Mechanism Of Irradiation Creep Research Articles

Irradiation creep due to point defect absorption at dislocations

AbstractThe stress-induced preferential absorption (SIP A) mechanism of irradiation creep is briefly reviewed. Its origin is explained in terms of the first-order size effect interaction and the induced interaction between the intrinsic point defects and the dislocations. These two interactions combine in a body under uniaxial stress to ensure that preferential absorption of interstitials will occur at those edge dislocations oriented with their extra planes normal to the stress axis and vice versa for the vacancies. A simplified rate theory argument is used to deduce the magnitude and microstructural dependence of the resulting creep strain. It is shown that the calculated creep rate is linear in applied stress and irradiating flux and is fairly independent of temperature.

The influence of cascade damage on irradiation creep and swelling

The irradiation of metals by fast neutrons or heavy ions produces displacement cascades. In many metals a substantial fraction of these cascades collapse to form vacancy loops. In this paper the effect of these vacancy loops on irradiation swelling and irradiation creep is examined. It is found that the presence of the vacancy loops reduces the effective displacement damage rate and consequently the irradiation creep and swelling rates are less than they would otherwise be. In addition it is shown that irradiation creep mechanisms which depend on the thermal emission of vacancies from vacancy loops are untenable. L'irradiation des métaux par les neutrons rapides ou par des ions lourds produit des cascades de déplacement. Dans de nombreux métaux une fraction substantielle de ces cascades se rassemblent pour former des boucles de lacunes. Dans ce mémoire, l'effet de ces boucles de lacunes sur le gonflement par irradiation et le fluage par irradiation est examiné. On a trouvé que la présence des boucles de lacunes réduit la vitesse d'endommagement par déplacement et en conséquence que les vitesses de fluage et de gonflement sous irradiation sont inférieures à ce qu'elles seraient autrement. En outre, il est montré que les mécanismes de fluage sous irradiation qui dépendent de l'émission thermique de lacunes à partir des boucles de lacunes ne peuvent être retenus. Durch Bestrahlung von Metallen mit schnellen Neutronen oder schweren Ionen werden Verlagerungskaskaden erzeugt. In vielen Metallen bricht ein wesentlicher Teil dieser Kaskaden unter Bildung von Leerstellenversetzungsringen zusammen. Deren Einfluss auf das bestrahlungsinduzierte Schwellen und Kriechen wird in dieser Arbeit untersucht. Durch das Auftreten der Leerstellenversetzungsringe wird die effektive Schädigungsrate durch Verlagerungen erniedrigt, und somit sind die bestrahlungsinduzierten Kriech- und Schwellraten geringer als sie sonst wären. Ferner wird gezeigt, dass die Mechanismen des bestrahlungsinduzierten Kriechens, die auf dem thermisch induzierten Austritt von Leerstellen aus Leerstellenversetzungsringen beruhen, unhaltbar sind.

Vacancy loops and irradiation induced creep

It is shown that the mechanism of irradiation creep recently proposed by Krishan and Ray produces a transient strain only. A calculation of an upper limit to the transient strain shows it to be much less than observed, irradiation creep strains.

On the statistical model of irradiation creep

Abstract The mechanism of irradiation creep, originally proposed by Gittus (1972), entailing purely random rather than systematic climb of dislocations is re-examined. It is concluded that his original calculations grossly over estimate the magnitude of the effect.

Irradiation-creep due to point defect absorption

A rate theory model for the analysis of the stress-induced preferred absorption (SIPA) mechanism of irradiation-creep is outlined and used to devise a simple design formula for the expected creep. This result is compared with an accurate numerical solution and with the relevant experimental data for solution-treated and cold-worked M316 steel. The importance of the high dislocation density arising from the athermal vacancy loop formation is emphasized and used to demonstrate the insensitivity of the predicted irradiation-creep to the usual metallurgical and physical variables.

The stress-induced point defect-dislocation interaction and its relevance to irradiation creep

Abstract The general interaction energy between an applied stress and a misfitting inhomogeneity is presented and used to deduce the accurate elastic interaction between edge dislocations and point defects in a body subjected to uniaxial tension. It is shown that the external stress does, as conjectured by Heald and Speight (1974), cause a preferential drift of interstitials and vacancies to dislocations with particular orientations to the external stress axis. Moreover, for a physically plausible range of the inhomogeneity representation of the point defects the particular preferences found are consistent with those necessary to validate the mechanism of irradiation creep proposed by Heald and Speight.

Irradiation-creep in a materials testing reactor

Helical springs made from titanium, zirconium, Nimonic PE 16 alloy and two austenitic stainless steels namely Firth-Vickers FV 548 and a steel with a composition within the AISI 316 specification have been irradiated in the Dounreay Materials Testing Reactor, at temperatures between 85 °C and 100 °C and to neutron fluences up to about 4 × 10 24 m −2 (> 1 MeV), whilst loaded in tension. Irradiation-creep was observed in all the materials studied and initial strain rates/unit stressneutron ranged from 1 to 1.9 × 10 −35 m 4/N for Nimonic PE 16 and zirconium respectively. Data obtained from an earlier experiment are re-presented and compared with the present results. Springs which received no heat-treatment after coiling unwound during irradiation at rates which were independent of the supported loads. The phenomenon is attributed to the relaxation of internal stresses (produced during the manufacture of the springs) with an irradiation-creep constant which is an increasing function of prior cold-work. A mechanism of irradiation-creep is proposed which involves the re-arrangement of the dislocation network in a crystal as the dislocations climb by absorbing interstitials produced by irradiation.

A possible mechanism of irradiation creep

A possible mechanism of irradiation creep

On the mechanisms of irradiation creep in zirconium-base alloys

A multiple-mechanism explanation was presented previously to explain the creep rates of zirconiumbase alloys under neutron irradiation. This interpretation involves defect loop alignment at low stresses, radiation-enhanced climb over radiation obstacles at intermediate stresses and cutting of obstacles at high stresses. Other authors have made alternative proposals. Recent data on stressdependencies, obtained principally in the intermediatestress region, are shown to offer support for the multiple-mechanism scheme and counter-arguments are presented concerning the differing interpretations.

The mechanisms of irradiation creep in graphite

Abstract The motion of edge and screw dislocations in a graphite polycrystal subjected to external forces and fast neutron irradiation is considered. A transient creep strain, proportional to the external stress and proportional to the ratio of the dimensions of the component crystallites in the a and c directions, is deduced. The calculated and experimentally observed magnitudes agree. From the time constant of this transient a value for the number of defect pairs created by a neutron during its moderation is calculated. There is fair agreement with accepted estimates. Graphite crystals show irradiation growth and a polycrystal must therefore either disintegrate or show yielding creep. The equations of yielding creep predict a creep rate greater than that observed, but they do not take into account that graphite is not a continuous solid. The discrepancy may thus be resolved. The possibility of an increase in the number of modes of deformation during neutron irradiation is discussed.

A possible mechanism of irradiation creep and its reference to uranium

The Anderson-Bishop-Cottrell mechanism of irradiation creep in alpha uranium does not explain some of the experimental results, e.g., recovery of creep strain after irradiation. A second mechanism effective at high temperatures is derived which involves the reversion of loop nuclei to clusters when the thermal spikes disappear (thermal annealing of creep). (D.L.C.)