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This examination is aimed to study the various densification mechanisms in the ZrB2-SiC system at different sintering temperatures. For such an objective, hot-pressing process was implemented to fabricate three ceramics at sintering temperatures of 1650, 1850, and 2050 degrees C under 10 MPa for 1 h. According to the results, particle rearrangement and fragmentation were the predominant densification mechanisms at the lowest sintering temperature. Additionally, it was found that the activation of the liquid phase sintering mechanism was advantageous in the particle rearrangement at low temperatures. However, rising the sintering temperature to 1850 degrees C changed the dominant mechanism to the plastic deformation. Such a phenomenon was accompanied by the creation of many dislocations in both ZrB2 and SiC grains. Implementing a higher sintering temperature (2050 degrees C) activated another consolidation mechanism called diffusion. This occurrence, together with the evaporation of the majority of the liquid phase at elevated temperatures, resulted in emerging transgranular fracture mode in the sample.

As a newly developed gun barrel steel, the novel steel has shown excellent high-temperature strength, high resistance to wear and erosion, contributing to the superior ballistic life of gun barrels. As ballistic life increases, the fatigue life becomes essential for the safety and reliability of gun barrels. This paper presents a comparison of the low cycle fatigue (LCF) behaviors between a novel steel and 30SiMn2MoV steel at 700 &deg, C. A strain-controlled fatigue test was carried out on the novel steel and 30SiMn2MoV steel in the strain range from 0.2 to 0.6%. The cyclic stress response behaviors of the novel steel and 30SiMn2MoV steel show cyclic softening behavior. In addition, the shape of the hysteresis rings of the novel steel and 30SiMn2MoV steel exhibit no-Masing model behavior. Energy&ndash, life relationships results show that the novel steel has higher fatigue resistance than the 30SiMn2MoV steel at 700 &deg, C. The results of fatigue fracture analysis show that the failure mode of the 30SiMn2MoV steel is a mixed mode of intergranular fracture and transgranular fracture, while the failure mode of the novel steel is intergranular fracture. The cyclic softening of the two materials can be attributed to the lath structure with a high density of dislocations gradually transforms into low energy subcrystalline and cellular structures at 700 &deg, C. The novel steel has a better fatigue life than the 30SiMn2MoV steel at 700 &deg, C and different strain amplitudes, which is mainly related to the carbides and lath martensite in the materials.

In this paper, nickel-based alloy ZSGH4169 was used as the test material and the fretting fatigue test of standard test pieces was carried out to study the variation rule of its fretting fatigue life under different temperatures and loads. The results show that the fretting fatigue life of ZSGH4169 decreases with the increase of temperature and load. Through fracture analysis under different working conditions, it can be seen that ZSGH4169 fretting fatigue failure form is basically the same, the crack expands in the form of transgranular fracture propagation. Temperature and load have significant influence on the depth of fretting fatigue crack initiation zone and the fatigue band spacing in the propagation zone, which affects its fretting fatigue life. According to the analysis of crack initiation and propagation path, the crack initiation angle of fretting fatigue crack is mainly affected by temperature. With the increase of temperature, the crack initiation angle increases, however the load has little influence on the crack initiation angle.

In this study, the Ni-Cr-W superalloy GH3230 is used as the test material. According to the actual structure of the flame tube, a porous laminate structure specimen is designed. The structure consists of impact holes, overflow holes and pin fins. High-temperature tensile tests at 650 &deg, C, 750 &deg, C and 850 &deg, C were carried out to study the high-temperature mechanical properties and fracture mechanism of the specimens of porous laminate structure, and the strain nephogram of the specimens were obtained by digital image correlation (DIC) technique. Due to the large number and dense arrangement of overflow holes, an obvious hole interference effect can be found from the strain nephogram. The stress concentration around the pore and the interference between the pores provide priority places and paths for the initiation and propagation of microcracks. The test found that the microcracks of the porous laminate structure first occurred around the hole, the overflow surface fractured first, after which the impact surface fractured. The strength of the alloy exhibits a significant temperature sensitivity to temperature. From 650 &deg, C to 750 &deg, C, the ultimate strength (&sigma, b) and yield strength (&sigma, 0.2) decrease slightly, but they decrease significantly at 850 &deg, C. The microstructure of the fracture surface shows that all microcracks occur at the interface between the matrix and the carbides but that the fracture mode of the specimens gradually changes from intergranular fracture to transgranular fracture as the temperature increases. Due to the pinning effect of the intracrystalline diffusive solute atoms on the dislocations, the stress-strain curves of the high-temperature tensile tests at 650 &deg, C and 750 &deg, C showed zigzag characteristic fluctuations during the strengthening stage.

To explore the influence of sintering temperature on mechanical properties of Al2TiO5 (AT) flexible ceramics, the XRD/Raman, SEM and acoustic emission (AE) technology were used to estimate the phase composition, microstructure evolution and damage behaviors of AT ceramics, respectively. Results show that the diffraction peaks of AT increase before rising sintering temperature from 1300 °C to 1500 °C and then start to decrease at 1600 °C due to the decomposition of AT grains. In specimens with sintering temperature of 1300 °C and 1400 °C, intergranular fractures are more likely to occur, which controls the origination of AE events in the whole stage. And the fracture of big bodies composed by large number of fine AT grains is the main source of AE events with high energy level at initial stage. In addition, Thanks to the more transgranular fracture in specimens with sintering temperature of 1500 °C and 1600 °C, the number of AE events in these two specimens are more than that of specimens with sintering temperature of 1300 °C and 1400 °C.

In this work, the crack propagation behavior and mechanism of dual-phase steel were studied under low temperature environment. The results show that the strength of dual-phase steel increases from −20 °C to −60 °C, and the fracture toughness presents the opposite trend. When the crack began to extend at low temperature, the size of plastic zone at the crack-tip is reduced, inducing the opening displacement rate, strain rate and temperature-strain parameters are reduced, which shows the extreme sensitivity to ambient temperature. More importantly, the fracture of dual-phase steel presents the transgranular fracture along {1 0 1} and {1 1 1} crystal planes, due to the accumulation of dislocation density. Therefore, the crack propagation of dual-phase steel at low temperature is caused by the dislocation accumulation and movement behaviors.

