Grain Boundary Wetting Research Articles

Liquid metal embrittlement and fracture behavior of three Mo metals in liquid lead-bismuth eutectic

In this work, liquid metal embrittlement (LME) and the related fracture behavior of Mo, TZM and Mo-14Re were assessed in oxygen-saturated and -depleted LBE at 350°C by means of slow strain rate tensile (SSRT) tests. Neither tensile mechanical property data nor fractographic micrographs show strong evidence of LME. Nevertheless, the observation of deep secondary cracking at the fiber-like grain boundaries of Mo and Mo-14Re tested in LBE suggests occurrence of grain boundary wetting (GBW). Moreover, oxidation-assisted crack initiation and propagation were also identified. Both fracture modes may be a concern when Mo metals with a fiber-like grain structure are used in high-oxygen LBE environment.

Understanding the growth of discontinuous precipitates in a Ni-45%Cr alloy driven by twins and stacking faults

The Growth of discontinuous precipitates (DPs) is crucial in alloys that are strengthened by the DPs. Our research has demonstrated that DPs nucleate at grain boundaries and grow along twins and stacking faults in the (111) direction as a result of the twinning effect. Moreover, the growth rate is influenced by the wetting of grain boundaries. It restates how DPs evolute and provides a strategy by manipulating twins and stacking faults.

Multiscale Investigation of Microcracks and Grain Boundary Wetting in Press-Hardened Galvanized 20MnB8 Steel

Grain boundary wetting as a preliminary stage for zinc-induced grain boundary weakening and embrittlement in a Zn-coated press-hardened 20MnB8 steel was analyzed using electron backscatter diffraction, Auger electron spectroscopy, energy dispersive X-ray analysis, atom probe tomography and transmission electron microscopy. Microcracks at prior austenite grain boundaries were observed. Structures that developed after microcrack formation were identified: for example, Zn/Fe intermetallic phases with grain sizes smaller than 100 nm were present at the crack surfaces and the wedge-shaped crack tips. An electrolytically coated reference sample that underwent the same heat treatment as the press-hardened material but without the application of tensile stress was investigated in order to find the initial cause of the microcracks. On this sample, Zn, in the order of one atomic layer, was found along prior austenite grain boundaries several micrometers away from the actual Zn/Fe phases in the coating. The resulting grain boundary weakening with the Zn wetting of prior austenitic grain boundaries during austenitization and/or hot forming is a necessary precondition for microcrack formation.

First-principles study of grain-boundary wetting in Fe-Σ5(013)[100] tilt boundary

We report an ab-initio study of grain-boundary wetting (GBW) in three systems (Fe/Zn, Fe/Pb & Fe/Bi) on a Σ5(013)[100] tilt grain boundary. An energy-based approach is pursued and adapted to the study of liquid-metal/solid-metal interfaces. We provide a sound calculation of the wettability parameter S (i.e., defined as the minimum energy difference between the unbroken initial state and final state with liquid-metal/solid-metal interfaces). Solid/liquid interfaces are modelled by ab-initio molecular dynamics, also allowing a study of structural effects in the liquid phase near the solid interface. We show how surface energies for solid-metal/liquid-metal interfaces need a special treatment for the liquid-metal chemical potential due to the change in crystallinity near the interface. The computation of the wettability criterion for the three systems correctly predicts the GBW trend known experimentally for these systems. The computations are compared with small-scale TEM experiments on GBW of two systems (Fe/Pb and Fe/Bi) and it is shown to provide reasonable results. This approach offers new perspectives in predicting GBW trends across various systems.

Coexistence of Intermetallic Complexions and Bulk Particles in Grain Boundaries in the ZEK100 Alloy

