Microvoid Coalescence Research Articles

Hydrogen embrittlement of an interstitial equimolar high-entropy alloy

We investigated the hydrogen embrittlement mechanism in an interstitially carbon alloyed equimolar CoCrFeMnNi high-entropy alloy (HEA) through low strain rate tensile testing under in-situ hydrogen charging. The tensile ductility was significantly reduced by hydrogen charging. The failure mode of the interstitial HEA in presence of hydrogen was a combination of intergranular and transgranular fracture as well as microvoid coalescence. Aggregated nano-carbides act as potential sites for crack initiation. These findings show that the carbon alloyed equimolar high-entropy alloy is susceptible to hydrogen embrittlement.

Fracture mechanisms of a Mo alloyed CoCrFeNi high entropy alloy: In-situ SEM investigation

This paper is focused on an experimental investigation on the fracture property and mechanisms of a Mo alloyed high entropy alloy (HEA) CoCrFeNiMo0.2 that demonstrates a promising damage tolerance capacity under room temperature. The employed experimental means is the recently developed in-situ scanning electron microscope (SEM) testing system that allows observation on the deforming material surface in real time and under variable spatial resolutions. A dedicated fracture test on the HEA is performed, and the overall process of crack nucleation and propagation is recorded thoroughly via the in-situ SEM. A remarkable finding of the tested specimen is the elevated steady-state crack-tip-opening angle (CTOA) during the stable crack extension, indicating a satisfactory crack resistance capacity of the studied material. Further analysis on the crack propagation demonstrates that the widespread Cr-rich hard intermetallic particles act as the weak sites for microvoid nucleation, and the high energy ductile fracture mechanism characterized by the microvoid nucleation, growth and coalescence is well identified. Intensified plastic straining along the crack propagation path in a zigzag pattern is remarked, and multiple deformation mechanisms are recognized in the crack tip plastic zone, involving both slip and twinning. The observed microstructural changes are consistent with the macroscopic evaluation on the fracture toughness of the material, and they are accountable for the excellent damage tolerance capacity of the investigated HEA.

Thermomechanical response of aluminum alloys under the combined action of tensile loading and laser irradiations**Project supported by the National Natural Science Foundation of China (Grant No. 61605079) and the Fundamental Research Funds for the Central Universities, China (Grant No. 30916014112-020).

This study reports the investigation of the thermomechanical behavior of aluminum alloys (Al-1060, Al-6061, and Al-7075) under the combined action of tensile loading and laser irradiations. The continuous wave ytterbium fiber laser (wavelength 1080 nm) was employed as the irradiation source, while tensile loading was provided by the tensile testing machine. The effects of various pre-loading and laser power densities on the failure time, temperature distribution, and the deformation behavior of aluminum alloys are analyzed. The experimental results represent the significant reduction in failure time for higher laser power densities and for high preloading values, which implies that preloading may contribute a significant role in the failure of the material at elevated temperature. Fracture on a microscopic scale was predominantly ductile comprising micro-void nucleation, growth, and coalescence. The Al-1060 specimens behaved plastically to some extent, while Al-6061 and Al-7075 specimens experienced catastrophic failure. The reason and characterization of material failure by tensile and laser loading are explored in detail. A comparative behavior of under-tested materials is also investigated. This work suggests that studies considering only combined loading are not enough to fully understand the mechanical behavior of under-tested materials. For complete characterization, one should consider the effect of heating as well as loading rate and the corresponding involved processes with the help of thermomechanical coupling and the thermal elastic-plastic theory.

Impact butt welding of NiTi and stainless steel- An examination of impact speed effect

Decreasing the impact speed can extend the effective heating time and escalate the heating rate. Additional flash is generated near the NiTi/SS interface due to the lengthening of the effective heating time. With decreasing impact speed, the plastic deformation zone of the SS is enlarged, and the microstructure in the heat-affected zone (HAZ) is increasingly coarsened, but those of NiTi demonstrate the opposite trends. In all of the NiTi/SS joints, the weld consists of a diffusion layer with a thickness that increases slightly from 1 μm to 1.7 μm as the impact speed decreases from 40 mm/s to 27.5 mm/s. The mechanical properties of the joint deteriorate with decreasing impact speed due to the increased remnant of semi-molten NiTi at the interface. The joint welded at an impact speed of 40 mm/s has the highest strength of 522 ± 41MPa with (7 ± 2)% rupture elongation, and it fractures via micro-void coalescence.