B4C&ndash, TiB2 composite ceramics with ultra-high fracture toughness were successfully prepared via spark plasma sintering (SPS) at 1900 &deg, C using B4C and Ti3SiC2 as raw materials. The results showed that compared with pure B4C ceramics sintered by SPS, the hardness of B4C&ndash, TiB2 composite ceramics was decreased, but the flexural strength and fracture toughness were significantly improved, the fracture toughness especially was greatly improved. When the content of Ti3SiC2 was 30 vol.%, the B4C&ndash, TiB2 composite ceramic had the best comprehensive mechanical properties: hardness, bending strength and fracture toughness were 27.28 GPa, 405.11 MPa and 18.94 MPa&middot, m1/2, respectively. The fracture mode of the B4C&ndash, TiB2 composite ceramics was a mixture of transgranular fracture and intergranular fracture. Two main reasons for the ultra-high fracture toughness were the existence of lamellar graphite at the grain boundary, and the formation of a three-dimensional interpenetrating network covering the whole composite.

Effect of different Ag content on microstructure, internal and external fatigue cracks of SAC lead-free solder balls (Sn0.7Cu, SAC0307, SAC105) under the rapid thermal fatigue was studied. The result shown that the surface cracks of balls with three kinds of silver content were found after similar rapid thermal fatigue cycles. With the increase of thermal fatigue cycles, surface cracks became wider and deeper under the thermal stress. The number of crack was largest on the surface of SAC105 solder ball and cracks grew up fastest. The recrystallization and grain directional growth preferentially occurred at the bottom of solder ball under the heat flow, and the inhomogeneity of microstructure and coarsening of grains were most significant inside the SAC105 solder ball. Internal cracks were appeared near the heat source of solder ball preferentially, cracks mainly originated from brittle phase and cavity, and the fracture pattern was intergranular fracture and transgranular fracture. The number of internal crack was largest inside SAC105 solder ball, The internal cracks were honeycomb-shaped and grew toward the heat source. Therefore, the research had a guiding significance for the performance evaluation of lead-free solder for high-power devices.

International audience; Dense tricalcium phosphate, Ca3(PO4)2 (TCP) – diopside, CaMg(SiO3)2, composites present better mechanical properties than single phase TCP. In this work, it is investigated whether the mechanical behaviour improvement by diopside is maintained in porous scaffolds. The processing parameters to obtain cylinders with ≈ 50% of aligned pores of elliptical cross sections with major axis up to 100 μm by freeze casting and sintering were established. Pore channels were introduced in the green specimens by laser ablation. After sintering, the diameter of the cross sections of the channels was ≈ 700 μm. The ceramic composite microstructure was constituted by a substructure of small diopside particles (2 −7 μm) and dense β-TCP zones of larger dimensions (up to 40 μm). Strength values, determined by diametral compression (DCDT) (≈ 2.5–4 MPa) are in the range of strength of cancellous bone. Diopside presents transgranular fracture, hindering crack propagation from the β-TCP areas and the pores, as occurred in the dense materials

Marked degradation of tensile properties induced by plastic deformation after dynamic interactions between strain-induced martensite transformation and hydrogen has been investigated for type 316L stainless steel by hydrogen thermal desorption analysis. Upon modified hydrogen charging reported previously, the amount of hydrogen desorbed in the low temperature range increases, the degradation of tensile properties induced by interactions between plastic deformation and hydrogen at 25 &deg, C or induced by interactions between martensite transformation and hydrogen at &minus, 196 &deg, C occurs even for the stainless steel with high resistance to hydrogen embrittlement. The hydrogen thermal desorption behavior is changed by each interaction, suggesting changes in hydrogen states. For specimen fractured at 25 &deg, C, the facet-like morphology and transgranular fracture are observed on the outer part of the fracture surface. At &minus, C, a quasi-cleave fracture is observed at the initiation area. Modified hydrogen charging significantly interacts both plastic deformation and martensite transformation, eventually enhancing the degradation of tensile properties. Upon plastic deformation at 25&deg, C after the interactions between martensite transformation and hydrogen by straining to 0.2 at &minus, C, cracks nucleate in association with martensite formed by the interactions at &minus, C and marked degradation of tensile properties occurs. It is likely that the interactions between martensite transformation and hydrogen induce damage directly related to the degradation, thereby affecting subsequent deformation. Upon dehydrogenation after the interactions between the martensite transformation and hydrogen, no degradation of tensile properties is observed. The damage induced by the interactions between martensite transformation and hydrogen probably changes to harmless defects during dehydrogenation.

International audience; In this article, we reported a processing route for fabricating extremely uniform and ultrafine Zr(Y)O2 particles diffusely distributed in the tungsten matrix. More than 90% of oxide particles are between 100 and 650 nm in diameter. The refinement mechanism of Zr(Y)O2 particles was discussed. Under TEM observation, lots of Zr(Y)O2 particles with a diameter of less than 50 nm are dispersed in tungsten matrix. HRTEM analysis reveals that the Zr(Y)O2 particles exhibit a good bonding with the tungsten phase, but without coherent relation at interface. The fabricated W alloy reinforced by 0.25wt% Zr(Y)O2 exhibits excellent compressive strength of 1577 MPa and fracture strain of 0.262, higherthan those of the pure W and the published experimental results. The fracture surface mainly consists of transgranular fracture and involvessignificant W grain tearing and grain deformations, confirming the strengthening effect caused by nanosized Zr(Y)O2 particles. The hotdeformation behavior of W-Zr(Y)O2 alloy at high temperature was investigated through establishing constitutive equation based on theArrhenius model.

Brazed monolayer diamond grinding wheels have advantages of a high abrasive bonding strength, high protrusion, and a large chip disposal space. However, it is difficult to prepare ordered and fine-grained brazed diamond grinding wheels. This study presents a new method for grain-arranged, brazed diamond grinding wheels with microtextures with similar performance to ordered and fine-grained brazed diamond grinding wheels. First, coarse diamond grains (18/20 mesh) were orderly brazed to fabricate the end grinding wheels. Next, a series of microtextures were ablated on the diamond grains using a pulsed laser, and two types of textured end grinding wheels—TG-G (ablated microgrooves only) and TG-GH (ablated microgrooves and microholes)—were prepared. Then, an experiment involving the grinding of alumina ceramics was performed, and the grinding characteristics and grinding mechanism were analyzed. The results indicated that compared with untextured diamond end grinding wheels (TG), the textured diamond grinding wheels (TG-G and TG-GH) significantly reduced the grinding force and the roughness of the machined surface. The local stress concentration at the microtextures promoted the formation of microcracks in the diamond grains of TG-G and TG-GH, and the self-sharpness of the grinding wheel was significantly improved. The brittle fracture mode of ceramic materials in grinding included intergranular fracture and transgranular fracture. Ironing pressure action was a key material-removal mechanism. It had an important influence on the cutting force and plasticity characteristics of the TG machined surface. For the surfaces processed by TG-G and TG-GH, the effect of ironing was weakened, while shearing played a more important role. The TG-GH grinding wheel ablated with microgrooves and microholes was superior to the TG-G grinding wheel ablated with only microgrooves, with regard to the grinding force, roughness, and self-sharpening.

