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NANOCRYSTALLINE GROWTH AND GRAIN-SIZE EFFECTS IN AU-CU ELECTRODEPOSITS

Description: The processing-structure-property relationship is investigated for electrodeposited foils of the gold-copper alloy system. A model is presented that relates the deposition process parameters to the nanocrystalline grain size. An activation energy of 1.52 eV {center_dot} atom{sup -1} for growth is determined for a long pulse (>10 msec) mode, and is 0.16 eV {center_dot} atom{sup -1} for short pulses (<5 msec). The affect of nanocrystalline grain size on the mechanical properties is assessed using indentation measurements. A Hall-Petch type variation of the Vickers microhardness with nanocrystalline grain size (>6 nm) is observed for Au-Cu samples with 1-12 wt.% Cu as tested in cross-section. The hardness increases three-fold from a rule-of-mixtures value <1 GPa to a maximum of 2.9 GPa.
Date: February 25, 2005
Creator: Jankowski, A F; Saw, C K; Harper, J F; Vallier, R F; Ferreira, J L & Hayes, J P
Partner: UNT Libraries Government Documents Department

Baseline Microstructural Characterization of Outer 3013 Containers

Description: Three DOE Standard 3013 outer storage containers were examined to characterize the microstructure of the type 316L stainless steel material of construction. Two of the containers were closure-welded yielding production-quality outer 3013 containers; the third examined container was not closed. Optical metallography and Knoop microhardness measurements were performed to establish a baseline characterization that will support future destructive examinations of 3013 outer containers in the storage inventory. Metallography revealed the microstructural features typical of this austenitic stainless steel as it is formed and welded. The grains were equiaxed with evident annealing twins. Flow lines were prominent in the forming directions of the cylindrical body and flat lids and bottom caps. No adverse indications were seen. Microhardness values, although widely varying, were consistent with annealed austenitic stainless steel. The data gathered as part of this characterization will be used as a baseline for the destructive examination of 3013 containers removed from the storage inventory.
Date: July 31, 2005
Creator: Zapp, Phillip E. & Dunn, Kerry A
Partner: UNT Libraries Government Documents Department

Spinodal Decomposition and Ordering Transformation in U-6 wt% Nb

Description: Phase stability and aging mechanisms in a water-quenched (WQ) U-6wt% Nb (WQ-U6Nb) alloy artificially aged at 200 C (16 hours) and naturally aged at room temperature for 15 years have been investigated. Age hardening/softening phenomenon is recorded from the artificially aged samples by microhardness measurement. The age hardening can be readily rationalized by the occurrence of spinodal decomposition (or fine-scaled Nb segregation), which results in the formation of a modulated structure in the aged samples. Prolonged aging leads to age softening of the alloy by coarsening of the modulated structure. Disorder-order or chemical ordering transformation is found within the naturally aged alloy according to TEM observations of antiphase domain boundaries (APBs) and superlattice diffraction patterns. The formation of a partially ordered phase in the naturally aged alloy is proposed and identified.
Date: August 15, 2005
Creator: Hsiung, L M
Partner: UNT Libraries Government Documents Department

Low-energy deposition of high-strength Al(0) alloys from an ECR plasma

Description: Low-energy deposition of Al(O) alloys from an electron cyclotron resonance (ECR) plasma offers a scaleable method for the synthesis of thick, high-strength Al layers. This work compares alloy layers formed by an ECR-0{sub 2} plasma in conjunction with Al evaporation to 0-implanted Al (ion energies 25-200 keV); and it examines the effects of volume fraction of A1{sub 2}0{sub 3} phase and deposition temperature on the yield stress of the material. TEM showed the Al(O) alloys contain a dense dispersion of small {gamma}-Al{sub 2}0{sub 3} precipitates ({approximately}l nm) in a fine-grain (10-100 nm) fcc Al matrix when deposited at a temperature of {approximately}100C, similar to the microstructure for gigapascal-strength 0-implanted Al. Nanoindentation gave hardnesses for ECR films from 1.1 to 3.2 GPa, and finite-element modeling gave yield stresses up to 1.3 {plus_minus} 0.2 GPa with an elastic modulus of 66 GPa {plus_minus} 6 GPa (similar to pure bulk Al). The yield stress of a polycrystalline pure Al layer was only 0.19 {plus_minus} 0.02 GPa, which was increased to 0.87 {plus_minus} 0.15 GPa by implantation with 5 at. % 0.
Date: December 31, 1995
Creator: Barbour, J.C.; Follstaedt, D.M.; Knapp, J.A.; Myers, S.M.; Marshall, D.A. & Lad, R.J.
Partner: UNT Libraries Government Documents Department

