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Numerical Integration of Elastoviscoplasticity Model with Stiff Hardening and Softening

Description: The constitutive equations for viscoplasticity typically are stiff differential equations and require special numerical methods to integrate them efficiently. The objective of this paper is to propose a class of rate-dependent viscoplastic constitutive equations which can be integrated by an efficient explicit scheme that includes the first order effect of pressure and plastic strain hardening.
Date: February 1, 2000
Creator: Vorobiev, O.Y.; Lomov, I.N; Glenn, L.A. & Rubin, M.B.
Partner: UNT Libraries Government Documents Department

Effect of Microstructure on the Deformation Mechanism of Friction Stir-Processed A₁₀.₁CoCrFeNi High Entropy Alloy

Description: This article reports grain refinement using friction stir processing and a detailed study on the microstructural evolution at various locations in the processed region using scanning electron microscopy, in addition to the effect of grain refinement on the tensile properties and the underlying deformation mechanisms in both coarse-grained and fine-grained materials.
Date: August 7, 2014
Creator: Komarasamy, Mageshwari; Kumar, N.; Tang, Z.; Mishra, Rajiv & Liaw, P.K.
Partner: UNT College of Engineering

Structural influences on the work hardening behavior of aluminum

Description: Effects of various grain and subgrain morphologies on low temperature work hardening of pure Al is studied using tensile tests. Plotting the work hardening rate as a function of true stress, the work hardening is separable into two distinct regimes. Both regimes are approximated by a line {Theta} = {Theta}{sub 0} {minus} K{sub 2}{sigma}, where {Theta}{sub 0} is theoretical work hardening rate at zero stress and K{sub 2} is related to dynamic recovery rate. The first or early deformation regime exhibits greater values of {Theta}{sub 0} and K{sub 2} and can extend up to the first 10% strain of tensile deformation. This early deformation regime is contingent on the existence of a pre-existent dislocation substructure from previous straining. The {Theta}{sub 0} and K{sub 2} associated with the early deformation regime are dependent on the strength and orientation of the pre-existent dislocation substructure relative to the new strain path. At high enough temperatures, this pre-existent dislocation substructure is annealed out, resulting in the near elimination of the early deformation regime. In comparison, the latter regime is dominated by the initial grain and/or subgrain morphology and exhibit lower values of {Theta}{sub 0} and K{sub 2}. The actual value of K{sub 2} in the latter regime is strongly dependent on the existence of a subgrain morphology. Recrystallized or well-annealed microstructures exhibit greater values of K{sub 2} than microstructures that remain partially or fully unrecrystallized. The higher K{sub 2} value is indicative of a more rapid dynamic recovery rate and a greater degree of strain relaxation. The ability to achieve a more relaxed state produces a low-energy cellular dislocation substructure upon deformation. The introduction of subgrains hinders the evolution of a low-energy dislocation cell network, giving way to a more random distribution of the dislocation density.
Date: December 1, 1994
Creator: Chu, D.
Partner: UNT Libraries Government Documents Department

Uniaxial Tensile Properties of Zircaloy Containing Oxygen : Summary Report

Description: The uniaxial stress-strain behavior of Zircaloy-2 and -4, Zircaloy-oxygen alloys with a uniform oxygen distribution, and composite specimens with a ZrO2/alpha/beta layer structure was investigated over the range of experimental conditions: temperature 25-1400 degrees C; strain rate ; oxygen content 0.11 - 4.4 wt %; grain size 5-50 micrometers; texture longitudinal, transverse, and diagonal orientations; and microstructural state, which consists of the equiaxed alpha phase and various transformed beta acicular structures. The work-hardening and strain-rate sensitivity parameters were determined from the experimental results, and the tensile properties were correlated with oxygen concentration, oxygen distribution in the material, and microstructure. Dynamic strain-aging phenomena were observed in Zircaloy at 200, 400, and 700 degrees C, and super-plastic deformation occurred at 850 and 1000 degrees C. An increase in the oxygen concentration in homogeneous Zircaloy-oxygen alloys increased the ultimate tensile strength and decreased the total strain, particularly below approximately 900/sup 0/C. In composite specimens with the ZrO2/alpha/beta structure, the total oxygen content had little effect on the ultimate tensile strength below approximately 1000 degrees C, but the strength increased with oxygen content at higher temperatures. Information on the effects of grain size, oxygen content, texture, and strain rate on the stress-strain behavior suggests that the dominant mechanism of super-plastic deformation in Zircaloy near approximately 850/sup 0/C is grain-boundary sliding at the alpha-beta interface with accommodation by diffusional creep, dislocation slip, and grain-boundary migration. Good correlation was obtained between ductility and values of the strain-rate sensitivity parameter.
Date: June 1977
Creator: Garde, A. M.; Chung, H. M. & Kassner, T. F.
Partner: UNT Libraries Government Documents Department

The correlation of indentation size effect experiments with pyramidal and spherical indenters.