Bulk polycrystalline zirconia-based shape memory ceramics (SMCs) generally crack as they undergo martensitic transformations, largely due to the transformation mismatch stresses. Polycrystalline SMCs subjected to cyclic transformations bodily lose individual grains and progressively comminute, but it is not yet clear how the grain size of a polycrystal affects such cyclic degradation. We explore this issue in 1.5 mol% yttria-doped zirconia SMCs by varying the grain size from 0.6 to 7.9 µm in pellets of fixed composition and sample size and subjecting them to multiple thermal cycles through the transformation. A smaller grain size is found to increase the number of cycles required to disaggregate the pellet because of the larger amount of grain boundary area that must crack. Calorimetry analysis shows that the energy relieved through cracking decreases with increasing grain size and reveals an apparent material length scale of ~2 micrometers for the stress relief zone. For grain diameters below this critical length scale, complete stress relief is suggested, while at larger grain sizes, intergranular cracking apparently does not fully relieve the transformation mismatch stresses. Alternate accommodation mechanisms are required including the formation of multiple variants and even some transgranular fracture.

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

Comprehensive study on effect of YAG amount on densification, creep resistance and room-temperature mechanical properties of Al2O3-YAG composite pressureless sintered at 1600 °C was conducted. The main goal was to optimize the amount of YAG in order to fabricate a composite with improved creep resistance and sufficiently good room-temperature mechanical properties. The composite was made by mixing a commercially available Al2O3 powder with fine YAG powder obtained by glycine-nitrate combustion synthesis starting from aluminum nitrate and yttrium nitrate. Increased driving force for sintering of fine YAG powder allowed fabrication of dense Al2O3-YAG composite with up to 30 vol% YAG. The presence of YAG was found to be very effective in improving creep resistance of Al2O3-YAG composite. Large Y3+ ions blocked diffusion along Al2O3 grain boundaries, reduced diffusivity and therefore enhanced creep resistance of Al2O3-YAG composite which continuously increased as the YAG amount increased. Тhe presence of YAG was also found to improve mechanical properties such as hardness and elastic modulus. The improvement of these properties was ascribed to increased density of Al2O3-YAG composites owing to high sintering activity of YAG powder. While fracture strength of the composite can be as high as that of monolithic Al2O3, fracture toughness of composite decreased continuously as the YAG content increased. The decrease was ascribed to transgranular fracture of both YAG and Al2O3 grains in samples containing larger amounts of YAG. The proper balance between fracture toughness and creep resistance was found in composite containing 18 vol% YAG which had considerably improved creep resistance accompanied by a relatively small decrease in fracture toughness.

Microstructural evolutions of EK61 superalloy during long-term aging until 1000 h at 700°C and 750°C, respectively, are studied by combination of Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). Impact fracture morphologies after aging for different time are observed by the SEM. The microstructure is found to be relatively stable during aging at 700°C, and the fracture morphologies are characterized by transgranular fracture. At 750°C, the coarsening of γ′ phase leads the reduction of the quantity of dimples, the chainization of carbides on grain boundaries leads to intergranular fracture, and the netting of η phases within grains leads to the formation of lamellar cleavage steps. It is obvious that the destabilization of precipitated phases affects fracture morphology significantly. The relationship between fracture morphology and the microstructure promotes the evaluation of service reliability of EK61 superalloy.

The Mo-10Si-(0, 5, 10, 15)B (at.%) alloys located in Moss (Mo solid solution) +Mo3Si +T2 (Mo5SiB2) region were prepared by arc melting. The effect of B content on the microstructure, microhardness, fracture toughness and oxidation resistance of the alloys was studied. In the Mo-10Si alloy, there are primary Moss and peritectic Mo3Si phases. With the addition of B, (Moss + T2) binary and (Moss + T2 + Mo3Si) ternary eutectics emerge in Mo-10Si-(5, 10)B alloys. For Mo-10Si-15B alloy, primary Moss disappears and the microstructure is only composed of binary and ternary eutectics. With the increase of B content, the proportion and grain size of Moss decrease. The microhardness of alloys increases with the increase of B content due to the increasing intermetallics. The fracture toughness of alloys decreases obviously in single-edge notched three-point bending testing (SENB) and the fracture mode transforms from transgranular fracture to intergranular fracture because of the refinement of Moss. In the high-temperature oxidation test at 1250 °C, the oxidation weight loss of the alloys decreases with the addition of B due to the densification of the borosilicate layer.

The influence of peak compressive stress on the dyamic tensile fracture (spall) in a multi-phase steel, austenite, ferrite and martensite phases, was investigated in as-received and pre-strained conditions. Plate impact experiments were done at ⁓3.0 GPa and ⁓6.0 GPa compressive peak stresses. Results showed that peak stress increase results in about 20% increase in the spall strength for samples tested in the as-received condition, and about 15% increase for the pre-strained samples. These increases could be related to the effects of peak stress on the tensile stress that causes spallation. However, introducing plastic deformations in the pre-strained samples had negligible effects on the spall strength. Microstructural examinations revealed that incipient spall damage was parallel to the phase boundaries and mainly accommodated by ferrite grains as quasi-cleavage transgranular fracture. Samples shocked in the as-received condition with full spallation experienced a quasi-cleavage fracture within the ferrite compared with a cleavage fracture within the ferrite for samples shocked in the pre-strained condition. Deformation within the austenite phase due to plate-impact testing was dependent on the initial sample condition, the pre-strained samples showed more deformation than the as-received samples.

In this paper, experiments were conducted to investigate the penetration characteristics of low-frequency impact of pulsed water jet. The pulsed jet was generated with pulse length of 217 mm at the frequency of 120 Hz, using a round nozzle with diameter of 1 mm within the standoff distance of 40 mm. The macro characteristics (entrance shape, cross-section features, penetration depth, entrance area, and penetration rate), inner wall roughness, and micro morphology of penetration cavity on different targets (sandstone and granite) were measured to investigate the penetration performance and mechanism of low-frequency liquid impact. Results showed that there were two different penetration patterns on different targets based on the macro characteristics and the analysis of flow field loading characteristics in penetration cavity. The penetration cavities were present as funnel-shaped pits with narrow entrance and large depth on sandstone samples, and shallow craters with wide entrance and small depth on the granite samples. The roughness on entrance zone and bottom zone of penetration cavity on sandstone decreased with impact time and impact velocity, and that of middle zone increased. The roughness of cavity wall for granite increased with the expansion of penetration cavity. It was found that the main destruction pattern of sandstone was in the form of intergranular fracture, and transgranular fracture for granite. Radial confined holes were observed on the cavity wall at different depths, and the holes mainly existed along the grains boundary of sandstone, present on the transgranular fracture surface of granite. Based on the study about the macro failure characteristics, roughness, and micro morphology of penetration cavity, it was expected to provide some useful information for the application of penetration technology by low-frequency liquid jet.

