Cold-worked Specimens Research Articles

Changes in residual stress, domain size and microstrain during the fatigue of AISI 1008 steel

Changes in the positions and shapes of X-ray diffraction peaks have been examined after low cycle and high cycle strain-controlled fatigue of normalized and cold-worked AISI 1008 steel. Measurements were made at and below the surface. The domain (mosaic) size is sometimes the major contributor to peak breadth, but sometimes it is controlled by microstrain, or by both quantities. Microstrains are larger after low cycle fatigue than after high cycle fatigue. The surface residual stress is altered by fatigue, and the pattern of change with cycling is similar for low or high strains, and for initially normalized of cold-worked specimens. Near failure in the low cycle fatigue of a cold-worked specimen, the domain size and microstrain approach those for a (cycled) annealed specimen. Dislocation densities are of the order of 10 13–10 14 m −2. Cycling in the initially normalized condition increases both the dislocation density and the dislocation arrangements with low long-range strains. However, with cold-worked AISI 1008 steel the density decreases and, near failure, the dislocation array is nearly random. In all cases the peak breadth versus depth below the surface passes through a minimum. In the low cycle fatigue of the normalized condition the damage is less in the interior than at the surface, but after high cycle fatigue (and after the high or low cycle fatigue of cold-worked samples) the damage is greater in the interior of a specimen. The dislocation arrangement oscillates with depth below the surface, at least for the first few hundred microns.

Uniaxial In-Reactor Creep of Zircaloy-2: Stress, Flux, and Temperature Dependence

Results of several uniaxial in-reactor creep tests, carried out in a temperature range of 553 to 623 K on Zircaloy-2 cold-worked specimens in fast flux (E > 1 MeV) from 1.2 × 1017 to 1.1 × 1018 n·m−2 · s and in a stress range from 98 to 157 MPa, are presented. The effects of instantaneous flux and applied tensile stress are investigated, and the available data correlated by functional relationship. The effect of the temperature on the creep rate in the presence and absence of flux is also investigated and discussed.

Microstructure and dimensional changes of neutron-irradiated zirconium alloys

Experimental observations concerning the neutron-irradiation-induced defect structure in zirconium-based alloys are analyzed within the framework of an irradiation growth theory developed in the past years. The competition of those defects and the microstructure present in the material prior to irradiation as point defect sinks is studied as a function of irradiation temperature and dose. Owing to the different growth behavior of recrystallized and of cold-worked specimens at reactor temperatures, the cellular microstructure of the latter is considered in detail. In view of the highly anisotropic dislocation system in these materials, cell boundaries are reasoned to form with essentially edge components in the walls parallel to the c-axis, while the boundary segments normal to that axis should be of screw type. Since the latter would induce no dimensional change if the cell boundary absorbs defects by dislocation climb, it is argued — on the basis of growth data — that it must behave as a point defect sink/source of a different nature than that of free dislocations. The possibility of dislocation segments with a ( c + a)-type Burgers vector in the cell boundary or rapid point defect diffusion along it are also discussed. The existing growth model is then enlarged in order to account quantitatively for the dimensional changes of cold-worked materials, and its results are compared with available experimental data.

HIGH TEMPERATURE INTERNAL FRICTION AND DISLOCATION MOTION IN POLY AND SINGLE CRYSTALS OF F.C.C. METALS

High temperature internal friction experiments were performed on different poly and single crystals of four (Al, Ag, Cu, Ni) FCC metals in the Hz frequency range. Damping spectra exhibits relaxation peaks even for vibration amplitude corresponding to strains as low as 2.5 10-7. For higher stress levels (corresponding to strains ranging between 2.5 10-7 and 5 10-6) damping proved to be very sensitive to the strain amplitude. In the 0.4 - 0.5 TM temperature range, one peak have been found associated with an activation energy of about 1 ev. This peak is not present on strongly cold-worked specimens and appear after annealing at medium temperature. He disappears after high temperature annealing whereas new peaks appear at 0.6 - 0.7 TM, associated to a large activation energy. The importance of the microstructural state and specially of the dislocation network, as regards the presence of internal friction maxima, was confirmed by the fact that same peak behaviour was observed on single and poly crystals.

Interaction between different kinds of dislocations and point defects created by irradiation in copper

The variation of the Bordoni peaks and also the modulus defect and internal friction at very low temperature are studied during the electron bombardment of copper samples and their subsequent annealing. A substructure is apparent in each of the two Bordoni peaks which is different in well annealed and cold-worked specimens. Comparison with electron microscope observations allows a tentative identification of the different dislocation classes responsible for the relaxation processes. The temperature range of the pipe diffusion of self-interstitials on the dislocations is also determined.

