Brittle Intergranular Fracture Research Articles

Hydrogen-induced cracking behaviors of Ni–Cr–Mo-based superalloy fabricated by wire arc additive manufacturing under different solution temperature

The hydrogen embrittlement behavior of Ni–Cr–Mo-based superalloy fabricated by wire arc additive manufacturing (WAAM) with or without solution treatment was investigated by electrochemical hydrogen charging. The results show that the dissolution of the Laves phase promotes the precipitation of the δ phase in the matrix of WAAM superalloy after solution treatment at 980 °C. When the solution temperature goes up to 1080 °C, the large Laves phase disappears and only MC exists in the matrix. The interfaces between the Laves phase/δ phase and matrix generally could act as hydrogen traps, which would reduce the motion of hydrogen and lead to high hydrogen embrittlement resistance of the WAAM and ST-980 °C samples. However, hydrogen mainly accumulates on grain boundaries in the ST-1080 °C due to the disappearance of the Laves phase and δ phase. Hydrogen-induced intergranular brittle fracture occurs during the slow strain rate tensile test and the ST-1080 °C samples exhibit higher hydrogen embrittlement susceptibility than that of the WAAM and ST-980 °C samples.

Comparative Analysis of Dip-Brazing and TIG Welding on the Properties of Al-64430 Joints

This study compares dip brazing and TIG welding operations carried out under optimal conditions on the parameters of strength, deformation, and ability to resist shocks for Al-64430 joints, which find extensive use in automobile and telecommunication industries. The fabricated samples were tested for bump test, surface deformation, microhardness and tensile strength, and the joint properties were studied through SEM, EDAX mapping and EDX. Compared to the dip-brazed samples, the TIG welded samples achieved 161% greater strength and 1000% greater elongation but subsequently 1330% higher surface deformation. Both joints were able to pass the bump test; however, some degree of cracks were observed post testing in both samples. Both samples achieved similar joint microhardness values, but notably, compared with the TIG welded sample, the brazed sample showed no HAZ zones in terms of microhardness variation, where a substantial decrease in microhardness was recorded. This absence of the HAZ region was also confirmed through the microstructure. Both primary and secondary silicon were observed in brazed samples, which had finer grain sizes than did the welded samples. Dendrite formation was observed for the brazed samples, whereas a uniform distribution of aluminium was observed for the welded samples. Fractographic analysis revealed brittle intergranular fracture with small dimples present at the fracture point for the brazed joints, while out-of-plane fracture with sharp edges indicated a more ductile nature for welded joints.

Effect of direct-quenching and tempering on hydrogen-induced delayed fracture resistance of high-strength bolt steel

This investigation attempts to explore the potential significance of controlled forging, i.e., direct-quenching at two different finish-forging temperatures of ∼930 °C (B8H sample) and ∼780 °C (B8L sample), and tempering at 600 °C in improving the hydrogen-induced delayed fracture (HIDF) resistance of a 1300-MPa-grade high-strength bolt steel by constant load tensile testing using circumferentially notched round bar specimens and hydrogen thermal analysis. The microstructural characteristics of the tested samples were analyzed and their influences on HIDF were discussed. The results showed that the B8L sample demonstrated a considerably increased HIDF resistance expressed by critical notch delayed fracture stress of 1825 MPa and a decreased hydrogen embrittled index of as low as 21% compared with those of 1675 MPa and 27% for the B8H sample, although the former exhibited slightly higher strength and absorbed hydrogen content. Scanning electron microscopy observation of the fracture surface demonstrated that the B8L sample prevented the brittle intergranular fracture with smaller quasi-cleavage facet and decreased area fraction of the brittle crack initiation zone compared with the B8H sample. The increased resistance to HIDF of the B8L sample is primarily because of its fine microstructure with increased number of finely distributed V-rich nanoscale MC precipitates and decreased dislocation density. It is proposed that controlled forging coupled with V-microalloying is a feasible and economical way to further increase the HIDF resistance of high-strength bolt steel.

