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The microstructure, mechanical properties, and high/very‐high cycle fatigue behaviors of 9Cr3W3Co turbine rotor steel and its welded joint were investigated. The welded nugget zone (WNZ) exhibited the highest microhardness value, followed by the heat‐affected zone (HAZ), while the base material (BM) showed the lowest hardness value. The WNZ exhibited a higher yield strength compared to the BM with reduced elongation, while maintaining similar ultimate tensile strength. The fatigue strength of the BM at 5 × 107 cycles is roughly 53.7% of the yield strength, whereas that of the WNZ increases to approximately 55.0%. In comparison to the fatigue strength of the BM at 5 × 107 cycles, the WNZ exhibits a notable increase of 10.5%. Smaller grain size and higher dislocation density were observed in the WNZ, which contributed to improved microhardness, yield strength, and fatigue resistance through grain boundary strengthening and dislocation hardening. Highlights: Microstructure and mechanical properties of 9Cr3W3Co steel welded joints were studied.Microhardness values of various zones are non‐uniform and the maximum value in WNZ.Dislocation and grain boundary strengthening contributed to strengthening in WNZ. [ABSTRACT FROM AUTHOR]

The study aims to analyze the tensile behavior and microstructural changes in MAR‐M247 nickel‐based superalloy across different temperatures. Tensile behavior is examined under a constant strain rate of 2.5 × 10−4 s−1 at varying temperatures. Results indicate a temperature‐dependent nature of the alloy's tensile properties. At 550 °C, a Portevin–Le Chatelier effect is observed, attributed to twinning nucleation and carbon atom diffusion. Dynamic recrystallization occurs at 950 °C, manifesting in a sinusoidal stress–strain curve. At room temperature, the primary fracture mechanism involves dislocation shearing in the γ matrix, with minor dislocation shearing in the γ′ phase. At 550 °C, carbides along grain boundaries and the γ′ phase impede dislocation motion. Concurrently, dislocations shear the γ′ phase, leading to the formation of superlattice intrinsic stacking faults and Kear–Wilsdorf locks, thereby enhancing tensile strength. However, at 950 °C, dislocation motion primarily involves climb, carbides decompose, the γ′ phase softens and enlarges, resulting in a notable decrease in tensile strength. [ABSTRACT FROM AUTHOR]

Calcium hexaluminate (CA6) porous ceramics were prepared by foaming and pore-forming agent methods, with Al2O3 and calcium aluminate cement (CAC) as raw materials and magnesium aluminum spinel hollow spheres (MASHSs) as pore-forming agent. The effects of the amount of magnesium aluminum spinel hollow sphere on the phase composition, bulk density, apparent porosity, flexural strength, microstructure, and thermal conductivity of CA6-based porous ceramics were systematically studied. The pores of such porous ceramics are mainly formed by the enhanced amount of MASHSs and the generation of CA6 phase. The results demonstrate that with the elevation of MASHSs content, the microstructure of the samples transforms from a uniform distribution of closed pores to an interconnected micro-pore arrangement. Notably, the fracture paths transition from cohesive to interfacial interactions between MASHSs. This intricate interplay culminates in a remarkable outcome – our ceramics exhibit thermal conductivities ranging from 0.210 to 0.451 W/(m/k), an augmented gas permeability of 25.96 × 10−11m2, and exceptional compressive strengths reaching up to 22.3 MPa. Overall, this study provides insights into the influence of MASHSs content on the microstructure and thermal-mechanical properties of porous materials, which may contribute to the development of advanced ceramic materials with tailored properties. [ABSTRACT FROM AUTHOR]

This study investigated the high cycle fatigue crack initiation and propagation behaviors in coarse‐grained FeCrAl alloys, with a focus on the relationship between the path of small cracks and the local microstructure. The results show that the small cracks initiate along the {100} slip plane due to the synthetical influence of local microstructure and shear stress. The initiation zone forms inclined areas with layered fine‐grain areas caused by cyclic shear stress. Quantitative analysis of the stress intensity factor (SIF) indicates that in the realm of mixed‐mode loading, the SIF of Mode I takes the lead in determining the intensity of stress and strain fields at the crack tip. As the crack propagates further, the SIF steadily increases. Once it reaches a specific threshold, the propagation mode within a coarse grain shifts from Mode II to Mode I. Moreover, fatigue life increases with a tendency for decreased SIF during crack deflection. Highlights: The influence of microstructure on crack initiation and early propagation behavior was studied.The cyclic shear stress leads to the formation of layered fine‐grain areas.The quantitative analysis elucidates the reason for the deviation of crack direction within a coarse grain. [ABSTRACT FROM AUTHOR]

The materials with grain size gradient variation on the surface, which were prepared with mechanical-induced severe plastic deformation, always show high resistance to high and low cycle fatigue and frictional wear because of their good strength–ductility synergy. The ultrasonic surface rolling treatment (USRT) has the advantages of high processing efficiency, good surface quality, and large residual compressive stress introduced to the surface after treatment. The USRT was used to prepare aluminum alloy (AA7075) samples with a surface gradient structure; meanwhile, the microstructural evolution mechanism of the deformation layers on the gradient structure was studied with XRD, SEM, and TEM. The microstructure with gradient distribution of grain size and dislocation density formed on the surface of AA7075 aluminum alloy after USRT. The surface layer consists of nanocrystals with random orientation distribution, and high-density dislocation cells and subgrains formed in some grains in the subsurface layer, while the center of the material is an undeformed coarse-grained matrix. The results show that the dislocation slip dominates the grain refinement process, following the continuous cutting and refinement of dislocation cells, subgrains, and fragmentation of the second precipitates. This study systematically clarified the mechanism of grain refinement and nanocrystallization on the surface of high-strength aluminum alloys and laid a theoretical foundation for further research on mechanical behavior and surface friction and wear properties of high-strength non-ferrous materials with gradient structure. [ABSTRACT FROM AUTHOR]

In the current work, a multiscale model was developed coupling a macro-model with the macromechanical physically based yield strength and a crystal plasticity model with micromechanical properties and realistic grain orientation based on the representative volume element. The simulation results show that the effect of microstructure on the macromechanical properties can be considered in the macro constitutive model due to a good consistency between experimental and computed results; whereas solid strengthening, grain boundaries, and dislocation density played a more crucial role than others. Besides coupling simulation and microstructure by EBSD, the microstructure evolution can be well explained by the micromechanical model. Strain is related to the grain orientation, leading to inhomogeneous deformation, forming the various Schmid factor and slip systems. A plastic strain occurs close to the grain boundaries and declines into the grain, resulting in higher kernel average misorientation (KAM) and geometry necessary dislocations (GNDs) in the grain boundaries. The higher the loading, the higher the local strain. Shear bands with around 45 degrees can be formed, resulting in crack initiation and tensile shear failure. This work has developed the guidance of structural integrity assessment and prediction of mechanical properties for the engineering material and components. [ABSTRACT FROM AUTHOR]

