Anelastic recovery of pure magnesium quantitatively evaluated by acoustic emission

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Manabu Enoki Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan (Received 20 May 2011; accepted 3 October 2011)

Anelastic recovery of pure magnesium was monitored quantitatively by means of acoustic emission (AE) in cyclic compression–quick unloading–recovery process. The influences of grain size, strain rate, and the strain-controlled cyclic process on anelastic recovery were analyzed in details. A detwinning model in anelastic recovery process was proposed, and the results showed good agreement with the experimental data. Three stages of anelastic recovery behavior as a function of strain were observed: stages I and III were considered to be dominated by various detwinning processes and stage II was dominated by thermal motion of dislocation. A quantitative relationship between anelastic recovery strain and AE signals was obtained, and from which the anelastic recovery strains from dislocation motion and detwinning were separated for the first time. In the strain-controlled cyclic process, both the amount of AE signals and the anelastic recovery strain were observed to decrease while the fraction of anelastic recovery strain from dislocation motions was observed to decrease more rapidly than that from detwinning with increasing cyclic number.

I. INTRODUCTION

Anelastic recovery of materials has several origins such as reversible dislocation motion,1 twinning,2 stressinduced phase transitions, etc.3 The anelastic recovery by dislocation motion is essentially a thermal-activated process through their interaction with defects, the speed of which is dependent on both the environmental temperature and the holding time.1 Anelastic recovery from twinning is due to the movement of twin boundary2; twinning in the deformed state is not stable, and a reversal driving force in unloading may cause it to disappear or shrink as a result of movements of the twinning boundaries.2 Until now, the anelastic recovery of magnesium and its alloys was considered to be the result of detwinning when an external stress is removed or reduced, and the detailed mechanism investigated through microstructure observation is difficult to be carried out due to the short duration of this process. Caceres et al. showed that the hysteresis loops of pure magnesium in cyclic deformations are related to f10 12g twin, which grows when materials are stressed and partially reverts when unloaded.4 The behavior and mechanisms of magnesium in loading and unloading processes are different from those of materials that do a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.369 3098

J. Mater. Res., Vol. 26, No. 24, Dec 28, 2011

http://journals.cambridge.org

Downloaded: 13 Mar 2015

not exhibit anelastic recovery. For example, the response characteristics and energy absorption behavior of magnesium are strongly affected by cyclic movements of twinning boundaries when it is used as a damping structure. Therefore, a precise understanding in the dynami