Severe Embrittlement Research Articles

Enhancing strength-ductility combination of Fe–Ni–Cr–Al–Ti high-entropy alloys via co-precipitation of sigma and L21 phases

The sigma phase is a well-known harmful phase in transition metal alloys, leading to severe embrittlement due to its inherent brittleness. However, in this work, we develop a new strategy to obtain enhanced strength-ductility combination via the synergistic effects of interlocking co-precipitation of the sigma and L21 phases in Fe40Ni30Cr20Al5Ti5 (at. %) high-entropy alloy (HEA). This HEA exhibits a good strength-ductility combination after aging treatment (800 °C for 4h), with a yield strength (YS) of about 920 MPa and an ultimate tensile strength (UTS) of about 1300 MPa while maintaining a total elongation (TE) of 24 %. It is attributed to the interlocking co-precipitation of the sigma and L21 phases with equal volume fractions into the face-centered cubic (FCC) matrix through thermomechanical treatment. During the tensile deformation, the softer L21 phase coordinates the deformation around the sigma phase, suppresses the local stress concentration, and achieves a stable high work hardening capability, as well as stacking faults (SFs) networks and density Lomer-Cottrell (L-C) locks, contributes to the excellent strain hardening capacity and thus obtains the enhanced strength-ductility combination in the HEA. This study will provide a new strategy to obtain enhanced strength-ductility combination for alloys containing brittle and hard topologically close-packed (TCP) phases, such as the sigma phases.

Suppressing hydrogen embrittlement of a CrCoNi medium-entropy alloy by triggering co-segregation of carbon, boron, and Cr

The CrCoNi medium-entropy alloy has drawn extensive attention due to its outstanding mechanical properties, yet it suffers severe gaseous hydrogen embrittlement (HE). Here we report that co-doping of carbon and boron could greatly suppress such embrittlement, resulting in a significant reduction in ductility loss from 71 % to merely 16 %. The minor doping has no significant influence on the phase structure, grain size, and grain-boundary character. However, the co-segregation of carbon, boron, and Cr was triggered, which not only improves the grain-boundary cohesive strength but also hinders the diffusion and accumulation of hydrogen at GBs, thereby leading to improved HE resistance.

Synergistic grain boundary engineering for achieving strength-ductility balance in ultrafine-grained high-Cr-bearing multicomponent alloys

Precipitation strengthening is a crucial strategy for ensuring the overall performance of conventional and multicomponent alloys to meet industrial demands. However, the mechanical properties of high-Cr-bearing alloys are often compromised by brittle Cr-rich precipitates at grain boundaries (GBs), leading to severe embrittlement. In this work, a multi-step thermomechanical process is employed to regulate discontinuous dynamic recrystallization (DDRX) and static recrystallization, achieving an ultrafine-grained microstructure. This optimized approach simultaneously impedes the continuous precipitation of the ordered L12 nanocrystals within the matrix and actively encourages the synergistic discontinuous precipitations of submicron L12 and Cr-rich σ particles at GBs, thereby enhancing (yield) strength and high-temperature thermal stability. The ultrafine grains facilitate uniform plastic deformation, characterized by pronounced parallel dislocation slip and stacking faults (SFs) within face-centered cubic (fcc) grains, while second-direction slips, SFs, and Lomer-Cottrell (L-C) lock networks near GB precipitates greatly alleviate stress concentration. Critically, the submicron L12 particles enveloping σ precipitates at GBs also display plastic deformation via mechanical twinning and dislocations, effectively impeding rapid crack propagation along GBs. This research not only provides new insights into the ductility-strength balance in advanced alloys but also proposes a compelling route for optimizing biphasic precipitation, expanding the applicability of high-Cr multicomponent alloys.

