Materials Science and Engineering A 474 (2008) 363–370 Dynamic strain ageing of an austenitic superalloy—Temperature and strain rate effects Ajit K. Roy ∗ , Joydeep Pal, Chandan Mukhopadhyay Department of Mechanical Engineering, University of Nevada, Las Vegas (UNLV), 4505 Maryland Parkway, Box 454027, Las Vegas, NV 89154-4027, United States Received 5 December 2006; received in revised form 4 April 2007; accepted 15 May 2007 Abstract The tensile data involving austenitic Alloy C-276 suggest that this alloy is capable of maintaining appreciable structural strength at temperatures relevant to the sulfuric acid decomposition process related to the nuclear hydrogen initiative. Reduced failure strain and formation of serrations, characteristics of dynamic strain ageing (DSA), were noted within susceptible temperature regimes. An average activation energy of 55kJ/mol, and work hardening index ranging from 0.68 to 0.75 were determined as functions of different testing temperature and strain rates. The occurrence of DSA was also associated with enhanced dislocation density. Depending on the testing temperature, a combination of ductile and intergranular brittle failures was observed with the tested specimens. © 2007 Elsevier B.V. All rights reserved. Keywords: Alloy C-276; Dynamic strain ageing; Activation energy; Work hardening index; Dislocation density 1. Introduction A thermochemical process, known as the sulfur–iodine (S–I) cycle, is currently being considered to generate hydrogen (H 2 ) using nuclear heat. The S–I process involves chemical reactions to form and decompose sulfuric acid (H 2 SO 4 ) and hydrogen iodide at elevated temperatures [1]. A maximum temperature of 950 ◦ C has been proposed to achieve the highest possible effi- ciency in H 2 generation using this process. The identification and selection of suitable structural materials to accommodate chemical reactions at such an elevated temperature constitute a significant challenge to materials scientists. This challenge stems from the difficulty of identifying structural materials pos- sessing the desired tensile properties at elevated temperatures and superior corrosion resistance in the presence of aggressive chemical species. This investigation is focused on the charac- terization of Alloy C-276 for application in nuclear hydrogen initiative at temperatures relevant to the H 2 SO 4 decomposition process. The identification of Alloy C-276 was based on its excellent tensile properties at elevated temperatures, and mod- erate resistance to degradations in many hostile environments, ∗ Corresponding author. Tel.: +1 702 895 1463; fax: +1 702 895 5199. E-mail address: aroy@unlv.nevada.edu (A.K. Roy). as cited [2] in the open literature. This paper presents the results of tensile testing at temperatures up to 1000 ◦ C. A mechanis- tic understanding on the plastic deformation of this alloy has been developed as functions of temperature and strain rate. Fur- ther, the characterization of defects such as dislocations, and fractographic evaluations of all tested specimens are presented. 2. Experimental procedures Two different heats of nickel-base austenitic Alloy C-276 were procured from a vendor in a heat-treated condition. Their chemical compositions are given in Table 1. The thermal treat- ment imparted to these materials consisted of solution annealing at 2050 ◦ F (1121 ◦ C) followed by rapid cooling. The metallurgi- cal microstructures of these heat-treated materials consisted of large austenitic grains and annealing twins, as expected. No addi- tional thermal treatments were given to these materials prior to the machining of the specimens used in tensile testing. Smooth cylindrical specimens were machined from the round bars of both heats in such a way that the gage section was parallel to the longitudinal rolling direction. A ratio of 4 was maintained between the gage length and the gage diameter according to the ASTM designation E 08-2004 [3]. These cylindrical specimens had 4-in. (101.6 mm) overall length, 1-in. (25.4 mm) gage length and 0.25-in. (6.35 mm) gage diameter. 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.05.056