Inverse strain rate effect on cyclic stress response in annealed Zircaloy-2 G. Sudhakar Rao a , Preeti Verma a , J.K. Chakravartty b , Saibaba Nudurupati c , G.S. Mahobia a , N.C. Santhi Srinivas a , Vakil Singh a,⇑ a Center of Advanced Study, Department of Metallurgical Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India b Mechanical Metallurgy Group, Bhabha Atomic Research Center, Trombay 400 085, Mumbai, India c Nuclear Fuel Complex, Hyderabad 500 062, India article info Article history: Received 9 June 2014 Accepted 17 November 2014 Available online 22 November 2014 abstract Low cycle fatigue behavior of annealed Zircaloy-2 was investigated at 300 and 400 °C at different strain amplitudes and strain rates of 10 2 , 10 3 , and 10 4 s 1 . Cyclic stress response showed initial hardening with decreasing rate of hardening, followed by linear cyclic hardening and finally secondary hardening with increasing rate of hardening for low strain amplitudes at both the temperatures. The rate as well the degree of linear hardening and secondary hardening decreased with decrease in strain rate at 300 °C, however, there was inverse effect of strain rate on cyclic stress response at 400 °C and cyclic stress was increased with decrease in strain rate. The fatigue life decreased with decrease in strain rate at both the temperatures. The occurrence of linear cyclic hardening, inverse effect of strain rate on cyclic stress response and deterioration in fatigue life with decrease in strain rate may be attributed to dynamic strain aging phenomena resulting from enhanced interaction of dislocations with solutes. Fracture surfaces revealed distinct striations, secondary cracking, and oxidation with decrease in strain rate. Deformation substructure showed parallel dislocation lines and dislocation band structure at 300 °C. Persistent slip band wall structure and development of fine Corduroy structure was observed at 400 °C. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Zirconium and its alloys are widely used in nuclear reactor core components like fuel tubes, pressure tubes and calandria tubes both in light and heavy water reactors. These are well known for their inherent property of low neutron absorption cross section, good corrosion resistance, and acceptable mechanical properties like fatigue, and creep resistance in the complex nuclear reactor environment. In service the core components of zirconium alloys experience temperature of approximately 300 °C and it has been established that the core components of a reactor are susceptible to low cycle fatigue and it leads to their ultimate failure [1]. There have been several studies on low cycle fatigue of zirconium alloys [1–6]. Constant amplitude strain controlled fatigue tests were con- ducted on the Zircaloy-2 fuel tubes at 300 and 350 °C using the modified Moore type rotating bending fatigue testing machine. The dependence of fatigue life on strain range was expressed as N f 0.54 De p = 0.65 and N f 0.53 De p = 0.62 at 300 °C and 350 °C respec- tively. Cyclic stability was reported based on the monotonic strain hardening parameters [1]. Temperature dependence of fatigue life at constant strain amplitude showed higher fatigue life due to dynamic strain aging between 20 and 300 °C and fall in fatigue life above 300 °C [2]. Patterson studied low cycle fatigue properties of recrystallized Zircaloy-2 using a bending testing rig from room temperature to 400 °C, and showed that this alloy obeyed Coffin–Manson relation- ship [3]. Also irradiation effect on fatigue life was studied at 300 °C and no effect was found on fatigue life. Furthermore, twinning was observed along the path of fracture but not at the crack tip, and it was suggested that there was no effect of twining in promoting fatigue cracking [3]. Lin and Haicheng studied low cycle fatigue behavior of Zirca- loy-4 in recrystallized and stress-relieved conditions at 400 °C in terms of fatigue life and deformation behavior [4]. While there was cyclic hardening in the recrystallized condition, based on the cyclic stress strain curves at that temperature, there was cyclic softening in the stress relieved condition. Fatigue life was found to be higher in case of recrystallized condition compared with that in the stress relieved condition. Elongated dislocation cells and dislocation dipoles perpendicular to the cell boundary were observed in the recrystallized condition whereas rectangular cells were observed in case of stress relieved condition, and were attributed to deformation by prismatic and pyramidal slip respectively. http://dx.doi.org/10.1016/j.jnucmat.2014.11.058 0022-3115/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Fax: +91 542 2369478. E-mail address: vsingh.met@itbhu.ac.in (V. Singh). Journal of Nuclear Materials 457 (2015) 330–342 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat