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[en] The fracture behaviors of A516 Gr.70 steel for reactor support and Armco iron were investigated with special emphasis on the synergistic effects of interstitial impurities and radiation-induced defects in the dynamic strain aging (DSA) regime. The steel clearly depicted decreased fracture resistance in the upper shelf region where DSA is noted. In contrast, Armco iron showed improvement in fracture initiation energy during DSA. These contrasting behaviors are believed to arise from the existence or lack of precipitates for crack nucleation sites. The superimposed radiation effects revealed complex trends, while test results on the steel clearly demonstrated the suppression of DSA, resulting in apparent improvement in the ductility and fracture energy, due to the interaction of interstitials with radiation-induced defects
[en] The Mg-6%Gd-1%Zn alloy exhibits flow serrations when strained at intermediate temperatures due to the dynamic strain ageing phenomenon. Such flow serrations during deformation need the simultaneous and competitive movement of diffusing solutes and mobile dislocations. Although the alloy examined has a random texture, tension-compression asymmetry and significantly greater yield stress and work hardening in compression than in tension have been observed. During deformation at intermediate temperatures, and independently of the nature of the stress (tension or compression), the activation of -dislocations and tensile twin systems has been observed. The volume fraction of twins is always higher, however, in the case of compression testing. At the intermediate temperatures where flow serrations are observed, Gd and Zn atoms pin dislocations as well as twins. Above 250 °C, the flow serrations disappear and g´ and g´´ precipitates form in the basal plane which increase work hardening.
[es]La aleación Mg-6%Gd-1%Zn muestra el fenómeno de serrado durante la deformación a temperaturas intermedias debido al proceso de envejecimiento dinámico provocado por la presencia de átomos de soluto en solución sólida y dislocaciones móviles. Aunque la aleación tiene una textura al azar, se observa un comportamiento diferente en tracción y en compresión. El límite elástico y el endurecimiento es mayor cuando la aleación se ensaya en compresión. Durante la deformación a temperaturas intermedias se ha observado la activación de dislocaciones tipo < a > y maclas de tensión, independientemente del signo de la carga. Sin embargo, la fracción en volumen de maclas es siempre mayor cuando el material se somete a compresión. A temperaturas intermedias, los átomos de Gd y Zn anclan tanto las dislocaciones como las maclas. Por encima de 250 ºC, el fenómeno de serrado desaparece y la presencia de precipitados g´ y g´´ en el plano basal aumenta el endurecimiento.
[en] Aging at temperatures below 200 deg C causes hardening in polycristalline niobium previously strained at room temperature according to the oxygen content. The pinning ratio is maximum for a critical value of O2, over which external atmospheres are formed which reduce the interaction with dislocations
[fr]Un vieillissement effectue a temperature inferieure a 200 deg C provoque dans le niobium polycristallin, predeforme de 6% a 20 deg C, un durcissement directement fonction de la teneur en oxygene. Le taux d'ancrage des dislocations passe par un maximum pour une teneur critique en O2 au-dela de laquelle se forment des atmospheres exterieures conduisant a une interaction moins efficace
[en] Alloy 800 (UNS NO8800) is used in the nuclear steam generators as the tube material because of its inherent resistance to stress corrosion and high temperature mechanical properties. Tubes are in cold worked (CW) condition and in 'U' bend form. The external surface is shot-peened with glass beads to further improve the resistance to fatigue and stress corrosion. The material is well studied with respect to its corrosion, mechanical and creep properties. The behavior of this material under cyclic stress is not well documented. The influence of the elevated temperature on the cyclic fatigue and fracture behavior is studied in the present work. One of the important observations made during the study was existence of dynamic strain aging (DSA) phenomenon at 350-550°C in this alloy. The LCF behavior at elevated temperature is studied in light of several phenomena like softening in CW structure at lower temperature, strengthening at elevated temperature due to DSA and precipitation, and lowering of fatigue life at elevated temperature. The drop in the peak stress in LCF test with number of cycles indicates rapid flow softening occurring at room temperature. Similar behavior was observed at 550°C. But, in the case of the test at 350°C, softening is delayed or there is an inherent resistance to the softening process due to DSA. The slip markings observed reveal a strongly localized surface topology (extrusions and intrusions) being typical of persistent slip bands. (author)
