Boundary Cohesion Research Articles

Crack suppression in directed energy deposition of molybdenum

Additive manufacturing (AM) is a valuable tool for the fabrication and repair of refractory parts such as molybdenum alloys. Cracking is a common defect encountered during AM processing of refractory parts, that is generally associated with the segregation of light elements to grain boundaries which affect grain boundary cohesion and, ultimately, affect the final performance of the part. Similarly, because of the high melting points of refractory metals, lack-of-fusion defects are also common. The effect of build substrate and small alloying additions on suppression of defects during multilayer builds was investigated using directed energy deposition (DED) printing. Identical sample matrices were printed on three different build substrates: molybdenum (Mo), commercially pure titanium (Cp-Ti), and 316 stainless steel (316). Twenty-six usable samples were produced. Samples were cross sectioned, polished, and were characterized for total cross-section defect area. Additionally, samples from each substrate material were analyzed for grain boundary oxygen content. The strongest defect suppression, producing crack free material, was observed in samples printed on a Cp-Ti build substrate with a ten atomic percent addition of titanium in the molybdenum powder feed. The part quality was enhanced due to three factors: 1) the moderation of thermal diffusivity through a change in build plate material, 2) the suppression of light element segregation via increased solubility through titanium addition, and 3) a lack of brittle phase formation due to metallurgical compatibility of the build material with the build substrate. Analysis of defect area versus dimensionless number, π1, shows that increasing π1 reduced defects throughout the part.

Unveiling the enhancing mechanism of cyclic stability in Tb doped Ni-Mn-Sn Heusler alloys

Rare-earth (RE) doping has been demonstrated as a feasible approach to improve the cyclic performance of Ni-Mn-based Heusler alloys, but the current demonstration of underlying mechanism is simply ascribed to strengthening of the grain boundary cohesion. In this work, the effects brought by RE doping on microstructure, stress induced martensitic transformation behavior and cyclic stability of Ni-Mn-based alloys were systematically investigated. By Tb doping, a stable adiabatic temperature change (|ΔTad|) of ∼ 5.5 K over 1000 cycles with no significant degradation was obtained in (Ni43Mn47Sn10)99.5Tb0.5. (1) Through the in-situ digital image correlation strain measurement and in-situ SEM observation upon loading process, it revealed a multipoint nucleation of the martensitic transformation across the specimen arising from the intergranular NiSnTb secondary phase as nucleate sites, which weakens the strain concentration and suppresses the crack tendency. (2) The multipoint nucleation promotes the transformation volume fraction, making the large |ΔTad| could be driven by a small transformation strain (Δεtr) and (Ni43Mn47Sn10)99.5Tb0.5 yields a |ΔTad/Δεtr| up to 8.7 K %−1. Such reduction in applied strain further alleviates the extension of the cracks. Both the two factors brought by Tb doping in (Ni43Mn47Sn10)99.5Tb0.5 give rise to the enhancement in cyclic stability of the large elastocaloric effect.

The Role of Extrusions and Intrusions in the Initiation and Intergranular Growth of Fatigue Cracks

The role of extrusions and intrusions in the initiation and growth of intergranular fatigue cracks is studied by performing strain‐controlled fatigue experiments on polycrystalline copper and by analyzing the development of intergranular crack initiation and resulting fracture surfaces. Fatigue cracks initiate either in the persistent slipbands within a grain or on the grain boundaries. The grain boundary initiation mechanism is due to the production of extrusions and intrusions on the grain boundaries that decrease the grain boundary cohesion forming submicroscopic crack nuclei. The alternating extrusions and intrusions are found also on the grain boundary facets of the fracture surface of a growing fatigue crack. Up to three slip systems are identified on the facets of cracked grain boundaries. Their form and height are found using scanning electron microscope observations and focused ion beam sectioning. A mechanism similar to intergranular grain boundary crack initiation is considered to explain the formation of grain boundary facets. A novel mechanism of intergranular fatigue crack growth is proposed based on the damaging effect of the extrusions and intrusions produced in the cyclic plastic zone of the growing fatigue crack.