A TiAl–Nb alloy was prepared by casting, and its tensile properties and microstructure evolution at different temperatures were investigated. The results show that the brittle-ductile transition temperature (BDTT) of the alloy is 880–920 °C. At low temperatures, deformation mainly occurs in the γ lamellae and coarsening γ phase between the lamellar colonies. As the temperature increase in the transitory stage, a small number of dislocations may pass through the interface from the coarsening γ phase into the γ lamellae. During the brittle stage, the dislocation and dislocation array bow out from the phase interface, and twins are emitted from the phase interface, which may contribute to the accommodation of stress concentration. During the ductile stage, the γ lamellae coarsen due to the thinner and smaller α2 lamellae dissolving and phase boundary transfer. The dislocations interact with the dislocation walls to form subboundaries in the γ lamellae and transform into equiaxed crystals by dynamic recrystallization (DRX), which increases the plasticity of the alloy. With the transition from the brittle model to the ductile model, the fracture mode of the alloy changes from the mixed fracture mode of transgranular fracture and translaminar fracture to intergranular fracture.

A twinning-induced plasticity (TWIP) steel was prepared by mechanical alloying and spark plasma sintering (SPS), and its microstructure and mechanical behavior were examined. The ultrahigh microhardness and considerable tensile strength of the sintered bulk sample were mainly attributed to grain-size strengthening and particle strengthening by numerous Mn-rich particles dispersed in the matrix. Micropores coupled with large internal inclusions easily produced cracks in the bulk sample under uniaxial tensile stress, resulting in poor ductility with rapid intergranular brittle fracture. The ductility of the material showed significant improvement with the elongation increased from 4% to 13% after annealing the bulk sample at 850 °C for 1 h. The size of the inclusions in the annealed sample dramatically reduced and several small dimples with high densities and depths were formed, presenting a mixed fracture mode in which intergranular fracture was the dominant mechanism, whereas transgranular fracture was the supplementary mechanism.

This study aims to combine Y2O3 and Ti to improve the mechanical and electrical properties of Al2O3 ceramics. Al2O3-based composites containing 20 vol% of metallic Ti were chosen. Y2O3 was used as the effective additive to optimize the mechanical and electrical properties of Al2O3/Ti composites. The effects of Y2O3, ranging from 1 to 5 wt%, on mechanical and electrical properties of Al2O3/Ti composites were studied. Energy-dispersive X-ray spectroscopy indicated that the added yttrium was present at the grain boundaries. The formation of yttrium aluminum garnet (Al5Y3O12, YAG) resulting from the solid-state reaction between Y2O3 and Al2O3 was identified by X-ray diffraction (XRD). All Y2O3 doped Al2O3/Ti composites showed an increase in the density. The observation of microstructure revealed that the microstructure was refined and the grain size of Al2O3 decreased. Vickers hardness, Young's modulus, and fracture toughness of the composite having Y2O3 content of 2 wt% reached their maximum values, which were 15.2 GPa, 355.2 GPa, and 4.5 MPa•m1/2, respectively. Ductile fracture mode and transgranular fracture were visible in all the composites. The toughening mechanism, which arises due to the presence of YAG, was demonstrated. In addition, the electrical resistivity of composites increased monotonically with an increase in the Y2O3 content. Nevertheless, the increase was negligible and the composites could be regarded as a good electrical conductor having 5 wt% Y2O3 addition.

The research of Nickel-based superalloys with excellent mechanical properties is of significance for aerospace. Here we report the effect of sintering temperature of 850 °C, 900 °C and 950 °C with the heating rate of 100 °C/min via spark plasma sintering (SPS) on Inconel 718 (IN718) superalloy reinforced with 0.25 wt. % reduced oxide graphene (RGO). The microstructures and mechanical properties of IN718-RGO composite were investigated by scanning electron microscopy (SEM) and MTS servo hydraulic testing machine, respectively. SEM results demonstrate that the RGO is almost uniformly distributed into the IN718 powders after 3D rock-milling for 8 h. The mechanical properties of the composite increase with increasing the sintering temperature. When the sintering temperature is 950 °C, a relative density of 92.5%, a microhardness of 375 Hv and an ultimate compressive strength of 1748.20 MPa after aging heat treatment was obtained for the IN718–0.25 wt. % RGO, respectively. The fracture mechanism of the composite changes from intergranular fracture to transgranular fracture with the sintering temperature increasing.

Effects of creep aging on strength and toughness and microstructure evolution of Al-Cu-Li alloy were investigated by tensile test and Kahn tear test at room temperature, combined with fractograph and transmission microstructure analysis. The specimens of pre-strain creep aging state (CA) have been studied and compared with artificial aging state (AA) and pre-strain artificial aging state (PA). It is demonstrated that pre-strain before artificial aging process can improve the strength property and the fracture toughness of Al-Cu-Li alloy. On top of that, both of them can be further improved by creep aging process: the yield strength was increased by 8.7% and the unit initiation energy of Kahn tear was increased by 11.7%. The increasing of fracture toughness has also been verified by Kahn tear fractographs that the dominating fracture type changes from the ductile intergranular fracture to the ductile transgranular fracture. Transmission electron microscopy revealed that creep aging promotes the nucleation of T1 phase, resulting in a fine and dense distribution of precipitates in the matrix, and the enrichment of subgrain boundary T1 precipitates was greatly reduced.

The hydrogen embrittlement of 12Cr2Mo1R(H) steel at different strain rates were investigated after hydrogen precharging for 4 h in a 0.5 M H2SO4 solution with 2 g/L ammonium thiocyanate. Results showed that the embrittlement index increased and gradually reached a relative stable value of about 20% at the strain rate of 5 × 10−5 s−1 with the decrease of strain rates. SEM images depicted small and deep flakes at high strain rates, while flakes grew larger at slow strain rates. Most hydrogen-induced cracks (HICs) were transgranular fracture through lath grain of bainitic ferrite. High strain field surrounds the crack tips, which makes the crack tips of two close and parallel cracks deflect toward each another and even form crack coalescence. The electron backscatter diffraction technique was used to investigate the effects of grain boundaries, recrystallization fraction, kernel average misorientation map, texture component, and coincidence site lattice boundaries on the HIC propagation. High densities of dislocations and strain concentrations were found around the cracks, where grains are highly sensitive to HIC.

