脆性材料断裂韧性测试及表现断裂韧性的估算

用２种测试技术测试了脆性材料的断裂韧韧性,一种是利用楔入法在悬臂梁试件预裂出自然裂纹;另一种是用研磨法在三点弯曲试件上作出微米级的尖缺口裂纹。分别测试了３种材料的断裂韧性,并用提出的模型分别评估了多晶和非晶脆性材料的断裂韧性。     

Properties and rapid consolidation of ultra-hard tungsten carbide

Extremely dense WC with a relative density of up to 99% was obtained within 3 min under a pressure of 80 MPa using the high frequency induction heating sintering method (HFIHS) method. The average grain size of the WC was about 87 nm. The advantage of this process is not only rapid densification to obtain a near theoretical density but also the prohibition of grain growth in nanostructured materials. The hardness and fracture toughness of the dense WC produced by the HFIHS were investigated.     

Notch effect of surface compression and the toughening of graded Al2O3/TiC/Ni materials

The fracture behavior of graded Al₂O₃/TiC/Ni materials with a symmetric structure was investigated using single-edge notch-bend (SENB) specimens with surface compression. The fracture toughness of the graded materials was determined according to ASTM Standard E399. The results show that the effective fracture toughness increases with an increase in notch depth in the compressive stress zone, and reaches the maximum of 39.2 MPa m^1/2 at the interface of compressive/tensile stress zones. Finite element analysis reveals that the surface compression will be intensified at the notch root once the specimen is edge-notched because of the stress concentration, and the degree of the compressive stress intensification increases with an increase in notch depth. The dependence of the effective fracture toughness of the graded materials on the notch depth shows a behavior similar to the R-curve that is usually associated with microstructural toughening mechanisms. This toughening behavior is caused by the intensification of the compressive stress concentration with the increase of the notch depth. A theoretical analysis based on fracture mechanics verifies that the mechanical reliability of brittle ceramics can be improved effectively by tailoring and controlling the internal stresses.     

High-frequency induction-heated consolidation of nanostructured WC and WC-Al hard materials and their mechanical properties

The rapid sintering of nanostructured WC-Al composites in a short time was investigated with a focus on the mechanical properties (hardness and fracture toughness) and consolidation using high-frequency induction heated sintering. This process allowed very quick densification to near theoretical density and prohibited grain growth in the nano-materials. The addition of Al to WC facilitated consolidation and improved fracture toughness. The hardness and fracture toughness of WC with 5 vol.% Al and WC with 10 vol.% Al composites were higher than those of monolithic WC.     

Predictive modeling of transition undeformed chip thickness in ductile-regime micro-machining of single crystal brittle materials

This paper proposes a predictive model to determine the undeformed chip thickness in micro-machining of single crystal brittle materials, where the mode of chip formation transitions from the ductile to the brittle regime. The comprehensive model includes a force model considering the rounded tool edge radius effect and ploughing. Irwin's model for computing the stress intensity factor is adopted here as it gives a relation between the stress intensity and applied normal stress including effects of crack size and crack inclination. The occurrence of plastic deformation is built upon the condition that the shear stress in the chip formation region must be greater than the critical shear stress for chip formation and the stress intensity factor must be less than the fracture toughness of the material. The point of transition takes place when the fracture toughness is equal to the stress intensity factor. The above conditions form the theoretical basis for the proposed model in determining the transition undeformed chip thickness. End-turning experiments have been conducted using a single crystal diamond cutting tool on (1 1 1) single crystal silicon, and the results compared to the model predictions for validation. The proposed model would support the determination of the cutting conditions for the micro-machining of a brittle material in ductile manner without resorting to trial and error.     

Fracture of metallic glasses at notches: Effects of notch-root radius and the ratio of the elastic shear modulus to the bulk modulus on toughness

The Anand–Su theory for large elastic–plastic deformations of metallic glasses is modified to account for the strongly nonlinear and eventually softening dilatational volumetric elastic response of these materials. Using this theory, we have conducted finite-element simulations of fracture initiation at notch tips in a representative metallic glass under Mode-I, plane-strain, small-scale-yielding conditions. We show that our theory predicts three important experimentally observed phenomena: (a) fracture initiates ahead of the notch root, where the mean normal stress reaches a maximum value; (b) the fracture toughness increases linearly with the square-root of the notch-tip radius; and (c) the fracture toughness decreases as the ratio of the elastic shear modulus to the bulk modulus increases.     

