Crack Propagation Resistance Research Articles

Fatigue behaviour of laser shock peened AISI D2 tool steel

Laser shock peening (LSP) is an advanced surface enhancement technique capable of imparting beneficial compressive residual stresses, thereby improving metal parts’ fatigue resistance and crack propagation resistance. This study presents the fatigue behaviour of AISI D2 tool steel samples subjected to LSP with a power density of 15 GW/cm2 and a double laser scanning sequence. The effect of LSP on quasi-static tensile properties was also addressed. The fatigue crack propagation was suppressed in LSP-treated samples when tested at ΔK = 10 MPam. The fatigue strength of treated samples was 32.9% higher compared to untreated samples. The LSP-treated samples have shown a larger fatigue fracture surface area than the as-received (AR) sample. In addition, the tendency to form secondary cracks in the fatigue fracture paths decreased due to LSP treatment. LSP treatment can effectively improve the fatigue resistance and crack propagation behaviour of AISI D2 steel. These improvements are due to significant compressive residual stresses in a superficial layer of the material. The present study provides a reference for improving the mechanical properties of AISI D2 steel by laser shock peening.

Effect of Precipitates on the Mechanical Performance of 7005 Aluminum Alloy Plates.

In this study, the strength, elongation, and fatigue properties of 7005 aluminum alloy plates with different configurations of precipitates were investigated by means of tensile tests, fatigue tests, and microstructural observation. We found that the number and size of GP zones in an alloy plate matrix increased and the distribution was more uniform after the aging time was extended from 1 h to 4 h at 120 °C, which led to a rise in both strength and elongation of alloy plates with the extending aging time. The fatigue life of the alloy plates shortened slightly at first, then significantly prolonged, and then shortened again with the aging time extending from 1 h to 192 h and a fatigue stress level of 185 MPa and stress ratio (R) = 0. After aging at 120 °C for 96 h, the precipitates in the alloy plate matrix were almost all metastable η′-phase particles, which had the optimal aging strengthening effect on the alloy matrix, and the degree of mismatch between the α-Al matrix and second-phase particles was the smallest; the fatigue crack initiation and propagation resistances were the largest, leading to the best fatigue performance of alloy plates, and the fatigue life of the aluminum plate was the longest, up to 1.272 × 106 cycles. When the aging time at 120 °C was extended to 192 h, there were a small number of equilibrium η phases in the aluminum plates that were completely incoherent with the matrix and destroyed the continuity of the aluminum matrix, easily causing stress concentration. As a result, the fatigue life of alloy plates was shortened to 9.422 × 105 cycles.

Silver-loaded mesoporous silica nanoparticles enhanced the mechanical and antimicrobial properties of 3D printed denture base resin

The aim of this study is to develop a novel 3D printed denture base resin material modified with mesoporous silica nanocarrier loaded with silver (Ag/MSN) to enhance mechanical and antimicrobial properties. Acrylate resin-based was incorporated with various proportion of Ag/MSN (0.0–2.0 wt%). Specimens with different geometry were printed and characterized accordingly for the effect of modification on properties such as: mechanical and physical properties, chemical composition and degree of conversion, as well as biological response in term of biocompatibility and antimicrobial against oral fibroblast and candida biofilm (C. albicans), respectively. The consecutive addition of Ag/MSN improved significantly surface hardness and crack propagation resistance, while flexural strength remained similar to control; however, a negligible decrease was observed with higher concentrations ≥1 wt%. No significant difference was noticed with water sorption, while water solubility had a remarkable trend of reduction associated with filler content. The surface roughness significantly increased when concentration of Ag/MSN was ≥1.0 wt%. A significant reduction in C. albicans biofilm mass, as the inhibition proficiency was correlated with the proportion of the filler. With respect to the amount of Ag/MSN, the modification was compatible toward fibroblast cells. The sequential addition of Ag/MSN enhanced significantly the mechanical and antimicrobial properties of the 3D printed resin-based material without affecting adversely compatibility. The acrylic resin denture base material has susceptibility of microbial adhesion which limits its application. Silver loaded MSN showed a significant performance to enhance antimicrobial activity against C. albicans which is the main cause of denture stomatitis. The proposed invention is a promise technique for clinical application to provide an advanced prosthesis fabrication and serve as long-term drug delivery.

