41 Matching Results

Search Results

Advanced search parameters have been applied.

Kinetic theory of radiation effects

Description: To help achieve the quantitative and mechanistic understanding of these processes, the kinetic theory of radiation effects has been developed in the DOE basic energy sciences radiation effects and fusion reactor materials programs, as well as in corresponding efforts in other countries. This discipline grapples with a very wide range of phenomena and draws on numerous sub-fields of theory such as defect physics, diffusion, elasticity, chemical reaction rates, phase transformations and thermodynamics. The theory is cast in a mathematical framework of continuum dynamics. Issues particularly relevant to the present inquiry can be viewed from the standpoints of applications of the theory and areas requiring further progress.
Date: January 21, 1987
Creator: Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Irradiation creep by climb-enabled glide of dislocations resulting from preferred absorption of point defects

Description: A mechanism of irradiation creep is proposed that arises from the climb-enabled glide of dislocations due to stress-induced preferred absorption of radiation-produced point defects. This creep component we term preferred absorption glide, PAG. PAG-creep operates in addition to the previously studied components of creep from climb by stress-induced preferred absorption, PA-creep, and the climb-enabled glide due to excess absorption of interstitials on dislocations during swelling, l-creep. A formulation of the various climb and climb-enabled glide processes is presented which includes earlier results. PAG-creep is comparable in magnitude to PA-creep in the parameter range of applications. While the PA-creep rate and the l-creep rate are linear in stress, the PAG creep rate is quadratic stress and thus dominates at high stress.
Date: August 1, 1978
Creator: Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Effects of point defect trapping and solute segregation on irradiation-induced swelling and creep

Description: The theory of irradiation swelling and creep, generalized to include impurity trapping of point defects and impurity-induced changes in sink efficiencies for point defects, is reviewed. The mathematical framework is developed and significant results are described. These include the relation between vacancy and interstitial trapping and the effectiveness of trapping as compared to segregation-induced changes in sink efficiencies in modifying void nucleation, void growth, and creep. Current understanding is critically assessed. Several areas requiring further development are identified. In particular those given special attention are the treatment of nondilute solutions and the consequences of current uncertainties in fundamental materials properties whose importance has been identified using the theory.
Date: January 1, 1978
Creator: Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Applications of the theory of cavity growth to dual-ion swelling experiments

Description: The rate theory of cavity growth is applied to study the effects of helium gas on cavity swelling. The variation of swelling with temperature is emphasized: (1) expressions are derived showing that the primary effect of the helium is in pressurizing cavities and that a secondary effect is in altering the microstructural sink strengths. The expressions simplify in the parameter range of engineering interest such that the temperature regime of swelling is predicted to shift upward in approximately direct proportion to the cavity gas pressure; (2) recent experimental data on swelling of a pure stainless steel type alloy under dual-nickel and helium-ion bombardment is interpreted. Helium-free, helium-coimplanted, and helium-preimplanted swelling results can be explained by the theory. It is necessary to account for the partitioning of the helium to dislocations as well as to cavities in order to explain the experimental results for helium coimplantation; (3) model studies for physically reasonable parameters reveal the importance of the He/dpa ratio.
Date: January 1, 1980
Creator: Hayns, M.R. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Cascade-induced fluctuations and the transition from the stable to the critical cavity radius for swelling

Description: Recently, a cascade diffusion theory was developed to understand cacade-induced fluctuations in point defect flux during irradiation. Application of the theory revealed that such fluctuations give rise to a mechanism of cascade-induced creep that is predicted to be of significant magnitude. Here we extend the investigation to the formation of cavities. Specifically, we explore the possible importance of cascade-induced cavity growth excursions in triggering a transition from the gas-content-dictated stable radius to the critical radius for bias-driven growth. Two methods of analysis are employed. The first uses the variance of fluctuations to assess the average effect of fluctuations. The second is based on the fact that in a large ensemble of cavities, a small fraction will experience larger than average excursions. This prospect is assessed by estimating upper limits to the processes. For the conditions considered, it is concluded that cascade-induced fluctuations are of minor importance in triggering the onset of swelling in a population of stable gas-containing cavities.
Date: January 1, 1985
Creator: Hayns, M.R. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Radiation effects on microstructures and properties of irradiated materials

