Luboš Kloc

Transition from power-law to viscous creep behaviour of P-91 type heat-resistant steel

Abstract Short-term creep tests were performed on a 9% Cr (P-91 type) steel at temperatures from 873 to 923 K and at stresses below 100 MPa by means of the helicoid spring specimens technique. The steady state creep rates correspond to viscous behaviour under the above conditions, characterized by the apparent stress exponent close to 1. Since the stress exponent at higher stresses is about 10, the change in the deformation mechanism at lower stresses is evident. The deformation mechanisms map resulting from the presented data shows that the service loading conditions respond to the viscous creep. Extrapolation from power-law creep regime to low stresses can cause serious underestimation of predicted deformation rates. The primary stage can be described by the Lis equation. Preliminary annealing at 873 K for 1.8 × 10 7 s reduces the primary strain, but it has no effect on the steady state creep rate.

Comparison of low stress creep properties of ferritic and austenitic creep resistant steels

Abstract Creep tests at stresses below 30 MPa and temperatures from 923 to 1073 K were conducted on an austenitic AISI-316H creep resistant steel. The helicoid spring specimen technique was used with the average test duration of ∼2000 h. The results were compared to the results obtained on modified 9% Cr ferritic P-91 type creep resistant steel by the same experimental technique under comparable conditions. The shapes of creep curves for both steels are rather similar, with a deep decrease in creep rate during primary stage followed by a secondary stage with constant strain rate. Viscous creep (i.e. creep having the stress exponent close to one) was demonstrated in both steels under the loading conditions described above. The secondary creep rate for AISI-316H is about two times lower than that of P-91 under the same testing conditions. The difference can be easily explained by the difference in mean grain size of both materials, supposing that the diffusional creep is the prevailing deformation mechanism. Unfortunately, it is not possible to identify creep mechanism clearly due to no possibility to obtain specimens with different grain size and the same microstructure. Further, the creep mechanism identification is complicated by the fact, that the measured apparent activation energy is almost the same for both materials. This result is in contradiction to the diffusion data published in the literature, where a considerably lower value of activation enthalpy in γ -phase than in α -phase have been reported for both volume and grain boundary diffusion.

Threshold creep behaviour of an MgZnCaCeLa alloy processed by rapid solidification

Abstract The creep behaviour of an Mg 3Zn Ca 1.5Ce 0.5La alloy processed by rapid solidification was investigated at temperatures ranging from 473 K to 523 K and applied stresses up to 82.6 MPa. It is shown that the minimum or steady-state creep strain rate is not attained up to a true creep strain of about 0.35. At least up to this creep strain, the true creep strain rate decreases in time essentially linearly for strains higher than 0.03. Thus primarily the true creep strain rate e 5 (the subscript 5 refers to a true creep strain of 0.05) is used to describe the creep behaviour of the alloy. In double logarithmic coordinates, the true creep strain rate e 5 depends on the applied stress in a way that strongly suggests a true threshold stress. The threshold stress σ TH decreases with increasing temperature and the same holds for the true applied stress exponent n . The values of this stress exponent are low, decreasing from 3 at 473 K to 2 at 523 K. At applied stresses ranging from about 10 MPa to about 85 MPa, the activation enthalpy (true activation energy) of creep ΔH is significantly higher than the activation enthalpy of diffusion in the alloy matrix lattice ΔH L . Analysis of the creep data suggests that the process (or dislocation mechanism) controlling the creep strain rate in the alloy is either dislocation slip in the basal planes dependent on cross slip to non-basal planes or, more probably, dislocation glide in non-basal slip planes dependent on the nucleation of kink motion.

Interpretation of constant-load and constant-stress creep behavior of a magnesium alloy produced by rapid solidification

Abstract The creep behaviour of an Mg–Zn–Ca–Ce–La alloy produced by rapid solidification was investigated by means of constant-load creep tests carried out at 498, 523 and 548 K. The analysis of strain rate as a function of applied stress suggests that in the ranges of stress and temperature studied, the creep mechanism is glide on basal plane controlled by cross-slip on non-basal planes or dislocation glide limited by the nucleation of kinks. This behaviour contrasts with previous results obtained in pure Mg where cross-slip on non-basal planes was the rate-controlling mechanism only at higher temperatures. The different behaviour of this alloy is attributed to the very fine grain size typical of rapidly solidified material. The enhancement in strain rate observed in the present alloy can be attributed to grain-boundary sliding accommodating the strain produced by dislocation creep mechanisms (glide on non-basal plane controlled by cross-slip or dislocation glide limited by the nucleation of kinks) that operate in the subgrain interior.

