K.L. Murty

Compressive mechanical behavior of nanocrystalline Fe investigated with an automated ball indentation technique

Abstract Nanocrystalline (nc) iron was produced by mechanical attrition and compacted into near fully dense samples. Isothermal annealing at 800 K resulted in grain sizes between 15 and 24 nm. A newly available Automated Ball Indentation system was used to study the compressive mechanical properties of the samples. The ABI method proved useful in examining the mechanical properties of nc iron on a more quantitative level than previously possible by conventional hardness testing methods. Stress–strain curves were obtained which indicated a compressive behavior similar to that of perfectly plastic materials: low strain hardening at high flow stresses around 3 GPa and a low room-temperature strain-rate sensitivity. The flow stresses were independent of the grain size in the range of the present study. The deformation pile-up around the indentations seems to have formed inhomogeneously, exhibiting intense plastic deformation in localized shear bands.

Ball indentation studies on the effect of aging on mechanical behavior of alloy 625

Abstract The effect of aging on mechanical behavior of Alloy 625 was investigated using the non-destructive Stress–Strain Microprobe (SSM) system. SSM is based on an automated ball indentation (ABI) technique, and involves multiple indentations by a small spherical indenter at a single penetration location under strain-controlled conditions. The technique permits evaluation of mechanical properties such as yield strength, ultimate tensile strength, strength coefficient and strain hardening exponent. Alloy 625 was aged at six different temperatures in the range of 873–1173 K for 500 h each. ABI tests were carried out at room temperature and at 473 K. The variation of yield and ultimate tensile strengths with aging temperature exhibited a peak in strength following aging at 973 K. The peak stress was 1.5 times the strength of the unaged material, and the strength after aging at 1173 K was nearly equal to that of the unaged material. The peak in strength is attributed to the precipitation of the γ” phase. The fall in strength due to aging above 973 K is attributed to the precipitation, growth and dissolution of δ -phase precipitates. These studies demonstrate that ABI can be used as a non-destructive technique to determine changes in mechanical properties of nickel base alloy components due to aging.

Low temperature aging embrittlement of CF-8 stainless steel

Abstract Cast stainless steels such as CF-8 are used for several components such as pump and valve casings, elbows, fittings and primary coolant pipes in nuclear power plants. CF-8 stainless steel has a duplex microstructure consisting of austenite and about 15–20% δ-ferrite. Long-term exposure at the reactor operating temperatures results in aging-induced embrittlement of the material due to spinodal decomposition of the ferrite phase. In this paper, we report the effects of low temperature aging (673 K) up to 18 months on the mechanical and fracture properties of cast CF-8 stainless steel in the range of 173–423 K. These studies have been carried out using non-destructive ball indentation tests. Standard Charpy and tensile tests were also carried out to complement the ball indentation data. Increase in strength and decrease in toughness have been observed following aging. These changes in the properties tend to saturate on aging for more than 12 months.

Impurity-Defect Interactions and Radiation Hardening and Embrittlement in BCC Metals

The interaction of interstitial impurity atoms (IIA’s) with radiation-produced defects in bcc metals and its influence on radiation hardening and embrittlement are reviewed. Special emphasis is placed on the role of oxygen in vanadium and niobium and of nitrogen and carbon in iron and steel. Upon postirradiation annealing at temperatures where the IIA’s are mobile (about 50 °C to 250 °C), resistivity stages and decreases in internal friction (Snoek damping) are observed. Evidence is examined that leads to the conclusion that IIA’s are trapped at radiation-produced defects upon postirradiation annealing,which removes the IIA’s from solid solution. The consequences of this IIA trapping on mechanical properties are summarized, particularly in terms of the phenomena of radiation anneal hardening, static strain aging, and dynamic strain aging (DSA). Static and dynamic strain aging are shown to be retarded or suppressed in irradiated metals. Recent investigations, which demonstrate that the suppression of DSA in steel can lead to an increase in ductility upon irradiation, are described.

A study on the decrease of fracture resistance curve under reversed cyclic loading

Abstract Fracture resistance ( J – R ) curves, which are used for elastic–plastic fracture mechanics analyses, decrease under reversed cyclic loading condition. Several former researchers reported this phenomenon, but the causes have not been clear. The objective of this paper has been to investigate the causes of this phenomenon. Here, we attempt to provide an explanation for the decrease in J – R curve based on an increase in strain-hardening, the corresponding stress redistribution during cyclic loading and crack opening phenomenon. Hardness tests, automated ball indentation tests, and theoretical stress field analyses were performed on SA516 steel to prove the hypotheses. The results revealed that the decrease in the J – R curve is related to the increased strain-hardening and generation of tensile residual stress at the crack tip during cyclic loading.

