Radiation Hardening of Neutron‐Irradiated Copper Single Crystals. II. Interpretation

Abstract

BY Interpretation W. FRANK, M. RUHLE, and M. SAXLOVA~)It is shown that the vacancy-type Frank dislocation loops on (111) planes observed in neutron-irradiated copper by means of electron microscopy lead to a decrease in the mobility of glide dislocations during plastic deformation which accounts quantitatively for the enhancement of the critical shear stress of copper single crystals caused by neutron irradiation. This follows from the analysis of measurements of the critical shear stress and of electron-microscopical determinations of the loop density in terms of a statistical theory of thermally activated dislocation motion over barriers of different heights. No adjustable parameters enter this analysis.Es wird gezeigt, da13 die in neutronenbestrahltem Kupfer im Elektronenmikroskop sichtbaren Frankschen Leerstellen-Versetzungsringe auf { 11 1 }-Ebenen bei plastischer Verformung zu einer Verminderung der Beweglichkeit der Gleitversetzungen fiihren, durch welche sich die nach Neutronenbestrahlung beobachtete Erhohung der kritischen Schubspanniing von Kupfereinkristallen quantitativ erklaren lafit. Dies geschieht mit Hilfe einer statistischen Theorie der thermisch aktivierten Versetzungsbewegung iiber Hindernisse verschiedener Hohe. I n diese Auswertung gehen keine frei verfiigbaren Parameter ein. IntroductionMany papers on radiation hardening of neutron-irradiated copper (for references see [l, 2,3]) have been published without too much progress in the theoretical understanding of this phenomenon since Seeger's pioneering work [4, 51. I n Seeger's approach to radiation hardening it was proposed that the increase of the critical shear stress to of copper after neutron irradiation is due to a reduction of the mobility of glide dislocations by "diluted zones" created during irradiation. More specifically, these zones were regarded as dislocation barriers of uniform heights surmountable with the aid of thermal activation and the applied stress.This model, which permits a qualitative description of the temperature dependence of z , , has failed to explain quantitatively tests involving changes of deformation temperature T and strain rate . & [l]. Therefore, Diehl and co-workers [2] extended Seeger's theory with respect to two essential points: They allowed for a height distribution in the radiation-induced dislocation barriers and for the experimentally observed inhomogeneity of the deformation of irradiated copper single crystals [S]. On the experimental side, the correlation between radiation hardening experiments and electron-microscopical observations on neutron-irradiated copper has recently become a powerful tool in clarifying the nature of radiation hardening (see Section 1.2). l ) Visiting Scientist from Mathematical-Physical Faculty of Charles University, Prague. physica 26jZ

The mechanisms of metal fatigue (I)

“…Whatever the source, these defects would seem t o offer the most likely source of friction stress. Recently, Frank, Riihle, and Saxlovh [29] have analysed the hardening of neutron-irradiated copper in terms of a "spectrum" of barriers provided by the various sizes and distribution of the irradiation-induced point defect clusters. Such an approach may prove successful in the case of fatigue.…”

“…Yet, it is known t8hat individual vacancies and interstitials produced in quenched copper do not produce any increase in the flow stress. Aging, to convert the point defects to clusters (or to jogs), is required to produce hardening [29]. However, fatigue produces numerous prismatic dislocation loops, which may be the result of collapsed vacancy clusters or of dissociated dipoles.…”

Determination of the Velocity and Local Density of Mobile Dislocations Within Slip Bands in Neutron-Irradiated Copper by High Speed Cinematography

“…where the obstacle spacing i i s eliminated so that ac-immediately follows from the measured vL without difficulties and assumptions concerning the spectrum of obstacles, themobile dislocation density, the active slip volume etc. [14,15,23]. Using a measured valueof vL=1.6mm/s( found by Niemann [29] for n-irradiated Cu crystals (dose 2 x 10l8 n/cm2) from a thermal activation analysis of macroscopic measurements (strain rate and temperature cycling).…”

On slip band growth in neutron-irradiated copper single crystals

“…(A2) has been solved by a Runge-Kutta routine. Independent of the initial distribution, the moving group is found to achieve, in a very short time or after a short distance of movement (of about 100 ms or 30 µ m, respectively, with reasonable parameters [56,75,76] : ∆ G 0 = 1.3 eV, V = 23 · 10 -27 m 3 , τ o = 10 MPa, g = 1.7 · 10 3 MPa/m, µ = 4.64 · 10 10 N/m 2 , b = 2.55 · 10 -10 m, ν D = 7 · 10 12 s -1 ), a quasi-stationary distribution which is quite similar to that in the static case [92,93] with a higher density in its center than towards the ends of the group. This result can also be inferred directly by adding up all ln(ẋ i ) terms of Eqs.…”