Contributions from research on irradiated ferritic/martensitic steels to materials science and engineering

Ferritic and martensitic steels are finding increased application for structural components in several reactor systems. Low-alloy steels have long been used for pressure vessels in light water fission reactors. Martensitic stainless steels are finding increasing usage in liquid metal fast breeder reactors and are being considered for fusion reactor applications when such systems become commercially viable. Recent efforts have evaluated the applicability of oxide dispersion-strengthened ferritic steels. Experiments on the effect of irradiation on these steels provide several examples where contributions are being made to materials science and engineering. Examples are given demonstrating improvements in basic understanding, small specimen test procedure development, and alloy development. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Review of small specimen test techniques for irradiation testing

Small specimen test technology has evolved out of the necessity to develop and monitor materials proposed for or used in nuclear power generation systems. Development of materials for improved cladding and in-core structures for fission reactors and assessment of core materials and pressure vessel steels already under irradiation necessitated the use of specimens which fit into existing irradiation space or which could be extracted from irradiated structures, such as cladding or ducts. Interest in simulating neutron irradiation by light and heavy ion irradiation led to the development of thin foil and wire geometry specimens. Further, interest in developing materials for fusion reactors has added additional constraints on specimen sizes associated with available irradiation volumes in existing and proposed high-energy neutron irradiation facilities. Consequently, a wide array of specimen geometries and test techniques has now been developed. It is the purpose of this paper to review these techniques and examine their status, problems, and potential for future applications. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Review of small specimen test techniques for irradiation testing

Small specimen test technology has evolved out of the necessity to develop and monitor materials proposed for or used in nuclear power generation systems. Development of materials for improved cladding and in-core structures for fission reactors and assessment of core materials and pressure vessel steels already under irradiation necessitated the use of specimens which fit into existing irradiation space or which could be extracted from irradiated structures, such as cladding or ducts. Interest in simulating neutron irradiation by light and heavy ion irradiation led to the development of thin foil and wire geometry specimens. Further, interest in developing materials for fusion reactors has added additional constraints on specimen sizes associated with available irradiation volumes in existing and proposed high-energy neutron irradiation facilities. Consequently, a wide array of specimen geometries and test techniques has now been developed. It is the purpose of this paper to review these techniques and examine their status, problems, and potential for future applications. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Ferritic/martensitic steels for advanced nuclear reactors

Design concepts for the next generation of nuclear power reactors include water-cooled, gas-cooled, and liquid-metal-cooled reactors. Reactor conditions for several designs offer challenges for engineers and designers concerning which structural and cladding materials to use. Depending on operating conditions, some designs favor elevated-temperature ferritic/martensitic steels for in-core and out-of core applications. Such steels have been investigated in previous work on international fast reactor and fusion reactor research programs. Steels from these fission and fusion programs will provide reference materials for future fission applications. In addition, new elevated-temperature steels have been developed in recent years for conventional power systems that also need to be considered.     

Fluctuations in composition in dilute alloys under irradiation

Solute segregation in alloys and steels during irradiation has been studied both experimentally and theoretically for a number of years. In this paper, a rigorous theory for the segregation of solute in a dilute binary alloy is described which is based on the kinetic theory of diffusion. This theory is then used to predict irradiation-induced instabilities in the diffusion of the solute which cause spatial oscillations in composition. It is shown that trapping of both vacancies and interstitials at solute atoms can cause such instabilities. The results of numerical calculations are compared with experimental results. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Experimental studies of U-Pu-Zr fast reactor fuel pins in the experimental breeder reactor-ll

Argonne National Laboratory’s Integral Fast Reactor (IFR) concept has been under demonstration in the Experimental Breeder Reactor II (EBR-II) since February 1985. Irradiation tests of U-Zr and U-Pu-Zr fuel pins to >15 at. pct burnup have demonstrated their viability as driver fuel prototypes in innovative design liquid metal reactors. A number of technically challenging irradiation effects have been observed and are now under study. Microstructural changes in the fuel are dominated early in exposure by grain boundary cavitation and fission gas bubble growth, producing large amounts of swelling. Irradiation creep and swelling of the austenitic (D9) and martensitic (HT-9) candidate cladding alloys have been measured and correlate well with property modeling efforts. Chemical interaction between the fuel and cladding alloys has been characterized to assess the magnitude of cladding wastage during steady-state irradiation. Significant interdiffusion of the uranium and zirconium occurs producing metallurgically distinct zones in the fuel. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Effect of oxygen on vacancy cluster morphology in metals