AlxCrFeNi2.5Mo high entropy alloys (HEAs) were fabricated. Investigations were done into how Al content affected phase development, microstructure and mechanical characteristics. As the amount of Al growing, the phases constitution of AlxCrFeNi2.5Mo HEAs altered from face-centered cubic (FCC) + σ phase to FCC + body-centered cubic (BCC) + σ phase, and the microstructure varied from hypoeutectic to dendrite + interdendritic + eutectic structure. The Al0.2CrFeNi2.5Mo HEA had an exceptional overall performance with high tensile strength and compression strength of 1160 MPa and 1728 MPa, and good fracture strain and plastic strain of 8.0% and 22.3%, respectively. [ABSTRACT FROM AUTHOR]

The aim of this study is to fabricate high-strength steel with exceptional yield strength and superior ductility by employing a novel design approach of nanolamellar/equiaxial crystal "sandwich" heterostructures, utilizing rolling and electron-beam-welding techniques. The microstructural heterogeneity of the steel is manifested in the phase content and grain size, ranging from nanolamellae comprising a small quantity of martensite on both sides to the completely coarse austenite in the center, which are interconnected via gradient interfaces. The structural heterogeneity and phase-transformation-induced plasticity (TIRP) offer remarkable strength and ductility for the samples. Furthermore, the synergistic confinement of the heterogeneous structures leads to the formation of Lüders bands, which exhibit stable propagation under the TIRP effect and impede the onset of plastic instability, ultimately resulting in a significant improvement in the ductility of the high-strength steel. [ABSTRACT FROM AUTHOR]

The uniaxial tensile behavior of MarBN steel with a constant strain rate of 5 × 10−5 s−1 under various temperatures ranging from room temperature to 630 °C was analyzed. This study aimed to identify the effect of the temperature on the tensile behavior and to understand the microstructure deformation by electron backscatter diffraction. The tensile results showed that the yield and ultimate tensile strength decreased with increasing temperature. Serrated flow was observed from 430 °C to 630 °C. The electron backscatter diffraction analysis showed that the low-angle grain boundaries decreased at the medium deformation and increased at the maximum deformation. In contrast, they decreased with increasing temperatures. In addition, the number of voids increased with the increasing plastic strain. As the strain increased, the voids joined together, and the tiny cracks became larger and failed. Three mechanisms were responsible for the tensile deformation failure at various temperatures: grain rotation, the formation and rearrangement of low angle grain boundaries, and void nucleation and propagation. Finally, the formation of the low-angle grain boundaries and voids under different degrees of deformation is discussed. [ABSTRACT FROM AUTHOR]

The present study envisaged the behavior and relevant crack initiation mechanism of super martensitic stainless steel (0Cr13Ni5Mo) at elevated temperature (ET) under very‐high‐cycle fatigue (VHCF). Fractographic analysis revealed that all the VHCF cracks of the failure specimens initiated from the surface and formed fine granular area (FGA) morphology at ET in an absence of vacuum. The initiation characteristic area (ICA) consisted of an initiation site, a relatively smooth area, and the annular continuous FGA. FGA was formed in the peaks and valleys of local microstructures, and the smooth regions connected these peaks and valleys dominated by shear stress. It was observed that the FGA was unevenly distributed on the crack surface related to the local microstructure and below the grain surface relevant to local strain distribution. The stress intensity factor (SIF) and the crack tip plastic zone of the ICA slightly changed with temperature. Highlights: The vacuum environment was no longer necessary for the formation of FGA.FGA was unevenly distributed related to local microstructure and local strain condition.The fatigue cracks' initiation and early propagation were affected by local microstructure. [ABSTRACT FROM AUTHOR]

To reduce harmful gas emission and improve the operational efficiency, advanced ultra-supercritical power plants put forward higher requirements on the high temperature mechanical properties of applied materials. In this paper, the tensile behavior and deformation mechanisms of MarBN steel are discussed at different strain rates (5 × 10−3 s−1, 5 × 10−4 s−1, and 5 × 10−5 s−1) under room temperature and 630 °C. The results show that the tensile behavior of the alloy is dependent on temperature and strain rate, which derived from the balance between the average dislocation velocity and dislocation density. Furthermore, observed dynamic recrystallized grains under severe deformation reveal the existence of dynamic recovery at 630 °C, which increases the elongation compared to room temperature. Finally, three typical constitutive equations are used to quantitatively describe the tensile deformation behavior of MarBN steel under different strain rates and temperatures. Meanwhile, the constitutive model of flow stress for MarBN steel is developed based on the hyperbolic sine law. [ABSTRACT FROM AUTHOR]

The purpose of this research was to evaluate integrity and microstructure changes of the electron beam-welded Nimonic 80A and the base metal after aging heat treatment process, i.e., 700 °C/72 h/air cooling. Here, microstructure, microhardness and tensile were investigated by various tools, i.e., scanning electron microscope with energy-dispersive spectroscopy and electron backscatter diffraction, x-ray diffraction analysis and Vickers hardness. Evaluation of microstructure shows that the weld zone mainly consists of columnar dendrites. The effect of aging heat treatment on the microstructure morphology and phase structure is very limited. Also, it was observed that there are obvious segregation and precipitates in intracrystalline and along the grain boundaries after aging heat treatment. Electron backscatter diffraction results show that the weld zone and base metal have a clear difference in the grain boundary characterization and kernel average misorientation at room temperature and after aging heat treatment, which leads to a difference in microhardness of the weld zone. In addition, the relationship between microhardness and tensile strength is discussed. [ABSTRACT FROM AUTHOR]

Fly ash geopolymer concrete (FAGC) and ordinary Portland cement concrete (OPCC) specimens were immersed in 5% MgSO4 solution undergoing 32 wetting-drying and heating-cooling cycles. Their compressive behavior was investigated after every 8 cycles. Several microstructure analysis techniques were applied on the samples to identify the materials formed due to magnesium sulfate attack, including XRD, FTIR, SEM, and EDS. Experimental results elucidated that the compressive strength loss ratio in the heating group of FAGC was 12.7%, while that of OPCC was 17.8%, which means that FAGC had better magnesium sulfate resistance than OPCC. The compressive strength loss of OPCC was due to the formation of gypsum under the magnesium sulfate attack exposed to wetting-drying and heating-cooling cycles. The deterioration mechanisms of FAGC against MgSO4 solution were discovered to be that sodium aluminum silicate hydrate (N-A-S-H) gels reacted with MgSO4, leading to the creation of low strength magnesium aluminum silicate hydrate (M-A-S-H) gels. [ABSTRACT FROM AUTHOR]