Comparison of mechanical properties for temper-rolled and temper-annealed strips of aluminium and α-brass

The mechanical properties and microstructures of cold-deformed (temper-rolled or cold-stretched) and temper-annealed specimens of aluminium and α-brass were studied. It was shown experimentally that for aluminium the temper annealing gives a lower total elongation for a given strength level than does cold deformation, whereas for brass the cold deformation in general yields the lowest total elongation. A model developed previously for uniform elongation can accurately describe experimental values for temper-annealed specimens of both materials. The improvement in the strength-ductility properties of the temper-annealed brass was found to be due mainly to a substantially refined recrystallized grain size. The resulting micron-sized grain size raises the flow stress by up to 250 MPa without markedly influencing the uniform elongation. For aluminium the strength-ductility properties of temper-annealed specimens were found to be inferior to those of cold-worked specimens, contrary to the results of investigations on aluminium and aluminium alloys of higher purity reported earlier. The reason for this is probably the high iron content which leads to the precipitation of large inclusions which reduce the ductility.

予歪を与えたSUS 304鋼の沸騰MgCl<SUB>2</SUB>溶液中応力腐食割れにおける水素添加の影響

Effect of hydrogenation on the stress corrosion cracking (SCC) was studied in boiling 45%MgCl2 solution. Hydrogenation was carried out in 28 MPa hydrogen at 673 K for 57.6 ks (16 h). Little change in SCC susceptibility with hydrogenation was observed in solution treated Type 304 steel, but pre-stress in the range of 200–400 MPa produced an increase in SCC susceptibility with hydrogenation. Similar tendency was observed by charging with hydrogen through immersion in 5%HF-30%HNO3 solution or 0.5 kmol/m3 H2SO4 solution, as well as in boiling 45%MgCl2 solution containing 25 g/m3 NaAsO2. A corrosion rate of hydrogenated steel was greater than that of hydrogen-free one in boiling 45%MgCl2 solution, and selective corrosion of martensite was observed in a cold worked specimen containing hydrogen. It is considered from these results that the increase in SCC susceptibility of Type 304 steel with hydrogenation might be produced by selective corrosion of martensite.

SUS 304ステンレス鋼の拡散溶接におよぼす溶接前の組織の影響

Fine grained microstructure enhances grain boundary diffusion, so diffusion weldability of fine grained material may be better than that of coarse grained material.Three types of diffusion welding specimens were prepared. Each abutting surface had coarse grained microstructure, fine grained microstructure, which was prepared from friction weld interface, and cold-worked microstructure. Third one was expected to change into small recrystal at diffusion welding temperature. These were welded by various diffusion welding conditions. And, tesnile strength and microstructure of weld, and deformation of specimens were examined in order to estimate the effect of prior microstructures on diffusion weldability.Obtained results were as follows;(A) Very fine grained microstructure, in which grain diameter was about 7 μm, was hardly to change into coarse grain even keeping at 1000°C.This specimen could be joined at lower diffusion welding temperature than coarse grained specimen, in which grain diameter was about 100 μm.(B) Cold-worked specimen started joining with onset of recrystallization into small grain.Diffusion weldability of this one was between that of fine grained one and that of coarse grained one.

Stress-relaxation in bending, at 673 K, of stress-relieved and cold-worked Zircaloy-4

Stress-relaxation measurements in bending, performed in Zry-4 at 673 K up to times of the order of 1000 h, in stress-relieved and cold-worked specimens, are reported. The stress-relieving treatments were made at three different temperatures: 773, 793 and 813 K, during 1 and 2 h. It is shown that the results can be analyzed in terms of a plastic equation of state suggesting a contribution of grain boundary sliding to the plastic deformation.

