The development of microstructural damage during high temperature creep–fatigue of a nickel alloy

Alloy 617 is the leading candidate material for an Intermediate Heat Exchanger (IHX) of the Very High Temperature Reactor (VHTR). To evaluate the behavior of this material in the expected service conditions, strain-controlled cyclic tests that include hold times up to 9000 s at maximum tensile strain were conducted at 950 °C. The fatigue resistance decreased when a hold time was added at peak tensile strain, owing to the mechanisms resulting in a change in fracture mode from transgranular in pure fatigue to intergranular in creep–fatigue. Increases in the tensile hold duration beyond an initial value were not detrimental to the creep–fatigue resistance. An analysis of the evolving failure modes was facilitated by interrupting tests during cycling for ex situ microstructural investigation.     

Generation of intergranular strains during high temperature creep fatigue loading of 316H stainless steel

Abstract

This paper investigates the development of intergranular strains and stresses in AISI type 316H austenitic stainless steel during cyclic loading at high temperature. Isothermal cyclic creep tests at 650°C with 2 h displacement controlled creep dwells at maximum strain are conducted with in situ neutron diffraction monitoring at a time of flight facility. The evolution of intergranular strains in five independent {hkl} grain families were successfully measured during consecutive cycles. The grain family with {111} lattice planes normal to the loading direction exhibited linearly reversible behaviour with cyclic load whereas the {200} planes deformed in a non-linear manner forming a hysteresis loop. Intergranular strains during the first dwell remained unchanged with time, but relaxed with time during later dwells. The start of dwell intergranular strains increased from cycle 1 to cycle 2, but markedly less moving from cycle 2 to cycle 3.     

Deformation and life assessment of high temperature materials under creep fatigue loading

The knowledge of mechanical long term behaviour under static and cyclic loading for high temperature components requires methodologies for life assessment in order to employ the full potential of materials. A phenomenological life time prediction concept which was developed for multi‐stage creep fatigue loading demonstrates the applicability of rules for synthesis of stress strain path and relaxation including an internal stress concept, as well as mean stress effects. Further, a creep fatigue interaction concept which was also developed covers a wide range of creep dominant loading as well as fatigue dominant loading. Service‐type experiments conducted at different strain rates and hold times for verification purposes demonstrate the acceptability of life prediction method for variation of conventional 1 %Cr‐steels as well as modern high chromium 9‐10 %Cr‐steels. Generally, the service life of components is influenced by multi‐axial behaviour. Multi‐axial experiments with e.g. notched specimens and with cruciform specimens accompanied by advanced methods for calculation of stress strain path and life time prediction stress conditions are of future interest.     

Investigation of the process of diamond formation from SiC under high pressure and high temperature

SiC powder or graphite in contact with cobalt, nickel or a Ni₇₀Mn₂₅Co₅ alloy was treated at high pressure and high temperature in stable region of diamond. It was found that Ni₇₀Mn₂₅Co₅ alloy is more effective in the process of diamond formation from SiC than the others, but the difference was not apparent when graphite was used instead of SiC. Using the Ni₇₀Mn₂₅Co₅ alloy, diamond formed rapidly with the decomposition of SiC at a pressure of 5.4–6.0 GPa and temperature 1350–1570°C, and the growth tended to stagnate after 6 min, when SiC was completely exhausted. X-ray diffraction showed that the relative intensity of the diffraction lines of diamond and graphite was nearly constant in the samples synthesized under the same conditions for 2, 4 and 6 min. The results suggest that diamond and graphite may be formed directly and respectively from separated carbon atoms in a short time.     

The crack initiation and growth under high temperature creep, fatigue and creep-fatigue multiplication

This article concerns some of our recent studies on the crack initiation, early stage crack growth and its subsequent crack growth under high temperature creep, fatigue and creep-fatigue multiplication. The criteria for these and some new ideas are proposed. For instance, the relative notch opening displacement (RNOD) criterion for the crack initiation and the Q^* parameter for the crack growth are critically reviewed. Early stage crack growth and its subsequent crack growth as affected by notch tip acuity were studied. The behaviour of the tail part in the log da/dN vs log C^* curve has been attempted to explain in terms of the curve of the creep behaviour and of the crack length against time. Furthermore it was proposed that early stage crack growth, say, the so-called first stage crack growth in terms of log da/dN vs log K curve may be characterized by the parameter different from those for the so-called second stage crack growth.     

