB energy are inversely related to the radius difference between TM and matrix Ni atoms. The segregation of 11 elements were identified to enable the Ni Σ11 [110](113) STGB to be stable according to Schuh criterion for GB stability. The electronic mechanism of GB stabilization through TM segregation is mainly due to the less anti-bonding states of Ni-X pairs than those of Ni–Ni pairs in unsegregated GB, from the analysis of crystal orbital Hamiltonian populations. The GB embrittlement resulted from TM segregation is mainly from the mechanical contribution caused by atomic size mismatch between segregation atom and matrix atom. The segregation caused GB strengthening comes from the chemical contribution, which is due to the fact that the segregation of X leads to the formation of stronger Ni-X covalent-like bonds instead of Ni–Ni metallic bonds. This work could provide better understanding on designing high-stable Ni nanograined alloys.

A Co-rich chemically complex intermetallic alloy with extraordinary strength-ductility synergy

High-strength ordered intermetallic alloys are always sought-after in sophisticated structural applications. However, their practical applications have long been hindered by the severe grain-boundary embrittlement. Here, we designed a Co-rich chemically complex intermetallic alloy (CCIMA) with the L12-type ordered structure, which exhibits an ultrahigh tensile strength of ∼1,611 MPa and a substantial ductility of ∼37%. These exceptional strength-ductility combination outperforms the majority of common Co-rich intermetallic alloys. The multi-addition of Ta, Ti, and V elements is largely responsible for the significantly increased antiphase boundary energy of the L12 superlattice, which accounts for the high strength. This high degree of ductility is attributed to both the increased fracture resistance of grain boundaries brought about by co-segregation-induced order-to-disorder phase transformation, as well as the high work-hardening capability.

Trace Ce addition for inhibiting Bi segregation induced embrittlement in ultrahigh-purity copper

Bismuth (Bi), as an impurity element in copper and copper-based alloys, usually leads to the catastrophic intergranular fracture even with a ppm concentration. However, the intrinsic mechanisms of the embrittlement remain largely unclear and the control of such a brittleness has been rarely reported. Here, we investigate the influence of trace amounts of Bi on the ductility of ultrahigh-purity copper (99.99999%) at temperatures between room temperature (RT) and 900 °C. On this basis, the ductility improvement of CuBi alloy by adding different concentration of Ce is also systematically studied. Compared to the ultrahigh-purity copper, the Bi-containing alloy exhibits a remarkable decrease in ductility, with the severe reduction occurs at 450–900 °C. This is caused by the grain boundary (GB) segregation of Bi layers, which reduces the GB cohesion because of the weak BiBi bonds. However, after trace Ce are added, the ductility of CuBi alloy in the temperature range of 450–900 °C is significantly increased. The obvious enhancement is due to the reaction behavior of Ce and Bi to form the precipitated phases, as well as the preferential segregation of Ce to GBs and phase boundaries, which change the existing form of Bi in copper. Our findings provide a comprehensive understanding on the microstructural origin of Bi-induced GB embrittlement and an effective way for inhibiting such an embrittlement.

Investigation of the O phase in the Ti–22Al–25Nb alloy during deformation at elevated temperatures: Plastic deformation mechanism and effect on B2 grain boundary embrittlement

The Ti–22Al–25Nb alloy, which possesses a complex microstructure, is a new type of lightweight high-temperature structural material. Revealing the effects of microstructure and temperature on the mechanical properties of this alloy is crucial for applications in the aerospace industry. In this study, we designed three types of microstructures of the Ti–22Al–25Nb alloy, namely, single-phase B2, dual-phase α2 + B2, and triple-phase O + α2 + B2, and investigated their deformation and fracture behavior with increasing temperature (25,750,and 930 °C). The O + α2 + B2 sample possessed the best comprehensive mechanical properties, and the softening of the O phase at elevated temperatures was mainly due to massive {001}<110>O basal slip and twinning. The evolution of O phase twins with increasing temperature can be described as follows: (021) → (021) + (110) → (110). At 750 °C, (021) twins formed in the strip-shaped O phase in B2 grains, while (110) twins formed in the spheroidized O phase at B2 grain boundaries. In addition, B2 grain boundary embrittlement was observed at elevated temperatures, and its mechanism can be summarized as follows. Elevated temperatures induced element segregation at the grain boundaries between hypersaturated-state B2 grains, enriching Ti and Al and diffusing Nb into the B2 grains on the nanometer scale, leading to a thin-shell O phase with a thickness of tens of nanometers precipitating at the B2 grain boundaries. During elevated-temperature fracturing, cracks propagated quickly along the interface between the thin-shell O phase and B2 phase, leading to embrittlement of the B2 grain boundary. The precipitation of many fine granular O or α2 phases at the B2 grain boundaries can inhibit the precipitation of the thin-shell O phase and hinder the propagation of cracks along the grain boundaries. Therefore, the microstructure of this alloy should be controlled to precipitate many fine granular O or α2 phases to occupy the B2 grain boundaries and inhibit its elevated-temperature embrittlement.

