Plastic Instability Stress Research Articles

Effects of pre-stress on the mechanical properties and microstructure of neutron-irradiated high-purity aluminum

Although structural materials in nuclear reactors are usually serviced under pre-stressed conditions, the irradiated mechanical properties and microstructures for prestressed metals have not been well understood due to the experimental challenges. In this study, the annealed high-purity aluminum was subjected to a low temperature (313 K) neutron irradiation under a stress of 1.5 MPa, which was ten percent of its unirradiated yield strength. Post-irradiation tensile tests were carried out at 300 K with a strain rate of 1.1 × 10−3 s−1. A strong radiation-induced hardening as well as a loss of uniform ductility was observed on all the specimens, and the pre-stress could defer irradiation hardening and embrittlement and decrease plastic instability stress, which was involved with a possible asymmetry distribution of voids induced by the pre-stress. Transmission electron microscopy (TEM) was conducted on the samples to quantify the void size and density as a function of pre-stress and dose. It was shown that the pre-stress could accelerate the void nucleation, increase the density but decrease the size of voids. Although previous studies have reported that the dislocation channel could be probably attributed to the micro-mechanism for irradiation embrittlement, none of dislocation channels were discovered in the deformed irradiated high-purity aluminum samples, which implied that the dislocation channel is unnecessary for irradiation embrittlement. In addition, a neutron irradiation embrittlement model for metals, which had been built on the basis of Johnson-Cook constitutive model, was found applicable to metals irradiated with pre-stress. The stimulated results also well agreed with the experimental data.

Irradiation hardening and ductility loss of Eurofer97 steel variants after neutron irradiation to ITER-TBM relevant conditions

Ten Eurofer97 steel variants, produced by non-standard fabrication-processing routes and modified alloying chemistries, were studied by neutron irradiations in the high flux isotope reactor. The irradiations were performed to ITER-TBM relevant conditions of ∼255–350 °C, 2.94–3.24 dpa. We quantified the irradiation-induced degradation of the steels using mechanical property tests. All the steels suffered from irradiation hardening, where a significant increase in Vickers microhardness and yield stress (σYS) occurred, accompanied with severe loss of tensile elongation. The extent of hardening was material dependent. For Tirr = 300±30 °C, most steels showed σYS increase in the range of ∼30% to as high as ∼66%, except for a low temperature tempered steel with σYS increase below 15%. Despite large losses in elongation, most failures were ductile. Significant post-necking ductility was retained with reduction in area (RA) between 65–75%, but <50% for low temperature tempered steels. The ultimate tensile stress to yield stress (σUTS/σYS) ratios decreased significantly after irradiation, highlighting irradiation-induced strain hardening capacity reduction. No major effect of irradiation on the plastic instability stress (σPIS) and true fracture stress of the steels was observed. By comparing the tensile stresses in true stress units and with literature, the results suggest that RAFM steel designing should target materials with a large separation between σPIS and σYS, to ensure the materials can maintain large work hardening and uniform deformation capability after irradiation. The tensile data of the steels additionally revealed a compelling evidence of an inverse trend between the change in RA and increase in σYS of the neutron irradiated Eurofer97 type steels.

Parameters of Necking Onset during Deformation of Chromium–Nickel Steel Irradiated by Neutrons

Mechanical tests of 12Cr18Ni10Ti (AISI 321 analogue: 10 Ni, 0.12 C, 0.5 Ti, 18 Cr, 1 MeV). The plastic instability stress and strain of the necking onset have been estimated. The results of three independent methods are in good agreement. The true local strain at the beginning of the necking process in the 12Cr18Ni10Ti steel has been established to decrease with increasing fluence, whereas the true plastic instability stress remains almost the same.

