High Stress Creep Research Articles

Grain boundary precipitation strengthening in high temperature creep of Fe15Cr25Ni alloys

Grain boundary carbide precipitation gives rise to a dramatic decrease in creep rates compared with those of a single phase alloy. The high stress exponent (n=7) and creep activation energy (Q C > Q sd) have been found due to these intergranular carbides, indicating that they both increase creep resistance and change creep characteristics of alloys. The model present here rationalizes the observed behavior in single phase and two phase alloys with grain boundary carbides by a unified power law equation involving boundary obstacle stress. The predictions of this model are in close agreement with experimental results.

Dispersion processing of creep resistant whisker-reinforced ceramic-matrix composites

SiC whisker-reinforced Al2O3 composites, with enhanced strength and creep failure strains, have been fabricated by dispersion processing, pressure filtration and hot pressing. This method minimizes flaws that otherwise cause premature failure. High stress creep measurements demonstrate that true steady-state creep does not develop in these composites for whisker loadings above some critical level; creep rates continue to decrease with increasing creep strain. Also, an anelastic strain recovery is observed on load removal at creep remperatures. Existing creep models are inadequate to explain these observations. The development of new models for composite creep requires measurements on composites with systematically varied microstructures, which currently are being prepared.

Creep and ductility in an Al-Cu solid-solution alloy

High-temperature creep was investigated in an Al-3 wt pct Cu alloy at temperatures in the range of 773 to 853 K and at a normalized shear stress range extending from 10-5 to 7 × 10-4. The results show the presence of three distinct regions. In region I (low stresses), the stress exponent is 4.5 and the activation energy is 155 kJ/mole. In region II (intermediate stresses), the stress exponent is 3.2 and the activation energy is 151 kJ/mole. In region III (high stresses), the stress exponent is 4.5 and the activation energy is 205 kJ/mole. Creep curves obtained in the three regions exhibit a normal primary stage, but the extent of the stage is less pronounced in region II than in regions I and III. The creep characteristics in regions I and II, along with the values of the transition stresses between the two regions, are in conformity with the prediction of the deformation criterion for solid-solution alloys. While the advent of region III (high stresses) correlates well with dislocation breakaway from a solute-atom atmosphere, the creep characteristics in this region are not entirely consistent with any of the existing high-stress creep mechanisms. The plot of elongation to fracturevs initial strain rate at 853 K exhibits two peaks at strain rates of 1 × 10-4 and 6 × 10-4 s-1. The first peak (1 × 10-4 s-1) is attributed to the variation of the stress exponent for creep in the alloy with strain rate, and the second peak (6 × 10-4 s-1) appears to reflect the effect of solute drag on dislocation velocity.

Analysis of Test System Misalignment in the Creep Test

An analysis of test system misalignment is presented for the creep test. Sheet type rectangular 1100-0 aluminum specimens are used for discussion. It is found that the creep strain at the geometric centerline of the specimen is different than that at the neutral axis. However, this difference in the creep strain decreases with time. Generally, the effect of misalignment decreases with creep time. Creep tests conducted with long pullrods and large initial strain level (high creep stress) will tend to minimize the effect of misalignment.

INTERNAL FRICTION DURING STEADY STATE CREEP OF 25Cr-20Ni AUSTENITIC STAINLESS STEEL

Internal friction was measured to know the changes in the dislocation density and loop length during the steady state creep. Main experimental results are (1) Internal friction of fine grain specimens (d=30µm) is constant during the steady state creep. (2) The decrease in internal friction is observed in medium (d=150µm) and coarse (d=600-850µm) grain specimens, and it is remarkable at high creep stress. The decrease is considered to correspond to that of Al4.

Anomalous primary creep behaviour of dilute aluminium-silicon alloys

Low temperature (750 °C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the 〈001〉 direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work.

