Toughening mechanisms for a zirconia-lithium aluminosilicate glass-ceramic

The mechanical properties of a lithium aluminosilicate glass-ceramic and the same glass-ceramic containing 5 and 15 wt% zirconia were investigated. The aim of the study was to assess the contributions to toughening from various toughening mechanisms. For the zirconia-containing compositions, zirconia initially precipitated, upon heat treatment of the glass, as tetragonal zirconia (t-ZrO₂), and upon further heat treatment, transformed to monoclinic zirconia (m-ZrO₂). This transformation could also be induced by grinding samples containing t-ZrO₂. By heat treating, the fracture toughness of all compositions increased with increasing matrix grain size until the matrix grain size exceeded ∼ 1 μm, whereupon both the fracture toughness and strength decreased sharply. The matrix phases, lithium metasilicate and β-eucryptite, have either high thermal expansion mismatch or high thermal expansion anisotropy resulting in large thermal stresses. The initial toughness increases observed in each composition were attributed to the formation of a microcrack zone around the propagating crack. At larger grain sizes, thermal stresses caused spontaneous cracking and loss of strength. Zirconia additions also contributed to the fracture toughness improvement; however, the predominant toughening mechanism was not by transformation but due to crack deflection by the stress fields around the transformed, i.e. m-ZrO₂, particles.     

Effect of notch root radius on fracture toughness of polycrystalline cubic boron nitride

The fracture toughness of five grades of polycrystalline cubic boron nitride (PCBN) has been determined using Single Edge V-Notched Beam specimens. Both coarse and fine grade materials were considered, containing CBN grain sizes of between 1 μm and 22 μm. The influence of notch root radius on the measured fracture toughness was examined. The notch root radius was found to have a major effect for materials with smaller CBN grain sizes while only a small effect was noted for the material with large CBN grain sizes. A simple analytical model was developed to explain the effect of the notch root radius on the fracture toughness and was found to agree well with experiment for all the materials tested. It was shown that the effect of notch root radius is directly linked to the size of the CBN grain. It is proposed that this effect results from the interaction between the microstructure and the stress field around the notch tip.     

The effect of heat treatments on the creep–rupture properties of a wrought Ni–Cr heat-resistant alloy at 973 K

The effect of heat treatments on the creep–rupture properties was investigated on a wrought Ni–Cr heat-resistant alloy at 973 K. Short-time aging (aging for 3.6 ks (1 h) at 973 K) was made on the solution-treated specimens with different grain sizes. The fine-grained specimen (the grain diameter, d = 45.2 μm) produced by short-time solution treatment exhibited almost the same rupture life and superior creep ductility as those of the medium-grained specimen (d = 108 μm) produced by normal solution treatment. The fine-grained specimen and medium-grained specimen showed the longer rupture life compared with the specimen with recommended aging. The principal strengthening of specimens was attributed to the precipitation hardening by γ′ phase particles. The fine-grained specimen had the highest hardness, and the increase of the hardness was observed in both the fine-grained and the medium-grained specimens during creep at 973 K. However, coarse-grained specimen (d = 286 μm) with high-temperature long-time solution treatment exhibited significantly short rupture life owing to insufficient precipitation hardening after the short-time aging and during creep. Ductile intergranular fracture with dimples occurred in the fine-grained specimen, while brittle intergranular fracture was observed in the medium-grained specimen and in the specimen with recommended aging. Both transgranular fracture and brittle intergranular fracture were observed in the coarse-grained specimen. A simple heat treatment composed of short-time solution treatment and short-time aging is applicable to high-temperature components of wrought Ni–Cr alloys.     

The effect of ceria co-doping on chemical stability and fracture toughness of Y-TZP

The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia polycrystals (Y, Ce-TZP) were evaluated as a function of grain size and ceria content. Very fine grained, fully dense materials could be produced by sinter forging at relatively low temperatures (1150–1200 °C). The ageing resistance in hot water (185 °C) of 2 mol% Y₂O₃-stabilized TZP is strongly enhanced by alloying with ceria. The ceria content necessary to avoid degradation completely, decreases with grain size. The toughness of fully dense Y, Ce-TZP is 7–9 MPa m^1/2 for grain sizes down to 0.2 m. No or very little transformation took place during fracturing and no clear variation with grain size was observed for the toughness at grain sizes up to 0.8 m. Reversible transformation and crack deflection may explain the observed toughness values.     