Rheological analysis of stress change experiments during high temperature creep

The main results of stress drop experiments during high temperature creep (i.e. the occurrence of zero and sometimes negative creep rates, the existence of two parts with different slopes in the curves of the incubation time t_r versus the stress decrement , the zero or positive values of the stress variations in stress relaxation tests) are analysed from a rheological point of view. The basis of the proposed interpretation is the existence of a creep criterion, represented by a limit surface in the stress space, which explains through the usual plastic flow rules the occurrence of zero or negative creep rates. The work hardening splits into a translation (kinematic hardening) and a deformation of this limit surface. The recovery of kinematic hardening is slower than the recovery of deformation. On this basis, the concept of internal stress used either in the activated glide of dislocations or in the Bailey-Orowan relationship and the nature of negative creep rates are discussed.     

Analysis of constrained cavity growth during high temperature creep deformation

Abstract

A general model for constrained cavity growth is developed for cavitation during creep deformation. The model incorporates the effect of the separation between cavitated facets, the contributions of local power-law creep in the cavitated regions of a specimen, and the possibility of coupled cavity growth. A detailed analysis of the model reveals that, during constrained cavity growth at low stresses, interaction effects enhance the cavity growth rates when L/d<10, where L is the separation between the cavitated facets and d is the grain size. An assessment of the stress redistributions occurring during cavity growth indicates that constrained cavity growth may lead to increases in the specimen creep rate by significantly increasing the stress in the uncavitated regions of a specimen when L/d<5. It is also demonstrated that, under the appropriate limiting conditions, the present general model gives results similar to those obtained in previous studies on constrained cavity growth.

MST/610     

Scatter bands in creep and fatigue crack growth rates in high temperature plant materials data

As a part of the European Commission supported project BE 1702: ‘HIDA’ Creep, creep-fatigue and high temperature fatigue crack growth data for five high temperature plant steels were accessed from a number of published and unpublished sources. These large sets of data were reviewed, and re-analysed where necessary, and plotted in terms of various crack growth rate correlating parameters. Thus limits of scatter bands and mean and upper 95% confidence limit creep and fatigue crack growth correlations are proposed. The present work covers a wide range of variables such as test specimen geometries, sizes, loading conditions and temperatures. Therefore, the correlations proposed are considered universal. However, it is envisaged that these correlations will be refined in future by enlarging the database and exploring the effect of the variables described above. The five materials studied are AISI 316 stainless, 2.25CrlMo, P91, 1CrMoV (forged), and 1CrMoV (cast) steel.     

Interaction between dislocations and precipitated water bubbles during high temperature creep of quartz

The modes of interaction between dislocations and precipitated water bubbles in quartz are discussed. It is shown that dislocations may be locally anchored by bubbles, but are able to break away under the action of applied stress. To escape, dislocations pinned by bubbles must bow to equilibrium curvature. It was observed that escape via thermal activation alone was not readily possible; however, unpinning of the dislocation through pipe diffusion was found to be possible if the dislocation could drag the bubbles under applied stress. This could result in very small strains leading to a microcreep rate linearly dependent on stress and independent of the concentration of bubbles.     

On the relation between creep cavitation and grain boundary orientation

• 1.1. The influence of grain boundary inclination on the creep cavity nucleation rate and the cavity growth rate has been studied using SEM and a stereo microscopy technique in a CuSb alloy.
• 2.2. Both the cavity growth rate and the cavity nucleation rate were found to be higher on transvers boundaries.
• 3.3. A possible explanation for the higher nucleation rate on boundaries by stochastic grain boundary sliding has been proposed.

     

Surface and interior measurements of grain boundary sliding during creep

Measurements of grain boundary sliding have been made on polycrystalline specimens of Magnox AL80, a magnesium-0.78 wt% aluminium alloy, at successive strains during creep at 200° C under a stress of 2800 psi. Three independent methods were used to determine the strain due to sliding ( _gb) at the surface and two to determine _gbin the interior of the specimens. The one direct method of measuring _gbin the interior used oxide markers introduced by extruding a composite billet. The values of _gbobtained from the offsets in these interior markers were found to agree with those given by the three sets of measurements made on the surface, but not with those from the indirect method for the interior which relies on the measurement of grain strain via grain shape changes.