Study of Fatigue and Fracture Behavior of Cr-Based Alloys and Intermetallic Materials

Description: The microhardness, and tensile and fracture-toughness properties of drop-cast and directionally-solidified Cr-9.25 at.% (atomic percent) Ta alloys have been investigated. Directional solidification was found to soften the alloy, which could be related to the development of equilibrium and aligned microstructures. It was observed that the tensile properties of the Cr-Ta alloys at room and elevated temperatures could be improved by obtaining aligned microstructures. The directionally-solidified alloy also showed increased fracture toughness at room temperature. This trend is mainly associated with crack deflection and the formation of shear ribs in the samples with aligned microstructures. The sample with better-aligned lamellar exhibits greater fracture toughness.
Date: January 31, 2001
Creator: He, YH
Partner: UNT Libraries Government Documents Department

Microhardness and elastic modulus of nanocrystalline Al-Zr

Description: An investigation of the mechanical properties of nanocrystalline Al-Zr alloy composites has been conducted via nanoindentation and Vickers microhardness experiments. The microhardness of the samples exhibits a four-fold increase over the concentration range of 0-30 wt.% Zr, from {approximately}0.7 GPa to nearly 3 GPa. The aluminum grain size is found to be strongly correlated with the level of zirconium present in the samples, suggesting that the observed hardness increase can be attributed to the combined effects of alloying and grain size reduction. The elastic moduli of the nanocrystalline Al-Zr samples are determined to be similar to the modulus of coarse-grained aluminum and independent of zirconium content.
Date: November 1, 1995
Creator: Rittner, M.N.; Weertman, J.R. & Eastman, J.A.
Partner: UNT Libraries Government Documents Department

PROCESSING OF NANOSTRUCTURED COPPER BY REPETITIVE CORRUGATION AND STRAIGHTENING (RCS)

Description: A new process, Repetitive Corrugation and Straightening (RCS), has been developed to create bulk, nanostructured copper. In this investigation, a high purity (99.99%). copper bar measuring 6 x 6 x 50 mm with an average grain size of 765 {micro}m was used as the starting material. It was repetitively corrugated and straightened for 14 times with 90{degree} rotations along its longitudinal axis between consecutive corrugation-straightening cycles. The copper was cooled to below room temperature before each RCS cycle. The grain size obtained after the RCS process was in the range of twenty to a few hundred nanometers, and microhardness was increased by 100%. Both equilibrium and non-equilibrium grain boundaries are observed. This work demonstrates the capability of the RCS process in refining grain size of metal materials. The RCS process can be easily adapted to large-scale industrial production and has the potential to pave the way to large-scale structural applications of nanostructured materials.
Date: October 1, 2000
Creator: Zhu, Y.T. & Jiang, H.
Partner: UNT Libraries Government Documents Department

