Microvoid Coalescence Research Articles

Fracture strain model for hydrogen embrittlement based on hydrogen enhanced localized plasticity mechanism

Hydrogen energy is a candidate of the “ultimate energy”. However, the quantified evaluation of hydrogen embrittlement phenomenon remains to be a challenging topic. This paper provides a continuum damage model to quantify hydrogen embrittlement. Firstly, theoretical model of the proposed method was introduced. The proposed model applied multi-axial fracture strain based ductile damage evolution law coupled with HELP mechanism to evaluate hydrogen accelerated micro-void coalescence (MVC) fracture. Void growth and coalescence were described by hydrogen enhanced plastic strain and hydrogen reduced fracture strain. Then, the proposed method was implemented on commercial finite element software package. Compact tensile (CT) test and slow strain rate tensile (SSRT) test of two commercial steels were investigated numerically and compared with available experimental results to verify the rationality of the proposed model. The results indicated the proposed method reproduced CT and SSRT tests result with different hydrogen concentrations precisely. Hydrogen's effect on MVC process was discussed. It is suggested HELP based MVC could be a viable model to quantify hydrogen embrittlement. The proposed method was also highlighted with the advantage of describing hydrogen's effect with only one parameter.

Influence of Intercritical Quenching Temperature on Microstructure, Mechanical Properties and Corrosion Resistance of Dual-Phase Steel

This work investigated the microstructure, mechanical properties and corrosion behavior of dual-phase (DP) steel after different intercritical quenching treatment and compared the results with commercial AH32 steel. The results indicate that the microstructure of all tested DP steels was composed of lath martensite and polygonal ferrite. As the temperature increased from 750 to 850 °C, the martensite lath became slender, whereas martensite volume fraction (Vm) increased from 53 to 71%. The change in the morphology and content of martensite not only led to an increase in hardness and yield strength, but also to a decrease in the plasticity and impact toughness, which was consistent with the mixture law. However, the ultimate tensile strength did not increase but declined, which was an unusual behavior. It was related to the micro-void coalescence fracture mechanism caused by the ferrite/martensite interface decohesion. Moreover, the increasing temperature led to the enhanced galvanic effect between ferrite and martensite, accelerating the corrosion rate of DP steels in a 3.5% NaCl solution. A comparison of all results in our work indicates that DP750 steel with 53% martensite had excellent comprehensive properties.

The metallurgical behaviors and crystallographic characteristic on macro deformation mechanism of 316 L laser-MIG hybrid welded joint

A 10 mm 316 L stainless steel plate was joined using multi-layer laser-MIG hybrid welding. Results showed that unmixed zone and “real” fusion zone (FZ) existed near the fusion line. Frequently met dendritic ferrite (δ) and newly encountered spherical nanoscale particles trapped in austenite (γ) were only observed in the latter region. Chemically, these particles turned out a SiO2 phase. From base metal to FZ, δ morphology was altered from originally linear to relatively diversely dendritic. Meantime, γ exhibited a converse manner where previously randomly oriented equiaxed was replaced by coarse, columnar and textured. Accordingly, a comparably high Schmidt factor in γ phase was obtained in FZ. Moreover, δ exhibited a relatively higher stress level than γ which presented a decrease in residual stress with grain coarsening in heat affected zone. In FZ, δ fraction displayed a dramatical increase with special BCC-FCC orientation relations established. Finally, tensile test and fracture appearance implied that FZ was exactly the weakest link of the whole welded joint in spite of a qualified strength achieved. In summary, lack in Σ3, largely increased δ fraction, high Schmidt factor in γ and especially being rich in SiO2 particles collectively led to the micro-void coalescence fracture in FZ.

