Enhanced Crack Propagation Resistance Research Articles

Fabrication of carbon fiber reinforced SiC composites based on laser directed energy deposition

Carbon fiber reinforced silicon carbide ceramic matrix composites (CF/SiC) are widely applied in aerospace and other industries fields for its extraordinary properties. Nevertheless, traditional preparation processes encounter the challenges of long cyclicality, high economic cost due to the increasing requests. Consequently, this work integrates the CF/SiC composites with the novel laser directed energy deposition (L-DED) printing technology, which is differing from conventional 3D printing technology for it omits the post-processing procedure. CF/SiC composites are designed and manufactured efficiently, and the effects of carbon fiber addition are analyzed. Mechanical properties are utilized to characterize the internal bonding strength and the results reveal that the maximum flexural and compressive reach 119.33 ± 4.04 MPa and 396.20 ± 10.21 MPa, respectively, attributed to the enhanced crack propagation resistance caused by the carbon fiber addition. At the same time, this paper not only characterizes the performance through objective experimental data, but also expounds the formation mechanism of pores and cracks through material analysis. In conclusion, this work pioneers the application of L-DED molding technology to CF/SiC composites fabrication and validated its preparation capability, which not only improves the efficiency but also opens up a new path for CF/SiC composites preparation in the field of 3D printing.

Enhanced crack propagation resistance after healing treatment by addition of boron in FeNi-base superalloy

The objective of the present study was to investigate whether the crack propagation resistance of boron-containing FeNi-based superalloy could be enhanced by healing treatment. The healing treatment was performed by pre-straining and subsequent heat treatment, resulting in a pronounced boron segregation zone at the interface. Formation of boron segregation zone reinforced the crack propagation path, thereby effectively enhancing the mechanical properties of FeNi-based superalloy. The tested alloys were prepared by hot-rolling followed by aging treatment, resulting in precipitation of γ′-Ni3Ti and γ''-Ni3Nb embedded in the γ matrix. After pre-straining and healing heat treatment, η-Ni3Ti phase formed at the grain boundary, leading to the formation of a boron segregation zone at the interface between the η-Ni3Ti phase and γ matrix. Both strength and ductility were improved with increasing boron content from 0 at% to 0.2 at% after healing treatment. The spontaneous segregation of boron at the precipitate-matrix interface played a role in enhancing both strength and ductility by interface strengthening. Moreover, it was revealed that a critical amount of boron segregation is required for interface strengthening. Dislocation density analysis revealed that the recovery effect was weaker in alloys with boron than in those without it, leading to a more pronounced work-hardening effect.

Fabrication of bamboo-inspired continuous carbon fiber-reinforced SiC composites via dual-material thermally assisted extrusion-based 3D printing

Ceramic matrix composites (CMCs) structural components encounter the dual challenges of severe mechanical conditions and complex electromagnetic environments due to the increasing demand for stealth technology in aerospace field. To address various functional requirements, this study integrates a biomimetic strategy inspired by gradient bamboo vascular bundles with a novel dual-material 3D printing approach. Three distinct bamboo-inspired structural configurations Cf/SiC composites are designed and manufactured, and the effects of these different structural configurations on the CVI process are analyzed. Nanoindentation method is utilized to characterize the relationship between interface bonding strength and mechanical properties. The results reveal that the maximum flexural strength and fracture toughness reach 108.6 ± 5.2 MPa and 16.45±1.52 MPa m1/2, respectively, attributed to the enhanced crack propagation resistance and path caused by the weak fiber-matrix interface. Furthermore, the bio-inspired configuration enhances the dielectric loss and conductivity loss, exhibiting a minimum reflection loss of −24.3 dB with the effective absorption band of 3.89 GHz. This work introduces an innovative biomimetic strategy and 3D printing method for continuous fiber-reinforced ceramic composites, expanding the application of 3D printing technology in the field of CMCs.