Micro- and macro-mechanical testing of transparent MgAl2O4 spinel

Advanced transparent ceramics with high chemical and thermal stability are gaining increasing interest as replacement of glass-based materials in technical window applications. The mechanical reliability and performance of transparent MgAl₂O₄ with a grain size of 5 μm has been characterized at ambient temperature using micro-mechanical indentation and macroscopic bending tests. The measurements focused on elastic modulus, fracture toughness, crack kinetics, and strength, the latter analyzed with Weibull statistics. The effect of slow crack growth is assessed using a strength–probability–time plot. Complementary fractography by optical, confocal and scanning electron microscopy provided a correlation between failure origin and fracture stress. The results and reliability aspects are discussed in terms of linear elastic fracture mechanics.     

Effect of test method and crack size on the fracture toughness of a chain-silicate glass-ceramic

The fracture toughness of a canasite glass-ceramic with a highly acicular, interlocked grain structure was measured by a number of different methods. The values at room temperature obtained by the chevron-notch, short-bar and notched-beam methods ranged from 4 to 5 M Pa m^–1/2, well above literature values for other glass-ceramics. Similar values of toughness were obtained by the fracture of bars with indentation cracks introduced with loads ranging from 1.96 to 400 N, but only for crack sizes >200 m, with lower values for cracks of smaller size. The toughness values obtained by the direct measurement of the size of the indentation cracks were appreciably lower than the values obtained by all other methods over the total range of indentation loads and corresponding crack size. SEM fractography showed that the surface within the indentation cracks was appreciably smoother than the surrounding fracture surface. The high values of fracture toughness were attributed to the combined mechanisms of crack-deflection and microcrack-toughening due to the stress-enhanced creation of microcracks caused by the residual stresses which arise from the thermal expansion anisotropy of the canasite monoclonic crystal structure. The strong negative temperature dependence of the fracture toughness suggests that at room temperature microcrack toughening represents the primary contributing mechanism to the fracture toughness. The combined effects of crack-deflection and microcrack-toughening can lead to the development of glass-ceramics with greatly improved resistance to crack propagation.     

Effect of grain morphology and grain size on the mechanical properties of Al2O3 ceramics

Bending strength, fracture toughness, fracture energy and crack extension resistance were evaluated for Al₂O₃ ceramics with equi-axed and platelet grains. Bending strength was proportional to grain size^–1/2, but flaws with a size of 10 m controlled the strength when the microstructure was finer than 10 m. Fracture toughness, measured by the single etched precracked beam (SERB) method, was proportional to fracture energy^1/2, and increased with the grain size of Al₂O₃ ceramics with equi-axed and platelet grains. However, the toughness of platelet grain ceramics was higher than the ceramics with equi-axed grains, and increased up to 6.6 MPam^1/2 with grain size. Therefore, it is thought that fracture toughness not only depends on grain size, but also on grain morphology; equations were derived to account for this phenomenon.     

The effects of texture and grain morphology on the fracture toughness and fatigue crack growth resistance of nanocrystalline platinum films

The fracture toughness and fatigue crack growth resistance of nanocrystalline materials are significantly affected by the thickness of the specimen. In this work we relate the mechanical properties of nanocrystalline platinum films to their texture and grain morphology. Tensile, creep and fatigue testing of annealed, ∼1 μm films resulted in mechanical properties similar to the as-received films (yield strength of ∼1.2 GPa, fracture toughness ∼17.8 MPa √m, and a fatigue crack growth power law exponent of ∼4.2). However, the breakdown of the initially columnar grain morphology had a marked effect on the transition point from an intergranular to transgranular fatigue cracking mode. Finite element modeling suggests that cyclic (fatigue) grain coarsening and the transition from inter- to transgranular cracking modes are a result of the relative importance of dislocation slip accommodation on in-plane and through-thickness oriented slip directions.     