Fly ash-based geopolymer concrete (FAGC) has been widely recognized in engineering for its environmentally friendly feature. However, the application is limited due to the high-temperature curing condition. This paper attempts to find a measurement to make it possible to cure low-calcium FAGC at ambient temperature. In this study, low-calcium fly ash was substituted with cement by 0, 5, and 10% in weight. One batch of specimens were cured at 176°F (80°C), the other batch of specimens were cured at 68°F (20°C). The acting mechanism of cement was illustrated in terms of slump, compressive strength, and microstructure. The results demonstrated that the optimum use ratio of cement was 5%, which could significantly improve microstructure and the properties of low-calcium FAGC. Additionally, appropriate ordinary portland cement (OPC) content could make it feasible to cure the geopolymer concrete at ambient temperature. [ABSTRACT FROM AUTHOR]

• The influence of various alkali on hydration and microstructure of HPGP was investigated. • Two opposite effects of alkali resulted by neutralization effect and ion strength effect on hydration of HPGP were observed. • Gypsum crystal morphology was significantly affected in alkaline environment except of excessive lime. • The influence effect of alkaline environment is related to its alkalinity and the impurity content in HPG. Although lime neutralization is widely recommended for raw phosphogypsum pretreatment, the influence mechanism of alkaline environment on hydration of hemihydrate phosphogypsum (HPG) is not clear. In the present paper, effect of alkaline environment created by different kinds of alkali including Mg(OH) 2 , NH 3 ·H 2 O, NaOH and Ca(OH) 2 with variable contents on hydration and microstructure of HPG were investigated. Abnormal setting and strength degradation were observed in the low-alkaline environment, while the setting time was normalized and crystal size developed in the high-alkaline environment regardless of the alkali category. Interestingly, the strength of HPG in the alkali environment only increased with excessive Ca(OH) 2. It is verified that the formation of Ca 3 (PO 4) 2 from neutralization reaction in the low-alkaline environment served as retarder, and the excessive alkali beyond the threshold value of neutralization reaction played as accelerator in the high-alkaline environment, while only excessive lime particles promoted the development of gypsum crystal at c axis probably due to its absorption of Ca 3 (PO 4) 2. This finding would promote the application of HPG in an alkaline environment and thus improve the utilization rate of phosphogypsum. [ABSTRACT FROM AUTHOR]

• The hydration process of HPG in alkaline environment were analyzed by a muti-technique approach. • Both the dissolution of hemihydrate and crystallization of gypsum were frustrated in alkaline environment. • As an increase in pH, HPG plaster sets slow with a fall in strength. • The morphology of gypsum crystal in hydrated HPG plaster were modified contributing to loose microstructure. The hydration of hemihydrate phosphogypsum in the alkaline environment was investigated aiming to recycle phosphogypsum-a promising gypsum resource in gypsum-based materials that generally incorporated alkaline binders. Isothermal conduction calorimetry and transmission electron microscopy showed that the hydration reaction of hemihydrate phosphogypsum was significantly changed by alkali. Both the dissolution of hemihydrate particles and crystallization of gypsum were impeded, as a result of phosphate precipitation from neutralization reaction. Consequently, the setting times decreased with the increase of pH and the time span of heat liberation was extended. The impurities in hemihydrate phosphogypsum, especially phosphate, served as setting retarders and crystal modifiers in alkaline environment, contributing to loose crystal microstructure with lower strength. This important finding called for the extra attention to the utilization of hemihydrate phosphogypsum in an alkaline environment. [ABSTRACT FROM AUTHOR]

Fatigue properties of Ti-6Al-4V (TC4) subjected to 0.9% physiological saline solutions were presented in this paper. The ultrasonic fatigue test method has been used to investigate the very high cycle fatigue properties of commercial titanium alloy TC4 with the microstructure of tramsformed lamellar dipped into 0.9% physiological saline solution prior to testing. Three estimated times have been decided for subjecting at 37 °C, such as 24 hours, 2 weeks and 4 weeks. The results show that physiological saline solution treatment makes a little influence on the ultrasonic fatigue life of TC4. The slope of S-N curve decreases as the dipping time increases. Dipping tests for three estimated times also have been performed. When TC4 has been dipped into physiological saline solution for 4 weeks, homogeneous white points have been observed on specular facet specimen. And in the fracture surface of which has been dipped for 4 weeks, new inclusion sized 5-10 µm in the crack initiation has been observed. The fractography pictures show the fracture to be transcrystalline plastic. Cracks are initiated because of the subsurface inclusion and the facet slip of α-phase. [ABSTRACT FROM AUTHOR]

Tensile deformation behavior and microstructure of nickel-base superalloy Inconel 625 are investigated under different strain rates of 5 × 10−4 s−1 and 5 × 10−5 s−1. According to the experimental results, yield strength and ultimate tensile strength of the alloy increase with the increase in strain rate in room temperature. Microstructure results indicate that the size of dimples is smaller in the tensile fracture surface at low strain rate than the high strain rate, and the number of dimples is also related to the strain rates and twins appear earlier in the specimens with higher strain rates. Apart from Hollomon and Ludwik functions, a new formula considering the variation trend of strength in different deformation stages is deduced and introduced, which fit closer to the tensile curves of the 625 alloy used in the present work at both strain rates. Furthermore, the Schmid factors of tensile samples under two strain rates are calculated and discussed. In the end, typical work hardening behavior resulting from the dislocations slip behavior under different strain rates is observed, and a shearing phenomenon of slip lines cross through the δ precipitates due to the movement of dislocations is also be note. [ABSTRACT FROM AUTHOR]

Effect of microstructure on the crack initiation and early propagation mechanism in the very high cycle fatigue (VHCF) regime was studied in 316L stainless steel (316L SS) by atomic force microscope (AFM) and electron back scattered diffraction (EBSD). The results show that small fatigue cracks initiate from the slip band near the grain boundaries (GBs) or the twin boundaries (TBs). Early crack propagation along or cross the slip band is strongly influenced by the local microstructure such as grain size, orientation, and boundary. Besides, the gathered slip bands (SBs) are presented side by side with the damage grains of the run-out specimen. Finally, it is found that dislocations can either pass through the TBs, or be arrested at the TBs. [ABSTRACT FROM AUTHOR]