Enhanced reliability with bimodal microstructure and transformation-induced toughening in Al2O3-YSZ based thermal barrier coatings

Thermal barrier coatings (TBCs) are thermally insulating coatings used in hot sections of gas turbines with temperature > 1400 °C. Present work pursues controlled bimodal microstructure consisting of micrometre-sized Al2O3 and spray-dried nanometre-sized Al2O3, reinforced with 20 wt% 3 mol% yttria stabilized zirconia (3YSZ) and 4 wt% carbon nanotube (CNT) as a thermal barrier coating (TBC) deposited onto Inconel 718 substrate with NiAl bond coat via atmospheric plasma spraying. Bimodal microstructure depicts fully melted and partially melted (~32–34 vol%) regions eliciting denser microstructure with less defects than conventional deposited Al2O3 TBC. Reinforcement with nanostructures 3YSZ, Al2O3 and CNT resulted a reduction in average stiffness of coatings (using nano indentation) from 139.25 GPa (μ-Al2O3) to 125.9 GPa (with 3YSZ) and 135.2 GPa (with 3YSZ+ CNT). However, the elastic recoverability of composite coatings is superior (50–55 %) compared to that of μ-Al2O3 coatings (47 %), indicating an enhanced crack propagation resistance. A pop-out behavior in nanoindentation is attributed to stress induced phase transformation of ZrO2 from tetragonal to monoclinic phase. Further, statistical analysis of nanohardness, and Young's modulus of composite coatings reveals bimodal Weibull moduli corroborating bimodality of microstructure. An increased reliability of bimodal coatings in conjunction with transformation toughening validates the superiority of Al2O3-based composite coating as a prospective TBC.

Enhancing the impact resistance of HVAF-sprayed Fe-based amorphous coatings via hierarchical structure design

Hierarchical structure has been proposed as an effective strategy for improving mechanical properties in many traditional alloy systems. In this work, using a similar structural design strategy, hierarchical structured Fe-based amorphous coatings comprising two layers with different porosities were successfully prepared by controlling the thermal spraying process parameters. Impact test revealed that hierarchical structure designed coatings exhibited enhanced impact resistance than that of the monolithic structured counterpart. Microstructure examinations demonstrated that compared with the monolithic structured coating, the introduction of hierarchical structure could not only effectively passivate the cracks, but also deflect the propagation path of the cracks, which directly or indirectly enhanced crack propagation resistance. Based on this, theoretical analysis and finite element simulations were systematically performed to reveal the intrinsic reasons for the improved impact toughness in hierarchical structured coatings, centering around the influence of hierarchical structure on the absorption rate of impact energy and the stress distribution of coating during the impact process. In summary, the current study opens another promising window to enhance the impact resistance of amorphous coatings.

Fracture process analysis in Magnesia-Hercynite refractory materials by combining an enhanced Digital Image Correlation method with Wedge Splitting Test

Magnesia-Hercynite bricks destined for thermal shock applications in cement rotary kilns often show an enhanced crack propagation resistance due to an engineered microstructure design. In these materials, microcrack networks, resulting from the thermal expansion mismatch between magnesia matrix and Hercynite aggregates, promote the activation of energy dissipating mechanisms within the so-called Fracture Process Zone (FPZ) during loading. In this research, the fracture behaviour of a Magnesia-Hercynite material has been investigated by coupling an enhanced Digital Image Correlation method (2P-DIC) with the Wedge Splitting Test (WST). The coupling of these advanced characterisation methods is very effective in measuring important fracture parameters accurately and in highlighting characteristic fracture mechanisms, such as crack-branching. A refined R-curve approach is proposed with effective fracture energy calculations based on 2P-DIC measurements. The results demonstrate interesting correlations between FPZ development and an enhanced crack propagation resistance.

