Creep Rupture Properties Research Articles

Effect of HIP on Creep Properties in the Single Crystal Superalloy

The Rene N5 is a second-generation single crystal superalloy containing 3wt% Re, commonly used in gas turbine blades for power generation at temperatures up to 1600°C due to its excellent creep life. Generally, single crystal (SX) and directionally solidified (DS) blades are not applied to hot isostatic pressing (HIP) because their porosities are lower than conventionally cast (CC) blades, and it causes recrystallization issue. However, in this study, HIP was chosen as a candidate to improve the mechanical properties of the second-generation single crystal superalloy Rene N5. HIP followed by ultra-rapid cooling rate of 660K/minute was applied in an attempt to address this issue. This approach was expected to enhance creep rupture properties through pore reduction and microstructural homogenization. Furthermore, by setting the HIP temperature and time identical to the solution treatment condition, the possibility of replacing solution treatment with HIP was discussed. As results of creep rupture tests, the rupture life was excellent in the order of specimens treated with HIP + solution + aging, solution + aging, and HIP + aging treatments. It is interesting that this is found to be primarily influenced by the size and shape of carbides and γ’ phases, with the effects of crystallographic orientation, shrinkage porosity, and eutectic phase being negligible.

Creep rupture properties of dissimilar welded joint of ferritic/martensitic P92 steel and Inconel 617 alloy

The dissimilar welded joint (DWJ) comprising P92 steel and Inconel 617 (IN617) is frequently utilized in advanced ultra-supercritical (AUSC) boilers. In present work the creep rupture behaviour, rupture mechanism and microstructure evolution of DWJ of P92 steel and IN617 with Ni-based ERNiCrCoMo-1 filler, were examined at 650 °C under the stress range of 120–200 MPa. The results of the creep test indicated that the location of creep rupture varied across different testing conditions, encompassing areas such as P92 base metal and the fine-grained heat-affected zone (FGHAZ). The creep tested specimens within the stress range of 180–200 MPa exhibited failure originating from P92 base metal, predominantly influenced by plastic deformation. The failure manifested in a transgranular mode, driven by the formation, growth, and eventually coalescence of microvoids. The creep tested specimens within the stress range of 120–150 MPa displayed the characteristic ofType IV failure, primarily attributed to matrix softening and the absence of adequate precipitates to pin the PAGBs. Microstructural characterization unveiled the presence of microvoids along the PAGBs, facilitating microvoid nucleation primarily owing to the existence of the coarse brittle carbide phase. The growth of these microvoids over a period of time during tertiary stage of creep which lead to their coalescence and the formation of microcracks, ultimately resulting in premature intergranular failure. The specimen creep exposed at 650 °C under the lower stress of 120 MPa exhibited higher creep rupture life for 432 h. The SEM/EDS study of the crept sample of 120 MPa is also confirmed the presence of intermetallic laves phases in regions near the fracture tip and in the heat-affected zones (HAZs). Although the creep tested specimens at 150 MPa and 120 MPa exhibited failure in the FGHAZ and their elemental SEM/EDS analysis confirmed the presence of an oxide layer and notch formation near the interface of P92 base metal and ErNiCrCoMo-1 filler weld. The FESEM study revealed the growth of the oxide layer in the notch region, and it also showed the presence of multiple cracks and microvoids in the oxide layer. The formation and propagation of the oxide notch mainly led to the interfacial failure. The crept specimens subjected to 180 MPa and 200 MPa did not exhibit any cracks or microvoids in HAZs. However, specimens that failed under applied stresses of 120 MPa and 150 MPa revealed the presence of a high density of microvoids in the FGHAZ, inter-critical heat-affected zone (ICHAZ), and in the region of the coarse-grained heat-affected zone (CGHAZ) adjacent to the interface attached with an oxide layer.