Effect of twins and non-basal planes activated by equal channel angular rolling process on properties of AZ31 magnesium alloy

Ultrafine-grained (UFG) metallic materials because of their superior properties have received considerable research interest. Recently, severe plastic deformation (SPD) processes are widely used for refining the grain size in magnesium alloys. Equal channel angular rolling (ECAR) is a SPD process based on equal channel angular pressing (ECAP) which is carried out on large, thin sheets. After doing this process, no significant change is occurred in cross-sectional area of specimen. In this research, an AZ31 magnesium alloy was subjected to ECAR. After completing eight passes of process, significant grain refinement was occurred, and the average grain size of about 3.9 μm was achieved. The distribution of grain size becomes more limited by increasing number of passes. Rotation of basal plane and activation of non-basal and twin planes were clearly observed in X-ray diffraction (XRD) pattern results. Mechanical properties were studied via tensile and hardness tests at room temperature. Tension tests indicated that better ductility due to the rotation of basal plane was achieved. Elongation-to-failure was increased from 8% of as-received material to 19% after two passes of process. Hardness values showed an increase of about 53% at eighth pass.     

Toughening of thermoset polymers by rigid crystalline particles

The toughening of an aromatic amine-cured diglycidyl ether of bisphenol-A epoxy with particles of crystalline polymers was studied. The crystalline polymers were poly (butylene terephthalate), nylon 6, and poly(vinylidene fluoride). Nylon 6 and poly(vinylidene fluoride) were found to toughen the epoxy about as well as did an equivalent amount of CTBN rubber. Poly(butylene terephthalate) was found to toughen the epoxy about twice as well as did the rubber. The toughness of poly(butylene terephthalate)-epoxy blends was independent of particle size for sizes in the range of tens of micrometres, but the toughness of the nylon 6-epoxy blends decreased with increasing particle size for sizes smaller than about 40 μm. There was no loss of either Young's modulus or yield strength of the epoxy with the inclusion of either nylon 6 or poly(butylene terephthalate) and less loss of these with the inclusion of poly(vinylidene fluoride) than with the inclusion of rubber. Toughness seems to have arisen from a combination of mechanisms. The poly(butylene terephthalate)-epoxy blends alone seem to have gained toughness from phase-transformation toughening. Crack path alteration and the formation of steps and welts and secondary crack bridging seem to have accounted for an especially large part of the fracture energy of the poly(vinylidene fluoride)-epoxy blends. Secondary crack nucleation contributed to the toughness of the nylon 6-epoxy blends.     

Tensile and compressive test in nanocrystalline and ultrafine carbon steel

Plastic deformation behavior was investigated in near fully dense nanostructured and ultrafine-grained bulk samples of carbon steel (0.55 wt% C) under compression and tension tests. The specimens were obtained by hot pressure from mechanically milled powder at 400 and 500 °C. Subsequent heat treatments at temperatures going from 600 to 900 °C produced samples with ferrite grain sizes from 30 nm to 17 μm. Nanocrystalline grained steel samples presented very high strength with low ductility. Once, in the ultrafine range, as the ferritic grain size was increased, the strength was decreased and the ductility was improved. The porosity and carbon atoms within the structure were analyzed in order to explain the results of strength and strain obtained.     

Grain size effect on high-speed deformation of Hadfield steel

The influence of the microstructure on the tensile properties and fracture behavior of Hadfield steel at high strain rate were studied. Hadfield steel samples with different mean grain sizes and carbon phases were prepared by rolling at medium temperatures and subsequent annealing. A sample with an average grain size larger than 10 μm, and a small number of carbides shows ductility with local elongation (post uniform elongation) at a high-speed tensile deformation rate of 10³ s⁻¹. In addition, the fracture surface changes from brittle to ductile with increasing strain rate. In contrast, a fine-grained sample with carbides undergoes brittle fracture at any strain rate. The grain size dependence is discussed by considering the dynamic strain aging as well as the emission of dislocation from cracks. The accelerated diffusion of carbon due to grain refinement is considered as one of the important reason for brittle fracture in the fine-grained Hadfield steel.     