Defeating hydrogen-induced grain-boundary embrittlement via triggering unusual interfacial segregation in FeCrCoNi-type high-entropy alloys

Metallic materials are mostly susceptible to hydrogen embrittlement (HE), which severely deteriorates their mechanical properties and causes catastrophic failures with poor ductility. In this study, we found that such a long-standing HE problem can be effectively eliminated in the Fex(CrCoNi)1-x face-centered-cubic (fcc) high-entropy alloys (HEAs) by triggering the localized segregation of Cr at grain boundaries (GBs). It was revealed that increasing the Fe concentration from 2.5 to 25 at. % leads to substantially improved HE resistance, i.e., the ductility loss decreases from 70% to 6%. Meanwhile, the fracture mode transformed from the intergranular to the transgranular mode. Multiscale microstructural analyses demonstrated that the Fe2.5Cr32.5Co32.5Ni32.5 and Fe25Cr25Co25Ni25 alloys show negligible differences in the phase structure, grain size, and grain-boundary (GB) character. However, interestingly, the near atomic-resolution elemental mapping revealed that an increased Fe concentration promotes the nanoscale Cr segregation at the GBs, which is primarily motivated by the strong repulsive force between Cr and Fe and the low self-binding energy of Cr. Such unusual interfacial segregation of Cr, which has not been reported before in the Fe25Cr25Co25Ni25 alloy, helps enhance the GBs’ cohesive strength and suppresses the local hydrogen segregation at GBs due to the deceased GB energy, leading to the outstanding HE resistance. These findings decipher the origins of the vastly-improved HE resistance in current FeCrCoNi-type HEAs, and meanwhile, provide new insight into the future development of novel high-performance structural alloys with extraordinary immunity to hydrogen-induced damages.

The application of in situ TEM on the grain boundary brittleness of precipitation-strengthened Ni-based superalloys: Recent progress and perspective

Due to their superior strength and oxidation resistance at high temperatures, precipitation-strengthened Ni-based superalloys are widely used in load-bearing hot components in energy generation systems, such as gas/steam engines in aircraft or power plants. However, brittleness originating from a grain boundary (GB) in a certain temperature range is one of the greatest deadlocks, which desperately restricts their thermal-mechanical processing capabilities and also industrial applications. Experimental and theoretical investigations of the origin of GB brittleness with aims to overcome it still attract many research efforts in the high-temperature material field. It is desirable to understand the GB embrittlement mechanism by dynamically investigating the entire GB cracking process in real time under stress/temperature combination, which might be hardly revealed by a traditional experiment on precipitation strength/theoretical technologies. Current advances in high-temperature mechanical testing systems, which can be operated in a transmission electron microscope (TEM), provide unique opportunities for in situ exploration of the mechanistic origins of GB brittleness of superalloys with the resolution up to an atomic scale. Here, we first briefly give an overview of the phenomenon and current understanding on GB brittleness, followed by introducing the state-of-art techniques in an in situ TEM/mechanical testing system (MTS). In the end, we will also discuss the potential application of the in situ TEM/MTS on GB brittleness and a perspective overlook.

