Intergranular Decohesion Research Articles

Mechanical properties of (Ni, Fe)Cr2O4 polycrystal spinels studied by molecular dynamics simulations.

The elastic moduli and mechanical properties at the onset of crack in nanocrystalline and nanoporous (Ni, Fe)Cr2O4 compounds with a spinel structure are investigated by molecular dynamics simulations. The polycrystalline structures generated contain nanograins from 2.5 to 30nm in diameter. These structures are representative of the internal corrosion layer in nickel-based alloys. These simulations enabled us to establish the evolution of elastic moduli as a function of the composition, porosity, and grain size of the polycrystals. From this evolution, the initial database for the elastic properties of corrosion layers based on von Bertalanffy growth functions was determined. The onset of crack in polycrystals is also investigated via uniaxial tensile and shear deformation. Under shear deformation, flow stress as a function of grain size follows normal and inverse Hall-Petch regimes. The regime change occurs for grain sizes around 10nm. For grain sizes under this threshold, shear banding involving collective translation and rotation of nanograins dominates the plastic deformation. For grain sizes greater than 10nm, phase transition inside grains from a spinel to a post-spinel-like structure is observed as well. In that case, phase transition dominates the plastic deformation. Under uniaxial tensile deformation, intergranular decohesion occurs. The general law as a function of grain size for toughness, which is the material's capacity to absorb elastic and plastic energy prior to failure, is also established.

The Effect of Impact Load on the Atomistic Scale Fracture Behavior of Nanocrystalline bcc Iron.

Nanocrystalline metals have many applications in nanodevices, especially nanoscale electronics in aerospace. Their ability to resist fracture under impact produced by environmental stress is the main concern of nanodevice design. By carrying out molecular dynamics simulations under different fast loading rates, this work examines the effect of impact load on the fracture behavior of nanocrystalline bcc iron at an atomistic scale. The results show that a crack propagates with intergranular decohesion in nanocrystalline iron. With the increase in impact load, intergranular decohesion weakens, and plastic behaviors are generated by grain boundary activities. Also, the mechanism dominating plastic deformation changes from the atomic slip at the crack tip to obvious grain boundary activities. The grain boundary activities produced by the increase in impact load lead to an increase in the threshold energy for crack cleavage and enhance nanocrystalline bcc iron resistance to fracture. Nanocrystalline bcc iron can keep a high fracture ductility under a large impact load.

The hydrogen embrittlement of pure Ni fabricated by additive manufacturing

This work investigated the hydrogen embrittlement mechanism of Ni fabricated by laser-based powder bed fusion (L-PBF). In the presence of hydrogen, the L-PBFed Ni failed with a brittle mode, while its fracture surface had a “transgranular-like” appearance. This unusual fracture morphology is rooted in the special grain shape induced by the laser-based manufacturing process, and the failure process is actually predominated by the intergranular decohesion. An annealing process of the as-printed sample enhanced its elongation and mitigated the hydrogen embrittlement. The special dislocation cellular pattern formed in additive manufacturing is considered to be detrimental to hydrogen embrittlement resistance.

The dual role of TRIP effect on ductility and toughness of a medium Mn steel

In this study, the tensile and fracture behaviors of a medium Mn steel fabricated by two thermomechanical processes, namely intercritical annealing (IA) and room-temperature quenching and partitioning (RT Q&P), were investigated. The IA steel consists of ultrafine-grained, fully recrystallized ferrite and austenite, while the RT Q&P steel is comprised of martensite matrix and retained austenite. The austenite in the IA steel is less stable due to its lower carbon content. High-resolution micro-digital image correlation (micro-DIC) results reveal moderate strain localization at boundaries in the RT Q&P steel. On the contrary, strain is highly localized in the austenite of the IA steel, making austenite transform into fresh martensite quickly and promoting the work hardening rate, i.e., transformation-induced plasticity (TRIP) effect. However, this intensive TRIP leads to premature decohesion at boundaries and sudden fracture without post-elongation. The J-integral-based resistance curves (J-R curve) were also measured for both steels. The crack-initiation toughness of the IA steel is 22% less due to its frequent martensite cracking and intergranular decohesion. In contrast, the hierarchical microstructure in the RT Q&P steel suppresses brittle fracture and enables significant crack tip blunting, resulting in a high fracture toughness. The present work illustrates the dual role of TRIP on ductility and fracture toughness, namely, intensive TRIP is useful for tensile strength and uniform elongation, but is harmful to fracture toughness, which contradicts the common belief that TRIP is beneficial for both ductility and toughness.