The effect of temperature on tensile behavior, low cycle fatigue, and microstructure evolution considering the damage mechanisms of Cr-based heat-resisting steel was investigated. The results present that significant monotonic and cyclic softening was observed at both room temperature and 630 °C. However, the cyclic softening factor is independent of the strain amplitudes and temperatures. At room temperature, the steady fatigue behavior until 80% fatigue life was presented, by contrast, great changes were offered below 80% fatigue life at 630 °C. this is directly related to the microstructure properties during fatigue tests, where dislocation network, wall, and dynamic recovery were formed, resulting from the dislocation movement, interaction, and annihilation. The density of dislocation and intensity of slip is inversely proportional to the temperature. Besides, a prediction fatigue life model fitted by the quadratic function was established through the hysteresis energy, which can consider the effect of temperature on the different fitting parameters and offer the method to optimize the fatigue behavior of Cr-based steel at different temperatures Unlabelled Image • The low cycle fatigue of 9Cr at room temperature and 630 ℃ were performed to evaluate cyclic behavior and microstructure. • The softening behavior is observed at both temperatures. The higher the temperature, the great the softening behavior. • The dynamic recovery happened due to the dislocations annihilation and low angle boundary disappearance. • The higher the temperature, the lower the density of dislocation and the greater intensity of slip. • A prediction model of low cycle fatigue considering the temperature was constituted and confirmed. [ABSTRACT FROM AUTHOR]

• The formation of FGA is attributed to continuous dynamic recrystallization. • The formation of fine grains promotes the extension of microcracks. • A mechanism for VHCF crack initiation in Q450NQR1 weathering steel is proposed. The fatigue behavior of Q450NQR1 high-strength weathering steel under a loading ratio of R = 0.1 was experimentally investigated in the very high cycle fatigue (VHCF) regime, elucidating the formation mechanism of the fine granular area (FGA). The results indicated that Q450NQR1 high-strength weathering steel mainly fails due to non-inclusion-induced crack initiation (NIICI) in VHCF regime. Slip in {1 1 0} and {1 1 2} plane families dominates in ferrite. Intergranular cracking results in fatigue microcracking, which in turn leads to the formation of NIICI in Q450NQR1 high-strength weathering steel. The formation of FGA was attributed to the localized severe plastic strain-induced continuous dynamic recrystallization at the tip of microcrack propagation. Moreover, it can be found that the formation of fine grains promotes the extension and aggregation of microcracks. Based on all experimental findings, a mechanism governing crack initiation in Q450NQR1 high-strength weathering steel during VHCF loading was proposed. [ABSTRACT FROM AUTHOR]

• The initial cyclic softening followed by cyclic hardening behavior is observed. • The dislocation rearrangement process promotes initial cyclic softening behavior. • The martensite formation results in the cyclic hardening behavior of SLM 304L. Considering the relationship between austenite stability and strain level, the cyclic response curves are obtained for SLM 304L under different strain amplitude at R = 0.3. The results indicate that since the microstructure characteristics shift from stacking faults (SFs) at low strain amplitude to martensite and twins at high strain amplitude, the cyclic softening behavior transitions to cyclic hardening behavior. Besides, it also indicates that high strain levels can overcome the influence of initial microstructure on fatigue performance, presenting a new sight to understand the relationship between cyclic hardening and martensite transformation. [ABSTRACT FROM AUTHOR]

• To improve the computational efficiency and accuracy of SP simulation, a coupled DEM-FEM model was established. • Dislocations are plugged in the γ matrix to form subgranular boundaries that can fix γ' to form small-sized grains, and in addition some of the dislocations shear γꞌ further leading to grain refinement. • The SP treatment produced a high density of dislocations, work hardening, and residual compressive stresses, also reduces porosity defects left on the surface. The effects of shot peening on the surface microstructure, tensile properties, and high-cycle fatigue properties of MAR-M247 nickel-based superalloy were investigated. The predicted surface roughness, magnitude of residual stress, and depth of plastic strain in the surface layer were simulated by the numerical simulations coupled Finite Element Method-Discrete Element Method (FEM-DEM). The results show that the shot peening treatment induced significant plastic deformation in the surface layer, leading to the accumulation of a large number of dislocations in the γ matrix, forming dislocation walls, resulting in the formation of fine grains. A high density of dislocations, work hardening, and residual compressive stresses occur after the shot peening treatment. These beneficial changes enhanced the resistance to crack initiation and propagation, thereby inhibiting surface crack initiation, and increasing the fatigue life and tensile strength of the alloy. [ABSTRACT FROM AUTHOR]

[Display omitted] • The evolution of the deformation mechanism with stress loading is analyzed. • The alteration of atomic arrangement due to dislocation motion is illustrated in conjunction with HRTEM. • The magnitude of the SFE of AM 939 at intermediate temperature is estimated from the critical shear stresses. Tensile interruption experiments with different strains were conducted on additively fabricated nickel-based Inconel 939 superalloys at 700 ℃ to investigate the evolution of multiple deformation mechanisms. The microstructures of different strain groups captured by transmission electron microscopy (TEM) were analyzed, and it was concluded that a total of four various deformation mechanisms were identified, of which the first to be activated was the Orowan loop bypassing γ', and a tendency of transition from double to single loop was observed. Anti-phase boundary (APB) shearing was subsequently initiated, with a needed critical resolved shear stress level of approximately 195.82 MPa. Stacking faults (SFs) and deformation twinnings (DTs) were observed at higher stress, accompanied by the dissociation of dislocations. In addition, the magnitude of the stacking fault energy of AM 939 superalloys at intermediate temperature was calculated and estimated by matching the experimental phenomena with the critical shear stress. [ABSTRACT FROM AUTHOR]

• The strain mismatch between coarse grains and small grain cluster regions leads to the initiation of fatigue cracks. • The surrounding microstructure influences the crack propagation mode, thereby intervening in the crack propagation rate. • Multiple slip will trigger cross slip, causing uneven local strain within the grains, thus altering the crack propagation path. • The retarding effect of small grain cluster regions on grain growth is weaker compared to high-angle grain boundaries and pinning by small grains. The objective of this study was to investigate the cyclic deformation mechanisms at the microstructural level and their correlation with the initiation and early propagation of small fatigue cracks in super austenitic stainless steel 654SMO. The findings revealed that localized microstructural variations in grain size and orientation led to strain concentration, which subsequently triggered crack initiation. The behavior of surrounding microstructural evolution influences the crack propagation mode, further affecting the crack propagation rate. The retarding effect of small grain cluster regions on grain growth is weaker compared to high-angle grain boundaries and pinning by small grains, and multiple slip will trigger cross slip, causing uneven local strain within the grains, thus altering the crack propagation path. [ABSTRACT FROM AUTHOR]