Interfacial microstructure and mechanical property of the discontinuous short tungsten fiber-reinforced tungsten matrix composites

For tungsten fiber-reinforced tungsten matrix (Wf/W) composites, the interface between W matrix and W fiber has a dominant effect on the mechanical properties. In this study, the interfacial microstructure in the Wf/W composites with or without Y2O3 coating deposited on W fiber is investigated. The crystal structure of the Y2O3 coating composed of nano-sized columnar grains on the W fibers is cubic. And the crystal structure of Y2O3 coating changes to be face centered cubic after sintering in the Wf/W composites. Two kinds of interfaces, including the interface between W matrix and Y2O3 coating, the interface between W fiber and Y2O3 coating, are present in the Wf/W composites. Different orientation relationships are observed in these two kinds of interface. The stable orientation relationship with 111Y2O3 // 01¯1W have the lowest interfacial energy. The Y2O3 interface is essential for improving the toughening behavior of Wf/W composites and impeding the abnormal grain growth on the surface of W fiber sintering at low temperature. The recrystallization nucleus is developed by the bonding between W powder and W fiber during sintering, and then grow fast into W fiber. The apparent activation energy of abnormal grain growth of W fibers is about 242.7 KJ/mol sintering at temperature between 1500 and 1800 °C. The abnormal large grains usually show intergranular brittle fracture and make the W fiber be brittle.

The liquid metal embrittlement of a reactive system at room temperature: α-brasses in contact with the liquid eutectic Ga-In

We studied the liquid metal embrittlement (LME) at room temperature of α-brasses by the liquid eutectic Ga-In (EGaIn). The EGaIn partially wets the α-brasses and forms the CuGa2 intermetallic. 3-point bending tests, Finite Element Analysis, and in situ testing were conducted. When LME occurs, there is a ductile fracture followed by an intergranular brittle fracture that propagates through the zones with the highest plastic strain. The LME sensitivity increases with higher strain rates, higher Zn content, and higher work hardening rates, suggesting a significant role of dislocation mobility in LME and a lack of influence from the CuGa2 intermetallic.

Enhanced hydrogen embrittlement resistance of additive manufacturing Ti-6Al-4V alloy with basket weave structure

The hydrogen embrittlement (HE) behavior of tungsten inert gas arc additive manufacturing (AM) Ti-6Al-4V alloy was studied through tensile tests with/without hydrogen charging. The influence of hydrogen on the microstructure and crack propagation of AM Ti-6Al-4V alloy was investigated. The results indicated that the AM Ti-6Al-4V alloy with a basket weave structure alleviated local stress at grain boundaries during deformation, thus avoiding intergranular cracking and brittle fracture. The AM alloy displayed less HE susceptibility than that of the wrought alloy, which was due to the basket weave structure hindering the propagation of hydrogen-induced cracks, thereby improving the HE resistance.

Influence of microstructure evolution and element segregation on high temperature tensile behaviors in GH4706 alloy

This work systematically investigated the influence of microstructure evolution and element segregation on tensile properties and fracture behaviors of GH4706 superalloy under different temperatures (25 °C, 400 °C, 500 °C, 600 °C and 650 °C). The advanced methods of TEM, AES and SIMS were conducted to characterize the variation of microstructure and element distribution. Experimental results showed that the yield and tensile strength of the test alloy decreased while the corresponding elongation increased as the increment of test temperature. The dominated fracture mode was intergranular brittle fracture at the temperature of 25 °C, gradually transforming to ductile dimple fracture as the temperature increased. This phenomenon was determined to be closely related to the increment of average grain size and size of γ'/γ″ coprecipitates as temperature rose. It was worth noticing that the elongation performed abnormally low (16.3%) at the temperature of 500 °C, exhibiting intermediate temperature brittleness (ITB) phenomenon. The SIMS analysis showed that the segregation of S towards grain boundary was the most obvious at the temperature of 500 °C, which was considered as the main factor leading to ITB. In addition, the coarsening of carbides at grain boundaries and local strain non-uniformity caused by dislocations could also resulted in ITB. The main reason for maintaining good strength and plasticity at the temperature of 650 °C was contributed to the softening of γ' phases by repeated cutting of dislocations and deformation twins, reducing the hindrance to the movement of dislocations. Moreover, composite strengthening effect of P and B could improve bonding strength, stabilize grain boundaries, and effectively suppress the initiation of intergranular cracks.