Microstructure and mechanical properties of hot-pressed WC–MgO composites with Cr3C2 or VC addition

The influence of Cr₃C₂ and VC addition on the microstructure and mechanical properties of WC–MgO composites hot-pressed at 1650 °C for 90 min was comprehensively investigated. The grain growth of WC was significantly retarded and the homogeneity of MgO particulate dispersion was effectively improved with the addition of 0.5 wt.% Cr₃C₂ or 0.5 wt.% VC. The indentation size effect (ISE) on hardness was restrained and the load-independent hardness was increased by doping grain growth inhibitors. Improvements on fracture toughness of hot-pressed samples were also observed due to the refined WC grains and uniformly dispersed MgO particulates. In addition, experimental results demonstrated that Niihara's equation was preferable for estimating the indentation fracture toughness, by comparing the fracture toughness evaluated using the single-edge V-notch beam (SEVNB) method with the values estimated through the Vickers indentation technique.     

Fracture mechanism of Ni3Al alloys and their composites with ceramic particles at elevated temperatures

Ni₃Al alloys and their matrix composites reinforced with fine ceramic particles have been successfully fabricated by reactive hot-pressing. This paper investigates the embrittlement mechanism of these materials at intermediate temperatures using a mechanical fracturing technique, i.e. a single edge chevron-notched beam method with variation of loading rate. In the case of monolithic alloys, extrinsic embrittlement originating from diffusion of atomic oxygen into plastic deformation zone coincides with the inherent brittleness connected with deterioration of grain boundary cohesion and unique dislocation motion at 673–1073 K. Oxygen embrittlement predominates over other mechanisms at 673 K, because significant loading rate dependence of fracture toughness is observed in air. The fracture toughness of the alloys intrinsically decreases at 873–1073 K. However, the mechanical behavior of their matrix composites is quite different, depending on the kind of reinforcement particles. Although the composites with TiN particles have high strength and ductility, their fracture toughness decreases at intermediate temperatures, in a similar manner to the monolithic alloys. The fracture toughness of TiC particle reinforced composites is exceptionally constant between 300 and 900 K.     

High temperature mechanical properties of WC–Co hard metals

Understanding of the load situation and consequently the lifetime of cutting tools made of WC–Co hard metal requires quantitative data for thermo-mechanical properties. For the elevated temperatures present in application, these data are currently rather rare. The present work does discuss elastic material properties up to 1100 °C and compressive yield strength up to 900 °C, both as a function of Co content. The fracture toughness was determined as a function of the WC grain size and Co content up to 800 °C. Young's modulus and yield strength decrease with increasing temperature. A significant rise in fracture toughness was observed at 800 °C with increasing Co content and decreasing WC grain size. A possible reason for this increase is an increase in the plastic zone size at elevated temperatures.     

On the toughness enhancement in hydroxyapatite-based composites

Among various biologically compatible materials, hydroxyapatite (HA) has excellent bioactivity/osteointegration properties and therefore has been extensively investigated for biomedical applications. However, its inferior fracture toughness limits the wider applications of monolithic HA as a load-bearing implant. To this end, HA-based biocomposites have been developed to improve their mechanical properties (toughness and strength) without compromising biocompatibility. Despite significant efforts over last few decades, the toughness of HA-based composites could not be enhanced beyond 1.5–2 MPa m^1/2, even when measured using indentation techniques. In this perspective, the present work demonstrates how spark plasma sintering can be effectively utilized to develop hydroxyapatite–titanium (HA–Ti) composites with varying amounts of Ti (5, 10 and 20 wt.%) with extremely high single edge V-notch beam fracture toughness (4–5 MPa m^1/2) along with a good combination of elastic modulus and flexural strength. Despite predominant retention of HA and Ti, the combination of critical analysis of X-ray diffraction and transmission electron microscopy investigation confirmed the formation of the CaTi₄(PO₄)₆ phase with nanoscale morphology at the HA/Ti interface and the formation of such a phase has been discussed in reference to possible sintering reactions. The variations in the measured fracture toughness and work of fracture with Ti addition to the HA matrix were further rationalized using the analytical models of crack bridging as well as on the basis of the additional contribution from crack deflection. The present work opens up the opportunity to further enhance the toughness beyond 5 MPa m^1/2 by microstructural designing with the desired combination of toughening phases.     