Evading the intermediate-temperature embrittlement of L12-strengthened high entropy alloys by alleviating intergranular oxidation

L12-strengthened high entropy alloys (HEAs) usually suffer from severe intermediate-temperature embrittlement. In this study, we revealed that the intermediate-temperature embrittlement of a (Ni2Co2FeCr)92Nb4Al4 L12-strengthened HEA originates from the severe oxidation of intergranular D019 precipitates which weakens the grain-boundary cohesion. With the minor addition of 1.5 at% Si, we successfully achieved a brittle-to-ductile transition in the alloy via alleviating intergranular oxidation. The alloyed Si preferentially partitioned into the intergranular precipitates, significantly improved their oxidation resistance and pinning effects on the grain boundaries, which effectively mitigated environmentally assisted grain-boundary damage and thus suppressed the crack propagation upon deformation.

Degradation of PET – Quantitative estimation of changes in molar mass using mechanical and thermal characterization methods

Polyethylene terephthalate (PET) films are used when mechanical strength, thermal and chemical stability, and barrier properties to atmospheric gases are required in combination with good processability. Hydrolysis leading to embrittlement of the material is a major concern as PET is used in a variety of applications with expected lifetimes of up to decades (e.g., for use in buildings, textiles, or photovoltaic backsheets). Therefore, a comprehensive understanding of the degradation processes and the effects on the molecular mass distribution is of great importance.Usually, the direct determination of molar mass and molar mass distribution involves high effort and sophisticated equipment. Therefore, the main objective of this work is to quantify molar mass changes due to accelerated aging using thermal and mechanical methods. Two stabilized PET films were subjected to seven different accelerated aging conditions (heat; combined heat-humidity). The samples were then characterized by size exclusion chromatography (SEC), tensile tests and differential scanning calorimetry (DSC).A linear correlation was found between crystallization temperature and average molar mass. The values of fracture stress from tensile tests indicate a ductile-brittle transition at a molar mass of 15 000 g mol−1. The study concludes that the crystallization temperature obtained from DSC measurements can be used to estimate changes in the average molar mass of PET after hydrolysis. Crystallization temperatures between 208 °C and 211 °C correspond to a critical reduction in molar mass and severe embrittlement.

Effect of neutron irradiation on tensile properties of advanced Cu-based alloys and composites developed for fusion applications

The effect of neutron irradiation on tensile properties and fracture mode has been investigated for several advanced CuCrZr alloys in the frame of the European fusion material development program. Five material grades utilizing different strengthening principles have been exposed to neutron irradiation up to ∼2.5 dpa (displacement per atom) in the target operational temperature range of 150–450 °C. The strengthening mechanisms are based on the application of: i) tungsten particles; ii) tungsten foils (laminate structure); iii) tungsten fibers; iv) Y2O3 particles; v) vanadium addition (0.22%). Neutron irradiation was performed in the BR2 material test reactor inside the fuel channel in order to maximize the fast neutron flux. The upper irradiation temperature of 450 °C was selected to validate the ability of the pre-selected advanced grades to sustain the high temperature irradiation, since the baseline ITER specification CuCrZr is known not to retain sufficient tensile strength above 400 °C in non-irradiated conditions and shows strong irradiation induced softening above 300 °C. Neutron irradiation at 150 °C caused severe embrittlement of tungsten-copper laminates as well as a considerable reduction of the total elongation of all other grades. The irradiation at 450 °C led to the reduction of the yield strength and ultimate tensile strength (i.e. irradiation softening) in the vanadium-doped alloy similar to CuCrZr, while all other materials preserved or increased their strength (irradiation hardening). The fracture surfaces of the tested samples were analysed to investigate the modification of the deformation mechanisms in each particular case.

Simultaneously improving mechanical properties and oxidation resistance of Ti-bearing high-entropy superalloys at intermediate temperature via silicon addition

Ti-bearing high-entropy superalloys (HESAs) often suffer from severe intergranular embrittlement and terrible oxidation degradation at intermediate temperatures. Here we showcase that minor Si addition can effectively mitigate the intergranular embrittlement and improve the oxidation resistance of the a (Ni2Co2FeCr)92Ti4Al4 HESA at 700 °C simultaneously. Experimental analysis revealed that the intergranular G phase induced by 2 at% Si addition can effectively suppress the inward diffusion of oxygen along grain boundaries at 700 °C, thus enhancing the tensile ductility of the alloy from ∼8.3% to ∼13.4%. Besides, the 2 at% Si addition facilitated the formation of a continuous Al2O3 layer during oxidation, contributing to a remarkable reduction in the growth rate of the oxide scale to a quarter of the Si-free HESA. Our results demonstrate that Si can be a favorable alloying element to design advanced HESAs with synergistically improved thermal-mechanical performance.