Effect of phase decomposition on the mechanical properties of Ti-Zr-Nb-Ta-Mo multi-principal element alloys

Body-centered cubic Ti-Zr-Nb-Ta-Mo multi-principal element alloys (MPEAs), boasting a yield strength exceeding one gigapascal, emerge as promising candidates for demanding structural applications. However, their limited tensile ductility at room temperature presents a significant challenge to their processability and large-scale implementation. This study identifies phase decomposition as a critical factor influencing the plasticity of these alloys. The microscale phase decomposition in these MPEAs during solidification, driven by miscibility gaps, manifests as dendritic structures within grains. Closer examination reveals that the MPEAs with a pronounced thermodynamic propensity for phase decomposition are also susceptible to analogous phenomena at the atomic level. The atomic phase decomposition is characterized by the localized aggregation of some elements across nanometric domains, culminating in the establishment of short-range orderings (SROs). It is observed that phase decomposition for these MPEAs, occurring at both microscale and atomic scale, adheres to thermodynamic principles and can be predicted using the CALPHAD approach. The impact of phase decomposition on the plasticity of MPEAs fundamentally stems from the induced heterogeneities at three distinct levels: (1) Fluctuations in mechanical properties at the micron scale; (2) Variations in the strain field at the atomic scale; (3) Bond polarization and bond index fluctuations at the electronic scale. Consequently, the key to designing high-strength and high-plasticity MPEAs lies in maximizing lattice distortion while simultaneously minimizing the adverse effects of phase decomposition on the alloy's plasticity (grain boundary cohesion). This research not only clarifies the mechanisms underpinning the ductile-to-brittle transition in high-strength Ti-Zr-Nb-Ta-Mo MPEAs but also offers crucial guidelines for developing advanced, high-performance alloys.

Enhanced grain boundary cohesion mediated by solute segregation in a dilute Mg alloy with improved crack tolerance and strength

Effects of solute segregation at grain boundaries (GBs) on the deformation mechanism and fracture behavior remain obscure for magnesium (Mg) alloys. Here, by introducing Zn segregation at GBs, we obtained an Mg-0.5Al-0.4Mn-0.2Ce-0.4Zn (wt.%) alloy achieving fracture elongation (FEL) of ∼33.6 %, with a remarkable FEL improvement by 100 % in comparison to Zn-free counterpart. Meanwhile, the tensile yield strength (TYS, 195.5 MPa) is increased by ∼27.5 MPa after trace Zn addition. Although trace Zn addition improves the fraction of GBs with high misorientation, it reconciles crack tolerance with enhanced strength. The introduction of Zn not only promotes pyramidal 〈c + a〉 slips and inhibits twinning nucleation, but also enhances the GB cohesion by Zn segregation. Based on in-situ microstructure observation, we found that the enhanced GB cohesion enables the segregation-inspired hierarchical crack buffering, as well as deflecting and branching. Enhanced GBs can also facilitate the continuous emission of 〈c + a〉 dislocations in neighboring grains irrespective of the onset of microcracks, forming a plastic zone to retard local strain concentration, thus avoiding microcrack percolation and attaining a crack-mediated elongation reserve of above 15 %. Besides, the higher TYS in the Zn-containing alloy mainly stems from the enhanced solid-solution strengthening of Zn solutes, thus achieving strength and crack tolerance synergy.

Effects of vanadium content on the microstructure and tensile properties of NbTiVxZr high-entropy alloys

Vanadium boasts a relatively small atomic radius among numerous refractory alloying elements, yet its effects on the microstructure and mechanical behavior of refractory alloys remain enigmatic. In this work, a series of NbTiVxZr high-entropy alloys (HEAs) with low density were formulated to explore the evolution of microstructures and mechanical properties with varying vanadium content. When the vanadium content was low, the matrix exhibited the single-phase body-centered cubic structure. Excessive vanadium contents (NbTiV0.75Zr, NbTiVZr) disrupted phase stability and tended to aggregate at grain boundaries, forming V-rich and Zr-rich phases. These precipitated phases pinned grain boundaries, impeding grain growth and resulting in both grain boundary strengthening and precipitation strengthening, albeit at the expense of grain boundary cohesion, ultimately leading to brittleness. However, these deleterious phases were redissolved, and fracture elongation increased more than 2 times after water quenching at 1200 °C. The microstructural changes induced by heat treatment were corroborated by alterations in grain boundary relaxation behavior, where the redissolution of precipitated phases after water quenching caused a shift of the grain boundary relaxation internal friction peak to a lower temperature and an increase in peak height. This study elucidates the intrinsic relationship between mechanical properties and microstructures with vanadium content in NbTiVxZr alloys, offering insights for the design of TiV-based lightweight refractory HEAs with high strength and excellent ductility.

