The tensile and creep behavior of Mg–Zn Alloys with and without Y and Zr as ternary elements

Tensile–creep experiments were conducted in the temperature range 100–200 °C and stress range 20–83 MPa for a series of magnesium–zinc–yttrium (Mg-Zn-Y) and mangnesium-zinc–zirconium (Mg-Zn-Zr) alloys ranging from 0 to 5.4 wt% Zn, 0 to 3 wt% Y, and 0 to 0.6 wt.% Zr. The greatest tensile–creep resistance was exhibited by an Mg–4.1Zn–0.2Y alloy. The room-temperature yield strength increased with increasing Y content for Mg–1.6–2.0Zn alloys. The greatest tensile strength and elongation was exhibited by Mg–5.4Zn–0.6Zr. This alloy also exhibited the finest grain size and the poorest creep resistance. The measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress. For applied stresses greater than 40 MPa, the creep exponents were between 4 and 8. For applied stresses less than 40 MPa, the creep exponent was 2.2. The calculated activation energies (Qapp) were dependent on temperature where the Q _app values between 100 and 150 °C (65 kJ/mol) were half those between 150 and 200 °C for the same applied stress value (30 MPa). Deformation observations indicated that the grain boundaries were susceptible to cracking in both tension and tension-creep, where at low applied stresses grain boundary sliding was suggested where strain accommodation occurred through grain boundary cracking. Thus grain size and grain boundaries appeared to be important microstructural parameters affecting the mechanical behavior. Microstructural effects on the tensile properties and creep behavior are discussed in comparison to other Mg-based alloy systems.

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The influence of grain size,stress and temperature on the steady state creep of a 25 Cr-20 Ni austenitic stainless steel without precipitates

This work was performed in order to study the steady state creep behaviour of a modified 25 Cr-20 Ni stainless steel which has no precipitates. The test temperature range was 1171 to 1211 K, the stress range 4.9 to 19.6 MPa, and the grain size was 40 to 600m. The steady state creep rate, , decreases with increase in grain size, especially at the lowest stress; is proportional to 1/d ² at 4.9 MPa, whered is a mean grain diameter. The variation of with grain size is smaller in the middle and coarse-grained specimens than in the fine-grained specimens, the stress exponent,n, gradually decreases from ~ 4 to ~ 1.5 with reducing stress, but in the middle- and coarse-grained specimens, a discontinuous point appears on a log versus log plot. The activation energy for the steady state creep of the coarse-grained specimens tends to be larger than that of the fine-grained specimens, and the tendency is remarkable in the higher stress level. It is indicated that the creep mechanisms in the fine-grained specimens are essentially different from those in the coarse-grained specimens, and that the creep at the lowest stresses and smallest grain size is similar to that predicted by a vacancy creep model involving grain-boundary diffusion.     

Reverse cyclic fatigue of a hot isostatically pressed silicon nitride at 1370 °C

The uniaxial, reverse cyclic fatigue performance of a commercially available hot isostatically pressed silicon nitride was examined at 1370 °C in air and with a 1 Hz sinusoidal waveform using button-head tensile specimens. Specimens did not fail in less than 10⁶ cycles when the applied stress amplitude was less than 280 MPa. Slow crack growth occurred at stress amplitudes 280 MPa and failure always occurred during the tensile stroke of the waveform. Multi-grain junction cavities resulted (i.e., the accumulation of net tensile creep strain) as a consequence of the reverse cyclic loading even though the specimens endured half their life under tensile stresses and the other half under compressive stresses. The presence of multi-grain junction cavities was a consequence of the stress exponent of tensile creep strain being greater than the stress exponent of compressive creep strain. Lastly, it was observed that the static creep resistance of this material improved when it was first subjected to reverse cyclic loading at 1370°C for at least 10⁶ cycles at 1 Hz. Silicon nitride grain coarsening (which was a consequence of the completion of the to silicon nitride solution/reprecipitation process that occurred during the history of the reverse cyclic loading) lessened the capacity for grain boundary sliding resulting in an improved static creep resistance.     

