Secondary Creep Regimes Research Articles

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

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite (CFCC) was investigated at 1000 and 1100 °C in laboratory air and in steam. The composite consists of a porous alumina 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. The influence of the loading rate on tensile stress–strain response and on tensile properties at 1100 and 1200 °C in steam was also explored. A change in loading rate by four orders or magnitude had a profound effect on tensile strength. Tensile creep behavior was examined for creep stresses in the 80–150 MPa range. Primary and secondary creep regimes were observed in creep tests at all temperatures investigated. Creep rates increased with increasing temperature and creep stress. The presence of steam accelerated creep rates and dramatically reduced creep life. The detrimental effects of steam become progressively more pronounced with increasing temperature. At 1000 °C, creep run-out defined as 100 h at creep stress was achieved in all tests. At 1100 °C, creep run-out was achieved in all tests in air and only in the 100 MPa test in steam. The residual strength and modulus of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.

Creep behavior in interlaminar shear of Nextel ™720/alumina ceramic composite at elevated temperature in air and in steam

The creep behavior in interlaminar shear of an oxide–oxide ceramic matrix composite (CMC) was evaluated at 1200 °C in laboratory air and in steam using double-notch shear test specimens. The composite consists of a porous alumina 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 interlaminar shear properties were measured. The creep behavior was examined for interlaminar shear stresses in the 4–6.5 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air. In steam, the composite exhibited primary, secondary and tertiary creep. In air, creep run-out defined as 100 h at creep stress was achieved in all tests. In the presence of steam, creep performance deteriorated rapidly and run-out was achieved only at 4 MPa (∼50% of the interlaminar shear strength at 1200 °C). The retained properties of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated. Matrix degradation appears to be the cause of reduced creep lifetimes in steam.

Creep behavior of Nextel™720/alumina ceramic composite with ±45° fiber orientation at 1200 °C

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite (CFCC) with ±45° fiber orientation was investigated at 1200 °C in laboratory air, in steam and in argon. The composite consists of a porous alumina 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 46 GPa and the ultimate tensile strength (UTS) was 55 MPa. Tensile creep behavior was examined for creep stresses in the 15–45 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in all test environments for creep stress levels ⩽35 MPa. At creep stresses >35 MPa, creep performance was best in laboratory air and worst in argon. The presence of either steam or argon accelerated creep rates and reduced creep life. Composite microstructure, as well as damage and failure mechanisms were investigated.

Extrapolation of short-term creep rupture data—The potential risk of over-estimation

This work deals with the creep behaviour of 9–12% Cr steels in the steady-state (secondary) creep regime in order to enable a more detailed and exact description of the creep rupture strength on the basis of the Monkman–Grant relation. The stationary creep behaviour has been investigated by evaluating the creep rate and the change of stress exponent of established grades of high temperature creep resistant steels using the so-called back-stress concept. A change in creep mechanism with applied stress is clearly identified in the creep rupture curves. The impact of this change is discussed and the huge potential for over-estimation of creep strengths from extrapolated short-term creep rupture data is emphasized.

Creep and fatigue behaviors of the lead-free Sn–Ag–Cu–Bi and Sn60Pb40 solder interconnections at elevated temperatures

Creep and fatigue behaviors of the interconnections soldered by the lead-free Sn–Ag–Cu–Bi solder were investigated at different elevated temperatures (with the homologue temperature in the range of 0.71– 0.82), with a comparison to that of a traditional Sn60Pb40 solder. The results show that the lead-free Sn–Ag–Cu–Bi solder shows a superior anti-creep performance over the Sn60Pb40 solder, in terms of a much lower creep strain rate and a vastly elongated creep fracture lifetime; in the secondary creep regime, the calculated creep-activation energy for two solders is reasonably close to other published data. In addition, it has also been shown that the joints soldered by the lead-free Sn–Ag–Cu–Bi solder exhibits a superb fatigue property.