Experimental analysis of dissimilar metal weld joint: Ferritic to austenitic stainless steel

The dissimilar metal weld (DMW) joint between SA508Gr.3Cl.1 ferritic steel and SS304LN using Inconel 82/182 consumables was required in the nuclear power plants. The joint integrity assessment of these welds requires mechanical and metallurgical properties evaluation in weldment regions. The joint was subjected to 100% radiography test and bend test and transverse tensile test. Welding and testing were carried out as per the requirements of ASME Sec-IX and acceptance criteria as per ASME Sec-III. The transverse tensile test results indicated the failure from the weld metal although it satisfies the minimum strength requirement of the ASME requirements; therefore, the DMW joint was analyzed in detail. Straight bead deposition technique, fine slag inclusion, less reliable radiograph technique, plastic instability stress, yield strength ratio and metallurgical deteriorations have been contributed to failure of the DMW joint from the weld region. In the present work, the factors contributing to the fracture from weld metal have been discussed and analyzed.

Local mechanical properties of Alloy 82/182 dissimilar weld joint between SA508 Gr.1a and F316 SS at RT and 320 °C

The distributions of mechanical and microstructural properties were investigated for the dissimilar metal weld joints between SA508 Gr.1a ferritic steel and F316 austenitic stainless steel with Alloy 82/182 filler metal using small-size tensile specimens. The material properties varied significantly in different zones while those were relatively uniform within each material. In particular, significant gradient of the mechanical properties were observed near the both heat-affected zones (HAZs) of F316 SS and SA508 Gr.1a. Thus, the yield stress (YS) was under-matched with respect to the both HAZs, although, the YS of the weld metal was over-matched with respect to both base metals. The minimum ductility occurred in the HAZ of SA508 Gr.1a at both test temperatures. The plastic instability stress also varied considerably across the weld joints, with minimum values occurring in the SA508 Gr.1a base metal at RT and in the HAZ of F316 SS at 320 °C. The transmission electron micrographs showed that the strengthening in the HAZ of F316 SS was attributed to the strain hardening, induced by a strain mismatch between the weldment and the base metal, which was evidenced by high dislocation density in the HAZ of F316 SS.

Deformation and fracture properties in neutron irradiated pure Mo and Mo alloys

The effect of neutron irradiation on the mechanical properties of select molybdenum materials, unalloyed low carbon arc-cast (LCAC) Mo, Mo–0.5% Ti–0.1% Zr (TZM) alloy, and oxide dispersion-strengthened (ODS) Mo alloy, was characterized by analyzing the temperature dependence of mechanical properties. This study assembles the tensile test data obtained through multiple irradiation and post-irradiation experiments, in which tensile specimens were irradiated up to 13.1dpa at 80–1000°C and tested at −194 to 1000°C. Irradiation at 80–609°C increased yield stress significantly, up to 170%, while the increase of yield stress after irradiation at 784–936°C was not significant. The plastic instability stress was strongly dependent on test temperature but was nearly independent of irradiation dose and temperature. The true fracture stress showed weak dependences on test temperature, irradiation dose and temperature when ductile failure occurred. Among the test materials the stress-relieved ODS material in the longitudinal direction (ODS-LSR) displayed the highest resistance to irradiation embrittlement due to its relatively high fracture stress. The critical temperature for shear failure (CTSF) was defined and evaluated for the test materials and the CTSF values were compared with the ductile-to-brittle transition temperatures (DBTT) based on ductility data.

Dose dependence of mechanical properties in tantalum and tantalum alloys after low temperature irradiation