Effects of different metallic additives and sintering parameters on microstructure and mechanical properties of TiB2-HfC ceramics were investigated. The results showed that the core-rim structure and HfC particle dispersion were discovered in these ceramics. Only Co as the metallic additive could more easily promote the formation of the strip-shaped TiB2 than the others; TiB2-HfC-Ni-Mo ceramic had more serious agglomeration of HfC particles than the others; for TiB2-HfC ceramic, using Ni-Co as the metallic additive could obtain better mechanical properties than using the others. In addition, with sintering temperature increasing from 1500°C to 1650°C, grains grew gradually and the core-rim structures became fewer and unobvious. Extending the holding time from 15 min to 60 min, grain size just had a slight increase, and the influence of holding time on mechanical properties was not significant. The increase of sintering temperature (1500–1650°C) could lead to more obvious grain growth and change of core-rim structure than the increase of holding time (15–60 min). The better sintering parameters for TiB2-HfC-Ni-Co ceramic were 1600°C and 30 min. Besides, better mechanical properties of TiB2-HfC-Ni-Co ceramic sintered at 1600°C with 30 min holding time were that Vickers hardness was 21.2 ± 0.3 GPa, flexural strength was 1169 ± 16 MPa, and fracture toughness was 6.7 ± 0.3 MPa m1/2. Meanwhile, its toughening mechanisms included core-rim structure, particle dispersion, crack deflection and crack bridging. The mixed intergranular/transgranular fracture could also improve fracture toughness.

ZrB 2 –SiC composite ceramics with different compositions (20 and 60 vol% ZrB 2 –SiC, 20ZS and 60 ZS, respectively) were prepared. Wear tests were conducted on the obtained ceramics in multiple distances using ball-on-flat tribotester. Volume loss and cross-sectional profiles of samples were measured by three–dimensional (3D) profilometer to study the onset of track wear damages. Pressure–depth curves and hardness were measured by indentation to investigate defects produced in the tribo-film by the debris. The debris of 20ZS was found to be joined to the tribo-film and accumulated with distance, shifting from microcrack (＜10,000 cycles) to abrasive wear (50,000 cycles). Compared to 20ZS, lower debris accumulation of 60ZS resulted in better wear resistance, leading to thinner and more stable non-substrate regions for this sample. These differences between both samples basically resulted from different particle sizes. Fine grains were easily pulled out in the experiment, resulting in abrasive wear of the specimen. While transgranular fracture of grains and the pinning led to larger grains with less debris, the damage mode remained transgranular fracture.

Cu-based bearing shell containing Ni layer was prepared by centrifugal casting. The effect of Ni layer on the microstructure evolution and mechanical property of Cu-based bearing shell with aging treatment was studied. The microstructure of the Sn-Sb-Cu alloy (ZChSnSbCu8-4) without aging is consisted of Sn-based solid solution and Cu6Sn5 phases which distributed uniformly in the matrix. After aging treatment, the grain boundary segregation is slowly disappeared, and the matrix transforms into a more uniform Sn-based solid solution. The Ni layer hinders the diffusion of Cu and Sn atoms at the interface, and suppresses the volume fraction increment of Cu6Sn5 phase during aging. The Cu6Sn5 phases are gradually coarsened during aging. The average bonding strength of Cu-based bearing shell with Ni layer is 70.3 MPa. After aging at 175 °C with 500 h, the bond strength is still higher than 50 MPa, which is greater than the specimen without Ni layer. The hardness of Cu matrix, Ni layer and Sn-Sb-Cu alloy decreases with increasing aging time. While the hardness of the region near the Ni/Sn-Sb-Cu interface increases from 47.5Hv to 88.4Hv during aging. The fracture surface of the samples after aging treatment is changed from intergranular/transgranular fracture mode to intergranular fracture mode. The change of the fracture mode is related to the coarsening and growth of Cu6Sn5 phases near the interface between Ni layer and Sn-Sb-Cu matrix.

Native protective oxide scales offer resistance against corrosion for high temperature materials, which often work in extreme conditions of varying mechanical and thermal loads. The integrity of such layers is of critical importance, since their damage can lead to significant reduction in material life. Mechanical data such as fracture strain and elastic modulus are required to include oxides in material life estimation models for high temperature materials, but there is lack of such data. Their thickness is in the μm range, which makes mechanical testing for property determination difficult. Here we present a micro-mechanical testing method, based on bending of micro-cantilevers produced by focused ion beam milling, capable of circumventing the limitations of conventional approaches. We apply this method to chromia thermally grown on pure chromium, and measure fracture strains at room and high temperatures (600 °C). The measured fracture strains were found to be higher at room temperature, due to a larger fraction of transgranular fracture. Surprisingly, a large fraction of transgranular fracture was seen even in the presence of stress concentrations at grain boundaries. Removal of the stress concentrations accentuated the propensity for transgranular cracking at room temperature. Realistic values of room temperature elastic modulus were obtained as well. The observed mixed trans- and intergranular cracking points towards the need for dedicated investigations of both oxide grain boundary strength and cleavage resistance of single crystals in order to fully understand the failure mechanisms in thermally grown oxide scales. acceptedVersion acceptedVersion

Ultrafine-grained tungsten shows strong potential for ballistic applications because of its increased strength, and higher propensity for strain localization compared with coarse-grained materials. However, tungsten typically exhibits poor ductility at room temperature and fractures by a brittle intergranular mechanism. This study evaluates the effects of boron as a sintering aid and grain-boundary strengthening additive for ultrafine-grained tungsten prepared by powder metallurgical methods. While boron did not improve the sinterability of nanocrystalline tungsten in this study, low concentrations of boron delivered a notable increase in strength, accompanied with a transition from intergranular to transgranular fracture. These significant changes in fracture behavior show promise for improving the low-temperature ductility of tungsten; however, the corresponding improvements to plastic strain to failure were less significant than anticipated.

In the present study, two types of diamond films with a diameter of 123 mm were deposited by DC arc plasma jet chemical vapor deposition (CVD) operating in the gas recycling mode. These two diamond films were found to possess peculiar grain family morphologies. The fracture strengths of the samples were 182.65 and 995.58 MPa, respectively. To investigate the possible reason for the large difference in the fracture strength between the samples, the surface and fracture morphologies of the two types of diamond films as well as their grain size were characterized. The results revealed that the sample with low fracture strength suffered from a typical intergranular fracture. Weak grain boundary, impurities distributed in the boundary, and low strength of diamond grains led to a decrease in the fracture strength. However, the sample with high fracture strength mainly suffered from transgranular fracture and the columnar grain exhibited numerous voids. Owing to the impact of twinning, the voids could be occupied, thus increasing the contact area between the adjacent grains. Moreover, large grain family on the growth side contained numerous small grains and twins. These small grains, twins on growth side along with the difference of stress on surfaces inhibit fracture strength from decreasing rapidly. All these features benefitted fracture strength. Moreover, when the sample was loaded, the tips located in the voids probably resulted in increase of risk of stress concentration, which might have led to initiation of crack origin and eventual fracture.