Creep behaviour of AZ61 magnesium alloy at low stresses and intermediate temperatures

Abstract

Tensile creep response was investigated for AZ61 alloy (Mg - 6.4Al - 0.9Zn - 0.2Mn, wt-%) of mean linear intercept grain size ~ 25 μm at stresses in the range 0.9 - 4 MPa over the temperature range 250 - 346°C. Bingham behaviour is obtained with strain rate ? under stress σ given by ?∝σ - σ_o with a threshold stress σ_o decreasing from 1.25 MPa at 210°C to ~ 0.5 MPa at 346°C, which is similar to earlier work on pure magnesium. The corresponding Arrhenius plot of log (Td?/d σ) versus T^-1 indicates an activation energy comparable with that expected for the grain boundary self-diffusion coefficient D _B, and values of D _Bδ (where δ is the effective grain boundary thickness) derived from the Coble equation are also similar to those for pure magnesium. Grain elongation in the direction of the tensile stress is also consistent with the key indicative feature of diffusional creep: deposition of material at grain boundaries nearly transverse to the axis of tensile stressing. Strain rates versus stress are shown to be continuous with published results for superplastic flow of AZ61 at comparable temperatures but higher stresses.     

The influence of a threshold stress for grain boundary sliding on constitutive response of polycrystalline Al during high temperature deformation

A continuum polycrystal plasticity model was used to estimate the influence of a threshold stress for grain boundary sliding on the relationship between macroscopic flow stress and strain rate for the aluminum alloy AA5083 when subjected to plane strain uniaxial tension at 450 °C. Under these conditions, AA5083 deforms by dislocation glide at strain rates exceeding 0.001 s⁻¹, and by grain boundary sliding at lower strain rates. The stress–strain rate response can be approximated by , where A and n depend on grain size and strain rate. We find that a threshold stress less or equal to 4 MPa has only a small influence on flow stress and stress exponent n in the dislocation creep regime (a threshold stress of 2 MPa increases n from 4.2 to 4.5), but substantially increases both flow stress and stress exponent in the grain boundary sliding regime (a threshold stress of 2 MPa increases n from 1.5 to 2.7). In addition, when the threshold stress is included, our model predicts stress versus strain rate behavior that is in good agreement with experimental measurements reported by Kulas et al. [M.A. Kulas, W.P. Green, E.M. Taleff, P.E. Krajewski, T.R. McNelley, Metall. Mater. Trans. A 36 (2005) 1249].     

The role of Coble creep and interface control in superplastic Sn-Pb alloys

The creep behaviour of superplastic Sn-2 wt% Pb and Sn-38.1 wt % Pb is investigated at temperatures between 298 and 403 K and for grain sizes between 2.5 and 260m. In Sn-2 wt% Pb with grain sizes larger than 50 m, diffusion-controlled Coble creep is found and it is experimentally shown that this type of creep is inhibited in smallgrained specimens. Measurements covering low stresses ( 0.1 MPa) and strain rates ( 10^–10 sec^–1) rule out any explanation which relies on a threshold stress for plastic deformation. The observations are explained by a model in which, at low stresses or small grain sizes, Coble creep is rate-limited not by diffusion of vacancies but by the rate of emission and absorption at the curved dislocations in the grain boundaries which are the ultimate sources and sinks of vacancies.     

High-temperature creep of yttrium-aluminium garnet single crystals

High-temperature creep in single crystals of Y₃Al₅O₁₂ (YAG) was studied by constant strainrate compression tests. The creep resistance of YAG is very high: a stress of ~ 300 MPa is needed to deform at a strain rate of 10^–6 (s^–1) at a temperature as high as 1900 K (~0.84 T _m, (melting temperature)). YAG deforms using the 111 {1¯10} slip systems following a power law with stress exponent n ~ 3 and activation energy E* ~ 720 kJ mol^–1. However, a small dependence of n on temperature and of E* on stress was observed. This stress-dependence of activation energy combined with the observed dislocation microstructures suggests that the high creep resistance of YAG is due to the difficulty of dislocation glide as opposed to the difficulty of climb. Present dislocation creep data are compared with diffusion creep data and a deformation mechanism map is constructed. Large transition stresses (2–3 GPa for 10 m grain size) are predicted, implying that deformation of most fine-grained YAG will occur by diffusion creep.     

