Intergranular Fracture Surfaces Research Articles

Structure and mechanical properties of PR-03N18K9M5TYu steel grade fabricated by selective laser melting and post-processing

We fabricate samples of PR-03N18K9M5TYu steel (equivalent to ChS4) using selective laser melting (SLM) in a nitrogen atmosphere. Our research focused on the influence of hot isostatic pressing (HIP) combined with heat treatment (HT), specifically hardening and aging, on the steel's structure and its physical and mechanical properties (σucs, σys, δ, ψ). Through tensile testing, we evaluated the impact of post-processing treatments (HIP followed by HT) on the material's strength. We also assessed how different post-processing protocols affected residual porosity. Our findings indicate that samples exhibiting the highest strength and plastic properties correspond to those with the least structural defects and minimal residual porosity. In-depth microstructural analysis revealed that the optimal structure–a fine-grained, homogeneous configuration–is achieved via the combined application of SLM, HIP, and subsequent HT. The improvement in mechanical properties can be primarily attributed to the dispersed hardening effect, which is a consequence of the precipitation of the superfluous Ni3Ti phase. Fractographic examination revealed that the post-processing leads to a ductile and dimple fracture, occurring through mechanisms of shearing and detachment, giving rise to mixed-type fractures. The samples that displayed superior mechanical properties were characterized by a homogenous ductile intergranular fracture surface with clear evidence of plastic deformation. We measured the hardness (Н), modulus of elasticity (Е), and elastic recovery via indentation methods. The post-processing treatments notably enhanced material hardness and elastic modulus, with an increase from H = 4.6 GPa and E = 194 GPa in the sample post-HIP to H = 8.5 GPa and E = 256 GPa following HIP coupled with hardening and aging.

Lattice Defects Underneath Hydrogen-induced Intergranular Fracture Surface of Ni-Cr Alloy Evaluated by Low-energy Positron Beam

Lattice Defects Underneath Hydrogen-induced Intergranular Fracture Surface of Ni-Cr Alloy Evaluated by Low-energy Positron Beam

Fracture Characteristics of Super Duplex Stainless Steel after Hot Tensile Testing

This study’s temperature range of hot tensile testing is 1000–1200 °C, which is consistent with ordinary industrial hot working temperatures. The fractography of super duplex stainless steel S32750 after hot tensile testing is investigated with Optical Microscopy and Scanning Electron Microscopy, and Electron Back-scatter Diffraction and Transmission Electron Microscopy study hot deformation behaviors of the necking area. The results reveal that ~1050–1150 °C is the optimal hot working temperature range regarding hot ductility and cracking sensitivity. The micro-voids nucleate at the ferrite-austenite interphase boundaries following the Kurdjumov-Sachs (K-S) or Nishiyama-Wasserman (N-W) orientation relationship at 1000 °C. Therefore, a quasi-cleavage fracture forms along the interphase boundaries. At the temperature range 1050–1150 °C, a uniform distribution of fracture surface is formed, and the highest Reduction in Area appears at 1150 °C. Dynamic phase transformation occurs with the formation of secondary austenite at ferrite grain boundaries at 1200 °C, which results in an intergranular fracture surface morphology.

Improvement of resistance against hydrogen embrittlement by controlling carbon segregation at prior austenite grain boundary in 3Mn-0.2C martensitic steels

This study challenged to improve the resistance against hydrogen embrittlement by increasing the concentration of carbon segregated at prior austenite grain boundary (PAGB), XPAGB, in low-carbon martensitic steels. The specimens with/without carbon segregation treatment (Non-seg and Seg specimens, respectively) had almost the same microstructure, other than higher XPAGB in the Seg specimen. While the uncharged Non-seg and Seg specimens exhibited similar mechanical properties, the maximum stress of the hydrogen-charged specimen was much higher in the Seg specimen than that in the Non-seg specimen even when diffusible hydrogen contents were almost the same. In addition, the fraction of intergranular fracture surface was much smaller in the Seg specimen. Based on these results, we conclude that the segregated carbon suppressed the accumulation of hydrogen around PAGB by site competition and increased cohesive energy of PAGB, leading to the significantly improved resistance against hydrogen-related intergranular fracture.