Description: Development of structural materials to withstand aggressive radiation environments has been carried out on an international scale over the past four decades. Major radiation-induced changes in properties include swelling, creep and embrittlement. The basic work, stimulated by technology, to understand and control these phenomena, has been heavily oriented toward the evolution of microstructures and their effects on properties. Microstructural research has coupled analyses by high resolution techniques with theoretical modeling to describe and predict microscopic features and the resulting macroscopic properties. A short summary is presented of key physical considerations that drive these changes during irradiation. Such processes begin with displacement cascades, and lead to property changes through the diffusion and clustering of defects.
Date: December 1, 1996
Creator: Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Effect of precipitate-matrix interface sinks on the growth of voids in the matrix

Description: A qualitative discussion of the differing roles played by coherent and incoherent precipitates as point defect sinks is presented. Rate theory is used to obtain semiquantitative estimates of the growth of cavities in the matrix when either type of precipitate is present. Methods for deriving the sink strengths of precipitates of arbitrary shape are developed. In three materials where available microstructural information allows an analysis, precipitates are found to cause only a small relative suppression of cavity growth via the mechanisms here considered.
Date: January 1, 1981
Creator: Brailsford, A.D. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Toward a comprehensive theory of radiation-induced swelling and creep - the point defect concentrations

Description: The theory of void swelling and irradiation creep is now fairly comprehensive. A unifying concept on which most of this understanding rests is that of the rate theory point defect concentrations. Several basic aspects of this unifying conept are reviewed. These relate to local fluctuations in point defect concentrations produced by cascades, the effects of thermal and radiation-produced divacancies, and the effects of point defect trapping.
Date: January 1, 1979
Creator: Mansur, L.K. & Yoo, M.H.
Partner: UNT Libraries Government Documents Department

Advances in the theory of radiation effects in metals and alloys

Description: Recent advances in the theory of swelling are reviewed. These include the development of a cascade diffusion theory to treat quantitatively the local fluctuations in point defect concentrations, the incorporation of mobile helium into the rate theory, and the spatial variation in swelling during charged particle bombardment.
Date: January 1, 1979
Creator: Mansur, L.K. & Yoo, M.H.
Partner: UNT Libraries Government Documents Department

Effect of point defects on the amorphization of metallic alloys during ion implantation. [NiTi]

Description: A theoretical model of radiation-induced amorphization of ordered intermetallic compounds is developed. The mechanism is proposed to be the buildup of lattice defects to very high concentrations, which destabilizes the crystalline structure. Because simple point defects do not normally reach such levels during irradiation, a new defect complex containing a vacancy and an interstitial is hypothesized. Crucial properties of the complex are that the interstitial sees a local chemical environment similar to that of an atom in the ordered lattice, that the formation of the complex prevents mutual recombination and that the complex is immobile. The evolution of a disorder based on complexes is not accompanied by like point defect aggregation. The latter leads to the development of a sink microstructure in alloys that do not become amorphous. For electron irradiation, the complexes form by diffusional encounters. For ion irradiation, complexes are also formed directly in cascades. The possibility of direct amorphization in cascades is also included. Calculations for the compound NiTi show reasonable agreement with measured amorphization kinetics.
Date: January 1, 1985
Creator: Pedraza, D.F. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Inclusion of mobile helium in a rate theory model of void swelling

Description: The effect of mobile helium on swelling has been studied by using a rate theory model of void growth. The partitioning of mobile helium to various internal sinks and the trapping of interstitial helium atoms by vacancies were included in the model. Helium gas generated during neutron irradiation increases swelling at temperatures above the peak void swelling temperature. Two temperature regimes of enhanced swelling are related to gas-assisted void growth and gas-driven bubble growth. Swelling due to void and bubble growth in a fusion reactor first wall is predicted using nickel as an example, and the effects of dislocation density and grain size on swelling are discussed. It is found that, as compared to simultaneous helium injection during heavy-ion bombardment, the method of helium preinjection is inadequate in simulating a fusion reactor condition.
Date: January 1, 1979
Creator: Yoo, M.H. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Mechanism of swelling suppression in phosphorous-modified Fe-Ni-Cr alloys