Microstructural and mechanical properties of cBN-Si composites prepared from the high pressure infiltration method

The grain size dependence of the mechanical properties of cBN-Si composites prepared using the high pressure infiltration method has been investigated. Indentation testing indicates that cBN-Si composites have hardness values of 38–43 GPa, which increase with increasing grain size and are harder than traditional polycrystalline cBN composites (PcBNs). Thermostability analyses display that cBN-Si composites with a grain size of > 9 μm also possess a higher temperature of oxidation, compared to traditional PcBNs, and the thermostability increases with increasing cBN grain size. Fracture toughness tests show that almost no cracks appear on the polished cBN-Si samples when the loading forces are increased to 294 N and the fracture toughness is better than for commercial samples. Scanning electron microscopy illustrates that deformations and close pores occurred easily between coarse BN grains, leading to denser cBN-Si compacts with better mechanical performances.     

A wedge fracture toughness test for intermediate toughness materials: Application to brazed joints

A fracture toughness test for intermediate toughness materials is developed. The test configuration is a wedge-driven double cantilever beam, with design guided by analytical solutions for the energy release rate and compliance. Actual toughness measurements require finite element methods. To promote crack stability, a pre-cracking fixture is employed. The method is illustrated for a brazed joint. Measurements of the fracture resistance used both fractographic and compliance methods to ascertain crack length. The ensuing fracture resistance, Γ_R ≈ 1 kJ m⁻², is significantly greater than that for the intermetallic constituents. Approximately half of the toughening is attributed to plastic stretch of the ductile phase within the eutectic. The remainder is attributed to dissipation within a plastic zone that forms in the primary γ-Ni regions. A rationale for improving toughness is presented.     

Enhancement of the fracture toughness of bulk L12-based (Al+12.5 at.% M)3Zr (M=Cu,Mn) intermetallics synthesized by mechanical alloying

The microstructural evolution and the mechanical properties of L1₂-type bulk (Al+12.5 at.% M)₃Zr (M=Cu, Mn) intermetallic compounds with a nanocrystalline structure were investigated. The (Al+12.5 at.% M)₃Zr (M=Cu, Mn) powders synthesized by planetary ball milling (PBM) could be successfully consolidated into nearly pore-free bulk compacts at 580 and 620 °C without taking holding time by spark plasma sintering (SPS). Their grain sizes were in the range from 8 to 10 nm. The micro-hardness of the SPS-processed bulks was measured to be 975.8 and 983.9 Hv, respectively. On the other hand, their fracture toughness was barely ∼2 MPam^1/2. It was lower than those (∼4–6 MPam^1/2) of the coarse-grained (∼100 nm) bulk specimens annealed. This result indicates that a grain refinement towards the nanoscale does not have an appreciable effect on improving fracture toughness in brittle intermetallics. Thus, it was found that the fracture toughness could be enhanced by proper annealing and addition of the boron. Furthermore, the effect of grain size on the fracture toughness in nano-sized level was investigated in the bulk specimen prepared by arc melting, using mechanical alloying powders with ball-milling.     

Enhanced properties of nanostructured TiO<Subscript>2</Subscript>-graphene composites by rapid sintering

Despite of many attractive properties of TiO₂, the drawback of TiO₂ ceramic is low fracture toughness for widely industrial application. The method to improve the fracture toughness and hardness has been reported by addition of reinforcing phase to fabricate a nanostructured composite. In this regard, graphene has been evaluated as an ideal second phase in ceramics. Nearly full density of nanostructured TiO₂-graphene composite was achieved within one min using pulsed current activated sintering. The effect of graphene on microstructure, fracture toughness and hardness of TiO₂-graphene composite was evaluated using Vickers hardness tester and field emission scanning electron microscopy. The grain size of TiO₂ in the TiO₂-x vol% (x = 0, 1, 3, and 5) graphene composite was greatly reduced with increase in addition of graphene. Both hardness and fracture toughness of TiO₂-graphene composites simultaneously increased in the addition of graphene.     