Magnesium-based alloys are highly sought after in the industry due to their lightweight and reliable strength. However, the hexagonal crystal structure of magnesium results in the mechanical properties’ anisotropy. This anisotropy is effectively addressed by alloying magnesium with elements like zirconium, zinc, and rare earth metals (REM). The addition of these elements promotes rapid seed formation, yielding small grains with a uniform orientation distribution, thereby reducing anisotropy. Despite these benefits, the formation of intermetallic phases (IP) containing Zn, Zr, and REM within the microstructure can be a concern. Some of these IP phases can be exceedingly hard and brittle, thus weakening the material by providing easy pathways for crack propagation along grain boundaries (GBs). This issue becomes particularly significant if intermetallic phases form continuous layers along the entire GB between two neighboring GB triple junctions, a phenomenon known as complete GB wetting. To mitigate the risks associated with complete GB wetting and prevent the weakening of the alloy’s structure, understanding the potential occurrence of a GB wetting phase transition and how to control continuous GB layers of IP phases becomes crucial. In the investigation of a commercial magnesium alloy, ZEK100, the GB wetting phase transition (i.e., between complete and partial GB wetting) was successfully studied and confirmed. Notably, complete GB wetting was observed at temperatures near the liquidus point of the alloy. However, at lower temperatures, a coexistence of a nano-scaled precipitate film and bulk particles with nonzero contact angles within the same GB was observed. This insight into the wetting transition characteristics holds potential to expand the range of applications for the present alloy in the industry. By understanding and controlling GB wetting phenomena, the alloy’s mechanical properties and structural integrity can be enhanced, paving the way for its wider utilization in various industrial applications.

Reducing stress corrosion cracking susceptibility of high-strength aluminum alloy and its fastener by a novel electromagnetic shocking treatment

A novel electromagnetic shocking treatment (EST) is put forward to reduce the stress corrosion cracking (SCC) susceptibility of high-strength aluminum alloys and its fasteners by selectively modifying grain boundary (GB) microstructure. The effect of EST on the microstructure, mechanical properties and SCC susceptibility of T73-Al7075 alloy and its high-lock nuts has been investigated. Mechanical properties, electrochemical performance and constant strain SCC test have been carried out for T73-Al7075 alloy, microhardness test and SCC test under wet-dry cyclic condition have been carried out for its high-lock nuts, as well as microstructure characterizations via SEM, EBSD and TEM have been carried out for T73-Al7075 alloy. Comparing to the received sample, the strength of EST sample basically keep consistent while the elongation increases slightly, besides, the electrical conductivity and electrochemical corrosion performance improves. The SCC susceptibility of T73-Al7075 alloy and its high-lock nuts have been reduced after EST. Microstructure characterization shows that the grain morphology and matrix intragranular precipitate distribution of the EST sample keep almost the same as those of the received sample, which is consistent with the almost unchanged mechanical properties of alloy sample. However, EST induces the transformation of grain boundary precipitates (GBPs) and the formation of stripy/wavy GB as observed experimentally, which indicates that EST promotes nonlinear GB wetting or pre-melting and even grain bridging. The phase transformation and GB complexion transition induced by EST are main factors attributing to the obvious improvement of SCC resistance of T73-Al7075 alloy and its high-lock nuts. It can be deduced that anodic dissolution cracking will be relieved because of the dissolution and coarsening of GBPs. Hydrogen induced cracking and crack propagation can be relieved with the occurrence of grain bridging. They are both beneficial to improve SCC resistance of Al7075 alloys. This work provides a novel EST method to targetedly embellish GB microstructure, optimize GB complexion and connectivity, and then enhance the service performance of solid alloys and even finished parts.

Grain Boundary Wetting by the Second Solid Phase: 20 Years of History

Grain boundaries (GBs) can be wetted by a second phase. This phase can be not only liquid (or melted), but it can also be solid. GB wetting can be incomplete (partial) or complete. In the case of incomplete (partial) wetting, the liquid forms in the GB droplets, and the second solid phase forms a chain of (usually lenticular) precipitates. Droplets or precipitates have a non-zero contact angle with the GB. In the case of complete GB wetting, the second phase (liquid or solid) forms in the GB continuous layers between matrix grains. These GB layers completely separate the matrix crystallites from each other. GB wetting by a second solid phase has some important differences from GB wetting by the melt phase. In the latter case, the contact angle always decreases with increasing temperature. If the wetting phase is solid, the contact angle can also increase with increasing temperature. Moreover, the transition from partial to complete wetting can be followed by the opposite transition from complete to partial GB wetting. The GB triple junctions are completely wetted in the broader temperature interval than GBs. Since Phase 2 is also solid, it contains GBs as well. This means that not only can Phase 2 wet the GBs in Phase 1, but the opposite can also occur when Phase 1 can wet the GBs in Phase 2. GB wetting by the second solid phase was observed in the Al-, Mg-, Co-, Ni-, Fe-, Cu-, Zr-, and Ti-based alloys as well as in multicomponent alloys, including high-entropy ones. It can seriously influence various properties of materials.