Fracture prediction on hydrogen-charge notched samples using a stress-state-dependent phenomenological model

There is a consensus for transitioning to clean energy sources to meet the world’s energy demands. Hydrogen is a candidate to supply green energy. However, hydrogen embrittlement (HE) in materials is an issue that needs to be assessed. Based on the damage concept, a straightforward and robust computational framework is presented to predict the harmful effects on material resistance caused by the HE phenomenon. The adopted model framework builds upon dealing with the coupled mechanical diffusion problem in a simplified approach by considering the local equilibrium between hydrogen concentration at interstitial sites and Cauchy stresses. A modified version of the Mohr–Coulomb criterion dependent on the stress triaxiality and Lode angle introduces a damage indicator parameter. The proposed model is applied to study the brittle intergranular fracture of a high-strength AISI 4135 steel. A detailed description of the stress state and hydrogen-driven failure parameters are presented for two specimens with different notch radii. The proposed model is implemented in a VUMAT subroutine and made available for download to the technical community for research and educational use. After calibrating the model parameters, the numerical results yield good agreements of the predicted fracture strength of notched hydrogen-charged samples compared to the experimental ones. Maximum differences of around 10% among experimental and numerical tests are observed.

Effect of V and Mn addition on the HAZ softening and tensile properties of friction stir welded martensitic steel

This study explores the suppression of heat-affected zone (HAZ) softening in friction stir welding (FSW) joints formed above the A3 temperature of martensitic steels by leveraging short-time secondary hardening induced by vanadium (V) addition. The HAZ microstructure was examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT). Mechanical properties were evaluated through Vickers hardness (HV) measurements and tensile tests. Analysis of hardness distribution indicates effective HAZ softening suppression with V addition, attributed to the precipitation of fine and high-density V carbides within the HAZ. The inhibition of HAZ softening was correlated not only with V content but also with the A1 temperature of the steels, regulated by the manganese (Mn) content. However, at 1 mass% V content, intergranular brittle fracture occurred within the HAZ due to continuously distributed fine carbides and Mn segregation along the prior austenite boundary.

Substantially improved room-temperature tensile ductility in lightweight refractory Ti-V-Zr-Nb medium entropy alloys by tuning Ti and V content

Lightweight high/medium-entropy alloys (H/MEAs) possess attractive properties such as high strength-to-weight ratios, however, their limited room-temperature tensile ductility hinders their widespread engineering implementation, for instance in aerospace structural components. This work achieved a transformative improvement of room-temperature tensile ductility in Ti-V-Zr-Nb MEAs with densities of 5.4–6.5 g/cm3, via ingenious composition modulation. Through the systematic co-adjustment of Ti and V contents, an intrinsic ductility mechanism was unveiled, manifested by a transition from predominant intergranular brittle fracture to pervasive ductile dimpled rupture. Notably, the modulated deformation mechanisms evolved from solitary slip toward collaborative multiple slip modes, without significantly compromising strength. Compared to equimolar Ti-V-Zr-Nb, a (Ti1.5V)3ZrNb composition demonstrated an impressive 360% improvement in elongation while sustaining a high yield strength of around 800 MPa. Increasing Ti and V not only purified the grain boundaries by reducing detrimental phases, but also tailored the deformation dislocation configurations. These insights expanded the applicability of lightweight HEAs to areas demanding combined high strength and ductility.

Tensile creep behavior of the Nb45Ta25Ti15Hf15 refractory high entropy alloy

The tensile creep behavior of a vacuum arc-melted Nb45Ta25Ti15Hf15 refractory high entropy alloy was investigated over a constant true stress range of 50–300 MPa at a temperature of 1173 K. Creep tests were carried out in both high vacuum (5 × 10−6 torr) and ultrahigh purity Ar gas to examine the environmental effect. The samples tested in vacuum exhibited power law behavior with a stress exponent of 4.1 and exceptional tensile creep ductility, whereas those tested in Ar suffered significant embrittlement due to HfO2 formation at grain boundaries, which was exacerbated at low applied stresses where extended exposure to residual O2 gas resulted in more extensive brittle intergranular fracture. Phase decomposition occurred after long-term thermal exposure, where a second Hf-rich body-centered cubic phase formed predominantly at grain boundaries but did not cause embrittlement. Compared to the equiatomic TaNbHfZrTi (Senkov alloy) and face-centered cubic multiple-principal element alloys, Nb45Ta25Ti15Hf15 has superior creep resistance, especially at high applied stresses, while maintaining excellent creep ductility. Transmission electron microscopy revealed that creep deformation in Nb45Ta25Ti15Hf15 at 1173 K is controlled by cross-kink collisions from screw dislocations that results in dipole drag at lower strain rates and jog drag at higher strain rates.