• Very high cycle fatigue behavior of titanium alloy welded joints under asymmetric loading was investigated. • Mean stress causes the fatigue failure to transfer from the FZ to the BM. • The size of the plastic zone at the crack tip for FGA is related to the lamella α structures in FZ. • The crack non-propagation region and critical safe size of welded pores are obtained. Very high cycle fatigue behavior of electron beam welded TC17 titanium alloy joints under different stress ratios was investigated. The results revealed that the welded joints exhibited different failure modes depending on the stress ratios applied. As the stress ratio increased, the failure probability attributed to welded pores in the fusion zone (FZ) gradually decreased, shifting the failure mode towards fatigue failure induced by the primary α phase in the base metal (BM). Moreover, the stress intensity factor at the edge of fine granular area (FGA) was determined to be the threshold of long crack growth. It was observed that the size of the plastic zone at the crack tip of FGA was related to the acicular α phase structures precipitated through recrystallization in the FZ. The influence of effective size and location of welded pores on fatigue life was further discussed. To gain further insights, improvements were made to the K-T diagram. Consequently, the non-crack propagation zone and the critical safety size of welded pores responsible for initiating cracks in the welded joint were identified. Additionally, the Weibull probability function was employed to determine the average size of welded pores that led to fatigue failure in the welded joints. [ABSTRACT FROM AUTHOR]

In the current work, uniaxial tensile tests, including interrupted tests at room temperature, intermediate temperatures (650 ℃ and 700 ℃), and high temperatures (850 ℃ and 950 ℃), were conducted on the additive manufacturing 939 superalloy to investigate the mechanical properties, fracture behaviors, and deformation mechanisms. According to the experimental results, the tensile mechanical properties exhibit a significant temperature sensitivity, as the yield, tensile strength, and elongation decrease with the increase in temperature. The fracture surfaces show that the material exhibits obvious plastic fracture characteristics at room temperature. Moreover, many multi-slip system steps can be observed when the temperature rises. Furthermore, according to the result of the transmission electron microscopy, the deformation mechanism at room temperature is primarily controlled by the single slip system. As temperature rises to the intermediate temperature region, more slip systems are activated by thermal energy, which promotes the movement of dislocations. The deformation mechanism is mainly dominated by dislocation shearing γ'. When the temperature reaches the high temperature region, the deformation mechanism firstly demonstrates large-scale stacking faults, and then transforms into the dislocation by-passing and climbing mechanism. Finally, the relationship between temperature and the deformation mechanism is discussed and deduced. [Display omitted] • Tensile behavior and flow stress are sensitive to temperature and evaluated by the Hollomon model. • Dislocation shearing of γ' exhibits a complex process, with the dominant mechanism evolving with temperature. • The critical shear stress was calculated to illustrate the competition among the dominant mechanism. [ABSTRACT FROM AUTHOR]

• The fatigue crack growth tests of 9%Cr martensitic steel are conducted under R = 0.1, 0.3 and 0.5 at 630 ℃. • The FCG path and secondary crack growth behavior under different stress ratios are discussed. • The martensitic lath structures can lead to a more tortuous crack growth path. • The finite element method is introduced to calculate the evolution of the plastic zone and the secondary crack growth behavior. To understand the effect of mean stress and microstructure on the fatigue crack growth (FCG) behavior of 9%Cr martensitic steel under high temperatures, the FCG experiments are conducted on three stress ratios (R = 0.1, 0.3, and 0.5) at 630 ℃. The results indicate that the high-stress ratios have a higher fatigue crack growth rate, and the Paris region can be divided into two regions by the transition point (da / d N = 1 × 10 - 4 mm/Cycle) with different fracture morphology. For da / d N > 1 × 10 - 4 mm/Cycle, due to the influence of high crack driving force and the microstructure, the FCG path presents in a zigzag way. For da / d N ⩽ 1 × 10 - 4 mm/Cycle, the FCG path presents the characteristic of purely horizontal and no obvious crack deflection. Meanwhile, the martensitic lath fracture angles near the FCG path have an obvious statistical relationship with the stress ratio, which is mainly concentrated at 60° to 80° and 100° to 130°. Additionally, according to the EBSD map of secondary cracks, when the secondary cracks encounter the HAGBs, the larger angle of secondary crack deflection occurs. [ABSTRACT FROM AUTHOR]

The study aimed to investigate the high and very-high fatigue properties of 9Cr3W3Co turbine rotor steel at room temperature (RT) and 650 °C. The resulting S–N curves demonstrated a continuous downward trend, without exhibiting a conventional fatigue limit. The fatigue strength corresponding to 5 × 107 cycles is approximately 53.9% of yield strength at RT, increasing to around 55.4% at 650 °C. Meanwhile, compared to the fatigue strength at 5 × 107 cycles at RT, it at 650 °C decreased by 53.3%. Surface crack initiation was found to be the main fatigue fracture failure model, observed at both RT and 650 °C. Microscopic analysis revealed that severe dislocation motion and grain deformation occurred within the material matrix during the cyclic loading process at both RT and 650 °C, resulting in the formation of abundant low-angle grain boundaries, dislocation lines and sub-grains. At RT, the presence of the M 23 C 6 precipitated phase imposed a strong pinning effect, while the MX phase within the martensite laths effectively impeded the movement of dislocation lines, thereby enhancing fatigue properties. However, at 650 °C, the emergence of newly precipitated Cu-rich phases was observed, which can reduce the free path of dislocations, hinder slip motion of dislocations, and thus compensate for the negative effect caused by high-temperature microstructural degeneration. [ABSTRACT FROM AUTHOR]

Highlights • Welding asymmetrical softening effects govern tensile failure mechanism. • Static mechanical property governs the fatigue crack initiation at lower fatigue life. • Porous micro-defects govern the fatigue crack initiation at higher fatigue life. Abstract The effects of microstructural inhomogeneities and micro-defects on tensile and very high cycle fatigue behaviors of a friction stir welded joint of ZK60 magnesium alloy have been investigated. The transitional zone between thermo-mechanically affected zone and heat affected zone on advancing side is prone to tensile fracture. With increasing of fatigue life, fatigue fractures transfer from the weak zone of static mechanical properties to the welding nugget zone, where softening effects of welding heat input govern fatigue failure at lower fatigue life and the porous micro-defects govern that at higher fatigue life. [ABSTRACT FROM AUTHOR]

Post-fire-curing is an effective method to recover fire-damaged concrete. In order to investigate the effects of cement dosage and cooling regimes on the recovery of fire-damaged concrete, three kinds of concrete specimens, with different cement dosages of 412, 392, 372 kg/m 3 , were prepared and tested. After cooling either in a furnace or ambient environment to the room temperature, these specimens were first soaked in water for 24 h and then subjected to 29 days’ post-fire-curing. Based on temperature history, residual compressive strength and cracks in the cross sections were tested and analyzed. And scanning electronic microscope (SEM) was used to help the morphology analysis. A new parameter named heat accumulation factor was proposed to estimate the strength loss and the recovery potency of the fire-damaged concrete. It was known from the experiments that the difference of relative residual compressive strength of specimens with three cement dosages is less than 3 percentage points. After post-fire-curing, the relative residual compressive strength of the specimens cooled in the air recovered from 77% to 83%, while that of the specimens cooled in the furnace recovered from 65% to 87%. The longer exposure under high-temperature due to the cooling in the furnace was responsible for the lower relative residual compressive strength and higher recovered strength. This is because the high temperature can promote dehydration processes, accompanied with more rehydration products with a denser microstructure. The crack lengths were observed much longer in the specimens cooled in the ambient environment due to the higher temperature gradient during the cooling process. The higher temperature and temperature gradient near the surface can result in more intensive cracks. [ABSTRACT FROM AUTHOR]