Analysis of grain boundary embrittlement by Cu and Sn in paramagnetic γ−Fe by first-principles computational tensile test

Ironmaking processes using steel scrap are becoming increasingly important in the effort to realize a decarbonized society, but their utilization is limited by the surface hot shortness related to cracking at paramagnetic \ensuremath{\gamma}-Fe grain boundaries (GBs) due to tramp elements such as Cu and Sn. To understand its electronic origin, GB embrittlement due to Cu and Sn was investigated by first-principles computational tensile tests (FPCTTs) on paramagnetic \ensuremath{\gamma}-Fe GBs. The paramagnetic \ensuremath{\gamma}-Fe GB was modeled as the $\mathrm{\ensuremath{\Sigma}}5(310)\phantom{\rule{0.28em}{0ex}}\mathrm{GB}$ in the antiferromagnetic double-layer (AFMD) configuration. Both Cu and Sn reduce the fracture stress, fracture energy, and fracture strain of the \ensuremath{\gamma}-Fe GB. The effect of Sn was more pronounced than that of Cu, which agrees well with experimental results on surface hot shortness. Crystal orbital Hamiltonian population (COHP) analysis of the electronic structure during the tensile processes revealed that the atomic orbitals of Cu are more localized than those of Fe in \ensuremath{\gamma}-Fe, resulting in the breaking of Cu--Fe bonds at a lower strain and stress, which leads to GB embrittlement. In addition to this effect, Sn increases the bond lengths between adjacent Fe atoms and weakens the Fe--Fe bonds, causing significant GB embrittlement. The obtained knowledge contributes to the elucidation of the mechanism of the surface hot shortness. Furthermore, the FPCTT using the \ensuremath{\gamma}-Fe GBs in the AFMD configuration is useful to investigate the effects of various solute elements on paramagnetic \ensuremath{\gamma}-Fe GBs, and is expected to be utilized to improve material properties governed by GB fracture.

Statistical perspective on embrittling potency for intergranular fracture

Embrittling potency is a thermodynamic metric that assesses the influence of solute segregation to a grain boundary (GB) on intergranular fracture. Historically, authors of studies have reported embrittling potency as a single scalar value, assuming a single segregation site of importance at a GB and a particular cleavage plane. However, the topography of intergranular fracture surfaces is not generally known a priori. Accordingly, in this paper, we present a statistical ensemble approach to compute embrittling potency, where many free surface (FS) permutations are systematically considered to model fracture of a GB. The result is a statistical description of the thermodynamics of GB embrittlement. As a specific example, embrittling potency distributions are presented for Cr segregation to sites at two Ni $\ensuremath{\langle}111\ensuremath{\rangle}$ symmetric tilt GBs using atomistic simulations. We show that the average embrittling potency for a particular GB site, considering an ensemble of FS permutations, is not equal to the embrittling potency computed using the lowest energy pair of FSs. A mean GB embrittlement is proposed, considering both the likelihood of formation of a particular FS and the probability of solute occupancy at each GB site, to compare the relative embrittling behavior of two distinct GBs.

First-principles computational tensile test of γ-Fe grain boundaries considering the effect of magnetism: Electronic origin of grain boundary embrittlement due to Zn segregation

The development of high-strength steels requires detailed understanding of the effect of solute elements on \ensuremath{\gamma}-Fe grain boundaries (GBs). In this study, first-principles computational tensile tests (FPCTT) were conducted on \ensuremath{\gamma}-Fe GBs to elucidate the mechanism of GB embrittlement due to Zn segregation. The paramagnetic \ensuremath{\gamma}-Fe GB was simulated by the $\mathrm{\ensuremath{\Sigma}}5\phantom{\rule{0.28em}{0ex}}(310)$ GB in the antiferromagnetic double-layer (AFMD) configuration. The FPCTTs revealed that the fracture stress and fracture energy of the \ensuremath{\gamma}-Fe GB were reduced by Zn segregation, which is consistent with experimental results. Crystal orbital Hamilton population analysis was also performed to investigate the change in electronic states during the tensile process, and the enhancement of GB fracture by Zn segregation is caused by the breaking of the covalent-like bonds between Fe and Zn at a relatively small strain compared to the Fe--Fe bonds. This behavior is attributed to the localized nature of the $3d$ orbitals of Zn in \ensuremath{\gamma}-Fe. The FPCTTs of \ensuremath{\gamma}-Fe GBs using the AFMD properly consider the effect of magnetism in paramagnetic \ensuremath{\gamma}-Fe under tensile strain and is useful for investigating the effects of various solute elements on GB fracture and for the development of high-strength steels.

