Internal friction in copper and α-brasses during plastic deformation

The internal friction Q ^–1 of annealed copper and -brass wires containing 10, 20, 30 and 35 at. % of zinc was studied by a torsional oscillation method during plastic deformation. The results are interpreted in terms of two theoretical models ascribing the amplitude-dependent internal friction, observed in the pre-yield stage, to coupling of the cyclic stress with the creep component of the deformation, and the amplitude-independent internal friction at larger, plastic, strains to losses arising from contributions of the torsional stress to the plastic deformation. Up to the maximum tensile strain of 1 % used in the experiments, the influence of zinc content on Q ^–1 is not pronounced.     

Static creep behaviour of Al-Zn-Mg and Al-Zn-Mg-Cu alloys

The creep behaviour of Al-Zn-Mg (7039) and Al-Zn-Mg-Cu (7075) alloys is evaluated at elevated temperatures (443T533 K and 483T563 K) under constant stresses between 49 and 123 MPa, respectively, in a custom-built creep testing facility. The measured activation energies of these alloys are 172–185 kJ mo^–1 and 248–272 kJ mol^–1. As the stress increases, the activation energy in both cases decreases due to the high density of dislocations. The average exponent values of these alloys are 7 and 9. The microstructure observation reveals that the dominant fracture mode of 7039 alloy is intergranular and that of 7075 alloy is transgranular.     

Environmental and time dependence of fracture toughness and crack growth in glass-fibre reinforced polyester resins

The validity of simple (maximum load) fracture toughness testing of glass-fibre reinforced polyester resin laminates has been examined for SEN specimens using slow strain rate test techniques developed for metallic materials. Laminates were tested both in air and whilst immersed in an acidic environment. In general, valid test conditions were only established at test speeds <10^–2 mm min^–1 which are slower than those generally reported in the past. Fracture toughness was found to be greatly reduced by the presence of the chemical environment, as would be expected from the known sensitivity of these materials to environmental stress cracking. The slow strain rate test technique indicated a threshold level of critical stress intensity for environmental stress cracking and hence could be useful in ranking materials.Environmental effects were also found to determine crack growth rates as measured on DCB specimens. Crack growth data was analysed in terms of initial and/or inherent flaws and its use in predicting creep-rupture life examined. In principle it was concluded that use of crack growth data could provide an alternative to the present qualitative approach to design and a more predictable alternative to long term creep-rupture testing.     

Dimensional changes and creep of silica core ceramics used in investment casting of superalloys

Dimensional changes and creep deformation of a silica/zircon (74%/24%, respectively) and a high silica (93% silica and 3% zircon) ceramic were characterized and compared. All specimens were tested with a thermal profile that consisted of a 300°C/h heating rate to 1475 or 1525°C, followed by a one-hour isothermal hold (where each specimen was compressively crept under a static stress of 2.07, 4.14, or 6.21 MPa). The specimens were cooled at a rate of 900°C/h under stress. Dimensional changes were interpreted from apparent thermal expansion behavior during heating as well as before-and-after dimensional measurements. The silica/zircon ceramic generally exhibited less total contraction than the high silica ceramic for a specific test condition even though it crept faster at all stresses and temperatures during the one-hour isothermal/isostress segment. This indicates that the total contraction for both was dominated by reinitiated sintering and subsequent cristobalite formation that occurred during the heating segment. Minimum creep rate during the one-hour isothermal/isostress segment was examined as a function of stress and temperature for both ceramics using a power-law creep model. Creep-rate stress exponents (n) and activation energies (Q) were equivalent (within 95% confidence) for both ceramics showing that their different contents of zircon (3 vs. 24%) did not affect them. Lastly, n 1.3–1.4 and Q 170 kJ/mol indicate that diffusion-assisted crystallization of cristobalite, combined with power-law sintering owing to the high concentration of porosity (28–30%) was likely the rate-limiting mechanism in the creep deformation for both ceramics.     

Creep-sintering of polycrystalline ceramic particulate composites

The sintering of particulate composites consisting of a polycrystalline zinc oxide matrix with 10 vol % zirconia inclusions of two different sizes (3 and 14 m) was investigated at a constant heating rate of 4 °C min^–1 under an applied stress of 300 kPa. The presence of the inclusions produced a decrease in both the creep rate and the densification rate but the ratio of the densification to creep rate remained constant during the experiment. The ratio of the densification rate to creep rate for the composites was 1.5 times greater than that of the unreinforced matrix regardless of inclusion size. The creep viscosity of the composites was higher than that of the unreinforced matrix and increased slightly with decreasing inclusion size.     