Creep behavior of Nextel TM610/Monazite/Alumina composite at elevated temperatures

The creep behavior of a N610/LaPO 4/Al 2O 3 composite was evaluated in this work. The composite consists of a porous alumina matrix reinforced with Nextel 610 fibers coated with monazite in a symmetric cross-ply (0°/90°/0°/90°) S orientation. The tensile stress–strain behavior was investigated and the tensile properties measured at room temperature and in the 900–1200°C range. The addition of monazite coating resulted in ∼50% improvement in ultimate tensile strength (UTS) at temperatures ⩽1100 °C, and in 37% improvement in UTS at 1200 °C. Tensile creep behavior was examined at temperatures in the 900–1100 °C range for creep stresses ranging from 40 to 150 MPa. Primary and secondary creep regimes were observed in creep tests at 900 °C. At temperatures above 900 °C, the composite exhibited primary, secondary and tertiary creep. Minimum creep rate was reached in all tests. Creep rates accelerated with increasing temperature and creep stress. At 900 °C creep run-out, defined as 100 h at creep stress, was achieved for stress levels ⩽120 MPa. The residual strength and modulus of all specimens that achieved run-out were characterized. Comparison with results obtained for N610/Al 2O 3 (control) specimens revealed that the use of the monazite coating resulted in improved creep resistance at 900 °C. Creep performance deteriorated rapidly as temperatures increased above 900 °C. Composite microstructure, as well as damage and failure mechanisms were investigated.

Effects of steam environment on high-temperature mechanical behavior of NextelTM720/alumina (N720/A) continuous fiber ceramic composite

Abstract Mechanical behavior of an oxide–oxide continuous fiber ceramic composite (CFCC) consisting of a porous alumina matrix reinforced with laminated, woven mullite/alumina fibers (Nextel TM 720) was investigated at 1200 and 1330 °C in laboratory air and in 100% steam environments. CFCC has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tension–tension fatigue behavior was studied for fatigue stresses ranging from 100 to 170 MPa at 1200 °C, and for fatigue stresses of 50 and 100 MPa at 1330 °C. Tensile creep behavior was examined for creep stresses ranging from 80 to 154 MPa at 1200 °C, and for creep stresses of 50 and 100 MPa at 1330 °C. At 1200 °C, the CFCC exhibited excellent fatigue resistance in laboratory air. The fatigue limit (based on a run-out condition of 10 5 cycles) was 170 MPa (88% UTS at 1200 °C). The material retained 100% of its tensile strength. Presence of steam caused noticeable degradation in fatigue performance at 1200 °C. Fatigue resistance at 1330 °C was poor. In creep tests, primary and secondary creep regimes were observed. Minimum creep rate was reached in all tests. At 1200 °C, creep rates were 10 −8 –10 −5 s −1 and maximum time to rupture was 255 h. At 1330 °C, creep rates were 10 −7 –10 −5 s −1 and maximum time to rupture was 87 h. Presence of steam accelerated creep rates and dramatically reduced creep life.

A critical damage criterion for creeping solids

A critical damage criterion for creeping solids

Analysis of steady state creep behaviour of 9–12% chromium ferritic–martensitic steels

The present work deals with the creep behaviour of 9–12% Cr steels in the steady state (secondary) creep regime in order to enable a more detailed and exact description of the creep rupture strength on the basis of the Monkman– Grant relationship. Special attention is paid to the microstructural aspects. The steady state creep behaviour has been investigated by evaluating the creep rate and the change of stress exponent of already established grades of high temperature creep resistant steels.

Thermal creep of Zr–Nb1%–O alloys: experimental analysis and micromechanical modelling

Zirconium alloys present a large variability of their mechanical behaviour with respect not only to their chemical composition but also to their microstructure. We analyze here the creep behaviour at 400 °C of two Zr–Nb1%–O alloys presenting identical chemical composition, crystallographic texture, grain size and grain shape. Both alloys only differ by the thermal cycles imposed during the fabrication process, either below (alloy A) or alternatively above and below (alloy B) the monotectoı̈d transition. This sole difference gives rise to creep rates varying by a factor of about 4 between the two alloys. From a microstructural point of view, alloys A and B differ by the precipitates distribution and the thermodynamical state (alloy B is in a metastable equilibrium state). Our experimental analysis based on mechanical tests, transmission electron microscopy (TEM) observations and phase analysis by X-ray diffraction strongly suggests an hardening effect of Nb in solid solution to explain the differences between alloys A and B. This result is confirmed by TEM X-ray spectrometry which gives a weight content of Nb in solid solution differing by about 0.1% between the two alloys. A predictive micromechanical model, based on the self-consistent affine scheme, is then applied. This model well captures the anisotropy of the specimens, and describes accurately both transient and secondary creep regimes. As a result of the identification procedure, identical hardening laws are obtained for the two alloys at the grain scale, and the saturating reference stress for prismatic slip is found to be higher for alloy B by about 30 MPa with respect to alloy A.

