Miniature Tensile Specimens Research Articles

Isotopic tailoring with 59Ni to study the influence of helium/dpa ratio on tensile property changes

Irradiation of miniature tensile specimens of simple model Fe-Cr-Ni alloys is proceeding in the Fast Flux Test Facility to study the interactive effects of alloy composition, cold work level, helium/dpa ratio and irradiation temperature on the evolution of microstructure and associated changes in mechanical properties. The addition of 59Ni to these alloys allows side-by-side irradiation of otherwise identical specimens, differing only in helium/dpa ratio. In the absence of differences in displacement rate and temperature history, it is shown that helium in this experiment exerts a rather small influence on tensile properties at 365 and 600 °C. While the saturation level of yield stress in Fe-25Ni-15Cr is strongly dependent on irradiation temperature, it is relatively insensitive to cold work level and helium/dpa ratio. The yield strength of Fe-25Ni-15Cr, both unirradiated and irradiated at 365 ° C, is increased slightly by addition of 0.04 wt% phosphorus.

“In Situ” observation by Transmission Electron Microscope of the deformation and fracture micromechanisms in 7050 aluminum alloy

During the last ten years the "in situ" deformation experiments by transmission electron microscopy have been used in metals to study the micromechanism of plastic deformation and fracture propagation mode occuring from the crack tip.These studies were performed in mono and policrystals metals with the aim to investigate the dislocation emission from crack tip phenomena relating it with the ductil versus brittle behavior of materials.The material used in this experiment was a high strength aluminum alloy 7050 contained recrystal1ized and unrecrystal1ized microstructures, as the obtained by thermomechanical treatment in previous work .Due to the small dimensions of the miniature tensile specimens (2.7 × 6.8 × 0.02 mm ) and the requisite to obtain a lot of samples without introduction of strain during manufacture, the specimens was performed using a lithographic process.

Dislocation and grain boundary interactions in metals

The passage of dislocations through grain boundaries in face centered cubic and body centered cubic polycrystalline metals was studied using dynamic in situ high voltage electron microscopy (HVEM), static transmission electron microscopy (TEM), and anisotropic elastic stress analysis. Several conclusions were reached: (1) when dislocations propagate across grain boundaries, the activated slip system can be predicted from pile-up properties and grain boundary orientation using a combined criterion based on boundary geometric factors and internal stresses; (2) different grain boundaries impede dislocation slip propagation to different degrees, the calculated value of the pile-up obstacle stress varying from 280 to 870 MPa for dislocation transmission through a grain boundary in 304 stainless steel; (3) dynamic in situ straining of miniature tensile specimens reveals additional modes of dislocation and grain boundary interactions that were hidden from static TEM observations. In connection with the last conclusion, simultaneous dislocation transmission and reflection was activated by a stressed pile-up and a complex mechanism involving coordinated movements of four sets of dilocations in and near a grain boundary was observed.

The effects of low doses of 14 MeV neutrons on the tensile properties of three binary copper alloys

Abstract Miniature tensile specimens of high purity copper and copper alloyed respectively with five atom percent of Al, Mn, and Ni were irradiated with D-T fusion neutrons in the RTNS-II to fluences up to 1.3 × 10 22 n / m 2 at 363 K. To compare fission and fusion neutron effects, some specimens were also irradiated at the same temperature to similar damage levels in the Omega West Reactor (OWR). Tensile tests were performed at room temperature, and the radiation-induced changes in tensile properties are compared as functions of displacements per atom (dpa). The irradiation-induced strengthening of Cu-5% Mn is greater than that of Cu-5% Al and Cu-5% Ni, which behave about the same. However, all the alloys sustain less irradiation-induced strengthening by 14 MeV neutrons than pure copper. The effects of fission and fusion neutrons on the yield stress of the copper alloys correlate well on the basis of dpa, in contrast to the behavior of pure copper.

Effects of the neutron spectrum on mechanical property changes in low dose irradiations

Mechanical property measurements from comparative low dose irradiations of metals and alloys are reviewed. The emphasis is on recent experiments involving miniature tensile specimens irradiated at the Rotating Target Neutron Source II (RTNS-II) at Lawrence Livermore National Laboratory and the Omega West Reactor at Los Alamos National Laboratory. Pure metals, model alloys and structural alloys have been irradiated in RTNS-II from room temperature to 723 K to fluences as high as 8 × 10 22 n/m 2. Where temperature effects are apparent, the effect of increasing the irradiation temperature is to delay the onset of irradiation hardening to higher fluences. Companion fission reactor irradiations have been more limited in scope, but comparative information has been obtained on tensile properties at irradiation temperatures of 363 K and 563 K. For most of the materials tested the differences in the neutron energy spectrum can be accounted for by using displacements per atom (dpa) as a correlation parameter. The exceptions are all pure metals: copper, niobium and vanadium. There is also evidence for rate effects in some of the data. The rate effects obscure the spectral effects and prevent examination of dpa as a damage correlation parameter.