Phase Transformations Caused by Heat Treatment and High-Pressure Torsion in TiZrHfMoCrCo Alloy

In this work the high-entropy alloy studied contained six components, Ti/Zr/Hf/Mo/Cr/Co, and three phases, namely one phase with body-centered cubic lattice (BCC) and two Laves phases C14 and C15. A series of annealings in the temperature range from 600 to 1000 °C demonstrated not only a change in the microstructure of the TiZrHfMoCrCo alloy, but also the modification of phase composition. After annealing at 1000 °C the BCC phase almost fully disappeared. The annealing at 600 and 800 °C leads to the formation of new Laves phases. After high-pressure torsion (HPT) of the as-cast TiZrHfMoCrCo alloy, the grains become very small, the BCC phase prevails, and C14 Laves phase completely disappears. This state is similar to the state after annealing at high effective temperature Teff. The additional annealing at 1000 °C after HPT returns the phase composition back to the state similar to that of the as-cast alloy after annealing at 1000 °C. At 1000 °C the BCC phase completely wets the C15/C15 grain boundaries (GBs). At 600 and 800 °C the GB wetting is incomplete. The big spread of nanohardness and Young's modulus for the BCC phase and (C15 + C14) Laves phases is observed.

High Entropy Alloys Coatings Deposited by Laser Cladding: A Review of Grain Boundary Wetting Phenomena

High-entropy alloys (HEAs) are called also alloys without a main component or multiprincipal alloys. They consist of five, six or more components in more or less equal proportions and possess unique properties. Several dozens of thousands of publications have already been devoted to bulk HEAs, while HEA coatings are just beginning to develop. More than half of the works on the deposition of HEA coatings are devoted to laser cladding. In the laser cladding process, a mixture of powders on a substrate is melted in a focused laser beam, which sequentially scans the substrate. In the heated zone, the powder mixture melts. At the end of the crystallization process, a solidified polycrystal and a small amount of residual melt are found in the heated zone. It is possible that the grain boundaries (GBs) in the solidified polycrystal are incompletely or fully wetted by this liquid phase. In this way, the GB wetting with a melt determines the morphology and microstructure of HEAs coatings. This review analyzes GB wetting in single-phase HEAs, as well as in HEAs containing two or more phases. We analyze how the HEAs’ composition, laser scanning speed, laser beam power, external magnetic field or ultrasonic impact affect the microstructure and GB wetting. It is also shown how the microstructure and GB wetting change over the thickness of the rather thick as well as multilayer coatings deposited using a laser cladding.

Grain Boundary Wetting Phenomena in High Entropy Alloys Containing Nitrides, Carbides, Borides, Silicides, and Hydrogen: A Review

In this review, we analyze the structure of multicomponent alloys without principal components (they are also called high entropy alloys—HEAs), containing not only metals but also hydrogen, nitrogen, carbon, boron, or silicon. In particular, we discuss the phenomenon of grain boundary (GB) wetting by the melt or solid phase. The GB wetting can be complete or incomplete (partial). In the former case, the grains of the matrix are completely separated by the continuous layer of the second phase (solid or liquid). In the latter case of partial GB wetting, the second solid phase forms, between the matrix grains, a chain of (usually lenticular) precipitates or droplets with a non-zero value of the contact angle. To deal with the morphology of GBs, the new GB tie-lines are used, which can be constructed in the two- or multiphase areas of the multidimensional HEAs phase diagrams. The GBs in HEAs in the case of complete or partial wetting can also contain hydrides, nitrides, carbides, borides, or silicides. Thus, GB wetting by the hydrides, nitrides, carbides, borides, or silicides can be used in the so-called grain boundary chemical engineering in order to improve the properties of respective HEAs.

Grain Boundary Wetting by a Second Solid Phase in the High Entropy Alloys: A Review.

In this review, the phenomenon of grain boundary (GB) wetting by the second solid phase is analyzed for the high entropy alloys (HEAs). Similar to the GB wetting by the liquid phase, the GB wetting by the second solid phase can be incomplete (partial) or complete. In the former case, the second solid phase forms in the GB of a matrix, the chain of (usually lenticular) precipitates with a certain non-zero contact angle. In the latter case, it forms in the GB continuous layers between matrix grains which completely separate the matrix crystallites. The GB wetting by the second solid phase can be observed in HEAs produced by all solidification-based technologies. The particle chains or continuous layers of a second solid phase form in GBs also without the mediation of a liquid phase, for example by solid-phase sintering or coatings deposition. To describe the GB wetting by the second solid phase, the new GB tie-lines should be considered in the two- or multiphase areas in the multicomponent phase diagrams for HEAs. The GB wetting by the second solid phase can be used to improve the properties of HEAs by applying the so-called grain boundary engineering methods.