High-Temperature Creep and Microstructure Evolution of Alloy 800H Weldments with Inconel 625 and Haynes 230 Filler Materials

Alloy 800H stands as one of the few code-qualified materials for fabricating in-core and out-of-core components operating in high-temperature reactors. Welding is a common practice for assembling these components; however, the selection of a suitable filler material is essential for enhancing the high-temperature creep resistance of Alloy 800H weldments in high-temperature applications. In this study, Inconel 625 and Haynes 230 filler materials were used to weld Alloy 800H plates by employing the gas tungsten arc welding technique. The high-temperature tensile and creep rupture properties, microstructural stability, and evolution of the weldments after high-temperature exposure were investigated and compared with those of Alloy 800H. The results show that both weldments exhibit enhanced tensile and creep behavior at 760 °C. The creep rupture times of the weldments with Inconel 625 filler and Haynes 230 filler materials were about two and three time longer, respectively, than those of Alloy 800H base metal when tested at 80 MPa and 760 °C. Carbides (MC and M23C6) were commonly observed in the microstructures of both the weld and base metals in the two weldments after high-temperature creep tests. However, the Inconel 625 filler weldment displayed detrimental δ and Laves phases in the fusion zone, and these precipitates could be potential sites for initiating cracks following prolonged high-temperature exposure. This study shows that the weldment with Haynes 230 filler material exhibit better phase stability and creep rupture properties than the one with Inconel 625, suggesting its potential for use as a candidate filler material for Alloy 800H for further investigation. This finding also emphasizes the critical consideration of microstructural evolutions and phase stability in evaluating high-temperature materials and their weldments in high-temperature reactor applications.

Influence of Ru on high-temperature creep rupture properties and oxidation behavior of Ni-based single-crystal superalloys

The present study investigates the microstructure, high-temperature creep rupture properties and oxidation resistance of three fourth-generation single-crystal Ni-based superalloys containing 0, 3.5 and 7 wt% of Ru. The addition of Ru changes the partitioning behavior of the alloying elements, decreases γ' size and increases the γ/γ' lattice misfit. The creep resistance keeps increasing as Ru content rises up 7 wt% under 1100 °C/140 MPa. However, it was discovered that 7 wt% Ru alloy has inferior creep resistance compared with the 3.5 wt% Ru one under 1150 °C/140 MPa, although no TCP precipitation was observed in this 7 wt% Ru-containing alloy. It was revealed that 7 wt% Ru-addition caused significant γ' dissolution and induced the generation of creep cavities, which may account for the anomaly. Furthermore, the increase in Ru content up to 7 wt% keeps degrading the oxidation resistance of the superalloys by facilitating the formation of pores and cracks within the oxide scales, which also accelerate scale spallation. This work suggests that excess Ru addition may adversely affect both the mechanical properties and the oxidation resistance of the fourth-generation Ni-based superalloys.

Correlation between reliability and safety factor based methods in characterizing uncertainty of creep rupture properties

Reliability and safety factor based methods are usually employed to characterize uncertainty of creep rupture properties, but the correlation between these two methods is rarely reported. In this work, a framework of correlation between reliability and safety factor based methods was proposed, and application of this framework in representation of minimum stress to creep rupture was reported. Effect of standard deviation on correlation results was discussed and comparisons of minimum stress to creep rupture in codes and standards were conducted. Results indicated that for a given reliability, the safety factor included in the minimum stress to creep rupture is higher at a small stress level and a high temperature. Accordingly, the minimum stress to creep rupture by reliability based method in ASME code (i.e. 95 %) is lower than that by safety factor based method in RCC-MRx code (i.e. 1.25), presenting a smaller allowable stress value and more conservative evaluation. Contrarily, a smaller safety factor is gained at a high stress level and a low temperature for a given reliability. The opposite conclusion is drawn when the reliability at a given safety factor is analyzed. In addition, the safety factor of 1.5 is larger than that at the reliability of 99 % for a wide range of stress levels, inducing an over-conservative design and excessive manufacturing costs.

Influence of section thickness of casting on the microstructure characteristics and creep rupture properties of Ti-45Al-2Nb-2Mn-1B

Ti-45Al-2Nb-2Mn-1B (atomic percent, 4522XD) cast step plate with different thicknesses was prepared by investment casting, and the variations of microstructure and creep rupture properties with thickness were examined. With decreasing thickness, lamellar colony size, lamellar thickness, and the size of equiaxed γ grains at the colony boundaries decreased, and the colony boundaries varied from smooth to serrated. Boride particles are mainly long flat needles in the region with 12 mm thickness, curvy flakes with considerable length and width in the region with 8 mm thickness, and curvy ribbons with small length in the region with 4 mm thickness. B27-TiB particles were observed in all the thicknesses, Ti3B4+ TiB2 lamellar particles were observed in the region with 8 mm and 12 mm thicknesses, and Bf-TiB+ TiB2 lamellar particles were observed in the region with 4 mm thickness. The smallest creep rupture elongation is obtained in samples taken from the plate with 12 mm thickness, while the shortest creep rupture life is obtained in samples taken from the region with 8 mm thickness. The former is attributed to the smooth colony boundaries which have weaker resistance to grain rotation, while the latter is attributed to the curvy boride particles that tend to nucleate cracks at the corner with small curve radius, providing a fast track for crack propagation along the boride/matrix interfaces, their considerable size makes the crack reach the critical size for fracture more easily. Weak creep rupture properties of some samples are mainly attributed to the aggregation of colonies whose lamellar interfaces are nearly perpendicular to the loading direction.