A fully coupled chemo-mechanical cohesive zone model for oxygen embrittlement of nickel-based superalloys

For nickel-based superalloys subjected to high temperatures and oxygen-rich environments, mechanical loading in combination with oxygen diffusion along grain boundaries leads to an acceleration of crack propagation. To account for these phenomena, a fully coupled thermodynamically consistent chemo-mechanical modeling framework for stress-assisted oxygen embrittlement of grain boundaries in polycrystals is proposed. We formulate an extended cohesive zone model where the grain boundary strength is reduced by the presence of oxygen and the oxygen diffusion is enhanced by tensile mechanical loading. We show that the model can qualitatively predict experimental results such as: reduction of ultimate tensile strength and accelerated crack growth due to dwell time combined with mechanical loading and saturation of crack growth rates for increasing environmental oxygen pressure levels. In addition, numerical simulation results of intergranular crack growth are shown for a 2D polycrystalline structure. An emphasis is put on the difference in cracking behavior after dwelling with or without mechanical loading.

Relation of embrittlement to phosphorus grain-boundary segregation for an advanced Ni–Cr–Mo RPV steel

Embrittlement is an essential issue for reactor pressure vessel (RPV) steels. Grain-boundary segregation (GBS) of impurity phosphorous is one of the major reasons for non-hardening embrittlement of an RPV steel. The main aim of the present work is to establish a relation of embrittlement to phosphorous GBS for an advanced SA508-4N Ni–Cr–Mo RPV steel so that one can forecast its embrittling tendency. To obtain this type of relation, the steel samples doped with phosphorous are quenched from 890 to 1100 °C to obtain different prior austenite grain sizes (PAGSs) and then tempered at 650 °C, followed by ageing at different lower temperatures so as to achieve different phosphorous segregation levels at prior austenite grain boundaries. The characterization work is done with the use of mini-Charpy impact tests (specimen: 2.5 mm × 2.5 mm in cross section) together with Auger electron spectroscopy, scanning electron microscopy, and optical microscopy. The results indicate that there is a linear relationship between ductile-to-brittle transition temperature (DBTT/oC) and phosphorous GBS concentration (Cp/at.%) for both cases (quenching from 890 °C: PAGS = 34 μm; quenching from 1100 °C: PAGS = 112 μm). The relations can be expressed as DBTT=13.13Cp−335.70 (PAGS = 34 μm) and DBTT=6.69Cp−223.87 (PAGS = 112 μm). The rate of material embrittlement, characterized by the slope of the line, is associated with both the phosphorous segregation-induced grain-boundary (GB) embrittlement and the total GB area per unit volume. Interestingly, there is a critical phosphorous GBS concentration, below which the DBTT of the coarse-grained specimen is higher due to its higher intrinsic brittleness and above which that of the fine-grained specimen is higher at the same phosphorous GBS level due to the considerable phosphorous segregation-induced GB embrittlement along with a much larger total GB area per unit volume.

Microstructural stability and mechanical properties of GH742 Ni-based wrought superalloy for turbine disk applications

Microstructural stability and mechanical properties were investigated in a Ni-based wrought superalloy during long-term thermal exposure at 650 °C and 700 °C for up to 7500h. The results demonstrate that the morphology of primary γ′ precipitate has no significant change after long-term thermal exposure, and it exhibits excellent thermal stability both at 650 °C and 700 °C. However, the morphology of secondary γ′ precipitates transformed from complex flower-like to rounded cubic during long-term exposure at 700 °C. The coarsening of tertiary spherical γ′ precipitates occurs with increasing thermal exposure time, and its coarsening behavior is consistent with Ostwald ripening theory. M23C6 carbides coarsen at grain boundaries and form continuous film with increasing time, resulting in the embrittlement of grain boundary. The multimodal distribution of γ′ precipitates results in high strength of the alloy after long-term thermal exposure. Both the strength of the alloy after exposure at 650 °C and 700 °C for 7500 h are not less than that of the alloy after standard heat treatment. The stress rupture lifetime tests at 650 °C/873 MPa increases firstly and then decreases with increasing exposure time at 650 °C and 700 °C. The increase of stress rupture lifetime is attributed to the supplementary precipitation and growth of finest tertiary γ′ precipitates, while the decrease of it is attributed to the γ′ evolution and the coarsen of M23C6 carbides.

