Effect of frequency and environment on fatigue behavior of a CVI SiC/SiC ceramic matrix composite at 1200 °C

The fatigue behavior of a SiC/SiC CMC (ceramic matrix composite) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a SiC matrix reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had boron nitride fiber coating applied and were then densified with CVI SiC. Tensile stress-strain behavior and tensile properties were evaluated at 1200 °C. Tension-tension fatigue tests were conducted at frequencies of 0.1, 1.0, and 10 Hz for fatigue stresses ranging from 80 to 120 MPa in air and from 60 to 110 MPa in steam. Fatigue run-out was defined as 10⁵ cycles at the frequency of 0.1 Hz and as 2 × 10⁵ cycles at the frequencies of 1.0 and 10 Hz. Presence of steam significantly degraded the fatigue performance. In both test environments the fatigue limit and fatigue lifetime decreased with increasing frequency. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength, yet modulus loss up to 22% was observed. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Compressive behavior of C/SiC composite sandwich structure with stitched lattice core

C/SiC composite sandwich structure with stitched lattice core was fabricated by a technique that involved polymer impregnation and interweaving. The mechanical behaviors of C/SiC composite sandwich structure were investigated at room temperature. The out-of-plane compressive strength was 20.97 MPa while modulus was 1473.55 MPa. The microstructural evolution on compression fracture surfaces of the stitching yarns was investigated by scanning electron microscopy, and the damage pattern of fibers on compression fracture surface was presented and discussed. Under an in-plane compression loading, the C/SiC composite sandwich structure displayed a linear-elastic behavior until failure. The peak strength and average modulus are 165.61 MPa and 19.74 GPa, respectively. The failure of the specimen was dominated by the fracture of the facesheet.     

Multi-walled carbon nanotube/nanostructured zirconia composites: Outstanding mechanical properties in a wide range of temperature

Multi-walled carbon nanotube (MWCNT)/nanostructured zirconia composites with a homogenous distribution of different MWCNT quantities (ranging within 0.5-5 wt.%) were developed. By using Spark Plasma Sintering we succeeded in preserving the MWCNTs firmly attached to zirconia grains and in obtaining fully dense materials. Moreover, MWCNTs reduce grain growth and keep a nanosize structure. A significant improvement in room temperature fracture toughness and shear modulus as well as an enhanced creep performance at high temperature is reported for the first time in this type of materials. To support these interesting mechanical properties, high-resolution electron microscopy and mechanical loss measurements have been carried out. Toughening and creep hindering mechanisms are proposed. Moreover, an enhancement of the electrical conductivity up to 10 orders of magnitude is obtained with respect to the pure ceramics.     

Creep deformation of Alloys 617 and 276 at 750–950 °C

Alloys 617 and 276 were subjected to time-dependent deformation at elevated temperatures under sustained loading of different magnitudes. The results indicate that Alloy 617 did not exhibit strains exceeding 1 percent (%) in 1000 h at 750, 850 and 950 °C when loaded to 10% of its yield strength (YS) values at these temperatures. However, this alloy was not capable of sustaining higher stresses (0.25YS and 0.35YS) for 1000 h at 850 and 950 °C without excessive deformation. Interestingly, Alloy 617 showed insignificant steady-state creep rate at 750 °C irrespective of the applied stress levels. Alloy 276 almost met the maximum creep deformation criterion when tested at 51 MPa–750 °C. Severe creep deformation of both alloys at 950 °C could be attributed to the dissolution of carbides and intermetallic phases remaining after solution annealing or precipitated during quenching.     

Effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites

The effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites was evaluated. Alkali treatment of the fibers and reaction with organosilanes as coupling agents were applied to improve fiber–matrix adhesion. Fiber loadings of 1, 3, 5, and 7 wt% were incorporated to the phenolic matrix and tensile, flexural, morphological and thermal properties of the resulting composites were studied. In general, mechanical properties of the composites showed a maximum at 3% of fiber loading and a uniform distribution of the fibers in such composites was observed. Silane treatment of the fibers provided derived composites with the best thermal and mechanical properties. Meanwhile, NaOH treatment improved thermal and flexural properties, but reduced tensile properties of the materials. Therefore, the phenolic composite containing 3% of silane treated cellulose fiber was selected as the material with optimal properties.     

