Search

Laser cladding is considered to be a highly complex process to set up and control because it involves several parameters, such as laser power, laser scanning speed, powder flow rate, powder size, etc. It has been widely studied for metal-part coating and repair due to its advantage in controllable deposited materials on a small target substrate with low heat-affected distortion. In this experiment, laser cladding of U75V and U20Mn rail steels with Inconel 625 powder was captured by an infrared camera with image analysis software to monitor the laser cladding process in order to determine the quality of the cladded substrates. The cladding temperature, thermal gradient, spot profile, and cooling rate were determined from infrared imaging of the molten pool. The results showed that cladding temperature and molten pool's spot closely related to the laser cladding process condition. Infrared imaging provided the cooling rate from a temperature gradient which was used to correctly predict the microhardness and microstructure of the HAZ region. This approach was able to effectively detect disturbance and identify geometry and microstructure of the cladded substrate. [ABSTRACT FROM AUTHOR]

Under appropriate conditions, surface pearlite can form on the outer surfaces of specimen. The microstructure evolution during the decarburization of surface pearlite was investigated. Two cementite dissolution mechanisms were observed. The first involved the direct dissolution of cementite into the pearlitic ferrite. The second mechanism involved the formation of austenite along the cementite/ferrite interface. Both mechanisms were active near the surface of specimens, and the second mechanism was observed for cementite plates adjacent to the austenite core. [ABSTRACT FROM AUTHOR]

C-axis textured α-SiC ceramics were prepared by gel-casting in a strong magnetic field of 6 T and subsequent sintering at 1950 °C for 2 h. The effects of sintering aids on the microstructure and properties of textured SiC ceramics were discussed. Results showed that highly textured α-SiC green bodies were obtained by gel-casting in a strong magnetic field of 6 T. The sintering process slightly suppressed the texture development of α-SiC ceramic. The degree of texture in the α-SiC green bodies and sintered bodies both showed a decreasing trend with the increase of sintering aids content. And the increase of the sintering aids content promoted the densification and improved the bending strength of α-SiC ceramics. Besides, the bending strength of textured SiC ceramics showed an anisotropy based on the grain orientation. [ABSTRACT FROM AUTHOR]

Al2O3‐based ceramic cores with a uniform microstructure were fabricated successfully by a traditional pressing forming method, in which Al2O3 powders were used as matrix and yttrium oxide as additive. The influences of yttrium oxide content and sintering temperature on properties of ceramic cores were studied carefully. Results indicated that a higher sintering temperature benefited the preparation of ceramic cores with excellent properties. As the temperature was above 1400°C, the reaction of Al2O3 and yttrium oxide occurred, leading to the formation of YAG phases. And, YAG was uniformly adhered on the surface of Al2O3 particles, exerting a good role in connecting Al2O3 particles. Based on XRD analyses, it was found that the increase in the sintering temperature could promote the formation of more YAG phases. When sintering temperature was adjusted to 1600°C, with the increase in the yttrium oxide content, their relative density developed a trend of decreasing first and then increasing, while the apparent porosity had an opposite change tendency. With the increase in the sintering temperature, the line shrinkage and bending strength of Al2O3‐based ceramic cores both increased gradually. In our research, their bending strength reached to 53.5 MPa and apparent porosity was 33.9% when the ceramic cores were prepared with 9 wt% yttrium oxide at 1600°C. [ABSTRACT FROM AUTHOR]

Isothermal rolling is conducted at 1050 °C (α2 + B2 phase region) and 1100 °C (B2 phase region) for a spark plasma sintering (SPS)‐processed Ti‐22Al‐25Nb alloy. The effects of rolling temperature, pass, reduction per pass, and solution and aging on microstructure evolution, phase transformation, and tensile property at ambient temperature are systematically investigated. The results show that as the alloy is rolled at 1050 °C for 1 pass (P), the optimal tensile strength (TS) and the yield strength (YS) are 964.78 and 935.16 MPa, respectively, and the elongation (δ) reaches 23.65%. Then, followed by a subsequent solution and aging at 940 °C/30 min/water quenching (WQ) + 760 °C/24 h/air cooling (AC), it exhibits a decrease in mechanical properties. Moreover, a mixed fracture mechanism dominated by ductile fracture is shown for the as‐rolled alloy. However, after a solution and aging, it is transformed to a mixed fracture mechanism dominated by brittle fracture, which attributes to the void aggregation and the development of the discontinuous brittle α2 precipitates along B2 matrix grains during the solution and aging treatment. Finally, fracture models of the as‐rolled alloy and the solution and aging‐treated alloy are established. [ABSTRACT FROM AUTHOR]

