A study on the combined effect of in-situ (TiC-TiB 2 ) reinforcement and aging treatment on the yield asymmetry of magnesium matrix composite B.N. Sahoo, S.K. Panigrahi * Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai- 600036, India article info Article history: Received 11 September 2017 Received in revised form 2 December 2017 Accepted 4 December 2017 Available online 6 December 2017 Keywords: In-situ magnesium matrix composite Tension-compression yield asymmetry Mechanical properties Microstructural analysis abstract The present work aims to study the individual as well as the combined effect of in-situ particles and precipitates on the tension-compression asymmetricity of AZ91 magnesium (Mg) alloy and AZ91 þ TiC-TiB 2 Mg matrix composites. Investigations have been done on four different material conditions: (a) alloy without particles or precipitates, (b) alloy with precipitates, (c) composite with particles, (d) composites with both particles and precipitates. Both the particle and precipitate significantly contribute towards lowering of tension to compression yield asymmetry. These particles/ precipitates increase the critical stress for twinning by exerting a back stress which hinders the twin propagation. Due to decrease in the twinning propensity, the tension to compression asymmetry reduces from 1.30 (at base condition) to 1.04 (peak aged condition of the composite). This phenom- enon has been experimentally validated by means of tension and compression tests and the results have been correlated with the pre and post-deformation microstructures. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Magnesium alloys are one of the most attractive materials used in the field of automotive and electronic industries due to its high specific strength and low density. However, the potential engi- neering applications of commercial magnesium alloys are limited due to its low strength and high tension-compression yield asymmetry behavior [1]. This is because of its hexagonal close pack (HCP) crystal structure and having limited number of slip systems. The strength and asymmetric mechanical behavior can be optimized through (a) thermal treatment, (b) alloy develop- ment, (c) texture modification and (d) reinforcement addition, or with the combined effect of all possible treatments [2e5]. Tension-compression yield asymmetry of a material is defined as the ratio between tensile yield stress (TYS) and compressive yield stress (CYS). The higher yield asymmetry in magnesium (Mg) and its alloys is due to HCP crystal structure which have c-axis to a-axis ratio (c/a) of around 1.633 [6]. Ishihara et al. [7] found that in the rolled Mg alloy, c-axis of the HCP crystal was aligned parallel to the thickness direction of the rolled plate. During compression deformation along rolling direction (RD) or tensile deformation along transverse direction (TD), the f10 12g twinning is found to be the dominant deformation mechanism due to the extension of c-axis [8,9]. Whereas, in case of tensile deformation along RD or compression deformation along TD, slip is the dominant mechanism [10]. The critical resolved shear stress (CRSS) of ð0002Þ basal slip system is larger than that of f10 12g twin system. Hence the stress required to activate basal slip sys- tem is higher than that of f10 12g twinning. Due to this difference in stress during deformation, Mg and its alloy give rise to the tension-compression yield asymmetry [11,12]. The tension-compression yield asymmetry of Mg alloys can be minimized mainly in four ways: (a) by weakening the texture, (b) by grain refinement, (c) by heat treatment and (d) by adding aluminum as an alloying element. Reduced texture is one of the major reason for lower yield asymmetry in cast alloys as compared to wrought alloys [13,14]. Grain refinement also effectively reduces the yield asymmetry [15,16]. Xin et al. [17] found a CYS/TYS of 0.4e0.5 in coarse-grained Mg alloys which increases to 0.9 with refining grain size to 1.9 mm. Barnett et al. [18] found that with decreasing grain size and increasing tem- perature, there is a transition of deformation mechanism from twinning to slip, which leads to reduced yield asymmetry. Besides * Corresponding author. E-mail address: skpanigrahi@iitm.ac.in (S.K. Panigrahi). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2017.12.027 0925-8388/© 2017 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 737 (2018) 575e589