The dose dependence of mechanical properties was investigated for tantalum and tantalum alloys after low temperature irradiation. Miniature tensile specimens of three pure tantalum metals, ISIS Ta, Aesar Ta1, Aesar Ta2, and one tantalum alloy, Ta–1W, were irradiated by neutrons in the high flux isotope reactor (HFIR) at ORNL to doses ranging from 0.00004 to 0.14 displacements per atom (dpa) in the temperature range 60–100 °C. Also, two tantalum–tungsten alloys, Ta–1W and Ta–10W, were irradiated by protons and spallation neutrons in the LANSCE facility at LANL to doses ranging from 0.7 to 7.5 dpa and from 0.7 to 25.2 dpa, respectively, in the temperature range 50–160 °C. Tensile tests were performed at room temperature and at 250 °C at nominal strain rates of about 10 −3 s −1. All neutron-irradiated materials underwent progressive irradiation hardening and loss of ductility with increasing dose. The ISIS Ta experienced embrittlement at 0.14 dpa, while the other metals retained significant necking ductility. Such a premature embrittlement in ISIS Ta is believed to be due to high initial oxygen concentrations picked up during a pre-irradiation anneal. The Ta–1W and Ta–10W specimens irradiated in spallation condition experienced prompt necking at yield since irradiation doses for those specimens were high (⩾0.7 dpa). At the highest dose, 25.2 dpa, the Ta–10W alloy specimen broke with little necking strain. Among the test materials, the Ta–1W alloy displayed the best combination of strength and ductility. The plastic instability stress and true fracture stress were nearly independent of dose. Increasing test temperature decreased strength and delayed the onset of necking at yield.

Deformation in metals after low-temperature irradiation: Part I – Mapping macroscopic deformation modes on true stress–dose plane

Macroscopic deformation modes, elastic, uniform plastic, and unstable plastic deformation modes, are mapped in tensile true stress–dose space for more than two dozen metallic materials consisting of 13 body-centered cubic (bcc), 11 face-centered cubic (fcc), and two hexagonal closed packed (hcp) metals. The boundaries between different deformation zones are set by the true stress versus dose curves: the yield stress (YS), plastic instability stress (PIS), and true fracture stress (FS) plotted as functions of dose. Values for these true stresses are obtained from uniaxial tensile tests or calculated from engineering tensile data using a linear strain-hardening model for necking deformation. The relatively low-strength annealed fcc metals display large uniform plasticity regions, while unstable deformation regions are dominant in the harder bcc and hcp metals. PIS values for all materials are independent of dose except for the precipitation-hardened IN718 alloy, where a decrease of PIS occurs due to an irradiation-induced change in second phases. In the bcc materials for high-temperature application, such as 9Cr ferritic/martensitic steels, sintered molybdenum, vanadium, and tantalum, the radiation-induced embrittlement is characterized in terms of FS decreasing with dose at relatively high doses. FS is nearly dose-independent below the critical dose for embrittlement. It is concluded that the tensile stress-based deformation mode maps effectively integrate mechanical property information and characterize differences in radiation effects between crystalline structures or material groups.

Low temperature neutron irradiation effects on microstructure and tensile properties of molybdenum

Polycrystalline molybdenum was irradiated in the hydraulic tube facility at the High Flux Isotope Reactor to doses ranging from 7.2 × 10 −5 to 0.28 dpa at ∼80 °C. As-irradiated microstructure was characterized by room-temperature electrical resistivity measurements, transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS). Tensile tests were carried out between −50 and 100 °C over the strain rate range 1 × 10 −5 to 1 × 10 −2 s −1. Fractography was performed by scanning electron microscopy (SEM), and the deformation microstructure was examined by TEM after tensile testing. Irradiation-induced defects became visible by TEM at ∼0.001 dpa. Both their density and mean size increased with increasing dose. Submicroscopic three-dimensional cavities were detected by PAS even at ∼0.0001 dpa. The cavity density increased with increasing dose, while their mean size and size distribution was relatively insensitive to neutron dose. It is suggested that the formation of visible dislocation loops was predominantly a nucleation and growth process, while in-cascade vacancy clustering may be significant in Mo. Neutron irradiation reduced the temperature and strain rate dependence of the yield stress, leading to radiation softening in Mo at lower doses. Irradiation had practically no influence on the magnitude and the temperature and strain rate dependence of the plastic instability stress.