Transverse Stress Rupture Properties of a Nickel-Base Superalloy Bicrystal

Effects of low angle boundaries (LABs) on the stress rupture properties of bicrystals of a nickel-based third generation single crystal superalloy at 1093 °C/158 MPa were investigated. The results show that the effect of LABs on the stress rupture elongation of the alloy is higher than that of the stress rupture life at 1093 °C/158 MPa. As the misorientation angle of the LABs reaches 9.0°, the stress rupture life of the alloy with LABs can still retain nearly 50% of that with LABs of 0° at 1093 °C/158 MPa; while the stress rupture elongation of the alloy with LABs drops obviously when the misorientation angle of the LABs is larger than 6.5°. The fracture surfaces of stress ruptured alloy with LABs of 0°~2.9° are characterized by dimple features, while those with LABs of 6.5°~12.3° all exhibit intergranular fracture features. Apparent dimple features can be observed at the intergranular fracture surface of the alloy with LABs of 6.5° and the elongation of it is high. However, obvious dendrite features can be observed at the intergranular fracture surfaces of the alloy with LABs of 7.6°~12.3° and the elongations of them are relatively low.

Preparation of an overall intergranular fracture surface caused by hydrogen and identification of lattice defects present in the local area just below the surface of tempered martensitic steel

An overall intergranular (IG) fracture surface was prepared in a load reduction test and analyzed by low-temperature thermal desorption spectroscopy (L-TDS). The purpose was to clarify on the atomic scale lattice defects present in the local area just below the surface of a hydrogen-induced IG fracture in tempered martensitic steel. The tracer hydrogen peak corresponding to vacancies and vacancy clusters was present just below the IG fracture surface. Local plasticity and nano-voids were observed on and near the prior austenite grain boundaries by the combined use of electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). These findings clearly indicate that local plastic deformation and associated vacancy formation play an essential role in hydrogen-induced IG fracture in tempered martensitic steel.

Statistical perspective on embrittling potency for intergranular fracture

Embrittling potency is a thermodynamic metric that assesses the influence of solute segregation to a grain boundary (GB) on intergranular fracture. Historically, authors of studies have reported embrittling potency as a single scalar value, assuming a single segregation site of importance at a GB and a particular cleavage plane. However, the topography of intergranular fracture surfaces is not generally known a priori. Accordingly, in this paper, we present a statistical ensemble approach to compute embrittling potency, where many free surface (FS) permutations are systematically considered to model fracture of a GB. The result is a statistical description of the thermodynamics of GB embrittlement. As a specific example, embrittling potency distributions are presented for Cr segregation to sites at two Ni $\ensuremath{\langle}111\ensuremath{\rangle}$ symmetric tilt GBs using atomistic simulations. We show that the average embrittling potency for a particular GB site, considering an ensemble of FS permutations, is not equal to the embrittling potency computed using the lowest energy pair of FSs. A mean GB embrittlement is proposed, considering both the likelihood of formation of a particular FS and the probability of solute occupancy at each GB site, to compare the relative embrittling behavior of two distinct GBs.

The effect of an Al-induced ferritic microfilm on the hydrogen embrittlement mechanism in martensitic steels

The addition of 2 wt% aluminium to a martensitic Fe-0.4C steel leads to a ferritic microfilm along the prior austenitic grain boundaries, which was demonstrated to significantly increase the ductility without affecting the bulk hardness. This study investigates its effect on the hydrogen embrittlement behavior. For Al-free martensitic steel without grain boundary ferrite, the presence of hydrogen causes a brittle cleavage type of fracture due to the embrittlement of the martensitic packets/blocks according to the hydrogen enhanced decohesion mechanism. Al-containing steels, with a ferritic grain boundary microfilm, on the other hand, experience a delay in hydrogen-induced fracture, and the crack propagation is redirected along the ferrite/martensite interface resulting in an intergranular fracture surface. This can be linked to an interplay between the hydrogen enhanced decohesion and hydrogen enhanced localized plasticity mechanism. However, by applying slow deformation rates in a hydrogen rich environment, the ferritic microfilm itself embrittles and loses its capability to influence the crack propagation, thus resulting in a similar fracture surface and ductility as the samples without the ferritic microfilm.

Rapid Evaluation of Hydrogen Embrittlement Resistance for Spot-Welds of High Tensile Strength Steel Sheet by Slow Rate Tensile Shear Test under Hydrogen Charging Conditions

Automobile manufacturers are accelerating adoption of spot welding of Advanced High-Strength-Steels (AHSS) sheets to reduce weight of automobile bodies. Rapid evaluation of the hydrogen embrittlement (HE) resistance for the spot-welds of AHSS sheets is required, since it is worried that the HE resistance of the nugget will deteriorate compared to the base metal due to the difference in microstructure caused by rapid cooling and solidification during spot welding. However, evaluation of the HE resistance for the spot-welds has not been established. In this study, we prepared spot-welded specimens using AHSS sheets and performed tensile shear tests with varying tensile rates under hydrogen charging to evaluate the relationship between diffusible hydrogen content and tensile shear strength. As a result, the tensile shear strength of spot welds decreased as the amount of diffusible hydrogen increased. The quasi-cleavage fracture surface and intergranular fracture surface were observed at the nugget and inside the crack generated at the nugget-heat affected zone interface. Furthermore, as the results of crack growth behavior and hydrogen thermal desorption spectroscopy analysis, hydrogen embrittlement in spot welds can be attributed to the stress-induced diffusion of hydrogen and the hydrogen trapped in dislocation and vacancy clusters at crack tips. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 75–83.