Defect evolution during high temperature tension-tension fatigue of SLM AISi10Mg alloy by synchrotron tomography

Lack of fusion defects and porosity are inevitable characteristics of additive manufacturing and these are expected to play a key role in determining fatigue life and fatigue failure when excluding the influence of inhomogeneous microstructures. This work followed damage accumulation under tension-tension cyclic loading at 250 °C in situ by time-lapse synchrotron radiation X-ray micro-computed tomography (SR-μCT) for AlSi10Mg test-pieces, produced by selective laser melting (SLM) over their complete fatigue lives (ranging from 180 to 38,000 cycles). These samples were found to accumulate widespread plastic strain each cycle in common with ultra-low cycle fatigue (UCLF) at low levels of triaxial constraint. The defects were found to elongate plastically at a rate approximately 10 times larger than their growth rate laterally. This elongation behaviour at room and elevated temperature fatigue is proportional to the accumulated longitudinal strain increment each cycle. Rotation under the influence of shear is also observed for those defects close to the surface of samples. Some defect coalescence was observed, but final failure was found to be associated with the nucleation of a high density of secondary microvoids (occurring at eutectic Si platelets) that form just prior to failure and link up by microvoid coalescence. These steps may take up approximate 90% of the fatigue life. The final stage of cyclic plasticity occurs when the longitudinal strain exceeds ~ 0.9. Our results are in line with previous models of strain accumulation and defect growth under ULCF conditions.

Stress/Strain Induced Void?

Fracture micro mechanisms of ductile porous solids are substantially researched worldwide since last 60 years through different experiments, theories, thermodynamics and computer models. It is still attracting immense interests to the scientists/engineers as evidenced by a slew of many interesting and innovative techniques, different perceptions and philosophies to elucidate ductile fracture micro mechanisms of materials. The damage accumulation (i.e., void volume fraction, fv) inside a ductile material under tensile deformation is strongly dependent on many engineering/metallurgical variables with their complex and unknown interactions. The role of micro void nucleation and growth during ductile fracture of materials under different environments and loading conditions is well established and documented, but the details of some micro mechanisms governing this fracture process are not still clearly understood. Such as, the coalescence of micro voids has not been clarified clearly, since it is an unstable and rapidly occurred phenomenon in materials. A comprehensive and exhaustive literature review has been performed to realize these facts completely. This article also critically reviews the standard computational methods often widely used for fracture mechanics analysis, which have been proposed/developed by eminent scientists to simulate the ductile fracture of materials. Many studies monitoring the damage accumulation during tensile deformation of different ductile alloys which are, in principle, affected by the imposed stress triaxiality, applied stress and the resulting plastic strains, are already available in the open published literatures. But it is still not clear in these circumstances, whether this damage accumulation is stress assisted or strain induced. In the current investigation, it has been demonstrated through experiments, modeling and reviewing from existing literature that damage accumulation inside a material can be effectively explained by imposed stress triaxiality. This article would be truly being a gift to the structural materials and solid mechanics communities as a whole.

Improvement of stress-relief cracking resistance in coarse-grained heat-affected zone of T23 steel by refining sub-structure through second thermal cycle

Simulated coarse-grained heat-affected zone (CGHAZ), second reheated CGHAZ (UA-CGHAZ), super-critical reheated CGHAZ (SCR-CGHAZ) and inter-critical CGHAZ (ICR-CGHAZ) of T23 steel were produced via thermal simulation of welding. Their corresponding stress-relief cracking (SRC) susceptibility were assessed using isothermal slow strain rate tensile test. The fracture features and microstructures were characterized to reveal the cracking mechanisms. The simulated CGHAZ of T23 steel was highly susceptible to SRC at temperatures of 550–750°C and the fracture mode exhibited micro-void coalescence. M23C6 carbides precipitated at grain boundary may promote the micro-void nucleation and weaken the grain boundary, which resulted in grain boundary cracking preferentially. The strain was concentrated on the weakened grain boundary under tensile load, and the micro-voids gradually grew and coalesced into micro-crack, which ultimately propagated along grain boundary until complete fracture occurred. The SRC susceptibility of SCR-CGHAZ was reduced partially and the SRC susceptibility of ICR-CGHAZ was eliminated. After a second thermal cycle with a peak temperature slightly higher than Ac3 (SCR-CGHAZ), the grain size of CGHAZ was significantly refined and the crack resistance was elevated. When the peak temperature of the second thermal cycle was limited between Ac1 and Ac3, partial grains near grain boundaries were austenitized and transformed to fine ferrite grains along the prior austenite grain boundaries. The fine ferrite grains inhibited M23C6 precipitation and improve the resistance of cracking propagation. Thus, the ductility was improved and the SRC susceptibility was reduced.