An advanced approach to improve the high-temperature property for Ni-based superalloys: Interface segregation manipulation

In response to the requirements for the long-term service reliability of Ni-based superalloys, the correlations between the microstructure and creep property are investigated in present work. Creep experiments were conducted on a designed Ni-based single crystal superalloy at 900 °C/330 MPa, the results indicate that the creep rupture properties deteriorated following thermal exposure at both 900 °C and 1000 °C. The γ′-phase, γ-matrix and γ/γ′ interface undergo varying degrees of degradation, including the coarsening and the decrease in the volume fraction of γ′-phase, the decrease in the γ-matrix strength, the decrease in the lattice misfit and the increase in the dislocation network spacing. When the values of the microstructural parameters (γ′-phase size and volume fraction) were numerically similar for different thermal exposure times at 900 °C and 1000 °C, taking 900 °C/10,000h and 1000 °C/2000h samples as examples. The experimental alloy exhibited better creep resistance after being exposed to 1000 °C compared to 900 °C. Through the combination of atom probe tomography and first-principles calculations, an interface strengthening mechanism driven by interface segregation is identified. Finally, a design concept for high-strength metal alloys based on the decoration of solutes interface segregation is proposed, also called “Interface Segregation Manipulation”.

Enhancing mechanical properties and creep performance of 304H and inconel 617 superalloy dissimilar welds for Advanced Ultra Super Critical power plants

This investigation is conducted to find improvements in the mechanical properties of the dissimilar welds between austenitic stainless steel and Nickel superalloy used in the manufacturing of Advanced Ultra Super Critical (AUSC) power plants boiler components. Nickel superalloys (IN617) have excellent creep rupture properties, so they are employed in higher temperature regions (above 700 °C) of the boiler. In contrast, austenitic stainless steel (304H) is used in moderate temperature regions (up to 650 °C) part of the boiler, also striking off the economic balance need. The welding is carried out with manual multipass Gas Tungsten Arc Welding using an IN617 filler to compare 304H and IN617 weld without and with a buttering layer. Multiple attenuated buttering material layers are deposited at the 304H side, further machining the butter weld layer to the weld groove and closure weld to the IN617 part. The buttering technique improves mechanical properties and high-temperature creep rupture life through microstructure transformation. The dissimilar weld characterization shows a supplementary dendritic growth layer for the buttering deposition. The tensile strength is found to be 728.214 MPa with buttering higher than without buttering weld (620.25 MPa). The V-notch Charpy impact toughness value of the buttered weld sample is 66 J, greater than the non-buttered sample, i.e., 63.25 J. The creep rupture duration significantly increased with buttering. Strain hardening at higher applied stress helps to resist creep failure even at higher temperatures.

Deformation Behavior of AEREX350 Alloy during Bolt Hot Heading Processing

AEREX350 is a nickel-cobalt fastener superalloy developed for high temperature applications such as aerospace and petrochemical industries. The AEREX350 alloy has superior creep resistance and rupture properties similar to the other MP family alloys. As for AEREX350 alloy, scholars have carried out many studies on its cold working process, heat treatment process, stress relaxation performance and identification of precipitated phase [3-9], while very limited research on the hot deformation behavior of this interesting alloy during hot heading process. This paper is concentrated on the deformation behavior of the alloy during hot heading process.