Role of Pd interlayer on NiTi to Ti6Al4V laser welded joints: Microstructural evolution and strengthening mechanisms

Obtaining a reliable NiTi to Ti6Al4V dissimilar joint exhibiting NiTi's superelasticity can provide design flexibility in aerospace and biomedical fields via integrating distinct material benefits. However, this materials couple is vulnerable to severe embrittlement due to the development of excessive intermetallic compounds (IMCs), namely Ti2Ni. Pd-free and Pd-interlayered NiTi-Ti6Al4V laser joints were evaluated for their microstructure, compositional changes, thermodynamic mechanism, and mechanical properties. The presence of Pd constrained the formation of Ti2Ni IMC, and NiTi-Ti6Al4V joints with excellent mechanical properties demonstrating superelastic behavior were achieved for the first time. The tensile strength and rupture strain of the Pd-added NiTi-Ti6Al4V joint improved more than twofold, reaching 520 MPa and 5.6%, respectively. During cyclic tensile testing, the Pd-added joint demonstrated superelasticity and a comparable irrecoverable strain to NiTi (1 versus 0.65%). Multiscale characterization revealed that the fraction of Ti2Ni decreased from 83 to 10% near the NiTi boundary and 24 to 6% at the weld center compared to the Pd-free joint. The superior thermodynamic formation tendency of Ti-Pd compounds over Ti2Ni IMC favored their development, and thus Ti-Pd and NiTi compounds dominated the fusion zone (FZ) at the expense of Ti2Ni IMC, explaining the improved mechanical performance of the Pd-added joint.

Role of low-level void swelling on plasticity and failure in a 33 dpa neutron-irradiated 304 stainless steel

Radiation damage introduces a wide range of defects in structural materials depending on the irradiation conditions. Irradiation-induced loops are known to lead to dislocation-channeling, which reduces the work hardenability and increases the susceptibility to intergranular stress corrosion cracking. In a higher temperature regime, void formation is an integral radiation-induced defect type in addition to radiation-induced loops. Previous bulk mechanical tests suggest that a high level of void swelling can lead to severe embrittlement while a low level of void swelling does not negatively affect intergranular cracking and ductility. This study utilized microscale bicrystal testing to evaluate the role of low-level void swelling on dislocation-channel suppression and intergranular cracking in a 33 dpa neutron-irradiated 304 stainless steels with 2% and 3.7% void swelling. The results provided direct observation on the suppression of dislocation-channel and intergranular cracking in the presence of low-level void swelling.

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.

Hydrogen Embrittlement of Inconel 718 Manufactured by Laser Powder Bed Fusion Using Sustainable Feedstock: Effect of Heat Treatment and Microstructural Anisotropy

This study investigated the in-situ gaseous (under 150 bar) hydrogen embrittlement behaviour of additively manufactured (AM) Inconel 718 produced from sustainable feedstock. Here, sustainable feedstock refers to the Inconel 718 powder produced by vacuum induction melting inert gas atomisation of failed printed parts or waste from CNC machining. All Inconel 718 samples, namely AM-as-processed, AM-heat-treated and conventional samples showed severe hydrogen embrittlement. Additionally, it was found that despite its higher yield strength (1462 ± 8 MPa) and the presence of δ phase, heat-treated AM Inconel 718 demonstrates 64% lower degree of hydrogen embrittlement compared to the wrought counterpart (Y.S. 1069 ± 4 MPa). This was linked to the anisotropic microstructure induced by the AM process, which was found to cause directional embrittlement unlike the wrought samples showing isotropic embrittlement. In conclusion, this study shows that AM Inconel 718 produced from recycled feedstock shows better hydrogen embrittlement resistance compared to the wrought sample. Furthermore, the unique anisotropic properties, seen in this study for Inconel 718 manufactured by laser powder bed fusion, could be considered further in component design to help minimise the degree of hydrogen embrittlement.