Description: Five simple alloys were ion irradiated at 948/sup 0/K in an experiment designed to investigate the mechanism of swelling suppression associated with phosphorous additions. One of the alloys was the simple ternary Fe-15Ni-13Cr, another had 0.05% P added and the other three had further additions of the phosphide precipitate-forming elements Ti and/or Si. Ion irradiations were carried out with heavy ions only (Ni or Fe) or with heavy ions followed by dual heavy ions and helium. The ternary with and without P swelled readily early in dose with or without helium. The other three alloys only showed swelling in the presence of helium and exhibited a long delay in dose prior to the onset of swelling. These displayed fine distributions of Fe/sub 2/P type phosphide precipitates enhanced by irradiation. The phosphide particles gave rise to very high concentrations of stable helium filled cavities at the precipitate matrix interfaces. The results were analyzed in terms of the theory of cavity swelling. The accumulation of the critical number of gas atoms in an individual cavity is required in the theory for point defect driven swelling to begin. It is concluded that the primary mechanism leading to swelling suppression is therefore the dilution of injected helium over a very large number of cavities. It is suggested that this mechanism may offer a key for alloy design for swelling resistance in high helium environments.
Date: January 1, 1986
Creator: Lee, E.H. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Reduction of irradiation-induced creep by point defect trapping

Description: The theory of point defect trapping was applied to predict the effect of solutes on the steady-state dislocation climb creep rate. Vacancy loops generated in the cascades of point defect production by heavy particles were included. It is shown that the effect of vacancy loops on the creep rate is mathematically equivalent to a simultaneous reduction in the vacancy trapping rate at solutes as well as in the free point defect generation rate and recombination coefficient. These parameters are reduced by the ratio of the sink strength without vacancy loops to the total sink strength. Solute trapping and vacancy loops do not reduce the unperturbed creep rates by more than an order of magnitude for the parametric range considered.
Date: July 10, 1978
Creator: Mansur, L.K. & Wolfer, W.G.
Partner: UNT Libraries Government Documents Department

Experimental determination of the critical cavity radius in Fe-10% Cr for ion irradiation

Description: An ion bombardment experiment was designed to investigate the minimum critical radius, r/sub c/sup c*/, for the ferritic alloy Fe-10% Cr. Specimens were implanted with 300 appM Helium, annealed, and then irradiated to 30 dpa at 850K with 4-MeV Fe/sup 2 +/ ions. The specimens contained a bimodal cavity distribution consisting of a population of larger cavities (average radius 7.6 nm) and a population of smaller cavities (average radius 1.2 nm). The upper cut-off of the cavity radii for the smaller cavities, 2.5 nm, is interpreted as r/sub c/sup c*/. Theoretical calculations of r/sub c/sup c*/ for physically allowable combinations of bias, surface energy, vacancy migration energy, and vacancy formation energy and entropy were performed using the measured minimum critical radius and microstructural data. Thus, an indirect determination of these fundamental parameters was made for this alloy. The results suggest that a bias of approx. 0.2 is reasonable. An assessment of the possible values of the other fundamental parameters is given. In addition, the low irradiation-induced dislocation density (1 x 10/sup 13/m/sup -2/), which results in the ratio of the dislocation and cavity sink strengths, Q, being much less than unity, may be partially responsible for the low cavity growth rate in this alloy.
Date: January 1, 1984
Creator: Horton, L.L. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Critical radius and critical number of gas atoms for cavities containing a Van der Waals gas

Description: The effect of gas on void nucleation and growth is particularly important for structural materials in fusion reactors because of the high production of helium by neutron-induced transmutation reactions. Gas reduces the critical radius for bias driven growth and there is a critical number of gas atoms, n/sub g/*, at which the critical radius is reduced essentially to zero. The significance of this is that the time interval to the accumulation of n/sub g/* gas atoms may determine the time to the onset of bias driven swelling where n/sub g/* is large. In previous papers these critical quantities were given for an ideal gas. Recently, we presented the results for a Van der Waals gas. Here the derivation of these relations is presented and further results of calculations are given. At low temperatures (high pressures) the results depart from those of the ideal gas, with the critical number affected more strongly than the critical radius. Comparisons are made with earlier calculations.
Date: January 1, 1983
Creator: Coghlan, W.A. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Characteristics of irradiation creep in the first wall of a fusion reactor

