Microstructural Toughening Research Articles

Effect of densification technology on the microstructure and mechanical properties of high-entropy (Ti, Zr, Hf, Nb, Ta)C ceramic-based cermets

High-entropy ceramic-based cermets represent a new and promising direction in improving the mechanical properties of conventional hardmetals through the formation of complex microstructures during synthesis. This has been systematically studied in two Co-free, high-entropy (Ti,Zr,Hf,Nb,Ta)C ceramic-based cermets using 10 wt% Ni and 10 wt% FeCrAl metallic binders during hot-press and spark plasma sintering. Fully densified microstructures were achieved in the temperature range of 1400–1500 °C, which is below the melting points of the pure Ni and FeCrAl alloy, owing to the liquid-phase assisted sintering. The optimal densification routes resulted in Vickers hardness (HV30) of 16.77 ± 0.72 and 18.32 ± 0.99 GPa, and fracture toughness (KIc_SENB) of 5.31 ± 0.41 and 4.83 ± 0.50 MPa m0.5, respectively for the Ni and FeCrAl bonded cermets. The improved damage tolerance of these cermets compared to the base (Ti,Zr,Hf,Nb,Ta)C high-entropy carbide is related to the reduced grain size and microstructural toughening mechanisms (e.g. crack deflection and bridging).

Micro/nano-reinforcements in bimodal-grained matrix: A heterostructure strategy for toughening particulate reinforced metal matrix composites

To overcome strength-ductility trade-off in particulate metal matrix composites (PRMMCs), we developed a novel and controllable strategy, for the first time, via a combination of micro/nano-reinforcements and ultrafine/coarse-grained (UFG/CG) matrices. A powder assembly was developed to fabricate micro B4Cp/6061Al composites, which features UFG/CG matrices containing ex/in-situ nano dispersoids and exhibits superior combination of strength and ductility. It was found that such a novel architecture possesses both intrinsic and extrinsic toughening. As the intrinsic toughening mechanisms, enhanced dislocation storage capability of UFG regions, twinning, and hetero-deformation induced toughening were detected. Moreover, crack-deflection induced by nano dispersoids as well as crack-blunting at UFG/CG regions were considered as the main extrinsic toughening mechanisms. The proposed strategy can be applied to design the architecture of other PRMMCs with both high strength and high ductility. Aluminum alloys; Powder processing; Heterostructures; Particulate reinforced composites; Microstructural toughening

Physico‐mechanical, rheological and gas barrier properties of organoclay and inorganic phyllosilicate reinforced thermoplastic films

AbstractInsertion of 2:1 organo‐modified phyllosilicate tactoids into rheologically tough thermoplastics has extraordinary potential candidate in oxygen permeability and microstructural toughening. Herein, two commercially abundant clays have been taken for improvement of the thermoplastic's gas barrier property in reasonably low loading. The cause of low loading has been accounted to the usage of maleated polyethylene (MA‐g‐PE) during the melt mixing tenure. The optimized nanocomposite compression molded film has been tested against uniaxial stretching, which showed a negligible change in the residual permanent set with sacrificing the elongation at break feature. Moreover, nanoindentation was also performed to get the hardness of the sample surface. The flow behavior of the nanocomposites showed thixotropic likely with increasing the frequency. Oxygen transmission rate (OTR) has significantly decreased for tallow amine‐modified nanoclay system (cloisite 15A) in comparison to cloisite Na+ providing 'tortoise path' formation inside the matrix. Thus, hitherto, the work could demonstrate and provide the information of comparative studies between organo‐clay and simple phyllosilicates, which could be remediation of the loopholes in mechanical toughening and gas barrier lineaments.

Microstructural toughening mechanisms in nanostructured Al2O3/GdAlO3 eutectic composite studied using in situ microscale fracture experiments

This work uses in situ transmission electron microscopy (TEM) based micromechanical testing to isolate and quantify the microstructural toughening mechanisms active in nanostructured Al2O3/GdAlO3 eutectic samples. The effect of fracture direction across orthogonal sections of the rod-like eutectic was used to reveal the influence of different fracture paths and mechanisms on toughening. The average fracture toughnesses of the rod-like structures in the longitudinal cross-section and transverse cross-section are 2.4 MPa·m1/2 and 2.7 MPa·m1/2, respectively. Multiple samples tested in the longitudinal cross-section show significant R-curve toughening response, and obtain values greatly exceeding the initial values upon crack extension. It is concluded that nanoscale crack bridging induces deflection of the crack path, which leads extrinsic energy dissipation as the crack opens. Micropillar compressions are also performed to investigate the composite’s strength. Sample orientation strongly affects the deformation mode and interfacial sliding occurs when the maximal shear stress is parallel to the interface.