Deformation in metals after low-temperature irradiation: Part II – Irradiation hardening, strain hardening, and stress ratios

Effects of irradiation at temperatures ⩽200 °C on tensile stress parameters are analyzed for dozens of body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close packed (hcp) pure metals and alloys, focusing on irradiation hardening, strain hardening, and relationships between the true stress parameters. Similar irradiation-hardening rates are observed for all the metals irrespective of crystal type. Typically, irradiation-hardening rates are large, in the range 100–1000 GPa/dpa, at the lowest dose of <0.0001 dpa and decrease with dose to a few tens of MPa/dpa or less at about 10 dpa. However, average irradiation-hardening rates over the dose range of 0 dpa− D C (the dose to plastic instability at yield) are considerably lower for stainless steels due to their high uniform ductility. It is shown that whereas low-temperature irradiation increases the yield stress, it does not significantly change the strain-hardening rate of metallic materials; it decreases the fracture stress only when non-ductile failure occurs. Such dose independence in strain-hardening behavior results in strong linear relationships between the true stress parameters. Average ratios of plastic instability stress to unirradiated yield stress are about 1.4, 3.9, and 1.3 for bcc metals (and precipitation hardened IN718 alloy), annealed fcc metals (and pure Zr), and Zr-4 alloy, respectively. Ratios of fracture stress to plastic instability stress are calculated to be 2.2, 1.7, and 2.1, respectively. Comparison of these values confirms that the annealed fcc metals and other soft metals have larger uniform ductility but smaller necking ductility when compared to other materials.

Strain Hardening During Mechanical Twinning and Dislocation Channeling in Irradiated 316 Stainless Steels

Abstract Localized deformation mechanisms and strain-hardening behaviors in irradiated 316 and 316LN stainless steels were investigated, and a theoretical model was proposed to explain the linear strain-hardening behavior during the localized deformation. After low temperature irradiation to significant doses, the deformation microstructure changed from dislocation tangles to channels or mechanical twins. It was also observed that irradiation hardening straightened gliding dislocations and increased the tendency for forming pileups. Regardless of these microstructural changes, the strain-hardening behavior was relatively insensitive to the irradiation. This dose-independent strain-hardening rate resulted in dose independence of the true stress parameters, such as the plastic instability stress and true fracture stress. In the proposed model, the long-range back stress was formulated as a function of the number of pileup dislocations per slip band and the number of slip bands in a grain. The calculation results confirmed the experimental observation that strain-hardening rate was insensitive to the change in deformation mechanism because the long-range back stress hardening became as high as the hardening by tangled dislocations.

Dose dependence of true stress parameters in irradiated bcc, fcc, and hcp metals

The dose dependence of true stress parameters has been investigated for nuclear structural materials: A533B pressure vessel steels, modified 9Cr–1Mo and 9Cr–2WVTa ferritic martensitic steels, 316 and 316LN stainless steels, and Zircaloy-4. After irradiation to significant doses, these alloys show radiation-induced strengthening and often experience prompt necking at yield followed by large necking deformation. In the present work, the critical true stresses for deformation and fracture events, such as yield stress (YS), plastic instability stress (PIS), and true fracture stress (FS), were obtained from uniaxial tensile tests or calculated using a linear strain-hardening model for necking deformation. At low dose levels where no significant embrittlement was detected, the true fracture stress was nearly independent of dose. The plastic instability stress was also independent of dose before the critical dose-to-prompt-necking at yield was reached. A few bcc alloys such as ferritic martensitic steels experienced significant embrittlement at doses above ∼1dpa; and the true fracture stress decreased with dose. The materials fractured before yield at or above 10dpa.