Environmental Degradation Effect of High-Temperature Water and Hydrogen on the Fracture Behavior of Low-Alloy Reactor Pressure Vessel Steels

Structural integrity of reactor pressure vessel (RPV) in light water reactors (LWR) is of highest importance regarding operation safety and lifetime. The fracture behaviour of low-alloy RPV steels with different dynamic strain aging (DSA) & environmental assisted cracking (EAC) susceptibilities in simulated LWR environments was evaluated by elastic plastic fracture mechanics tests (EPFM) and by metallo- and fractographic post-test analysis. Exposure to high temperature water (HTW) environments at LWR temperatures revealed only moderated reductions in the fracture initiation and tearing resistance of low alloy RPV steels with high DSA or EAC susceptibility, accompanied with a moderate, but clear change in fracture morphology, which indicates the potential synergies of hydrogen/HTW embrittlement with DSA and EAC under suitable conditions. The most pronounced degradation effects occurred in a) RPV steels with high DSA susceptibility, where the fracture initiation and tearing resistance reduction increased with decreasing loading rate and were most pronounced in hydrogenated HTW and b) high sulphur steels with high EAC susceptibility in aggressive occluded crevice environment and with preceding fast EAC crack growth in oxygenated HTW. The moderate effects are due to the low hydrogen availability in HTW together with high density of fine-dispersed hydrogen traps in RPV steels. Stable ductile transgranular tearing by microvoid coalescence was the dominant failure mechanism in all environments with additional varying few % of secondary cracks, macrovoids and quasi-cleavage in HTW. The observed behavior suggests a combination of plastic strain localisation by the Hydrogen-enhanced Local Plasticity (HELP) mechanism, in synergy with DSA, and Hydrogen-enhanced Strain-induced Vacancies (HESIV) mechanism with additional minor contributions of Hydrogen-enhanced Decohesion Embrittlement (HEDE) mechanism.

Investigation of In-service Creep Failures of Aircraft Ti-6Al-4V Bolts

Three Ti-6Al-4V bolts from mid-life jet aircraft failed during service operation. Each of the failed bolts were installed on a landing gear component. Metallurgical failure analysis indicated that the fracture mechanism is low temperature creep, possibly resulting from a sustained tension load over a long time. Visual inspection revealed ductile fracture morphology with no macroscopic deformation or corrosion. For all bolts fracture occurred close to the root at the first pitches of the thread. Scanning Electron Microscope (SEM) analysis revealed dimple morphologies with micro-void coalescence. Energy Dispersive X-Ray (EDX) analysis did not reveal any material deficiency that would have precipitated failure.

KIc Testing for Micro-void Coalescence Type Fracture with Small Specimens and Assessment of Plane Strain Condition

<I>K_Ic</I> Testing for Micro-void Coalescence Type Fracture with Small Specimens and Assessment of Plane Strain Condition

A modified Gurson-type plasticity model at finite strains: formulation, numerical analysis and phase-field coupling

The modeling of failure in ductile materials must account for complex phenomena at the micro-scale, such as nucleation, growth and coalescence of micro-voids, as well as the final rupture at the macro-scale, as rooted in the work of Gurson (J Eng Mater Technol 99:2–15, 1977). Within a top–down viewpoint, this can be achieved by the combination of a micro-structure-informed elastic–plastic model for a porous medium with a concept for the modeling of macroscopic crack discontinuities. The modeling of macroscopic cracks can be achieved in a convenient way by recently developed continuum phase field approaches to fracture, which are based on the regularization of sharp crack discontinuities, see Miehe et al. (Comput Methods Appl Mech Eng 294:486–522, 2015). This avoids the use of complex discretization methods for crack discontinuities, and can account for complex crack patterns. In this work, we develop a new theoretical and computational framework for the phase field modeling of ductile fracture in conventional elastic–plastic solids under finite strain deformation. It combines modified structures of Gurson–Tvergaard–Needelman GTN-type plasticity model outlined in Tvergaard and Needleman (Acta Metall 32:157–169, 1984) and Nahshon and Hutchinson (Eur J Mech A Solids 27:1–17, 2008) with a new evolution equation for the crack phase field. An important aspect of this work is the development of a robust Explicit–Implicit numerical integration scheme for the highly nonlinear rate equations of the enhanced GTN model, resulting with a low computational cost strategy. The performance of the formulation is underlined by means of some representative examples, including the development of the experimentally observed cup–cone failure mechanism.