Effect of Solution Aggressiveness on the Crack Growth Resistance and Cracking Mechanism of AA2024-T3

Aluminum alloy 2024-T3 was examined using a range of microscopy techniques at the early stages of corrosion attack to investigate the corrosion-induced cracking mechanism. Two different corrosive environments—exfoliation corrosion (EXCO) and 3.5 wt% NaCl—were used for the exposure of tensile and prenotched compact-tension C(T) specimens of AA2024-T3. Different embrittlement mechanisms are noticed for the two investigated corrosive environments. Significant intergranular corrosion (IGC) and grain boundary embrittlement are evident in the specimens exposed to EXCO solution, whilethis was not the case for the milder solution comprising of 3.5 wt% NaCl. With regard to the milder solution, the corrosion attack is not only restricted to the grain boundary, but evolves transgranularly to the neighboring grains of the IGC attacked region and, consequently, the grain boundary strength in the direct vicinity is not notably affected. The extent of secondary cracks, after the exposure of C(T) specimens to EXCO solution and the subsequent crack-growth resistance evaluation, were found to correlate with the diameter of the plastically affected zone (≈3.78±0.04 mm). Additionally, the depth of these cracks was found to correlate well with the thickness of the intergranular fracture surface, giving evidence that the secondary cracks form due to grain boundary embrittlement; probably attributed to hydrogen embrittlement phenomena.

Fracture Behavior and Mechanical Properties of a Thermomechanical TRIP Steel Under Simulated Heat Treatment Conditions

In this research, hypereutectoid Si–Mn TRIP steel was prepared via casting method followed by thermomechanical processing and different heat treatments. Subsequently, fracture behavior and mechanical properties of the prepared steel were investigated. The time–temperature–transformation curves of the specimens were predicted by a simulation software. Their mechanical properties were studied through tensile, hardness, and impact tests. The microstructure and fracture behavior of the specimens were evaluated using an optical microscope, scanning electron microscope and fractographic analysis. Results indicated that the heat-treated sample containing a multi-phase microstructure of bainite, martensite, pearlite and retained austenite, had the highest ultimate tensile strength around 1,300 MPa. The software predictions of the heat treatments were in good accordance with the microstructural results. The samples have undergone thermomechanical process and heat treatment including isothermal transformation showed brittle behavior with intergranular and cleavage fracture surfaces. In addition, the two-stage quenching cycles resulted in dual-phase microstructure composed of martensite embedded in retained austenite with a combination of brittle and ductile fracture behavior.

The influence of carbonitriding conditions on microstructure and mechanical properties of 25CrMo4 low-alloy steel

Carbonitriding is an important industrial process applied for the improvement of the mechanical characteristics of the component of many steels employed in several machines parts like cam shafts, crank shafts and gears to enhance fatigue strength and wear resistance. In this study, the influence of the gaseous carbonitriding on the enhancement of surface characteristics and mechanical properties was investigated using different parameters for low alloy steel. The analysis and characterization of the treated material were carried out employing optical microscopy (OM) and scanning electron microscopy (SEM) and X-ray diffraction methods (retained austenite and residual stresses). Microhardness and tensile tests were performed and the fracture mechanisms of the various materials were ultimately examined and discussed. The main contribution of this research work is to show and optimize the selection of the appropriate parameters for this kind of steel in industrial field. Diffusion mechanism and microstructure proved that the process of high temperature gas carbonitriding allowed enhancing the mechanical properties of material. This process resulted in a rise of the yield strengths and a loss in a pronounced ductility associated with the brittle intergranular fracture surface caused by the nitride precipitation in the grain boundaries.

