Static and cyclic creep behaviour of SiC whisker reinforced aluminium composite

Abstract

Static and cyclic creep tests were carried out in tension at 573–673 K on a 20 vol.-%SiC whisker reinforced aluminium (Al/SiC_w ) composite. The Al/SiC_w composite exhibited an apparent stress exponent of 18·1–19·0 at 573–673 K and an apparent activation energy of 325 kJ mol^-1 for static creep, whereas an apparent stress exponent of 19·6 at 623 K and an apparent activation energy of 376 kJ mol^-1 were observed for cyclic creep. A cyclic creep retardation (CCR) behaviour was observed for the Al/SiC_w composite. The steady state creep rate for cyclic creep was three orders of magnitude lower than that for static creep. Furthermore, the steady state creep rates of the composite tended to decrease continuously with increasing percentage unloading amount. The static creep data of the Al/SiC_w composite were rationalised by the substructure invariant model with a true stress exponent of 8 together with a threshold stress. The CCR behaviour can be explained by the storage of anelastic strain delaying non-recoverable creep during the onload cycles.     

Creep and strain recovery in hot-pressed silicon nitride

It is observed that creep response in hot-pressed silicon is characterized by two parallel phenomena; one accounts for a persistent non-recoverable plastic deformation and the other for a transient viscoelastic recoverable deformation. The persistent creep component is time-dependent, and apparently follows parabolic time kinetics. It is further observed that creep is characterized by a power law stress exponent of about 4 and an activation energy of 848 kJ mol^–1. The viscoelastic recoverable component of strain is found to be independent of the total plastic strain in the material. The recovery rate at any given time is directly proportional to the preceding creep stress and therefore can be considered linear viscoelastic. The creep compliance of the viscoelastic transient is temperature-dependent with an activation energy of about 722 kJ mol^–1. It is further observed that the viscoelastic recovery is characterized by a spectrum of retardation times and can be modelled by a series of Kelvin analogue models. Finally, the viscoelastic recovery and the viscoelastic component of subsequent creep appear to be inversely related and apparently obey Boltzman superposition. A model is developed for the creep and recovery behaviour of hot-pressed silicon nitride consistent with all experimental observations and based in relative grain motion accommodated by the fluid grain-boundary glass liquid flow, cavitation and wedge opening.     

Plastic deformation kinetics of fine-grained MgO in tension

The plastic deformation kinetics of 99.4% MgO (d _o = 5.3 m) was investigated at 1500°–1600°C in uniaxial tension. It was determined that the stress exponent n 1 and the activation energy Q = 204 kJ/mole. Neither the present results on MgO, nor data in the literature on MgO and other oxide ceramics, can be fully explained by the models usually proposed for the plastic deformation of fine-grained oxide ceramics. The present results are however in good agreement with the model for grain boundary diffusion plastic flow recently developed by Kim, Estrin and Bush.     

Study on creep deformation mechanism of 316LN stainless steel from impression creep tests

Abstract

Impression creep tests were carried out on 316LN stainless steel (SS) at various temperatures in the range 898–973 K. The stress dependence of the steady state impression velocity followed the power-law with stress exponent n?=?6. The temperature dependence of the steady state impression velocity obeyed Arrhenius type rate equation. The apparent activation energy for creep deformation (Q_c) was estimated to be 500 kJ mol^?1. Based on the n and Q_c values, it is concluded that the rate controlling mechanism is dislocation creep.     

Transient creep behaviour of hot isostatically pressed silicon nitride

Transient creep is shown to dominate the high-temperature behaviour of a grade of hot isostatically pressed silicon nitride containing only 4 wt% Y₂O₃ as a sintering aid. Contributing factors to transient creep are discussed and it is concluded that the most likely cause of longterm transient creep in the present study is intergranular sliding and interlocking of silicon nitride grains. In early stages of creep, devitrification of the intergranular phase, and intergranular flow of that phase may also contribute to the transient creep process. The occurrence of transient creep precluded the determination of an activation energy on the as-received material. However, after creep in the temperature range 1330–1430°C for times exceeding approximately 1100 h, an apparent activation energy of 1260 kJ mol^–1 was measured. It is suggested that the apparent activation energy for creep is determined by the mobility and concentration of diffusing species in the intergranular glassy phase. The time-to-rupture was found to be a power function of the minimum strain rate, independent of applied stress or temperature. Hence, creep-rupture behaviour followed a Monkman-Grant relation. A strain rate exponent of – 1.12 was determined.     