Evaluation of Internal Pressure Creep Rupture Life of Boiler Tube Using Miniature Tensile Creep Test

Abstract Many studies have been made for evaluating the creep rupture life of a boiler tube. The working stress of a tube under internal pressure is calculated mostly by the mean-diameter formula adopted in design for pressurized components. However, this stress is not always representative of a complex stress state under internal pressure. So, focusing on the representative stress, internal pressure and uniaxial creep tests were conducted using unused and 90,000-h used boiler tubes of Type 321 stainless steel in this study. A miniature specimen (1.5 mm in width, 1.5 mm in thickness, and 25 mm in length) for the uniaxial test was taken from the middle of the tube wall in the direction of the maximum principal stress. As a result, the mean-diameter formula did not present good agreement between internal pressure and uniaxial creep rupture lives. Hence, as an alternative stress, von Mises equivalent stress, which presents good correlation between uniaxial and multiaxial creep rupture properties, was chosen. The equivalent stress, σ*, at the middle of a tube wall was calculated using Rimrott’s solution to internal pressure creep of thick-walled tubes considering large deformation. Because σ* increases with creep strain under internal pressure, σ* at εθo = εθof / 2 (where, εθo and εθ0f are tangential and tangential rupture strains on the outer diameter, respectively) was employed as the representative stress. Consequently, this stress resulted in good agreement between both creep rupture lives for the unused tube. However, the internal pressure creep rupture life of the used tube was inconsistent with the uniaxial life, which was attributed to the existence of an outside layer due to exposure to combustion gas. Using a reduction rate of the rupture ductility of the layer, the internal pressure creep rupture life of the used tube was evaluated well with the unused tube.

Optimization and Verification of Ultra-Miniature Specimen for Evaluating Creep Property of In-Service Component Material under Uniaxial Loading

Abstract A practical procedure of creep life prediction has already been applied to an in-service boiler steam pipe in Japan. The procedure is based on a few creep tests by a small punch creep (SPC) specimen machined from a thin coupon sample, which is successfully taken from the outer surface of the pipe by an electric discharge sampling equipment. However, a coefficient converting the applied load into stress is required in the application of the SPC test data to the life prediction. Thus, in this study, as an alternative specimen with a dimension equivalent to a SPC disc specimen (8 mm in diameter and 0.5 mm in thickness), a thin plate specimen referred to as the ultra-miniature creep (UMC) specimen (2 mm in width, 10 mm in length, and 0.5 mm in thickness) was devised for a uniaxial tension creep test. Compared to the creep and creep-rupture properties obtained by a standard round bar (φ6 mm) specimen machined from a replaced steam pipe material (ASME Grade 91 steel), the stress-rupture relationship of the UMC specimen was found to coincide with that of the standard one. However, the creep strain and creep rate of the UMC specimen measured by a pull-rod displacement were larger than those of the standard one. From an elastic-creep analysis by a finite element method, this was attributed to superimposition of creep deformation in a grip portion clamping the UMC specimen on the creep strain in the parallel portion of the specimen. So, from the analytical results, an adjustment factor was derived to modify the creep property of the UMC specimen. As a result, application of this factor was found to lead to coincidence of the creep property between UMC and standard specimens.

Role of Thermo-Mechanical Treatment on Creep Deformation Behaviour of Reduced Activation Ferritic Martensitic Steel

<div class="section abstract"><div class="htmlview paragraph">To enhance the microstructural and mechanical properties of Reduced Activation and Ferritic-Martensitic (RAFM) steel thermo mechanical treatment (TMT) was performed on as received Normalised and Tempered steel (N+T). The RAFM with 9Cr-1W-0.06Ta major alloying elements steel was used in the study in N+T and TMT conditions. The refinement in microstructure and precipitates was observed in TMT steel in comparison to N+T steel. There is a drastic improvement in mechanical properties such as hardness, tensile properties are observed without losing the ductility. The creep deformation behaviour of thermo-mechanical treated steel and N+T steel were studied at different stress levels at 823 K. The comparative creep rupture and strain properties were studied, and various creep deformation regimes were also analysed for both the conditions of steel. The rate of exhaustion of primary creep (r), minimum creep rate and time spent in primary and tertiary creep regime, rate of acceleration of tertiary creep and time to reach the onset of tertiary creep. Microstructural studies on Optical and Scanning electron microscopy and transmission electron microscopy results were correlated with the mechanical properties of steels. On a whole, a comparative investigation was performed on various reasons for drastic improvement in tensile and creep rupture properties of TMT steel than the steel in N+T steel and necessary microstructural investigation support is considered.</div></div>

Correlation microstructural evolution with creep-rupture properties of a novel directionally solidified Ni-based superalloy M4706