Effects of Pb and Bi on mechanical properties and fracture modes of bcc iron symmetrical tilt grain boundaries

The internal structural material T91 in the accelerator driven subcritical reactor system (ADS) has encountered the challenge of liquid metal embrittlement caused by Pb and Bi. Therefore, it is of utmost importance to gain a comprehensive understanding of their embrittlement mechanism. The effects of Pb and Bi on the surface energy of surfaces and separation work and fracture toughness of grain boundaries in body center cubic (BCC) Fe are examined using first-principles calculation. Pb and Bi atoms reduce the surface energy and grain boundary cohesion strength of BCC Fe, with a more significant decrease in surface energy. Meanwhile, the segregation of Pb and Bi in the grain boundary reduces the fracture toughness of the grain boundary. By comparing different surface energy and separation work, it is found that BCC pure Fe is more prone to intergranular fracture, while the presence of Pb and Bi causes cleavage fracture. This elucidates the low-temperature fracture mode of BCC-Fe and reveals the potential mechanism of brittle cleavage fracture caused by Pb and Bi in BCC-Fe. The derived results are discussed in terms of electronic structure and show good agreement with experimental results in the literature, ultimately contributing to a thorough understanding of the intrinsic mechanism of liquid metal embrittlement of T91 steel.

The Pt, Hf dopants in aluminide coating against S impurity: Dependence on grain boundary cohesion and energetic properties

The Pt and Hf additions in aluminide coating can pronouncedly change corrosion resistance against sulfur attack. To provide a comprehensive picture of this issue, by first-principles simulations we study the sulfur-induced embrittlement of ∑5(310)[001] symmetrical tilt grain boundary (STGB) in NiAl, with a focus on roles of Pt, Hf dopants played in sulfur-attacked grain boundary cohesion. The results show that the Hf-doping tends to increase GB combination with larger pre-breaker strength and theoretical tensile strength after sulfur introduction, while Pt-doping hardly makes contribution to the GB mechanical properties. The sulfur-induced embrittlement can be attributed to the charge density depletion and breaking of chemical bonding during stretching process, evaluated by the charge density distributions and bond length analysis. By analyzing the sulfur formation energy at NiAl bulk and NiAl(110) surface, we find that the presence of Pt enhances the energy criterion forming S impurity, while the significant reduction in adsorption energy after Hf-doping results from HfS strong atomic bonding. Alternatively, the existence of Pt decreases probability of S occurrence and becomes the barrier against S invasion. Instead, Hf tightly ‘embraces’ S impurity and impedes its diffusion into depth.

Insight into grain boundaries with reduced liquid metal embrittlement susceptibility in a boron-added 3rd generation advanced high strength steel

The addition of boron to the advanced high-strength steels (AHSS) enhances grain boundary cohesion and can play a significant role in intergranular degradation phenomena such as Zn-assisted liquid metal embrittlement (LME). The objective of this study is to reveal a thorough understanding of the effect of B on LME behaviour, where the welding test results indicated substantial improvement even at relatively low B concentration. Here, we adopted the strategy of identifying LME-sensitive grain boundaries, followed by characterizing grain boundary chemistry. EBSD measurements and prior austenite grain (PAG) reconstruction provided strong evidence of intergranular LME crack propagation via PAGBs. Further transmission electron microscopy and energy dispersive X-ray spectroscopy investigations demonstrated an asymmetric Mn profile along the PAGB in B-free steel. Therefore, we propose that B addition and consequently, lower grain boundary energy appears to prevent Mn segregation. Based on our observation, we conclude that B seems to have the similar desired effect on Zn diffusion along the PAGBs and eventually mitigates Zn-assisted LME.

Metallographic Evaluation of Increased Susceptibility to Intermediate Embrittlement of Engine Valve Forgings Made of NCF 3015 High Nickel and Chromium Steel.