The powder metallurgy (P/M) Ti-22Al-25Nb alloy obtained by spark plasma sintering (SPS) of Ti-22Al-25Nb prealloyed powders at 950 °C/80 MPa/10 min (α2 + B2 + O three-phase region) was adopted as the starting material to systematically investigate the evolution of microstructure, phase transition, B2 grain size uniformity and microhardness with solution temperature and holding time. The solution treatment was conducted at temperatures from 940 to 1100 °C (α2 + B2 + O, α2 + B2 and B2 phase regions) and holding times range of 10-120 min (10, 30, 60, and 120 min). After solution treatment, the microhardness of the alloy was measured. The results showed that the microstructure and microhardness of the alloy were closely related to the solution temperature and holding time. The B2 phase grain size and its uniformity increased with the increase in the solution temperature and holding time, and the microhardness decreased with the growth of the grains. The P/M Ti-22Al-25Nb alloy tested at 940 °C/10 min had the best performance, exhibiting three phases of α2 + B2 + O and a grain size of 43.57 μm. The microhardness was 359.52 ± 3.73 HV. In addition, the grain growth activation energy and grain growth index of different solution temperatures and holding times were obtained by linear fitting, and the relationship between the B2 grain size and the solution temperature and holding time was further established. [ABSTRACT FROM AUTHOR]

Three kinds of high-entropy metal diboride composites were successfully prepared by pressureless sintering. The as-synthesized (Hf 0.2 V 0.2 Ta 0.2 Ti 0.2 Nb 0.2)B 2 (HEB-Hf), (Zr 0.2 V 0.2 Ta 0.2 Ti 0.2 Nb 0.2)B 2 (HEB-Zr) and (Cr 0.2 V 0.2 Ta 0.2 Ti 0.2 Nb 0.2)B 2 (HEB-Cr) powders were used as the matrix, and Ni metal was used as the binder phase. The results demonstrated that three kinds of single-phase high-entropy metal diboride powders (HEB-Hf, HEB-Zr, and HEB-Cr) were successfully synthesized by a facile high-energy ball milling-assisted boro/carbothermal reduction method, at 1600 °C for 2 h. Each HEB powder had a hexagonal structure, and the five main elements were evenly distributed. Further, Ni as the binders effectively improved the density and fracture toughness of the sintered HEB composites. In the case of maintaining high hardness, the fracture toughness of the HEB composites was greatly improved. Meanwhile, The composites exhibited excellent properties when the heat-treating temperature was increased to 1600 °C. The hardness of the HEB-Hf, HEB-Zr, and HEB-Cr composites were 24.2 ± 0.6, 23.8 ± 1.2 and 21.8 ± 0.8 GPa, and their fracture toughness were 9.2 ± 0.3, 7.1 ± 0.7 and 7.3 ± 0.5 MPa∙m1/2, respectively. This manuscript reports a novel method for future commercial production of high-entropy diboride. Further, there will be more applications for high-entropy diborides because of their increased toughness when prepared by this method. [Display omitted] • Three single-phase high entropy diboride powders were successfully prepared by boro/carbothermic reduction reaction. • High-dense high-entropy boride composites were successfully prepared using Ni metal as the additive by pressureless sintering at 1600 °C. • The Vickers hardness of HEB-Hf composites, HEB-Zr composites and HEB-Cr composites are 24.2 ± 0.6, 23.8 ± 1.2 and 21.8 ± 0.8 GPa, respectively and the fracture toughness values are 9.2 ± 0.3, 6.1 ± 0.7 and 7.3 ± 0.5 MPa∙m1/2, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Preferred orientation of porous Si3N4 ceramics with β‐Si3N4 powders as raw material is obtained by gel‐casting in a longitudinal rotating magnetic field and subsequent pressureless sintering. The effects of rotational speed on crystal orientation and properties of porous Si3N4 ceramics are analyzed. The results show that the c‐axis of β‐Si3N4 grains tended to align along the direction of the rotation axis. The degree of orientation in the samples increases with the increasing rotational speed. The textured porous samples show higher bending strength than that of untextured porous samples owing to the integrative effect of the grain orientation and pores. [ABSTRACT FROM AUTHOR]