Effect of Applied Tensile Stress on Hydrogen-Induced Delayed Fracture Mode of Fe-Ni-Cr Austenitic Alloy Weldment

Fe-Ni-Cr austenitic alloys are widely used in hydrogen environments as structural materials. Their weld normally shows higher hydrogen-embrittlement sensitivity than the base metal, endangering large-scale applications. Herein, by using electron microscopy and numerical calculations, the influence of applied load on the fracture mode of hydrogen-embrittled JBK-75 alloy weldment is revealed and correlated with a competition between hydrogen-prompted intergranular decohesion (HPID) and hydrogen-enhanced localized plasticity (HELP). Therefore, independent of the load levels, the weld featuring a lower strength and smaller grain sizes is always more vulnerable to hydrogen embrittlement than the base metal.

The effect of boron and zirconium on the microcracking susceptibility of IN-738LC derivatives in laser powder bed fusion

The effect of boron (<0.01 to 0.03 wt%) and zirconium (<0.01 to 0.07 wt%) on the microcracking susceptibility of the γ’-strengthened Ni-base superalloy IN-738LC during laser powder bed fusion (LPBF) was studied using custom designed powder grades. It was found that both elements have a strong effect on the microcracking susceptibility, the microcracks are located at high angle grain boundaries based on EBSD measurements and crack density increases with the content of both elements. High crack density in the material with high boron and zirconium content corresponds to a large fraction of intergranular decohesion facets exhibiting a dendritic morphology on the fracture surface, typical for solidification cracking. Investigation of the fracture surface chemistry by X-ray photoelectron spectroscopy (XPS) indicates that considerable amounts of B and Zr are present in oxide state. Auger electron spectroscopy (AES) confirms that both elements are segregated to the intergranular decohesion facets on the fracture surface. Thin layers of B- and Zr-containing oxide on the microcrack surfaces were indicated by atom probe tomography (APT) as well. Hence, it is suggested that the cracking susceptibility of the studied alloying system is caused by formation of B- and Zr-containing oxide at high-angle grain boundaries during solidification.

Liquid Metal Embrittlement of Advanced High Strength Steel: Experiments and Damage Modeling.

In the automotive industry, corrosion protected galvanized advanced high strength steels with high ductility (AHSS-HD) gain importance due to their good formability and their lightweight potential. Unfortunately, under specific thermomechanical loading conditions such as during resistance spot welding galvanized, AHSS-HD sheets tend to show liquid metal embrittlement (LME). LME is an intergranular decohesion phenomenon leading to a drastic loss of ductility of up to 95%. The occurrence of LME for a given galvanized material mainly depends on thermal and mechanical loading. These influences are investigated for a dual phase steel with an ultimate tensile strength of 1200 MPa, a fracture strain of 14% and high ductility (DP1200HD) by means of systematic isothermal hot tensile testing on a Gleeble® 3800 thermomechanical simulator. Based on the experimental findings, a machine learning procedure using symbolic regression is applied to calibrate an LME damage model that accounts for the governing quantities of temperature, plastic strain and strain rate. The finite element (FE) implementation of the damage model is validated based on the local damage distribution in the hot tensile tested samples and in an exemplary 2-sheet resistance spot weld. The developed LME damage model predicts the local position and the local intensity of liquid metal induced cracking in both cases very well.

