Effects of Steam Environment on Creep Behavior of Nextel™610/Monazite/Alumina Composite at 1,100°C

The tensile creep behavior of a N610™/LaPO₄/Al₂O₃ composite was investigated at 1,100°C in laboratory air and in steam. The composite consists of a porous alumina matrix reinforced with Nextel 610 fibers woven in an eight-harness satin weave fabric and coated with monazite. The tensile stress-strain behavior was investigated and the tensile properties measured at 1,100°C. The addition of monazite coating resulted in ~33% improvement in ultimate tensile strength (UTS) at 1,100°C. Tensile creep behavior was examined for creep stresses in the 32–72 MPa range. Primary and secondary creep regimes were observed in all tests. Minimum creep rate was reached in all tests. In air, creep strains remained below 0.8% and creep strain rates approached 2 × 10⁻⁸ s⁻¹. Creep run-out defined as 100 h at creep stress was achieved in all tests conducted in air. The presence of steam accelerated creep rates and significantly reduced creep lifetimes. In steam, creep strain reached 2.25%, and creep strain rate approached 2.6 × 10⁻⁶ s⁻¹. In steam, creep run-out was not achieved. The retained strength and modulus of all specimens that achieved run-out were characterized. Comparison with results obtained for N610™/Al₂O₃ (control) specimens revealed that the use of the monazite coating resulted in considerable improvement in creep resistance at 1,100°C both in air and in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Effects of steam environment on creep behavior of Nextel™720/alumina–mullite ceramic composite at elevated temperature

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1000 and 1100 °C in laboratory air and in steam. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel?720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. The presence of steam accelerated creep rates and dramatically reduced creep lifetimes. The degrading effects of steam become more pronounced with increasing temperature. At 1000 °C, creep run-out (set to 100 h) was achieved in all tests. At 1100 °C, creep run-out was achieved in all tests in air and only in the 87.5 MPa test in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Cyclic and sustained loading behaviors of oxide/oxide Nextel™720/alumina composite with double edge sharp notch

This study investigated an oxide/oxide CMC consisting of Nextel?720 (meta-stable mullite) fibers in alumina matrix, N720/A, with 0°/90° fiber orientation having double edge sharp notch under sustained and cyclic loading conditions at 1200 °C in laboratory air environment. Monotonic tensile tests at 1200 °C were also conducted. Fracture surfaces were examined to analyze failure and damage mechanisms. Comparisons with counterparts from unnotched geometry showed N720/A is mildly sensitive to the sharp notch under monotonic tensile, creep and fatigue loading conditions. The ultimate tensile strength of the composite was reduced by about 15% in the presence of the sharp notch. The rupture strength of the sharp notched geometry was reduced by about 15% of unnotched geometry for a given rupture time. The fatigue strength was reduced by about 20% of unnotched geometry for a given number of cycles to failure. Deformation under cyclic loading condition had contributions both from fatigue and creep. Damage mechanisms were identical under cyclic and sustained loading conditions.     

Creep of Nextel™720/alumina–mullite ceramic composite at 1200 °C in air,argon, and steam

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1200 °C in laboratory air, in steam and in argon. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured at 1200 °C. The elastic modulus was 74.5 GPa and the ultimate tensile strength was 153 MPa. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in laboratory air for creep stress levels ?91 MPa. The presence of either steam or argon accelerated creep rates and reduced creep lifetimes. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Nextel™ 610 alumina fibre reinforced aluminium: influence of matrix and process on flow stress

Continuous alumina fibre reinforced aluminium matrix composites are produced using two different liquid metal infiltration methods, namely direct squeeze casting and gas pressure infiltration. Net-shape fibre performs for longitudinal parallel tensile bars are prepared by winding the Nextel™ 610 alumina fibre (3M, St Paul, MN) into graphite moulds. High purity aluminium, two binary (Al–6% Zn and Al–1% Mg) and one ternary (Al–6% Zn–0.5% Mg) aluminium alloys are used as matrix materials. The composite is tested in uniaxial tension–compression, using unload–reload loops to monitor the evolution of Young's modulus. A linear dependence between Young's modulus and strain is observed; this is attributed, by deduction, to intrinsic elastic non-linearity of the alumina fibre. This conclusion is then used to compare on the basis of the in situ matrix flow curve the influence of matrix composition and infiltration process on the composite stress–strain behaviour.     