Multiscale analysis of grain boundary microstructure in high strength 7xxx Al alloys

The size, distribution and chemical composition of grain boundary η-phase precipitates (GBPs) and micro-segregation present in thick plate (140 mm) 7xxx Al alloys has been quantified across a range of length scales. To address the known limitations of individual characterisation methods, a number of cross-correlated, high resolution techniques have been used, including atom probe tomography (APT). A new-generation high-Zn alloy (AA7085) has been compared to a more established material, AA7050, in T7651 temper conditions. The results show that high angle grain boundaries in both alloys are dominated by quench-induced GBPs (Q-GBPs), covering up to ~40% of the area in AA7050. When viewed on brittle intergranular fracture surfaces and in 3D, the Q-GBPs appear much larger than previously reported and exhibit complex branched, dendritic-like morphologies. In AA7050, the Q-GBPs contain substantially higher levels of Cu (by 29 %) and Al (by 37%) and lower Zn (by 33 %) than AA7085. Classical modelling demonstrates that these differences result from different transformation pathways, with precipitates in the more quench sensitive AA7050 alloy nucleating at higher temperatures, which exaggerates the effect of alloy chemistry. In both alloys GB segregation was limited, with low levels of Zn detected relative to the matrix, but more Mg and less Cu in AA7050. It is, therefore, proposed that the higher Cu and Al, and lower Zn content, of the large Q-GBPs present in AA7050 is the main difference in GB microchemistry between the two materials and is the primary reason its GBs are less chemically active. The implications for the relative susceptibilities of the alloys to environmentally assisted cracking are briefly discussed.

Qualification of the in-situ bending technique towards the evaluation of the hydrogen induced fracture mechanism of martensitic Fe–C steels

This paper proposes a new in-situ hydrogen (H) charging bending technique to investigate the susceptibility to hydrogen embrittlement (HE) of high strength steels with limited ductility. The methodology is tested with generic martensitic Fe–C steels with a carbon (C) content of 0.2 wt%, 0.4 wt% and 1.1 wt%, respectively. The in-situ bending technique is developed to evaluate the hydrogen susceptibility of these brittle materials and is compared to uncharged samples as a reference. Moreover, as a crucial step in the validation of the technique, a comparison with conventional in-situ tensile testing for the most ductile material (i.e. Fe-0.2C) is performed. The bending results show that charging with H causes a significant ductility loss, which is characterized by a transition from a microvoid (Fe-0.2C), intergranular (Fe-1.1C) or mixed (Fe-0.4C) fracture surface for the uncharged samples to a hydrogen induced cleavage fracture appearance with additional cracking. The transition to the cleavage fracture type is found to be caused by the Hydrogen Enhanced Plasticity Mediated Decohesion mechanism, indicating that hydrogen is preferentially trapped at packet or block boundaries in high carbon steels without alloying additions. The fracture surface of the Fe-0.2C alloy after in-situ tensile testing was very similar to the fracture surface obtained after in-situ bending testing, which indicates that the fracture mode during bending is mainly dominated by the tensile field. This supports the applicability of the in-situ bending technique for intrinsically brittle materials.

Relationship between solidification sequence and toughness of carbon steel weld metal

This work aimed to investigate the segregation sequence of impurity elements in the weld metal of carbon steel with various solidification sequences, such as δ single-phase, hypo-peritectic, hyper-peritectic, and γ-single-phase solidification. Moreover, the relationship between the solidification sequence and toughness of the weld metal of carbon steel was studied. The solidification sequence was controlled by the nickel content of the base metals. Various amounts of sulfur and phosphorous were added to the base metals as impurity elements, and the segregation amount of the impurity elements in the weld metals increased in the order of δ-single-phase, hypo-peritectic, hyper-peritectic, and γ-single-phase. The amount of sulfur was higher than that of phosphorous in each solidification sequence. In δ-single-phase solidification, sulfur segregated during solidification and relaxed during cooling after solidification. In contrast, in γ-single-phase solidification, sulfur continued to segregate during and after solidification. The segregated sulfur was in the grain boundaries of the prior austenite at low temperatures. Charpy impact test results revealed that the ductile-brittle transition temperature (DBTT) increased, and the upper shelf energy decreased with an increase in the nickel content, specifically, only for the specimen of γ-single-phase solidification with S addition. In particular, the tendency in the specimens containing sulfur was more significant compared to the specimens containing phosphorous. Moreover, the intergranular fracture surface was only observed in the γ-single-phase solidification specimen, and sulfur was concentrated on the surface. Therefore, the superposition of segregation during and after solidification induced intergranular embrittlement and deterioration of toughness.

Coarse-grain CeO2 doped ZrO2 ceramic prepared by spark plasma sintering

In the present work, coarse grain cerium stabilized zirconia bulk ceramic was prepared by spark plasma sintering technique. The relatively high temperature of 2000 °C used for sintering led to enormous grain growth up to approximately 100 μm. Sintering at high temperatures and in the vacuum caused oxygen depletion and thus transformation from tetragonal to cubic phase during the sintering process. The tetragonal phase was recovered by annealing at 1400 °C in air. This led to a change in fracture behavior. Mostly transgranular fracture of the cubic phase was changed to intergranular fracture after recovering the tetragonal phase. On the intergranular fracture surface, twinning-like structure and structures similar to antiphase domain were observed.Mechanical properties represented by indentation hardness of prepared samples were evaluated.