Correlation of Nanoindentation and Conventional Mechanical Property Measurements

Description: A series of model ferritic alloys and two commercial steels were used to develop a correlation between tensile yield strength and nano-indentation hardness measurements. The NanoIndenter-II{reg_sign} was used with loads as low as 0.05 g{sub f} (0.490 mN) and the results were compared with conventional Vickers microhardness measurements using 200 and 500 g{sub f} (1.96 and 4.90 N) loads. Two methods were used to obtain the nanohardness data: (1) constant displacement depth and (2) constant load. When the nanohardness data were corrected to account for the difference between projected and actual indenter contact area, good correlation between the Vickers and nanohardness measurements was obtained for hardness values between 0.7 and 3 GPa. The correlation based on constant nanoindentation load was slightly better than that based on constant nanoindentation displacement. Tensile property measurements were made on these same alloys, and the expected linear relationship between Vickers hardness and yield strength was found, leading to a correlation between measured changes in nanohardness and yield strength changes.
Date: February 14, 2001
Creator: Rice, P. M.
Partner: UNT Libraries Government Documents Department

Hardness and microstructure of internally oxidized silver alloys

Description: Alloys of Ag, Ag/1.12 at.% Mg, and Ag/0.25 at.% Mg-0.25 at.% Ni were internally oxidized at 450 to 825 C and their hardnesses and microstructure were observed. Microhardness profiles showed that hardness was high near the surface and decreased with depth into the sample. Microstructure contained regions of small and large grains, where region sizes were dependent on treatment temperature. Transitions between small- and large-grained regions were abrupt. Treatments at higher temperatures failed to alter grain structure, indicating that Mg-O clusters had indeed pinned the grain boundaries. Clustering was shown to affect the hardness of oxidized alloys, not only by inhibiting grain growth, but also by inhibiting dislocation motion.
Date: November 11, 1999
Creator: Prorok, B. C.; Park, J. H.; Goretta, K. C.; Balachandran, U. & McNallan, M. J.
Partner: UNT Libraries Government Documents Department

High-temperature internal oxidation of Ag/1.2at.% Mg and Ag/0.25at.% Mg-0.25at.% Ni.

Description: High-temperature oxygen diffusion and internal oxidation in Ag, Ag/1.2 at.% Mg (Ag-Mg), and Ag/0.25 at.% Mg-0.25 at.% Ni (Ag-Mg-Ni) have been studied, mostly in air and 8% O{sub 2}, at 450-835 C. The focus of the studies was on thermogravimetric analysis, microhardness tests, and optical and electron microscopy observations of grain growth and its inhibition by oxidation. The internal oxidation of both alloys exhibited nearly identical activation energies (0.81 eV for Ag-Mg and 0.83 eV for Ag-Mg-Ni) and rate constants. The maximum O content of both alloys was superstoichiometric (e.g., O/Mg > 1.0) and the maximum O/Mg ratios were higher at lower temperatures than at higher temperatures (e.g., 1.25 at 500 C and 1.05 at 800 C). Diffusion of O in pure Ag was {approx}60 times faster at 825 C and {approx}400 times faster at 500 C than internal oxidation of either of the Ag alloys. Grain growth of both alloys and of the Ag was quantified between 450-800 C and related to internal oxidation.
Date: September 8, 1999
Creator: Balachandran, U.; Goretta, K. C.; McNallan, M. J.; Park, J.-H. & Prorok, B. C.
Partner: UNT Libraries Government Documents Department

Fabrication and properties of a tin-lithium alloy.

Description: A Sn-25 at.% Li alloy has been fabricated by starting with pure Sn and Li. The alloy was cast into various shapes and sent to researchers in several organizations in the United States and abroad for property measurements. The fabrication procedure involved gradual addition of Li from a separate vessel into a vessel that contained Sn; continuous stirring of the mixture during Li addition increased the Li content of the alloy gradually from 0 to the final value of 25 at.%. The melting temperature of the alloy was 334 C; its density was 6.36 g/cm{sup 3}. Results are presented on microstructure, chemical composition, phase distribution, and microhardness of the alloy. Several experiments were conducted to evaluate the chemical reactivity of the alloy with air, water, and steam. Results showed that the alloy undergoes catastrophic oxidation when T >250 C in air and develops an oxide scale in water at temperatures up to 95 C. Exposure in low-pressure steam at 200 C caused significant swelling and cracking of the alloy.
Date: April 26, 2002
Creator: Natesan, K. & Ruther, W.E.
Partner: UNT Libraries Government Documents Department