Carbon influence on fracture toughness of niobium carbides

This paper explores the fracture behavior of niobium carbides of varying compositions between NbC1.0 and NbC0.5. The surface crack in flexure (SCF) method was used to evaluate the fracture toughness as a function of carbon concentration. Additionally, hardness measurements were conducted with a Knoop indenter, and X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to identify the phase content and microstructures. As the carbon content decreased, the hardness increased from 8 GPa for NbC1.0 to 12 GPa for NbC0.5 and the fracture toughness decreased from 2.5 MPa m to 0.44 MPa m. Notably, the NbC0.67 sample exhibited a secondary precipitate lath-like microstructure with the laths indexed to β-Nb2C and a KIC near 2 MPa m. Though similar lath like structures in tantalum carbides have been reported to yield a KIC of approximately 15 MPa m, the laths in these two materials have fundamentally different structures where bonding in the former is comprised of β-Nb2C and the latter of ζ-Ta4C3-x. This results in the observed different fracture properties, which can be explained through concepts of microstructural toughening.

Strength and fracture mechanism of iron reinforced tricalcium phosphate cermet fabricated by spark plasma sintering.

The present work studies the microstructure and mechanical performance of tricalcium phosphate (TCP) based cermet toughened by iron particles. A novelty arises by the employment of spark plasma sintering for fabrication of the cermet. Results showed partial transformation of initial alpha TCP matrix to beta phase and the absence of oxidation of iron particles, as well as a lack of chemical reaction between TCP and iron components during sintering. The values of compressive and tensile strength of TCP/Fe cermet were 3.2 and 2.5 times, respectively, greater than those of monolithic TCP. Fracture analysis revealed the simultaneous action of crack-bridging and crack-deflection microstructural toughening mechanisms under compression. In contrast, under tension the reinforcing mechanism was only crack-bridging, being the reason for smaller increment of strength. Elastic properties of the cermet better matched values reported for human cortical bone. Thereby the new TCP/Fe cermet has potential for eventual use as a material for bone fractures fixation under load-bearing conditions.

Water effects on the deformation and fracture behaviors of the multi-scaled cellular fibrous bamboo.

The mechanical behaviors during the whole fracture process of bamboo were investigated by acoustic emission (AE). During the fracture process there were three kinds of fracture behaviors recognized by AE: matrix (parenchyma cells) failure, interfacial (fiber/fiber or fiber/parenchyma cell walls) dissociations and fiber breakage. The mechanical performance was greatly influenced by water contents (0%, 6% and 22%). Wet bamboos had higher fracture energy than the dry ones. There was more interfacial dissociation behaviors detected as more water was absorbed within the multi-scaled structures. The micro structural toughening mechanisms were strengthened by the macromolecular complexes and water molecules, which are working together and ensuring the excellent mechanical properties of the natural bamboo.

3D Crack‐tip Microscopy: Illuminating Micro‐Scale Effects on Crack‐Tip Behavior

AbstractTraditionally the driving force for crack growth is described in terms of global parameters which, for continua, nevertheless reflect the local conditions at the crack‐tip. Mimicking nature, materials microstructures are now being designed at the microscale, or even the nanoscale, employing interfaces and heterogeneities to shield the propagating crack for improved resistance to sub‐critical crack growth. In such cases global approaches simply will not do. Information is needed about intrinsic damage occurring ahead, and extrinsic shielding mechanisms behind, the crack. Here, high resolution X‐ray imaging and diffraction modes analogous to those in 2D electron microscopy are combined to form a 3D “crack‐tip microscope”. In this way one can quantify the effect of microstructural scale events on the crack‐tip environment. The technique is applied to study overload phenomena under fatigue, to characterize the bridging ligaments under stress corrosion cracking and fiber bridging during fatigue of a metal matrix composite. Besides identifying shielding mechanisms, it provides three methods for calculating the effective stress intensity at the crack tip; namely from the near tip stress field, from crack face tractions and from crack opening displacements. The wider opportunities opened up by this approach to study self‐healing, transformation toughening and other microstructural toughening mechanisms are also discussed.

Loading rate effects on the R-curve behavior of cortical bone

Rising resistance curve ( R-curve) behavior in bone during quasi-static experiments has demonstrated the importance of microstructural toughening mechanisms in resisting fracture. However, despite clinical bone fracture primarily occurring under dynamic loading and the significant changes in material behavior observed with increasing strain rates, there have been no previous investigations into whether crack growth resistance is maintained during dynamic fracture. Using a novel modified split-Hopkinson pressure bar coupled with a high-speed camera to measure crack propagation, we present the first evidence of rising R-curve behavior in bone under dynamic loading (∼2 × 10 5 MPa m 1/2 s −1). Results indicate that rising R-curve behavior is maintained, although with lower crack initiation toughness and propagation resistance than observed in quasi-static experiments. Observations of crack initiation and propagation in double-notched specimens using confocal fluorescence microscopy and electron microscopy suggest that this is due to subtle differences in toughening mechanisms between quasi-static and dynamic fracture.