Ultrasonic fatigue tests were carried out to investigate the effect of stress ratio on the very high cycle fatigue (VHCF) behaviors of titanium alloy (Ti-8Al-1Mo-1V). The fatigue strength at stress ratios of 0.1 and 0.5 were lower than the safely modified Goodman approximation, and a significant deviation was observed in the VHCF regime. Based on fractography and fracture mechanics, the fatigue failure process was congruously divided into four stages: (1) crack initiation induced by cleavage of primary α grains and its coalescence; (2) microstructure-sensitive slow crack propagation; (3) microstructure-insensitive fast crack propagation; and (4) final fatigue failure. Meanwhile, for surface crack initiation, fatigue crack propagation life only occupied less than 3% of total life in the VHCF regime, and over 95% of the fatigue crack propagation life was expended in the microstructure-sensitive crack propagation stage. The effect of stress ratio on crack propagation life was not distinct. [ABSTRACT FROM AUTHOR]

A series of Al x CrFeNi 2.5 Mo 1-x high-entropy alloys (HEAs) were prepared by vacuum arc melting. The effects of Al/Mo content on phase, microstructure, mechanical and corrosion properties of as-cast Al x CrFeNi 2.5 Mo 1-x alloys were investigated. With the increase of Al content, the phases of Al x CrFeNi 2.5 Mo 1-x alloys evolve from FCC + σ phase (x = 0, 0.2) to FCC + BCC + σ phase (x = 0.5, 0.8) and finally to FCC + BCC phase (x = 1.0) and the microstructure evolves from typical hypoeutectic composed of the eutectic lamellar and interdendrite structure to hypereutectic composed of dendritic and lamellar eutectic structure. Among all the alloys, the Al 1.0 CrFeNi 2.5 alloy exhibits excellent comprehensive mechanical properties with the yield strength, ultimate tensile strength, elongation and hardness of 502 MPa, 1090 MPa, 16.8% and 336 HV, respectively. Moreover, Mo element is beneficial to improving the pitting corrosion resistance of Al x CrFeNi 2.5 Mo 1-x alloys in 3.5 wt.% NaCl solution but it could promote the formation of σ phase, which leads to an uneven distribution of Cr elements and reduce the resistance to localized corrosion. • The Al x CrFeNi 2.5 Mo 1-x (x = 0, 0.2, 0.5, 0.8, 1.0) alloys were prepared by arc-melting and subsequent copper mold suction casting. • With the increase of Al/Mo molar ratio, the crystal structure transforms from FCC + σ phase to FCC + σ + BCC phase, and then to FCC + BCC phase. • Al 1.0 CrFeNi 2.5 alloy with dual-phase phase exhibited better mechanical properties. • The Al 0.5 CrFeNiMo 0.5 alloy possesses excellent comprehensive corrosion resistance in 3.5 wt% NaCl. [ABSTRACT FROM AUTHOR]

• Slip bands initiated inside martensitic laths, are oriented parallel to the lath. • Internal slip bands of the martensitic laths cause cracks to initiate. • Crack are more likely to propagation along the martensitic laths. • The complex structure of martensite has a greater resistance to small crack growth. Martensitic steel presents high fatigue crack resistance due to its complex and hierarchical microstructure, but its mechanisms are unclear. This work experimentally investigated effect of the lath and the prior austenitic grain (PAGs) on crack initiation and the early propagation behavior in high cycle fatigue. It is found that micro cracks initiated from the slip bands in the interior of thin monocrystalline laths, in which the dislocation intensity zones deflected the crack path. Subsequently, the micro cracks coalesced through the lath boundaries, and resulted in an intermittent and staggered appearance of crack path in the PAGs. Finally, the crack propagation across the boundaries of the primary PAG are extremely slow and presents significant zigzag manners due to the misorientation of the laths in neighboring PAGs. The above observations demonstrate that the crack initiation and early propagation are significantly hindered by the multi-scaled microstructure of the martensitic steel. [ABSTRACT FROM AUTHOR]

In consideration of growing human health and ecological environment issues, lead-free piezoelectric ceramics have aroused heated research to develop their potential applications in electronic fields instead of toxic lead-based ones. In the present work, a ternary lead-free material system of (0.97- x)K 0.48 Na 0.52 Nb 0.96 Sb 0.04 O 3 –0.03Bi 0.5 Na 0.5 Zr 0.8 Sn 0.2 O 3 - x BiFeO 3 (KNNS-BNZS- x BF, 0 ≤ x ≤ 0.007) ceramics was designed, and the evolution mechanisms of microstructures and electrical behaviors at multiple applied fields were revealed in detail. The bimodal microstructure with an alternative fine and coarse grain size distribution feature is generally observed, while the BF doping tends to decrease grain size and increase the diffusion phase transition behaviors. As x = 0.005, the R-O-T multiphase boundary is formed at room temperature, accompanied by the presence of striped nanodomains with smaller size and regular distribution feature that exhibit the better polarization switching and the fast domain wall movement under external stimuli. In particular, an enhanced d 33 ∼ 470 pC/N and a simultaneous higher T c ∼ 260 °C are achieved in KNNS-BNZS-0.005BF, reflecting a better balanced development of performance by the present doping strategy. Most importantly, benefiting from the coexistence of R-O-T multiphase boundary and the lower domain wall energy, an excellent d 33 ⁎ ∼ 530 pm/V is attained as well in this ceramic, which favors the better applications in high precision displacement or strain actuators. This work sheds insight into understanding the underlying physical mechanism of enhanced performance of KNN-based ceramics, substantially expanding the more applications of lead-free piezoelectric materials in electronic fields. • A new lead-free material system of KNNS-BNZS- x BF is designed. • Enhanced comprehensive performance is obtained by constructing R-O-T multiphase boundary. • The domain structures and polarization switching behaviors are revealed in detail. • The physical origin of enhanced performance is elaborated. [ABSTRACT FROM AUTHOR]