The extensive literature on oxygen chemisorption and solubility in metals is briefly reviewed, with special emphasis on the reduction of surface tension associated with oxygen adsorption. A thermodynamic model based on the adsorption equations of Gibbs and Langmuir is developed to determine the relative stability in the presence of oxygen of the void compared to the dislocation loop and stacking fault tetrahedron. Representative calculations are performed for copper, nickel, and austenitic stainless steel. Atomistic and elastic continuum calculations predict that void formation should not occur in most pure face-centered cubic metals during quenching or irradiation. However, the thermodynamic model predicts that oxygen concentrations of 30 to 1000 appm will stabilize void formation in copper, nickel, and stainless steel. Foils of copper and several Fe-Cr-Ni stainless steels containing various amounts of oxygen have been examined with electron microscopy following ion bombardment. The presence of 30 to 1000 appm O resulted in significant amounts of void formation, whereas no voids were observed in low-oxygen specimens, in agreement with the model predictions. Oxygen introduced by ion implantation was more effective in promoting void formation than residual oxygen. Solutes such as phosphorus in stainless steel reduced the effectiveness of oxygen as a void-stabilizing agent. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD     

Effect of oxygen on vacancy cluster morphology in metals

The extensive literature on oxygen chemisorption and solubility in metals is briefly reviewed, with special emphasis on the reduction of surface tension associated with oxygen adsorption. A thermodynamic model based on the adsorption equations of Gibbs and Langmuir is developed to determine the relative stability in the presence of oxygen of the void compared to the dislocation loop and stacking fault tetrahedron. Representative calculations are performed for copper, nickel, and austenitic stainless steel. Atomistic and elastic continuum calculations predict that void formation should not occur in most pure face-centered cubic metals during quenching or irradiation. However, the thermodynamic model predicts that oxygen concentrations of 30 to 1000 appm will stabilize void formation in copper, nickel, and stainless steel. Foils of copper and several Fe-Cr-Ni stainless steels containing various amounts of oxygen have been examined with electron microscopy following ion bombardment. The presence of 30 to 1000 appm O resulted in significant amounts of void formation, whereas no voids were observed in low-oxygen specimens, in agreement with the model predictions. Oxygen introduced by ion implantation was more effective in promoting void formation than residual oxygen. Solutes such as phosphorus in stainless steel reduced the effectiveness of oxygen as a void-stabilizing agent. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD     

Analytical electron microscopy studies of radiation damage

Analytical electron microscopy (AEM) has provided structural, crystallographic, and compositional characterization to aid in the understanding of radiation damage processes, especially in multiphase materials. The range of AEM techniques is based on the use of as many of the signals produced by the interaction of an electron beam with a specimen as possible. This paper briefly discusses the origins, capabilities, and current developments of AEM, including the spatial resolution of the various techniques. Several important applications of AEM in radiation damage studies, including radiation-induced segregation and phase instability in austenitic stainless steels, will be reviewed. From the comparison of phase equilibria under irradiation to that under thermal aging, principles for alloy development in non-nuclear applications will be discussed. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Structure-property relationships in bainitic steels

Bainitic microstructures can be produced in a variety of steels either as a result of a deliberate attempt to achieve a particular combination of strength and toughness or in response to welding during fabrication. In addition, such microstructures can offer advantages in terms of their resistance to creep or fatigue deformation or susceptibility to hydrogen embrittlement. The relationships among chemical composition, processing, microstructure, and the mechanical properties will be reviewed. Particular emphasis will be placed on recent advances in alloy design. These developments rely on an improved understanding of the mechanisms of bainitic transformation, and the relevance of recent research in this area to the design of new alloy systems will be discussed. Bainitic structures which arise during welding can have a significant and sometimes detrimental effect on the fracture toughness of the welded joint. The fracture toughness of bainitic microstructures in so-called “local brittle zones” in the heat-affected zone and in weld metals and the importance of controlling the bainitic morphology will be considered and the transformation mechanisms discussed. In summary, the aim of this review will be to indicate the prospects for improved microstructural control of structure-property relationships in steels containing a significant proportion of bainite. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Ion beam characterization and treatment