Fatigue crack propagation and fracture toughness of cortical bone are radiation dose-dependent.

Cortical bone allograft sterilized with a standard γ-radiation dose of 25-35kGy has demonstrated reduced static and cyclic fracture resistance compared with unirradiated bone. To mitigate radiation damage, we recently observed a dose-dependent response of high-cycle fatigue behavior of human cortical bone from 0 to 25 kGy, with lower doses exhibiting logarithmically longer fatigue lives. The objectives of this study were as follows: (1) to determine whether fracture toughness, work-to-fracture, and fatigue crack propagation resistance of human cortical bone are also radiation dose-dependent, and (2) to determine the associations of radiation dose and a Raman biomarker for collagen disorder with fracture properties. Compact tension specimens were machined from two donor femoral pairs and allocated to four treatment groups: 0 (unirradiated control), 10, 17.5, and 25 kGy. Fracture toughness specimens were monotonically loaded to failure and the critical stress intensity factor (KC ) was determined. Work-to-fracture was calculated from the load versus displacement integral up to fracture. Fatigue crack propagation specimens were cyclically loaded under constant room-temperature irrigation and fatigue crack growth rate (da/dN) and cyclic stress intensity (∆K) were calculated. Fracture toughness, work-to-fracture, and fatigue crack propagation resistance decreased 18%, 33%, and 15-fold from 0 to 25 kGy, respectively (p < 0.05). Radiation dose was more predictive of fracture properties than collagen disorder. These findings support that quasi-static and fatigue fracture properties of cortical bone are radiation dose-dependent within this dose range. The structural alterations arising from irradiation that cause these losses in fracture resistance remain to be elucidated.

Tough Hydrogels with Isotropic and Unprecedented Crack Propagation Resistance

AbstractMuscles and some tough hydrogels can maintain perfect mechanical properties after millions of loading cycles owing to the anisotropic microstructures inside them. However, applications of intrinsic anisotropic microstructures in biological tissues and tough hydrogels are limited by the poor mechanical performance in the perpendicular direction relative to the alignment direction. Here, a universal strategy is proposed for developing hydrogels with unprecedented isotropic crack propagation resistance only depending on the interpenetrating entanglements of polymer chains (polyacrylamide (PAAM) or poly‐(1‐acrylanmido‐2‐methylpropanesulfonic acid) (PAMPS)) in deformable polymeric microspheres (PAMPS or PAAM). The deformable interpenetrating network in microspheres can transform the hydrogel from isotropic to anisotropic instantaneously in any load direction, and effectively alleviate the stress concentration at the crack tip, dissipate energy, and eliminate notch sensitivity. The best isotropic hydrogel displays an ultimate strain of 5300%, toughness of 18.9 MJ m–3, fracture energy of 157 kJ m–2, and fatigue threshold of 4.2 kJ m–2. Furthermore, the mechanical strength of hydrogels can be simply tuned by solvent replacement. The strategy presented here can be expanded to prepare other isotropic hydrogels with super tear‐resistant and anti‐fatigue properties, based on a wide variety of deformable microspheres and matrix polymers.

Effect of laser heat treatment on AlxTi1-xN-based PVD coatings, deposited on carbon and tool steel substrates