Effect of S on H-induced grain-boundary embrittlement in γ-Fe by first-principles calculations

The effect of interstitial impurities (H and S) on the atomic, electronic structure, and mechanical properties of the γ-Fe Σ5 (021) grain boundary (GB) was investigated via first-principles calculations. H atoms act as an intergranular embrittler in the Σ5 GB due to a reduction in the charge density between the Fe atoms connected to the grains, whereas H and S co-segregation produces more pronounced embrittlement behavior, resulting in intergranular fracture. The S-induced embrittlement plays a crucial role in the H and S segregation, due to a combination of the structural and chemical effects. The fracturing of Σ5 GB due to S and H segregation is a two-step process. The first step is the breaking of Fe–Fe bonds in the GB, followed by the breaking of the remaining Fe–S bonds in the second step, resulting in the complete separation of the two grains. Moreover, the S atom can slightly compensate for the embrittlement induced by H, because some of the Fe atoms that obtain electrons from the S atoms can provide more electrons to the H atoms, and thus, they can compensate for the electrons that must be acquired from other Fe atoms. We call this “the electrons compensating effect” and this effect is helpful in the design and alloying of steels that are resistant to H embrittlement.

Mechanical and Fracture Properties of Polycrystalline Graphene with Hydrogenated Grain Boundaries

Molecular dynamics (MD) is employed to study the mechanical and fracture properties of polycrystalline graphene with hydrogenated grain boundaries. Polycrystalline graphene sheets with an average grain size of 4, 6, and 8 nm are considered. The impact of hydrogenation percentage on the mechanical strength and fracture toughness of polycrystalline graphene sheets is investigated. Polycrystalline graphene with and without an initial crack are considered. The results show that due to the hydrogenation the strength of polycrystalline graphene sheets reduces. The impact of hydrogenation is significantly greater on the strength of polycrystalline graphene without an initial crack. The strength of polycrystalline graphene sheets with hydrogenated grain boundaries decreases with increase in temperature. The impact of temperature on the strength increases with increase in hydrogenation percentage at the grain boundaries. The results also show that the hydrogenation of the polycrystalline graphene affects the crack propagation path. As the hydrogenation percentage of grain boundaries increases, the crack path changes from intragranular to intergranular. Furthermore, the increase in the hydrogenation reduces the critical stress intensity factor of polycrystalline graphene. Such changes in the strength, fracture toughness, and crack path are due to the hydrogen embrittlement of grain boundaries which reduces their fracture energy.

Effect of element Te on alterations of microstructure and mechanical property of nickel-based superalloy Inconel 718 through alloy infiltration

Alloying elements have significant effects on material physical and mechanical properties. The present work studies the effect of element Te on the alterations of microstructure and mechanical property of nickel-based superalloy Inconel 718. Te was diffused into the surface layer of treated samples with a new alloy infiltration method. The microstructure and element distribution within the modified layer were tested and quantitatively characterized with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The depth of infiltration layer presented an approximately linear relationship with the treating temperature range from 700 °C to 900 °C. Furthermore, the mechanical property of the modified layer was evaluated through nano-indentation experiments. The results show that the Te infiltration into the surface layer of Inconel 718 presents a gradient distribution from the outmost surface to the matrix, and it results in a substantial weakening of the modified layer. A gradual decrease in nano-hardness was detected from the surface to the matrix, and the nano-hardness value of infiltration layer was weakened by 31% for the Te infiltration treatment at 900 °C. The underlying weakening mechanism of Te on the superalloy Inconel 718 was revealed. Te element penetrates along the grain boundary leading to the formation of new intercrystalline phases. Consequently, the grain boundary is “opened” which causes the embrittlement of grain boundaries and weakening of the treated surface. This study can not only help to understand the influence of alloying elements on the variation of material properties, but also provide guidance for design of material compositions.