Effect of small amount of alumina doping on superplastic behavior of tetragonal zirconia

The effect of Al₂O₃ doping of around 0.1 wt% on superplastic behavior was studied in 3 mol% yttria-stabilized tetragonal zirconia polycrystal (TZP) which was free from SiO₂ contamination and had a grain size of 0.4 m. Compression creep tests revealed that high-purity TZP with less than 0.07 wt% Al₂O₃ had two deformation regions: the low stress region had a stress exponent of three and an apparent activation energy of 640 kJ/mol, and the high stress region had two and 460 kJ/mol. On the other hand, TZP containing more than 0.12 wt% Al₂O₃ had only one region which had a stress exponent of two and an activation energy of 480 kJ/mol. The region of diffusion control with a stress exponent of one was not observed in any samples. High resolution transmission electron microscopy revealed that no amorphous grain boundary phase was produced even with 0.18 wt% Al₂O₃ doping. Energy dispersive X-ray spectroscopy near grain boundaries revealed that yttrium was segregating at the grain boundaries with denuded zones of 30 nm width, which were created during slow cooling from the sintering temperature.     

Creep behaviour of Si3N4/Y2O3/Al2O3/AIN alloys

The compression creep behaviour of pressureless sintered Y₂O₃/Al₂O₃/AIN-doped Si₃N₄ was studied between 1473 and 1673 K, under stresses ranging from 100–300 MPa. Strain rate versus stress and temperature analysis give a stress exponent n1 and an activation energy Q=860 kJ mol^–1. Microstructural change was investigated by transmission electron microscopy. The observed strain whorls, the stress exponent and the activation energy are indicative of a solution-diffusion-precipitation accommodated grain-boundary sliding where the diffusion through the glass is rate controlling.     

Grain growth of ZnO in binary ZnO-V2O5 ceramics

Grain growth of ZnO during liquid-phase sintering of binary ZnO-V₂O₅ ceramics has been studied for V₂O₅ contents from 0.5 to 4 mol% and sintering from 900°C to 1200°C. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G ⁿ – G _o ⁿ = K _o t exp(–Q/RT).Addition of V₂O₅ is found to decrease the ZnO grain growth exponent, n, as well as the apparent activation energy, Q. The activation analysis also reveals a change in the rate-controlling mechanism for ZnO grain growth. Following a low-V₂O₅-content (2 mol%) of nearly constant Q values of about 88 kJ/mol, further V₂O₅ additions cause an increase of the Q value to about 115 kJ/mol. Consistent with accepted models of liquid-phase sintering, it is concluded that the rate-controlling mechanism of ZnO grain growth during liquid-phase sintering in the presence of V₂O₅ changes from one of a phase-boundary reaction at low V₂O₅ levels to one of diffusion through the liquid phase at more than 2 mol% V₂O₅ levels.     

Effect of silicon on recrystallisation of V–Ti microalloyed steel

Abstract

The dynamic recrystallisation (DRX) and static recrystallisation (SRX) behaviours of three V–Ti microalloyed steels were studied by the analysis of the true stress–strain curves and the stress relaxation curves under different deformation conditions. The results of DRX showed that deformation activation energy Q_def, peak stress and peak strain increased, as a result of the solute strengthening and dragging effect due to Si. The results of SRX showed that Si increased the SRX activation energy Q_SRX. The solute retardation parameter for static recrystallisation of Si was calculated. Based on the SRX results, to quantify the drag effect of Si and V, a new model was proposed to describe the time for 50% recrystallisation (t_0·5), which was tested and verified by previously published data on similar steels. Precipitation during recrystallisation could lead to a lower value of the Avrami exponent.     