Computer analysis of the cemented carbides’ microstructure

Cemented carbides have been known for about a hundred years and are now widely used in mining, civil and road construction, and mechanical engineering. Their improvement requires the development of novel technologies based on fundamentally new approaches. Here the method for microstructure analysis using computer processing with elements of machine learning and artificial intelligence is proposed. It has been applied to the analysis of micrographs obtained by the scanning electron microscopy of the WC-Co cemented carbides. The geometric parameters of the WC / Co interphase boundaries are extracted using mathematical methods for processing digital pictures. This method is applied to the micrographs of three samples with different WC mean grain sizes. It has been found that the distribution of the contact angles of WC / Co interphase boundaries has a pronounced bimodal structure, and the values of the peak angles are practically the same for the samples of the fine-, medium-, and coarse-grain cemented carbide grades. The distributions of the semiaxes for the isolated areas of the cobalt binder are also obtained. The probability of finding a particular value of long and short semiaxis decreases exponentially with increasing semiaxis length. Contrary to the contact angles, the exponents are different for samples with different WC grains sizes. The obtained results are discussed using the ideas of wetting of grain boundaries and faceting-roughening of interphase boundaries.

The Grain Boundary Wetting Phenomena in the Ti-Containing High-Entropy Alloys: A Review

In this review, the phenomenon of grain boundary (GB) wetting by melt is analyzed for multicomponent alloys without principal components (also called high-entropy alloys or HEAs) containing titanium. GB wetting can be complete or partial. In the former case, the liquid phase forms the continuous layers between solid grains and completely separates them. In the latter case of partial GB wetting, the melt forms the chain of droplets in GBs, with certain non-zero contact angles. The GB wetting phenomenon can be observed in HEAs produced by all solidification-based technologies. GB leads to the appearance of novel GB tie lines Twmin and Twmax in the multicomponent HEA phase diagrams. The so-called grain-boundary engineering of HEAs permits the use of GB wetting to improve the HEAs’ properties or, alternatively, its exclusion if the GB layers of a second phase are detrimental.

Self-Induced Internal Corrosion Stress Transgranular Cracking in Gradient-Structural Ploycrystalline Materials at High Temperature

Self-induced internal corrosion stress transgranular cracking is investigated theoretically and experimentally linking grain boundary wetting (GBW) and grain boundary diffusion (GBD) to improve the ability to reveal the micro mechanism of crack in compositional gradient-structural intermetallic materials. Theoretical analysis shows that the grain boundary wetting and diffusion induce the diffusion-coupled dynamic internal stresses, and their interaction leads to crack nucleation. The experimental results show a stress concentration zone have been established at the grain boundary interface where the cracks preferentially nucleate and then extend through the inside of the grain to both sides, forming a typical transgranular fracture.

The high temperature wetting and corrosion mechanism analysis of Nb by TiAl alloy melt

High temperature corrosion behavior of Nb containing a TiAl melt is studied connecting with grain boundary wetting (GBW). Three intermetallic compounds (IMC) layers are formed and corroded step by step via the TiAl melt. The interfacial morphology and wetting angle evolution are investigated theoretically and experimentally. The morphology of the grooves controlled by interfacial tension, while nonzero wetting angles θd controlled by surface diffusion are established at the roots. By quantitative assessment, when θd is more than 140°, the grooves terminating on a thin sheet, which is responsible for the effective inhibition of further wetting and corrosion.

Supermodulus effect by grain-boundary wetting in nanostructured multilayers

The effect of thermal treatments on mechanical properties was systematically investigated in Ni/Mo multilayers with a constant modulation period (160 nm) prepared by magnetron sputtering deposition. A supermodulus effect was found in the annealed multilayers as compared to the as-deposited state. A large tensile stress development was observed in the multilayers. The evolution of grain-boundary (GB) wetting was observed at the interfaces of the multilayers, which results in an enhanced modulus based on the mechanism of GB-wetting-induced interfacial stress/strain. The GB wetting phenomenon was further supported by a thermodynamic calculation. The results not only bring clear evidence of the important role of interfacial structures in governing the elastic behavior of metallic multilayers, but also allow designing the multilayers with special properties through atomic diffusion and wetting at the interfaces based on the thermodynamic calculation.