Highly c-axis textured TiB 2 ceramics were prepared by slip-casting in a strong magnetic field of 6 T and subsequent pressureless sintering. By analyzing the zeta potential and viscosity, the optimal slurries with good dispersibility and stability were prepared when the pH was about 10.5. In 6 T, c-axis of TiB 2 grain was oriented to parallel to the direction of magnetic field and its a or b-axis was perpendicular to the direction of magnetic field in TiB 2 ceramics. The degree of texture was 0.80 on the top surface perpendicular to the direction of magnetic field in the green body. After sintering without the magnetic field, the degree of texture reached 0.94. Sintering process promoted the improvement of degree of texture in TiB 2 ceramics by grain growth. [ABSTRACT FROM AUTHOR]

Optical Coherence Tomography (OCT) is a valuable technology that has been used to obtain microstructure images of tissue, and has several advantages, though its applications are limited in high-scattering tissues. Therefore, semiconducting polymer nanoparticles (SPNs) that possess strong absorption characteristics are applied to decrease light scattering in tissues and used as exogenous contrast agents for enhancing the contrast of OCT imaging detection. In this paper, we prepared two kinds of SPNs, termed PIDT-TBZ SPNs and PBDT-TBZ SPNs, as the contrast agents for OCT detection to enhance the signal. Firstly, we proved that they were good contrast agents for OCT imaging in agar–TiO2. After that, the contrast effects of these two SPNs were quantitatively analyzed, and then cerebral blood vessels were monitored by a home-made SD-OCT system. Finally, we created OCT images in vitro and in vivo with these two probes and performed quantitative analysis using the images. The results indicated that these SPNs created a clear contrast enhancement of small vessels in the OCT imaging process, which provides a basis for the application of SPNs as contrast agents for bioimaging studies. [ABSTRACT FROM AUTHOR]

In this study, a corrugated constrained groove pressing (CCGP) technology based on a semicircle groove was proposed, improving the uniformity of strain accumulation in deformed metal sheets through staggered groove pressing. First, 6061-O aluminum alloy was processed at room temperature via conventional CCGP (Conv-CCGP) and 90° cross CCGP (Cross-CCGP). The effects of strain route and deformation pass on the microstructure evolution of the CCGPed alloy were investigated and correlated with mechanical properties. Changes in the mechanical properties of the CCGPed alloy at room temperature measured via tensile in different directions and hardness tests were studied. Results showed that the strength and hardness of the CCGPed sheet in both routes were considerably improved, but ductility was decreased. Cross-CCGP can effectively alleviate the anisotropy of the deformed sheet and achieve optimal strength in roll direction (RD) and transverse direction (TD) at two passes. Yield strength (YS) and ultimate tensile strength (UTS) along RD were increased by 190.2% and 15.5% relative to the initial alloy, reaching 119 MPa and 134 MPa, respectively. Along TD, YS and UTS were increased by 255.0% and 36.0%, reaching 142 MPa and 151 MPa, respectively. Characterization techniques, including electron backscatter diffraction and high-resolution transmission electron microscopy, were used to establish comprehensive models of dynamic recovery-based substructure evolution and dynamic recrystallization-dominated grain refinement. Meanwhile, the integrated microstructural changes and theoretical calculations identified dislocation strengthening and local grain refinement as the major contributors to the strengthening of the CCGPed alloy. In addition, tensile fracture surface analysis showed that the CCGPed alloy still underwent plastic fracture, but the fracture mode shifted from tension fracture into shear fracture. • A CCGP technology to uniform strain by staggered groove pressing was proposed. • CCGPed sheets show significant improvement in microhardness, YS and UTS. • Combined models of substructure evolution and grain refinement were developed. • Dislocations and local grain refinement on enhancement of strength were discussed. • The tensile fracture mechanisms of the CCGPed alloy were analyzed. [ABSTRACT FROM AUTHOR]

• Continuous observations of microstructure are achieved by quasi in-situ method. • The moment when the transformations of retained austenite take place is disclosed. • The preferential sites where the phase transformations occur are revealed. • The evolution of the damage until the formation of cracks is investigated. • A comprehensive microstructure evolution model is proposed. Microstructure evolution during fatigue is crucial to the initiation and propagation of fatigue cracks in carbide-free bainitic steels. Utilizing quasi in-situ electron back-scattered diffraction method, the morphology of retained austenite and the accumulated plastic deformation within the bainitic laths at different fatigue stages were characterized. The observations disclosed that the retained austenite in various locations has different mechanical stability, and the dislocations in the bainitic laths will evolve to low-energy substructures continually throughout the fatigue process. Furthermore, combined with hysteresis curves analysis, a microstructural evolution model that comprehensively considered the retained austenite stability and the evolution of dislocations was proposed. [ABSTRACT FROM AUTHOR]