An evaluation of the creep properties of two Al-Si alloys produced by rapid solidification processing

Tensile creep tests were conducted on two Al-Si alloys produced by rapid solidification: an Al-Si-Ni-Cr alloy and an Al-Si-Cu-Fe alloy, designated alloys A and B, respectively. The creep curves of these two alloys in the temperature range from 493 to 573 K were markedly different, with alloy A exhibiting a normal creep curve with a very short tertiary region and alloy B exhibiting an extended tertiary stage associated with strain localization. The minimum creep rates varied, with the applied stress raised to exponents of ∼9.0 and ∼8.5 for the two alloys, respectively. The hardness of alloy B decreased with time during the creep testing, but there was little or no change in the hardness of alloy A. These differences in the creep and hardness characteristics are attributed to the evolution of precipitates within the two alloys during creep testing. A detailed analysis shows that, over the temperature range examined experimentally, alloy A exhibits a creep strength that is superior to conventional Al-based alloys and comparable to, or even higher than, some SiC-reinforced Al composites.

Significance of continuous precipitation during creep of a powder mettallurgy aluminum alloy

Abstract Experiments were conducted to evaluate the creep properties of a 2024 aluminum alloy fabricated by powder metallurgy processing. The creep curves exhibit a minimum creep rate followed by an extended tertiary stage prior to failure. Using the values of the minimum creep rates, the apparent stress exponents are high and variable suggesting the presence of a threshold stress. Observations using transmission electron microscopy (TEM) reveal the occurrence of a continuous precipitation of fine particles during the tests. Although the density of these particles is dependent upon the testing conditions, quantitative measurements show that their average size is of the order of ∼ 60 nm under all conditions. A temperature compensated time is introduced to describe the evolution of fine particles during testing, and this permits the development of a relationship which can be used to estimate the density of the particles under any selected conditions.

Creep behaviour of a Ni15Cr solid solution alloy at low stresses and intermediate temperatures

Abstract Creep in a Ni15Cr solid solution alloy at low stresses and intermediate temperatures was investigated using the helicoidal spring specimen technique. The applied stress ranged from 0.2 to 5.0 MPa, the testing temperatures from 973 to 1123 K (homologous temperatures T T m from 0.59 to 0.65, where Tm is the solidus temperature). The intercept grain size of creep specimens ranged from 47 μm to 254 μm. It was found that under the above conditions, viscous creep of the Bingham type occurs, characterized by a low threshold stress which decreases with increasing temperature. The temperature, applied stress and grain-size dependence of the steady-state creep strain rate can be interpreted in terms of the transition from Coble diffusional creep to Harper-Dorn dislocation creep controlled by dislocation core diffusion.

Measurement of Very Low Creep Strains: A Review

Various experimental techniques using mechanical springs for high strain sensitivity creep testing are described. A theory of stress and strain using these techniques is briefly summarized. Problems and advantages of the helicoid spring specimen technique are also analyzed. The helicoid spring specimen technique provides high efficiency for low-stress creep experiments in the self-loaded mode, while the external load mode allows extraordinary strain sensitivity. A new machine for conducting high-sensitivity torsion creep tests is presented together with some examples of creep curves obtained by the technique. The machine was designed to enable high sensitivity creep tests of inherently brittle materials such as intermetallics and ceramics.

Small Punch Testing of Sanicro 25 Steel and its Correlation with Uniaxial Tests

Small punch testing under constant deflection rate, constant force and constant deflection (i.e. force relaxation) were performed on the new austenitic steel Sanicro 25. Constant deflection rate experiments were correlated to uniaxial tensile tests at room temperature and 700°C with the help of several empirical relationships. Small punch creep testing was performed in as received state. Correlation of the small punch results with uniaxial creep test results was done and the force/stress ratio Ψ and kSP parameter were determined. The constant deflection small punch test was correlated with the uniaxial stress relaxation test and good agreement was reached.

Pre-Primary Stage of Low Strain Rate Creep in Metals

Primarystageofcreepprocessisaperiodwhenthecreeprateisdecreasingduetomi-crostructuredevelopment.Thedislocationdensityevolvestothedynamicequilibriumvaluerelatedtotheappliedstress.Inlow-strainratecreepexperiments,similarstageofdecreasingstrainrateisobservedalongwithapparenttransitiontosteadystate.Nevertheless,deeperanalysisofthevariousparametersofthestageshowsthatthesubstanceoftheprocessmustbedifferent.Themainreasonsare:i)thestrainistoosmallfordislocationsubstructurerearrangement,ii)stressandtemperaturedependenciesofthetransitionstrainandrelaxationtimeoftheprocessdoesnotcorrespondtoanyoftheknownprimarycreepprocesses,iii)thestrainreachedduringthestageisalmostfullyrecoverable,sothatthedeformationisratheranelasticthanplasticone.Thetransitionstrainobservedinlowstrainrateexperimentsthusshouldbetreatedasaspecial“pre-primary”stage,whiletheapparentsecondary(steadystate)stageisinfactpartoftheprimarycreepstage.