Superhardness Effect in Au/Ni multilayers

Description: The presence of an enhanced hardness, that is a `Superhardness Effect, is found as a behavior parallel to the Supermodulus Effect in Au/Ni multilayer structures. The submicron thick Au/Ni coatings are prepared by magnetron sputter deposition. A microindenter is used to measure load as a function of indentation depth. An increase in hardness is measured as the Au/Ni layer pair spacing decreases. A local maximum in hardness occurs for samples with layer pairs consisting of 4-6 atomic planes of each metal component.
Date: March 1, 1993
Creator: Jankowski, A. F.
Partner: UNT Libraries Government Documents Department

Spinodal Decomposition and Order-Disorder Transformation in a Water-Quenched U-6wt%Nb Alloy

Description: A combinative approach of microhardness testing, tensile testing, and TEM microstructural analysis has been employed to study phase stability and aging mechanisms of a water-quenched U-6wt%Nb (WQU6Nb) alloy subjected to different aging schedules that include artificial aging of WQ-U6Nb at 200 C, natural aging of WQ-U6Nb at ambient temperatures for 15 to18 years, and accelerative aging of the naturally aged (NA) alloy at 200 C. During the early stages of artificial aging at 200 C, the microhardness values continuously increase as a result of the development of a fine-scale compositional modulation (wavelength: 3 nm) caused by spinodal decomposition. Coarsening of the modulated structure occurs after prolonged aging of WQ-U6Nb at 200 C for 16 hours, which leads to a decrease of microhardness. Phase instability has also been found to occur in the NA alloy, in which the formation of partially ordered phase domains resulting from an atomic-scale spinodal modulation (wavelength: 0.5 nm) renders the appearance of antiphase domain boundaries (APBs) in TEM images. Although 18-year natural aging does not cause a significant change in hardness, it affects dramatically the aging mechanism of WQ-U6Nb subjected to the accelerative aging at 200 C. The result of microhardness measurement shows that the hardness values continuously increase until after aging for 239 hours, and the total hardness increment is twice in magnitude than that in the case of the artificial aging of water-quenched alloy at 200 C. The anomalous increment of hardness for the accelerative aging of NA alloy can be attributed to the precipitation of an ordered U{sub 3}Nb phase. It is accordingly concluded that the long-term natural aging at ambient temperatures can detour the transformation pathway of WQ U-6Nb alloy; it leads to the order-disorder transformation and precipitation of ordered phase in the alloy.
Date: September 12, 2006
Creator: Hsiung, L & Zhou, J
Partner: UNT Libraries Government Documents Department

Synthesis and properties of erbium oxide single crystals

Description: Erbium oxide (Er{sub 2}O{sub 3}, erbia) is a highly stable cubic rare earth oxide with a high melting point of 2,430 C. Because of this, it may have potential applications where high temperature stability and corrosion resistance are required. However, relatively little is known about the properties of this oxide ceramic. The authors have employed a xenon optical floating zone unit with a temperature capability of 3,000 C to grow high quality single crystals of erbia. The conditions for single crystal growth of erbia have been established. The mechanical properties of erbia single crystals have been initially examined using microhardness indentation as a function of temperature.
Date: April 1, 1999
Creator: Petrovic, J.J.; Romero, R.S.; Mendoza, D.; Kukla, A.M.; Hoover, R.C. & McClellan, K.J.
Partner: UNT Libraries Government Documents Department

