Serrated plastic flow

Abstract

This paper attempts an assessment of the current understanding of the phenomenon of "serrated plastic flow", which manifests itself as serrations, load drops, jerkiness or other discontinuities in the stress-strain curves obtained in constant extension rate tensile tests, and as sudden bursts of strain in constant loading rate tests and in constant load (stress) creep tests (the so called staircase creep). Though one can identify at least seven physical processes that can cause serrations, the discussion here is restricted mainly to serrated yielding in tension tests originating from dynamic strain ageing (DS^). The characteristics of the five types of serrations that have been identified so far and the experimental conditions under which they occur are discussed. The various models of serrated flow that have been put forward are reviewed critically. Some recent results on 316 stainless steel are presented to illustrate the effects of grain size, temperature and strain rate on serrated flow. Manifestations of DS^ other than serrations such as a negative strain rate sensitivity, positive temperature dependence for flow stress and work hardening, and the ductility minimum are also discussed. Finally the various issues to be resolved are enumerated.

Serrated Yielding in Alloy 718

“…The inverse behavior is noticed at high temperatures in the present study in the case of only ST and STA specimens and the average activation energy is around 210 kJ/ mole (Table 1). This value is very close to the activation energy for diffusion of solutes like chromium, niobium, and molybdenum in nickel [18]. In fact, Hale et al [6] have explained the appearance of serrations at high temperature to the diffusion of carbide-forming chromium solute to dislocations.…”

“…2b and c). Generally, five types of serrations, termed A, B, C, D and E, have been observed in the stress strain curves depending upon the test temperature and strain rate [18]. The serrations of types A, B and C, are commonly observed, while D and E type serrations are rarely seen.…”

“…This type of behavior has been reported by Venkadesan et al [19] in 316 stainless steel. A broad view of the test temperature regimes for the occurrence of serrations and their nature, for different strain rates, as per the classification scheme given by Rodriguez [18], is presented in Fig. 2d to e.…”

The influence of C and Si on the flow behavior of NiAl single crystals

“…Such observations are indicative of the occurrence of DSA [22][23][24]. Classical theory [22,23] indicates that during DSA, dislocation motion is characterized by a waiting time, L, during which dislocations are temporarily arrested at obstacles in the slip path.…”

Dynamic strain aging in Haynes 282 superalloy

“…This is well known to occur in precipitation hardened Ni-base superalloys, and has been correlated to the presence of carbon at intermediate temperatures (< 550 • C) [4][5][6][7][8][9] and solute elements (typically Cr or Mo) at higher temperatures (> 550 • C) [6][7][8][9]. Except for the serrated yielding (Portevin-Le Chatelier effect) DSA typically leads to increased stresses and reduced ductility during monotonic loading [10]. During LCF increased cyclic hardening, reduced plastic strain amplitudes and decreased fatigue lives are typically observed [11].…”