The influence of grain size on the creep of uranium dioxide

The creep of uranium dioxide has been investigated as a function of grain size. At high stresses, when creep is controlled by dislocation movement, grain boundaries exert a strengthening effect and this strengthening is correlated with the Hall-Petch equation. The degree of strengthening diminishes with increases in temperature. At lower stresses, when creep is controlled by mass transport, grain boundaries exert a weakening effect owing to the reduction in diffusion path length as grain size is reduced. In this range behaviour is correlated with the Nabarro-Herring equation with stress replaced by an effective stress _E=–₀ where ₀ is a threshold stress for diffusional creep associated with the limitation of the ability of boundaries to emit and absorb vacancies. ₀ appears to decrease as grain size is increased.     

The influence of temperature cycling on the creep properties of Nickel 201 and Inconel 600 in combustion gas

The effect of temperature cycling on the creep behaviour of Nickel 201 and Inconel 600 in combustion gas has been studied. Specimens were tested both at constant temperature, 900° C, and at 900° C interrupted by temperarature drops down to 510° C. The creep straining has been analysed with respect to a weighted time parameter which includes the creep contribution during the lower temperatures of each cycle. With respect to this compensated time parameter, the temperature variations were generally observed to result in a strong acceleration in creep. The effect seemed to increase with increasing frequency of temperature drops, increasing grain size and decreasing stress. Thus, at low stress levels, large-grained specimens of both alloys experienced an acceleration even inabsolute creep rate upon cycling. The grain size dependency indicates that the destructive effect of the cycles is caused by crack formation. Surface cracking associated with grain boundary oxidation seemed to be the dominant cracking mode. It is suggested that, during creep in oxidizing environments, repeated periods of cooling might strongly accelerate the growth of surface creep cracks due to the difference in thermal expansion between metals and oxides. This difference causes high tensile stresses to arise in the metal in front of the grain boundary oxides, and the stresses are assumed to be high enough to nucleate microcracks along the boundary.     

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.     

Creep in a Cu-14at.%Al solid solution alloy at intermediate temperatures and low stresses

The helicoid spring specimen technique was applied to investigate creep of a Cu-14at.%Al solid solution alloy at homologous temperatures from 0.54 to 0.65 and stresses ranging from 0.2 to 5.0 MPa. At stresses lower than about 1 MPa, Coble-type creep was found to dominate, associated with a threshold stress apparently independent either of grain size or of temperature. At stresses above about 1 MPa, another creep mechanism obviously contributes to the measured creep rate. This mechanism operating in parallel with Coble creep is characterized by the fact that the steady state creep rate is proportional to the second power of stress and inversely proportional to the third power of grain size and is most probably grain boundary diffusion controlled. This mechanism, called the non-viscous mechanism in the present work, is similar to that considered by Gifkins and Kaibyshev et al. to result from the motion of grain boundary dislocations (grain boundary sliding) accomodated by slip of lattice dislocations in thin layers along grain boundaries, although these workers assumed the creep rate to be inversely proportional not to the cube but to the square of the grain size.     

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.     