Spontaneous local redox reaction to passivate CNTs as lightweight current collector for high energy density lithium ion batteries

Extensive usage of highly conductive carbon materials with large specific surface area (e.g., carbon nanotubes, CNTs) in lithium ion batteries (LIBs), especially as current collector of anodes, suffers from low initial coulombic efficiency (ICE), large interfacial resistance, and severe embrittlement, as the large specific surface area often results in severe interfacial decomposition of the electrolyte and the formation of thick and fluffy solid electrolyte interphase (SEI) during cycling of LIBs. Herein, we demonstrate that when the CNT-based current collector and Na foil (which are being stacked intimately upon each other) are being placed in Na+-based organic electrolyte, local redox reaction between the Na foil and the electrolyte would occur spontaneously, generating a thin and homogeneous NaF-based passivating layer on the CNTs. More importantly, we found that owing to the weak solvation behaviors of Na+ in the organic electrolyte, the resulting passivation layer, which is rich in NaF, is thin and dense; when used as the anode current collector in LIBs, the pre-existing passivating layer can function effectively in isolating the anode from the solvated Li+, thus suppressing the formation of bulky SEI and the destructive intercalation of solvated Li+. The relevant half-cell (graphite as anode) exhibits a high ICE of 92.1%; the relevant pouch cell with thus passivated CNT film as current collectors for both electrodes (LiCoO2 as cathode, graphite as anode) displays a high energy density of 255 Wh kg−1, spelling an increase of 50% compared with that using the conventional metal current collectors.

Development of the high-strength ductile ferritic alloys via regulating the intragranular and grain boundary precipitation of G-phase

• Although the nano-dispersed precipitation of G-phase within the grain hardens the alloy remarkably, its large amount of precipitation at the grain boundary leads to serious alloy embrittlement. • For the first time, the crystal structure transition of nano-dispersed precipitates from the Fe 2 TiSi-L2 1 to the Ni 16 Ti 6 Si 7 -G phase is observed within the ferrite at the early stage of aging. • Guided by current TEM and DFT results, the interfacial energy is predicted to play a leading role in forming the cube-on-cube orientation relationship between α-Fe and G-phase. • Adopting the “cold-rolling and aging” process to effectively avoid its precipitation at grain boundaries, a class of high-performance ferroalloys with both high strength and good ductility is realized. A typical G-phase strengthened ferritic model alloy (1Ti: Fe-20Cr-3Ni-1Ti-3Si, wt.%) has been carefully studied using both advanced experimental (EBSD, TEM and APT) and theoretical (DFT) techniques. During the classic “solid solution and aging” process, the superfine (Fe, Ni) 2 TiSi-L2 1 particles densely precipitate within the ferritic grain and subsequently transform into the (Ni, Fe) 16 Ti 6 Si 7 -G phase. In the meanwhile, the elemental segregation at grain boundaries and the resulting precipitation of a large amount of the (Ni, Fe) 16 Ti 6 Si 7 -G phase are also observed. These nanoscale microstructural evolutions result in a remarkable increase in hardness (100 - 300 HV) and severe embrittlement. When the “cold rolling and aging” process is used, the brittle fracture is effectively suppressed without loss of nano-precipitation strengthening effect. Superhigh yield strength of 1700 MPa with 4% elongation at break is achieved. This key improvement in mechanical properties is mainly attributed to the pre-cold rolling process which effectively avoids the dense precipitation of the G-phase at the grain boundary. These findings could shed light on the further exploration of the precipitation site via optimal processing strategies.