Atomistic scale behaviors of crack propagation in nanocrystalline bcc iron

Nanocrystalline materials have an extensive application in engineering due to their excellent performance on strength and toughness. This work carries out molecular dynamics simulation to identify the atomistic scale behaviors of crack propagation in nanocrystalline bcc iron, where the effect of grain size less than 15 nm is examined. The crack, instead of the grain boundary, is found to play a major role in the engineering stress-strain behavior. The maximum stress of the cracked model is no longer consistent with the inverse Hall-Petch relationship. Based on three-dimensional investigation, the diverse propagation behaviors on different cross sections along crack front are revealed, and the mechanism of intergranular decohesion is clarified. Through intergranular decohesion, the fast brittle cleavage on one cross section can accelerate the ductile propagation on the other cross sections. With a decrease in grain size, the intergranular decohesion effect weakens, and the crack propagation becomes more ductile. The threshold for crack ductile growth also increases with grain size decrease, which is very different from the inverse Hall-Petch relationship. The nanocrystalline iron with a small grain size has lower tensile strength but higher crack ductile growth threshold. Besides, the convictive evidence of asymmetrical propagation in nanocrystalline bcc iron is found. The asymmetrical propagation behavior is determined by the different abilities of leading partial dislocation nucleation on the asymmetrically distributed atomic close-packed planes. Since the grain orientation is random, the atoms are usually distributed asymmetrically along the crack plane. The asymmetrical behavior dominates the crack propagation inside grain for nanocrystalline bcc iron.

Mechanical properties and fatigue crack growth in tungsten deposited by RF-plasma

Fracture toughness and fatigue crack growth rates of free-standing thick W deposits produced in a protective atmosphere using radio frequency inductively-coupled plasma process were investigated. Stress-strain behavior of the deposits was studied using four-point bending and their fracture toughness was estimated from three-point bending following the ASTM E1820 standard. The fatigue crack growth rates were measured in symmetrical bending using a resonance approach. The results were compared with commercially available bulk tungsten prepared by powder metallurgy that served as a reference. The results showed that the deposits had significantly lower stress-strain properties: the elastic modulus and tensile strength ranged from 30% to 50% of the reference bulk values. The crack growth rate tests together with fractographic analysis suggested that a static failure by inter-granular decohesion predetermines the fatigue properties of the deposits and causes high slopes of the corresponding fatigue crack growth rate curves. Contrary to this, the reference sheet failed trans-granularly. The inter-granular failure mode of the deposits was caused by grain boundary embrittlement, a phenomenon probably related to oxides. The lowest obtained oxygen concentration in deposit was nearly by two orders of magnitude higher, than in reference sheet. The distribution of oxygen was studied by TEM and the amorphous and crystalline oxide phases were identified at splat interfaces. The presence of oxygen in the deposits produced by the inherently non-oxidizing RF technology calls for control of oxygen content in the feedstock powder. The performed characterization provided baseline data of mechanical and fatigue properties of RF plasma sprayed tungsten.

Anisotropy in creep properties and its microstructural origins of 12Cr oxide dispersion strengthened ferrite steels

The anisotropy in creep properties and its microstructural origins in two oxide dispersion strengthened (ODS) steels (i.e., 12Cr-ODS-IMR and 12Cr-ODS-NIFS) were systematically investigated. Both ODS steels exhibited the anisotropic creep properties. The creep strength in longitudinal directions (LD) or extrusion directions (ED) specimens was better than that in transverse directions (TD) specimens. The microstructural characterizations revealed that both steels had elongated grains along the ED and relatively equiaxed grains in other orientations. Both steels had a streak-like distribution of precipitates along the ED that was composed of Ti-enriched precipitates and nano-scale oxide particles in local area. The fractography of the crept TD and LD specimens suggested that both steels fractured by the mixed ductile and brittle fracture modes. The propagation of cracks and intergranular decohesion along the boundaries of elongated grains were more severe in the TD specimens than in the LD specimens. The combined effects of elongated grain morphology with low grain shape aspect ratio and segregation of precipitates along the ED can explain the anisotropy in creep properties. The anisotropy in 12Cr-ODS-IMR steel was relatively higher than that in 12Cr-ODS-NIFS steel, and the additional cold rolling and lower final heat treatment temperature in 12Cr-ODS-IMR steel were responsible for it.

Effect of strain rate, soaking time and alloying elements on hot ductility and hot shortness of low alloy steels