Effects of Cf/C density on microstructure and properties of 3-D Cf/ZrC composites prepared by liquid metal infiltration

Three-dimensional braided carbon fiber-reinforced ZrC matrix composites, 3-D C_f/ZrC, were fabricated by Liquid metal infiltration process at 1200 °C. Porous carbon/carbon (C_f/C) composites with various densities were used as preforms, and the effects of C_f/C density on microstructure and properties of the 3-D C_f/ZrC composites were investigated. The results show that the composites are composed of carbon, ZrC and residual metal. Both microstructure and properties of the 3-D C_f/ZrC composites are apparently affected by C_f/C density. With increasing density of C_f/C preform, the density of 3-D C_f/ZrC composites decreases while the open porosity increases. The composites obtained from the C_f/C preform with a density of 1.12 g/cm³ have the best mechanical properties, with flexural strength of 286.2 ± 11.4 MPa, elastic modulus of 83.5 ± 6.8 GPa and fracture toughness of 9.2 ± 0.6 MPa m^1/2. The composites exhibit excellent ablation resistance, and the mass rate and the linear ablation rate under an oxyacetylene torch are as low as 5.1 ± 0.4 mg s⁻¹ and 1.1 ± 0.3 μm s⁻¹, respectively.     

Oxidation mechanism of ZrB2/SiC ceramics based on phase-field model

In this paper, the phase-field oxidation mechanism of ultra-high temperature ZrB₂/SiC ceramics is investigated theoretically. Firstly, a phase-field model is developed to analyze the oxidation behaviors of multiphase materials. Secondly, the evolutions of the porosity and the oxidation stress for the oxidized ZrB₂/SiC ceramics with different temperatures and different oxygen partial pressures are predicted, and the influences of the mechanical factors on the oxidation behaviors of ZrB₂/SiC ceramics are discussed. Finally, two-dimensional oxidation behaviors of ZrB₂/SiC ceramics are simulated and analyzed.     

Comparative study on tensile fracture behavior of monofilament and bundle C/C composites

Strength controlling factors in C/C composites were experimentally examined using monofilament fiber reinforced C/C composites and those reinforced by one carbon fiber bundle. Tensile strength of the monofilament C/C composites was almost the same level with that of the carbon fiber. This result indicated that carbon fibers in the C/C composites were intact even after the processing. On the other hand, remarkable reduction was observed in the bundle C/C composites. It was indicated that the fracture of the C/C composite is dominated by the brittle fracture of the sub-bundles, in which the fiber/matrix interface is bonded well.     

Assessment of tensile behaviour of an Al–Mg alloy composite reinforced with NiAl and oxidized NiAl powder particles helped by nanoindentation

AA5056 matrix composites have been reinforced with as-received and oxidized NiAl particles and their nanohardness investigated as a function of distance to reinforcement. Results indicate that a non-heat treatable aluminium matrix, as is the present case, does not require that the intermetallic particles are surrounding by a protective Al₂O₃ layer to avoid reactions at matrix-reinforcement interfaces. On the other hand, the quality of the matrix-reinforcement bonding has been quantified by the reinforcement influence distance, defined as the distance from the particle at which the nanohardness of the matrix drops to its asymptotic value.     

Fracture behavior and mechanism of 2D C/SiC-BCx composite at room temperature

2D C/SiC composite was modified with partial BC_x matrix by low pressure chemical vapor infiltration technique (LPCVI), which was named as 2D C/SiC-BC_x composite. The flexural fracture behavior, mechanism, and strength distribution of 2D C/SiC-BC_x composite are investigated. The results indicate that the flexural strength, fracture toughness, and fracture work are 442.1 MPa, 22.84 MPa m^1/2, and 19.2 kJ m⁻², respectively. The flexural strength of C/SiC-BC_x composite decrease about 20% than that of C/SiC composite. However, the fracture toughness and fracture work increase about 19% and 18.5%, respectively. The properties varieties between C/SiC-BC_x composite and C/SiC composite can be attributed to the weak-bonding interface between BC_x/SiC matrices according to the results of detailed microstructure analysis. The strength distribution of 2D C/SiC-BC_x composite follows as Normal distribution or Weibull distribution with σ_u = 0, and m = 8.1393. The mean value of flexural strength for 2D C/SiC-BC_x composite is 443 MPa obtained by theory calculation, which is consistent with experiment result (442.1 MPa) very well.     