This paper highlights the effect of laser heat treatment on the adhesion and sliding wear of physical vapour deposited (PVD) multilayer AlxTi1-xN and nanocomposite AlxTi1-xN/α-Si3N4 coatings on carbon (C45E, preliminarily bulk hardened) and tool steel (Vanadis® 6, preliminarily bulk hardened and tempered), as well as substrate steel microstructure and hardness. The hardened zone in carbon steel generally comprises martensite and retained austenite. The hardened zone in tool steel contains martensite and retained austenite, as well as M7C3 (M = Fe, Cr) and MC (M = Fe, V) carbides. Laser heat treatment increased the average surface hardness of the carbon steel by 1.5–3.1 times and that of the tool steel by 1.1–1.2 times. The critical adhesion loads Lc1 and Lc2 enlarged by 1.22.6 times and by 1.2–1.3 times, respectively, in the case of the coatings deposited on the carbon steel. However, only slightly positive or no changes in the critical adhesion loads were observed for the tool steel case. The scratch crack propagation resistance (CPRs) of the coatings increased by 1.1–4.8 times, being more pronounced for the carbon steel substrate. The improvement of adhesion was assumed to be the result of the increased hardness (H) to Young's modulus (E) H/E and H3/E2 ratios of the substrate steel. Wear resistance of the coatings improved by 1.3–1.7 times. Scuffing and surface fatigue wear were the principle wear mechanisms in all the cases. However, the first mechanism was more remarkable for laser heat treated samples, and the second for the samples that remained untreated by laser. Apart from the above-mentioned increment of H/E and H3/E2 ratios, the improvement of wear resistance was explained by the increased CPRs values of the coatings and by the presumed precipitation of the AlN phase within them.

Hydrogen as a Temporary Alloying Element for Establishing Specific Microstructural Gradients in Ti-6Al-4V

Parts of vehicles, such as landing gear components of aircrafts, are subject to growing demands in terms of sustainability via lightweight design and durability. To fulfill these requirements, the development of thermochemical processes is auspicious. Titanium alloys allow a heat treatment in hydrogen-containing atmosphere for temporary hydrogen alloying, often called thermohydrogen treatment (THT). The investigation presented intends to realize a local microstructure modification of Ti-6Al-4V by means of THT. The study aims to use hydrogen (H) as a promoter for changing the local distribution and morphology of strengthening precipitates during THT as well as the local grain size (microstructural gradient). Both shall improve the fatigue properties of the material after hydrogen degassing. To derive suitable thermohydrogen treatment process parameters, the resulting fatigue crack propagation resistance and fracture toughness after different solution heat treatments are determined experimentally and compared to each other. Moreover, various graded microstructures are evaluated after hydrogen uptake (hydrogenation) and hydrogen degassing (dehydrogenation) using numerically simulated hydrogen concentration profiles, observed hardness curves, metallographically determined microstructure gradients and the corresponding results of the phase analysis by means of X-ray diffraction. The study shows that hydrogenation at 500 °C and dehydrogenation at 750 °C enables the generation of a promising microstructural gradient.

Enhancing the impact resistance of HVAF-sprayed Fe-based amorphous coatings via hierarchical structure design

Hierarchical structure has been proposed as an effective strategy for improving mechanical properties in many traditional alloy systems. In this work, using a similar structural design strategy, hierarchical structured Fe-based amorphous coatings comprising two layers with different porosities were successfully prepared by controlling the thermal spraying process parameters. Impact test revealed that hierarchical structure designed coatings exhibited enhanced impact resistance than that of the monolithic structured counterpart. Microstructure examinations demonstrated that compared with the monolithic structured coating, the introduction of hierarchical structure could not only effectively passivate the cracks, but also deflect the propagation path of the cracks, which directly or indirectly enhanced crack propagation resistance. Based on this, theoretical analysis and finite element simulations were systematically performed to reveal the intrinsic reasons for the improved impact toughness in hierarchical structured coatings, centering around the influence of hierarchical structure on the absorption rate of impact energy and the stress distribution of coating during the impact process. In summary, the current study opens another promising window to enhance the impact resistance of amorphous coatings.