Creep in pure and two phase nickel-doped alumina

Creep in pure and two phase nickel-doped alumina has been investigated in the stress range 0.70 to 4.57 kgf mm^–2 (1000 to 6500 psi), and temperature range 1450 to 1800° C, for grain sizes from 15 to 45 m (pure alumina) and 15 to 30 um, (nickel-doped alumina). The effect of stress, grain size and temperature on the creep rate suggests that diffusion controlled grain-boundary sliding is the predominant creep mechanism at low stresses and small grain sizes. However, the stress exponents show that some non-viscous boundary sliding occurs even at the lowest stresses investigated. This mechanism is confirmed by metallographic evidence, which shows considerable boundary corrugation in the deformed aluminas. At higher stresses and larger grain sizes the localized propagation of microcracks leads to high stress exponents in the creep rate equation. The nickel dopant, which introduces an evenly distributed spinel second phase into the alumina matrix, increases the creep rate and enhances boundary sliding and localized crack propagation. The weakening effect of the second phase increases with grain size, and tertiary creep occurs at strains of 0.5% and below in large grained material.     

Creep of magnesium strengthened with high volume fractions of yttria dispersoids

Creep experiments were performed on dispersion-strengthened-cast magnesium (DSC-Mg), consisting of unalloyed magnesium with 1 μm grain size containing 30 vol.% of 0.33 μm yttria particles. Strain rates were measured for temperatures between 573 and 723 K at compressive stresses between 7 and 125 MPa. DSC-Mg exhibits outstanding creep strength as compared with other magnesium materials, but is less creep resistant than comparable DSC-Al and other dispersion-strengthened aluminum materials. Two separate creep regimes were observed in DSC-Mg, at low stresses (σ<30 MPa), both the apparent stress exponent (n_app≈2) and the apparent activation energy (Q_app≈48 kJ mol⁻¹) are low, while at high stresses (σ>34 MPa), these parameters are much higher (n_app=9–15 and Q_app=230–325 kJ mol⁻¹) and increase, respectively, with increasing temperature and stress. The low-stress regime can be explained by an existing model of grain-boundary sliding inhibited by dispersoids at grain-boundaries. The unexpectedly low activation energy (about half the activation energy of grain boundary diffusion in pure magnesium) is interpreted as interfacial diffusion at the Mg/Y₂O₃ interface. The high-stress regime can be described by dislocation creep with dispersion-strengthening from the interaction of the submicron particles with matrix dislocations. The origin of the threshold stress is discussed in the light of existing dislocation climb, detachment and pile-up models.     

High-temperature creep of the particlehardened commercial Al-Li-Cu-Mg alloy 8090

Creep of the particle-hardened commercial Al-Li 8090 alloy has been studied at temperatures of 425 and 445 K. The measured stress sensitivity of the minimum creep rates changes abruptly at a given applied stress with stress exponents being around 4–6 at low stresses and 30–40 at high stresses. Creep activation enthalpies were determined by both temperature cycling and by comparing creep rates at two temperatures at a given applied stress, the results from both gave the same unrealistically high values. The internal stresses, _i, developed during creep were determined using the strain-transient dip test. These increased linearly with the applied stress, _a, at low stresses and were effectively constant at high stresses. The minimum creep rate was found to be a simple function of the effective stress, _a-_i, with a stress exponent of between 5 and 6, at all applied stresses. The dislocation and precipitate structure of the alloy was examined before and after creep using thin-film electron microscopy. The initial structure consisted of pancake grains with a well-developed {1 1 0}1 1 2 type texture. The grains contained well-developed sub-cells and and S precipitates. The structure developed during creep consisted of dislocation pairs, single dislocations and dislocations loops. There was evidence to suggest that slip took place on both {1 0 0} and {1 1 1} planes. The dislocation loops were most likely to have been Orowan in character and around the rodlike S precipitate, with the coherent precipitate being sheared by pairs of dislocations. The measured internal stresses result from inhomogeneity of plastic deformation. These stresses increase continuously with applied stress up to the observed macroscopic yield stress, and then become constant. The internal stresses are likely to have arisen from the Orowan loops around S and the behaviour of sub-grain boundaries. The increases in internal stress may have resulted from an increased loop density with increasing applied stress. This rate of increase is likely to slow down if S particles are sheared or fractured at high applied stresses.     

