Origin of the reversed yield asymmetry in Mg-rare earth alloys at high temperature P. Hidalgo-Manrique, a,⇑ V. Herrera-Solaz, b J. Segurado, a,b J. Llorca, a,b F. Ga ´lvez, b O.A. Ruano, c S.B. Yi d and M.T. Pe ´rez-Prado a a IMDEA Materials Institute, C/Eric Kandel 2, 28906 Getafe, Madrid, Spain b Department of Materials Science, Polytechnic University of Madrid, E.T.S. de Ingenieros de Caminos, 28040 Madrid, Spain c Department of Physical Metallurgy, CENIM-CSIC, Av. Gregorio del Amo 8, 28040 Madrid, Spain d Magnesium Innovation Centre MagIC, Helmholtz-Zentrum Geesthacht, Max-Planck-Strasse 1, D-21502 Geesthacht, Germany Received 10 October 2014; revised 30 March 2015; accepted 30 March 2015 Abstract—The mechanical behaviour in tension and compression of an extruded Mg–1 wt.% Mn–1 wt.% Nd (MN11) alloy was studied along the extrusion direction in the temperature range 175 °C to 300 °C at both quasi-static and dynamic strain rates. Microstructural analysis revealed that the as-extruded bar presents a recrystallized microstructure and a weak texture that remain stable in the whole temperature range. A remarkable reversed yield stress asymmetry was observed above 150 °C, with the compressive yield stress being significantly higher than the tensile yield stress. The origin of this anomalous reversed yield stress asymmetry, which to date remains unknown, was investigated through the analysis of the macro and microtexture development during deformation, as well as by means of crystal plasticity finite element simulations of a representative volume element of the polycrystal. The critical resolved shear stresses of slip and twining for simulated single crystals were obtained as a function of the temperature by means of an inverse optimisation strategy. Experimental and simulation results suggest that the reversed yield asymmetry may be primarily attributed to the non-Schmid behaviour of pyramidal hc+ai slip, which is the dominant deformation mechanism at high temperatures. It is proposed, furthermore, that the asymmetry is enhanced at quasi-static strain rates by the stronger interaction of hc+ai dislocations with the diffusing solute atoms and particles in compression than in tension. Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium alloys; Reversed yield stress asymmetry; Non-Schmid; Portevin–Le Chatelier; Finite element simulation 1. Introduction Mg can deform by crystallographic slip and mechanical twinning [1,2]. Slip may take place both along the h11  20i or hai direction mainly on basal and prismatic {1 0  10} planes as well as along the h11  23i or hc+ai direction on pyramidal {1 1  2 2} planes. Twinning occurs predominantly on the pyramidal {1 0  1 2} planes [3,4] and it plays a key role during deformation. Indeed, due to the low symmetry of the hcp lattice, the number of independent slip systems capable of accommodating deformation along the c-axis is limited and twinning has to occur in order to avoid inter- granular incompatibilities. As the c/a ratio of Mg is lower than the ideal one, pyramidal twinning is denominated ten- sion or extension twinning, since it can only be activated when the resolved applied stress results in an extension of the c-axis [5]. Different combinations of deformation systems may be activated under different deformation modes for a given texture due to the polar nature of the mechanical twinning. Wrought processes, such as extrusion or rolling, gener- ally give rise to a crystallographic texture where a signifi- cant fraction of grains have their {0 0 0 1} basal planes preferentially oriented parallel to the extrusion (ED) or the rolling direction (RD) [6]. Basal slip, which is usually the most easily activated deformation mechanism at room temperature (RT) [2], is severely hindered when loading along the ED or the RD due to the low Schmid factor of the basal planes in this condition [7]. In contrast, tension twinning is easily activated under compression, while pris- matic slip is mainly activated under tension [7]. Since the critical resolved shear stress (CRSS) of non-basal systems at RT is much higher than that of basal slip and twinning [8,9], Mg wrought products are characterised by a distinct RT tension–compression asymmetry [4,5]. The addition of certain rare earth (RE) elements, even in dilute concentrations, has been recently claimed to be an effective method for reducing the RT yield asymmetry of Mg wrought products [10–15]. This is related to the relative weak texture of RE-containing Mg alloys as compared with pure Mg or conventional Mg alloys. The mechanism responsible for the texture weakening is not still clear http://dx.doi.org/10.1016/j.actamat.2015.03.053 1359-6462/Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Available online at www.sciencedirect.com ScienceDirect Acta Materialia 92 (2015) 265–277 www.elsevier.com/locate/actamat