A novel single-crystal L12-strengthened Co-rich high-entropy alloy with excellent high-temperature strength and antioxidant property

Severe intermediate-temperature embrittlement as well as rapid softening and oxidation at high temperatures are significant problems for high-entropy alloys (HEAs) in polycrystalline structures, which greatly limit their applications in modern industries. In this study, a novel Co-rich single-crystal high-entropy alloy (HEA), Co41Ni35Al11·5Ta2·5Cr4Ti6 (at.%), was innovatively developed. After isothermally aged at 1250 °C, this alloy exhibits an ultrahigh volume fraction of cuboidal L12-type nanoprecipitates (∼80%) which is higher than those of previous Co-based superalloys. More importantly, the present HEA shows excellent mechanical performance at both ambient and elevated temperatures, and no intermediate temperature embrittlement and rapid softening were observed. Noticeably, the yield strength can still maintain at ∼800 and ∼500 MPa when tested at 900 and 1000 °C, which are superior to other conventional single-crystal alloys. Moreover, extraordinary oxidation resistance can be achieved at the high temperature of 1000 °C due to the formation of a compact sandwich-structured oxide layer.

Correlation between grain size variation and hydrogen embrittlement in a cost-effective Fe40Mn40Ni10Cr10 austenitic medium entropy alloy

The effect of grain size variation (11 μm, 34 μm) on the hydrogen-induced tensile properties degradation of a Co-free cost-effective Fe40Mn40Ni10Cr10 austenitic medium entropy alloy was investigated using a slow strain rate test. Despite improving both strength and ductility with decreasing the grain size in non-charged conditions, the fine-grain alloy showed a higher relative elongation loss after electro-chemical hydrogen charging. The larger ductility loss in the fine-grain alloy was ascribed to the fast propagation rate of major intergranular cracks and the drastic strain hardening rate drop of the alloy under hydrogen charging. The Schmid factor analysis showed that the enhanced dislocation activity in the fine-grain alloy compared with coarse-grain one was responsible for rapid hydrogen transfer to the grain boundaries, fast dislocation pile-up behind the grain boundaries, and, consequently, more severe hydrogen embrittlement. The significant stress concentration near the grain boundaries and fast intergranular crack propagation were recognized to be the main reason for premature fracture in fine-grain alloy.

The effect of stress-annealing on the mechanical and magnetic properties of several Fe-based metal-amorphous nano-composite soft magnetic alloys

Fe-based metal-amorphous nanocomposites (MANCs) are the subject of many contemporary studies for their unique tunable soft-magnetic properties. The alloys can exhibit some of the smallest losses and highest power storage capabilities of available materials used in electromagnetic inductors. Here, the magnetic and mechanical properties of Fe72.5Nb2Mo2Cu1Si15.5B7 at.%, Fe68.5Co5Ta3Cu1Si16.5B6 at.%, Fe68.5Co5Ta3Cu1Si16B6.5 at.%, and Fe73.5Ta3Cu1Si16B6.5 at.% melt-spun alloys were investigated in relation to their ability to be stress-annealed to tune permeability while simultaneously minimizing losses in tape-wound inductor cores. Mechanical and physical properties of the as-cast and processed ribbons including relaxation and crystallization temperatures and failure strains were measured using thermomechanical analysis (TMA), uniaxial tension, and bend tests to help inform the stress-annealing parameters. Magnetic properties were measured for the as-cast and stress-annealed materials in both wound-cores and flat ribbon strips. The alloys exhibited severe embrittlement during heating, thus creating significant challenges when tuning magnetic properties with an in-line stress-annealing (SA) process. However, when conditions were optimized, core losses as low as 1.14 W·kg−1 were obtained when cores were excited at 10 kHz to a maximum induction field of 0.1 T, which are among the lowest reported in the literature. Based on this preliminary effort, understanding of the stress state in amorphous precursor ribbons prior to SA could reveal the key to developing more robust in-line SA processes for brittle Fe-based MANCs.

Mechanism of tellurium induced nickel alloy corrosion in molten LiCl-KCl salt

Fission product tellurium causes severe embrittlement of nickel alloys in nuclear molten fuel salt systems. This paper investigates the tellurium chemistry in molten LiCl-KCl-Li2Te salts and the tellurium reactions at the alloy interface. The reactions for the two-step reduction of Te0 and the electrochemical formation of nickel tellurides have been identified from cyclic voltammograms. The corrosion of UNS N10003 alloy with different tellurium salt chemistries and applied potentials found that tellurium corrosion attack is only present at potentials more positive than the Ni1.43Te/Te2− potential. The critical potential is thus suggested for mitigating tellurium embrittlement using the redox control technique.