Effects of temperature and stress on the oxidation behavior of a 3D C/SiC composite in a combustion wind tunnel

High-temperature oxidation of a 3D C/SiC composite has been conducted under various tensile creep loads in a combustion wind tunnel at 1200–1500 °C. The effects of temperature and stress on the oxidation behavior were evaluated according to length change, lifetime and morphology of the specimens. The damage mechanisms of the composite are changed from superficial oxidation to non-uniform even uniform oxidation by a tensile stress. The stressed oxidation process is controlled by a normalized threshold stress (NTS), which is increased with rising temperature. When the normalized stress (NS) is below the threshold value, the oxidation of carbon fibers is controlled by the in-crack diffusion, starts from the windward and develops region by region along the combustion gas flow. The specimen displays a multiple creep behavior because the applied tensile load is borne by several load-bearing regions in turn and each region manifests a typical creep behavior after the tensile load transferred from an oxidized region to it. When NS is above NTS, the oxidation of carbon fibers is limited by the boundary layer diffusion, and the specimen exhibits a typical creep behavior.     

Effect of excessive bending on residual tensile strength of hybrid composite rods

Excessive bending has been identified as a concern for the hybrid composite core that is currently being used as the structural member for the Aluminum Conductor Composite Core Trapezoidal Wire (ACCC/TW™) transmission line. In this work the flexure strength of the ACCC core was measured in a series of four point bend tests while monitoring acoustic emissions. To quantify the stress state within the rods and to evaluate its flexure strength, an analytic solution for the bending stress was derived and numerically verified using the finite element method. In the second part of the study several specimens that had been subjected to excessive bending were subsequently tested for their residual tensile strength. It was found that wrapping the ACCC core around a 1 m mandrel, which is a common loading condition in practice, will not generate significant structural damage in the composite core. It was determined that the diameter of the mandrel that would cause failure of the composite core is 467 mm. From this work it was found that excessive bending, up to 90% of the flexural strength of the ACCC core, had no detrimental effect on the residual tensile strength of the hybrid composite. It was observed that the majority of the micro-structural damage that was accrued during the excessive bending of the cores presented itself in the form of matrix damage without any significant fiber kinking.     

Effect of oxidation on the creep behaviour of copper–chromium in situ composite

The creep deformation and fracture behaviours of a Cu–Cr in situ composite were investigated in air and in vacuum over a temperature range of 400–650 °C to study the effect of environment. The similarities of the activation energy and the stress exponent in air and in vacuum strongly suggest that the oxygen and/or the oxide have no direct effect on the deformation mechanism of Cu–Cr in situ composite. The higher creep rate of the composite in air than in vacuum is due to the gradual decrease of the cross-sectional area of the matrix due to increasing thickness of the oxide layer. The mechanism of damage was found to be similar for all the creep tests performed.     

Stress-dependence and time-dependence of the post-fatigue tensile behavior of carbon fiber reinforced SiC matrix composites

The major objective of this paper is to phenomenally report the stress-dependence and time-dependence of fatigue damage to C/SiC composites, and to tentatively discuss the effects of the fatigue stress levels and the fatigue cycles on the post-fatigue tensile behavior. Results show that compared with the virgin strength of the as-received C/SiC specimens, the tensile strengths of the as-fatigued specimens after 86,400 cycles were increased by 8.47% at the stresses of 90 ± 30 MPa, 23.47% at 120 ± 40 MPa, and 9.8% at 160 ± 53 MPa. As cycles continued, however, the post-fatigue strength of the composites gradually decreased after the peak of 23.47%, at which the optimal strength enhancement was obtained because the mean fatigue stress of 120 MPa was the closest to thermal residual stress (TRS), and caused TRS relieve largely during the fatigue. Most interestingly, there was a general inflexion appeared on the post-fatigue tensile stress-strain curves, which was just equal to the historic maximum fatigue stress acted upon the as-fatigued specimens. Below this inflexion stress the tensile curves revealed the apparent linear behavior with little AE response, and above that nonlinearity with new damage immediately emitted highly increase rate of AE activities. This ‘stress memory’ characteristic was strongly relevant to damaged microstructures of the as-fatigued composites in the form of the coating/matrix cracks, interface debonding/wear, and fiber breaking, which resulted undoubtedly in reduction of modulus but in proper increase of strength via TRS relief.     