Nanocrystal formation via yttrium ion implantation into sapphire

Description: Ion implantation has been used to form nanocrystals in the near surface of single crystal {alpha}-Al{sub 2}O{sub 3}. The ion fluence was 5 x 10{sup 16} Y{sup +}/cm{sup 2}, and the implant energies investigated were 100, 150, and 170 keV. The morphology of the implanted region was investigated using transmission electron microscopy, x-ray energy dispersive spectroscopy, Rutherford backscattering spectroscopy and ion channeling. The implantation causes the formation of an amorphous surface layer which contains spherical nanosized crystals with a diameter of {approximately}13 nm. The nanocrystals are randomly oriented and exhibit a face-centered cubic structure with a lattice parameter of {approximately}4.1 A {+-} .02 A. Preliminary chemical analysis shows that these nanocrystals are rich in aluminum and yttrium and poor in oxygen relative to the amorphous matrix.
Date: December 31, 1995
Creator: Hunt, E.M.; Hampikian, J.M. & Poker, D.B.
Partner: UNT Libraries Government Documents Department

Investigation of the possibility of using continuous electron beams for electromotive liner hardening. Final report (Task 3)

Description: Continuous laser and electron beams are presently used for surface hardening of steel and cast iron articles. The main advantage of the sources of continuous electron beams over lasers is their high (over 90%) efficiency of conversion of an electric energy to the energy of an electron beam. A promising version of electron-beam system is that including a plasma electron source of the type developed at the Institute of High Current Electronics (IHCE, Tomsk). This type of source has a long lifetime and can operate at a poor vacuum ({approximately} 10{sup {minus}2} torr) in the working chamber. Owing to this they are successfully used for electron-beam welding, melting-on of hardening coatings, and surface hardening of machine parts. The goal of the present work is to elucidate whether it is possible to use a plasma-cathode continuous electron gun for hardening the internal surface of cast iron liners.
Date: November 1, 1998
Creator: Proskurovsky, D.I.; Rotshtein, V.P.; Rempe, N.G.; Osipov, I.V. & Goncharenko, I.M.
Partner: UNT Libraries Government Documents Department

Mitigation of sub-surface crack propagation in railroad rails by laser surface modification

Description: The authors address the mitigation of sub-surface crack propagation in railroad rails via laser surface modification. The goal is to reduce the shear forces from rail-wheel friction, which contribute significantly to the nucleation and propagation of cracks in the sub-surface region at rail gage corners. Microhardness scans and tensile tests were performed on samples from cross-sections of unused and heavily used rail heads. The results of these tests indicate that the severe cyclic plastic deformation that occurs at the gage corners, during service, significantly hardens the sub-surface region there, which leads to cracking. Laser glazing, the rapid melting and rapid solidification of a thin surface layer, was used to reduce the friction coefficient of rail steel. The advantages of this process are that specific regions of the rail surface can be targeted; the treatment does not wash away as the currently used liquid lubricants do; it is more environmentally sound than liquid lubricants; and it can be applied in service, during re-work or during rail fabrication. A number of laser treatments were conducted on AISI 1080 steel plates, similar to rail steel, from which friction samples were extracted. Static block-on-ring friction experiments performed on a variety of laser treated surfaces showed reductions in the friction coefficient by about 25% relative to untreated surfaces at loads corresponding to prototypic rail service loads. The authors laser-glazed two areas on the top surface of a 6-ft length of rail with multiple pass treatments, one with adjacent passes overlapping, and one with adjacent passes separated by 1 mm. Friction measurements were made after they were subjected to 20,000 run-in cycles. The laser treatments remained intact after these cycles. Reductions of friction coefficient of ca. 40%, relative to untreated surfaces, were observed, corresponding to a reduction in the calculated mixed mode crack propagation rate by ...
Date: October 1, 1997
Creator: DiMelfi, R.J.; Sanders, P.G.; Hunter, B.; Eastman, J.A.; Leong, K.; Kramer, J.M. et al.
Partner: UNT Libraries Government Documents Department