Experimental investigation of fatigue failure in an ASTM A401 compression spring of a passenger car

Abstract This work outlines the fatigue failure in a failed helical coiled compression spring of a passenger car used as a commercial vehicle. As a failure investigation case study, the fractured left rear axle suspension spring of the car is thoroughly examined. The fractographic investigations included the visual examination, spectroscopy, microstructure analysis, SEM and micro-hardness testing. The foremost causes of failure have been identified. From the SEM fractograph, microvoid coalescence is observed at the grain boundaries on of the fractured surface which eventually resulted in intergranular dimple rupture. Residual stress is also measured on the fractured surface through two-dimensional (2-D) imaging using a Debye Scherrer ring, with the conclusion that the spring under investigation experiences micro-stress due to the variation of theintergranular spacing resulting from the vibrations induced by overloading, road conditions and prolonged use which eventually results in a rupturing of the compression spring.

A Fractographic Analysis of Additively Manufactured Ti6Al4V by Electron Beam Melting: Effects of Powder Reuse

Metal additive manufacturing (AM) is being rapidly adopted in the aerospace and biomedical industries. Powder bed fusion AM processes are leading this trend. To maximize process economy, excess “unmelted” powder retrieved from the build chamber is used in subsequent build cycles. The metal properties and component reliability could undergo degradation with powder reuse. This study investigates the effects of powder reuse on fracture surface characteristics of Ti6Al4V specimens fabricated by electron beam melting AM over 30 sequential build cycles. Optical microscopy and scanning electron microscopy were used to evaluate the changes in fracture surface features of tensile failures with powder reuse. Macroscopically, slant fractures were most common in early builds, which transitioned to orthogonal fracture surfaces with poorly defined shear lips with increasing reuse. Regardless of the build number, the fracture origins were consistently from the as-built surfaces. Microscopically, ductile features such as micro-void coalescence were evident throughout the 30 build cycles. However, increasing flute content with reuse suggests that rising oxygen levels causes solution strengthening and limits the participation of active slip systems. These results highlight the importance of surface roughness and powder oxidation to metal performance in AM, and the evolution of fractographic features with powder reuse.

Damage micromechanisms in high Mn and Zn content 7XXX aluminum alloys

The nucleation, growth, and coalescence of microvoids are examined in high Zn and Mn content 7XXX aluminum alloys using high-resolution synchrotron X-ray microtomography. The results have clearly shown that the addition of Mn content (0.6% mass) increases the ultimate tensile strength with limited ductility. The loading step in which each microvoid was nucleated together with its nucleation site is determined by tracking the microvoids in reverse chronological order from the final loading step towards the initial stress-free loading step. It was observed that microvoids were initiated due to particle cracking, and void nucleation occurs continuously with the applied strain. Furthermore, it was also observed that the particle underwent multiple fractures. It was concluded the ductile fracture was dominated by the nucleation and growth of microvoids due to particle fracture.

Hydrogen embrittlement of the coarse grain heat affected zone of a quenched and tempered 42CrMo4 steel

The coarse grain heat affected zone (CG-HAZ) of welds produced in a quenched and tempered 42CrMo4 steel was simulated by means of a laboratory heat treatment consisting in austenitizing at 1200 °C for 20 min, oil quenching and finally applying a post weld heat treatment at 700 °C for 2 h (similar to the tempering treatment previously applied to the base steel). A tempered martensite microstructure with a prior austenite grain size of 150 μm and a hardness of 230 HV, similar to the aforementioned CG-HAZ weld region, was produced. The effect of the prior austenite grain size on the hydrogen embrittlement (HE) behaviour of the steel was studied comparing this coarse-grained microstructure with that of the fine-grained base steel, with a prior austenite grain size of 20 μm.The specimens used in this study were charged with hydrogen gas in a reactor at 19.5 MPa and 450 °C for 21 h. Cylindrical specimens were used to determine hydrogen uptake and hydrogen desorption behaviour. Smooth and notched tensile specimens tested under different displacement rates were also used to evaluate HE.Embrittlement indexes, EI, were generally quite low in the case of hydrogen pre-charged tensile tests performed on smooth tensile specimens. However, very significant embrittlement indexes were obtained with notched tensile specimens. It was observed that these indexes always increase as the applied displacement rate decreases. Moreover, hydrogen embrittlement indexes also increase with increasing prior austenite grain size. In fact, the embrittlement index related to the reduction in area, EI(RA), reached values of over 20% and 50% for the fine and coarse grain size steels, respectively, when tested under the lowest displacement rates (0.002 mm/min).A comprehensive fractographic analysis was performed and the main operative failure micromechanisms due to the presence of internal hydrogen were determined at different test displacement rates. While microvoids coalescence (MVC) was found to be the typical ductile failure micromechanism in the absence of hydrogen in the two steels, brittle decohesion mechanisms (carbide-matrix interface decohesion, CMD, and martensitic lath interface decohesion, MLD) were observed under internal hydrogen. Intergranular fracture (IG) was also found to be operative in the case of the coarse-grained steel tested under the lowest displacement rate, in which hydrogen accumulation in the process zone ahead of the notch tip is maximal.