Effect of inclusion size on mechanical properties of alumina toughened cubic zirconia

The relationship between alumina inclusion size and mechanical properties of particulate cubic zirconia-alumina composites was studied. The composites of the diverse size and content of alumina inclusions and of the nearly constant size of zirconia grains were used. Physical mixtures of the 8 mol% Y₂O₃-ZrO₂ nano-powder and the γ-Al₂O₃ or α -Al₂O₃ micro-powder were cold isostatically pressed and then pressurelessly sintered for 2 h at 1300^∘C in air. The γ -Al₂O₃ and α -Al₂O₃ powder was composed of the particles of 0.17 and 0.36 μ m in size, respectively. Crystallites of the zirconia powder had the size of 6 nm. Microstructural features of the composites have been characterised quantitatively. Hardness, critical stress intensity factor and bending strength of the composites was measured and correlated with the microstructural features. Depending on the size and content, the alumina inclusions influenced strength of the composites by influencing their fracture toughness and the presence of flaws of critical size. An increase in size of the alumina inclusions was accompanied by the increase of fracture toughness due to the additional contribution of large alumina inclusions to the crack deflection mechanism. It was found that decreasing the alumina inclusion size significantly below the cubic zirconia matrix grain size (more than 3 times) did not lead to the increased values of fracture toughness of the composites. The highest increase in fracture toughness (up to 3.9 MPa⋅ m^0.5) has been found when the inclusion size was comparable to the matrix grain size.     

Fracture toughness correlation with microstructure and other mechanical properties in near-eutectoid steel

The variation of yield strength and fracture toughness was investigated for four different heat treatments attempted on specimens of a near-eutectoid steel. The aim of this study was to optimize the microstructure for simultaneous improvements in strength and toughness. Further, the fracture toughness deduced through empirical relations from tensile and charpy impact tests was compared with those measured directly according to ASTM Designation: E 399. Among the four different heat treatments attempted in this study, the plane strain condition was valid in the fracture toughness tests for (i) normalized and (ii) hardened and tempered (500°C for 1 h) treatments only. The latter of the two heat treatments resulted in simultaneous improvement of strength and plane strain fracture toughness. The finely-dispersed carbides seem to arrest the crack propagation and also increase the strength. The pearlitic microstructure of the former leads to easy crack propagation along cementite platelets and/or cementite/ferrite interfaces. The nature of variation of empirically determined toughness values from tensile tests for different heat treatments is similar to that measured directly through fracture toughness tests, although the two sets of values do not match quantitatively. On the other hand, the toughness data deduced from charpy impact test is in close agreement with that evaluated directly from fracture toughness tests.     

Mechanical properties of ultra-fine grained zirconia ceramics

The mechanical properties of tetragonal zirconia (TZP) materials doped with Y, Ce or Ti were studied as a function of temperature and grain size. Fine grained Y-TZP (grain size < 0.3 m) shows values for fracture toughness and strength at room temperature, which are comparable with the coarse grained transformation toughened materials, despite lacking transformation toughening. The morphology of the fracture surface points to crack deflection as the most important toughening mechanism. At 800 °C fracture toughness and strength are higher than in coarse grained Y-TZP materials. Doping Y-TZP with Ce or Ti results in a similar trend in mechanical properties, for fine grained material, as for the Y-TZP materials.     

Notch sensitivity and brittleness in fracture testing of S2 columnar freshwater ice

Two loading configurations (four-point-bend, three-point-bend) were used in the laboratory at Clarkson to test for the fracture toughness of carefully grown S2 columnar freshwater ice. For one specific crack orientation and one grain size, the crack length was varied ranging from very short to very deep. The crack length effects were studied in this way for three specimen sizes (the in-plane dimensions of the specimen size were geometrically scaled; the specimen thickness was essentially constant). These crack length and specimen size tests are primarily directed towards designing fracture toughness tests for ice that both satisfy small scale yielding requirements and provide material properties (in the sense of (1)) — toughness values independent of the size and geometry of the specimen. Considerations of sufficient notch sensitivity in terms of brittleness numbers provide a means to determine the necessary specimen size. The results reported in this paper suggest that the specimen sizes used in testing S2 ice to date have largely been sub-size.     