Correlation microstructural evolution with creep-rupture properties of a novel directionally solidified Ni-based superalloy M4706

Effect of Grain Size on Mechanical and Creep Rupture Properties of 253 MA Austenitic Stainless Steel

The effect of grain size on the mechanical properties and creep rupture of 253 microalloyed (MA) austenitic stainless steel (ASS) was investigated. The cold rolling process with a 53% reduction in thickness was applied to the steel followed by annealing at 1100 °C over 0, 900, 1800, and 3600 s to obtain grain sizes of 32.4, 34.88, 40.35, and 43.77 µm, respectively. Uniaxial tensile and micro-Vickers hardness tests were carried out to study the effect of grain size on mechanical properties at room temperature. The creep rupture test was performed at 700 °C under a load of 150 MPa. The results showed that there was a correlation between grain size, mechanical properties, and creep rupture time. The fine initial grain size showed relatively good mechanical properties with a short creep rupture time, while the coarse initial grain size produced low mechanical properties with a long creep rupture time. The initial grain size of 40.35 µm was the optimum grain size for a high value of creep rupture time due to the low hardness and elongation values at room temperature and low creep ductility value. The intergranular fracture was found on the initial grain size below 40.35 µm, and a mixed mode of intergranular and transgranular fracture was found on the initial grain size above 40.35 µm after the creep rupture test.

Laser additive manufacturing of Inconel 718 at increased deposition rates

Inconel 718 (IN718) is a nickel-based superalloy designed to display exceptionally high yield, tensile and creep-rupture properties at temperatures up to approx. 700 °C. For aerospace applications, it is the most widely used superalloy. The additive manufacturing process, laser-based direct energy deposition (DED) with powdery filler material can be used to repair, geometrically modify or manufacture components made of IN718. However, despite the devious advantages, its typical deposition rate of <0.5 kg/h is significantly lower than that of other DED processes such as metal inert gas (MIG) of wire + arc additive manufacturing (WAAM) – at about 10 kg/h – and electron beam additive manufacturing (EBAM®) with wire - at approx. 12 kg/h. Here, using IN718 as additive material, we targeted to increase the deposition rate of laser-based DED by a factor of more than ten and achieved a deposition rate of about 6 kg/h. For this purpose, we first developed a suitable process management strategy in combination with adapted system technology. Then, we built up samples with developed process parameters, from which tensile specimens for mechanical property tests were manufactured. Thereafter, the macro- and microstructures of the as-deposited material were analyzed. Based on the results, we applied a post heat treatment for modifying the microstructure and improving the mechanical properties. We demonstrated finally, the aerospace material specifications for the static mechanical properties can be achieved. This work shows a great potential of laser-based DED with increased deposition rate.

Role of Cr Content in Microstructure, Creep, and Oxidation Resistance of Alumina-Forming Austenitic Alloys at 850–900 °C

Creep-rupture properties and oxidation behavior of a series of alumina-forming austenitic (AFA) alloys with variations of Cr contents, based on Fe-(13.5-18)Cr-25Ni-4Al-1.5Nb-0.1C in weight percent, have been evaluated at 850–900 °C. The study investigates material responses in the properties and microstructure through compositional modifications in AFA alloys, targeting performance optimization of alloys under high-temperature, corrosive industrial environments. The creep-rupture life of the alloys at 850 °C and 30MPa monotonically decreased with increasing Cr content, which was correlated with changes in secondary phase volume fractions, such as the reduction in B2-NiAl + Laves-Fe2Nb and increase in Sigma-FeCr with Cr content. The oxidation test at 900 °C in a water-vapor containing environment revealed a range of Cr content from 13.9 to 15.7 wt.%, enabling the formation of stable, protective external alumina scale as well as preventing internal oxidation/nitridation for up to total 7000 h exposure. On the other hand, the alloys with &gt;16.7 wt.% Cr formed Sigma precipitates, which caused a reduction in not only Cr but also Al in the austenite matrix, resulting in less oxidation resistance than other alloys. The findings will guide the further optimization of material performance in the AFA alloy series.