The textured porous Al2O3 ceramics were prepared by slip casting in a strong magnetic field of 6 T and subsequently sintering. The c axis of Al2O3 grain was oriented parallel to the direction of the magnetic field and the textured porous microstructure with plate-shape grains was formed. The porosity of textured porous Al2O3 ceramic was 30.37% and the relative density reached 66.29% when the sintering temperature is 1600°C. The textured porous Al2O3 green body showed the linear shrinkage anisotropy. The bending strength of the textured porous Al2O3 ceramics depended on the alignment direction of plate-shape grains. [ABSTRACT FROM AUTHOR]

ABSTRACT Series of polyimide (PI)/mesoporous silica nanospheres (MSNs) nanocomposite films with different contents of MSNs were successfully prepared via a simple wet impregnation method. The morphologies, microstructures, mechanical properties, transmittance, and thermal properties of the prepared PI and the PI/MSNs nanocomposite films were investigated in detail. As a result, the thermal stability and mechanical performances of PI were obviously improved by incorporating MSNs into PI. The tensile stress and Young's modulus of the nanocomposite film with 5 wt % MSNs were raised up to 97.65 MPa and 2220.06 MPa, which are greatly higher than the values of 82.51 MPa and 1440.86 MPa for the pure PI film. Experimental results confirmed that the designed polymerization tactic, which occurred in the pores of the MSNs, facilitated to enhance the mechanical and physical performances of the PI/MSNs nanocomposite films, and definitely induced better integration between organic matrix and inorganic nanofillers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41173. [ABSTRACT FROM AUTHOR]

In this paper, SiO 2 -based ceramic cores were prepared successfully by the preceramic polymer technique. Effects of sintering temperatures and silicone resin contents on the microstructure and properties of SiO 2 -based ceramic cores were investigated in N 2 atmosphere. Results showed that in N 2 atmosphere, the silicone resin transformed into the amorphous SiOC and C-nanowires. SiOC phases mainly adhered on the surface of ball-shaped SiO 2 grains and C-nanowires were mainly located in the gaps between ball-shaped SiO 2 grains. Meanwhile, no crystallized phases formed when the sample was sintered at 1300 °C. The sintering temperatures had an important effect on the formation of C-nanowires. When the temperature reached 1300 °C, a lot of C-nanowires were formed. The method provided a pathway to prepare the SiO 2 -based ceramic cores with high properties by in-situ formed C-nanowires. • C-nanowires were in situ formed between SiO 2 particles. • The existence of C-nanowires increased the bending strength of porous ceramic cores. • This process offered a pathway to prepare the ceramic cores with in situ grown nanowires. [ABSTRACT FROM AUTHOR]

Alumina-based ceramic cores pre-sintered at 1500 °C for 2 h were infiltrated by liquid silicone resin in a vacuum. Results showed that infiltrated silicone resin was well filled in the gaps among alumina particles by forming big polymer with network structure after curing. After sintering in N 2 atmosphere, nanowires were formed in the gaps among alumina particles, which was derived from the pyrolysis of silicone resin. SEM images showed that the formation of nanowires belonged to the mechanism of Vapor-Liquid-Solid owing to the existence of evident "bumps" formed by liquid phase during cooling. TEM and EDS analysis confirmed that C-nanowires were formed by the pyrolysis of liquid silicone resin in N 2 atmosphere. As a result, the liquid silicone resin infiltration dramatically improved the bending strength of alumina-based ceramic cores. This method using pure liquid silicone resin as reinforcement agent was an effective path for second improvement of the properties of ceramic cores. • Liquid silicone resin infiltration dramatically improved the bending strengt. • C-nanowires were formed by the pyrolysis of liquid silicone resin in N 2 atmosphere. • The infiltration method with liquid silicone resin dramatically improved the bending strength of alumina-based ceramic cores. [ABSTRACT FROM AUTHOR]