Susceptibility to hot shortness and hot cracking of Cu bearing steels is generally minimized with the addition of Ni. However, in this study, attempts have been made to reduce the hot working problems in such steels by the addition of Si, P and either very low content of Ni or no Ni, which will be understood after evaluating their hot ductility and hot shortness properties. The main techniques employed for this study are hot tensile test, analysis of fractography and characterization of scale/steel interface after long exposure at high temperature. The steels having 0.80–0.86% Si exhibited adequate hot ductility at all deformation temperatures ranging from 850 °C to 1200 °C during slow strain rate (0.005 s−1) testing conditions and the fracture morphology exhibited both ductile conical dimples and transgranular fracture. On the other hand, ~0.40% Si containing steels exhibited significantly reduced hot ductility at 850 °C and failed in intergranular brittle fracture mode. The improved hot ductility of high Si containing steels is explained using Fe-P phase diagram along with the proposition of inverse segregation of Si and repulsive interaction between P and Si. On increasing strain rate to 6.5 s−1, ductile failure was observed in all steels. The long duration of soaking at 1200 °C lead to intergranular decohesion and hot tearing during tensile testing in ~0.12% and ~0.19% P containing steels, respectively. A separate study of oxidation in air at 1200 °C indicated that presence of ~ 0.12% P in steel is beneficial in reducing hot shortness by keeping Cu enriched phase away from the scale/steel interface. In contrary to this, oxidation of ~0.19%P containing steel is detrimental to hot working as the grain boundaries of this steel were severely oxidized to the depth of few grains. Other alloying elements such as Ni and Cr in experimental steels had no effect on hot ductility and hot shortness properties.

Cohesive zone simulation of grain size and misorientation effects on hydrogen embrittlement in nickel

Abstract The size and misorientation effects on hydrogen embrittlement of a four grain nickel aggregate are studied with the help of hydrogen informed cohesive zone model. The grain misorientation angle is parameterized by fixing the lower grains while rotating the upper grains about the out-of-plane axis. Brittle failure of the grain aggregate is observed and nominal strength obtained. In the crack-free situation, the grain misorientation exerts an obvious weakening effect on the nominal strength, which is most pronounced at misorientation angles around 20°. Such trend applies to the pre-cracked situation but is much less pronounced. Both misorientation and pre-crack lead to size effect. The nominal strength shows a decreasing trend with the grain size, indicating that grain refinement tends to improve the load bearing capacity, which coincides with the observation in practice. Further, it is shown that the size effect diagram without hydrogen can be divided into three regimes. The conclusions apply to the case with hydrogen, except that the trend of the size effect curve can be affected by large grain sizes due to the longer absolute distance of hydrogen diffusion. These results provide guidelines for grain boundary engineering and for nanomechanical tests aiming at calibrating the intergranular decohesion parameters.

Metallurgical and Mechanical characterization of A319 Aluminum Alloy Casting Solidified Under Mold Oscillation

This paper deals with the effect of mold oscillation (during alloy solidification) on the metallurgical and mechanical properties of A319 alloy casting. For the evaluation of aforesaid effect, oscillatory amplitudes of 5, 10, 15, and 20 µm with a constant frequency of 100 Hz were used. Results obtained under oscillatory conditions were compared with those of castings prepared under stationary condition. Metallurgical properties were examined through microstructure, tensile fracture SEM and SEM image of test specimens prepared under different conditions of solidification. Grain refinement was obtained through mold vibration. SEM image of the tensile fractured surface in all the cases reveals a mixed mode of fracture consisting of fine dimples and intergranular decohesion fracture surfaces. Small dimples indicate that some plastic deformation has taken place before fracture. Cleavage formation may be attributed to the presence of hard and brittle Al2Cu and FeSi phases. Likewise, mechanical properties of oscillatory casting were examined through the tensile and microhardness test and compared with similar properties obtained under stationary condition.

Microstructure and mechanical properties of nonmagnetic Fe-25Mn-xCu-C steels by super solidus liquid phase sintering

In this work, nonmagnetic steels Fe-25Mn-xCu-C were prepared by high manganese pre-alloyed steel powders through powder metallurgy (PM) technique. Four types of steels specimen were created to investigate the microstructure evolving with sintering process, mechanical properties and magnetic properties. The microstructures, fracture surfaces, phase constitutions and mechanical properties of Fe-25Mn-xCu-C were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and tensile strength test. The results showed that super solidus liquid-phase sintering (SLPS) phenomenon was conclusively verified, for the first time, in the Fe-Mn pre-alloyed powders: Liquids generated by SLPS process from pre-alloyed powders could improve the binding condition between the particles and enhance the densification. X-ray diffraction (XRD) experiment and physical property measurement system (PPMS) measurement verified the nonmagnetic properties of steels with single austenite phase. It is confirmed that mechanical properties are intensively influenced by the characteristic and quantity of liquids between the matrix particles. The fracture mechanism of the steel is dominated by intergranular decohesion mode. The preliminary study found this kind of new non-magnetic steel exhibits relatively high density. With the efficiency in fabricating and the non-magnetic property, this work foresees good prospects for application in the steel components manufacturing industry.

