Dynamic Strain Aging of Ni-base Alloys Inconel 600 and 690

Dynamic strain aging (DSA) and jerky flow phenomena in the commercial Ni-base alloys Inconel 600 and 690 have been investigated. Tensile tests were performed in the strain rate range of 10 to 10 s at temperatures of 100 – 600 C. No remarkable difference in the DSA behaviour of Inconel 600 and 690 alloys was observed. Tensile properties of the studied alloys in the DSA temperature range, and type and characteristics of jerky flow were obtained and analysed. The map for occurrence of serrated flow as a function of strain rate and temperature was built for Inconel 600 and 690 alloys and the activation enthalpies of dynamic strain aging appearance were found to be 1.6 eV for both materials. The obtained enthalpy of DSA appearance corresponds well to the enthalpy of carbon diffusion (1.76 eV), which was estimated by means of internal friction for the studied Inconel 600 alloy. The mechanisms of DSA based on interstitial atom interactions with dislocations in the studied alloys are discussed and the results are analysed based on the susceptibility of these alloys to environmentally assisted cracking (EAC). Introduction The occurrence of dynamic strain aging (DSA) has been extensively studied in many engineering alloys, which are applied in the nuclear power industry [1 7]. Recently quite much attention is paid to austenitic stainless steels and especially AISI 316L steel [8 12]. There are only a few studies published on this phenomenon in Inconel alloys which play an important role in the nuclear power industry [13 18]. DSA occurs in alloys containing solute atoms which can rapidly and strongly segregate to dislocations during straining. This phenomenon leads to an inhomogeneous plastic flow or serrated yielding during straining at elevated temperatures and results often in a remarkable degradation of mechanical properties. In different materials various solute atoms and a number of mechanisms can participate in the solute atom – mobile dislocation interactions. For instance, in austenitic stainless steels carbon and nitrogen interstitials or interstitial-vacancy pairs and at higher temperatures substitutional solute atoms such as Cr and Mo can interact with dislocations and be the cause of the appearance of DSA serrations [8 12]. In nickel-base alloys and superalloys interstitial (H and C) and substitutional (Cr) solute atoms are usually responsible for jerky flow [13 18]. Mulford and Kocks have found that DSA phenomenon takes place in Inconel 600 alloy over a wide range of temperature and strain rate [13]. Interstitials play a key role in DSA of pure Ni which has been shown for Ni-C and Ni-H interstitial alloys [14]. In Ni-C alloys the lower critical temperature of DSA occurrence had an activation enthalpy of about half of that for bulk diffusion of carbon and it was associated with carbon diffusion in the dislocation core. Additionally it was observed that serrations were not affected by quenched-in vacancies [15]. Temperature of DSA occurrence in Inconel 600 alloy was found to be close to that of austenitic stainless steel and markedly higher than that of Ni-C binary alloy [15]. Based on this result it was concluded that C-Cr and N-Cr complexes may also play an important role in the DSA mechanism. Alloy 600 is used as material, e.g., for vessel head penetrations in pressurized water reactors (PWR). Primary water stress corrosion cracking (PWSCC) as well as intergranular stress corrosion cracking (IGSCC) in components other than steam generators are the problems of intensive research for nuclear power plants. Role of EAC mechanisms related to crack tip plastic deformation in the highly deformed material of the plastic zone is the key question of this study. Aim of the present investigation was to study the DSA phenomenon in Inconel 600 and 690 alloys and its effects on their mechanical properties. By means of internal friction (IF) technique the investigation of DSA mechanism of Inconel alloys was extended to interstitial atom diffusion. Experimental methods Two commercial Ni-base alloys Inconel 600 and Inconel 690 were studied. Chemical compositions of the materials are shown in Table 1. A series of uniaxial tensile tests was conducted in the laboratory air under axial displacement control with constant cross-head speeds corresponding to initial strain rates of 1×10, 1×10, 1×10 and 1×10 s. The tests were carried out over a range of temperatures from ambient up to 700 C. All tests were performed in laboratory air. Table I. Chemical compositions of the studied alloys in weight %. Ni Cr Fe Cu Al Co Nb Ti C Si Mn S P Inconel 600 rest 16.1 8.1 0.01 0.17 0.29 0.01 0.16 0.04 0.32 0.22 <0.01 <0.01 Inconel 690 rest 27.7 8.4 0.01 0.18 0.07 0.01 0.14 0.01 0.23 0.22 <0.01 <0.01 Proceedings of the 12th International Conference on Environmental Degradation of Materials in Nuclear Power System – Water Reactors – Edited by T.R. Allen, P.J. King, and L. Nelson TMS (The Minerals, Metals & Materials Society), 2005