Hydrogen-assisted failure in a bimodal twinning-induced plasticity steel: Delamination events and damage evolution

The effect of the bimodal grain size distribution on the hydrogen susceptibility of a high-Mn fully austenitic twinning-induced plasticity (TWIP) steel was investigated by tensile testing under ongoing electrochemical hydrogen charging. Observation of the surface microstructure of the hydrogen-charged specimen yielded a correlation between the microstructure, crack initiation sites, and crack propagation path. The observed embrittlement arose from crack initiation/propagation along the grain and twin boundaries and delamination governed crack growth. In the present bimodal TWIP steel, the fine grained regions mostly showed intergranular cracking along the grain boundaries between the fine and coarse grains. By contrast, the coarse grained region exhibited transgranular cracking along the twin boundaries. The delamination cracking phenomena is rationalized by the evident nucleation, growth, and coalescence of microvoids in the tensile direction. The results reveal that the bimodal grain size distribution of TWIP steel plays a major role in hydrogen-assisted cracking and the evolution of delamination-related damage.

Hydrogen embrittlement in compositionally complex FeNiCoCrMn FCC solid solution alloy

The influence of internal hydrogen on the tensile properties of an equi-molar FeNiCoCrMn alloy results in a significant reduction of ductility, which is accompanied by a change in the fracture mode from ductile microvoid coalescence to intergranular failure. The introduction of 146.9 mass ppm of hydrogen reduced the plastic strain to failure from 0.67 in the uncharged case to 0.34 and 0.51 in hydrogen-charged specimens. The reduction in ductility and the transition in failure mode are clear indications that this alloy exhibits the classic signs of being susceptible to hydrogen embrittlement. The results are discussed in terms of the hydrogen-enhanced plasticity mechanism and its influence on hydrogen-induced intergranular failure. Furthermore, a new additional constraint that further promotes intergranular failure is introduced for the first time.

Effects of initial microstructures on hot tensile deformation behaviors and fracture characteristics of Ti-6Al-4V alloy

The effects of heat treatment processing on the microstructures of Ti-6Al-4V alloy are systematically studied. Static globularization behavior of lamellar α phase is found when the alloy is cooled in air or furnace from 950–960°C (below the β-transus temperature, about 975°C). Three kinds of microstructures (basket-weave, globular-lamellar, and equiaxed microstructures) are obtained by different heat treatments. The uniaxial tensile tests of the studied alloy with different initial microstructures are conducted at the elevated temperature and different strain rates. It is found that the initial microstructures have obvious effects on the tensile properties and fracture mechanisms. The alloy with basket-weave microstructures exhibits the most obvious work hardening behavior and the highest strength. The alloy with globular-lamellar microstructures has the better ductility than that with basket-weave microstructures. Furthermore, the alloy with equiaxed microstructures has the best ductility, because the equiaxed α phases can delay the formation and coalescence of microvoids. Meanwhile, α phases are elongated, bent and spherized, which contributes to the flow softening during tensile deformation. Especially, the alloy with initial equiaxed microstructures finally transforms to bimodal microstructures after tensile fracture. Additionally, the alloys with three different initial microstructures all show a primary ductile fracture.

Microstructural characterization and mechanical properties of TiB2 reinforced Al6061 matrix composites produced using stir casting process

Particulate reinforced aluminum matrix composites are the most promising alternative for applications where the combination of high strength and ductility is essential. In the present study, Al-TiB2 composites with different amounts of reinforcement (3, 6 and 9wt%) were fabricated using stir casting method. after optimizing the process parameters such as preheating temperature, stirring speed and duration. Microstructure, mechanical properties and the fracture surfaces of tensile specimens were studied. Optical microstructure of prepared composites showed a uniform distribution of reinforcements in matrix and also SEM analysis demonstrated the strong bonding between matrix and reinforcements which is the consequence of improved wettability due to K2TiF6 addition and preheating of TiB2 powder before adding to the melt. In addition to microstructure uniformity, the tensile strength of composites was improved with increasing the volume fraction of TiB2 reinforcement particles without any significant decrease in elongation. SEM micrographs from fractured surfaces of tensile specimens approved that the ductile fracture is occurred in Al6061 alloy in all of prepared composites through nucleation and coalescence of micro-voids.