[Display omitted] • Tensile behavior and flow stress are sensitive to temperature and evaluated by the Chaboche model. • The brittle to ductile transition temperature is between 700 and 750 ℃. • The brittleness is related to the localization of deformation and stress concentrations. • Dynamic recovery and recrystallization are the primary processes for the ductile stage. To evaluate the tensile behavior, microstructural evolution, and deformation mechanisms of Ti-45Al-8Nb alloy additively manufactured by electron beam melting, uniaxial tensile experiments were performed at a constant strain rate of 2.5 × 10−4 s−1 at various temperatures. The experimental results indicated that the tensile behavior and flow stress are sensitive to temperature. The brittle to ductile transition temperature of Ti-45Al-8Nb alloy additively manufactured by electron beam melting is between 700 and 750 ℃. Below this temperature, the fracture was predominantly trans -granular, resulting in brittle failure. In addition, the brittle failure behavior is related to the localization of deformation within the grains and stress concentrations owing to plastic incompatibility at the interface of the α 2 + γ phases. However, above this temperature, the fracture transformed from the trans -granular type to the mixed mode of trans -granular and ductile dimples. Dynamic recovery and recrystallization are the primary processes leading to softening behavior. The flow behavior at elevated temperatures results from the competition between work hardening and softening. Furthermore, the Chaboche model was used to describe the tensile inelastic behavior at different temperatures. [ABSTRACT FROM AUTHOR]

Ultrafine lamellar-structured FeCrAl alloys with excellent mechanical properties were prepared via manipulating the content of Al 1.8 CrCuFeNi 2 particles by spark plasma sintering and mechanical milling. The microstructure shows a typical lamellar-structured with some fine equiaxed grains. Interestingly, the ultrafine lamellar-structured FeCrAl alloys show an ultrahigh tensile strength of 1445 Mpa and 151 Mpa at RT and 800 °C, respectively, which improves by about 247 % and 321 % than FeCrAl alloys, as well as a good elongation of 5.9 % and 15.1 %. [ABSTRACT FROM AUTHOR]

Controlled microstructures play a pivotal role in upgrading the microwave absorption properties of materials. Herein, ScFeO 3 ceramics with a controllable three-dimensional (3D) network structure were prepared by adjusting the sintering temperature. Specifically, the effect of sintering temperature on microstructure and microwave absorption properties were investigated. The results show that ScFeO 3 ceramics with the sintering temperature of 1300 ºC possess the superior dielectric and magnetic loss capabilities for incident microwaves, which attributes to the ideal 3D network microstructure enhancing the impedance matching characteristic. Besides, this unique 3D network microstructure not only provides multiple channels for multiple reflecting and scattering of microwaves, but also offers abundant interfaces to dissipate microwaves. Furthermore, the natural resonance effect efficiently increases the magnetic loss. The optimal ScFeO 3 samples possess the minimum reflection coefficient (RC min) of − 57.4 dB at 17.44 GHz with a thickness of 2.45 mm and an effective absorption bandwidth (EAB) covering the whole Ku band (5.6 GHz) with a thickness range of 1.35–1.55 mm. It is believed that ScFeO 3 ceramics can be new promising microwave absorption material with thin, broadband, and strong absorption performance. • The ScFeO 3 ceramics with three-dimensional (3D) network microstructures are prepared by sol-gel and calcination technology. • The 3D network microstructures exhibit good synergism of impedance matching characteristics and multiple dissipation mechanisms. • The ScFeO 3 ceramics display excellent microwave absorption performances. [ABSTRACT FROM AUTHOR]

• Crack initiation from the microstructural inhomogeneity was investigated in very high cycle fatigue. • Mode II cracking along the martensite lath dominant the crack initiation behavior. • Fatigue cracking prefers to occur at the boundaries of favorably oriented martensite lath due to slip-cracking process. Super martensitic stainless steel is known for its outstanding mechanical properties, which lead to its widely application in structural component bearing cyclic loadings. This study investigates the effect of local microstructure on the crack initiation behavior of a super martensitic stainless steel in the very high cycle fatigue (VHCF) regime. Results show that the non-inclusion-induced crack initiation is the dominating pattern of VHCF failure. Fatigue cracks initiate from the boundaries of the martensite lath due to the mode II cracking process, resulting in the formation of a slip area located at the core of the fine granular area (FGA). The size and the orientation of the martensite lath determine the morphology of the slip area but exert limited influence on the formation of the FGA at the fracture surface. These results may effectively improve the service safety and reliability of super martensitic stainless steel. [ABSTRACT FROM AUTHOR]

Sintering temperatures (1050–1150 °C) play an important role on influencing microstructures and electrical properties of Bi 4 Ti 2.95 W 0.05 O 12.05 + 0.2 wt%Cr 2 O 3 ceramics. Firstly, the lattice constants and relative density of samples varied with sintering temperatures. And the increase in sintering temperatures increased their grain size and decreased the porosity, resulting in the extreme grain growth of the sample sintered at 1150 °C. Based on these structural evolutions, ϵ r , TK ϵ and tan δ of samples deceased with the increase in sintering temperatures, while T c , d 33 and k p increased with it. ρ dc of samples also increased with the increase in sintering temperatures below 1125 °C, but decreased sharply at 1150 °C. The variation in 2 E c of samples with sintering temperatures may be related to the 90° domain walls. The sample sintered at 1150 °C presented a lower ρ dc due to the high aspect ratio, but possessed a higher d 33 due to the large grain size. [ABSTRACT FROM AUTHOR]

Magnetic-dielectric composites with multiple dissipation mediums are extensively applied in microwave absorption fields. However, it still remains bottleneck problem to combine magnetic and dielectric components in a rational way to achieve significant microwave absorption performance. In this study, rGO/ScFeO 3 composites were synthesized by freeze-drying and heat treatment methods. The ScFeO 3 particles decorated the rGO sheet notably improves microwave absorption performance, owing to the suitable impedance matching and abundant dissipation capabilities induced by the effective complementarities between ScFeO 3 particles and carbon sheet. These results show that the electromagnetic parameters and microwave absorption properties of rGO/ScFeO 3 composites are controlled by ScFeO 3 content. In particular, the prepared rGO/ScFeO 3 composites (content of 2%wt) exhibit the minimum reflection loss (RL min) of −19 dB. The effective absorption bandwidth (EAB) is 4.2 GHz at thickness ranging from 3.05 to 3.55 mm. The microwave dissipation abilities probably benefit from multiple reflections and scattering, interface and dipole polarization and natural resonance effect. Therefore, the composites prepared by embedding ScFeO 3 particles into rGO can improve the microwave absorption performance. More importantly, the microwave absorption mechanism of the composites is explored and analyzed, which provides a reliable reference for the construction of magnetic dielectric composites. • The rGO/ScFeO 3 composites were synthesized by freeze-drying and heat treatment methods. • The rGO/ScFeO 3 composites possessed the magnetic-dielectric property withRL min of −19 dB. • The microwave dissipation abilities benefited from dielectric and magnetic losses. [ABSTRACT FROM AUTHOR]

