[en] The formation of a gradient structure during the severe plastic deformation (SPD) by high-pressure torsion (HPT) of low- and medium-carbon steels under the influence of dynamic strain aging (DSA) is studied. Deformation mechanisms under various regimes of deformation processing are analyzed. The temperature ranges of the DSA effect during the ECAP processing of steel 10 and the fact of the formation of a gradient structure during the HPT processing under these conditions are established. It is shown that the deformation of carbon steels in the DSA temperature range enables controlling the structure of the deformed state. (paper)

No abstract available

[en] Using digital image correlation (DIC) analysis in an Fe-19Cr-13Ni-0.2C austenitic stainless steel (mass%), the tensile deformation behavior accompanied with the Portevin-LeChatelier (PLC) effect was investigated at high temperatures of 723-823 K under various applied strain rates of 10⁻⁴ s⁻¹–5 × 10⁻³ s⁻¹. With the help of the DIC analysis, the global strain in the gauge part of a tensile specimen and the local strain in a PLC band were precisely measured regardless of the lowering machine stiffness at a high temperature. The high-temperature DIC analysis could identify the critical strain and strain rate conditions where stress-strain curve indicated serrated flow. The analytical results revealed that dynamic strain aging in the austenitic stainless steel was controlled by the pipe diffusion of Cr atoms. Further, it was also confirmed that the local strain gently increased in a gauge part through a PLC band propagation, which promoted a repeatable A-type PLC band propagation in the same direction. Furthermore, the local strain was discontinuously developed by the repeatable formation of PLC bands; consequently, the local strain rate became remarkably high within a PLC band. In addition, the local strain rate in the PLC band monotonically increased with the increase in the number of PLC bands. From this viewpoint, it was discovered that necking deformation occurs suddenly at the PLC band, when the local strain rate exceeds the critical value, where dynamic strain aging is ineffective.

[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

[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] The 9 Cr-1 Mo steel exhibits dynamic strain aging akin to substitutional alloys but the activation energy is consistent with carbon diffusion. (orig.)

[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] 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.)