Impression creep behavior of different zones in friction stir welded Mg–Zn–Mn wrought alloy

The friction stir welded joint of wrought ZM21 alloy was divided into five parts, and their localized creep behavior was studied via the impression method. The tests were carried out in the stress range of 300–450 MPa (σ_imp/G ≈ 0.02–0.03) and in the temperature range of 448–523 K. Optical and SEM micrographs and EDS taken before and after the impression tests were used to study the microstructure of various zones of the FS welded joint. Power law was found to satisfactorily relate the stress and strain rates. The steady-state impression velocity was found to vary significantly between the advancing and retreating sides of TMAZ and HAZ. For TMAZ, the creep exponent on the AS was 4.8, and on the RS, it was 7.8. The activation energy on the AS was ~?133 kJ/mol, and on the RS, it was ~?101 kJ/mol. Similarly, for HAZ, the creep exponent on the AS was found to be 5.5 and on the RS, it was 4.9. The activation energy on the AS was ~?86 kJ/mol and on the RS, it was ~?232 kJ/mol. The cross-over of steady-state impression velocity of different zones indicates that the weak zone was temperature and stress dependent. Within the stresses and temperatures studied, the weld zone's creep resistance (i.e., lower minimum impression velocity) was found to be better than the base material. As it is with most magnesium alloys, dislocation climb was found to be the operative mechanism in the FS weldments of ZM21 alloy. The rate-controlling mechanism remains to be identified because the wide variation in n and Q values suggests that different creep mechanisms are in operation in different zones.

Graphical abstract

     

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.     

Flow behaviour and microstructural evolution in cold worked 70∶30 α-brass

Abstract

Deformation behaviour and microstructures at failure were investigated in a mill cold worked 70∶30 α-brass over the test temperature range of 298–973 K and strain rate range of 10^?5–5×10^?3 s^?1. Tensile properties as a function of temperature revealed three distinct regions, with their temperature sensitivity being maximum at intermediate temperatures (553–673 K) and much less towards the lower and higher temperature ranges. Two values of activation energy for high temperature deformation Q were obtained to be 117·5 kJ mol^?1 below 623 K and 196·4 kJ mol^?1 above this critical temperature. In the respective temperature range the values of stress exponent n were 5·6 and 3·8. Based on the values of Q and n, the deformation mechanism was suggested to be dislocation climb creep with a probable contribution from dislocation pipe diffusion on lowering the temperature. Both grain size and cavity size were found to increase with increasing test temperature, suggesting them to be interrelated and act as an alternative steps for accommodating grain boundary sliding. Static grain growth study, over the temperature range of 773 to 1073 K, led to activation energy for grain growth to be 71 kJ mol^?1, with the time exponent of 0·37.     

Creep of hot-pressed silicon nitride

Creep tests were undertaken on hot-pressed silicon nitride in the temperature range 1200 to 1400° C. The activation energy for creep was determined to be 140 kcal mol^?1 and the stress exponent of creep rate was 1.7. The creep behaviour is ascribed to grain-boundary sliding accommodated by void deformation at triple points and by limited local plastic deformation. Electron microscopic evidence supporting this mechanism is presented.     

Oxidation behavior of silicon nitride sintered with Lu2O3 additive

Kyocera SN282 silicon nitride ceramics sintered with 5.35 wt% Lu₂O₃ were oxidized in dry oxygen at 930–1,200 °C. Oxidation of SN282 follows a parabolic rate law. SN282 exhibits significantly lower parabolic rate constants and better oxide morphological stability than silicon nitride containing other sintering additives under similar conditions. The activation energy for oxidation of SN282 is 107 ± 5 kJ/mol K, suggesting inward diffusion of molecular oxygen in the oxide layer as the rate-limiting mechanism.     

Deformation behaviour of Fe-3Si steel

Abstract

The deformation behaviour of an Fe-3Si (wt-%) steel was studied in the temperature range 400-900°C over six orders of magnitude of strain rate. It was found that the Fe-3Si steel exhibits a threshold behaviour. A correlation between the deformation behaviour and the temperature dependence of the threshold stress was etermined. An analysis in terms of the threshold stress showed that two modes of deformation behaviour exist in the power law creep regime. At normalised strain rate ?kT/(D ₁ Eb) ranging from 2 × 10^-6 to 10^-3, the value of the true stress exponent n is equal to 7, and at lower values of ?kT/(D ₁ Eb) the value of n is ~ 5. The true activation energy for plastic deformation Q _c increases from 250 ± 15 to 290 ± 30 kJ mol^-1 with increasing temperature from 550 to 700°C, and remains virtually unchanged at high temperatures. The relationship between rate controlling mechanisms of plastic deformation and mechanisms of interaction between lattice dislocations and dispersoids is discussed.     