This paper focused on determining the increased tendency of cracking after the die forging process of high nickel and chromium steel. The increase in carbon content in austenitic nickel-chromium steel promoted the tendency of valve forgings to forging intergranular crack on the valve head. Attention was paid to issues related to the chemical composition of the material to be considered when hot forming nickel-chromium steel components. Optical and scanning electron microscopies were used to examine the microstructure and fracture features of the samples removed from a fractured valve head. The embrittlement was due to microcavity formation at grain boundaries. Creep theory at grain boundaries was used to explain crack formation. The tensile behavior was interpreted from the evolution of the microstructure during deformation and referred to intermediate brittleness to explain the effect of carbon. It was found that the increased carbon content of the nickel-chromium steel and the strong undercooling observed at the edges of the valve head are factors that promote a reduction in grain boundary cohesion and enhance intermediate temperature embrittlement. Finally, it was found that the formation of a heterogeneous structure manifested by the presence of grain boundary M23C6-type carbides in the austenitic matrix was most likely related to the occurring brittleness.

Fabrication of nanocrystalline and ultra-hard tungsten alloy by dual-particle dispersion strengthening

Alloying with heterogeneous atoms or second phase particles is an effective method for inhibiting grain growth and achieving ultrafine and nanocrystalline materials. In this work, by varying the component and content of Ti and ZrC, the synergistic effects of dual-particle reinforcement on sintering behavior and final bulk W alloys were investigated. The results revealed that adding both Ti and ZrC not only dramatically suppresses grain growth but also effectively promotes relative density. Nanocrystalline W-3Ti-1ZrC bulks were successfully fabricated with an ultra-high hardness of 1347 kgf/mm2. Further characterization showed that the alloying component and content strongly influence the milled-powder state agglomeration and morphology even in same milling condition. Additionally, the advantage of Ti and ZrC addition can be utilized simultaneously, as Ti retards grain growth and ZrC reacts with oxygen impurities to enhance grain boundary cohesion.

Hydrogen absorption and embrittlement of martensitic medium-Mn steels

In the present study, the H absorption and the H embrittlement (HE) resistance of medium (3–7 wt%) Mn martensitic steels were investigated. When the specimens were electrochemically H-charged, 7 wt% Mn specimen had the highest diffusible H content because of the highest reversible H trap density. When the diffusible H content was identical, HE resistance deterioration arose due to the Mn addition, probably caused by grain boundary decohesion and H segregation into the grain boundaries. However, this condition was improved by B addition due to the enhanced grain boundary cohesion and the suppression of H segregation into the grain boundaries.

Enhanced cyclic stability and enlarged working temperature window of NiFeGa elastocaloric refrigerant via introducing strong texture and ductile interfacial precipitate

The poor mechanical properties will result in low fatigue life and limited working temperature window of elastocaloric effect (eCE) for ferromagnetic shape memory alloys with lower critical stress. In this work, a microstructure design strategy, i.e., simultaneously introducing strong texture and ductile interfacial γ-phase, was adopted to reduce the strain discontinuity near the grain boundary and enhance the grain boundary cohesion, further improving the mechanical properties and eCE performance. With this strategy, the compressive strength and strain of the prototype Ni54Fe19Ga27 alloy can exceed 1.5 GPa and 30.0% at room temperature, respectively. Furthermore, a wide working temperature window of 120 K with an adiabatic temperature change of −8.5 to −1.2 K and superior eCE fatigue life of 2 × 104 cycles with an adiabatic temperature change of ∼3 K have been achieved. This work is expected to provide an effective strategy to enhance the eCE performance of ferromagnetic shape memory alloys.

Enhancing hot ductility of a cryogenic high manganese steel at a high strain rate by matrix homogenization

The high temperature brittle zone of cryogenic high manganese (Mn) steel often leads to transverse cracks during hot working process, which seriously hinders the production of its sheets or wires. In the present work, we found that the brittle zone (at temperatures between 800 and 900 °C) can be effectively enhanced by homogenizing the initial microstructure and composition. It was revealed that the elements such as Mn and C tend to be enriched in the inter-dendrite area of the as-cast samples, which is difficult to eliminate even during high temperature forging. The C segregation promotes the formation of fine precipitates, such as NbC and Cr23C6. These carbides are usually located at or near grain boundaries, which can weaken grain boundary cohesion and deteriorate the hot ductility. However, homogenizing the initial composition helps to increase the size of hybrid precipitates containing TiN, WC, MoO2 and WO3 and inhibit the precipitation at grain boundaries. Meanwhile, homogenizing the initial microstructure size and stored energy reduces the critical dynamic recrystallization (DRX) temperature to 850 °C. A lower Z value and critical DRX stress increase the DRX fraction from 24% to 66%, which recover microstructure and inhibit grain boundary slip.