High-temperature creep response of a commercial grade siliconized silicon carbide

Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained 90% -SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10×10^–10 s^–1) at temperatures 1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (0.8 to 3 × 10^–9 s^–1) at stresses below a threshold value of 190 MPa. At stresses >190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n=17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature 1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.     

Effect of loading rate on the monotonic tensile behavior and tensile strength of an oxide–oxide ceramic composite at 1200 °C

The influence of loading rate on monotonic tensile behavior and tensile properties of an oxide–oxide ceramic composite was evaluated in laboratory air at 1200 °C. The composite consists of a porous alumina matrix reinforced with woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tensile tests conducted at loading rates of 0.0025 and 25 MPa/s revealed a strong effect of rate on the stress–strain behavior as well as on the ultimate tensile strength (UTS), elastic modulus and failure strain. At 0.0025 MPa/s, increase in stress results in non-monotonic change in strain, with the rate of change of strain reversing its sign at stresses 25 MPa/s. Several samples were subjected to additional heat treatments prior to testing in order to determine whether this unusual stress–strain behavior was an artifact of incomplete processing of fibers in the as-received material. The unusual material response in the 0–30 MPa stress range was further investigated in creep tests conducted with the applied stresses ≤26 MPa. Negative creep (i.e. decrease in strain under constant stress) was observed. Porosity measurements indicate that a decrease in matrix porosity and matrix densification may be taking place in the N720/A composite exposed to 1200 °C at stresses <30 MPa for prolonged periods of time.     

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.     

Effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy

This paper examines the effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy. The processed material with an ultrafine-grained (UFG) structure (d ≈ 150 nm) was prepared by multiaxial forging. Uniaxial constant stress compression and constant load tensile creep tests were performed at 648–698 K and at stresses ranging between 300 and 600 MPa on the UFG processed alloy and, for comparison purposes, on its coarse-grained (CG) state. The values of the stress exponents of the minimum creep rate n and creep activation energy Q _c were determined. Creep behaviour was also investigated by nanoindentation method at room temperature under constant load. The microstructure was examined by transmission electron microscopy and scanning electron microscope equipped with an electron back scatter diffraction unit. The results of the uniaxial creep tests showed that the minimum creep rates of the UFG specimens are significantly higher in comparison with those of the CG state. However, the differences in the minimum creep rates of both states of alloy strongly decrease with increasing values of applied stress. The CG alloy exhibits better creep resistance than the UFG one over the stress range used; the minimum creep rate for the UFG alloy is about one to two orders of magnitude higher than that of the CG alloy. The indentation creep tests showed that annealing had little effect on the creep behaviour in UFG Ti alloy at room temperature.     

Fractal dimension of the grain boundary fracture in creep-fracture of cobalt-based heat resistant alloys

The effects of grain boundary configuration and creep conditions on the fractal dimension of the grain boundary fracture (D _f) were investigated using commercial cobalt-based heat resistant alloys, namely, HS-21 and L-605 alloys. Creep-rupture experiments were carried out under the initial creep stresses of 19.6–176 MPa in the temperature range from 1089–1422 K in air. The value of D _f was larger in specimens with serrated grain boundaries than in those with straight grain boundaries in the HS-21 alloy under the same creep condition, and the difference in the value of D _f between these specimens was large in the scale range of the analysis which was less than about one grain boundary length. However, there was almost no difference in the value of D _f between the specimens with serrated grain boundaries and those with straight grain boundaries in the L-605 alloy, because there was no obvious difference in the microstructure between these specimens. The value of D _f increased with decreasing creep stress in the scale range of the fractal analysis larger than about one grain boundary length in both HS-21 and L-605 alloys, while the stress dependence of D _f was larger in the HS-21 alloy. The stress dependence of D _f was explained by the stress dependence on the number of grain boundary microcracks linked to the fracture surface. The value of D _f estimated in the scale range smaller than about one grain boundary length showed essentially no stress dependence in both L-605 and HS-21 alloys.