Oxidation Behaviors of C-ZrB_2-SiC Composite at 2100℃ in Air and O_2

The oxidation behaviors of graphite and ZrB2-SiC modified graphite composite were investigated at 2100℃ in1X105 Pa air and 0.2X105 Pa O2. The oxidation tests were conducted in an induction heating furnace. The oxidation of these two materials followed the linear rate law. The determined radius loss rates of graphite and C-ZrB2-Si C at 2100℃ were 2.18X10-2and 1.05X10-2%/s in 1X105 Pa air, and 3.23X10 2 and 2.21X10 2%/s in 0.2X105 Pa O2, respectively. The incorporation of ZrB2 and SiC decreased remarkably the oxidation rate of graphite because the oxide scale formed on the sample surface during oxidation helps in reducing the exposed surface area of the underneath substrate. In two different atmospheres with the same oxygen partial pressure, both graphite and ZrB2-SiC experienced more severe oxidation at 2100℃ in0.2X105 Pa O2than in 1X105 Pa air. The oxidation rate-controlling step for graphite and ZrB2-SiC was proposed to be the inward diffusion of oxygen through the boundary layer and through the pores in the oxide scale, respectively. A model based on diffusion theory was established to discuss the effect of the total gas pressure on their oxidation behaviors.     

Effects of environment on creep behavior of two oxide/oxide ceramic–matrix composites at 1200 °C

The tensile creep behavior of two oxide/oxide ceramic–matrix composites (CMCs) was investigated at 1200 °C in laboratory air, in steam, and in argon. The composites consist of a porous oxide matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, have no interface between the fiber and matrix, and rely on the porous matrix for flaw tolerance. The matrix materials were alumina and aluminosilicate. The tensile stress–strain behavior was investigated and the tensile properties were measured at 1200 °C. Tensile creep behavior of both CMCs was examined for creep stresses in the 80–150 MPa range. Creep run-out defined as 100 h at creep stress was achieved in air and in argon for stress levels ≤100 MPa for both composites. The retained strength and modulus of all specimens that achieved run-out were evaluated. The presence of steam accelerated creep rates and reduced creep life of both CMCs. In the case of the composite with the aluminosilicate matrix, no-load exposure in steam at 1200 °C caused severe degradation of tensile strength. Composite microstructure, as well as damage and failure mechanisms were investigated. Poor creep performance of both composites in steam is attributed to the degradation of the fibers and densification of the matrix. Results indicate that the aluminosilicate matrix is considerably more susceptible to densification and coarsening of the porosity than the alumina matrix. The views expressed are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense or the U.S. Government.     

Numerical Modeling of Oxidized 2D C/SiC Composites in Air Environments Below 900 °C: Microstructure and Elastic Properties

A numerical model is presented for simulation of the oxidation-affected behaviors of two dimensional carbon fiber-reinforced silcon carbide matrix composite (2D C/SiC) exposed to air oxidizing environments below 900 °C, which incorporates the modeling of oxidized microstructure and computing of degraded elastic properties. This model is based upon the analysis of the representative volume cell (RVC) of the composite. The multi-scale model of 2D C/SiC composites is concerned in the present study. Analysis results of such a composite can provide a guideline for the real 2D C/SiC composite. The micro-structure during oxidation process is firstly modeled in the RVC. The elastic moduli of oxidized composite under non-stress oxidation environment is computed by finite element analysis. The elastic properties of 2D-C/SiC composites in air oxidizing environment are evaluated and validated in comparison to experimental data. The oxidation time, temperature and fiber volume fractions of C/SiC composite are investigated to show their influences upon the elastic properties of 2D C/SiC composites.     

Strain rate and oxidation effects on crack initiation at 600 and 650 °C in a nickel-based superalloy

Nickel-based superalloys are sensitive to an oxidation-assisted intergranular crack (OAIC) growth mechanism. Crack initiation during slow strain rate tensile tests is investigated at 600 and 650 °C, at different strain rates, with or without oxidation on a direct-aged material. A V-shaped sample geometry is used to promote damage initiation for a specific stress triaxiality. The critical mechanical loading paths inducing intergranular crack initiation as well as the effect of oxidation are discussed.     

Two‐Dimensional Calculation of the Parameters of a Steam‐Air Mixture in a Film‐Type Heat and Mass Exchanger

A mathematical model for twodimensional calculation of the temperature and density of the steam in a steamair mixture which rises between two water films flowing down adiabatic shields has been proposed. The parameters of the films have been calculated in a onedimensional approximation. The results of calculating the parameters of air according to the onedimensional and twodimensional models of the processes of heat and mass transfer have been compared. The range of applicability of the former to a filmtype heat and mass exchanger has been determined.     