Effects of age and loading rate on equine cortical bone failure

Although clinical bone fractures occur predominately under impact loading (as occurs during sporting accidents, falls, high-speed impacts or other catastrophic events), experimentally validated studies on the dynamic fracture behavior of bone, at the loading rates associated with such events, remain limited. In this study, a series of tests were performed on femoral specimens obtained post-mortem from equine donors ranging in age from 6 months to 28 years. Fracture toughness and compressive tests were performed under both quasi-static and dynamic loading conditions in order to determine the effects of loading rate and age on the mechanical behavior of the cortical bone. Fracture toughness experiments were performed using a four-point bending geometry on single and double-notch specimens in order to measure fracture toughness, as well as observe differences in crack initiation between dynamic and quasi-static experiments. Compressive properties were measured on bone loaded parallel and transverse to the osteonal growth direction. Fracture propagation was then analyzed using scanning electron and scanning confocal microscopy to observe the effects of microstructural toughening mechanisms at different strain rates. Specimens from each horse were also analyzed for dry, wet and mineral densities, as well as weight percent mineral, in order to investigate possible influences of composition on mechanical behavior. Results indicate that bone has a higher compressive strength, but lower fracture toughness when tested dynamically as compared to quasi-static experiments. Fracture toughness also tends to decrease with age when measured quasi-statically, but shows little change with age under dynamic loading conditions, where brittle “cleavage-like” fracture behavior dominates.

Influence of ceramic–metal interface adhesion on crack growth resistance of ZrO 2–Nb ceramic matrix composites

Yttria-stabilized zirconia strengthened with lamellar flaky-shape Nb metal particles was obtained by hot-pressing at 1500 °C for 1 h. The ZrO 2–Nb interface has been studied by atomistic, first-principles calculations and by high-resolution transmission electron microscopy. The influence of the ceramic–metal interface on the crack growth resistance has been investigated. Crack growth is shown to occur with a rising resistance, governed by intact metal ligaments in the crack wake. Crack extension occurs by a combination of plastic deformation on the metal particles and interface debonding. The connection between the interface adhesion and this microstructural toughening mechanism has been evaluated.

Plasma sprayed ceramic coatings without and with epoxy resin sealing treatment and their wear resistance

Plasma sprayed aluminum oxide-based coatings (Al 2O 3) are mainly used as a wear resistant surface covers in mechanical applications. Previous studies have shown that abrasion wear resistance of these coatings can be significantly improved by applying a sealing treatment. It was reported that this improvement is mainly due to microstructural toughening which takes place via the sealing process. Therefore, this study was conducted in order to determine the influence of the microstructural characteristics governed by the spray system settings and epoxy sealing on the wear resistance of these coatings. Unsealed and epoxy resin sealed plasma sprayed specimens prepared from alumina as well as alumina-13% titania were studied. Abrasion wear resistance of these coatings was evaluated employing the standard slurry abrasion response (SAR) and the standard dry sand rubber wheel (DSRW) abrasion tests. The coatings microstructure was characterized by light microscopy and their worn surfaces were analyzed using scanning electron microscopy. Porosity, microhardness and surface roughness of unsealed and sealed coatings were also compared. It was shown that the abrasion wear resistance of the epoxy resin sealed coatings is significantly better than that of the as-sprayed coatings for both types of applied tests.

Comparison between fatigue behavior of some ceramics: a new concept of intrinsic stress-corrosion exponent n0

The fatigue behavior was systematically investigated with three typical ceramic materials, including Y 2O 3–ZrO 2, Si 3N 4 and a machinable glass–ceramic. Cyclic, static and dynamic fatigue tests were conducted in three environments, i.e., moist air, distilled water and kerosene. The effects of environment and loading condition on fatigue behavior were analyzed. The difference between materials was discussed. Experimental results showed that, for all the three materials, fatigue life under cyclic load was the shortest and a low value of n (fatigue exponent) was obtained. Compared with the other materials, Si 3N 4 ceramic had a very large n value under static load. Therefore the static fatigue for Si 3N 4 ceramics may be ignored. Cyclic load decreases the fatigue life of transformation-toughening ceramics (Y–TZP) more seriously, while no remarkable difference was observed between cyclic and static fatigue for original glass of the glass–ceramic. A new idea about the physical meaning of n value and the concept of intrinsic stress-corrosion exponent n 0 were introduced. The exponent n can be separated into two terms, i.e., n= n 0+ n μ , where n 0 is a material constant and just related to the most basic properties, such as atom-bonds or crystalline structure, n μ reflects the contribution of microstructure toughening and is very sensitive to environments or loading conditions. This hypothesis can be used to describe and explain the experiment results successfully.