Phase transformations in some hafnium-tantalum-titanium-zirconium alloys

Description: Phase transformations in hafnium alloys are of interest as a means of achieving a material which exhibits flow softening and high localized strains during deformation at high strain rates. Hafnium transforms from a body-centered-cubic beta phase to a hexagonal alpha phase upon cooling below 1749{degrees}C. Hafnium-based alloys containing up to 17.5% Ti, up to 17.5% Ta, and up to 7.3% Zr by weight were button-arc melted and, in some cases, hot extruded to obtain a refined grain size. A number of alloys were shown to have beta solvus temperatures in the range of 1100 to 1300{degrees}C and showed evidence of a shear transformation upon water quenching. The Vickers microhardness of the quenched materials are typically above 350 HV as compared to 300 HV or less for materials with an alpha plus beta structure. Quenching dilatometry indicates a martensite start temperature of about 750{degrees}C for the Hf-7.5 Ta-10 Ti-1 Zr alloy and 800{degrees}C or more for the Hf-7.5 Ta-7.5 Ti-1 Zr alloy. Tensile tests at 1 s{sup {minus}1} strain rate show a constant ultimate tensile strength for temperatures up to 600{degrees}C for the above two alloys and a rapid decrease in strength with a further increase in temperature.
Date: November 1, 1997
Creator: Ohriner, E.K. & Kapoor, D.
Partner: UNT Libraries Government Documents Department

Welding and mechanical properties of cast FAPY (Fe-16 at. % Al-based) alloy slabs

Description: This report deals with the welding procedure development and weldment properties of an Fe-16 at. % Al alloy known as FAPY. The welding procedure development was carried out on 12-, 25-, and 51-mm (0.5-, 1-, and 2-in.) -thick plates of the alloy in the as-cast condition. The welds were prepared by using the gas tungsten arc process and filler wire of composition matching the base-metal composition. The preheat temperatures varied from room temperature to 350{degrees}C, and the postweld heat treatment (PWHT) was limited only for 1 h at 750{degrees}C. The welds were characterized by microstructural. analysis and microhardness data. The weldment specimens were machined for Charpy-impact, tensile, and creep properties. The tensile and creep properties of the weldment specimens were essentially the same as that of the base metal. The Charpy-impact properties of the weldment specimens improved with the PWHT and were somewhat lower than previously developed data on the wrought material. Additional work is required on welding of thicker sections, development of PWHT temperatures as a function of section thickness, and mechanical properties.
Date: May 1, 1995
Creator: Sikka, V.K.; Goodwin, G.M.; Alexander, D.J. & Howell, C.R.
Partner: UNT Libraries Government Documents Department

Property and lifetime prediction in aged U-Nb alloys: a statistical assessment

Description: This study was undertaken to better model the aging response of U-Nb alloys, particularly to predict properties and their scatter bands, from which lifetimes and their uncertainties can be evaluated. Predictive models of the aging time- and temperature-dependencies of seven age-sensitive properties were developed for nonbanded U-5.6 wt% Nb and U-7.7 wt% Nb alloys. These properties were total and uniform plastic tensile elongation to failure; first-yield, second-yield, and ultimate tensile strengths; first-yield elastic modulus; and Vickers microhardness. A more systematic and statistically aware kinetics modeling approach than employed previously gave reasonable models fits to accelerated aging property data in nonbanded U-5.6Nb and U-7.7Nb, and useful predictions for most of the properties studied. With minor modifications, the U-5.6Nb model was extended to banded U-6Nb. This modeling approach shared many of the key assumptions of the previous approach, including the assumption of Arrhenius behavior and the use of three adjustable parameters. Initial data returns from long-term aging experiments were used to validate the fitted models, a new feature to this study. The apparent activation energies of aging for the property of greatest interest, total elongation, were 32 kcal/mol for U-5.6Nb and 39 kcal/mol for U-7.7Nb, respectively; those for the other properties spanned 14–51 kcal/mol. Comparing the goodness of the model fits for the seven properties, the best fits were obtained for second-yield strength and hardness, the first-yield modulus fit the least well, and the other properties’ fits were in between. The U-5.6Nb models are more robust and therefore are expected to have better predictive power than those of U-7.7Nb, especially at the lower aging temperatures of interest. Model extrapolations to longer times (up to 5 years) and lower temperatures (as low as 40ºC) than those used for the model fitting agreed well with most of the validation data gathered for both nonbanded ...
Date: February 1, 2009
Creator: Hackenberg, Robert E. & Hemphill, Geralyn M.
Partner: UNT Libraries Government Documents Department