Directionality of metal-induced crystallization and layer exchange in amorphous carbon/nickel thin film stacks

In thin amorphous carbon (a-C) films being in contact with a thin nickel layer, metal-induced crystallization and layer exchange (LE) occur at temperatures lower than 700 °C. Analysis of thin film stacks with different architectures (a-C/Ni, Ni/a-C and Ni/a-C/Ni) by means of ion beam analysis, Raman spectroscopy, X-ray diffraction and transmission electron microscopy revealed that the degree of LE and the structural quality of the crystallized carbon layers depend on the initial layer sequence. A LE degree of approx. 93% was found for Ni/a-C bilayers, where graphenic layers formed on the Ni surface, whereas in a-C/Ni bilayers only 83% of carbon was transferred from the surface towards the fused silica substrate. The diffusion of carbon in the outward direction produces turbostratic carbon with basal planes oriented parallel to the Ni surface, while for the inward direction planar and curved turbostratic structures coexist. The crystallization and the LE are driven by the crystallization energy of a-C. The LE is mediated by the wetting of the Ni grain boundaries by carbon. The directionality of the LE was explained primarily by the difference in the surface and interface energies in the a-C/Ni and Ni/a-C stacks that were obtained from thermodynamic considerations.

Wetting of surfaces and grain boundaries in cemented carbides and the effect from local chemistry

Wettability is an important factor in the sintering of cemented carbides. We study wetting of surfaces and grain boundaries in WC–Co och WC–Ni cemented carbides using density functional theory (DFT). Based on experimental observations of WC grain orientations in cemented carbides, relevant model interfaces are created. The local chemical composition at the interfaces is taken into account by substitution of interface atoms, and the effect of temperature is evaluated based on a companion study of temperature dependent interface energies in cemented carbides.The results indicate that the wettability of Ni and Co on WC surfaces are similar. Furthermore, the wettability of Co on the commonly occurring basal WC surface is better in W-rich materials compared to C-rich materials. At liquid phase sintering temperatures we get perfect wetting in W-rich materials, while only partial wetting in C-rich materials, which is in agreement with recent experiments on wetting in WC–Co cemented carbides.The segregation of binder phase atoms to WC/WC grain boundaries stabilize grain boundaries and make them more resistance against infiltration (wetting) by binder phase. We find that the amount of dissolved binder atoms in essentially all studied WC/WC grain boundaries are of half a monolayer proportion, which is in agreement with experimental studies. Further, in WC–Co there is a stronger resistance against grain boundary infiltration compared to WC–Ni. We find that the continuous skeleton of WC grains seen after sintering and which is crucial for the superior mechanical strength of the material exists already during liquid phase sintering.

On the Formation of Nanoscale Intergranular Intermetallic Compound Films in a Cu-5 at. pct Zr Alloy

Grain boundary structure and chemical composition have been shown to play important roles in plasticity, strength, creep, diffusivity, and conductivity of fine-grained metallic materials. Wetting of grain boundaries in metallic materials with nanoscale intergranular intermetallic compound films (NIICFs) is suggested to offer enhanced strength and ductility of alloys. In the current study, the NIICF wetting Cu-Zr micrograins of the matrix is observed in a Cu-5 at. pct Zr alloy produced by powder metallurgy. The underlying mechanism responsible for the formation of these NIICFs is discussed, and the effect of these films on the strength and strain-hardening capacity of the alloy is evaluated.

In-situ study of microstructural evolution during thermal treatment of 6063 aluminum alloy

The present study reports microstructural observation during thermal treatment of 6063 aluminum alloy. Microscopic investigation employing low-energy electron and X-ray photoemission electron techniques was conducted. Work function pertaining thereto such techniques was analyzed. During the thermal treatment representing homogenization of the alloy, as-cast intermetallics transform morphologically from initial β-AlFeSi to α-Al(FeMn)Si. The transformation is governed by elemental diffusion between the as-cast intermetallics and the aluminum matrix and by reversible wetting of grain boundaries. These lead to formation of discrete and globular particles of α-Al(FeMn)Si all over the aluminum matrix. This in-situ study reveals that the phase transformation is not taken place by nucleation of α-Al(FeMn)Si onto surfaces of β-AlFeSi.