Description: Experiments were conducted in annealed iridium using pyramidal and spherical indenters over a wide range of load. For a Berkovich pyramidal indenter, the hardness increased with decreasing depth of penetration. However, for spherical indenters, hardness increased with decreasing sphere radius. Based on the number of geometrically necessary dislocations generated during indentation, a theory that takes into account the work hardening differences between pyramidal and spherical indenters is developed to correlate the indentation size effects measured with the two indenters. The experimental results verify the theoretical correlation.
Date: January 1, 2001
Creator: Swadener, J. G. (John G.); George, Easo P. & Pharr, G.M.
Partner: UNT Libraries Government Documents Department

MECHANICAL BEHAVIOR OF MOLYBDENUM DISILICIDE-BASED ALLOYS

Description: We have investigated the mechanical behavior of the following single-phase polycrystalline alloys with the MoSi{sub 2} body-center tetragonal structure: MoSi{sub 2} alloyed with {approximately}2.5 at.% Re, MoSi{sub 2} alloyed with 2 at.% Al, MoSi{sub 2} alloyed with 1 at.% Nb, and MoSi{sub 2} alloyed with 1 at.% Re and 2 at.% Al. Several anomalies in the mechanical behavior of alloyed materials were observed. For example, (1) addition of only {approximately}2.5 at. % Re results in an order of magnitude increase in compressive strength at 1600 C, (2) additions of Nb and Al cause solution softening at near-ambient temperatures, and (3) quaternary MoSi{sub 2}-Re-Al alloys show strengthening at elevated temperatures and reduction in flow stress with enhanced plasticity at near-ambient temperatures in compression. The mechanisms of anomalous solution hardening and softening are discussed.
Date: December 1, 2000
Creator: MISRA, A.; SHARIF, A. & AL, ET
Partner: UNT Libraries Government Documents Department

Extreme Precipitation Strengthening in Ion-Implanted Nickel

Description: Precipitation strengthening of nickel was investigated using ion-implantation alloying and nanoindentation testing for particle separations in the nanometer range and volume fractions extending above 10O/O. Ion implantation of either oxygen alone or oxygen plus aluminum at room temperature was shown to produce substantial strengthening in the ion-treated layer, with yield strengths near 5 GPa in both cases. After annealing to 550"C the oxygen-alone layer loses much of the benefit, with its yield strength reduced to 1.2 GP~ but the dual ion-implanted layer retains a substantially enhanced yield strength of over 4 GPa. Examination by transmission electron f microscopy showed very fine dispersions of 1-5 nm diameter NiO and y-A1203 precipitates in the implanted layers before annealing. The heat treatment at 550"C induced ripening of the NiO particles to sizes ranging from 7 to 20 nm, whereas the more stable ~-A1203 precipitates were little changed. The extreme strengthening we observe is in semiquantitative agreement with predictions based on the application of dispersion-hardening theory to these microstructure.
Date: May 3, 1999
Creator: Follstaedt, D.M.; Knapp, J.A.; Myers, S.M. & Petersen, G.A.
Partner: UNT Libraries Government Documents Department

A Strength and Damage Model for Rock Under Dynamic Loading

Description: A thermodynamically consistent strength and failure model for granite under dynamic loading has been developed and evaluated. The model agrees with static strength measurements and describes the effects of pressure hardening, bulking, shear-enhanced compaction, porous dilation, tensile failure, and failure under compression due to distortional deformations. This paper briefly describes the model and the sensitivity of the simulated response to variations in the model parameters and in the inelastic deformation processes used in different simulations. Numerical simulations of an underground explosion in granite are used in the sensitivity study.
Date: December 1, 1999
Creator: Vorobiev, O.Y.; Antoun, T.H.; Lomov, I.N. & Glenn, L.A.
Partner: UNT Libraries Government Documents Department

Cyclic Material Properties Test to Determine Hardening/Softening Characteristics of HY-80 Steel

