PhD Candidate , North Carolina State University
Creep Properties and Dynamic Strain Aging in Austenitic Stainless Steels
Stainless steels may be classified by their crystalline structure into three main types: austenitic, ferritic and martensitic. Austenitic stainless steels are frequently used for high temperature applications including conventional and nuclear power plants due to its higher creep properties coupled with more corrosion/oxidation resistance at high temperature than ferritic steels. Moreover, newly developed austenitic stainless steels with high Chromium and Nickle contents have superior mechanical properties at high temperature relative to the widely used stainless steel such as 304 and 316 type stainless steels make it primary options for Gen-IV nuclear reactors structural application. In general, the materials are deformed faster at high temperature. The time-dependent plastic (i.e. permanent) deformation of materials at a constant temperature and constant stress or load is known as creep and it is playing the major rules for design criteria at high temperature applications. The plastic elongation during creep test is a result of several mechanisms operating in the microstructure levels and understanding the creep mechanisms in austenitic stainless steels is essential not only to predict the performance of the steel under the operating conditions but also to help improve the microstructural design of such advanced creep resistant metals. In addition to the importance of creep properties at high temperature applications, austenitic stainless steels exhibit serrations in stress-strain curves commonly known as Portevin-Le Chatelier effect. The serrated yielding in austenitic stainless steel is commonly attributed to dynamic strain aging (DSA) resulting from the interaction between diffusing solute atoms and mobile dislocations during plastic deformation at intermediate temperatures. The DSA has been shown to degrade mechanical properties such as ductility and fracture toughness so investigation into this phenomenon should be carried out for a comprehensive understanding of the underlying mechanisms as well as for design and safety considerations. To investigate the creep mechanisms and dynamic strain aging in austenitic stainless steels used in nuclear power plant, extensive experimental and theoretical works have been done through the last decades. In this pinboard, recent works in these properties are to be mentioned and discussed.
Abstract: The stress-strain behaviour of three nitrogen-bearing low-nickel austenitic stainless steels has been investigated via a series of tensile tests in the temperature range 298–473 K at an initial strain rate of 1.6×10−5s−1. Experimental stress-strain data were analysed employing Rosenbrock's minimization technique in terms of constitutive equations proposed by Hollomon, Ludwik, Voce and Ludwigson. Ludwigson's equation has been found to describe the flow behaviour accurately, followed by Voce's equation. The resultant strain-hardening parameters were analysed in terms of variations in temperature. A linear relationship between ultimate tensile stress and the Ludwigson parameters has been established. The influence of nitrogen on the Ludwigson modelling parameters has also been explained.
Pub.: 01 Jan '95, Pinned: 03 Jul '17
Abstract: Zirconium alloys, commonly used as cladding tubes in water reactors, undergo complex biaxial creep deformation. The anisotropic nature of these metals makes it relatively complex to predict their dimensional changes in-reactor. These alloys exhibit transients in creep mechanisms as stress levels change. The underlying creep mechanisms and creep anisotropy depend on the alloy composition as well as the thermomechanical treatment. The anisotropic biaxial creep of cold-worked and recrystallized Zircaloy-4 in terms of Hill’s generalized stress formulation is described, and the temperature and stress dependencies of the steady-state creep rate are reviewed. Predictive models that incorporate anelastic strain are used for transient and transients in creep.
Pub.: 01 Oct '99, Pinned: 03 Jul '17
Abstract: All materials exhibit Newtonian viscous creep behavior at low stresses and high temperatures. We review here such creep behaviors in metals comprising of pure metals and alloys. The underlying creep mechanism(s) depends mainly on the grain size and test temperature while other factors such as the initial dislocation density might also be a factor. Coble creep due to diffusion of point defects through grain boundaries is known to be the dominant creep mechanism in metals with very small grain sizes and relatively low temperatures while Nabarro-Herring creep becomes important for intermediate grain sizes and/or high temperatures. Large grain size and bulk single crystalline metals exhibit Harper-Dorn creep due to dislocation motion rather than point defect diffusion-dominated mechanisms albeit the underlying mechanism is still unclear. Microstructural studies of the specimens deformed in the Harper-Dorn regime have provided some insights. Recent studies suggest that microstructural characterization of deformed specimens is necessary for accurate determination of the rate controlling mechanism. The aim of this paper is two fold namely, to first review the viscous creep mechanisms and to present recent results on Ti3Al2.5V alloy emphasizing the importance of post creep microstructural characterization in establishing the rate controlling mechanism(s).
Pub.: 23 Nov '10, Pinned: 03 Jul '17
Abstract: Only a limited number of creep investigations have been carried out on nanocrystalline materials to-date. These studies have remained largely inconclusive in establishing the mechanisms of creep in nanocrystalline materials. The stress exponent and activation energy values obtained from nanocrystalline materials do not correlate well with conventional well established creep models. Furthermore discrepancies between experimentally determined deformation rates and theoretical predictions suggest that an entirely new mechanism of creep could be operational in these exotic materials. In this work we aim to develop an understanding of the creep behavior of nanocrystalline materials by considering a stress assisted grain growth mechanism that has been recently identified in these materials. In turn a model has been developed that provides a quantitative understanding of some of the observations made in creep literature.
Pub.: 23 Nov '10, Pinned: 03 Jul '17
Abstract: We have examined irradiation induced creep of graphite in the framework of transition state rate theory. Experimental data for two grades of nuclear graphite (H-337 and AGOT) have been analyzed to determine the stress exponent (n) and activation energy (Q) for plastic flow under irradiation. We show that the mean activation energy (Q) lies between 0.15–0.32 eV with a mean stress-exponent of 1.0±0.2. A stress exponent of unity and the unusually low activation energies strongly indicate a diffusive defect transport mechanism for neutron doses in the range of 3 to 4×1022n/cm2.
Pub.: 20 Feb '16, Pinned: 03 Jul '17
Abstract: High temperature tensile creep tests were conducted on AZ31 Magnesium alloy at low stress range of 1 ~ 13 MPa to clarify the existence of grain boundary sliding (GBS) mechanism during creep deformation. Experimental data within the GBS regime shows the stress exponent is ~2 and the activation energy value is close to that for grain boundary diffusion. Analyses of the fracture surface of the sample revealed that the GBS provides many stress concentrated sites for diffusional cavities formation and leads to premature failure. Scanning electron microscopy images show the appearances of both ductile and brittle type fracture mechanism. X-ray diffraction line profile analysis (based on Williamson-Hall technique) shows a reduction in dislocation density due to dynamic recovery (DRV). A correlation between experimental data and Langdon's model for GBS was also demonstrated.
Pub.: 19 May '16, Pinned: 03 Jul '17