Disposing of excessive decomposition and destructive intercalation of solvated Li+ in CNT-based flexible 3D Si anode of flexible battery

• A sieving-out strategy by replacing Li + with Na + cations in the electrolyte in the first cycle. • A sieving layer disposing of excessive decomposition and destructive intercalation of solvated Li + in CNT. • A significant increase in ICE at the first cycle is obtained for the as prepared 3D Si/CNT electrode. • Excellent cycle performance of the flexible 3D Si/CNT anode is due to the precise sieving layer. Carbon nanotubes (CNTs) suffer from low initial coulombic efficiency, large interface resistance and severe embrittlement when employed in flexible lithium-ion batteries (LIBs). We reveal in this paper that these issues are essentially caused by the excessive decomposition and destructive intercalation of solvated Li + due to the large electrochemical surface area and the defects in CNTs. We further demonstrate that Na + is much less solvated than Li + in carbonate-based electrolyte and the solvated Na + possesses poor affinity with graphitic carbon materials. Therefore, to simply replace solvated Li + with solvated Na + in the first cycle enables electrochemical deposition of an ultrathin NaF-containing solid electrolyte sieving layer on the CNTs, which isolates them from solvated Li + and suppresses both the extensive formation of thick bulky SEI (which causes large interfacial resistance) and the destructive intercalation of solvent molecules (which embrittles CNTs) when being employed in LIBs. Si/CNT electrode with such NaF-containing solid electrolyte sieving layer displays excellent flexibility and high specific capacity of 1042 mAh g −1 after 500 cycles, which is 3 times higher than those with copper foil or pristine CNTs as current collectors. This study have revealed the dramatic differences between solvated Na + and solvated Li + , and provided new insight into practical application of CNTs in flexible LIBs. A sieving-out strategy by replacing Li + with Na + cations in the electrolyte in the first cycle, which reveals dramatic difference of the dissolution and solvation behaviors between these two alkali cations in the same solvent enables practical application of CNT-based current collectors for flexible electrodes. And verify the above speculations by using a 3D freestanding Si/CNT network as an example.

Severe embrittlement of copper pillar bumps electrodeposited using JGB as leveler

Severe embrittlement of copper pillar bumps electrodeposited using JGB as leveler

The effect of Nb on the hydrogen embrittlement susceptibility of Q&P steel under static and dynamic loading

In the present study, the effect of Niobium (Nb) on the hydrogen embrittlement resistance of Quenched and Partitioning (Q&P) steel is investigated. For this purpose, the hydrogen uptake level and its impact on the mechanical properties of a Nb-free and a 0.024 wt% Nb Q&P steel are thoroughly analysed. The hydrogen trapping capacity is evaluated via thermal desorption spectroscopy (TDS). In-depth analysis of the desorption kinetics at different heating rates allows identification and quantification of the available trapping sites. The hydrogen embrittlement sensitivity of both steels is characterized using static and dynamic tensile tests. The addition of Nb results in an increase of the hydrogen concentration by more than 25%. The larger hydrogen content in the Nb steel, as a result of the higher fraction of grain boundaries/interphases, gives rise to a more severe embrittlement of the Nb steel compared to the Nb-free one. In addition to the larger hydrogen fraction in the Nb Q&P steel, the larger retained austenite fraction of low stability is detrimental due to the larger fraction of high carbon martensite formed when straining. This results in higher susceptibility to hydrogen embrittlement of the Nb microalloyed steel due to the brittle character of the high carbon martensite that forms easily during straining. Under dynamic loading conditions, the hydrogen embrittlement of both steels is minimal, which is attributed to a reduced hydrogen diffusion and the suppression of the transformation induced plasticity (TRIP) effect due to adiabatic heating.