Almost every process that occurs when an ion beam hits a solid has been pressed into service as a technique in materials science. Some ions bounce off: backscattering is a routine technique for near-surface analysis in many laboratories. Atoms of the sample are knocked out: sputter profiling is used as an adjunct to many surface science measurements, and analysis of the ejected atoms yields information about the composition of the sample. Ions slow down in a solid, depositing energy in the sample and causing radiation damage. This makes ion beams useful in the development of radiation-resistant materials and has to be understood in order to apply other ion beam techniques. Finally, the ion stops and becomes incorporated into the sample, which is known as “implantation.” As well as being a vital industrial technique in the manufacture of semiconductor devices, implantation can be used in materials science wherever it is useful to change the composition of a near-surface layer. The implanted species doesn’t have to be soluble in the sample. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25—29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Positron spectroscopy for materials characterization

One of the more active areas of research on materials involves the observation and characterization of defects. The discovery of positron localization in vacancy-type defects in solids in the 1960’s initiated a vast number of experimental and theoretical investigations which continue to this day. Traditional positron annihilation spectroscopic techniques, including lifetime studies, angular correlation, and Doppler broadening of annihilation radiation, are still being applied to new problems in the bulk properties of simple metals and their alloys. In addition, new techniques based on tunable sources of monoenergetic positron beams have, in the last five years, expanded the horizons to studies of surfaces, thin films, and interfaces. In the present paper, we briefly review these experimental techniques, illustrating them with some of the important accomplishments in the field. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Positron spectroscopy for materials characterization

One of the more active areas of research on materials involves the observation and characterization of defects. The discovery of positron localization in vacancy-type defects in solids in the 1960’s initiated a vast number of experimental and theoretical investigations which continue to this day. Traditional positron annihilation spectroscopic techniques, including lifetime studies, angular correlation, and Doppler broadening of annihilation radiation, are still being applied to new problems in the bulk properties of simple metals and their alloys. In addition, new techniques based on tunable sources of monoenergetic positron beams have, in the last five years, expanded the horizons to studies of surfaces, thin films, and interfaces. In the present paper, we briefly review these experimental techniques, illustrating them with some of the important accomplishments in the field. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Unified theoretical analysis of experimental swelling data for irradiated austenitic and ferritic/martensitic alloys

Central concepts in the theory of swelling based on defect reactions are reviewed. The critical radius and critical number of gas atoms, which determine the initiation of swelling, and the ratio of the dislocation and cavity sink strengths, which dictates the swelling rate, are demonstrated to be of great utility in the understanding and control of swelling. Over the past two decades, a large data base has been accumulated covering austenitic and ferritic/martensitic alloys, the leading candidate materials for both fusion and fast fission reactor applications. This collection of data naturally serves as the largest source of information on which to develop and test mechanistic understanding of swelling. Over wide ranges in materials and irradiation parameters, including composition, temperature, damage rate, and helium generation rate, we find that apparently divergent swelling behaviors can be explained in a unified manner within the present theoretical framework. Principles for microstructures that insure swelling resistance, together with results from the necessary confirmatory experiments, are described. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Mechanical property changes in ion-irradiated metals: Part II. high-strength Cu-Ni-be alloy

Mechanical property changes in a high-strength copper alloy as a result of 14-MeV Cu ion irradiation have been investigated using a recently developed mechanical properties microprobe (MPM). A Cu-1.5 pct Ni-0.3 pct Be alloy was irradiated in both the cold-worked and aged and solution-annealed and aged conditions to a peak damage dose of 40 displacements per atom (dpa) (10 dpa at 1 μm) over the temperature range of 100 °C to 500 °C. Ultra-low load microindentation hardness changes were measured parallel to the ion beam and perpendicular to the beam, the latter being made possible by cross-sectional techniques. Both thermal and radiationenhanced softening was observed in the cold-worked and aged material, and the amount of softening increased as temperature increased. Irradiation had very little effect on the solutionannealed and aged material, and only at 500 °C was any thermally induced softening observed. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25—29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Unified theoretical analysis of experimental swelling data for irradiated austenitic and ferritic/martensitic alloys