Assessment of Initial Test Conditions for Experiments to Assess Irradiation Assisted Stress Corrosion Cracking Mechanisms

Description: Irradiation-assisted stress corrosion cracking is a key materials degradation issue in today s nuclear power reactor fleet and affects critical structural components within the reactor core. The effects of increased exposure to irradiation, stress, and/or coolant can substantially increase susceptibility to stress-corrosion cracking of austenitic steels in high-temperature water environments. . Despite 30 years of experience, the underlying mechanisms of IASCC are unknown. Extended service conditions will increase the exposure to irradiation, stress, and corrosive environment for all core internal components. The objective of this effort within the Light Water Reactor Sustainability program is to evaluate the response and mechanisms of IASCC in austenitic stainless steels with single variable experiments. A series of high-value irradiated specimens has been acquired from the past international research programs, providing a valuable opportunity to examine the mechanisms of IASCC. This batch of irradiated specimens has been received and inventoried. In addition, visual examination and sample cleaning has been completed. Microhardness testing has been performed on these specimens. All samples show evidence of hardening, as expected, although the degree of hardening has saturated and no trend with dose is observed. Further, the change in hardening can be converted to changes in mechanical properties. The calculated yield stress is consistent with previous data from light water reactor conditions. In addition, some evidence of changes in deformation mode was identified via examination of the microhardness indents. This analysis may provide further insights into the deformation mode under larger scale tests. Finally, swelling analysis was performed using immersion density methods. Most alloys showed some evidence of swelling, consistent with the expected trends for this class of alloy. The Hf-doped alloy showed densification rather than swelling. This observation may be related to the formation of second-phases under irradiation, although further examination is required
Date: April 1, 2011
Creator: Busby, Jeremy T & Gussev, Maxim N
Partner: UNT Libraries Government Documents Department

Microstructure and Mechanical Instability of Water-Quenched U-6wt% Nb Alloy Affected by Long-Term Aging

Description: A combinative approach of microhardness testing, tensile testing, and TEM microstructural analysis was employed to study the microstructure and mechanical instability of a water-quenched U-6wt.% Nb (WQ-U6Nb) alloy subjected to different aging schedules including artificial aging at 200 C, 15-year natural aging at ambient temperatures, and 15-year natural aging followed by accelerative aging at 200 C. The changes in mechanical property during and after the aging processes were examined using microhardness and tensile-testing methods. During the early stages of artificial aging at 200 C, the microhardness of WQ-U6Nb alloy increased, i.e., age hardening, as a result of the development of nanoscale modulation caused by spinodal decomposition. Coarsening of the modulated structure occurred after a prolonged aging at 200 C for 16 hours, and it led to a decrease of microhardness, i.e., age softening. Phase instability was also found to occur in WQ-U6Nb alloy that was subjected to a 15-year natural aging at ambient temperatures. The formation of partially ordered domains resulting from a spinodal modulation with an atomic-scale wavelength rendered the appearance of swirl-shape antiphase domain boundaries (APBs) observed in TEM images. Although it did not cause a significant change in microhardness, 15-year natural aging has dramatically affected the aging mechanisms of the alloy isothermally aged at 200 C. Microhardness values of the NA alloy continuously increased and no age softening was found after isothermal aging at 200 C for 96 hours as a result of the phase decomposition of partially ordered domains into Nb-depleted {alpha} phase and Nb-enriched U{sub 3}Nb ordered phase in the alloy. It is concluded that the long-term natural aging changes the transformation pathway of WQ-U6Nb, and it leads to order-disorder transformation, precipitation hardening, and ductility embrittlement of WQ-U6Nb alloy.
Date: December 6, 2005
Creator: Hsiung, L & Zhou, J
Partner: UNT Libraries Government Documents Department