Effects of filler materials on the microstructural characteristics and mechanical properties of laser-welded lap joints of TZM alloys

The poor weldability of molybdenum (Mo) alloys has always been the main problem that impedes their application as large-sized structural components. Two types of laser-welded lap joints, namely LW-Ti and LW-Nb joints, were prepared using titanium (Ti) and niobium (Nb) as filler materials to investigate the alterations in microstructures and mechanical properties of TZM (Mo-0.5Ti-0.08Zr-0.02C) alloy joints. The results indicate that both Ti and Nb have excellent metallurgical compatibility with Mo, resulting in well-formed joints. Due to solid-solution strengthening, the microhardness of the fusion zone (FZ) significantly increases from 269.9 HV to 397.1 HV for LW-Ti joints and reaches 389.6 HV for LW-Nb joints. Furthermore, the shear strength of LW-Ti joints is approximately 2.05 times that of LW-Nb joints, measuring at 110.97 MPa. Fractures in LW-Ti joints primarily occur at the base metal (BM) and display lamellar ductile fracture characteristics, while fractures in LW-Nb joints mainly occur at the FZ and exhibit coarse brittle intergranular fractures. The mechanical properties of LW-Nb joints are compromised due to the larger maximum pore diameter (300 μm) in the FZ compared to LW-Ti joints (50 μm), which can be attributed to the higher viscosity of liquid niobium. Additionally, the average grain size in the FZ of LW-Nb joints (86.37 μm) is larger than that in LW-Ti joints (53.28 μm). The reduction in grain size observed in LW-Ti joints also increases the grain boundary area, decreases oxygen concentration along the grain boundaries, and enhances binding strength at Mo grain boundaries. Given Ti's stronger affinity for oxygen compared to Mo, the abundant formation of second-phase TiO2 particles within LW-Ti joints hinders the specific formation of MoO2 along grain boundaries. Consequently, the proposed welding technology was employed to weld a large-scale cylinder-flange component made from Mo alloy, which is expected to greatly facilitate the widespread utilization of welded structures made from Mo alloys.

Influence of corrosion on ultra-low cycle fatigue performance of steel butt-welded joints with various welding methods

Preferential corrosion of welded joints in steel structures has been observed in a wide range of scenarios and should receive enough attention. The current work focuses on the corrosion and ultra-low cycle fatigue (ULCF) behavior of low-alloy steel weld joints. Firstly, 1600 h neutral salt spray (NSS) tests were conducted on welded joints made by different welding methods, and then topography scanning of corrosion pits was carried out. It was found that the welded region is more susceptible to corrosion. Dynamic polarization curves of the welded joints were measured, and the results show that the self-corrosion potential of the welded region is lower than that of the base metal (BM) region, while the corrosion current density is higher than that of the BM region. Subsequently, cyclic tests were carried out on the corroded weld joints. The results show that corrosion can cause a maximum reduction of 11.79% in yield load and 20.87% in ultimate load of the welded joints. Corrosion has slight influence on the energy dissipation capacity of welded joints, but the maximum reductions in ULCF life could reach 77.31% due to corrosion. Fractography analysis further shows that secondary cracks, map cracks, and intergranular brittle fracture, apart from more common fatigue fracture, ductile fracture, and quasi-cleavage fracture, appear on the fracture surfaces. Supplementary numerical simulation was conducted to enable a better understanding of the failure processes. In the simulation, a geometric modeling method using topography scanning data of corrosion surface was proposed. By using the proposed modeling approach, together with Chaboche cyclic plasticity constitutive model and cyclic void growth model (CVGM) for capturing fracture behavior, the predicted results show an acceptable agreement with the experimental ones.

