Serrated flow in nanostructured binary Mg-Al alloys

Abstract

Serrated flow, also known as the Portevin-Le Chatelier effect, has been extensively investigated in binary Mg-alloys such as Mg-Ag, Mg-Y and Mg-Li alloys. Interestingly, however, this effect has not been observed in Mg-Al alloys. In this work, we report the appearance of serrations in nanostructured Mg-Al alloys processed by cryomilling and spark-plasma-sintering during in-situ SEM microcompression tests at room temperature and strain rates between 2 × 10 −3 and 10. The observed serrated stress-strain behavior is attributed to the dynamic strain aging by which interstitial O and/or N impurities introduced during cryomilling diffuse and interact with in-grown dislocations during deformation.© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Mechanical millingIn-situ SEM micro-compression tests Magnesium alloys Portevin-Le Chetelier effectThe Portevin-Le Chatelier (PLC) effect is one of the best studied plastic instabilities in metallic materials. One of its main manifestations, in the appropriate strain rate and temperature range, is the appearance of "serrated flow" in the stress-strain curve. The PLC effect has been investigated extensively, from both experimental and theoretical points of view, in body and face centered cubic materials. However, the study of this phenomenon in hexagonal close-packed (hcp) materials, particularly in binary Mg-Al alloys, has been limited.Chaturvedi et al.[1] reported serrated flow in a binary Mg-Ag alloy in the 53-124°C temperature range, and attributed the locking of mobile dislocations to the diffusion of silver atoms during deformation. Serrated flow was also observed by Gao et al. [2] in binary Mg-Y alloys during tensile deformation between 150 and 250°C. In contrast, Caceres et al.[3] investigated the serrated behavior of as-cast binary Mg-Al alloys and did not observe serrations. Interestingly, Standford et al. [4] reported flow serrations during deformation of a Mg-Gd alloy at 200°C. However, they could not observe serrations in a Mg-Al alloy processed under the same conditions to obtain a similar extrusion texture and grain size.The PLC effect has also been found in ternary Mg-alloys, such as AZ91 by Corby et al. [5], and in alloys containing rare earth additions such as Mg-Y-Nd by Zhu et al. [6], Mg-Gd-Zn alloys by Wu et al. [7] and Mg-Mn-Nd by Dudamell et al. [8]. In all these systems, serrated flow was attributed to the dynamic interaction between solute atoms and dislocations, a phenomenon known as dynamic strain aging (DSA), first proposed by Cottrell in 1953 [9]. In the case of AZ91 at room temperature, Corby et al. [5] reported that both Al and Zn atoms were involved in the DSA. They suggested that Al might be the dominant solute, while Zn acted as a catalyzer enabling the formation of a forest of dislocations through which Al atoms might pipe diffuse.This study reveals the appearance of serrations at room temperature in a nanostructured binary Mg-10Al (wt.%) alloy processed via cryomilling and spark plasma sintering (SPS). Our p...

Effect of solute content and temperature on the deformation mechanisms and critical resolved shear stress in Mg-Al and Mg-Zn alloys

“…Mg-Al alloys [42]. The results indicate that thermal activation helps overcome the pinning of the dislocations by the solute atoms in Mg-9 at.%Al and Mg-Zn, so that after the initial elastic loading, yielding takes place smoothly by the gliding of pre-existing Supplementary Fig.…”

Effect of Auto-Tuning on Serrated Flow Behavior

“…The process of auto-tuning of a certain channel is served to accomplish the test with accurate results and control performance. Although the system was being stable during the tests, many researchers have reported the serrated flow in some materials under a certain strain rate and temperature range [7][8][9][10][11]. It was considered as the material property rather than the noise of machine 2 of 9 system, commonly referred to as the Portevin-Le Chatelier (PLC) effect in numerous papers and books, e.g., [12][13][14][15][16][17][18][19].…”

“…The PLC effect has been investigated by stress and strain-controlled tensile tests as well as digital image correlation and acoustic emission [21]. Recently some researchers have reported the PLC effect under compressive loading as well [9].…”

Spatiotemporal characteristics of Portevin-Le Chatelier effect in Ti-Mo alloys under thermo-mechanical loading

“…Focusing on the former, Amokhtar et al [23] investigated the evolution of serration morphology with different Mg contents and applied strain rates. Moreover, Pozuelo et al [24] studied the stress drop magnitude in nanostructured binary Mg-Al alloys and discovered that it follows a linear relationship with the number of stress drops. Sarkar et al [25] compared the stress drop magnitude in HT-9 steel at different temperatures, and the experimental results indicated that it is enlarged by increasing temperature.…”