Effect of boron doping on grain boundary cohesion in technically pure molybdenum investigated via meso-scale three-point-bending tests

Molybdenum has numerous advantageous functional and high-temperature properties. However, plastic deformation as well as structural applications are limited due to a propensity for brittle, intercrystalline failure, especially at low temperatures. It is well known that oxygen segregations have a detrimental effect, whereas it is assessed that carbon and/or boron have a beneficial effect on grain boundary cohesion. An advanced approach for the improvement of these interfaces is segregation engineering, e.g. the addition of cohesion enhancing elements segregating to the grain boundaries. To investigate early stages of crack formation, three-point bending tests on recrystallized commercially pure and boron micro-doped molybdenum were conducted between −28 °C and room temperature. The tensile-loaded top surface of the specimens was examined post-mortem close to the final fracture area via scanning electron microscopy. The occurring separations of grains are investigated for distinct features and the chemical composition of the interface is complementary measured by atom probe tomography.

Solute-solute interactions and their impacts on solute co-segregation and interfacial cohesion of {101¯2} twin boundary in zinc

• Zn-based biodegradable alloys are receiving increasing attention in recent years. Alloying is an effective approach to increase mechanical properties of such alloys for load bearing biodegradable applications, but it is unclear how solute atoms of the added alloying elements interact with each other and how these solute-solute interactions affect the mechanical properties of these alloys. In this work, the interactions between solutes (Li, Mg, Mn, Cu, and Ag) commonly used in biodegradable Zn alloys are investigated using first-principles calculations. The segregation ability of these solutes and the effects of the segregations of these solutes on { 10 1 ¯ 2 } twin boundary cohesion are also examined. This work uncovers the effect of solute-solute interactions on solute co-segregation and interfacial cohesion of twin boundaries along which fracture occurs. The findings of the present work are extendable to other hexagonal alloys such as Mg that often contain multiple alloying elements. Interactions of solute atoms in biodegradable zinc alloys and their effect on alloy mechanical properties have been less investigated. In this work, the interactions between the common solutes (Li, Mg, Mn, Cu, and Ag) used in the biodegradable Zn alloys, including a solute-solute pair with the same element or with two different elements, are investigated based on first-principles calculations. It is found that the energetically favorable configuration is the third nearest-neighboring for most solute-solute pairs in the bulk lattice because of the relatively strong electronic interaction between solute and Zn atoms or the relatively small local elastic deformation associated with the configuration. Considering that interfacial cleavage is a key fracture mode of zinc, the segregation ability of these solutes and their effect on the { 10 1 ¯ 2 } twin boundary cohesion are also examined. The result shows that Li tends to fully occupy its preferred site in the twin boundary, while Mg, Mn, Cu, or Ag has a concentration limitation in the twin boundary. The twin boundary cohesion can be significantly enhanced by the segregation of Mn, followed by Cu and Ag, because of the contribution of their d states close to the Fermi level. Furthermore, the co-segregation ability of two solute atoms in the twin boundary increases with increasing the binding tendency of these two solute atoms in the boundary. Mn and Li or Mg show a relatively strong co-segregation ability in the twin boundary. Adding Mn to Zn-Li or Zn-Mg alloys can significantly enhance the resistance to fracture of twin boundaries.

The enhanced Zn and Ca co-segregation and mechanical properties of Mg–Zn–Ce alloy with micro Ca addition

The micro addition of Ca is demonstrated to noticeably increase the strength and ductility of Mg–Zn–Ce alloy, which is closely correlated with the enhanced alloying segregation at grain boundary. Addition of Ca is found to yield the enhanced Zn segregation and co-segregation of Zn, Ce, and Ca. The mechanism behind is that Ca decreases the segregation energy while promoting the diffusion process of Zn, ultimately accelerating the thermodynamic and kinetic process of segregation. The enhanced alloying segregation induced by Ca effectively stabilizes grain boundary, which contributes to the refined grain size, retardant mobility of dislocation and ultimately improved strength. Furthermore, the intergranular fracture energy can be noticeably increased by Ca owing to the promoted alloying element segregation at grain boundary, which contributes to the improved grain boundary cohesion and ductility. The controllable segregation at grain boundary via alloying is intended to contribute to the development of advanced magnesium alloys.