Description: A number of significant differences in the irradiation environment of a fusion reactor are expected with respect to the fission reactor irradiation environment. These differences are expected to affect the characteristics of irradiation creep in the fusion reactor. Special conditions of importance are identified as the (1) large number of defects produced per pka, (2) high helium production rate, (3) cyclic operation, (4) unique stress histories, and (5) low temperature operations. Existing experimental data from the fission reactor environment is analyzed to shed light on irradiation creep under fusion conditions. Theoretical considerations are used to deduce additional characteristics of irradiation creep in the fusion reactor environment for which no experimental data are available.
Date: January 1, 1981
Creator: Coghlan, W.A. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Theoretical relationships between creep and swelling by point defect absorption during irradiation

Description: Relationships between irradiation creep and swelling implicit in the theories of these processes are derived. Four mechanisms of irradiation creep are treated. These are the climb-only process of preferred point defect absorption on dislocations; the climb and glide processes resulting from cumulative absorption of defects at dislocations, i.e., preferred absorption glide and swelling-driven creep; and the recently developed climb and glide process enabled by point defect concentration fluctuations resulting from cascades. The results are expressed both as differential equations for creep rate in terms of swelling rate and as integrated forms giving creep strain in terms of swelling for stabilized microstructures.
Date: November 1, 1981
Creator: Mansur, L.K. & Coghlan, W.A.
Partner: UNT Libraries Government Documents Department

Effects of pulsed dual-ion irradiation on phase transformations and microstructure in Ti-modified austenitic alloy

Description: The influence of pulsed 4 MeV Ni ion bombardment, with and without simultaneous helium injection, has been explored in a low swelling, Ti-modified austenitic stainless steel. Irradiations were carried out to 70 dpa at 950/sup 0/K; the pulsing frequencies were either 60 s on/off or 1 s on/off. Compared to continuous irradiation, pulsing caused a decrease in the interstitial loop diameter at 1 dpa, although at higher doses the overall dislocation density was not affected. Pulsing and helium both promoted the stability of MC precipitates and retarded the subsequent G phase formation; in some cases G-phase was suppressed and eta phase formed instead. Small bubble-like cavities were observed to grow into large voids after steady dual beam irradiation to 70 dpa. However, this conversion was suppressed by pulse irradiation to 70 dpa and furthermore the sizes of the small cavities were somewhat reduced. The results are explained in terms of current mechanistic understanding of mean point defect kinetics and the evolution of microstructure and microcomposition during irradiation with superimposed annealing periods.
Date: January 5, 1983
Creator: Lee, E.H.; Packan, N.H. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Swelling with inhomogeneous point defect production: a cascade diffusion theory

Description: A theoretical method is described for evaluating the effects of spatially and temporally discrete production in collision cascades on point defect concentrations and swelling in materials during irradiation. The concentrations of vacancies and interstitials at a point which result from their diffusion from all cascades in the material are calculated. Large fluctuations occur with time in the vacancy concentration. The interstitial concentration is nearly always zero except for extremely large spikes of very short duration, corresponding to the occurrence of a cascade anywhere within the sphere beyond which all generated defects are absorbed by sinks before reaching the reference point. The growth rate of a void in this cascade diffusion theory is compared to that given by the more approximate rate theory. The difference is small but increases rapidly at high temperature. Implications of this work for void nucleation, irradiation creep, and analysis of pulsed irradiations are mentioned.
Date: January 1, 1979
Creator: Mansur, L.K.; Coghlan, W.A. & Brailsford, A.D.
Partner: UNT Libraries Government Documents Department

