High temperature creep behaviour of nearly stoichiometric alumina spinel

Creep behaviour of nearly stoichiometric spinel, (Al₂O₃)_n MgO, n=1.1, is investigated for compression axis [001], at temperatures 0.77 to 0.83 T _m and constant loads 88.2 to 117.6 MPa. Experimental observations, including the mechanical creep law and the dislocation substructures as imaged by TEM and Berg-Barrett X-ray topography support the following picture: {100} 110 slip is activated in the very early creep stage, while no evidence for {111} is found; on the other hand {111} slip planes are observed for stress orientation [110], in agreement with Schmid's law. (ii) When in edge orientation slip dislocations become sessile by pure climb splitting. Their dissociation plane has been determinated unambiguously and observed to be perpendicular to their Burgers vector. As a result, it is suggested that slip should be inhibited and further creep should occur by pure climb strain only. This expected climb-creep accounts for experimental rates and, tentatively, for their dependence on stoichiometry n since the latter is observed to change only pre-exponential terms, the creep energies being much the same whatever the value of n.This paper is part of the work done by RD for a Thèse d'Etat dissertation.Associated with CNRS.     

The creep behaviour of isotropic polyethylene

Dead loading creep and constant strain rate yield experiments have been used to study the tensile creep behaviour of three grades of isotropic polyethylene. This has provided further evidence for the existence of two yield points in isotropic polyethylene. Two different models have been used to attempt to describe this behaviour. Although the results can be described by to both the two process model of Wilding and Ward and the co-operative jump model of Fotheringham and Cherry, it appears that the two process model provides a more convincing quantitative fit to the data. This revised version was published online in September 2006 with corrections to the Cover Date.     

The creep behaviour of ultra-high modulus polypropylene

The creep and recovery behaviour of highly drawn polypropylene monofilaments has been studied over the temperature range 20 to 50° C. A range of samples was examined to identify the influence of draw ratio and molecular weight. It is concluded that the permanent flow creep arises from the presence of two thermally activated processes, one of which relates to the -relaxation process and is associated with the crystalline regions of the polymer, and the second with the molecular network.     

The creep and fracture behaviour of tempered martensitic steels

Creep strength enhanced ferritic steels contain 9 to 12% Cr and were developed to exhibit excellent high temperature properties. These should be achieved when the microstructure exhibits a tempered martensitic matrix containing a substructure with a high dislocation density and a uniform dispersion of fine, second phase precipitates. It is interesting to note that when properly processed the typical alloy compositions for these steels provide reasonable strength but can exhibit brittle creep behaviour. The levels of ductility required in engineering applications necessitate proper control of composition (including trace elements), steel making and processing and all heat treatments. The properties needed for modern design methods can only be obtained using validated procedures for both uniaxial and multiaxial testing and documentation to establish the mechanisms controlling deformation and fracture for relevant stress states.     

The mechanism of creep behaviour in glassy polymers

A model is presented to describe the creep behaviour of glassy polymers below the glass transition temperature. It consists of a Hookean spring in series with a non-Newtonian dashpot having an entropy spring in parallel. The shape of the response of this spring is deduced from a master curve, giving the extension as a function of logarithm of time, built from creep data, reported here and obtained on polycarbonate over a wide range of times and temperatures. The model takes into account a number of aspects of creep behaviour and predicts a threshold stress beneath which delayed yielding no longer occurs. Torsional creep data, obtained on Polyvinylchloride by Mallon and Benham are found to be in excellent agreement with the proposed model.     

The compression creep behaviour of silicon nitride ceramics

A comparison has been made of the compression creep characteristics of samples of reaction-bonded and hot-pressed silicon nitride, a sialon and silicon carbide. In addition, the effects of factors such as oxide additions and fabrication variables on the creep resistance of reaction-bonded material and the influence of dispersions of SiC particles on the creep properties of hot-pressed silicon nitride have been considered. For the entire range of materials examined, the creep behaviour appears to be determined primarily by the rate at which the development of grain boundary microcracks allows relative movement of the crystals to take place. Now with the BNF Metals Technology Centre, Wantage.     

Transitions in creep behaviour

Attempts to identify the mechanisms operating during creep are often made by examining plots which yield apparent activation energies, or the stress or grain size-dependences of creep-rate. The forms of such plots are here examined and the ambiguities which arise near transitions from one regime to another are noted. The ranges of temperature, stress and grain size commonly used are inadequate and serious errors in interpreting the results of creep tests will continue to be made until a better understanding of the interaction of the basic processes is developed, so as to enable the positions of transitions to be predicted.At Dept. of Metallurgy, University of British Columbia, Vancouver 8, BC, Canada until 31 March 1970.     