Experimental investigation of fatigue failure in an ASTM A401 compression spring of a passenger car

Abstract This work outlines the fatigue failure in a failed helical coiled compression spring of a passenger car used as a commercial vehicle. As a failure investigation case study, the fractured left rear axle suspension spring of the car is thoroughly examined. The fractographic investigations included the visual examination, spectroscopy, microstructure analysis, SEM and micro-hardness testing. The foremost causes of failure have been identified. From the SEM fractograph, microvoid coalescence is observed at the grain boundaries on of the fractured surface which eventually resulted in intergranular dimple rupture. Residual stress is also measured on the fractured surface through two-dimensional (2-D) imaging using a Debye Scherrer ring, with the conclusion that the spring under investigation experiences micro-stress due to the variation of theintergranular spacing resulting from the vibrations induced by overloading, road conditions and prolonged use which eventually results in a rupturing of the compression spring.

Evaluation of the mechanical behavior of 2001 LDSS and 2205 DSS reinforcements exposed to simultaneous load and corrosion in chloride contained concrete pore solution

The present work is focused on the study and evaluation of the mechanical properties of the new low-nickel 2001 lean–duplex stainless steel (LDSS), 2205 duplex stainless steel (DSS) and B500SD carbon steel rebars, influenced by aggressive corrosion environment. Test specimens were loaded to 85% of the yield strength while simultaneously exposed to 8 wt% Cl− concrete pore solution for 315 days. Subsequently, the specimens were loaded until failure at a high deformation rate of 0.41 mm/s. The analysis of the stress‒strain curve showed a reduction of the ultimate tensile strength greater than 35% for the B500SD, while the 2001 LDSS and 2205 DSS show a reduction lower than 20% and 10%, respectively. Analyzing elongation to failure, neither 2001 LDSS nor 2205 DSS show evidence of stress corrosion under the simultaneous action of chlorides and load. The toughness change in the 2205 DSS is negligible, the 2001 LDSS slightly reduces its resilience, and the B500SD severely decreases both properties. The fractography study revealed that 2001 LDSS and 2205 DSS show a dimple pattern with features of ductile fracture by coalescence of microvoids.

On the impact toughness of gradient-structured metals

Gradient-structured (GS) materials are capable of displaying high strength without compromising ductility, which can result in damage-tolerant structures. However, due to the difficulties in fabricating bulk GS materials, there has been only limited studies on the fracture behavior in GS metals. In the present work, the impact toughness of the macroscale GS pure Ni plates was investigated using instrumented Charpy impact testing. The gradient orientation was found to have a significant influence on the impact toughness of GS Ni. For gradient structures that transition from coarse grains (CG) to nano-grains (NG), termed CG→NG gradients (in the present study from ∼8 μm to ∼30 nm), the absorbed energy and the tensile strength were increased, respectively, by 1.6 and 2.3 times from those exhibited by uniform coarse-grained structures, demonstrating a simultaneous enhancement in strength and impact toughness. Analysis of load-displacement curves revealed that the resistance to both crack initiation and propagation were significantly enhanced as the crack penetrated through the CG→NG gradient structure, leading to markedly rising dynamic R-curve behavior estimated from nonlinear-elastic fracture mechanics J-based measurements. The superior fracture resistance in the CG→NG gradient structure was found to originate from sustained ductile fracture by microvoid coalescence, taking place not only in the initial CG zone, but also within the latter NG regions where adiabatic shear bands form during impact; in these latter regions, plasticity becomes enhanced due to grain coarsening induced by recrystallization under the dynamic loading. The present work not only reveals how the dynamic fracture resistance can be significantly enhanced in GS metals, but also provides structure-design strategies for developing superior metallic materials for impact engineering applications.