Formation Mechanism and Control of Solidification Cracking in Laser-Welded Joints of Steel/Copper Dissimilar Metals

The solidification cracking behavior in laser welds of steel/copper dissimilar metals was systematically investigated. T2 copper and SUS304 stainless steel were used in the study. The results showed that the occurrence of solidification cracking in welds was the synergistic effect of ε phase liquation, inclusions and composition segregation. During the welding process, the liquation of grain boundaries substantially reduced the cohesion between adjacent grains, as well as the resistance for intergranular crack propagation. The composition segregation inside the grains could induce lattice distortion, thus reducing the plastic deformation capacity of the material itself and concurrently increasing the susceptibility to cracks. In addition, an effective solution for inhibiting solidification cracking was proposed by using an oscillating laser, and the inhibition mechanism was further discussed. Laser oscillating welding significantly promoted grain refinement, solute diffusion and the formation of uniformly distributed ε-Cu precipitated phases in welds. It can improve the intergranular bonding, reduce the susceptibility to solidification cracking and increase the resistance to plastic deformation. The tensile strength of joints using laser oscillating welding is 251 MPa, 35.7% more than 185 MPa using laser welding. Meanwhile, the strain of joints using laser oscillating welding is 3.69, a 96% increase compared to 1.88 using laser welding.

Fatigue performance differences between rolled and selective laser melted Ti6Al4V alloys

In this work, the performance differences between rolled and selective laser melted (SLM) Ti6Al4V alloys used commonly for orthopedic implants were investigated. By rotation bending fatigue (RBF) and in-situ fatigue tests, the SN curves, crack propagation rate (d a /d N ) curves and cracking behavior of the SLM samples (in 0°, 45° and 90° directions) and rolled samples (fabricated along the rolling and transverse directions (RD and TD)) were compared. Slight fatigue performance distinction was identified for the rolled TD and RD Ti6Al4V samples. However, the building directions exhibited an obvious impact on the SN and d a /d N curves of the SLM Ti6Al4V alloy. The SLM 45° samples revealed excellent crack propagation resistance and the fatigue limit of the SLM 0° samples reached to that of the rolled Ti6Al4V alloy. The fatigue performance difference of the SLM components in different SLM directions can be expected. Moreover, the crack aspect ratios ( a / b ) of the SLM Ti6Al4V alloy were obtained from sample fracture surfaces. The effect of SLM building directions on the a / b values was revealed. Based on the results, the residual fatigue lives of the SLM and rolled Ti6Al4V hip joint prostheses were compared by finite element simulation. The SLM prostheses exhibited excellent fatigue performance and could replace the existing components fabricated by traditional alloys in some fields. • The various material anisotropies were identified for the rolled (RD and TD) and SLM Ti6Al4V (0°, 45° and 90°) samples. • The SLM 0° and 45° samples exhibited excellent fatigue limit and crack propagation resistance, respectively. • The effects of AM directions, sample types and crack depths on the crack aspect ratios of the SLM samples were revealed. • A K IC estimation method was applied for the rolled and SLM Ti6Al4V alloys. The validity of the method was proved. • The residual fatigue lives of SLM Ti6Al4V prostheses were calculated, which exhibited excellent fatigue performance.

Effect of tempering heat treatment on the microstructure and impact toughness of a Ni–Cr–Mo–V steel weld metal

Microstructure evolution and impact toughness of Ni–Cr–Mo–V weld metal in as-welded (AW) and different post-weld heat treatment (PWHT) conditions were investigated using Transmission Electron Microscopy (TEM), Transmission Kikuchi Diffraction (TKD), and Charpy impact test. The enrichment of Mn and Ni in inter-dendritic (ID) regions caused by welding solidification segregation lowered the Ac1 temperature, making partial untempered martensite transform into reversed austenite during PWHT at 580 °C, which was confirmed directly by Laser Scanning Confocal Microscope (LSCM). The transformation kinetics and element distribution during austenite reversion were revealed by DICTRA simulation, suggesting the enrichment of C, Mn and Ni in reversed austenite. The microstructure at room temperature of ID regions depended on the tempering cooling rates. When weld metal was cooled by water after tempering, the microstructure in ID regions consisted of finer fresh martensite and over tempered martensite, which was a major contributor to the higher impact toughness. While weld metal was cooled at a rate of 5 °C/h, the reversed austenite in ID regions transformed into fresh ferrite and cementite, which was difficult to resist crack propagation, and resultantly the impact toughness was deteriorated. The impact absorbed energy of weld metal increased with the increase of tempering cooling rates, which was attributed to the microstructure change in ID regions from ferrite to martensite. The inherited substructures of martensite in ID regions had a hindering effect on crack propagation, and resultantly more energy was consumed.