On the Step Cooling Treatment for the Assessment of Temper Embrittlement Susceptibility of Heavy Forgings in Superclean Steels

When subjected to extended exposure to intermediate service temperatures, Cr–Mo steels, Ni–Cr steels, and 5% Ni steels can become embrittled, with an associated decrease in fracture toughness and a shift in the ductile-to-brittle transition temperature to higher temperatures. Two methods for the investigation of temper embrittlement phenomena are isothermal aging or the use of a step cooling aging treatment, which is less time consuming and is considered to be the most severe test to evaluate steel’s susceptibility to this phenomenon. In the present work, the effectiveness of the step cooling treatment in the assessment of temper embrittlement in a superclean 26NiCrMoV14.5 steel for heavy section forgings has been studied. Some isothermal aging treatments in the critical temperature range have also been carried out. Results of a Charpy V impact test on not-aged and aged specimens, and observation of the fracture surfaces led to the following conclusions: the steel does not undergo temper embrittlement upon step cooling treatment or after aging at different temperatures and times in the critical temperature range; the most negative effect on the shift of the ductile-to-brittle transition curve—compared with not aged steel—has been observed after aging at 593 °C for 2 h (ΔT54J = 9 °C); further aging up to 8 h produced a ΔT54J of only 1 °C. Neither step cooling nor aging at various critical temperatures gave rise to an intergranular brittle fracture; the amount of embrittling impurity elements in a superclean steel does not seem to be enough to cause embrittlement and a pure intergranular decohesion.

The Contribution of Hydrogen to the Loss of Mechanical Properties of a 7046 Aluminium Alloy Pre-Exposed in a Chloride Media

Automotive industry is increasingly affected by standards requiring a major cut of polluting emissions. Rolled sheets of Cu-free 7000-series alloys offer a very attractive compromise between mechanical properties, corrosion resistance, forming and weight. With the continuing need for further down-gauging and light-weighting in transportation sector, as well as other applications, these alloys are being considered for large volume use. Nevertheless, their susceptibility to hydrogen embrittlement (HE) and to stress corrosion cracking (SCC) is a limiting factor. The present work concerns HE susceptibility of a 7046-T4 aluminium alloy exposed to a 0.6M NaCl solution. First results showed a strong decrease of the ultimate elongation for tensile samples after exposure to the chloride solution Surface observations revealed that the alloy was mildly affected by corrosion in this environment: pits initiated rapidly around intermetallic particles but only few pits propagated after several days of exposure. Cross-section of pre-corroded samples showed no intergranular corrosion and the maximal depth for the corrosion defects was about 100 nm which was not enough to lead to a noticeable loss of mechanical properties. Therefore, it was assumed that the decrease of the mechanical properties was related to the penetration of hydrogen inside the materials and the results gave proofs that hydrogen could enter and embrittle the 7046 aluminium alloy even without an applied stress during its exposure to the corrosive solution. Global hydrogen content measurements by melting method confirmed this assertion. Furthermore, comparison of the fracture surface of a healthy specimen and that of a sample pre-exposed to the NaCl solution showed that the ductile fracture mode observed for a healthy specimen was partially replaced by cleavage and brittle intergranular rupture when the sample was pre-corroded. Both fracture modes could be attributed to different hydrogen contents and different trapping sites in the microstructure. The hydrogen localized in the grain boundaries was believed to lead to intergranular decohesion, while the reticular and trapped hydrogen presumably induced cleavage rupture by exacerbating the localization of the plasticity. In addition, tensile tests were performed on samples pre-hydrogenated in a pH 2 sulfuric acid solution with a cathodic potential applied. The aim was to precisely control the hydrogen amount introduced in the samples. The depth and the nature of the fracture mode were then analyzed and related to the hydrogen amount. In parallel, pre-hydrogenated coupons were analyzed by SIMS in order to measure the maximal depth affected by hydrogen and to localize the trapping sites in the microstructure. The coupling of these two methods allowed to conclude about the role of the hydrogen generated by corrosion and to propose a mechanism to explain the embrittlement.