[en] Crack jumps occurring in concert with stable cracking in a low-carbon steel, used in nuclear power plants, are examined. These jumps occur in steels susceptible to dynamic strain aging at liquid water reactor temperatures. Through fractographic studies, a connection between the nature of cracking and fracture morphology is attempted. Numerical simulation of a crack growth experiment is carried out. The results of the study indicate that there may be certain conditions, such as crack-tip constraint and triaxiality and the dominance of elastic stored energy over plastic dissipation (in addition to the material's susceptibility to dynamic strain aging), which need to be met for crack jumps to occur. The numerical results also indicate that approximate methods, such as ASTM E1152 method, for estimating driving force for growing cracks may not be viable for materials susceptible to dynamic strain aging
[en] Zr and Ti belong to the same group IV of the classification but the radius of the octahedral interstitial site is 0.67A for Zr and 0.64A for Ti. This suggests a difference between their strain-aging behaviors, particularly in the case of the insertion of oxygen atoms in Ti and Zr lattices. The influence of various parameters on strain-aging is examined: time, temperature, stresses. The analysis of the results obtained show that the strain-aging of Zr and Ti is related to the local rearrangement of oxygen atoms leading to the blocking of dislocations
[fr]Le zirconium et le titane appartiennent au meme groupe IV de la classification, mais le rayon du site interstitiel octaedrique est de 0,67A pour Zr et de 0,64A pour Ti. Ceci laisse prevoir une difference de comportement au durcissement par vieillissement apres deformation, en particulier lors de l'insertion de l'oxygene dans Ti et Zr. L'influence de divers parametres a ete examinee: temperature, duree, contraintes appliquees. L'analyse des resultats obtenus montre que le durcissement par vieillissement apres deformation de Ti et Zr est lie a une remise en ordre a courte distance des atomes d'oxygene entrainant un blocage des dislocations
[en] Torsion testing is used to determine the quality of steel wire used for beads in pneumatic tires. However, strain aging (dynamic and static) caused by interstitial carbon and nitrogen atoms bound to mobile dislocations increases yield strength and decreases bead formability. Processing parameters of bead wire, such as line speed, lead bath temperature and wire diameter, were investigated, and theoretical calculations were made to estimate the effect of these parameters on strain aging. Nitrogen concentration was measured in bead wire samples with varying numbers of twists to failure during torsion testing. Surface morphologies of twisted bead wires were examined by scanning electron microscopy. Experimental data showed that torsional properties of bead wire were a function of stress relief temperature on and theoretical calculations showed that line speed and temperature have to be optimized for optimum torsion ductility.
[en] Alloy 617M is the proposed candidate structural material for Advanced Ultra Supercritical (A-USC) Power Plant. The working temperature is approximately 973 K and hence the study around this is of technological importance. The start-ups, shut-downs and power transients of components produce low cycle fatigue (LCF) loading conditions and hence study on the LCF performance of this alloy is necessary from the design point of view. Objective of the present work is to perform fully reversed total strain controlled LCF experiments on Alloy 617Mat 300 K, 573 K and 773 K and strain amplitudes of ±0.25%, ±0.4%, ±0.6% and ±0.8%. The tests were conducted using triangular waveform with a strain rate of 3 x 10"-"3 s"-"1. The fatigue properties and the changing microstructure were related with the underlying deformation and damage mechanisms. Changes in the cyclic stress response, hysteresis loop evolution and microstructural studies have been used to understand the fatigue behavior of this material. Serrations observed in the hysteresis loops for the tests at 773 K have been ascribed to the occurrence of Dynamic Strain Aging (DSA). (author)
[en] Free nitrogen in pressure vessel steel might cause embrittlement through dynamic strain aging. Steels from 7 nuclear reactors have been analyzed. In most of the 19 samples sufficient nitrogen was present for this kind of embrittlement. (P.Aa.)
[en] The low cycle fatigue (LCF) behaviour of a near α Timetal 834 titanium alloy has been studied under fast-slow and slow-fast waveforms at peak dynamic strain aging (DSA) temperature (450 oC). The alloy exhibits inferior fatigue life under slow-fast waveform as compared to fast-slow waveform. This is attributed to slow rate of straining in tensile direction under slow-fast waveform which allows more interaction between mobile dislocations with solute atoms thereby enhancing the strength of DSA. A life prediction model based on the dynamic viscosity with waiting time correction as per dislocation kinetics predicts fatigue lives reasonably well under pronounced DSA effect.