Effect of the addition of titanium filler on high temperature strength and microstructural characteristics of laser welded tube-end plug socket joints of molybdenum alloy

Three types of Mo alloy socket joints were prepared in this study: a joint without alloying (LW joint), a joint with a trace amount of titanium (Ti) added to the fusion zone (FZ) (LW-Ti-A joint), and a joint with Ti separately added to the FZ and the heat affected zone (HAZ) (LW-Ti-B joint). Tensile tests were carried out on the three types of joints at 360, 900, and 1200 °C. The results showed that the tensile strengths of the base metal (BM) of the Mo tubes were 368.7, 239.9, and 189.6 MPa at 360, 900, and 1200 °C, respectively. At all temperatures, the tensile strengths of the welded joints increased in the order of LW, LW-Ti-A, and LW-Ti-B, which indicates that adding Ti to the FZ and HAZ separately had an obvious strengthening effect. The fracture positions of the joints were primarily affected by the Ti alloying scheme. The LW joints are always fractured at the FZ, showing brittle intergranular fractures; the LW-Ti-A joints fractured at the BM or the HAZ near the FZ; and the LW-Ti-B joints fractured at the BM far from the FZ, exhibiting ductile fracture. Adding Ti to the FZ can induced a solid-solution strengthening effect, which refined the grain size in the FZ by approximately 30–40% and reduced the amount of Mo oxide on the grain boundaries in the FZ, thus improving the joint strength. Presetting a Ti foil on the overlapping interface in the HAZ realized metallurgical bonding between the Mo tube and Mo end plug in the HAZ of the laser-welded joints, enlarge the bearing area, and improve the joint strength. And a new metallurgical bond zone is formed on the overlapping interface in the HAZ during the high-temperature tensile tests of the welded joints, which further improve the joint strength.

Revealing the room temperature superplasticity in bulk recrystallized molybdenum

Body-centered cubic refractory metallic materials exhibit excellent high-temperature strength, but often suffer from brittle intergranular fracture due to the recrystallization-induced enrichment of trace elements at grain boundaries (GBs). Here, we report a fully-recrystallized pure molybdenum (Mo) material with room temperature (RT) superplasticity, fabricated by a facile method of powder metallurgy, Y-type hot rolling and annealing. By engineering the ultralow concentration of O at GBs, the inherent GB brittleness of Mo can be largely eliminated, which, in conjunction with high fractions of soft texture and low angle GBs, enables a significant development of ordered dislocation networks and the effective dislocation transmission across low angle GBs. Synergy of these factors greatly suppress the brittle intergranular fracture of Mo, contributing to an enhanced deformability of 108.7% at RT. These findings should have general implication for fabricating a broad class of refractory metals and alloys toward harsh applications.

Enhancing the hydrogen embrittlement resistance of medium‑carbon high-strength steel by optimizing the tempering temperature

Hydrogen embrittlement (HE) and hydrogen diffusion in medium‑carbon high-strength steel tempered at various temperatures were investigated by in situ slow strain rate testing and electrochemical hydrogen permeation, respectively. Synergistic correlations between the various microstructural components and reversible/irreversible hydrogen traps were established. Dislocations and the size, shape, distribution, and volume fraction of cementite were found to considerably influence the HE susceptibility. High dislocation density and interconnected needle-like cementite with a high aspect ratio, oriented along grain boundaries, were produced at a low tempering temperature (560 °C); this facilitated the decohesion of atoms at the cementite–matrix interface, enabling them to act as crack propagation sites and inducing brittle intergranular fracture. However, a significantly improved HE resistance associated with a quasi-cleavage-based fracture morphology was achieved at a high tempering temperature (680 °C) owing to the low dislocation density and the presence of uniformly distributed spheroidised cementite.

Mechanism of enhanced room temperature ductility of cold–rolled tungsten under micro-bending test

The enhanced ductility of cold-rolled (CR) tungsten at room temperature is considered to be related to the activities of dislocations and the associated microstructures. However, the specific mechanism and the exact roles that dislocation activity and microstructure effects play in enhancing ductility in CR tungsten are still subjects of conjecture. In this context, three-dimensional electron backscatter diffraction reveals that a CR tungsten micro-cantilever fractures in a semi-brittle manner within the L-T fracture system. The ductile, blunted region shows high dislocation mobility away from the cracking tip, and its emission pathway is strongly correlated with low-angle grain boundaries. Meanwhile, strain and dislocations blocked at high-angle grain boundaries induces stress cracking and results in brittle intergranular fracture. These ductile and brittle failure modes trigger the crack bridging effect, ultimately leading to controlled crack propagation. This work would provide valuable idea for future investigations into the ductility enhancement of ultra-fine grain W.