Effects of HfN content and metallic additives on microstructure and mechanical properties of TiC 0.7 N 0.3 -based ceramics were investigated. HfN had very weak inhibition effect on TiC 0.7 N 0.3 grain growth. When HfN content increased from 10 wt% to 20 wt%, flexural strength and fracture toughness became higher, but Vickers hardness was around 20 GPa without significant changes. The optimal mechanical properties of TiC 0.7 N 0.3 -20 wt%HfN-Ni-Co were that Vickers hardness was 18.93 GPa, flexural strength was 876.17 MPa and fracture toughness was 7.58 MPa m 1/2 in this investigation. Ni-Mo additive exhibited better wettability with TiC 0.7 N 0.3 than the other metallic additives. Ni-Mo additive could inhibit TiC 0.7 N 0.3 grain growth effectively and improve mechanical properties of TiC 0.7 N 0.3 -based ceramic. The optimal mechanical properties of TiC 0.7 N 0.3 -20 wt%HfN-Ni-Mo ceramic was that Vickers hardness was 19.43 GPa, flexural strength was 1235.88 MPa and fracture toughness was 8.46 MPa m 1/2 in this investigation. Its toughening mechanism mainly included crack deflection and crack bridging. In addition, the mixed intergranular/transgranular fracture also could improve fracture toughness.

The scratching processes of monocrystalline and polycrystalline silicon carbide (SiC) with diamond grit were studied by molecular dynamics simulation to investigate the nanoscale material removal behavior. The results showed that, for both monocrystalline and polycrystalline SiC, the material removal processes were achieved by the phase transition to the amorphous structure. Large depth of cut and low scratching speed induced the large scratching forces, stress, and surface damage layer thickness. Less amorphous structure phase transition, smaller normal scratching force, and higher tangential stress were found in polycrystalline SiC, comparing to the monocrystalline SiC, due to the material soften caused by the microstructure, under all scratching conditions. Furthermore, the tangential stress showed highly dependent on the grain geometry and grain boundary (GB) location in polycrystalline. The subsurface damage layer in polycrystalline was little thinner than that in monocrystalline before the new GB generation at a low depth of cut and deteriorated at large depth of cut. In addition to the plastic deformation, which occurred in the monocrystalline SiC nanoscale scratching, the intergranular fracture and transgranular fracture were also observed through the GB generation and connection in polycrystalline SiC.

The influence of hydrogen on the mechanical properties of four, medium-strength, commercial, quenched-and-temped steels has been studied using the linearly increasing stress test (LIST) combined with cathodic hydrogen charging. The relationship was established between the equivalent hydrogen pressure and the hydrogen charging overpotential during cathodic hydrogen charging, though the use of electrochemical permeation experiments and thermal desorption spectroscopy. The cathodic hydrogen charging conditions were equivalent to testing in gaseous hydrogen at hydrogen fugacities of over a thousand bar. Under these hydrogen-charging conditions, there was no effect of hydrogen up to the yield stress. There was an influence of hydrogen on the final fracture, which occurred at the same stress as for the steels tested in air. The influence of hydrogen was on the details of the final fracture. In some cases, brittle fractures initiated by hydrogen, or DHF: Decohesive hydrogen fracture, initiated the final fracture of the specimen, which was largely by ductile micro-void coalescence (MVC), but did include some brittle fisheye fractures. Each fisheye was surrounded by MVC. This corresponds to MF: Mixed fracture, wherein a hydrogen microfracture mechanism (i.e., that producing the fisheyes) competed with the ductile MVC fracture. The fisheyes were associated with alumina oxide inclusion, which indicated that these features would be less for a cleaner steel. There was no subcritical crack growth. There was essentially no influence of hydrogen on ductility for the hydrogen conditions studied. At applied stress amplitudes above the threshold stress, fatigue initiation, for low cycle fatigue, occurred at a lower number of cycles with increasing hydrogen fugacity and increasing stress amplitude. This was caused by a decrease in the fatigue initiation period, and by an increase in the crack growth rate. In the presence of hydrogen, there was flat transgranular fracture with vague striations with some intergranular fracture at lower stresses. Mechanical overload occurred when the fatigue crack reached the critical length. There was no significant influence of hydrogen on the final fracture.

In this study, 3D molecular dynamics simulations were conducted to investigate the nanoscale scratching mechanism of polycrystalline SiC constructed by Voronoi site-rotation and cut method. The scratching process, SiC crystal structure evaluation, scratching force, stress, and temperature, surface morphology, and subsurface damage (SSD) were discussed after simulation. The results indicate that the ductile scratching process of polycrystalline SiC could be achieved in the nanoscale depth of cut through the amorphous crystal structure phase transition, which is the primary material removal mechanism in nanoscale SiC scratching. The silicon atom can penetrate into the diamond grit which may cause the wear of diamond tool. The disorder grain boundary (GB) atoms can transit to hexagonal diamond structure and generate dislocation during the scratching. Furthermore, the higher scratching speed results in smaller scratching force, smaller normal stress, and higher temperature due to the larger impaction breaking more Si C bonds, which makes the SiC material more ductile and easier to remove. The tangential stress shows great dependence on the grain and GB geometry and position due to the stress concentration in the GB. It is also found that the higher scratching speed encourages the pileup atoms in the front of grit to flowing to the grit side to form the groove protrusion, which can get a shallower SSD thickness and a wider SSD layer. SSD results also indicate that at least three types of the material removal mechanism – amorphous transition, intergranular fracture, and transgranular fracture – exists in the polycrystalline SiC scratching process.

Dense and fine-grained boron carbide (B4C) ceramics were fabricated via high-pressure hot pressing (100 MPa) using powders, which are prepared by high-energy ball milling. These powders were sintered at a low temperature (1800 °C) without any sintering aid. The dense and fine-grained B4C ceramics demonstrate super high hardness, outstanding fracture toughness and modern flexure strength. The milled powders were characterised by disordered crystal structure and ultrafine particle size that ranges from a few nanometres to a few hundred nanometres. The combined contributions of high pressure and the characteristic of the milled powders guaranteed that the dense fine-grained microstructure was achieved at only 1800 °C. The grain size distribution of the ceramics was inhomogeneous and ranged from 70 nm to 1.6 µm. However, the average grain size was fine at only 430 nm, which partially contributed to the super high hardness of the B4C ceramics. The locally concentrated areas of the small grains changed the fracture mode of the B4C ceramics from the complete transgranular fracture to a mixture of transgranular and intergranular fractures, thereby enhancing the toughness of the B4C ceramics. The relative density, Vickers hardness, flexure strength and fracture toughness of the obtained B4C ceramics reached up to 99.5%, 41.3 GPa, 564 MPa and 4.41 MPa m1/2, respectively.