Influence of solution heat treatment on microstructure and tensile properties of Inconel 718 formed by high-deposition-rate laser metal deposition

The influence of solution heat treatment on microstructure and tensile property of Inconel 718 formed by high-deposition-rate laser metal deposition has been investigated in this paper. Two different heat treatment regimes were employed: 1100 °C × 0.5 h/WQ+720 °C × 8 h/FC to 620 °C × 8 h/AC (this type of samples is called type-S) and 1100 °C × 1 h/WQ+720 °C × 8 h/FC to 620 °C × 8 h/AC (this type of samples is called type-L). The results showed that niobium segregation and Laves phases were eliminated both in type-S and type-L samples. This means that heat treatment at 1100 °C for no more than 0.5 h is enough, if initial epitaxial columnar grains are needed and niobium segregation and Laves phases are tend to be removed. Twins only existed in type-L sample, which was the main reason why the elongation of type-L was superior to that of type-S. The fracture mechanism of these two samples was microvoids coalescence ductile rupture. The separation of the granular sub-micron particles and the γ matrix was the main nucleus of the micro-voids.

Evolution of microstructure and mechanical properties of a new high strength steel containing Ce element

Abstract

Pre-exposure embrittlement of a commercial Al-Mg-Mn alloy, AA5083-H131

AbstractTwo distinct modes of environment-induce cracking (EIC) can initiate in AA5083-H131 during slow strain rate testing (SSRT) in laboratory air (50% RH) at nominal strain rates around 10−6/s, for material either sensitized in distilled water (DW) at 80°C or when material sensitized in dry air is subsequently pre-exposed to DW at room temperature. Type-1 EIC is the “classic” form of intergranular stress corrosion cracking (IGSCC), which during SSRT in laboratory air, initiates at intergranular corrosion (IGC) sites promoted during exposure to DW and provides the prerequisite local stress intensity factor of around 5 MNm−3/2required for crack initiation. When Type-1 EIC cracks become sufficiently deep for local plane-strain stress intensity factors to exceed 12–15 MNm−3/2, Type-2 EIC initiation triggers a series of sudden large load-drops and SSRT failures with extremely low fracture stresses (30–65 MPa). Pre-exposure to DW at room temperature can enhance propensity to both Type-1 and Type-2 EIC. SSRT in dry air, as opposed to humid laboratory air, exhibits failure consistently as a 45° slant failure via a predominantly ductile microvoid coalescence fracture mode, with Type-1 EIC very rarely initiating and any Type-2 EIC restricted to isolated internal patches that only form when relative elongations exceed ~9%.

Micro-mechanism of central damage formation during cross wedge rolling

Central damage is a serious defect in the solid products of cross wedge rolling. A combined experimental and modelling approach was used to study the micro-mechanism of central damage. The evolution process of the micro-voids initiation, growth and coalescence during cross wedge rolling of steel was observed, and the micro-damage morphology was linked to the stress-strain state to reveal the mechanism of central damage. The influences of process parameters on the central damage were investigated on the basis of analyzing the characteristics of stress and strain in the center of the workpiece. It is found that the micro-voids in the center of the workpiece initiate around non-metallic inclusions and develop into macroscopic damage in the directions of shear stress and tensile stress by growth and coalescence; the shear stress and tensile stress cause significant alternating shear and tensile deformation with the rotation of the workpiece, leading to the central damage. The larger the shear deformation and tensile deformation coefficient, the more the cyclic numbers, the greater the degree of damage; among the process parameters of cross wedge rolling, the forming angle has the greatest influence on the central damage.

Electrochemical Noise for Detection of Stress Corrosion Cracking of Low Carbon Steel Exposed to Synthetic Soil Solution

Summary of the mechanical properties obtained from SSRT of X52 steel. Environment YS (MPa) RS (MPa) UTS (MPa) E (GPa) Strain (%) EL (mm) TF (h) RA(%) PE (%) Air 368 243 450 245 19.1 4.85 52.31 78.95 17NS4 pH 5 330 212 422 259 15.7 3.99 45.28 72.72 15NS4 pH 8 463 273 545 279 15.5 3.94 45.24 67.90 14NS4 pH 10 372 153 462 209 15.6 3.95 45.44 68.16 15 Table 4. SCC index obtained from mechanical properties of SSRT. Environment RA (%) RAR TF (h) TFR PE (%) PER ELR (mm) ELR Strain (%) SR Air 78.95 - 52.31 - 17 - 4.85 - 19.1 -NS4 pH 5 72.72 0.92 45.28 0.87 15 0.88 3.99 0.82 15.7 0.82NS4 pH 8 67.90 0.86 45.24 0.86 14 0.82 3.94 0.81 15.5 0.81NS4 pH 10 68.16 0.86 45.44 0.87 15 0.88 3.95 0.81 15.6 0.82 was not observed. However, a decreases in YS, UTS, EL and deformation was observed, which can be attributed to diffusion of hydrogen atoms into the steel generating hydrogen embrittlement. 3.4 Surface fracture analysis Figure 5 shows the SEM images of the fracture surfaces of the X52 pipeline steel after carried out the SSRT in air and in the synthetic soil solution (NS4 solution). Measurements of final diameter of fracture were carried out in order to calculate the reduction area (RA). The SSRT of the X52 steel in air presented a ductile fracture, and it is characterized by extensive plastic (permanent) deformation of the material (figure 5a). One mechanism of ductile fracture is known as microvoid coalescence