The microstructure of AerMet100 steel after aging was observed and analyzed. Based on the microstructure information of AerMet100 with different aging temperature, equivalent radius of M 2 C carbides was calculated and used in modified Wang's model to simulate the relation between yield strength and aging temperature. The simulation results showed that AerMet100 was mainly strengthened by M 2 C precipitate and solid solution. Solid solution strengthening decreased with the increasing of aging temperature because the growth of M 2 C and austenite gradually consumed Cr, Mo, Ni and C in the matrix. M 2 C strengthening was controlled by the opposite effect of growth rate and nucleation rate of M 2 C on strength. By adding the effect of austenite into the model and making simplification, the simulation results of the modified Wang's model were closer to the experiment results than the original Wang's model. [ABSTRACT FROM AUTHOR]

Si 3 N 4 ceramic has been joined successfully to nickel-based superalloy by partial transient liquid phase (PTLP) bonding using Ni/Cu/Ti multi-interlayers. The interfacial microstructure and strength of the joints were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and a three-point bending test. According to the TEM analysis, it was found that a TiN reaction layer was formed at the Si 3 N 4 /filler alloy interface. The TiN reaction layer was composed of two zones: one next to the Si 3 N 4 ceramic with a thickness of about 0.4 μm and the other zone with grains of about 0.8 μm. The microstructure of the joint between the Si 3 N 4 and the Cu interlayer can be described as: TiN layer with fine grains/TiN layer with coarse grains/Ti 2 Ni layer. Ni 3 Ti and CuTi 2 phases were produced at the interface between the Cu interlayer and the DZ483 superalloy. The room-temperature average strength of the joint was 147 MPa, and 96 MPa was achieved when the joints were tested at 1073 K. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of ultrahigh strength steels (300M, Aermet100 and M54) were analyzed. The experiment results showed that the steels with austenite layers at the boundary of martensite laths had higher K IC than those without austenite layers. Finite element method (FEM) was used to establish models based on the actual microstructure of the steels. Extended finite element method (XFEM) was used to study the crack path and crack growth rate. Contour integral method (CIM) was used to study the best thickness of austenite layer, the effect of crack propagation direction and the value of K IC . According to the simulation results, 10–15 nm was the best thickness of austenite layer for the fracture toughness of the steels with martensite as matrix. The simulation results of normalized value of K IC were close to the experimental results. [ABSTRACT FROM AUTHOR]

Sag refers to the reduction in the free height of coil springs during actual usage, hence sag resistance is one of the crucial properties of spring steel and its improvement is essential for prolonging the service life of springs. However, limited research has been conducted on the effect of microstructure on sag resistance in recent years. This study aims to investigate the relationship between microstructure and sag resistance in 65SiCrV6 spring steel. The steel was quenched from various temperatures to obtain microstructures with varying sub-block size, and tempered at different temperatures to obtain microstructures with distinct tempered carbides morphology. The microstructures were characterized and the sag resistance was evaluated by hysteresis loop effect formed during cyclic tensile test. The results revealed that smaller sub-block sizes of martensite and higher proportions of high-angle grain boundary (HAGB) are beneficial for improving sag resistance. Incoherent granular cementite precipitated in the tempered microstructure improves sag resistance by enhancing interactions with dislocations, and coarsened cementite does not reduce the sag resistance due to the generation of back stress in heterogeneous microstructure. The hysteresis loop effect resulting from heterostructure as well as interactions between dislocations and tempered carbides increases with rising tempering temperature until it reaches a saturation state. This study highlights the importance of optimizing the microstructure with both small sub-block size and incoherent cementite precipitation through appropriate heat treatment process to enhance sag resistance, meanwhile provides valuable insights into understanding the micromechanism of sag resistance in spring steel. • The relationship between microstructure and sag resistance of spring steel was investigated in this work. • The smaller sub-block size and higher proportion of high-angle grain boundaries are conducive to sag resistance. • The incoherent granular cementite precipitated in the microstructure can enhance the sag resistance. [ABSTRACT FROM AUTHOR]

Austenite reversion treatment (ART) is usually used to obtain retained austenite (RA) in medium Mn steel (MMS). In this contribution, we proposed a flash-annealing strategy to produce MMS, which allows us to efficiently obtain a large amount of RA in MMS via the "explosive" nucleation of austenite and the fast C partitioning. Furthermore, flash-annealing can also render a heterogeneous ferritic matrix consisting of recrystallized/non-recrystallized ferrite grains, leading to heterogeneous deformation-induced hardening. As compared to the conventional ART processed MMS, the unique heterogeneous microstructure in the flash-annealed MMS leads to a higher yield and ultimate tensile strength but a marginal ductility loss. Flash-annealing provides an alternative route to tailor microstructure and optimize the mechanical performance of steels. [Display omitted] [ABSTRACT FROM AUTHOR]