Effects of Mo segregation on Charpy absorbed energy in Ti-12Mo alloys

Beta phase stabilizing elements such as Mo have strong tendency to segregate. We have introduced swirly type segregation of Mo in Ti-12Mo (mass %) alloy through groove bar rolling. After solution treatment and low temperature aging, hard omega phase was precipitated heterogeneously, which improved the room temperature tensile elongation values without sacrificing tensile strength. In this study, the effect of Mo segregation and heterogeneous distribution of omega phase on Charpy absorbed energy was investigated in Ti-12Mo alloy. Samples with two types of segregation were prepared; namely, swirly segregation in bar rolled sample and layered segregation in plate rolled sample. For comparison, we have also prepared Ti-12Mo bar samples with lesser Mo segregation, through high temperature thermomechanical treatment. Charpy impact tests were carried out at room temperature, 373 K and 473 K, respectively, using the samples after aging to introduce isothermal omega-phase. The samples with the segregation exhibited higher Charpy absorbed energy, especially at higher temperature of 473 K, while the sample with the swirly segregation showed higher Charpy absorbed energy than that with the layered segregation. The sample with lesser Mo segregation exhibited brittle intergranular fracture surface after Charpy testing. On the contrary, samples with Mo segregation exhibited ductile transgranular fracture surfaces.

Microstructure evolution mechanism near the fracture lip of 4Cr5MoSiV1 steel during deforming at 580 °C

The failure mechanisms under thermal-mechanical conditions have always been a critical issue in hot work steels. However, previous studies mainly focused on the mechanical properties of hot work materials, while the underlying deformation mechanism remains largely unclear. Here, we investigate the crack morphology and microstructure evolution mechanism near the fracture surfaces of 4Cr5MoSiV1 hot work die steels subjected to the uniaxial tension at 580°C. An evident deformation band was observed consisting of refined α-Fe grains with a width of about 100nm along both sides of the intergranular fracture surface. Additionally, characteristic slip band rings formed frequently, presumably due to the grain rotation near the crack. Finally, carbides (including MC, M7C3 and M23C6) were also considered as possible crack nucleation sources due to the existence of incoherent boundary between these carbides and the ferrite matrix.

Effects of θ′ precipitates on the mechanical performance and fracture behavior of an Al–Cu alloy subjected to overaged condition

In this work, the effects of shear-resistant plate-shaped θ′ (Al2Cu) precipitates on mechanical performance and fracture behavior of a heat-treatable Al–Cu alloy were studied. The precipitates, dislocations and micro-voids around grain boundaries were observed using transmission electron microscope and scanning electron microscope equipped with electron backscatter diffraction. The Al–Cu alloy was overaged to ensure that the θ′ phase was the only precipitate in the material. The results show that concentration of local strain and pile-up of dislocations occurred along grain boundaries during tensile deformation. A method was developed to calculate the total dislocation density nearby the grain boundaries in the deformed alloy, from which a quantitative relation was established between precipitate number density and void areal density on intergranular fracture surface. The effect of precipitates on the mechanical properties of Al–Cu alloy was also discussed qualitatively. The precipitate number density of the over-aged Al–Cu alloy acted as the dominating factor that influenced the dislocation density and void density on grain boundaries, which thus determined the mechanical properties and fracture behaviors of the alloy.

Crystallographic feature of hydrogen-related fracture in 2Mn-0.1C ferritic steel

The present paper investigated crystallographic feature of hydrogen-related fracture in a 2Mn-0.1C steel having a simple ferritic microstructure. We found that the mechanical properties (in particular post-uniform elongation) were degraded by concurrent hydrogen-charging. Most of the fracture surfaces (over 90%) of the concurrently hydrogen-charged specimens showed quasi-cleavage morphologies with serrated markings, but no intergranular fracture surface was observed. Through a detailed crystallographic orientation analysis using EBSD, we have clarified that micro-cracks formed at ferrite grain boundaries and the micro-cracks propagated inside grains along crystallographic {011} planes of ferrite, leading to the quasi-cleavage fracture. Hydrogen micro-print technique revealed that hydrogen accumulated along ferrite grain boundaries under tensile-loading. On the basis of the obtained results, we propose that the fracture on {011} planes is an intrinsic characteristic of hydrogen-related quasi-cleavage fracture in steels having BCC phases.