Aging behavior and tensile response of a SiCw reinforced eutectoid zinc-aluminium-copper alloy matrix composite

A SiC whisker-reinforced eutectoid zinc-aluminum-copper alloy matrix composite (SiCw/ZAC) was fabricated via a vacuum pressure infiltration technique and investigated comprehensively by comparison with the unreinforced ZAC alloy. The microstructure of the composite was composed of SiC whiskers, Al3Cu5Zn phases and eutectoids of α (Al-rich solid solution) and η (Zn-rich solid solution) with an amount of Al3Cu5Zn phase 1.5 times that in the ZAC alloy. The density and coefficient of thermal expansion of the composite were 4.81 g/cm3 and 21.45 ppm/K, respectively, lower than those of the ZAC alloy. Compared with the ZAC alloy, the composite had a lower peak aging temperature but a much higher peak aging hardness. The addition of SiCw dramatically increased the strength and elastic modulus of the ZAC alloy but greatly decreased the ductility. The ZAC alloy was ductile and fractured in the mode of microvoid coalescence. In contrast, the SiCw/ZAC composite was brittle. In addition to SiC whiskers, brittle Al3Cu5Zn phases were also the nucleation sites of microcracks and hence also responsible for the much lower ductility of the composite.

Microstructures and Elevated Temperature Mechanical Properties of As-extruded ZM61-Sn Alloys

Microstructures, elevated temperature mechanical properties and fracture mechanisms of the as-extruded ZM61- x Sn ( x =2, 4, 8, wt%) alloys were investigated by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and high temperature tensile tests. The results reveal that the addition of Sn can refine microstructures and the refinement effect increases with the increase of Sn content. The average grain sizes of the as-extruded ZM61- x Sn ( x =2, 4, 8) alloys are 11, 8 and 4 μm, respectively. With the increasing of Sn amount, the strength of experimental alloys increases at first and decreases afterward. ZM61-4Sn alloy shows the highest strength, whose ultimate tensile strength and yield strength are 216 and 173 MPa tested at 180 °C, respectively. The ductility increases with the content of Sn increasing, and the elongation of as-extruded ZM61- x Sn ( x =2, 4, 8) alloys are 183.8%, 235.8% and 258.6%, respectively, when tensile temperature reaches 300 °C. The ZM61-4Sn alloy has the optimal coalescence of strength and ductility. The localized necking leads to the final fracture of the specimens and the micro-void coalescence is the main fracture mechanism. Incomplete dynamic recrystallization occurs when tensile tests are carried out at 260 and 300 °C.

Microstructure and mechanical properties of resistance-welded NiTi/stainless steel joints

Resistance welding was employed to join NiTi and stainless steel (SS), with the aim to fabricate a NiTi/stainless steel joint for biomedical applications. The effects of welding current and post-welding cold drawing with 30% area reduction on the joint microstructures and mechanical properties were investigated. At a welding current of 40A, a NiTi/SS joint with a reaction layer of several microns in size was obtained. The tensile strength of this joint reached 440MPa, with 7.9% rupture elongation and a fracture on the NiTi side that occurred via micro-void coalescence mechanism. Good plasticity and bending performance of the joint were verified by the bending test. After cold drawing, the microstructures in the heat-affected zones (HAZs) were refined and the microhardness in the HAZs increased. The tensile strength of the joint increased to 830MPa, with 6.2% elongation. The 40-μm-thick weld obtained at the welding current of 45A consisted of a reaction layer and a NiTi molten zone. Local embrittlement occurred near the NiTi fusion line owing to grain coarsening, the existence of re-solidified grain boundaries in the HAZ and eutectics in the molten zone. The tensile strength of the joint was established as 340MPa, with 5.8% rupture elongation and a cleavage-mediated fracture on the NiTi side. This type of joint could not be drawn as a result of its reduced ductility.