• Local quenching causes gradient distribution of microstructure and hardness in wheel. • The statistical correlation is: Hardness = −2.77 × Fraction of proeutectoid ferrite + 328.07. • Wear and rolling contact causes microstructure evolution of wheel. • Microstructure and hardness show a significant synergistic evolution law. High-speed train wheels have complex service environments during wheel–rail contact, and a comprehensive understanding of the microstructural inhomogeneity at the rim is critical for service safety assessment. The local quenching process in the vicinity of the rim will cause a difference in microstructure, directly affecting the service performance of the material. In this study, high-throughput characterization method based on material genome engineering was used, the methods for characterising and identifying the microstructure of wheel steel were developed using high-throughput scanning electron microscopy, and the accuracy and reproducibility of the methods were evaluated. A point-to-point research scheme for microstructure image and hardness data was designed based on the concept of statistical mapping characterization. The area fraction of the proeutectoid ferrite and micro-Vickers hardness distribution in a large size range of the key area of the rim and their internal correlation was obtained, these helped avoid statistical and representativeness limitations of traditional local selection analysis of microstructure and hardness. The results indicates that the area fraction of proeutectoid ferrite and micro-Vickers hardness showed a clear gradient distribution, and opposite distribution trend, with a high negative correlation. In addition, by observing the correlation between microstructure and hardness, and studying the evolution of microstructure, it was found that the microstructure and hardness of the rim near the tread demonstrated a significant synergistic evolution law under the influence of service, this was observed both statistically and microscopically. These findings enhanced the application prospects of the microstructure-performance statistical mapping method, and were beneficial in efficiently evaluating the service life and safety of key components. [ABSTRACT FROM AUTHOR]

Retained austenite plays a significant role in the strength-ductility balance of medium Mn steels. In this contribution, a new processing route (Flash-Austenite Reversion Treatment, Flash-ART) was proposed to design retained austenite with a compositional core-shell structure in an Fe-0.20C-7.76Mn-1.99Al (wt%) steel. It was interestingly found that the austenite core, which should transform into martensite based on its composition and size, was stabilized by the Mn enriched shell. Flash-ART allows us to obtain more retained austenite compared with the conventional ART. Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract Modeling of crack deflection/penetration or manifested as intergranular/transgranular fracture in polycrystals has long been a challenge in both fracture mechanics and materials science. A phase-field model formulated with two-set order parameters describing the crack field and the microstructure field respectively, is herein established to investigate the competition between crack penetration and deflection at an interface. The advantages of the proposed model are three-fold: (1) the fracture toughness of grain boundaries and/or interfaces, besides single crystal properties (e.g., Young's modulus, Poisson ratio, and fracture toughness), can be automatically incorporated, (2) cohesive interfaces are treated in one phase-field framework in comparison with the integrated phase-field and cohesive zone model in which cohesive elements have to be manually placed at the interface, and (3) two-set order parameters enable to describe the concurrent crack propagation and microstructure evolution. The proposed model is validated by comparing the simulated critical deflection angles with the results from theoretical analysis of the linear elastic fracture mechanics and the experiments. The model is then applied to a heterogeneous Silicon Nitride polycrystal to determine the effect of interface fracture toughness on the intergranular/transgranular fracture transition and the effective toughness of the materials. Highlights • Competition between crack penetration and deflection at an interface is studied. • Interface and bulk fracture toughness are incorporated through a unified form. • Effect of grain boundary energy and interface fracture toughness on crack deflection. • Crack morphology and fracture toughness in polycrystals are simulated. [ABSTRACT FROM AUTHOR]

Cementite dissolution behavior of pearlitic steels subjected to rolling-sliding contact deformation is comprehensively investigated by combining experimental characterization and phase-field modeling. An elasto-plastic phase-field model, incorporating the elastic strain-induced free energy contribution from first-principles calculations and the plastic counterpart from a rolling-sliding contact finite element model assisted with a plastic strain accumulation model, is originally proposed to simulate the real-time evolution of cementite volume fraction, cementite morphology and carbon distribution for different rolling cycles and contact depths. Upon experimental validations, the proposed model predicts more accurate and realistic results than Sauvage’s model. A three-stage behavior of cementite dissolution is also revealed, which well explains an experimentally observed significant cementite dissolution gradient along the depth direction. Besides, the effect of ferrite/cementite interface thickness and the initial lamellae thickness of cementite on cementite dissolution kinetics is studied. The proposed phase-field model can not only help understand the mechanism of cementite dissolution, but also give new sights into quantitative predictions of the mechanical properties and even the rolling contact fatigue life of pearlitic rail steels in service. [ABSTRACT FROM AUTHOR]