Improving the fracture toughness of MgO–Al2O3–SiO2 glass/molybdenum composites by the microdispersion of flaky molybdenum particles

The flake-forming behaviour of powders of molybdenum, niobium, nickel, BS 316 S 12, Ni–17Cr–6Al–0.6Y, iron, titanium and Ti–6Al–4V, using a wet ball mill, was investigated. MgO–Al₂O₃–SiO₂ (MAS) glass composites reinforced with these flaked particles were fabricated, and improvements in flexural strength evaluated. The MAS glass composites reinforced with flaky metallic particles such as molybdenum, niobium, iron, nickel and Ni–17Cr–6Al–0.6Y, showed an improvement. The effect of molybdenum particle size on the flake-forming behaviour of molybdenum, flexural strength and fracture toughness of MAS glass/molybdenum composites, were investigated. The flake-forming behaviour shows a high degree of dependence on molybdenum particle size and, upto a size of 32 μm, becomes conspicuous with increasing particle size. At 32 μm, the aspect ratio reaches a value of 17 and, above 32 μm, flake forming saturates. Fracture toughness is closely related to flake-forming behaviour and the more marked the flake forming, the greater is the increase in fracture toughness. A composite of MAS glass with flaky molybdenum particles has a greater improvement effect on fracture toughness than composites with SiC whiskers, SiC platelets or ZrO₂ particles. This is closely linked to plastic deformation of the flaky metallic particles at the crack tip at the time of fracture. This revised version was published online in November 2006 with corrections to the Cover Date.     

The effect of the grain size of the ferritic-pearlitic steel 45 on the parameters of its fracture under impact loading

The article investigates the influence of the grain size of the ferritic-pearlitic steel 45 with standard composition in the range 2–50 Μm on the parameters of the energy of crack nucleation and propagation in the process of impact loading in the temperature range from ?100 to +150?C. It is shown that reduction of the grain size from 50 to 2 Μm has no effect on the fracture energy and on the “true” specific values of impact toughness at temperatures inducing completely brittle or ductile fracture, and that it increases the fracture energy and KCV_tru solely in the region of temperatures of the viscobrittle transition. Dissipation of the fracture energy expanded on the nucleation of secondary cracks and microcracks has no significant effect on the KCV_tru of the steel in all the examined ranges of grain sizes and temperatures. The article notes the structural insensitivity of the process of crack and microcrack nucleation in steel 45 under dynamic loading.     

High temperature fracture of boron carbide: experiments and simple theoretical models

The mechanical properties of theoretically dense boron carbide with a grain size of about 10μm have been investigated as a function of temperature. It was found that the fracture toughness remained constant at ∼3.7 MPa m^1/2 up to 1500 K and there was little or no decrease in strength from its room temperature value of ∼350 MPa. Both the order of magnitude and the temperature dependence of the fracture energy, calculated from the fracture toughness and elastic data, can be explained in terms of simple theoretical models.     

Abnormal grain growth enhanced densification of liquid phase-sintered WC–Co in support of the pore filling theory

This study examines the effect of grain growth on densification during liquid phase sintering of compacts with faceted grains. Two kinds of WC powders with different sizes were used to produce WC–Co alloys. Large pores of ~5 μm size were generated in 95WC–5Co (wt%) using spherical Co particles of the same size. The overall sintering behavior was observed by measuring grain growth and densification as a function of sintering time at a sintering temperature of 1350 °C. When the WC powder was fine (0.4 μm), large pores disappeared upon filling of pores by liquid with the formation of abnormal grains. On the contrary, when the WC powder was large (4.2 μm), grain growth is not observed, and large pores remained intact even after a long period of sintering (24 h). These observations confirm that densification during final stage liquid phase sintering occurs via filling of pores by liquid as a result of grain growth. This finding is consistent with the model of densification predicted by the pore filling theory.