Crack Initiation Mechanism of Z3CN20.09M Duplex Stainless Steel during Corrosion Fatigue in Water and Air at 290 °C

The crack initiation mechanism of a Z3CN20.09 M duplex stainless steel(DSS) during corrosion fatigue(CF) in water and air at 290 °C was investigated by using a CF cracking machine and a scanning electron microscopy(SEM). The cracks were initiated successively at the persistent slip bands(PSBs), phase boundaries(PBs) and pitting corrosion points(PCPs) of the specimens when they were tested in water at 290 °C,while in air at 290 °C the cracks were only initiated at the PSBs and PBs. And the cracks were found mainly to initiate at the PSBs and PBs when the specimens were tested in water and air at 290 °C, respectively.The results also reveal that the cracks were likely to be initiated at the first 20% of fatigue life of the specimens tested in water at 290 °C. However, the cracks were not found until 50% of fatigue life when tested in air at 290 °C. Moreover, the crack numbers of the specimens tested in water at 290 °C were much more than those tested in air at 290 °C.     

Comparison of oxidation resistance of NiCoCrAlTaY-coated and -uncoated Mar-M247 superalloys in the air at 1150 °C

Oxidation resistance of NiCoCrAlTaY coated and uncoated Mar-M247 superalloys were compared experimentally in the air at 1150 °C. It was found that the oxidation behavior of the NiCoCrAlTaY coated and uncoated Mar-M247 superalloys generally followed the parabolic kinetics with the rate of 1.5 and 9.8 × 10⁻³ mg²/cm⁴ h for each one. Microstructural observation and elemental analysis indicated that after 200 h the uncoated alloy covered with an oxide scale of 6–10 times thicker than the coated one, beneath which a fairly continuous and relatively thin α-Al₂O₃ could be found along the interface close to the substrate. Comparatively, the NiCoCrAlTaY coated alloy uniformly produced the continuous, dense and thick α-Al₂O₃ layer adhesive to the coating to prevent the metal elements against excessive oxidation, and finally formed the steady oxide scales: outer (Ni, Co)(Al, Cr)₂O₄ spinels, NiO, (Al, Cr)₂Ni₃; inner protective α-Al₂O₃ with a little Cr₂O₃. On the surface of the uncoated alloy, however, it mainly formed the Ni-rich oxides in which a large quantity of cracks propagated to result in the more oxidation.     

Mixed Convection of Water at 4 °C Along a Wedge with Variable Surface Temperature in a Porous Medium

In this study, the mixed convection of water at 4 °C along a wedge in a porous medium is investigated numerically using a finite difference method. To explore the effect of mixed convection, both forced and free convection-dominated regimes are considered. Non-similarity solutions are obtained for the variable walltemperature boundary condition. Velocity and temperature profiles as well as local dimensionless skin friction and the Nusselt number are obtained and compared with available numerical results for various values of different parameters. The wedge angle geometry parameter m and mixed convection parameter ξ ranged from 0 to 1 in both regimes, whereas different values of λ are considered for the purpose of comparison of heat transfer results.     

Oxidation behavior of a new Fe–Ni–Cr-based alloy in pure steam at 750 °C

The isothermal oxidation of a new Fe–Ni–Cr-based alloy has been investigated in pure steam at 750 °C for exposure time up to 500 h using secondary electron microscope (SEM)/ X-ray energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Results showed that the alloy was oxidized approximately following a parabolic law with a parabolic rate constant k_p of 2.36 × 10^?13 g²/m⁴/s. As revealed by SEM/EDS and XRD results, a duplex-layered external oxide scale was formed, consisting of a thin outer layer of Ni(Fe, Al)₂O₄ and a thicker inner layer of (Cr, Mn)₂O₃. Underneath the external oxide scale, the internal oxidation of Ti to be TiO₂ occurred particularly along the grain boundaries of the matrix alloy. Internal oxide of Al₂O₃ was also observed but at a deeper depth. Based on the detailed compositional and microstructural characterization of the oxidized zone, the mechanism of the external and internal oxidation in steam is presented.     

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C have been studied to analyse defect assessment in a steam generator. Different grades of alloy 800 have been investigated to reproduce the in service conditions. Fatigue crack growth (FCG) tests were conducted on CT20 and tubular specimens, then on welded tubes. Furthermore the influence of hold times on fatigue crack growth behaviour was studied.