The three-point bending test was performed on cemented tailing backfill prepared with different offset notches by a loading system with high-speed camera. And the effect of heated temperature on CTB fracture behavior has been investigated. These results indicate that the influence of pre-existing notch and temperature on CTB fracture feature is obvious. The peak load increases linearly with the increasing of the offset ratio when the depth-height ratio of notch is fixed at the same value, while the fracture peak loading decreases sharply with the increase of the notch depth when the offset ratios are the same level. The fracture toughness decreases by 63.5% as the heated temperature rises from 20 °C to 120 °C. The fracture toughness is directly proportional to the porosity of CTB. The increasing of porosity leads to the decrease of the fracture toughness mainly due to the decomposition of hydration products and the coarsening of the pore structure of CTBs. The crack propagates along the interface among tailing particles, resulting in a rough and jagged edge propagation path. The rack propagation pattern of CTB samples contains transgranular fracture and intergranular fracture. The intergranular fracture is clearly predominant during crack propagation. The rack propagation and failure pattern of CTB specimens from numerical modeling are conformed to the failure pattern of samples obtained by experimental testing. PFC is an effective numerical analysis method for exploring the fracture mechanism of micro-cracking in CTB.

This paper reports a study on mechanical behavior during tensile testing of nanostructured (NS) Al and ultra-fine grained (UFG) Al containing nanoscale oxide particles that were produced via consolidation of nano-sized Al powders using spark plasma sintering (SPS) only and SPS plus extrusion, respectively. Our results show that the NS Al exhibits ∼370 MPa in 0.2% offset yield strength and ∼0.9% in elongation to failure, whereas the UFG Al presents ∼430 MPa in 0.2% offset yield strength and ∼1.5% and ∼1.9% in uniform elongation and elongation to failure, respectively. Based on the tensile stress vs. strain curves, microstructural features and fractured surface characteristics, mechanical behavior of NS Al and UFG Al was analyzed and discussed in detail. We suggest that the lower yield strength and ductility in the NS Al than those in UFG Al can be attributed to the strong stress concentration at grain boundaries (GBs) as a result of (i) the presence of highly segregated nanoscale oxide particles and of residual porosity at GBs to induce, and (ii) the absence of dislocation sources and thus of activated dislocation slip at nano-sized grain interiors (GIs) to release, the stress concentration. Immediately after yield in the as-SPSed Al, the stress concentration debonded GBs and further tensile straining rapidly led to connection between debonded GBs and the occurrence of intergranular fracture. In contrast, in the UFG Al the extent of oxide particle segregation and the porosity at GBs were reduced due to materials flow during extrusion, avoiding the occurrence of intergranular fracture at a very low strain level and thus sustaining plastic straining; as a result, yield strength and ductility were improved. However, the residual porosity and nano-sized/ultra-fine grains still induced early strain localization shortly followed by transgranular fracture via growth, coalescence and connection of nano-sized pores, leading to limited ductility.

We investigated the hydrogen embrittlement mechanism in an interstitially carbon alloyed equimolar CoCrFeMnNi high-entropy alloy (HEA) through low strain rate tensile testing under in-situ hydrogen charging. The tensile ductility was significantly reduced by hydrogen charging. The failure mode of the interstitial HEA in presence of hydrogen was a combination of intergranular and transgranular fracture as well as microvoid coalescence. Aggregated nano-carbides act as potential sites for crack initiation. These findings show that the carbon alloyed equimolar high-entropy alloy is susceptible to hydrogen embrittlement.

ZrO2-based ceramics were fabricated by the hot pressing sintering technology at 1550 ℃. Effect of HfN, HfC and HfB2 additives on their phase transformation, microstructure and mechanical properties were investigated. The m-ZrO2 was not discovered in ZrO2-HfC ceramics but discovered in ZrO2-HfN and ZrO2-HfB2 ceramics, which indicated that HfC potentiated the effect of Y2O3 stabilizer on inhibiting phase transformation and that to some extend HfN and HfB2 undermined this effect. Meanwhile, HfC can inhibit the ZrO2 grain growth or amalgamation to form fine ZrO2 grains in ZrO2-HfC ceramics, whereas HfN and HfB2 probably induced the ZrO2 grain growth or amalgamation to form big ZrO2 grains in their ceramics. And the bonding strength between HfB2 and ZrO2 higher than the other two was approximately inferred from the observed grain boundary-interface dihedral angles between the ZrO2 grain and these particles (HfC, HfN and HfB2). Fracture model of these ceramics was the mixed inter/transgranular fracture model. When the additive content – HfN, HfC and HfB2 content - increased from 5 wt% to 15 wt%, the pinning effect of HfN particle in ZrO2-HfN ceramics as well as fine ZrO2 grains in ZrO2-HfC ceramics and the high bonding strength between the ZrO2 grain and HfB2 particle in ZrO2-HfB2 ceramics played a dominant role in improving the flexural strength and fracture toughness; meanwhile, the improved Vickers hardness of these ceramics was mainly attributed to the increase of hard additive content. However, when these additives content increased to 20 wt%, many big micro-voids in ZrO2-HfN and ZrO2-HfC ceramics and big ZrO2 grains in ZrO2-HfB2 ceramic produced negative effect on mechanical properties. In these ceramics, ZrO2-15 wt%HfB2 ceramic showed the optimum mechanical properties: flexural strength of 1093.06 ± 24 MPa, fracture toughness of 8.17 ± 0.18 MPa m1/2 and Vickers hardness of 14.37 ± 0.21 GPa.

Vacuum diffusion bonding (DB) of as-extruded Ti-47.5Al-Cr-V (at.%) alloy was carried out at 1000–1100 °C at a pressure of 10–30 MPa for 1–3 h, and the bonded joints were post-bond heat treated (PBHT) at 1360 °C for 0.5 h. Subsequently, the shear strength of all the joints was measured, and the microstructure was observed. The results showed that the shear strength and bonding interface quality of joints increased under as-bonded and PBHT conditions with the increase of each bonding parameter (temperature, pressure, and time). The shear strength of the base material was 486 MPa, and it was 665 MPa after PBHT. Furthermore, the maximum shear strength of the as-bonded joints was over 80% of that of the base material. After PBHT, the maximum strength of the joints was nearly equal to that of the base material. The fracture morphologies of the as-bonded and PBHT joints were analyzed, and the fracture mode was changed from transgranular fracture to intergranular fracture. The optimal bonding parameters were proposed: T ≥ 1050 °C, p ≥ 20 MPa, and t ≥ 2 h.