Description: The Cyclic Material Properties Test was structured to obtain and provide experimental data for determining cyclic hardening/softening characteristics of HY-80 steel. The inelastic strain history data generated by this test program and the resulting cyclic stress-strain curve will be used to enhance material models in the finite element codes used to perform nonlinear elastic-plastic analysis.
Date: April 30, 2003
Creator: Hodge, S. C.; Minicucci, J. M. & Trimble, T. F.
Partner: UNT Libraries Government Documents Department

The development of a ductility-based aging model for low temperature aged U-6Nb alloy

Description: This study focuses on the ductility evaluation of low-temperature (100 and 200 C) aged U-6Nb alloy. The objective is to develop a ductility-based aging model to improve lifetime prediction for weapon components in the stockpile environment. Literature review shows that the work hardening n-value and the strain-rate hardening mvalue are the two most important metallurgical factors for the uniform and the post-uniform (necking) ductility control, respectively. Unfortunately, both n and m values of the U-6Nb alloy are lacking. The study shows that the total ductility of U-6Nb is dominated by the uniform ductility, which deteriorates in both 100 C and 200 C aging. Further analysis shows that the uniform ductility correlates well with the work hardening n-value of the later stage deformation in which dislocation-slip is the mechanism. The kinetics of the loss of uniform ductility and the associated reduction in work-hardening n-value in low temperature aging will be used for the development of a ductility-based aging model. The necking ductility appears to be a minor but significant factor in the total ductility of U-6Nb. It does not show a clear trend due to large data scatter. The uncertain nature of necking failure may always hinder a reliable measurement of necking ductility. Consequently, a precise measurement of strain-rate hardening m-value could be a viable alternative to model the metallurgical contribution to the necking ductility. The conventional strain rate step-change method and the ABI (Automated-Ball-Indentation) test both show promising result in m-value measurement.
Date: March 24, 2005
Creator: Bridges, B
Partner: UNT Libraries Government Documents Department

Hardening of transistor circuits to ionizing radiation

Description: It is shown experimentally that transistor circuits may have their fundamental sensitivities to ionizing radiation reduced by factors of ten by addition of a simple reverse biased diode. Bipolar transistors, field effect transistors and microcircuits have been hardened. (auth)
Date: September 10, 1965
Creator: Crowe, J.W.
Partner: UNT Libraries Government Documents Department

MECHANICAL PROPERTIES AND MICROSTRUCTURE IN LOW ACTIVATION MARTENSITIC STEELS F82H AND OPTIMAX AFTER 800 MEV PROTON IRRADIATION

Description: Low-activation martensitic steels, F82H (mod.) and Optimax-A, have been irradiated with 800-MeV protons up to 5.9 dpa. The tensile properties and microstructure have been studied. The results show that radiation hardening increases continuously with irradiation dose. F82H has lesser irradiation hardening as compared to Optimax-A in the present work and DIN1.4926 from a previous study. The irradiation embrittlement effects are evident in the materials since the uniform elongation is reduced sharply to less than 2%. However, all the irradiated samples ruptured in a ductile-fracture mode. Defect clusters have been observed. The size and the density of defect clusters increase with the irradiation dose. Precipitates are amorphous after irradiation.
Date: October 1, 1999
Creator: DAI, Y. & AL, ET
Partner: UNT Libraries Government Documents Department

Surface Hardening by Nanoparticle Precipitation in Ni(Al,O)

Description: Ion implantation of O and Al were used to form nanometer-size precipitates of NiO or Al{sub 2}O{sub 3} in the near-surface of Ni. The yield strengths of the treated layers were determined by nanoindentation testing in conjunction with finite-element modeling. The strengths range up to {approximately}5 GPa, substantially above values for hard bearing steels. These results agree quantitatively with predictions of dispersion-hardening theory based on the precipitate microstructures observed by transmission electron microscopy. Such surface hardening by ion implantation may be beneficial for Ni components in micro-electromechanical systems.
Date: April 1, 2001
Creator: MYERS, SAMUEL M. Jr.; FOLLSTAEDT, DAVID M. & KNAPP, JAMES A.
Partner: UNT Libraries Government Documents Department

Modeling the post-yield flow behavior after neutron and electron irradiation of steels and iron-base alloys.