Material micromorphological design can effectively enhance electromagnetic properties, thus improving microwave absorption performance. In this paper, Re -SiC ceramics (Re: Sc, Y, Ce) were prepared using PDCs methods. The addition of various rare earth (Sc, Y, and Ce) produced improved microstructural, dielectric, and microwave absorption properties. The results indicate that Sc-SiC ceramic with an appropriately three-dimensional (3D) network structure improves the impedance matching characteristic and dielectric loss, therefore obtaining superior microwave absorption performance. The Sc-SiC ceramic effective absorption bandwidth (EAB) is 3.2 GHz with a thickness of 2.9 mm, and the minimum reflection coefficient (RC min) is −10.8 dB at the X band. This remarkable microwave absorption property is ascribed to the 3D structure, interface polarization, dipole polarization, conductance loss, multiple reflections, and microwave scattering. Consequently, PDCs-SiC ceramics with appropriate structural decoration can be prospective candidates for application in harsh environments. • Re -SiC ceramics (Re: Sc, Y, Ce) were prepared using PDCs approach. • Sc-SiC ceramic displayed the excellent microwave absorption performance. • Sc-SiC ceramic improved the polarization loss and impedance matching properties. [ABSTRACT FROM AUTHOR]

• A grain size dependent and thermo-mechanically coupled phase field model is newly proposed. • The effects of grain size, texture, gradient-nanograin and bimodal grain structures as well as geometric gradient on the elastocaloric effect of NiTi SMAs are revealed. • The COP mat of the optimal microstructure engineering scheme is up to 35.4, which is about 200% higher than that of uniform coarse-grained NiTi SMAs. • The microstructure engineering scheme with geometric gradient shows the lowest input stress of 195 MPa. By newly introducing a grain boundary energy, a grain size (GS) dependent and thermo-mechanically coupled phase field model was proposed to investigate the elastocaloric effect (eCE) of NiTi shape memory alloys (SMAs), and discuss the effects of GS, texture, gradient-nanograin and bimodal grain structures as well as geometric gradient on the eCE through simulations. The simulated results show that with decreasing the GS, the gradual reduction of the degree of martensite transformation (MT) and the variation of transformation mode in the nano-polycrystalline NiTi SMA system lead to the decreasing absorption of transformation latent heat, and the gradual weakness and even disappearance of local instability during MT decreases the hysteresis, resulting in the increase of the coefficient of performance (COP mat); the deformation dependence of COP mat and the grain orientation dependence of transformation instability lead to a dependence of COP mat on texture, which can be enhanced with decreasing the GS. It is further found that the microstructure engineering (from the perspectives of GS, texture, gradient-nanograin and bimodal grain structures) can significantly improve the COP mat , and the introduction of geometric gradient can effectively reduce the MT start stress. Several microstructure and geometric schemes with considerable adiabatic temperature change (Δ T ad) and high COP mat as well as relatively low MT start stress are reported, which can provide an important theoretical guidance for improving the eCE of NiTi SMAs and exploiting excellent SMA-based elastocaloric materials. The proposed phase field model and the characterization method for texture can be used as a reference to simulate the eCE and anisotropic MT of other SMAs. I. Microstructure engineering schemes: polycrystalline systems with different gradient-nanograin structures and bimodal grain structure; II. Simulated microstructure evolution; III. Improved elastocaloric effect [Display omitted] [ABSTRACT FROM AUTHOR]

The non-uniform temperature field and cooling rates in the molten pool during solidification process fabricate microstructural and mechanical heterogeneities in fusion zone (FZ) and heat-affected zone (HAZ) of thick titanium alloy weldment. In this study, electron beam welding was used to assemble thick Ti-5Al-2Sn-2Zr-4Mo-4Cr (TC17) alloy plates. Microstructural heterogeneity concerning recrystallized martensitic structures with different morphology and distribution features in FZ and retained microstructures after experiencing thermal exposure in HAZ were analyzed to unravel their effects on multiscale mechanical behavior. It was found that needle-shaped martensite (α') exhibited a comparative high nano hardness and the precipitation of needle-shaped martensite cluster contributed to a more severe tensile strain localization in FZ and premature brittle fracture of the joint. The bottom layer of weldment, where the FZ was verified to be martensitic free, exhibited a better combination of strength and ductility. Mo and Al exhibited a better thermostability than Cr, which held the responsibility for the microstructural stability in HAZ during severe thermal exposure after welding. • Effects of heterogeneous microstructure on nanomechanical and tensile behavior were studied. • The formation of needle-shaped martensite was verified to damage the ductility of the weld joint. • A comparatively lower diffusion rate of Al than Cr contributes to a better thermostability of α P than α L. [ABSTRACT FROM AUTHOR]

The mechanical behaviour and microstructure evolution of twinning-induced plasticity (TWIP) steel subjected to tensile tests at high strain rate and low temperature were investigated. Under extreme conditions (3 × 103 s−1 and liquid nitrogen temperature), the TWIP steel exhibits high strain hardening capability and high strength, which are intimately related to deformation twinning. Here we investigate the plasticity and failure processes of a TWIP steel under extreme conditions, such as shock and/or cryogenic loading, and identify that deformation twinning no longer serves the fine plasticity. Obvious ductility loss has been found in TWIP steel even with strong strain hardening capability, which always leads to large uniform tensile ductility against local necking. In this study, the microstructure observations suggest that both of the second twin systems and nanotwin structures are highly active under extreme conditions. Twin-twin and dislocation-twin interactions accelerate the crack nucleation at and propagation along the twin boundaries. The conversion from twinning-induced plasticity to twinning-assisted cracking is controlled by twin structure and its spatio-temporal evolution, which are most susceptible to strain rate and temperature. These results provide an underlying micromechanism that controls the plasticity and failure in TWIP materials under extreme conditions and contributes to the understanding of twin structure-related processes to a broad community. [ABSTRACT FROM AUTHOR]

Friction stir butt welding was carried out to assemble 6 mm-thick TC17 titanium alloy plates with a W–Re weld tool. Inhomogeneous microstructures were obtained in stir zone (SZ) and heat affected zone (HAZ) of the weldment. A series of tests were conducted to reveal the influence of inhomogeneity of microstructure on mechanical behavior. Comparative refined β phase without precipitating secondary metastable microstructure generated a soft SZ and favored on the ductility of weldment. Strain localization during tension was promoted by the interface of HAZ and SZ that formed by thermal-mechanical coupling effects. Coincidentally, fatal fatigue cracks were verified to nucleate around the same region, and presented in a stress dependent mode. • Comparative refined β grains without lamellar α generate a soft SZ. • W-contaminate traces exhibit higher nano hardness than intermediate banded regions. • Strain localization promoted by the HAZ/SZ interface causes tensile fracture. • Fatal fatigue cracks nucleate around HAZ/SZ interfaces. [ABSTRACT FROM AUTHOR]