Effect of plastic strain damage on the hydrogen embrittlement of a dual-phase (DP) and a quenching and partitioning (Q&P) advanced high-strength steel

The hydrogen influence on two 1180 MPa advanced high-strength steels was studied using electropolished U-bend specimens in HCl solutions of pH 1 and pH 3. Bending limit diagrams identified conditions for safe use. Hydrogen-assisted fracture occurred only near the bending limit, and was promoted by microvoids and microfractures, introduced by the severe plastic deformation of the U-bend specimens. The hydrogen fracture initiation and early propagation was by the mechanisms designated as hydrogen fracture due to hydrogen enhanced microvoid coalescence (HFMVC). Exploratory work indicated that fractures were critically dependent on the surface state of the specimens.

Effects of deformation parameters and stress triaxiality on the fracture behaviors and microstructural evolution of an Al-Zn-Mg-Cu alloy

Abstract Hot tensile tests are performed on an Al-Zn-Mg-Cu alloy at larger temperature and strain rate ranges. The influences of deformation parameters and stress triaxiality on hot tensile fracture characteristics, fracture mechanisms and microstructural evolution are discussed. It is found that the maximum tensile load increases with raising stress triaxiality. The fracture strain rises with increasing strain rate or decreasing stress triaxiality. When the deformation temperature is raised, the fracture strain firstly increases and then decreases. Dynamic recovery (DRV) is the main softening mechanism. At low strain rates or high deformation temperatures, the partial dynamic recrystallization (DRX) behavior occurs, and continuous dynamic recrystallization (CDRX) is the main DRX mode. The dominant fracture mechanism is the coalescence of micro-voids at the tested deformation conditions. Due to the occurrence of DRX, the intergranular fracture also occurs at low strain rates or high deformation temperatures. The low strain rate or high stress triaxiality easily induces the appearance of micro-voids, which accelerate the fracture failure.

Evaluation of Hydrogen-Assisted Cracking Susceptibility in Grade T24 Steel

The delayed hydrogen cracking test was performed to evaluate the hydrogen-assisted cracking (HAC) susceptibility of Grade T24 steel base metal and the simulated coarse-grained heat-affected zone (CGHAZ). The base metal did not fail after testing for up to 672 h. In contrast, the CGHAZ sample failed after about 2 h when charged from all four sides, and 4 h when charged only from the internal diameter (ID) surface. The higher HAC resistance of the base metal compared to the CGHAZ was due to the microstructure difference. The tempered bainitic-martensitic microstructure in the base metal was more resistant to HAC compared to the untempered martensite microstructure in the CGHAZ. Fractography analysis indicated the decarburized zone on the ID surface delayed the development of the critical hydrogen concentration in the CGHAZ, thus improving the HAC resistance. The HAC cracking initiated with an intergranular fracture, then transitioned to quasi-cleavage and microvoid coalescence. The fracture behavior was explained using Beachem’s model.

An SEM compatible plasma cell for in situ studies of hydrogen-material interaction.

An in situ hydrogen (H) plasma charging and in situ observation method was developed to continuously charge materials, while tensile testing them inside a scanning electron microscope (SEM). The present work will introduce and validate the setup and showcase an application allowing high-resolution observation of H-material interactions in a Ni-based alloy, Alloy 718. The effect of charging time and pre-straining was investigated. Fracture surface observation showed the expected ductile microvoid coalescence behavior in the uncharged samples, while the charged ones displayed brittle intergranular and quasi-cleavage failure. With the in situ images, it was possible to monitor the sample deformation and correlate the different crack propagation rates with the load-elongation curves. H-charging reduced the material ductility, while increasing pre-strain decreased hydrogen embrittlement susceptibility due to the possible suppression of mechanical twinning during the tensile test and, therefore, a reduction in H concentration at grain and twin boundaries. All the presented results demonstrated the validity of the method and the possibility of in situ continuously charging of materials with H without presenting any technical risk for the SEM.