Verification of Long-term Creep Rupture Properties and Microstructure Stability of Ni based Alloy 45Ni-24Fe-23Cr-7W-Ti Parent Metal and Weldment

ABSTRACT To clarify the long-term creep rupture strength and microstructure stability of Ni-based alloy HR6W (45Ni-24Fe-23Cr-7W-Ti, ASME Code Case 2684), creep tests were conducted up to about 70,000h and 117,000h for the weldment and parent metal respectively, and the microstructure of crept specimens was experimentally investigated. The weldment showed comparable strength with the parent metal. The metallurgical microstructure characterization and quantitative evaluation on the precipitation behavior indicated the precipitation strengthening of Laves phase, M23C6 carbide and αCr phase in the HAZ (Heat Affected Zone) and parent metal contributing to the long-term creep rupture strength stability of weldment. Satisfactory microstructural stability was proven without distinct degradation during long-term creep at 700°C up to around 117,000h. Furthermore, the internal pressure creep test of circumferential weldment was performed to verify the damage and failure morphology of actual components under the multiaxial loading

Correlation of hardness with maximum allowable stresses of Grade 91 steel

ABSTRACT Separately using the data at the individual hardness levels of 192-197, 206-211 and 216HBW, we have analysed the correlation of 550, 575, 600 and 650℃ creep-rupture properties of the specimens with the given hardness levels. The ranges of hardness values satisfying the requirement of maximum allowable stresses at the elevated temperatures specified by the ASME BPVC 2017 and the ASME BPVC 2019 are respectively presented by using an approach proposed. The maximum allowable stress values may directly correlate to the lower limits of the hardness values at the temperatures given. Typically, a lower limit of 205HBW is demonstrated to be suitable not only for the application of the present Type 1 and Type 2 Grade 91 materials at a temperature of ≤ 600℃ but also for the previous Grade 91 components with a wall-thickness of ＞ 75mm at a temperature of ≤ 575℃.

Creep Behavior and Phase Equilibria in Model Precipitate Strengthened Alumina-Forming Austenitic Alloys

Creep-rupture behavior and microstructural response in alumina-forming austenitic (AFA) alloys with two different precipitation strengthening mechanisms, “Laves-phase + M23C6 carbide” and “coherent L12 γ′-Ni3(Al,Ti),” were explored as “model” cases of multi-phase, multi-scale heat-resistant AFA alloys for 650–750°C use. These alloys will be used to guide and verify computational alloy design and life-prediction modeling under an on-going eXtremeMAT project through the Office of Fossil Energy and Carbon Management, US Department of Energy. Computational thermodynamics were used to design and predict the amounts of strengthening and deteriorating secondary phases at 750°C. Creep-rupture lives of the alloys tested at 750°C and 100 MPa were in a range of 4000–9000 h, and the microstructure at the gage/grip after creep-rupture testing was compared with isothermally aged alloys for 1500 h, as well as the calculated phases. Detailed microstructure characterization includes phase identification, volume fraction measurement, and compositional analysis, which were correlated with the creep-rupture properties. High-temperature oxidation resistance was also screened and compared with commercial, chromia-forming heat-resistant steels. These model alloys also provide the basis for further design and optimization of next generation AFA alloys with improved creep resistance.

Creep properties of melt infiltrated SiC/SiC composites with Sylramic™‐iBN and Hi‐Nicalon™‐S fibers

AbstractIsothermal tensile creep tests were conducted on 2D woven and laminated, 0/90 balanced melt infiltration (MI) SiC/SiC composites at stress levels from 48 to 138 MPa and temperatures to 1400°C in air. Effects of fiber architecture and fiber types on creep properties, influence of accumulated creep strain on in‐plane tensile properties, and the dominant constituent controlling the creep behavior and creep rupture properties of these composites were investigated. In addition, the creep parameters of both composites were determined. Results indicate that in 2D woven MI SiC/SiC composites with Sylramic™‐iBN or Hi‐Nicalon™‐S fibers, creep is controlled by chemical vapor infiltration (CVI) SiC matrix, whereas in 2D laminated MI SiC/SiC composites with Hi‐Nicalon™‐S fibers, creep is controlled by the fiber. Both types of composites exhibit significant variation in creep behavior and rupture life at a constant temperature and stress, predominantly due to local variation in microstructural inhomogeneity and stress raisers. In both types of composites at temperatures &gt;1350°C, residual silicon present in SiC matrix to reacts with SiC fibers and fiber coating causing premature creep rupture. Using the creep parameters generated, the creep behaviors of the composites have been modeled and factors influencing creep durability are discussed.