Role of Persistent Slip Bands and Persistent Slip Markings in Fatigue Crack Initiation in Polycrystals

The cyclic plastic deformation of polycrystals leads to the inhomogeneous distribution of the cyclic plastic strain. The cyclic plastic strain is concentrated in thin bands, called persistent slip bands (PSBs). The dislocation structure of these bands generally differs from the matrix structure and is characterized by alternating dislocation-rich and dislocation-poor regions. The mechanisms of the dislocation motion in the PSBs and the formation of the point defects and their migration are quantitatively described. It is shown that, due to localized cyclic plastic straining in the PSBs, persistent slip markings (PSMs) are produced where the PSBs emerge on the surface. They typically consist of a central extrusion accompanied by one or two parallel intrusions. The deep intrusion is equivalent to the crack-like surface defect. The concentration of the cyclic strain in the tip of an intrusion leads to intragranular fatigue crack initiation. The mechanism of the early crack growth in the primary slip plane is proposed and discussed. Numerous PSMs are produced on the surface of the cyclically loaded materials. PSMs contribute to the formation of the surface relief, as well as the relief on the grain boundary. PSMs from one grain impinging the grain boundary are sufficient to create sharp relief on the grain boundary. Void-like defects weaken the grain boundary cohesion and extra material push both grains locally apart. The conditions necessary for the weakening of the grain boundary are enumerated and examples of grain boundary crack initiations are shown. The relevant parameters affecting grain boundary initiation are identified and discussed. The collected experimental evidence and analysis is mostly based on the papers published by the author and his colleagues in the Institute of Physics of Materials in Brno.

Enhancing mechanical properties of ultrafine-grained tungsten for fusion applications

Tungsten, while showing many favorable properties, faces challenges in high-performance applications due to its brittle nature. One strategy to improve strength and toughness in tungsten is to refine the grain size down to the ultra-fine grained (ufg) regime. However, as the grain size is reduced, the fraction of grain boundaries that provide easy paths for crack growth increases, thereby limiting the gain in ductility. Therefore, strengthening the grain boundaries is of great importance if one wants to tap the full potential of this material. Using ab-initio calculations, potential grain boundary cohesion enhancing doping elements were identified, and doped ultra-fine grained tungsten samples were fabricated from powders and characterized extensively using small-scale testing techniques. We found that additions of boron and hafnium improve the mechanical properties of tungsten remarkably. Furthermore, an additional low-temperature heat treatment of the boron-doped sample promotes grain boundary segregation, enhancing the properties even further. Thus, in this work we provide an effective pathway of improving mechanical properties in ultra-fine grained tungsten using grain boundary segregation engineering. This opens the door for many challenging applications of ufg W in harsh environments. To further underline the potential employment of ufg W in nuclear fusion reactors, a favorable swelling behavior and mechanical property response after irradiation with helium is presented within this work.

Mechanisms of intergranular fatigue crack initiation in polycrystalline materials

Fatigue failure in polycrystalline materials occurs by initiation and propagation of fatigue cracks. Fatigue cracks can initiate transgranularly or intergranularly. While considerable progress has been achieved in the explanation of the mechanism of transgranular fatigue crack initiation intergranular initiation is still the subject of controversy. A recent study of the crack initiation in superaustenitic Sanicro 25 steel [1] revealed the important role of persistent slip markings (PSMs) in the grains adjoining the grain boundary. A novel mechanism of intergranular fatigue crack initiation has been proposed. Provided the cyclic slip cannot proceed from one grain to the neighbour one, extrusions and intrusions arise not only on the surface but also on the grain boundary. Growing intrusions represent void-like defects and growing intrusions push neighbour grain apart. A systematic study of intergranular cracking has been performed on polycrystalline copper cycled with high and low strain amplitudes. SEM observations of the early stages of fatigue crack initiation on the grain boundaries, EBSD study of grains orientation, FIB cutting of the grain boundaries and simultaneous documentation of the effect of extrusions and intrusions on the grain boundary cohesion are reported simultaneously with the conditions favourable for the initiation of the fatigue cracks.