K. L. MURTY, North Carolina State University, USA, S. GOLLAPUDI, Massachusetts Institute of Technology, USA, K. RAMASWAMY, Bhabha Atomic Research Center, India, M. D. MATHEW, Indira Gandhi Center for Atomic Research, India and I. CHARIT, University of Idaho, USA

DOI: 10.1533/9780857097453.1.81

Abstract: The time-dependent deformation of materials or creep governs the useful life of many engineering structures. It assumes even higher significance in the case of structures constituting a nuclear reactor, wherein materials bombarded with neutrons develop defects that assist faster diffusion leading to greater plastic deformation. As a result, an understanding of the creep deformation process and factors controlling it is necessary for gauging the usefulness of materials in a nuclear reactor as well as for predicting life-times of various structures. Thus in this work we discuss the various mechanisms of creep, the rate controlling factors, deformation mechanism maps and useful life prediction methodologies. We also identify a few cases where direct application of simple creep correlations might not be feasible. Finally, we discuss the various factors that control the creep behavior of materials in light water reactors.

Key words: creep, diffusion creep, dislocation creep, deformation mechanism maps, modeling, zirconium alloys, stainless steels, irradiation creep.

3.1 Introduction

Creep is time-dependent plastic strain under a constant load/stress at a given temperature and often becomes the life limiting criterion for many structures that experience loads and temperatures, and becomes signifi­cant for materials in light water reactors (LWRs) due to imposed radiation effects. A thorough understanding of the plastic deformation behavior of materials is essential for the sound design of engineering structures. Fail-safe designs are based on the ability to predict the response of a structure to applied loads and ensuing plastic deformation. While brittle materials such as ceramics fail after relatively low plastic strains, a significant number of engineering materials such as metals and alloys are characterized by large scale plastic deformation leading to failure. The extent of deformation is controlled by intrinsic factors such as bond strength, presence of secondary phases and defect concentration. At the same time extrinsic factors such as applied loads, temperature, deformation rates and geometry of the structure also determine the amount of plastic deformation. It has been well estab­lished that high applied loads and temperatures generally accelerate the rate of plastic deformation. This is because high temperatures and stresses provide the necessary activation energy required for defects to overcome barriers to plastic deformation. While plastic deformation at room tempera­ture or low homologous temperatures (T/Tm) occurs when the applied stress exceeds the yield stress ay, deformation at high temperatures can occur at stresses significantly smaller in comparison to the yield stress. The branch of metallurgy which attempts at understanding material deformability at high homologous temperatures and small applied stress has come to be known as creep. The kinetics of deformation processes become important with increasing temperatures and hence creep is defined as the time dependent plastic deformation of a material under constant load or stress.

The earliest studies on time dependence of plastic strain were carried out by Andrade.1 The time dependence of elongation under tensile loads was investigated at constant temperature. Andrade observed that the total deformation could be divided into three periods: (a) immediate extension upon loading (mainly elastic with relatively small instantaneous plastic), (b) an initial flow which gradually disappears and (c) a constant flow which takes place throughout the elongation. Subsequent studies by Hanson and Wheeler2 showed the presence of a period where the extension increases continuously until fracture. This period was found to occur following the period of constant flow and was understood to be due to decreased cross-sectional area accompanying the elongation. At constant loads, the cross-sectional area decrease leads to the increase in effective stress and a corresponding increase in strain rate.