A study of the Bailey-Orowan equation of creep

A new method was developed to study the Bailey-Orowan equation of creep, _c=r/h, where _c is the creep rate,r is the recovery rate andh is the work-hardening coefficient. The method was to vary the strain rate,, around the creep rate, _c, and to measure the corresponding stress rate,. In a plot of stress rate against strain rate, a straight line was obtained. The slope of the straight line was equal toh, and the intersection of the straight line with the stress axis was equal to –r, as in the equation=–r+h. The creep test under a constant stress is a special case of this equation when the stress rate,, is zero. The above measurement was carried out within a very small stress variation, less than 1% of the total stress, so that the values ofr andh were not disturbed. The creep test was performed on Type 316 stainless steel. The creep rate was shown to be equal to the ratior/h, but the value ofh was approximately equal to Young's modulus at the testing temperature, rather than, as is commonly believed, to the work-hardening coefficient.     

The thermal expansion of carbon fibre-reinforced plastics

Interferometric measurements of the linear thermal expansion coefficients of epoxy resin DLS 351/BF₃400 are reported over the approximate temperature range 90 to 500 K. Corresponding measurements in directions parallel and perpendicular to the fibres are also reported for unidirectional composite bars of Courtaulds HTS carbon fibre in this resin, at nominal fibre volume contents of 50, 60 and 80%. The results are qualitatively similar to earlier observations upon resin ERLA 4617/mPDA-based specimens, but effects associated with resin softening occur at significantly higher temperatures in the case of resin DLS 351/BF₃400. Current theoretical models account successfully for the influence of fibre volume fraction in the range 0.5 to 0.8 upon the value of the coefficient of thermal expansion at room temperature, within the limitations imposed by experimental uncertainty, provided that appropriate values are assigned to the linear thermal expansion coefficients of the fibres. In the directions parallel ( ^f ) and perpendicular ( ^f ) to the fibre axis these values have been selected from the ranges –10×10^–7< ^f <–9×10^–7 and 0.5×10^–5< ^f <1.9×10^–5. It is concluded that a more rigorous appraisal must await the availability of independent information concerning the directional thermoelastic properties of carbon fibres.     

Creep of (Mg,Fe)O single crystals

The creep behaviour of (Mg, Fe)O single crystals compressed along 1 0 0 has been investigated over the temperature range 1300 to 1500° C, at stresses between 20 and 70 MPa, for oxygen partial pressures between 10^–4 and 10² Pa, and with iron concentrations between 70 and 11 900 p.p.m. Under these conditions, the dependence of the steady-state strain rate on stress, temperature, oxygen partial pressure, and iron concentration can be summarized by the flow law, exp (–445 kJ mol^–1/RT. These results suggest that the steadystate strain rate is controlled by dislocation climb with a jog velocity which is limited by lattice diffusion of oxygen by a vacancy pair mechanism. The activation energy for creep, 445 kJ mol^–1 is larger than that reported for self-diffusion of oxygen, 330 kJ mol^–1, because the formation energy for jogs is relatively large, 115 kJ mol^–1.     

A substructure characterizing parameter in creep

A substructure characterizing parameter which is the ratio of applied stress, , in creep test to yield stress, _YS, of the material at the test temperature is introduced. A correlation is found to exist between this parameter and the creep rate for the data obtained in the temperature range 820–975 K when the initial yield strength is modified by (i) introducing different amounts of prior cold work by two modes of deformation at room temperature in a type 316 LN stainless steel and (ii) grain size, chemistry and grain size variation in a type 316 stainless steel. The correlation was found to exist also for a Cr-Mo-V steel at 823 K, in which different yield strengths were due to different heat treatments. Minimum creep rate when plotted against the substructure characterizing parameter yields an exponent similar to Norton's creep exponent and it is postulated that the value of the exponent reflects on the type of substructure developed in creep. Another parameter /F _ys where F _ys is a function of the ratio of the yield strength of a given microstructure to that of a reference microstructure (zero cold work for cold worked material, largest grain size when the microstructure variation is through grain size and solution annealed microstructure among heat treatments) also gives a unique correlation with the minimum creep rate at a test temperature with the exponent identical to Norton's creep exponent.     

Effects of heat treatments on the creep properties of a hot pressed silicon nitride ceramic

Effects of heat treatment in an argon atmosphere at high temperatures for varying times on the creep properties of a Y₂O₃-Al₂O₃ (8-2 wt%) doped hot pressed silicon nitride (HPSN) ceramic were investigated. It was observed from the creep measurements that higher temperature, i.e. 1360C, and longer time, i.e. 8 h, heat treatment in an argon atmosphere improved the creep properties, (e.g. secondary creep rate) of this material. Heat treatment at a lower temperature of 1300C and for a shorter time of 4 h did not change the creep behaviour. Improvement of the creep properties was related to the crystallization of an amorphous grain boundary phase by heat treatment. Secondary creep rate parameters of the as-received material: stress exponent, n (2.95–3.08) and activation energy, Q (634–818 kJ mol^S–1), were in the range of values found by other investigators for various hot pressed silicon nitride ceramics.     