Enhanced thermal shock resistance of ZrO2-reinforced Al2O3–CaAl12O19 composites prepared from ferrotitanium slag: Crack propagation resistance mechanisms

Al2O3–CaAl12O19 composites are ideal high-temperature structural ceramics, however, their poor thermal shock resistance limits their application under thermal shock conditions. In this work, ferrotitanium slag was used as the main raw material for composites preparation. Enhanced thermal shock resistance were obtained by adding 12.5 wt.% unstabilized monoclinic zirconia. A systematic analysis of the phase compositions and microstructural evolution with particular attention to crack propagation was performed. The improvement in thermal shock resistance is attributed to the enhanced crack propagation resistance. Furthermore, the enhanced crack propagation resistance is caused by multiple mechanisms, including the t-ZrO2-related stress-induced t → m phase transformation mechanism and the m-ZrO2-related microcracking mechanism. Coarsened ZrO2 particles doped with TiO2 from the ferrotitanium slag are beneficial to the occurrences of stress-induced transformation and microcracking.

Silicon nitride composites with magnesia and alumina additives: Toughening mechanisms and mechanical properties

This work was aimed to investigate the effect of magnesia (MgO) and aluminia (Al2O3) additives on the microstructure and mechanical properties of silicon nitride (Si3N4) ceramic composites, focusing on the salient toughening mechanisms. The application of MgO additives led to a denser and more uniform microstructure consisting of distinctive rod-like Si3N4 grains with higher aspect ratios, and thus a high flexural strength of 1039±78 MPa and fracture toughness of ~11.6 MPa m1/2, in comparison with the composite with Al2O3 additives, where inhomogeneous structure and abnormal grain growth occurred. Underlying toughening mechanisms in the Si3N4 ceramic composites, including crack deflection, crack bridging and rod-like grain pull-out were observed, leading to an enhanced crack propagation resistance. With increasing aspect ratio of Si3N4 grains, the crack deflection angles increased, corroborating the strong toughening mechanisms present in the Si3N4 ceramic composites. The rod-like grain pull-out mechanism was also analyzed.

Enhancement of impact toughness of β-type Ti–Mo alloy by {332}<113> twinning

The Charpy impact behavior was examined at room temperature on the basis of the metastable β-type Ti-15Mo alloy with {332} twinning-induced plasticity (TWIP) effect. This TWIP alloy exhibited an excellent impact toughness of approximately 250 J/cm2, which was approximately four times higher than those of common titanium alloys. The elongated dimples, fine dimples, and serpentine glide morphology were observed on the fracture surface. The large amounts of twins were formed under impact loading, which induced the adequate plastic deformation to absorb the impact energy. The formation of these dense and fine twins effectively released the stress concentration due to the dynamic microstructure refinement effect, and the abundant cracks along the twin boundaries remarkably increased the crack propagation path. The delayed crack initiation and enhanced crack propagation resistance further consumed the impact energy.

Microstructural morphology effects on fracture resistance and crack tip strain distribution in Ti–6Al–4V alloy for orthopedic implants

The effects of microstructural morphology on the fracture behavior of Ti–6Al–4V ELI (extra-low impurity) alloy in two different heat treatment conditions were examined. Alloy was solution treated above (β ST) and below (α+β ST) β transus temperature followed by furnace cooling (FC) in order to obtain the fully lamellar and equiaxed microstructures. Tensile and fracture toughness tests were conducted. The crack tip opening displacement (CTOD) and strain distribution near the crack tip were measured on the compact tension (CT) specimen surface by digital stereometric method. The crack propagation resistance (CTOD-R) curves were developed by applying the modified normalization method and critical CTOD values were determined. To identify the microstructural length scale controlling the fracture resistance of this alloy, the crack propagation path and fracture surface morphology were evaluated. It was found that the reduction in the characteristic microstructural dimension of an order of magnitude and significant change in the α phase aspect ratio contribute to drastic increase in the tensile properties and decrease in the crack initiation and propagation resistance. The fully lamellar microstructure displays slightly better biocompatibility because of the lower elastic modulus and superior fracture resistance. The enhanced crack propagation resistance of this microstructure is associated with the larger propensity for crack tip tortuousity, due to the coarser microstructural dimensions (lamellar colony size vs. primary α grain size). The difference in the crack propagation modes affects the shape and size of the actual crack tip strain distribution. These results were discussed correlating the complex multiple fracture mechanisms with the stress state in two microstructures.