Tailoring the fraction and stability of retained austenite in the medium manganese steel has always been an issue of great industrial interest. A novel cyclic austenite reversion treatment (ART) is proposed to obtain considerable amount of retained austenite in a Fe-0.21C-4.53Mn (wt%) steel, and it is found to be more efficient by comparison with the conventional ART. Transformation kinetics, alloying element partitioning and microstructure evolution during the cyclic and conventional austenite reversion process is discussed. [ABSTRACT FROM AUTHOR]

The microstructure evolution on the rolling surface of GCr15 steel subjected to rolling-sliding and pure rolling contact loading was systematically investigated. Experimental results showed that the pearlite structure of the surface layer in the rolling-sliding sample transformed into nanocrystalline α-Fe–C alloy in which cementite underwent severe decomposition while the pearlite lamellae appeared unperturbed in the pure rolling sample. A white etching layer (WEL) was also detected in the surface of the rolling-sliding sample. The WEL formation was found to be due to cyclic shear plastic deformation instead of frictional heating. A surface layer of Fe 3 O 4 was detected in the pure rolling samples. Microhardness depth profiles of the rolling-sliding and pure rolling samples also showed different trends. [ABSTRACT FROM AUTHOR]

Abstract: Partial transient liquid phase bonding (PTLPB) was applied to silicon nitride/nickel-based superalloy (DZ483) joints using Ti/Cu/Ni interlayers. Microstructure examination and mechanical properties evaluation of the joints show that bonding of Si3N4 to superalloy was successful. A uniform compact reaction layer about 2.7μm thickness was found at the Si3N4/Ti interface, which was formed by the reaction between Ti and Si3N4 substrate. With joining temperature rising, the flexural strength of the joining samples increased initially and then decreased. When the joining temperature was 1323K, the flexural strength of the joints reached to 170MPa. The fracture behavior of the joints was also discussed, with the ceramic reaction layer being the region where all failures occurred. [Copyright &y& Elsevier]

The mechanical properties and microstructure of reduced activation ferritic/martensitic (RAFM) steel after austenitizing at 930–1080 °C for 30 min and tempering at 760 °C for 90 min were investigated using field emission scanning electron microscopy, field emission transmission electron microscopy, electron back-scattered diffraction, X-ray diffraction, tensile, and impact tests. The results showed that the size of the M 23 C 6 particles decreased from 930 to 1030 °C and then increased from 1030 to 1080 °C, whereas their number densities showed the opposite trend with increasing austenitizing temperature. After austenitizing at 930 and 980 °C, the largest proportion consisted of the sub-grain structure, and when the austenitizing temperature was raised to 1030 and 1080 °C, the major component consisted of bundle martensite laths. The tensile strength increased gradually with increasing austenitizing temperature, and dislocation strengthening accounted for the largest proportion among the various strengthening mechanisms. Finally, fitting analysis showed that among the various hierarchical structural units in RAFM martensitic steel, the packets served as the effective "grain" that governed the increase in yield strength. [ABSTRACT FROM AUTHOR]

In order to deepen the understanding towards the creep rupture lifetime of G115 martensitic heat-resistant steel, a series of creep tests using 130–200MPa loading stresses at 923K were carried out to investigate the as-crept microstructural characteristics and cavity behaviors. The significant microstructure deformation and a dramatic increase in the areal density of high-angle grain boundaries occur when the stress loading is over 160MPa. Apart from the strain concentration at elongated grain boundaries and refined grains, the elevated stress also leads to the appearance of a strong α-fiber texture with {001}<110>//RD. The nucleation and growth of cavities can be effectively accelerated by the stress elevation, and ductile fracture is determined as the dominate mechanism within the test stresses. However, a transition towards brittle intergranular fracture is observed due to the rapid coarsening of Laves phase. Furthermore, a good prediction for creep lifetimes of G115 steel has been obtained based on the constrained diffusional cavity growth model. [ABSTRACT FROM AUTHOR]