Ordering process in Cu-18Ni-26Zn alloy

An investigation was made of the effect of low-temperature annealing on the structure and properties of strips from a Cu-18Ni-26 Zn alloy. Specimens taken from strips cold rolled at reductions from 20 to 90% and specimens from recrystallized strips were annealed within the temperature range from 473 to 773 K for 10 h. Changes in the apparent elastic limit, yield strength, tensile strength, elongation, and hardness were determined. Electron-microscope observations of changes in the alloy structure during annealing were also made. It was found that annealing of the alloy at temperatures up to 673 K improved its mechanical properties, depending on the annealing temperature and time as well as on the degree of reduction. Maximum elastic limit, yield strength, tensile strength, elongation, and hardness values were obtained in specimens subjected to the highest reductions, which were annealed at a temperature of 573 K for up to 2h. It was proved that hardening of the alloy was responsible for the ordering process taking place during the annealing treatment. The structure of an ordered alloy is characterized by the dispersional distribution of super lattice domains in the disordered or short-range ordered matrix. Changes in mechanical properties of cold-worked specimens produced by the low-temperature annealing process result from changes caused by two overlapping processes: the recovery and ordering processes. An analysis of observed changes in the mechanical properties in relation to reduction and annealing temperature and time was made.

Effect of fast neutron irradiation on the recovery and recrystallization of niobium

Effect of fast neutron irradiation ( E ⪢ 0.1 MeV ) on the recovery and recrystallization of high purity niobium has been studied, utilising transmission electron microscopy and isochronal microhardness measruements, in the temperature range 25–1200°C for 1 h. The sigmoidal recovery curves show an initial increase in microhardness in the temperature range 25–300° C, which is related to the migration of interstitial impurity atoms O, C and N to dislocations and defect agglomerates produced by cold work and irradiation. Recrystallization in cold worked niobium proceeds by subgrain growth (controlled by subgrain coalescence) and strain induced grain boundary migration. Fast neutron irradiation to a fluence of φ = 1.3 × 10 18 nvt facilitates the formation of recrystallization nuclei by subgrain coalescence and thus accelerates the initial nucleation process in cold worked and irradiated specimens by about 150°C.

Influence of Cold Work on the In-Pile Stress Rupture Strength of 16Chromium-16Nickel-Niobium Austenitic Stainless Steels

Abstract The in-pile stress rupture strength of three casts of 16Cr-16Ni-Nb stainless steel was investigated. Different pretreatments were used, namely solution annealing plus various cold-working levels up to 20%. Thermal stability under irradiation and the high-temperature embrittlement of the different material conditions were studied. The specimens were irradiated in the form of electrically heated pressurized tubes at irradiation temperatures ranging from 615 to 720°C. Irradiation time was up to 4800 h, corresponding to a maximum fast neutron fluence (E > 0.1 MeV) of 8.2 × 1025 n/m2. Neutron irradiation caused a marked reduction in stress rupture strength for the solution-annealed and the cold-worked specimens. For the solution-annealed tubes, this reduction was mainly attributed to high-temperature embrittlement; the decrease in rupture strain was less pronounced for the cold-worked tubes. The reduction in stress rupture strength came mainly from an increase in creep rate. As far as stress rupture strength and creep strength at high irradiation temperatures were concerned, the 15% cold-worked 16Cr-16Ni-Nb steel was superior to the other cold-work levels.

The topographical structure of grain boundaries in tungsten

SUMMARYThis paper discusses the observation of a variety of topographical defects in grain boundaries in tungsten as a function of thermomechanical treatment. It is shown that, for both cold worked and crept specimen material, the boundary topography is highly complex, whereas for secondary recrystallized specimen materials the boundaries approximate to that expected for an ‘equilibrium’ configuration. However, no evidence for any period step structure was observed in this specimen material although, in this and all specimen states, grain boundary dislocations were frequently observed.

The effect of rolling texture on the magnetoelastic properties of constant modulus alloys

The effect of rolling texture on both the elastic modulus and the temperature coefficient of the elastic modulus (TC E ) in an age-hardenable Fe-Ni-Cr alloy was investigated. Experiments were conducted on cold-rolled and magnetized strips. Tuning forks which were machined from cold-rolled strip to have differently oriented rolling textures were also studied. Fifty percent cold-work was sufficient to produce a strong orientation effect, and, in addition, resulted in an uneven rolling texture over the cross section of the specimen. With an increasing degree of cold-work the elastic modulus and the texture sharpness increased, whereas the dependence of TC E on the texture orientation decreased. Young's Modulus still increased significantly when a magnetic field was applied to 90% cold-worked specimens. To suppress the ΔE-effect, degrees of cold-work greater than 90% are required.

