International Journal of Minerals, Metallurgy and Materials Volume 24, Number 8, August 2017, Page 901 DOI: 10.1007/s12613-017-1476-4 Corresponding author: Uma Thanu Subramonia Pillai E-mail: utspillai@rediffmail.com © University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2017 Effects of gadolinium addition on the microstructure and mechanical properties of Mg–9Al alloy Lavish Kumar Singh 1,2) , Alok Bhadauria 1) , Amirthalingam Srinivasan 2) , Uma Thanu Subramonia Pillai 2) , and Bellambettu Chandrasekhara Pai 2) 1) Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India 2) Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology, Trivandrum 695001, India (Received: 26 November 2016; revised: 23 March 2017; accepted: 24 March 2017) Abstract: This research aims to study the significance of Gd addition (0wt%–2wt%) on the microstructure and mechanical properties of Mg–9Al alloy. The effect of Gd addition on the microstructure was investigated via X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The Mg–9Al alloy contained two phases, α-Mg and β-Mg 17 Al 12 . Alloying with Gd led to the emergence of a new rectangular-shaped phase, Al 2 Gd. The grain size also decreased marginally upon Gd addition. The ultimate tensile strength and microhardness of Mg–9Al alloy increased by 23% and 19%, respectively, upon 1.5wt% Gd addition. We observed that, although Mg–9Al–2.0Gd alloy exhibited the smallest grain size (181 μm) and the highest dislocation density (5.1 × 10 10 m −2 ) among the investigated compositions, the Mg–9Al–1.5Gd alloy displayed the best mechanical properties. This anomalous behavior was ob- served because the Al 2 Gd phase was uniformly distributed and present in abundance in Mg–9Al–1.5Gd alloy, whereas it was coarsened and asymmetrically conglomerated in Mg–9Al–2.0Gd. Keywords: magnesium; rare earth; alloying; gadolinium; microstructure; mechanical properties 1. Introduction Magnesium alloys, because of their high specific strength, low density (1.74 g/cm 3 ), improved damping capacity, elec- tromagnetic shielding features, and superior machinability, have enormous application potential in the railway, aerospace, and automotive industries [1–2]. Oil prices are high, and glob- al concern regarding vehicle emissions is increasing [3]. A re- duction in vehicle mass by 10% could lead to an improvement of 6%–8% in fuel efficiency. 1 kg of mass reduction will re- duce the CO 2 emissions of a vehicle by 17 to 20 kg over its lifetime [4]. Among the Mg-based alloys, the Mg–Al alloys have been studied extensively because of their ease of casting and low density. However, the inferior tensile properties of Mg alloys restrict their extensive applications as a structural material. The use of these alloys in automobiles has been li- mited to noncritical parts such as valve covers and instru- ment panels [5]. Li et al. [6] reported that substantial improvements in the microstructure and mechanical properties of Mg and its alloys could be attained by alloying, that is, by the intro- duction of alloying elements in small quantities. Recently, high-solubility rare-earth (RE) elements have been consi- dered as alloying elements for Mg alloys [7–8]. Mg–Al–RE alloys have been fabricated exclusively for high-temperature applications. Mg–4Al–2RE (AE42) is one such alloy, in which a combination of RE elements known as misch metal (MM) was added. The room-temperature mechanical properties were significantly enhanced as a result of alloy- ing. However, the creep resistance deteriorated rapidly at temperatures greater than 150°C because of partial de- composition of Al 11 RE 3 into Mg 17 Al 12 and Al 2 RE [9]. Yi et al. [10] added La–Pr–Ce MM to Mg–8Al alloy and ob- served a slight deterioration of the room-temperature me- chanical properties, which they attributed to the concentra- tion of stress and strain caused by the precipitation of aci-