Central concepts in the theory of swelling based on defect reactions are reviewed. The critical radius and critical number of gas atoms, which determine the initiation of swelling, and the ratio of the dislocation and cavity sink strengths, which dictates the swelling rate, are demonstrated to be of great utility in the understanding and control of swelling. Over the past two decades, a large data base has been accumulated covering austenitic and ferritic/martensitic alloys, the leading candidate materials for both fusion and fast fission reactor applications. This collection of data naturally serves as the largest source of information on which to develop and test mechanistic understanding of swelling. Over wide ranges in materials and irradiation parameters, including composition, temperature, damage rate, and helium generation rate, we find that apparently divergent swelling behaviors can be explained in a unified manner within the present theoretical framework. Principles for microstructures that insure swelling resistance, together with results from the necessary confirmatory experiments, are described. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Influence of gravity level and interfacial energies on dispersion-forming tendencies in hypermonotectic Cu-Pb-Al alloys

Nondirectional solidification experiments involving several hypermonotectic Cu-Pb-Al alloys were carried out aboard NASA's KC-135 zero-g aircraft in order to determine the influence of interfacial energies and gravity levels on dispersion-forming tendencies. For Cu-Pb-Al alloys, changes in Al content are thought to result in variations in the interfacial energy between the two liquid phases. It has been postulated that the interfacial energy between the two liquid phases may have a strong influence on the ability to form well-dispersed structures in these systems. In order to study the influence of interfacial energies, the Al content was systematically varied in the alloys. To eliminate gravity driven sedimentation of the more dense immiscible liquid phase during solidification, experimentation was carried out aboard NASA's KC-135 zero-g aircraft. The resulting structures have been analyzed and the dispersion-forming ability related to the gravity level during solidification, the interfacial energy between the immiscible phases, and the tendency for the minority immiscible phase to wet the walls of the crucible. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

The distribution of substitutional alloying elements during the bainite transformation

The behavior of substitutional alloying elements during and after the growth of upper bainite in Fe-Mn-Si-C and Fe-Mn-Si-C-Mo alloy steels has been examined using an atomic resolution microanalysis technique. From the results obtained, and judging from published data, it is concluded that manganese, nickel, silicon, chromium, and molybdenum do not redistribute during the growth of bainitic ferrite. Their concentrations are found to be uniform both at and in the vicinity of the transformation interface, with no indications of any segregation to the transformation interface during growth. However, prolonged annealing at the isothermal transformation temperature, after the formation of bainite has stopped, eventually stimulates the partitioning of substitutional alloying elements as the system tends toward equilibrium. The results demonstrate the existence of an atomic correspondence between the parent and product phases during transformation, the effect of substitutional alloying additions being manifestedvia a modification of the driving force for transformation. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

The bainite transformation in chemically heterogeneous 300M high-strength steel

A theoretical model of the bainite reaction in chemically heterogeneous steels is compared with experimental data obtained using commercially available 300M ultrahigh-strength steel. It is demonstrated that the development of microstructure is influenced strongly by variations in substitutional solute concentrations. The maximum degree of transformation to bainite at any given transformation temperature was, in general, higher for homogenized samples than for the steel in the as-received, segregated condition. The results are analyzed using a computer model in which the heterogeneous steel is represented in terms of a series of slices, each of a different, but uniform, composition. The transformation was allowed to proceed in each slice, both in circumstances where the slices did not interact and when carbon was allowed to redistribute between the slices. The model is found to be a good quantitative representation of the nature of the bainite reaction in heterogeneous 300M steel. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Shear Localization in Dynamic Deformation: Microstructural Evolution

Investigations made by the authors and collaborators into the microstructural aspects of adiabatic shear localization are critically reviewed. The materials analyzed are low-carbon steels, 304 stainless steel, monocrystalline Fe-Ni-Cr, Ti and its alloys, Al-Li alloys, Zircaloy, copper, and Al/SiC_p composites. The principal findings are the following: (a) there is a strain-rate-dependent critical strain for the development of shear bands; (b) deformed bands and white-etching bands correspond to different stages of deformation; (c) different slip activities occur in different stages of band development; (d) grain refinement and amorphization occur in shear bands; (e) loss of stress-carrying capability is more closely associated with microdefects rather than with localization of strain; (f) both crystalline rotation and slip play important roles; and (g) band development and band structures are material dependent. Additionally, avenues for new research directions are suggested. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.