Strain hardening and long-range internal stress in the localized deformation of irradiated polycrystalline metals

Low-temperature irradiation can significantly harden metallic materials and often results in microscopic strain localization such as dislocation channeling during deformation. In true stress–true strain analyses, however, the strain localization does not significantly affect macroscopic strain-hardening behavior. It was attempted to explain the strain-hardening behavior during strain localization in terms of long-range back stresses. In theoretical modeling the long-range back stress was formulated as a function of the number of residual pileup dislocations at a grain boundary and the number of localized bands formed in a grain. The strain-hardening rates in channel deformation were calculated for ten face-centered cubic (fcc) and body-centered cubic (bcc) metals. A few residual dislocations in each channel could account for the strain-hardening rates as high as those for uniform deformation. It was also shown that the strain-hardening behavior predicted by the long-range back stress model resembled the empirical strain-hardening behaviors, which result from both localized and non-localized deformations. The predicted plastic instability stress was comparable to the tensile test data.

Plastic instability behavior of bcc and hcp metals after low temperature neutron irradiation

Plastic instability in uniaxial tensile deformation has been investigated for the body centered cubic (bcc) and hexagonal close packed (hcp) pure metals, V, Nb, Mo, and Zr, after low temperature (60–100 °C) neutron irradiation up to 0.7 dpa. Relatively ductile metals, V, Nb, and Zr, experienced uniform deformation prior to necking at low doses and prompt plastic instability at yield at high doses. Mo failed in a brittle mode within the elastic limit at doses above 0.0001 dpa. V showed a quasi-brittle failure at the highest dose of 0.69 dpa. In the ductile metals, plastic instability at yield occurred when the yield stress exceeded the plastic instability stress (PIS), which was nearly independent of dose. The PIS values for V, Nb, Mo, and Zr were about 390, 370, 510, and 170 MPa, respectively. The coincidence of plastic instability at yield and dislocation channeling cannot be generalized for all metallic materials.

Temperature dependence of strain hardening and plastic instability behaviors in austenitic stainless steels

The temperature dependencies of true strain-hardening and plastic-instability properties are investigated for austenitic stainless steels; including annealed 304, 316, 316LN, and 20% cold-worked 316LN, at test temperatures from −150 to 450 °C. In both annealed and cold-worked conditions, strength decreases with increasing temperature, while ductility peaks below room temperature and is least at about 400 °C. At room temperature or below, the strain-hardening behavior exhibits two stages consisting of a rapid decrease for small strains and an increase-decrease cycle before plastic instability occurs. At higher temperatures the strain-hardening rate decreases monotonically with strain. The characteristics of these strain-hardening behaviors are explained by changes in deformation microstructure. Transmission electron microscopy (TEM) of the deformed 316LN steel shows that twins, stacking faults, and/or martensite laths, along with dislocations, are formed at subzero temperatures, and dislocation-dominant microstructures at elevated temperatures. It is also shown that the average strain-hardening rate during necking to failure is almost equal to the true stress at the onset of necking. This stress is called the plastic instability stress (PIS). Cold-worked specimens fail by prompt necking at yield when the yield stress exceeds the PIS of annealed material, indicating that the PIS is independent of prior cold work.

Plastic instability in polycrystalline metals after low temperature irradiation

The dose dependence of plastic instability behavior has been investigated for polycrystalline metals after neutron irradiation at low temperatures (<200 °C). The analyzed materials consist of 10 body-centered cubic (bcc), 7 face-centered cubic (fcc), and 2 hexagonal close packed (hcp) metals. It was found that the metals after irradiation showed necking at yield when the yield stress exceeded the true plastic instability stress, σ IS, for the unirradiated material. It was also shown that σ IS was almost independent of dose below a critical dose. The critical dose is called the dose to plastic instability at yield, D C, because at higher doses the material shows necking at yield. The D C values ranged from 0.002 to 0.2 dpa for bcc and hcp metals, except for a high purity iron, that had a D C value of 6 dpa; whereas the fcc metals gave generally high values ranging from 0.1 to 40 dpa. It is attempted to explain the dose independence of the plastic instability stress by a straightforward shifting of tensile curves by the appropriate strain corresponding to the radiation-induced increase in yield stress. The dose independence of strain-hardening behavior suggests that radiation-induced defects and deformation-produced dislocations give similar net strain-hardening effects.