Influences of Mo on stress corrosion cracking susceptibility of newly developed FeCrMnNiNC-based lean austenitic stainless steels

In this paper, newly developed high interstitial alloys, Febalance18Cr10Mn1Ni0.4N0.15CxMo (x=0, 0.91, and 1.76wt%), were briefly introduced, and susceptibility to stress corrosion cracking of the alloys was discussed based on slow strain rate test results and fractographic observation. It was found that the economical developed alloys exhibited superior mechanical properties and resistance to pitting corrosion to those of AISI 316L stainless steel. In addition, it was revealed that the addition of Mo ranging from 0 to 1.76wt% affected tensile properties only in a limited manner, but it increased the resistance to the stress corrosion cracking of the alloys. The improved resistance to stress corrosion cracking was attributed to the enhanced pitting corrosion resistance and suppressed intergranular decohesion of the matrix by addition of Mo.

The effect of Ag, Pb and Bi impurities on grain boundary sliding and intergranular decohesion in Copper

We investigate the changes in grain boundary sliding (GBS) and intergranular decohesion in copper (Cu), due to the inclusion of bismuth (Bi), lead (Pb) and silver (Ag) substitutional impurity atoms at a symmetric tilt grain boundary (GB), using a first-principles concurrent multiscale approach. We first study the segregation behavior of the impurities by determining the impurity segregation energy in the vicinity of the GB. We find that the energetically preferred sites are on the GB plane. We investigate the intergranular decohesion of Cu by Bi and Pb impurities and compare this to the effect of Ag impurities by considering the work of separation, and the tensile strength, . Both and decrease in the presence of Bi and Pb impurities, indicating their great propensity for intergranular embrittlement, whilst the presence of Ag impurities has only a small effect. We consider GBS to assess the mechanical properties in nanocrystalline metals and find that all three impurities strongly inhibit GBS, with Ag having the biggest effect. This suggests that Ag has a strong effect on the mechanical properties of nanocrystalline Cu, even though its effect on the intergranular decohesion properties of coarse-grained Cu is not significant.

Traction–separation relationships for hydrogen induced grain boundary embrittlement in nickel via molecular dynamics simulations

A statistical approach combined with molecular dynamics simulations is used to study the influence of hydrogen on intergranular decohesion. This methodology is applied to a Ni Σ3(112)[11¯0] symmetric tilt grain boundary. Hydrogenated grain boundaries with different H concentrations are constructed using an energy minimization technique with initial H atom positions guided by Monte Carlo simulation results. Decohesion behavior is assessed through extraction of a traction–separation relationship during steady-state crack propagation in a statistically meaningful approach, building upon prior work employing atomistic cohesive zone volume elements (CZVEs). A sensitivity analysis is performed on the numerical approach used to extract the traction–separation relationships, clarifying the role of CZVE size, threshold parameters necessary to differentiate elastic and decohesion responses, and the numerical averaging technique. Results show that increasing H coverage at the Ni Σ3(112)[11¯0] grain boundary asymmetrically influences the crack tip velocity during propagation, leads to a general decrease in the work of separation required for crack propagation, and provides a reduction in the peak stress in the extracted traction–separation relationship. The present framework offers a meaningful vehicle to pass atomistically derived interfacial behavior to higher length scale formulations for intergranular fracture.

Effects of hydrogen on the mechanical response of α-uranium

The presence of hydrogen is known to embrittle and drastically reduce the ductility of α-uranium. However, it has not been established whether aging amounts of hydrogen alter the mechanisms of plastic deformation (i.e. slip and twinning). In-situ neutron diffraction experiments and polycrystalline plasticity modeling were employed to address this issue. Samples were thermally charged to 0.3 and 1.8 wppm H then tested in-situ. It was seen that hydrogen charging decreased the ductility drastically. However, it did not affect the bulk plastic deformation processes. Specifically, the grain-level onset of dislocation glide and deformation twinning as well as the hardening behaviors of the individual mechanisms remained consistent between samples. The texture evolution did not change with hydrogen content either. What did clearly change with increasing hydrogen content was the fracture mechanism, which transitioned from transgranular ductile rupture to brittle intergranular decohesion.