Early Damage Mechanisms in Nuclear Grade Graphite under Irradiation

Using Raman and X-ray photoelectron spectroscopy, we delineate the bond and defect structures in nuclear block graphite (NBG-18) under neutron and ion irradiation. The strengthening of the defect (D) peak in the Raman spectra under irradiation is attributed to an increase in the topological, sp2-hybridized defects. Using transmission electron microscopy, we provide evidence for prismatic dislocations as well as a number of basal dislocations dissociating into Shockley partials. The non-vanishing D peak in the Raman spectra, together with a generous number of dislocations, even at low irradiation doses, indicates a dislocation-mediated amorphization process in graphite.

Effects of grain-shape anisotropy and texture on balanced-biaxial creep of Ti and Zr alloys

Biaxial creep behaviors of stress-relieved and recrystallized thin-walled tubing of Ti-3Al-2.5V and Zircaloy-4 are considered under equal hoop and axial stresses by internal pressurization superimposed with axial load. Both hoop and axial strains were monitored and the ratio of the strain rate along the hoop to that along the axial directions is considered to represent the degree of anisotropy. In both of these alloys, relatively weak hoop direction in cold-worked stress-relieved (CWSR) materials became slightly stronger following recrystallization. Crystallographic texture was considered in terms of x-ray pole figures from which the crystallite orientation distribution functions (CODFs) were derived and crystal plasticity model with slip dominant on prism planes was combined with the CODFs to predict the creep anisotropy. While good correlation was noted for recrystallized materials, distinct deviations are observed for CWSR that are believed to arise from grain shape anisotropy. The relatively small (equiaxed) grain size along the hoop and radial directions results in grain boundary sliding leading to stress enhancements along these directions. This anisotropy in grain boundary sliding is shown to explain the observed deviations.

Non-Destructive Evaluation of Deformation and Fracture Properties of Materials using Stress-strain Microprobe

Tensile deformation and fracture properties of several metallic materials, welds, and their heataffected-zones were determined non-destructively using the Stress-Strain Microprobe (SSM) system. The system is based on automated ball indentation (ABI) technique and involves straincontrolled multiple indentations at the same location on the material surface by a small spherical indenter. The technique permits evaluation of tensile deformation parameters such as yield strength, ultimate tensile strength, strength coefficient, and strain-hardening exponent, and a fracture energy parameter called indentation energy to fracture. ABI tests were conducted on carbon steels, stainless steels, nickel alloys, aluminum alloys, Zircaloys, electronic soldering materials and several nuclear pressure vessel steels (in the unirradiated, neutron irradiated, and irradiated and thermally annealed conditions). For all these test materials and conditions, the ABI-derived results were found to agree with the data from conventional standard test methods. In addition to the laboratory applications of SSM, it can be used as an in-situ testing instrument for non-destructive assessment of deformation and fracture properties of operating structural components.

Creep deformation of materials in light water reactors (LWRs)

Abstract: The time-dependent deformation of materials or creep governs the useful life of many engineering structures. It assumes even higher significance in the case of structures constituting a nuclear reactor, wherein materials bombarded with neutrons develop defects that assist faster diffusion leading to greater plastic deformation. As a result, an understanding of the creep deformation process and factors controlling it is necessary for gauging the usefulness of materials in a nuclear reactor as well as for predicting life-times of various structures. Thus in this work we discuss the various mechanisms of creep, the rate controlling factors, deformation mechanism maps and useful life prediction methodologies. We also identify a few cases where direct application of simple creep correlations might not be feasible. Finally, we discuss the various factors that control the creep behavior of materials in light water reactors.

A Comparative Study of Welded ODS Cladding materials for AFCI/GNEP Applications

This research project involved working on the pressure resistance welding of oxide dispersion strengthened (ODS) alloys which will have a large role to play in advanced nuclear reactors. The project also demonstrated the research collaboration between four universities and one nation laboratory (Idaho National Laboratory) with participation from an industry for developing for ODS alloys. These alloys contain a high number density of very fine oxide particles that can impart high temperature strength and radiation damage resistance suitable for in-core applications in advanced reactors. The conventional fusion welding techniques tend to produce porosity-laden microstructure in the weld region and lead to the agglomeration and non-uniform distribution of the neededoxide particles. That is why two solid state welding methods - pressure resistance welding (PRW) and friction stir welding (FSW) - were chosen to be evaluated in this project. The proposal is expected to support the development of Advanced Burner Reactors (ABR) under the GNEP program (now incorporated in Fuel Cycle R&D program). The outcomes of the concluded research include training of graduate and undergraduate students and get them interested in nuclear related research.