Tensile creep of fine grained (3–5 μm) Ti 3SiC 2 in the 1000–1200°C temperature range

In this paper we report on the tensile creep behavior of fine-grained (3–5 μm) Ti 3SiC 2 in the 1000–1200°C temperature, T, range and the 10–100 MPa stress, σ, range. The creep behavior is characterized by three regimes a primary, quasi-steady state and a tertiary. In the quasi-steady state range and over the entire range of testing temperatures and stresses, the minimum creep rate, ε ̇ min , is given by: ε ̇ min ( s −1)= ε ̇ 0· exp[19±1]·(σ/σ 0) 1.5±0.1· exp (−445±10) kJ/mol RT where σ 0=MPa and ε ̇ 0=1 s −1. The times to failure, t f, were fitted to an expression of the form: t f (s)=t 0· exp[−2±1]·[ ε ̇ min / ε ̇ 0] −0.9±0.1 where t 0=1 s. Interrupted creep tests show that volume-conserving plastic deformation is the dominant source of strain during the secondary creep regime, while cavities and microcracks are responsible for the acceleration of the creep rate during the tertiary creep regime. Like in ice, large internal stresses, especially at high stresses, are developed in Ti 3SiC 2 during deformation, as a consequence of its extreme plastic anisotropy. The response of Ti 3SiC 2 to stress appears to be determined by a competition between the rates of accumulation and relaxation of these internal stresses.

Evolution of the microstructure during high-temperature creep and oxygenation in directionally solidified YBa2Cu3O7-x

Abstract The microstructure of YBa2Cu3O7−x −Y2BaCuO5 melt-textured composities deformed in the secondary and tertiary creep regimes has been investigated by transmission electron microscopy. The high density of Y2BaCuO5 precipitates plays an important role in the microstructural development as pinning sites for gliding dislocations. In the secondary regime, trapped dislocations are dissociated, leaving a stacking fault with displacement vector [½—δ 0 ⅓]. A second stacking fault, ⅙〈301〉, is typically associated with the former stacking fault. At this stage, the deformation microstructure is dominated by diffusive processes between precipitates interconnected by the trapped dislocations. In the tertiary stage, dislocation multiplication is the main factor controlling the microstructure, which is characterized by a dramatic increase in the density of perfect dislocations with Burgers vectors 〈100〉 and 〈110〉. Since deformation is performed above the orthohombic-to-tetragonal transition temperature, the samples need to be oxygenated in order to achieve the superconducting phase. We have found that this oxygenation step, performed at 450°C, induces severe modifications of the as-deformed microstructure.

Microstructure and tensile creep mechanisms of an in situ reinforced silicon nitride

The creep behaviour of an acicular-grained Si 3N 4 in the temperature range of 1260–1400°C under 50–350 MPa tensile stress was investigated. The viscous flow of an amorphous grain boundary phase and uninhibited grain boundary sliding resulted in a fast initial creep regime characterized by a stress exponent = 1.8 and an activation energy = 509 kJ mol −1, which increased to 3.2 and 1054 kJ mol −1, respectively, in the secondary creep regime. Between these two regimes the creep rate dropped rapidly due to grain interlocking and loading of the acicular grains parallel to the tensile axis. An enhanced dislocation activity in the parallel grains and cavitation in the grain pockets were observed, but their roles in the creep deformation appear minor. It is proposed that the mechanism controlling the secondary creep regime is the nucleation and growth of surface steps on the atomically smooth (100) grain boundary plane through the solution/precipitation process.