Design of a single variable helium effects experiment for irradiation in FFTF using alloys enriched in nickel-59

Nickel enriched in Ni-59 was extracted from the fragments of a fracture toughness specimen of Inconel ∗ ∗ Inconel is a registered trade mark of the International Nickel Company. 600 irradiated in the Engineering Test Reactor (ETR). The nickel contained 2.0% Ni-59. Three heats of austenitic steel doped with Ni-59 were prepared and inserted in the Materials Open Test Assembly (MOTA) of the Fast Flux Test Facility (FFTF). The experiment is single variable in helium effects because chemically identical alloys without Ni-59 are being irradiated side by side with the doped material. The doped alloys will produce 10–100 times more helium than the control alloys. The materials include ternary and quaternary alloys in the form of transmission electron microscope (TEM) discs and miniature tensile specimens. The helium-to-dpa (displacements per atom) ratio will be in the range 5 to 35 and will be nearly constant throughout the irradiation. The exposures will range from 0.25 to 50 dpa over the duration of the experiment. The irradiation temperatures cover the range of 360–600°C.

The effects of low doses of 14 MeV neutrons on the properties of various commercial copper alloys

Miniature tensile specimens of high purity copper and five copper alloys were irradiated with D-T fusion neutrons in the RTNS-II to fluences up to 2.5 × 10 22 n/m 2 at 90°C and 290°C. The series of alloys includes solution-strengthened, precipitate-strengthened and dispersion-strengthened alloys. To compare fission and fusion neutron effects, some of the alloys were also irradiated at the same temperatures to similar damage levels in the Omega West Reactor. Tensile tests were performed at room temperature, and the radiation-induced changes in tensile properties are examined as functions of fluence and displacements per atom (dpa). All the alloys sustain less irradiation-induced strengthening than pure copper. In contrast to pure copper, the effects of fission and fusion neutrons on the yield stress changes in the copper alloys correlate well on the basis of dpa.

Spectral effects in low-dose fission and fusion neutron irradiated metals and alloys

Flat miniature tensile specimens were irradiated to neutron fluences up to 9 × 10 22 n/m 2 in the RTNS-II and in the Omega West Reactor. Specimen temperatures were the same in both environments, with runs being made at both 90°C and 290°C. The results of tensile tests on AISI 316 stainless steel, A302B pressure vessel steel and pure copper are reported here. The radiation-induced changes in yield strength as a function of neutron dose in each spectrum are compared. The data for 316 stainless steel correlate well on the basis of displacements per atom (dpa) while those for copper and A302B do not. In copper the ratio of fission dpa to 14 MeV neutron dpa for a given yield stress change is about three to one. In A302B pressure vessel steel this ratio is more than three at lower fluences, but the yield stress data for fission and 14 MeV neutron-irradiated A302B steel appears to coalesce or intersect at the higher fluences.

Correlation of the yield strength and mlcrohardnesss of high-strength, high-conductivity copper alloys

Vickers microhardness and miniature tensile specimen measurements were made on AMZIRC and AMAX-MZC copper alloys in the cold-worked plus aged and annealed conditions. It was determined that a large portion of the strength of these alloys is due to their cold-worked nature and this strength is lost when recrystallization occurs. The recrystallization temperature of both alloys is about 475°C for a 1 h anneal and is estimated to be about 320°C for a 20 y anneal. A linear correlation between microhardness and yield strength was observed for AMZIRC and AMAX-MZC. namely σ y(MPa) ∼- 3.0 VHN.

Miniature tensile test specimens for fusion reactor irradiation studies

Three miniature sheet-type tensile specimens and a miniature rod-tensile specimen are being used in irradiation-damage studies for fusion reactor materials. Miniature specimens are also being used to determine design properties of irradiated materials. Consequently, a comparison of the tensile properties determined with these different specimens was made by testing cold-worked and solution-annealed Type 316 stainless steel sheet and rod at room temperature, 300°C and 600°C. Good agreement was observed when the properties obtained with the different specimens were compared, and all of the specimens can be used for irradiation-damage studies. However, there were small differences in the properties obtained using the different specimens, and it was recommended that before any miniature specimens are used to determine design-type properties, further studies be conducted.