M23C6-assisted formation mechanism of grain boundary cellular structure and its role on the brittle fracture in an austenitic Fe-Mn-Al-C lightweight alloy

The austenitic Fe-Mn-Al-C lightweight alloy is limited using its full-hardened state due to its brittleness after long-term aging treatment. This study investigates the formation mechanism of grain boundary precipitates and their effect on crack initiation and propagation. The alloy showed typical age-hardening behavior but exhibited intergranular fracture after a critical aging time. TEM study revealed that the amount of Mn segregation increased and Al decreased at the grain boundary at the early aging stage. M23C6 is first formed at the grain boundary, and α-ferrite is formed at the coherent M23C6/γ interface. Finally, the grain boundary cellular structure of α-ferrite+κ-carbide is formed by the continuous formation of α + κ and growing into their incoherent interfaces with γ. As this cellular structure decorates the grain boundaries, the fracture type changed from ductile to intergranular brittle fracture. The grain boundary cracks initiated at the coarse grain boundary κ-carbide and propagated through the {100} planes of α and κ. The lowest theoretical cohesive strengths in the {100} planes of both phases are suggested as the origin of the crack initiation and propagation.

The effect of Al addition on the microstructure, magnetostrictive and mechanical properties of Fe73Ga27 alloy

Fe-Ga alloy exhibits two magnetostrictive peaks when the Ga content (at%) is 19 and 27, respectively but most studies have focused on improving the performance of Fe81Ga19. In order to reduce the manufacturing cost of the alloys and achieve better mechanical properties, Al element is used to replace part of the Ga of Fe73Ga27 alloy in this experiment. So long as the magnetostrictive properties does not decrease significantly, which is in line with expectations from an engineering application perspective. Fe73Ga27−xAlx (x = 0,1,2,3,4,5) alloys were prepared by vacuum arc furnace and the effect of Al addition on the microstructure, magnetostrictive properties, and mechanical properties of the alloys is studied. The results show that the phase structure of alloys is chiefly composed of A2 phase and the metallographic structure of the alloys is comprised of equiaxed and columnar grains. The fracture morphology of the alloys is principally intergranular brittle fracture and cleavage fracture, and the reasons for the alloys fracture include the detachment of Fe and Al elements and the segregation of Fe, Ga, Al elements. Except for Fe73Ga27 alloy, the tensile strength, elongation, and Vickers hardness of the alloys are ameliorated when the content of Al added is equal to 4 at% while the parallel magnetostrictive strain(λ∥) of the alloy crested 81 ppm, which is 24.3% higher than Fe73Ga27.

Pre-quenching induced lath structures and enhanced TRIP effect to optimize the strength-ductility of ultrahigh-strength hot-galvanized steel

Compared with the conventional hot-dip galvanized steel, an optimized strength-ductility combination was obtained by the combination of continuous galvanizing line (CGL) compatible quenching and austempering (QAT) treatment and pre-quenching, i.e., “pre-quenching + CGL-QAT”. The effects of the pre-quenching on the microstructure and mechanical performance were analyzed by using dilatometry, SEM, AES-EBSD, TEM and XRD technique. Compared with QAT samples, pre-quenching could eliminate the blocky M-A islands and promote the formation of lath reversed austenite and bainite ferrite in “pre-quenching + CGL-QAT” samples. Meanwhile, a large fraction (22.7–25.8%) of lath RA were obtained, such that brittle intergranular fracture can be prevented. The lath bainite/ferrite structures and sufficient TRIP effect enhanced the strength-ductility combination with an UTS reaching 1000 MPa and TEL around 30%. The fracture mainly presents ductile fracture and results in a ductile fracture surface with large dimples. The excellent strength-ductility combination with a UTS above 980 MPa, a YS of 625 MPa, a TEL of 29.1% and a higher PSE around 30 GPa·% can be obtained, which exceeds the current commercial hot-dip galvanized steel grades.