The Combined Influence of Molecular Weight and Temperature on the Physical Aging and Creep Compliance of a Glassy Thermoplastic Polyimide

The effect of molecular weight on the viscoelastic performance of anadvanced polymer (LaRC-SI) was investigated through the use of creepcompliance tests. Testing consisted of short-term isothermal creep andrecovery with the creep segments performed under constant load. Thetests were conducted at three temperatures below the glass transitiontemperature of five materials of different molecular weight. Through theuse of time-aging-time superposition procedures, the material constants,material master curves and aging-related parameters were evaluated ateach temperature for a given molecular weight. The time-temperaturesuperposition technique helped to describe the effect of temperature onthe timescale of the viscoelastic response of each molecular weight. Itwas shown that the low molecular weight materials have higher creepcompliance and creep rate, and are more sensitive to temperature thanthe high molecular weight materials. Furthermore, a critical molecularweight transition was observed to occur at a weight-average molecularweight of _w 25,000 g/mol below which, the temperature sensitivity of thetime-temperature superposition shift factor increases significantly. Theshort-term creep compliance data were used in association with Struik'seffective time theory to predict the long-term creep compliance behaviorfor the different molecular weights. At long timescales, physical agingserves to significantly decrease the creep compliance and creep rate ofall the materials tested. Long-term test data verified the predictivecreep behavior. Materials with higher temperature and lower molecularweights had greater creep compliance and higher creep rates.     

Stress relaxation and creep of cork

The stress relaxation and creep behaviour of cork under compression were characterized in tests done with the compression axis parallel to each of the three principal directions in the tree (radial, tangential and axial). All stress relaxations lead to a linear variation of stress with the logarithm of time, the slopes being nearly independent of stress and direction of compression. Creep stresses in the range 0.36 to 1.72 MPa were used. The strain rate continuously decreases during creep, from initial values around 10^–4sec^–1 to 10-7 sec^–1 after 8 h, but its dependence on the creep stress and direction of compression is not simple, mainly because different deformation regimes may operate during a single creep test. Compression loading-relaxation-unloading cycles were imposed on specimens, with compression either in the radial or in the tangential direction, with the purpose of simulating the performance of a cork stopper. A work softening is observed, i.e. the resistance decreases in successive compressions, particularly between the first two. This is explained in terms of an increased undulation of the cell walls produced in the first compression.     

Grain-boundary de-segregation and intergranular cohesion in Si-Al-O-N ceramics

The influence of heat-treatment on high-temperature creep and sub-critical crack growth in hot-pressed Si-Al-O-N ceramics has been analyzed from microstructural evidence and determination of stress exponents and activation energies. The most significant change is the suppression of cavitation during creep and of the cavity-interlinkage mechanism for slow crack propagation. A creep mechanism of grain-boundary diffusion is characterized by stress exponent n=1 and unusually high activation energy >820 kJ mol^–1. The microstructural origin of the transformation in grain-boundary dominated properties is mainly the removal of triple-junction glassy residues within which cavities are nucleated. This is caused by grain-boundary diffusion of metallic impurities (Mg, Mn, Ca) into a surface silica oxidation layer, and consequent crystallization of the remaining glass components as . There is a continued improvement in grain-boundary cohesion and increased difficulty of grain-boundary diffusion following the stage at which triple-junction glass is removed. The resultant ceramics, in addition to superior mechanical behaviour, have an increased temperature for application due to a marked reduction in susceptibility to dissociation above 1400° C.     

Tensile creep behaviour of a fibre-reinforced SiC-Si3N4 composite

The tensile creep behaviour of a SiC-fibre-Si₃N₄-matrix composite was investigated in air at 1350 C. The unidirectional composite, containing 30 vol % SCS-6 SiC fibres, was prepared by hot pressing at 1700 C. Creep testing was conducted at stress levels of 70, 110, 150 and 190 MPa. An apparent steady-state creep rate was observed at stress levels between 70 and 150 MPa; at 190 MPa, only tertiary creep was observed. For an applied stress of 70 MPa, the steady-state creep rate was approximately 2.5×10^–10 s^–1 with failure times in excess of 790 h. At 150 MPa, the steady-state creep rate increased to an average of 5.6×10^–8 s^–1 with failure times under 40 h. The creep rate of the composite is compared with published data for the steady-state creep rate of monolithic Si₃N₄.     