Characteristics of primary recrystallization and its role in the sintering of metal powders

1. The recrystallization threshold of metal powder compacts is lower, and their temperature range of primary recrystallization wider, compared with cold-worked cast specimens. The regularities observed are attributable to the fact that the total amount of stored energy is greater in metal powders than in cast specimens and that the distribution of strain in a compact is very uneven. 2. A correlation is shown to exist between the recrystallization threshold of nickel powder compacts and the intensity with which they undergo densification during sintering. The role of primary recrystallization in sintering processes is examined. 3. A Permalloy alloy produced by sintering a powder mixture having the lowest recrystallization threshold exhibits the best magnetic properties. Thus, the maximum magnetic permability of specimens prepared from carbonyl iron and nickel powders is three to five times higher than that of an alloy produced from reduced powders.

The effect of metallurgical pretreatment on the kinetics of oxidation of iron at 700°C in pure gaseous oxygen

Continuous weight gain/time measurements have been made using cold rolled sheet samples of Hellefors “Remko” iron and Metals Research “Ferrovac E” iron. The effects of cold work on the oxidation characteristics of these irons in oxygen at 700°C were obtained by comparison with the oxidation kinetics of the same cold rolled specimens after various metallurgical treatments. It is suggested that the observed higher oxidation rate of untreated cold worked specimens is due to the greater vacancy sink concentration in the substrate metal. Since the impurity content of the two irons differed, an attempt was made to assess the effect of impurity on iron oxidation. In agreement with the observations of other workers the less pure “Remko” iron oxidized at the slower rate in both the cold worked and annealed state even though initially the oxidation rates were similar for both irons. It is suggested that after an initial period of oxidation the impurities accumulate at the substrate metal surface and act as barriers restricting further oxidation.

Density of Dislocations in Heavily Cold-Worked Cu-Al Alloys

Flow stresses and stored energies were measured for Cu-Al alloy specimens cold-worked by swaging. Two calorimetric methods, the differential calorimetry and a specific heat measuring technique, were used for the energy release measurements. After analyzing the present data together with those by other authors, it was found that the linear relationship between the flow stress and the square root of the dislocation density could be reasonably assumed even for heavily cold-worked specimens. The most probable relation between the resolved shear flow stress τ and the dislocation density N is (µ the shear modulus, b the Burgers vector) τ/µb=α√N+β; α=0.33, β=0.30×105. The value of the constant α is not inconsistent with the forest cutting theory of the work hardening.

Effect of primary recrystallization on the recovery stage-V of neutron irradiated copper

Abstract In these experiments we have measured the recovery spectra of the electrical resistivity after neutron irradiation at 78 °K of previously cold-worked copper specimens at room temperature and compared with well annealed specimens after neutron irradiation.

The oxidation of tantalum-tungsten polycrystals and single crystals in the temperature range 850°–1100°C

Single crystals and polycrystals of a Ta-3.9 at. % W alloy have been oxidised in the temperature range 850°–1100°C in oxygen at approximately 740 Torr. Recrystallised polycrystalline coupons exhibit cracking at the metal grain boundaries after oxidation at 950°C; this is believed to occur on cooling, and to be due to stresses imposed by the oxide growth process and the differential thermal contraction, and to the increase in resistance to fracture of the oxide and of the metal/oxide interface. However, cold-worked coupons show no sign of this effect. At 1050°C, recrystallised specimens ignited on admission of oxygen but cold-worked specimens oxidised without burning to produce compact, adherent scales. Single-crystal cylinders show similarly anisotropic scale-fracture morphologies to those observed in pure tantalum, but the anisotropy in oxidation rate is much less marked. After oxidation at 1100°C the single crystals fractured along {100}FX planes on cooling; this appeared to be initiated again by the high stresses and to propagate along suboxide platelets. There are considerable intrusions of pentoxide along the suboxide platelets. However, unlike the intrusions observed in the oxidation of niobium below 600°C, they do not appear to lead to metal fragmentation and an increase in rate; instead they appear to improve the scale/metal adhesion. This amount of tungsten reduces the oxidation rate of tantalum throughout this temperature range.

Relationship between residual microstrain and electrode potential in miller copper powder

X-ray diffraction line-broadening analysis has been used to measure the residual microstrain in ball-milled copper powder. Residual strain energy has been calculated from the microstrains obtained for different degrees of cold working. The stored energy produces changes in the electrode potential; a comparison has been made between the strain energy, obtained from the X-ray data, and the change of surface free energy, derived from the electrode potential measurements. The relationship between the strain energy and the change in the electrode potential, relative to that of the annealed state, is linear. A slow decrease of the electrode potential of the cold worked specimen takes place with time. This is attributed to the exchange current operating at the surface of the electroce.