Description: Irradiation hardening is an issue of practical importance as it relates to the remanent life and the nature of failure of reactor components exposed to displacement-producing radiation. For example, irradiation-induced yield strength increases in pressure vessel steels are directly related to increases in the ductile-to-brittle-transition-temperature of these materials. Other issues associated with hardening, such as reductions in ductility, toughness and fatigue life of structural steels are also of concern. Understanding these phenomena requires studies of fundamental microstructural mechanisms of hardening. Because of the limited supply of neutron-irradiated surveillance material, difficulties posed by the radioactivity of neutron-exposed samples and the uncertainty of irradiation conditions in this case, fundamental studies are often conducted using well-controlled experiments involving irradiation by electrons instead of neutrons. Also, in such studies, simple model alloys are used in place of steels to focus on the influence of specific alloy constituents. It is, therefore, important to understand the relationship between the results of this kind of experiment and the effects of in-reactor neutron exposure in order to use them to make predictions of significance to reactor component life. In this paper, we analyze the tensile behavior of pressure vessel steels (A212B and A350) irradiated by neutrons and electrons. The results show that the post-yield true stress/true strain behavior can provide fingerprints of the different hardening effects that result from irradiation by the two particles, which also reflect the influence of alloy content. Microstructurally-based models for irradiation-induced yield strength increases, combined with a model for strain hardening, are used to make predictions of the different effects of irradiation by the two particles on the entire flow curve that agree well with data.
Date: January 13, 1999
Creator: Dimelfi, R. J.
Partner: UNT Libraries Government Documents Department

Solution hardening and strain hardening at elevated temperatures

Description: Solutes can significantly increase the rate of strain hardening; as a consequence, the saturation stress, at which strain hardening tends to cease for a given temperature and strain rate, is increased more than the yield stress: this is the major effect of solutes on strength at elevated temperatures, especially in the regime where dynamic strain-aging occurs. It is shown that local solute mobility can affect both the rate of dynamic recovery and the dislocation/dislocation interaction strength. The latter effect leads to multiplicative solution strengthening. It is explained by a new model based on repeated dislocation unlocking, in a high-temperature limit, which also rationalizes the stress dependence of static and dynamic strain-aging, and may help explain the plateau of the yield stress at elevated temperatures. 15 figures.
Date: October 1, 1982
Creator: Kocks, U.F.
Partner: UNT Libraries Government Documents Department

The Effect of Oversize Solute Additions on the Irradiation-Assisted Stress Corrosion Cracking Resistance of Austenitic Stainless Steels

Description: Solute additions of zirconium are believed to decrease RIS and dislocation density through point defect trapping and recombination, which in turn reduces grain boundary sensitization and IGSCC. In this work, the effect of zirconium on the microstructure, microchemistry, hardening and IGSCC behavior of 316SS doped with zirconium to levels of 0.31 and 0.45 wt% was studied. These alloys were then irradiated with 3.2 MeV protons to doses up to 7 dpa at a temperature of 400 C. Zr additions had relatively little effect on radiation hardening. Dislocation densities were reduced and average sizes slightly increased for the +Zr alloys relative to the 316SS. Although a low amount of swelling was seen in 316SS at 3 dpa, no voids were observed in either of the +Zr alloys at 3 or 7 dpa. The difference in RIS of Cr and Ni between 316SS and 316+LoZr at 3 dpa was negligible, though RIS for 316+HiZr was considerably less than 316+LoZr at 7 dpa. The link between the oversize solute addition of Zr and its effect on IASCC shows that although the percent strain to failure increased substantially for 316+LoZr compared to the 316SS, cracking behavior was substantially worse as the number of cracks and total crack length was increased by more than an order of magnitude.
Date: August 12, 2005
Creator: Hackett, M & Was, G
Partner: UNT Libraries Government Documents Department

Hardness of Carburized Surfaces in 316LN Stainless Steel after Low Temperature Neutron Irradiation

Description: A proprietary surface carburization treatment is being considered to minimize possible cavitation pitting of the inner surfaces of the stainless steel target vessel of the SNS. The treatment gives a large supersaturation of carbon in the surface layers and causes substantial hardening of the surface. To answer the question of whether such a hardened layer will remain hard and stable during neutron irradiation, specimens of the candidate materials were irradiated in the High Flux Isotope Reactor (HFIR) to an atomic displacement level of 1 dpa. Considerable radiation hardening occurred in annealed 316LN stainless steel and 20% cold rolled 316LN stainless steel, and lesser radiation hardening in Kolsterised layers on these materials. These observations coupled with optical microscopy examinations indicate that the carbon-supersaturated layers did not suffer radiation-induced decomposition and softening.
Date: January 31, 2005
Creator: Byun, TS
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

Spinodal Ordering and Precipitation in U-6 wt% Nb

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 (WQU6Nb) 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 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 and precipitation hardening of WQ-U6Nb alloy.
Date: December 19, 2005
Creator: Hsiung, L & Zhou, J
Partner: UNT Libraries Government Documents Department