The results obtained on material simulating the weld heat affected zone are in agreement with the tests conducted on welded tubes. Fatigue crack growth characteristics of aged and cold-worked aged material seem to be slightly improved in comparison with base material. Finally a hold time of one minute increases strongly the FCG threshold value determined in pure fatigue but has a negligible influence on crack growth rates.     

Residual Stresses and Deformation Behaviour in a SiC_p/Al Composite

Experiments were conducted to determine theresidual stresses with X-ray diffraction in the ma-trix of a SiC/Al composite after different thermaltreatments,and to investigate the stress-straincharacteristics and fracture behaviour of the com-posite.It was found that there existed a tensileresidual stress in the matrix and both thermal cy-cling between room temperature and 350℃ and lowtemperature treatment in liquid nitrogen reducedthe residual stress.The results of the strengthdifferential effect and Bauschinger effect were con-sistent with the results of residual stressmeasurements.The tensile residual stresses in the Almatrix enhanced the strength differential effect.Themagnitude of Bauschinger effect is greater for a testinitially started in compression than that in tension.     

Kinetics and Mechanism of Interfacial Reaction in a SiCf／Ti Composite

In order to evaluate the interracial reaction, a SiCf/Ti (TA1) composite was fabricated by a vacuum hot pressingmethod and then heat-treated in vacuum at 800℃ for up to 100 h. The elemental distributions of C, Si and Ti at the interracial reaction zone were investigated. It was found that the reaction zone occurs during the fabrication process and continuously grows at high temperature because the Si and C atoms diffuse from SiC fibers to the matrix and Ti atoms diffuse in the opposite direction. The growth of the reaction zone is diffusion controlled and the mechanism of the reaction can be described by a reactive diffusion model of solid-state growth of an AmBn layer between two elementary substances A and B.     

Oxidation behavior of a Mo(Si,Al)2 composite at 900–1600 °C in dry air

The oxidation of a Mo(Si,Al)₂-based composite is investigated in the temperature range 900–1600 °C in dry air. Exposure time was 72 h. Comparisons are made with the oxidation behavior of a conventional MoSi₂-based material. Cross-sections are examined with scanning electron microscopy and transmission electron microscopy; the phase composition is analyzed by X-ray diffraction and convergent beam electron diffraction. The material forms a continuous external α-alumina scale throughout the temperature range. Below the scale, there is a continuous Mo₅(Si,Al)₃ layer that overlies Mo(Si,Al)₂ in the bulk. The Mo(Si,Al)₂ phase immediately beneath the Mo₅(Si,Al)₃ layer is depleted in Al. No indications of MoO₃ volatilization could be found for the Mo(Si,Al)₂ material.     

Fatigue crack growth behaviour of A533B steel in pressurized water at 288 and 28 °C

Fatigue crack growth behaviour of A533B steel was investigated in pressurized water at 288 °C using specimens machined from four different orientations. When inclusions were oriented along the direction of crack propagation, fatigue crack growth rate (FCGR) was enhanced compared to when they were perpendicular to the direction of crack propagation. At low ΔK levels FCGR in ambient water was slightly higher than that in 288 °C water. This may be attributed to the occurrence of intergranular cracking in ambient water tested specimen. Though mainly ductile striations were observed on the fracture surfaces, isolated intergranular facets (in a specimen tested in ambient water) and fan shaped features were also present. Hydrogen induced damage was clearly evident in the ambient water tested specimen in the form of isolated intergranular facets.     

Isothermal Oxidation Behavior of Dysprosium/S-Doped β-NiAl Alloys at 1200°C

A β-NiAl alloy with normal purity, a S-doped and a Dy and S co-doped b-NiAl alloys were prepared by arc-melting and their corresponding S contents were less than 20×10 6, 33×10-6 and 22×10-6, respectively. The isothermal oxidation behavior of the alloys at 1200° C was investigated and the extent of S segregation at the scalee alloy interface was determined by scanning Auger microscopy. S-doping had no significant effect on the phase transformation rate from q- to a-Al2O3, while the addition of Dy retarded this process. For the Sdoped alloy, scale rumpling occurred only after 2 h thermal exposure and numerous large voids were observed at the scalee alloy interface where S segregated. In contrast to this, the oxide scale formed on the Dy and S co-doped alloy still remained flat even after 50 h isothermal oxidation and only small voids existed at the interface where S segregation was not detected.