Non-linear creep behaviour of PET

The non-linear creep behaviour has been studied on PET films at room temperature. A particular value of the stress, _c, was used to characterize the change between the linear to the non-linear domain. The variations of the elastic modulus, the relaxed modulus and _c revealed great sensitivity to the morphology of the crystallization. A molecular model of non elastic deformation, assuming (i) hierarchical correlated molecular motion, and (ii) nucleation and expansion of sheared-microdomains, was used to analyse the role of stress on anelasticity. To take into account the two-phase structure of semicrystalline films, a phenomenological series/parallel model was applied to express the mechanical coupling between amorphous and crystalline phases. Quantitative agreement was found between theoretical predictions and experimental data for low and high stresses. However, there was a discrepancy in the rate of recovery because the model predicts a strain recovery slower than the experimental behaviour. Consequently, it is proposed to develop further the molecular model mentioned above by specifying the energy profile of a sheared-microdomain and its stress dependence. Then, the difference between creep and recovery strain rate could be explained.Nomenclature A Anelastic equilibrium compliance - A Parameter proportional to the relaxation strength - b Shear vector - Correlation parameter - _i Particular value in the distribution - _e Average value in the material ( = 0.27) - Correlation parameter characterizing the ability of chain orientation - d _a Amorphous density - d _c Crystalline density - Parameter of the mechanical mixing law - E Tensile modulus - E _c Crystalline Young's modulus - g _i Statistical weight in the distribution - G ₀ Shear modulus at 0 K - J _max Creep compliance at the end of the creep time - J _max(0) Value ofJ _max for low stresses - J _u Unrelaxed compliance - J _i( _i;A _i) Calculated compliance for a couple ( _i;A _i) - J _exp Experimental creep compliance - J _a Compliance of the amorphous part - J _c Compliance of the crystalline part - J _sc Compliance of the semi-crystalline material - k Boltzmann's constant - Parameter of the mechanical coupling law - R Radius of a shear micro domain - ₀ Stress necessary to cross the energy barrier only by mechanical activation - T _c Crystallization temperature - t _c Creep time - _mol Time for a translational motion of a structural unit over a distance comparable to its size - Particular value of _mol in the time distribution - Characteristic time for the secondary relaxation - ₀ Time proportional to the Debye time - t ₀ Scaling time parameter determined by the experimental value of _mol - U Activation energy for an elementary molecular motion - X _c Crystallinity ratio - V _a Volume fraction of the amorphous part - V _c Volume fraction of the crystalline part     

The cyclic creep behaviour of Type 316 stainless steel

The cyclic creep behaviour of a Type 316 stainless steel at 625° C has been examined as a function of the maximum applied stress and frequency using trapezoidal loading cycling between zero and a maximum stress. The so-called static-to-dynamic creep transition observed is interpreted in terms of recoverable anelastic strain behaviour without using an internal stress argument. Over the range of experimental conditions examined, failure occurs by static creep modes, namely wedge crack nucleation and growth. The loading strain increments appear to be damaging to about the same extent as the much slower strain occurring at constant load, such that it is the overall strain rate that determines the rate of damage. A cursory examination of square wave load cycling shows that the behaviour is very similar to that observed during trapezoidal loading and suggests that the rate of loading and unloading does not play an important part in determining the creep and rupture behaviour.     

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.     

Immobilization of sodium and potassium in Synroc

Substitution of K for Na in certain nuclear fuel reprocessing cycles may allow an increase of waste loading in Synroc, because K can be incorporated in the barium hollandite phase more easily than Na. The use of rare-earth additions to stabilize Na in the perovskite phase may also have merit.     

The constant stress tensile creep behaviour of a superplastic zirconia-alumina composite

A detailed study was conducted to evaluate the constant stress tensile creep behaviour of a superplastic 3 mol% yttria-stabilized zirconia-20 wt% alumina composite. The comprehensive experimental results indicate that creep deformation may be expressed in the form exp(–585 000/8.3T), where is the steady-state creep rate, is the imposed stress, is the linear intercept grain size andT is the absolute temperature. Microstructural observations revealed that there is very little dislocation activity, or change in grain size or shape. A detailed analysis was conducted to evaluate the possible rate-controlling mechanisms in terms of the experimentally determined mechanical properties and the microstructural observations. Based on the maintenance of an equiaxed microstructure and the strong grain size and stress dependence, it is concluded that creep occurs by a grain-boundary sliding/grain rearrangement process.     

Transient and steady state creep behaviour of polycrystalline MgO

Stress change experiments during compressive creep tests at high stresses on polycrystalline MgO at 1596 K have shown that the creep rate at any instant during transient and steady state creep is predicted by the ratio,r/h, wherer is the rate of recovery (=??σ/t6t) andh is the coefficient of strain hardening (=?σ/?ε). Over most of transient and steady state creep, whenh is constant and the decrease in creep rate, \(\dot \in\) , is a direct result of a decrease inr, the variation of the creep strain,ε, with time,t, is accurately described as $$ \in = \in _0 + \in _T (1 - e^{ - mt} ) + \dot \in _s t$$ whereε ₀ is the instantaneous strain on loading,ε _T the transient creep strain,m relates to the rate of exhaustion of transient creep and \(\dot \in _s\) is the steady creep rate. Deviations from this equation occur during the initial 10 to 15% of the transient creep life, whenh increases rapidly. The variations in \(\dot \in\) ,r andh during transient and steady state creep are explained in terms of a model for creep in which the rate-determining process is the diffusion controlled growth of the three-dimensional dislocation network within subgrains to form dislocation sources allowing slip to occur.     

Determination of creep behaviour of Ti60 alloy by small punch creep test

The small punch (SP) creep test has distinct advantages in the creep property assessment of materials at elevated temperatures. However, there are few creep properties of Ti alloys obtained by the SP creep test in the current literature. In this paper, the SP creep behaviour of Ti60 alloy has been evaluated under various loads in the range 550–800 N over a temperature range 873–973 K. The SP creep curves obviously indicated the primary, secondary and tertiary stages of creep. The test results have been compared with those of conventional creep tests. The European Code of Practice (CoP) for Small Punch Testing, Dorn equation and Monkman–Grant relationship have also been used to analyse the results of the SP creep tests. The ratio of load of the SP creep tests to equivalent stress of conventional creep tests, the load exponent value of steady deflection rate and activation energy for creep deformation were estimated from the SP creep tests. In conclusion, it was found that dislocation creep may be the main mechanism that dominates the SP creep deformation of Ti60 alloy in the range of load and temperature.