Mechanical properties and rapid consolidation of nanostructured WC and WC–Al2O3 composites by high-frequency induction-heated sintering

The short-term rapid sintering of nanostructured WC and WC–Al₂O₃ hard materials was fabricated using the high-frequency induction-heating sintering (HFIHS) process. The sintering behaviors, microstructure, and mechanical properties of the WC and WC–Al₂O₃ composites were investigated. The addition of Al₂O₃ to WC can facilitate sintering, and the grain size of WC decreases as the addition of Al₂O₃ is increased; furthermore, the hardness and fracture toughness of WC-15 vol% Al₂O₃ are greater than those of monolithic WC and Al₂O₃.     

Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano- and nano-structured WC-Co cemented carbides

In this paper the influence of the consolidation process and sintering temperature on the properties of near nano- and nano-structured cemented carbides was researched. Samples were consolidated from a WC 9-Co mixture by two different powder metallurgy processes; conventional sintering in hydrogen and the sinter-HIP process. Two WC powders with different grain growth inhibitors were selected for the research. Both WC powders used were near nanoscaled and had a grain size of 150 nm and a specific surface area of 2.5 m²/g. Special emphasis was placed on microstructure and mechanical properties; hardness and fracture toughness of sintered samples. Consolidated samples are characterised by different microstructural and mechanical properties with respect to the sintering temperature, the consolidation process used and grain growth inhibitors in starting powders. Increasing sintering temperature leads to microstructure irregularities and inferior hardness, especially for samples sintered in hydrogen. The addition of Cr₃C₂ in the starting powder reduced a carbide grain growth during sintering, improved microstructural characteristics, increased Vickers hardness and fracture toughness. The relationship between hardness and fracture toughness is not linear. Palmqvist toughness does not change with regard to sintering temperature or the change of Vickers hardness.     

Wear-resistance and hardness: Are they directly related for nanostructured hard materials?

The major challenge in the field of cemented carbides and other hard materials is to obtain their better combination of hardness, wear-resistance and fracture toughness. It is well known that the dependence of abrasion wear on fracture toughness for WC–Co cemented carbides is represented by a relatively narrow band and it is hardly possible to “break away” out from it by the use of conventional approaches based on varying the WC mean grain size and Co content. Also, it is well known that the wear-resistance of conventional cemented carbides depends mainly on their hardness. The major objective of this paper is to establish what will happen with the wear-resistance of hard materials as a result of their nanostructuring when the hardness is nearly the same as for conventional WC–Co cemented carbides. The results obtained provide clear evidence that, if one enters the region of nanostructured materials with the mean grain size of less than 10 nm, traditional wisdom indicating that the wear-resistance is directly related to the hardness appears not to be valid. In some cases of such nanostructured materials, it can be possible to achieve the dramatically improved wear-resistance compared to that of conventional WC–Co cemented carbides at nearly the same level of hardness and fracture toughness. The abovementioned is based on considering hard nanomaterials of the following four types: (1) WC–Co cemented carbides with nanograin reinforced binder, (2) near-nano WC–Co cemented carbides, (3) cemented carbides of the W–C–Cr–Si–Fe system for hard-facing having a nanostructured Fe-based binder, and (4) CVD hard materials consisting of nanostructured W₂C grains embedded in a tungsten metal binder.     

Compressive plasticity and toughness of a Ti-based bulk metallic glass

The effects of changes in sample dimensions on the compressive plasticity and fracture toughness/energy have been determined for Ti₄₀Zr₂₅Cu₁₂Ni₃Be₂₀ bulk metallic glass (BMG). Changes in sample dimensions alone produced widely different values for compressive plasticity. While these were correlated with changes to the measured Poisson’s ratio for samples of the same composition cast into different sample sizes, significant effects of testing conditions apart from the Poisson’s ratio on the measured compressive plasticity are also demonstrated. Fracture toughness testing on the same material/size that exhibited zero compressive plasticity produced both notch and fatigue pre-cracked toughness in excess of 100 MPa m^1/2, while a size effect on the magnitude of toughness was similarly demonstrated. Discussions on the source(s) of the size effect on toughness are provided in addition to demonstrating that exceptional toughness can be obtained in this BMG which exhibits essentially zero compressive plasticity under certain test conditions. The apparent critical Poisson’s ratio for plasticity/toughness is thus different in these very different types of tests.