Notch effect of surface compression and the toughening of graded Al 2O 3/TiC/Ni materials

The fracture behavior of graded Al 2O 3/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.

Crack Healing and Stress Relaxation in Al2O3 SiC “Nanocomposites”

The crack‐healing behavior of A12O3 and Al2O3‐SiC nanocomposite was studied using Vickers indentations to generate precracks. After annealing in argon for 2 h at 1300°C, radial cracks in the nanocomposite healed: The cracks closed and there was a small degree of rebonding in the vicinity of the crack tip. In contrast, radial cracks in alumina grew when exposed to the same annealing treatment. The different responses are attributed to the fracture mode and toughening mechanism in each material: In the nanocomposite, the cracks close as the residual stresses surrounding the indentations relax. Radial cracks open and grow in A12O3 because microstructural toughening is diminished during heating to the annealing temperature. An implication is that strength‐limiting machining flaws in these materials behave similarly, thereby accounting for the strengthening effect of annealing in this “nanocomposite“ system.

Deformation mechanism and fibre toughening of nylon 6,6

The effects of glass-fibre reinforcement on the fracture toughness, K IC, of nylon 6,6 were examined and the deformation mechanisms of unreinforced nylon 6,6 were studied by varying the deformation rate, by dilatational measurements and by i.r. spectroscopy. In the unreinforced nylon 6,6 a flow stress plateau was observed in the stress-strain behaviour prior to the onset of necking. Of the 25–30% inelastic strains stored in this plateau a substantial portion appears to be related to a crystallographic plastic deformation due to the crystalline segments in nylon 6,6. In glass-fibre-reinforced nylon 6,6 a brittle to ductile transition was found to occur when the mean fibre-end spacing was less than a critical value. The observed brittle-ductile transition was found to originate from an observed enhanced matrix plasticity at fibre ends when the glass fibres are sufficiently closely spaced. Such enhanced localized plasticity at fibre ends was suggested to result from interactions of stress fields with nearby fibre ends when the fibre-end spacing is less than the critical value. It is further postulated that the enhanced localized fibre-end plasticity is made possible due to the ability of the matrix to exhibit a large degree of crystallographic plasticity. The toughening behaviour of fibre-reinforced nylon 6,6 was also compared qualitatively to that of rubber-toughened nylon 6,6 and a general principle for microstructural toughening in nylon 6,6 was addressed. Strategies for fibre toughening in fibre-reinforced nylon 6,6 were also discussed.

Microstructural toughening mechanisms in γ-TiAl · 20 pct TiCb as determined by FRASTA

The mechanisms of toughening in an intermetallic matrix composite were determined by FRASTA, a technique that reconstructs microfailure details of a fracture event by comparing the topographies of conjugate fracture surfaces. Graphic portrayals of the interaction of a crack front with microstructure features are presented for a forged gamma titanium aluminide reinforced with 20 vol pct, 300-µm-diameter, 30-µm-thick TiCb platelets. The threefold toughness increase relative to unreinforced material is attributed to (1) drag on the crack front by the TiCb platelets, (2) bridging of the crack faces by the TiCb platelets, and (3) deflection of the crack front by microcracks in the process zone that nucleate and grow out-of-plane at TiCb platelets. The fractographic results explain discrepancies between measured and predicted toughnesses, provide the mechanistic understanding and microfailure data needed to develop more reliable models, and suggest microstructural modifications for developing more fracture-resistant materials.

Crack Stability and T‐Curves Due to Macroscopic Residual Compressive Stress Profiles

Stabilization of the fracture process and resistance to strength degradation have been observed for materials with increasing T‐curves. In this study, the possibility of using residual compressive stresses to induce crack stabilization is examined theoretically. Nonmonotonic forms for the residual compressive stress profiles are assumed. The stress intensity factors for linear through‐the‐thickness cracks subjected to these profiles are derived. The stress intensity factors are then used to construct the T‐curves for the stress profiles considered. It is demonstrated that the presence of these T‐curves leads to crack stability under the action of applied tensile stresses, and to strength insensitivity to the initial flaw size. The effects of additional localized stress fields (similar to those produced by indentation) on crack growth in these materials are also considered. In this case, the strength is found to be relatively insensitive to the magnitude of the localized loading. It is therefore concluded that residual stresses can be used to improve mechanical reliability in ways which are usually associated with microstructural toughening mechanisms.