Characterization of anisotropic mode II fracture behaviors of a typical layered rock combining AE and DIC techniques

Shear-sliding mode fracture, i.e., mode II fracture, is a very common rupture mode of layered rocks. To achieve a better understanding of the anisotropy in the real mode II fracture, which propagates in a self-similar manner, a series of direct shear tests were conducted on single-notched specimens made of shale with three bedding angles, i.e., β = 0°, 45° and 90°. The ranking of the anisotropic mode II fracture toughness (KIIc) with different bedding angles is KIIc,45° >KIIc,90° >KIIc,0°, which is different from that of the anisotropic mode II fracture energy release rate (GIIc), i.e., GIIc,90° >GIIc,45° >GIIc,0°, suggesting that, unlike isotropic rocks, the crack propagation resistance of a layered rock cannot be completely characterized by its fracture toughness. From the acoustic emission monitoring results, it is found that the fracture process initiates and accelerates from the notch front once the applied mode II stress intensity factor (SIF) reaches its first and second thresholds, KII, FPI (approximately 50%∼70% of the fracture toughness) and KII, FPA (approximately 85%∼95% of the fracture toughness), respectively; generally, both KII, FPI and KII, FPA show noticeable anisotropies. The results of the b value indicate that more small-scale events occur in the specimen with β = 0°, and large-scale events account for a greater proportion in the specimen with β = 45°. A dominant high strain zone along the prefabricated crack direction and a secondary high strain zone along the bedding planes were discovered. Combining the acoustic emission and digital image correlation results, three typical fracture mechanisms were observed: (1) shear fracture along bedding planes when shearing along beddings; (2) shear fracture in matrix and along bedding planes for β = 45°; and (3) shear fracture in matrix and tensile fracture of bedding planes when shearing perpendicular to the beddings.

Breaking the hardness–toughness trade-off in WC–Co hardmetals: Introduction of dual-scale plate-like WC and modification of prediction models

The hardness–toughness trade-off relationship is an essential challenge when creating high-performance WC–Co hardmetals. In this work, by combining plasma milling and low-pressure sintering, a dual-scale plate-like WC–8Co hardmetal is fabricated. The hardmetal simultaneously exhibits ultra-high strength (1701 kgf/mm2) and fracture toughness (21.77 MPa*m1/2), with bimodal grain size distributions of plate-like WC with average diameter of 1.1 μm and 1.4 μm, respectively. This breaks the normal trade-off relationship between the strength and the fracture toughness, which is because of the enhancement of crack propagation resistance owing to the synergetic effect of the plate-like WC and the bimodal grain structure. Existing models for hardness and fracture toughness prediction are invalid for WC–Co hardmetals with oriented plate-like WC grains. Modified models, based on the Lee-Gurland hardness model and the Ravichandran fracture toughness model, are developed by inducing factors R, which is the proportion of the WC(0001) plane in the indented plane, and can be determined by XRD analysis, to represent morphology features of WC. The modified models are universal and can reliably predict the hardness and fracture toughness of WC–Co hardmetals with both equiaxial and plate-like WC grains.

Enlarged Zn Content Improves Fatigue Crack Propagation Resistance of Two Al-Zn-Mg-Cu Alloys with Multiple Aging Tempers

Fatigue crack propagation is closely associated to the chemical composition of Al-Zn-Mg-Cu alloys. In this work, two Al-Zn-Mg-Cu alloys with a variation of zinc content was investigated and multiple aging treatments were exerted on them and a same regime was selected for further fatigue analysis by tensile property tests. The corresponding precipitation characteristics and fracture surface were observed. The results showed that the alloy with lower zinc content (LZ alloy) possessed an inferior strength value compared with the alloy with higher zinc content (HZ alloy) under the same three stage aging treatments while the elongation had no obvious difference. In contrast, the LZ alloy had a higher fatigue crack propagation rate than the HZ alloy. The observation of fracture surface also proved it. The precipitation observation demonstrated that both had GP zones and η' phase, which possessed the majority. Quantitative analysis of precipitates exhibited that the HZ alloy had a larger proportion of large size precipitates than the LZ alloy. The mechanism of the interaction between dislocation and precipitate was employed to elaborate the difference.