Fatigue crack growth in friction stir welds of 6082-T6 and 6061-T6 aluminium alloys: A comparison

Friction stir welding (FSW) is a solid-state joining process which emerged as an alternative technology to be used in high strength alloys that were difficult to join with conventional techniques. Notwithstanding the widespread interest in the possibilities offered by FSW, data concerning the fatigue behaviour of joints obtained using this process still is scarce. In this work, a comparative study between fatigue crack growth behaviour of friction stir welds of 6082-T6 and 6061-T6 aluminium alloys is carried out. Fatigue crack growth curves were determined for cracks growing in different locations of the weldments, including the base material, the heat affected zone and the welded material. Generally, friction stir material exhibited lower strength and ductility properties than the base material. However, an enhanced crack propagation resistance is observed in the welded material. The 6082-T6 and 6061-T6 base materials exhibit very similar crack propagation behaviours. On the other hand the friction stir 6061-T6 material shows lower crack propagation rates than corresponding 6082-T6 friction stir material. Particular features of the distinct microstructures of the welded and surrounding material are illustrated using scanning electron microscopy.

Assessment of the Fatigue Behaviour of Friction Stir Welded Joints: Aluminium Alloy 6082-T6

A study on the fatigue behaviour of friction stir butt welds of 3mm thick 6082-T6 aluminium alloy was carried out. Monotonic tensile and cyclic tests of welded joints and base material were performed to understand the influence of the welding process on the static and fatigue properties. Microhardness profiles were measured and fatigue crack growth curves were determined for cracks growing in different locations of the weldments. Friction stir material exhibited lower strength and ductility properties than the base material. However, an enhanced crack propagation resistance is observed.

Fracture resistance of ceramics: Base diagram and R-line

Modern methods of determining crack resistance of ceramics are analyzed and it is shown that fracture of the specimens can be divided into the three stages: formation of a stress concentrator, nucleation and propagation of a crack. Note that standard methods provide determining at the second and third stages but the EF method effects measurements at all the three stages, with testing small-size specimens. The flaking resistance FR was estimated by the Rockwell indentation of the specimens of oxide and nonoxide linear elastic and inelastic ceramics as well as particulate ceramic composites. It was established that FR values for homogeneous linear elastic ceramics were invariant (independent of flaking loads and chip scar sizes). It was demonstrated on the flaking resistance-critical stress intensity factor diagram that the values for ceramics with enhanced damage resistance were found to the left of the base line, while those of homogeneous linear elastic ceramics fell on this line. The values for ceramics with enhanced crack propagation resistance are to the right of this line. Crack propagation resistance diagrams (R-lines) are proposed, they are similar to known R-curves but they are plotted on the basis of results obtained with flaking off the edges of ceramic specimens. For homogeneous linear elastic ceramics these lines are flat, which is indicative of their surface energy invariance.

The effect of microstructure on the fatigue crack propagation behavior of an Al-Zn-Mg-Cu alloy

The effect of slip distribution on the fatigue crack propagation behavior in vacuum of a high purity Al-5.9Zn-2.6Mg-l.7Cu alloy in various age-hardened conditions has been investigated. The crack propagation resistance was observed to be significantly higher for underaged microstructures containing shearable precipitates in comparison to overaged conditions with nonshearable precipitates. The improved crack propagation resistance is attributed in part to an increased amount of reversed slip in the plastic zone at the crack tip due to a higher degree of planar slip for conditions with shearable precipitates. The observed increase in fatigue crack propagation resistance with decreasing precipitate size for microstructures containing a constant volume fraction of shearable precipitates cannot be explained on the basis of such slip reversibility alone. The variation in ductility for the different microstructures has also to be taken into account. It was found that the enhanced crack propagation resistance can be correlated to the increased ductility with decreasing precipitate size. This explanation was supported by the experimental observation that microstructures containing different volume fractions and sizes of shearable precipitates but exhibiting the same ductility showed approximately the same resistance against fatigue crack propagation.