The creep of Beta-Cobalt at low stresses

The creep of β-Co in vacuum has been studied for the ranges of stress up to 12 MN m −2, temperatures from 773 to 1113 K (0.44–0.63 T M ), intercept grain size from 21 to 125 μm and test durations from 5 to 210 h. The measurements were made in situ on self-loaded helical specimens self-heated by electric current as reported previously for α-Fe. Creep rates at lower stresses were found to be proportional to stress in excess of a threshold stress which could be as large as 2.4 MN m −2 and grain size and temperature dependence of creep rate per unit stress indicate Coble creep as the dominant mechanism in this regime. Observed creep rates exceeded predicted values by factors less than ten, if the recent determination of the grain boundary diffusion coefficient in β-Co is used. Transition to creep with a stress exponent as high as six was observed especially at higher temperatures and larger grain sizes, the transition stresses ~5–9 MN m −2 then being factors of 1.3–3.5 below values derived by extrapolating reported high stress creep data fitted to the Dorn equation. Grain growth during creep occurred with an activation energy close to that for grain boundary diffusion.

Sous-structure de dislocations dans l'oxyde d'yttrium monocristallin

The relation between dislocation substructure and macroscopic aspect of the surfaces of yttrium oxide single crystals which have been annealed at high temperature with and without applied stress, is studied. Experimental conditions of temperature and gaseous environnement give rise to a thermo-chemical etching of the single crystal faces which show dislocation motion. From simple arguments, it is shown that climb of dislocations plays an important role in the high temperature and low stress creep of Y2O3. Direct measurement of the dislocation climb velocity is perhaps possible from this effect.

Creep of 316 stainless steel under high stresses

The dislocation substructure developed in 316 stainless steel after creep tests at temperatures between 600 and 800°C and stresses between 6 and 38 kg/mm 2 has been observed. A gradual transition from equiaxed subgrains at low stresses to a grain orientation dependent duplex substructure at high stresses occurs in the range where ge s D ≅ 10 9 cm −2 . It is suggested that at high stresses dislocation climb is replaced by a diffusion independent dislocation glide mechanism as the dominant recovery process. Thus, the high stress creep range at ge s D ≅ 10 9 appears to be equivalent to the low temperature creep range at T ⪝ 0.5 T m. In 316 stainless steel, thermally activated cross slip may constitute the rate-controlling mechanism at high stresses.

Creep recovery, internal friction and stress relaxation

The correlations between anelastic to elastic strain ratios determined by creep recovery, stress relaxation and internal friction are examined. It is shown that good agreement is obtained provided due recognition is made of the effective frequency of the respective testing method and the temperature range of interest. Data for various material conditions of copper analysed in the above manner show that grain boundary relaxation and dislocation bowing are the principal sources of strain during high temperature creep recovery and stress relaxation.

Strength of polymethylmethacrylate in air and in n‐propanol

AbstractA series of tests was conducted to measure the failure strengths of notched polymethylmethacrylate tensile specimens in air and n‐propanol. Tests were conducted on specimens which had been subjected to creep stresses of various magnitudes and for different times. The resulting data indicate that two opposing factors contribute to the fracture strength in n‐propanol. The presence of fluid during crack propagation increases the nominal fracture failure stress. Diffusion of the fluid into the polymer produces crazing resulting in the provision of a preferential crack path and consequent lowering of the fracture strength. The former mechanism dominates at small pre‐stresses or at short creep times, resulting in an increase in strength in n‐propanol over that in air. However, the latter mechanism is dominant at higher creep stresses and longer creep times with a resulting decrease in strength in n‐propanol compared to that in air.

An approximate study of void expansion by ductility or creep

The growth of a spherical void in an infinite plastic matrix is studied. Based on conclusions by Rice and Tracey the analysis is directed at the behavior under high stress triaxiality, strain-hardening and creep being presently included. A symmetric near-field solution is continued into the remote field in an approximate manner. Implications are carefully studied and conclusions on the effects of hardening and creep are ventured.