Mesoporous submicron α−Al2O3 green compacts were fabricated using slip casting of ultrafine alumina powders. The pre-sintered samples were sintered in air atmosphere at 1300 °C, 1350 °C, 1400 °C, and 1500 °C to obtain a variety of grain morphologies namely submicron equiaxed and micro rod structures. The resulting grain diameters lie between ∼ 270 nm and ∼ 1590 nm and total porosity fraction between 0.05% and 13%. The room temperature flexural strength (σ) evaluations and fractography analyses of sintered alumina samples were performed. It was observed that the total porosity fraction dictates the flexural strength as compared to grain diameter. Further, it was found that the flexural strength exhibited a decreasing trend for an increase in the total porosity fraction, and proved to be a better effective parameter than open porosity fraction. The fractography analyses suggest that samples sintered at 1300 °C and 1350 °C predominantly underwent intergranular fracture, while those sintered at 1400 °C and 1500 °C underwent a mixture of intergranular and transgranular fracture.

Hot salt stress corrosion cracking in Ti 6246 alloy has been investigated to elucidate the chemical mechanisms that occur. Cracking was found to initiate beneath salt particles in the presence of oxidation. The observed transgranular fracture was suspected to be due to hydrogen charging; XRD and high-resolution transmission electron microscopy detected the presence of hydrides that were precipitated on cooling. SEM-EDS showed oxygen enrichment near salt particles, alongside chlorine and sodium. Aluminium and zirconium were also involved in the oxidation reactions. The role of intermediate corrosion products such as Na 2 TiO 3 , Al 2 O 3 , ZrO 2 , TiCl 2 and TiH are discussed.

The effects of size distribution, morphology and volume fraction of γ′ phase and grain size on tensile properties of powder processed Ni-based superalloy were investigated by using two different quenching methods. Oil quenching and air cooling were adopted with cooling rate of 183 °C/s and 4–15 °C/s, respectively. The experimental results show that the average size of the secondary γ′ after oil quenching is 24.5 nm compared with 49.8 nm under air cooling, and corresponding volume fractions of γ′ are 29% and 34%, respectively. Meanwhile, the average grain size remains nearly equivalent from both oil-quenching and air-cooling specimens. The tensile strength at room temperature is higher for the oil-quenched specimen than the equivalent from the air-cooled specimen, but the difference approaches each other as the temperature increases to 650 °C. The fractography clearly demonstrates that transgranular fracture governs the failure process at ambient temperature, in contrast to the intergranular fracture at 650 °C or even higher temperature. These two mechanical responses indicate the strengthening effects of γ′ precipitates and grain boundary for polycrystalline Ni-based superalloys at different temperatures.

Edge-on impact experiments on AlON, a transparent ceramic with superior ballistic performance, show combined intergranular and transgranular fracture. An initial fast-propagating damage front transitions into slower-propagating localized cracks. We develop a peridynamic model for polycrystalline AlON to investigate the failure evolution observed in experiments. We use a computational polycrystalline structure with the same average grain size as the samples used in experiments. The peridynamic model helps explain the reasons behind the observed failure front supershear propagation speed (higher than 2 times the shear wave speed), and the subsequent transition to sub-Rayleigh propagating localized cracks. The computed propagation speeds for the damage front, and the cracks that emanate from it, match very well those measured in experiments. Elastic anisotropy, material microstructure, and brittle failure are the only ingredients used here to determine damage evolution in edge-on impact on brittle polycrystalline materials, under no confinement. Other possible dissipation mechanisms, like twinning, plasticity, friction, are not included in the model. These are likely second-order effects in the evolution of damage and failure for this material and at these impact speeds.

The design of metallic alloys with superior properties requires a deep understanding of the atomistic processes governing the materials behavior. In this paper, we present results providing a close link between grain boundary (GB) chemistry and fracture behavior in a Mo-Hf alloy. By combining atom probe tomography and ab-initio simulations of GBs, we unravel the origin for the transition between intergranular and transgranular fracture in technological Mo-Hf alloys. The main agent affecting GB strength is not the primary alloying element Hf, but rather the impurities O, C and B. With larger Hf additions, an intricate interplay between segregation and precipitation leads to a strong GB enrichment of C and B and to a depletion of O and Hf resulting in a higher cohesion of the GBs and thus, leading to a change in fracture mode. Our investigation exemplarily demonstrates that smallest additions of solutes can be decisive for understanding fracture behavior on the macroscale. Keywords: Atom probe tomography, Density functional theory, Grain boundary engineering, Intergranular failure, Molybdenum

With BN and graphite as weak interfacial phases, ZrB2-SiC/BN (LZrB) and ZrB2-SiC/graphite (LZrG) laminated ceramics were prepared by tape casting and hot pressing. The fracture behaviors of the above two laminated ceramics were investigated at room temperature and high temperature. At room temperature, the flexural strengths of LZrB and LZrG ceramics were 381 ± 21 MPa and 414 ± 17 MPa, respectively. When heated at 1300 °C and 1500 °C, the flexural strength of LZrG ceramic was higher than that of the LZrB ceramic. When heated at 1500 °C for 2 h, flexural strength of the LZrG ceramic was as high as 352 MPa, suggesting typical multiple fractures. Such excellent high temperature flexural strength of LZrG ceramic was attributed to the healing of surface flaws by the glassy phase SiO2 after oxidation, the viscous bridging of ZrO2 and SiO2, as well as the transgranular fracture mode of ZrB2 grains in ZrB2-SiC layers.

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

The influence of pre-stretching and aging treatment on the mechanical properties and corrosion behavior of 7050 alloy were studied by tensile testing, potentiodynamic polarization, and intergranular corrosion (IGC) testing associated with optical microscopy (OM), scanning electron microscopy, and transmission electron microscopy. The results exhibit both strength and elongation of T6 are higher than T74. The strength of retrogression and re-aging (RRA) samples are equal to or even in excess of T6 sample, especially, the strength of RRA-60 sample increased by 5.7% at the sacrifice of its elongation. Both yield strength of T651 and RRA51-60 samples with 4% pre-stretching are enhanced while their elongation decreased too much. The fracture feature of T6 and T651 samples are majorly intergranular fracture, while the fracture mode of RRA samples are mainly transgranular fracture. The corrosion resistance of T74 could be significantly improved through IGC tests and electrochemical polarization tests. Al...