Characterisation and simulation of the creep behaviour of Nicrofer 6025HT wire material at 650 °C

Thermal efficiency of combined cycle power plants can be improved by increasing temperature and pressure in the steam turbine. Since typical power plant materials have presently reached their operation limit with higher steam temperature, the application of a new cooling system could reduce the material temperature to tolerable conditions. For this purpose, a new sandwich structure was developed comprising a woven wire mesh interlayer between two plane sheets. Cooling steam is fed into the interlayer where it can flow without severe losses. This sandwich structure is applied to the steam turbine casing as a wall cladding.Due to the combination of constant overpressure of cooling steam and high temperature exposure of hot steam, the structures are stressed parallel and perpendicular to the intermediate layer primarily by creep loads. To simulate the creep behaviour via the finite element method the exact knowledge of the creep behaviour of the constituents, the wire and the sheet, is essential. Therefore, creep tests at 650 °C on wire material, manufactured from the nickel base alloy Nicrofer 6025HT, were carried out to determine constitutive equations. The creep behaviour was simulated on the basis of the concept of the internal backstress, which was implemented in an adequate user subroutine of the commercial FEM-software Abaqus.     

Viscoplastic behavior of a FeCrAl alloy for high temperature steam electrolysis (HTSE) sealing applications between 700 °C and 900 °C

High temperature steam electrolysis (HTSE) is one of the most promising technologies for the industrial production of hydrogen. However one of the remaining problems lies in sealing at high temperature. The reference solution is based on glass seals which presents several drawbacks. That explains why metallic seals are under development. The expected seal will be submitted to creep under low stresses between 700 °C and 900 °C, possibly involving complex loading and thermal history. The candidate material investigated in this work is a FeCrAl (OC404, Sandvik) supplied as a 0.3 mm thick sheet. The ability of this material to develop a protective layer of alumina was studied first, as well as grain size growth during thermal ageing. Creep and tensile tests were performed between 700 °C and 900 °C to determine its mechanical properties. This database was used to propose and identify an elasto-viscoplastic behavior for the material. Creep was described by the Sellars-Tegart law. This law was then used to simulate and predict creep indentation tests performed in the same range of temperatures.     

Enchanced high-temperature performances of SiC/SiC composites by high densification and crystalline structure

We report the enchanced in situ performances of tensile strength and thermal conductivity at elevated temperatures of the PCS-free SiC/SiC composite with a high fiber volume fraction above 50% fabricated by NITE process for nuclear applications. The composite was fabricated by the optimized combination of the fiber coating, the matrix slurry and the pressure-sintering conditions, based on our previous composites’ study history. The composite showed the excellent tensile strength up to 1500 °C, that it retained approximately 88% of the room-temperature strength. Also, the thermal conductivity of the composites represented over 20 W/m K up to 1500 °C, which was enough high to take the advantage of the assumed design value for nuclear applications. Microstructural observation indicated that the excellent high-temperature performances regarding tensile strength and thermal conductivity up to 1500 °C were the contribution to the high densification and crystalline structure in matrix.     

Consolidation of polymer-derived SiC matrix composites: processing and microstructure

SiC fiber reinforced SiC matrix composite (SiC/SiC composite) has been developed by polymer impregnation and pyrolysis (PIP) method, which consists of impregnation, curing, consolidation, and re-impregnation and pyrolysis. As a prospective approach to fabricate a high performance composite, consolidation conditions, such as curing temperature to make a green body, pressure and heating rate during consolidation, were systematically controlled for effective consolidation. Because of its advantage in controlling physical characteristic, polyvinylsilane (PVS) that is liquid thermosetting organo-silicic compound was utilized as the matrix precursor. Based on the pyrolytic behavior of PVS, effects of the process conditions on microstructure of the consolidated bodies were accurately characterized. To relate those microstructure with mechanical property, flexural tests were performed for the composites after multiple PIP processing. Consequently, process conditions to make a high performance composite could be appeared. Structural conditions to be optimized for further improvement in mechanical and environmental properties were also discussed.     

Microstructure and mechanical properties of in situ TiB reinforced titanium matrix composites based on Ti–FeMo–B prepared by spark plasma sintering

The in situ synthesized TiB reinforced titanium matrix composites have been prepared by spark plasma sintering at 800–1200 °C under 20 MPa for 5 min. The effects of sintering temperature and reinforcement volume fraction on flexural strength, Young’s modulus and fracture toughness of the composites are investigated. The titanium matrix consists of -Ti and β-Ti phases, and the volume fraction of β-Ti increases with increasing sintering temperatures. The in situ synthesized TiB reinforcements are distributed randomly and uniformly in matrix. The transverse section of TiB has a hexagonal shape aligned along [0 1 0] direction, and the crystallographic planes of the TiB needles are always of the type . The 10 vol% TiB reinforced composite sintered at 1000 °C exhibits excellent mechanical properties. The flexural strength, Young’s modulus and fracture toughness of this composite are 1560 MPa, 137 GPa and 8.64 MPa · m^1/2, respectively.