Mixed-mode (I and II) fracture behavior of a basal-textured magnesium alloy

Mixed-mode (I/II) fracture experiments on rolled AZ31 Mg alloy are conducted using notched four point bend specimens along with in-situ imaging. Digital Image Correlation (DIC) technique is used to analyze the images and map out the deformation and strain fields. With increase in mode II component of loading, a monotonic reduction in the notched fracture toughness (Jc) is observed. Fractographs reveal that the fracture mechanism transitions first from (ductile) micro-void growth and coalescence to twin induced quasi-brittle failure and then to shear cracking as the loading changes from mode I to II. Detailed optical micrographs and EBSD maps show profuse tensile twinning in the ligament, especially near the far-edge of the specimen leading to pronounced texture changes. However, the micro void size near the notch tip as well as the twin density in the ligament reduce with higher mode II component. The pronounced drop in fracture toughness and the changes in the associated mechanism with increase in mode-mixity are rationalized from the combined effects of hydrostatic stress, plastic strain localization and tensile twinning.

Effect of Pt-Al bond-coat on the tensile deformation and fracture behaviors of a second-generation SX Ni-based superalloy at elevated temperatures

An investigation was conducted into the influence of applying PtAl bond-coat on the tensile deformation and fracture behavior of second-generation single crystal superalloy substrate with the temperature ranging between room temperature RT and 1100 °C. In the meantime, the development of a two-layer PtAl bond-coat, consisting of an outer layer of single-phase β-(Ni,Pt)Al and an inter-diffusion zone of refractory metal precipitates, has also been discussed. As revealed by the results, the application of PtAl bond-coat contributed to a noticeable deterioration in both yield strength and ultimate tensile strength of the superalloy substrate at elevated temperatures. Nevertheless, the elongation of coated superalloy was shown to be 17.2% (at RT) and 10.4% (at 750 °C) lower and 9.6% (at 1100 °C) higher compared to uncoated samples. Following tensile test, the TEM analyses indicated that there were onsets of various slip systems, the traditional superlattice stacking faults were retarded at intermediate temperature and the dislocation debris could be increased to develop dislocation networks at higher temperature in the region of superalloy near the bond-coat. As the temperature was on the rise, the fracture mechanism of coated superalloy shifted from tearing ridges and dimples to quasi-cleavage and then to micro-void coalescence. While that of PtAl bond-coat was transformed from brittle to ductile rupture. Within the temperature range from RT up to 750 °C, a combination of long crack lengths and greater crack penetration depths into bond-coat caused more deterioration in the tensile properties of superalloy. When the temperature rose above 750 °C, the application of PtAl bond-coat improved the ductility of the substrate, as a result of the ductile failure in the bond-coat which hindered the development of through-thickness cracks.

Dynamic Mechanical Behavior and Adiabatic Shear Bands of Ultrafine Grained Pure Zirconium

Dynamic compression tests were carried out to investigate dynamic mechanical behavior and adiabatic shear bands in ultrafine grained (UFG) pure zirconium prepared by equal channel angular pressing (ECAP) and rotary swaying. The cylindrical specimens were deformed dynamically on the split Hopkinson pressure bar (SHPB) at different strain rates of 800 to 4 000 s-1 at room temperature. The temperature distribution of the shear bands was estimated on the basis of temperature rise of uniform plastic deformation stage and thermal diffusion effect. The results show that the true stress-true strain curves of UFG pure zirconium are concave upward trend of strain in range of 0.02-0.16 due to the effects of strain hardening, strain rate hardening and thermal softening. The formation of the adiabatic shear bands is the main reason of UFG pure zirconium failure. A large number of micro-voids are observed in the adiabatic shear bands, and the macroscopic cracks develop from the micro-voids coalescence. The fracture surface of UFG pure zirconium exhibits quasi cleavage fracture with the characteristic features of shear dimples and river pattern. The highest temperature within the shear bands of UFG pure zirconium is about 592 K.