In-reactor creep of zircaloy-2, zircaloy-4 and Zr-1.15 wt % Cr-0.1 wt % Fe at 568 K derived from their stress-relaxation behaviour

The in-reactor stress relaxation of zirconium alloys can be represented by the equation σ/σ 0 = D exp (−Kt) where the unrelaxed stress ratio is equal to a factor times the exponential of the relaxation rate. Creep rates derived from the bent-beam stress-relaxation data are compared with uniaxial creep results and a good correlation is shown for Zircaloy-2 through a continuous trend curve from low-stress stress relaxation to the high-stress creep behaviour. Specimens quenched from the β-phase had the lowest creep rates compared with those which were annealed or cold-worked, while increasing cold-work increased the creep rates for most specimens. The effects of cold-work and results from post-irradiation annealing of stress-relaxation test specimens suggest that for the low stress régime ( σ < 1 3 σ yield) the operation of a dislocation climb/glide mechanism in addition to the stress-directed loop formation mechanism is required for in-reactor creep.

Creep of dilute zinc-copper alloys

A series of zinc-copper alloys containing up to 2.0 wt % Cu have been prepared and tested in creep under constant load at temperatures up to 200° C. A metallographic study has also been made of the crept specimens. The creep resistance of Zn is shown to increase as the Cu content is raised, although the creep strength increment is small above 1 wt % Cu. Ageing the alloys also improves the creep strength, but precipitation during creep can generate voids which may lead to premature failure. The effect of increasing the Cu content is to make slip and grain-boundary sliding progressively more difficult, and to raiseΔHc, the apparent activation energy for creep. A higher copper content also enables a low value of the stress exponent to persist to higher creep stresses.

High temperature creep in Fe-3 pct Si

The kinetics and structure dependence of the high temperature-low stress creep of Fe-3 pct Si has been studied and compared with the predictions of current creep theories. It is shown that the steady state creep rate is diffusion controlled and exhibits a power law stress dependence in the temperature range 1393 to 1678 K and stress range 6.9×104 to 1.03×106 Pa. In this same range of experimental conditions the dislocation density present during steady state creep is essentially stress independent. It is shown that current creep theories are incompatible with the experimental results.

High temperature-low stress creep of Al and Al+0.5%Fe

The kinetics and structure dependence of high temperature-low stress Newtonian creep of Al and Al+0.5%Fe have been studied and compared with the predictions of the Nabarro-Herring (NH) theory of directional diffusion. It is shown that although the stress and temperature dependence of the Newtonian creep rate of pure Al are in agreement with the NH theory, the observed absolute creep rates in both single crystals and polycrystals are 1500 times faster than the theory predicts. Further it is shown that Newtonian creep in Al is structure sensitive. The dislocation density increases and subgrains form during creep. The subgrain size varies inversely with stress and extrapolates to subgrain size measurements for Al at lower temperatures and higher stresses while the dislocation density is essentially stress independent. Pre-straining at low temperatures results in inverse transient creep behavior similar to that observed in cold worked metals tested at lower temperatures and higher stresses. Also, the presence of precipitates of FeAl 3 reduces the creep rate of Al by about a factor of 100 at the same stress and temperature, in spite of the fact that the grain size in the Al-5% Fe alloy is smaller by a factor of 30. These experimental facts lead us to conclude that Newtonian creep in Al deformation is controlled by a dislocation process rather than by directional diffusion. A simple theory based on the climb controlled generation of dislocations from a fixed density of sources is developed to explain the observed behavior.

The fracture, by the triple point mechanism, of aluminium-20% zinc in the presence of a hydrostatic pressure

The effect of an applied hydrostatic pressure on the high stress creep fracture characteristics of an aluminium-20% zinc alloy at 260°C has been investigated. The application of a pressure equal to the applied stress increases both the fracture life and elongation to an extent which is more marked at low stresses. The results are interpreted in terms of current theories of crack nucleation and growth.

Tensile and Short-Time Creep Properties of N-155 Alloy Sheet

The special test equipment and procedures developed for the evaluation of ultrashort-time elevated-temperature mechanical properties of materials are described herein. The tensile and high-stress creep data obtained for Haynes N-155 alloy sheet are reported. At rapid strain rates, the high-temperature tensile strengths were up to 300 per cent higher than those obtained at conventional testing speeds. At high temperatures and high stress levels, however, creep rates were shown to be extremely rapid, and in most cases, the creep strength became a critical factor. The application of rapid-strain-rate tensile data and high-stress creep data in short-life designs is discussed.