Experimental and Analytical Modeling of Flexural Impact Strength of Preplaced Aggregate Fibrous Concrete Beams.

Preplaced aggregate fibrous concrete (PAFC) is a revolutionary kind of concrete composite that is gaining popularity and attracting the interest of academics from across the world. PAFC is a uniquely designed concrete prepared by stacking and packing premixed fibers and coarse aggregate in a steel mold. The gaps between the fibers and aggregates are subsequently filled by injecting a cement grout with high flowability. This study investigates the impact performance of three different sizes of PAFC beams. Steel and polypropylene fibers were used in a 3% dosage to make three different beam sizes, measuring 550 × 150 × 150 mm, 400 × 100 × 100 mm, and 250 × 50 × 50 mm. According to ACI Committee 544, all beams were subjected to a drop weight flexural impact test. Compressive strength, impact energies at initial crack and failure, ductility index, and failure mode were evaluated. Additionally, analytical modeling was used to compute the failure impact energy for the fibrous beams. The results showed that the addition of fibers increased the capacity of the tested beams to absorb greater flexural impact energy. Compared to polypropylene fibers, steel fibers had better crack propagation and opening resistance because of their higher tensile strength and crimped and hooked end configuration. For all large-size beams, the analysis of the percentage increase in impact energy at the failure stages was found to be 5.3 to 14.6 times higher than the impact energy at cracking.

Numerical Investigation of the Influence of Ultimate-Strength Heterogeneity on Crack Propagation and Fracture Toughness in Welded Joints.

The mechanical properties of dissimilar metal-welded joint materials are heterogeneous, which is an obstacle to the safety evaluation of key welded structures. The variation of stress–strain conditions at the crack tip caused by mismatch of material mechanical properties in dissimilar metal-welded joints is an important factor affecting crack propagation behavior. To understand the influence of uneven distribution of ultimate strength of the base metal and the welded metal on the crack propagation path, fracture toughness, as well as the mechanical field at the crack tip in the small-scale yield range, the user-defined field variable subroutine method is used to express continuous variation characteristics of welded joint ultimate strength in finite element software. In addition, the J-integral during crack propagation is calculated, and the effect of the ultimate strength on the J-integral and the stress field at the crack tip are analyzed. The results show that as the crack propagation direction is perpendicular to the direction of ultimate strength, the gradient of ultimate strength increases from |Gy|= 50 to |Gy|= 100 MPa/mm, the crack deflection angle increases by 0.018%, and the crack length increases by 1.46%. The fracture toughness of the material decreased slightly during crack propagation. Under the condition that the crack propagation direction is the same as the direction of ultimate strength, the crack propagation path is a straight line. As the gradient of ultimate strength increases from Gx = 50 to Gx = 100 MPa/mm, the crack propagation length decreases by 5.17%, and the slope of fracture toughness curve increases by 51.63%. On the contrary, as the crack propagates to the low ultimate strength side, the crack propagation resistance decreases, the ultimate strength gradient increases from Gx = −100 to Gx = −50 MPa/mm, and the slope of the fracture toughness curve decreases by 51.01%. It is suggested to consider the relationship between crack growth behavior and ultimate strength when designing and evaluating the structural integrity of cracks at the material interface of dissimilar metal-welded joints.