Deformation, stress relaxation, and crystallization of lithium silicate glass fibers below the glass transition temperature

The deformation and crystallization of Li₂O·2SiO₂ and Li₂O·1.6SiO₂ glass fibers subjected to a bending stress were measured as a function of time over the temperature range 50 to 150°C below the glass transition temperature (T _g). The glass fibers can be permanently deformed at temperatures about 100°C below T _g, and they crystallize significantly at temperatures close to, but below T _g, about 150°C lower than the onset temperature for crystallization for these glasses in the no-stress condition. The crystallization was found to occur only on the surface of the glass fibers with no detectable difference in the extent of crystallization in tensile and compressive stress regions. The relaxation mechanism for fiber deformation can be best described by a stretched exponential (Kohlrausch-Williams- Watt (KWW) approximation), rather than a single exponential model.The activation energy for stress relaxation, E _s, for the glass fibers ranges between 175 and 195 kJ/mol, which is considerably smaller than the activation energy for viscous flow, E (400 kJ/mol) near T _g for these glasses at normal, stress-free condition. It is suspected that a viscosity relaxation mechanism could be responsible for permanent deformation and crystallization of the glass fibers below T _g.     

The effect of heterogeneous deformation on the hot deformation of WE54 magnesium alloy

The high temperature forming behavior of WE54 magnesium alloy is studied by means of compression and tension tests. Metallographic investigation was performed to evaluate the heterogeneous deformation of the compression samples at high temperature. Dynamic recrystallization was found to be related to the amount of deformation in the various regions of the compression sample. The compression data allowed determination of the Garofalo equation describing the hot deformation behavior. The parameters n and Q, stress exponent and activation energy, of this equation were 4.4 and 237 kJ/mol respectively. This equation was used to predict the formability behavior for the hot rolling process and also to determine the maximum forming efficiency and stability of the alloy. The optimum rolling temperature was found to be 520 °C.     

Creep of chemically vapour deposited SiC fibres

The creep, thermal expansion, and elastic modulus properties for chemically vapour deposited SiC fibres were measured between 1000 and 1500°C. Creep strain was observed to increase logarithmically with time, monotonically with temperature, and linearly with tensile stress up to 800 MPa. The controlling activation energy was 480 ± 20 kJ mol^–1. Thermal pretreatments near 1200 and 145O° C were found to significantly reduce fibre creep. These results coupled with creep recovery observations indicate that below 1400°C fibre creep is anelastic with negligible plastic component. This allowed a simple predictive method to be developed for describing fibre total deformation as a function of time, temperature, and stress. Mechanistic analysis of the property data suggests that fibre creep is the result of -SiC grain boundary sliding, controlled by a small percentage of free silicon in the grain boundaries.     

Anisotropy of creep deformation rate in hot-pressed Si3N4 with preferred orientation of the elongated grains

Compressive creep deformation of hot-pressed silicon nitride with two different preorientations of grain was investigated at temperatures in the range of 1300–1400 °C under 30–100 MPa. The stress exponent of the creep rate was determined to be nearly unity of the apparent activation energy of creep rate was found to be about 500 kJ mol^-1. It means the creep deformation is due to diffusion controlled solution/precipitation. Creep rate of specimen with creep loading direction in parallel to the hot-pressing axis was determined to be higher than that in perpendicular to the hot-pressing axis. In addition, microstructural observation revealed that no cavity appeared and grain boundary glass was recrystallized during creep. X-ray diffraction (XRD) analysis confirms that needle-like Si₃N₄ grains were reoriented during creep test. These results indicate that the anisotropy of creep rate results from the disparity in the rate of solution–reprecipitation of grains rather than that in diffusion through the grain boundary, which is dependent on the preferred orientation of the needle-like grains. This revised version was published online in November 2006 with corrections to the Cover Date.