High temperature bending creep of a Sm-α-β Sialon composite

The four-point bending creep behavior of a Sm-- Sialon composite, in which Sm-melilite solid solution (denoted as M) was designed as intergranular phase, was investigated in the temperature range 1260–1350°C and stresses between 85 and 290 MPa. At temperatures less than 1300°C, the stress exponents were measured to be 1.2–1.5, and the creep activation energy was 708 kJ mol^–1, the dominant creep mechanism was identified as diffusion coupled with grain boundary sliding. At temperatures above 1300°C, the stress exponents were determined to be 2.3–2.4, and creep activation energy was 507 kJ mol ^–1, the dominant creep mechanism was suggested to be diffusion cavity growth at sliding grain boundaries. Creep test at 1350°C for pre-oxidation sample showed a pure diffusion mechanism, because of a stress exponent of 1. N^3– diffusing along grain boundaries was believed to be the rate controlling mechanism for diffusion creep. The oxidation and Sialon phase transformation were analyzed and their effect on creep was evaluated.     

Comparative creep damage assessments using the various models

The creep fracture characteristics of a conventionally cast (CC) MARM-002 superalloy were studied for creep conditions of 1173 K/200–400 MPa using different approaches including the Kachanov-Rabotnov type continuum damage mechanics, grain boundary damage accumulation, and Chen-Argon diffusional cavity growth. A rapid improvement in creep rupture life can be achieved by reducing the Kachanov-Rabotnov damage rate () below a critical value of this rate. It is possible that a large improvement in creep resistance would be made by decreasing grain boundary damage rate rather than continuum damage rate since the minimum creep rate (_m) accelerates rapidly without changing the parameter .     

High temperature compressive properties over a wide range of strain rates in an AZ31 magnesium alloy

High temperature compressive properties in AZ31 magnesium alloy were examined over a wide strain rate range from 10^–3 to 10³ s^–1. It was suggested that the dominant deformation mechanism in the low strain rate range below 10^–1 s^–1 was dislocation creep controlled by pipe diffusion at low temperatures, and by lattice diffusion at high temperatures. On the other hand, analysis of the flow behavior and microstructural observations indicated that the deformation at high strain rates of 10³ s^–1 proceeds by conventional plastic flow of dislocation glide and twinning even at elevated temperatures.     

The Measurement of Compressive Creep Deformation and Damage Mechanisms in a Single-Phase Alumina Part I Grain boundary sliding

Grain boundary sliding (GBS) has been hypothesized to act as the primary driving force for the nucleation and growth of grain boundary cavities in ceramics undergoing creep. In addition, GBS is often a major mode of deformation during high-temperature creep. This paper demonstrates the importance of GBS with mode II GBS measurements performed using a stereoimaging technique on a single-phase alumina tested under constant compressive stresses of 70 and 140 MPa at 1600 °C. Measurements were taken at constant time intervals during creep. The results support previous observations that GBS is stochastic and history independent. GBS displacements at given time intervals are shown to fit a Wiebull distribution. During steady-state creep, GBS displacements increased linearly with time at a constant sliding rate of 6.0 × 10–5 m s–1 at 70 MPa and 1.3 × 10–4 m s–1 at 140 MPa. Also, an average of 67% of the grain boundaries exhibited measurable sliding throughout the creep life of the 140 MPa test. Results of the GBS measurements are used to modify an existing creep model describing stochastic GBS. In part II of this paper [1], the GBS measurements reported are related to the associated creep cavitation measured in specimens tested under identical conditions.     

Optical Properties of Ca3Ga2Ge4O14 Crystals

The optical absorption, induced-absorption, luminescence, and excitation spectra and temperature-dependent luminescence intensity of thermochemically colored Ca₃Ga₂Ge₄O₁₄ crystals were measured. The results indicate that the induced-absorption bands at 34000–28000 and 28000–20000 cm^–1 and the emission band at 14800 cm^–1 are related to F-centers.     

Estimation of remaining creep life of an aluminium alloy from creep crack density measurements

Long transverse test pieces of fully aged RR58 plate were stressed in tension at 278 and 308 MPa at 120° C for various fractions of their creep lives. The test pieces were subsequently sectioned, mechanically and electrolytically polished and the numbers of cracks per square millimetre were measured by optical microscopy. The crack density, n, increased linearly with creep strain at both stress levels. No accurate assessment of the variation of n with time was possible. Good agreement between the crack densities measured on duplicate microsections was achieved when the crack density was greater than 10 cracks mm^–2. The crack densities in the uniformly strained portions of 11 test pieces from the same plate, fractured at 150° C at stresses within the range 200 to 290 MPa were also measured. The crack density decreased from 45 cracks mm^–2 at 200 MPa to 4 cracks mm^–2 at 290 MPa. A regression equation n/ge=164 – 0.57 (where is the applied stress) was derived assuming linear n versus relationships at 150° C. The 90% confidence limits were derived for the determination of an unknown stress level from a single measurement of n/. Of the creep life prediction methods discussed, only the correlation of creep crack density and creep strain is of sufficient accuracy and this only when the creep stress and creep temperature are low, i.e. only for those conditions which would develop a high crack density at small fractions of the creep life.