Miniature tensile test specimens for fusion reactor irradiation studies

Three miniature sheet-type tensile specimens and a miniature rod-tensile specimen are being used in irradiation-damage studies for fusion reactor materials. Miniature specimens are also being used to determine design properties of irradiated materials. Consequently, a comparison of the tensile properties determined with these different specimens was made by testing cold-worked and solution-annealed Type 316 stainless steel sheet and rod at room temperature, 300°C and 600°C. Good agreement was observed when the properties obtained with the different specimens were compared, and all of the specimens can be used for irradiation-damage studies. However, there were small differences in the properties obtained using the different specimens, and it was recommended that before any miniature specimens are used to determine design-type properties, further studies be conducted.

Miniature specimen tensile data for high energy neutron source experiments

A miniature tensile specimen technology has been developed in support of high energy neutron irradiated material testing. The work includes miniature specimen design and fabrication technique development, as well as both baseline and irradiated tensile testing. Baseline data obtained using miniature tensile specimens are in good agreement with published values obtained using larger specimen geometries. Tensile data from miniature specimens irradiated up to a high energy, E ∼14 MeV, dose level of 1.10 18 n/cm 2 or 0.003 dpa, have been obtained for annealed and cold worked AISI 316 stainless steel.

Dynamic studies of the deformation & fracture of pearlite using 200 Kv SEM

A Jeol 200 kV STEM equipped with a tensile deformation stage, a secondaray electron detector and a video recording chain has been used as a high voltage SEM to study in situ the deformation and fracture of miniature tensile specimens of pearlite. The advantage with high voltage SEM is that the increased gun brightness, compared with conventional SEMs, provides an improvement in resolution for a given signal/noise ratio and therefore allows higher magnifications to be used during the recording of dynamic deformation sequences at TV scanning rates. This facility proved to be essential for the TV monitoring of pearlites at the finest interlamellar spacings. The influence of interlamellar spacing on the mechanisms of deformation was of particular interest in this study and the materials examined ranged from coarse pearlite, with a mean interlamellar spacing λ ⁓ 0.4 μm, to fine pearlite with λ ⁓ 0.1 μm,. The problems of ex post facto reasoning regarding the modes of deformation were avoided by continuously following deformation with the TV monitor and recording high resolution slow scan micrographs from the same area both before and after straining.

The design and calibration of a soft straining/heating rod for in situ deformation experiments in a 1MV electron microscope

A side entry specimen rod is described which has been constructed to enable microstructural changes in miniature tensile specimens to be observed in an AEI EM7 one million volt electron microscope. The specimens may be deformed under constant or fluctuating load at elevated temperatures and may be tilted ±20° about the rod axis. The necessary friction and temperature calibration procedures are discussed and the values of the relevant parameters for quantitative analysis of in situ creep experiments are presented. The rod has been used to study the microstructureal changes occuring during the deformation of superplastic alloys and other materials under creep conditions.

The relationship between crack tip opening displacement, local strain and specimen geometry

A method is described whereby values of crack tip opening displacement (COD) can be measured at any section of a pre-cracked or slotted specimen. The relationship between COD and the degree of transverse strain at the notch root of a Charpy specimen has been investigated. A specimen will show some portion in plane strain up to a COD of about one twenty-fifth of the thickness. Transverse contraction results in smaller COD values, for a particular clip gauge displacement, at the outside than at the specimen midsection. An equation is presented relating COD to longitudinal notch root strain for Charpy specimens with root radius 0.002 in. COD and notch root strain are found to be not directly proportional. This finding is discussed in relation to the concept of a “miniature tensile specimen” at the notch root. The effect of specimen geometry on the value of COD at fracture initiation, (COD)c, has been investigated. Neither a change in specimen thickness nor a change from a fatigue pre-crack to a 0.002 in. root radius notch has been found to significantly affect (COD)c. The material used in this research was, for the most part, a low alloy pressure vessel steel, A533B.

Preparation of miniature tensile specimens for in situ electron microscope experiments

A technique has been developed for producing uniformly thin miniature tensile specimens for in situ electron microscope experiments at high voltages. The method has the advantage that specimens can be produced with any desired shape and several can be made at the same time.

Stress-strain behaviour of copper and zinc cubes subjected to cyclical compression

Cubes of pure copper in annealed and cold-worked conditions were subjected to compressive loading along each of the three principal axes in turn. With successive orientation cycles of loading the measured strains for the three directions reached stable values after strain hardening or softening, and a cyclical stress-strain curve was derived. Miniature tensile specimens cut from the cubes before and after cycling indicated that triaxial cycling retained initial isotropy and reduced previous anisotropy, while achieving a harder or softer state of the material. Cyclical stress-strain relationships from triaxial compression tests and previous uniaxial tension-compression tests on copper showed marked differences. The few tests conducted on cubes cut from a cast zinc ingot exhibited a similar pattern of behaviour to that for copper, except that the effects of anisotropy were more marked.