Proceedings of the international workshop on spallation materials technology

Description: This document contains papers which were presented at the International Workshop on Spallation Materials Technology. Topics included: overviews and thermal response; operational experience; materials experience; target station and component design; particle transport and damage calculations; neutron sources; and compatibility.
Date: October 1, 1996
Creator: Mansur, L.K. & Ullmaier, H.
Partner: UNT Libraries Government Documents Department

Cascade diffusion theory of displacement-induced point defect concentration and flux fluctuations

Description: A theoretical approach, which has been developed to assess the fluctuations in point defect concentrations and fluxes to sinks that are induced by the production of point defects in spatially and temporally discrete cascades, is summarized. Solutions for cascade dissipation are outlined for the case where cascades occur in a homogeneous lossy medium as well as for the more involved geometries where cascades occur in the presence of a nearby dislocation or cavity. By superposition of solutions representing discrete cascades under conditions of interest, time profiles of point defect concentrations and fluxes are generated. The profiles exhibit extreme fluctuations. Continuum rate theory results arise by applying limits and approximations to cascade diffusion theory. Application of the theory to microstructural processes shows that property changes, especially creep are affected by the fluctuations.
Date: January 1, 1986
Creator: Mansur, L.K.; Brailsford, A.D. & Coghlan, W.A.
Partner: UNT Libraries Government Documents Department

Precipitation and cavity formation in austenitic stainless steels during irradiation

Description: Microstructural evolution in austenitic stainless steels subjected to displacement damage at high temperature is strongly influenced by the interactions between helium atoms and second phase particles. Cavity nucleation occurs by the trapping of helium at partially coherent particle-matrix interfaces. The recent precipitate point defect collector theory describes the more rapid growth of precipitate-attached cavities compared to matrix cavities where the precipitate-matrix interface collects point defects to augment the normal point deflect flux to the cavitry. Data are presented which support these ideas. It is shown that during nickel ion irradiation of a titanium-modified stainless steel at 675/sup 0/C the rate of injection of helium has a strong effect on the total swelling and also on the nature and distribution of precipitate phases.
Date: January 1, 1981
Creator: Lee, E.H.; Rowcliffe, A.F. & Mansur, L.K.
Partner: UNT Libraries Government Documents Department

Mechanisms affecting swelling in alloys with precipitates

Description: In alloys under irradiation many mechanisms exist that couple phase instability to cavity swelling. These are compounded with the more familiar mechanisms associated with point defect behavior and the evolution of microstructure. The mechanisms may be classified according to three modes of operation. Some affect cavity swelling directly by cavity-precipitate particle association, others operate indirectly by precipitate-induced changes in sinks other than cavities and finally there are mechanisms that are mediated by precipitate-induced changes in the host matrix. The physics of one mechanism of each type is developed in detail and the results compared where possible to experimental measurements. In particular, we develop the theory necessary to treat the effects on swelling of precipitation-induced changes in overall sink density; precipitation-induced changes in point defect trapping by solute depletion and creation of precipitate particle-matrix interfacial trap sites.
Date: January 1, 1980
Creator: Mansur, L.K.; Haynes, M.R. & Lee, E.H.
Partner: UNT Libraries Government Documents Department

Spatial variation in void volume during charged particle bombardment: the effects of injected interstitials

Description: Experimental observations of the void volume at several depths along the range of 4 MeV Ni ions in 316 stainless steel are reported. The specimens were first preconditioned by neutron irradiation at temperatures of 450 and 584/sup 0/C to fluences of approximately 8 x 10/sup 26/ n/m/sup -2/. The void volume after ion bombardment to 60 dpa at the peak damage depth is significantly lower at the peak damage depth than in the region between that and the free surface. The ratio of the step height to void volume at the depth of peak energy deposition between regions masked from and exposed to the beam is strongly dependent on bombardment temperature. The reduction of void volume near the peak damage depth is larger for the 584/sup 0/C than for the 450/sup 0/C preconditioned material. These observations are consistent with recent theoretical results which account for the injection of the bombarding ions as self-interstitials. The theory necessary to understand the effect is developed.
Date: January 1, 1979
Creator: Lee, E.H.; Mansur, L.K. & Yoo, M.H.
Partner: UNT Libraries Government Documents Department