Interlaminar fracture property of Moso bamboo strips influenced by fiber distributions in bamboo internode and node

Bamboo is widely used as sustainable bio-composite material in the construction of houses, pavilions, furniture, and bridges, etc. As a unidirectional fibrous composite, it is usually damaged due to the interlaminar crack, which is directly related to the corresponding microstructure. In this study, the different fracture performances between bamboo internode and bamboo node are researched, as well as the fiber distributions in these zones. For the internode specimens, the results show that specimens with higher fiber contents tend to exhibit higher fracture energy than the case of lower fiber contents, so the fracture energy presents the functional gradient characteristics with fiber content. It is mainly because that more longitudinal vascular bundles make the fracture crack more tortuous and ultimately need more fracture energy. Furthermore, the node specimens have much-strengthened fracture performance than internode specimens with the same fiber contents, due to the radial fibers distributing in the bamboo node. The microscopic observation results indicate that the existence of radial vascular bundles causes the fracture process to include multiple damage modes such as interface delamination, vascular bundle pull-out, and crack deflection and finally play the effect of resisting crack propagation.

Homogenized constitutive equations for porous single crystals plasticity

Ductile fracture through void growth to coalescence occurs at the grain scale in numerous metallic alloys encountered in engineering applications. Classical models used to perform numerical simulations of ductile fracture, as the Gurson–Tvergaard–Needleman model and its extensions, are relevant for the case of large voids compared to the grain size, in which a homogenization of the material behavior over a large number of grains is used. Such modeling prevents assessing the effects of microstructure on both crack path and crack propagation resistance. Therefore, homogenized constitutive equations for porous single crystals plasticity are proposed, featuring void growth and void coalescence stages, hardening and void shape evolution. An original numerical implementation based on the coupling of Newton–Raphson and fixed point algorithms is described. In order to assess the accuracy of the proposed model as well as another one described recently in the literature, an extended database of porous unit-cell simulation results is gathered, investigating the effect of crystallographic orientations and hardening behavior for a FCC material. Strengths and weaknesses of both models are detailed with respect to the reference simulations, leading to the definition of the validity domain of the current model and to pinpoint necessary refinements.

Quasi-in-situ observations of crack propagation and microstructure evolution of high manganese austenitic steel at −196 °C

The crack propagation behaviors and microstructure characteristics in the high manganese austenitic steel were comprehensively investigated by quasi-in-situ observations at −196 °C. The steel exhibits a superior fracture toughness at −196 °C than at room temperature. It is found that a severe plastic deformation is necessary for a crack initiation. Although the main crack propagates along the {111} planes due to the plastic strain localization caused by an extensive planar slip, the steel still possesses a superior crack propagation resistance, which is attributed to the activations of multiple twinning and slipping systems, which provide two positive effects on blocking a crack propagation: (1) the plastic deformation further enhanced by multiple twinning and slipping systems as well as (2) a tortuous crack propagation path caused by numerous twin boundaries. Besides, the pronounced deflections at grain boundaries also have a large contribution to block a crack propagation.

Influence of DIMT on impact toughness: Relationship between crack propagation and the α′-martensite morphology in austenitic steel

The trade-off between transformation-induced plasticity (TRIP) strengthening and the intrinsically brittle nature of deformation-induced α′-martensite (DIM) has been a long-standing dilemma in optimizing the strength–toughness synergy of austenitic steels. This has limited their potential use, particularly in energy absorption applications. Here, we propose a new strategy to minimize the negative effect of intrinsically brittle α′-martensite without limiting the TRIP. In this study, austenitic 321 stainless steel samples with different austenite grain sizes (AGSs) were fabricated to tailor the DIM morphology. The effect of the DIM morphology on the crack propagation energy was simultaneously evaluated. Electron channeling contrast imaging (ECCI) and selected area electron backscattered diffraction (EBSD) of the Charpy fractures reveals that lath-like α′-martensite effectively deflects cracks and increases the crack propagation energy (Ep). As a consequence, coarse-grained and ultra-coarse-grained (CG/UCG) steels with lath-like α′-martensite can simultaneously achieve high strength and crack propagation resistance, while ultra-fine-grained (UFG) steel with blocky DIM exhibits the lowest Ep. Furthermore, the morphology of DIM is controlled by its nucleation sites and adjacent crystallographic/phase boundaries. The twins or ε-γ boundaries promote the formation of lath-like α′-martensite. The γ→ε→α′ transformation sequence can be explained by the change in stacking fault energy (SFE), which provides further guidance for the design of austenitic steels with high strength and high toughness.