Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea Influence of additions of Sb and/or Sr on microstructure and tensile creep behaviour of squeeze-cast AZ91D Mg alloy Punit Kumar a , A.K. Mondal a, ⁎ , S.G. Chowdhury b , G. Krishna b , Ashok Kumar Ray b a Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela 769008, India b Materials Science and Technology Division, National Metallurgical Laboratory, Jamshedpur 831007, India ARTICLE INFO Keywords: Magnesium alloy AZ91D Squeeze-casting Microstructure Tensile creep ABSTRACT The effects of individual and combined additions of Sb and Sr on microstructure and creep properties of the squeeze-cast AZ91D alloy have been investigated. For comparison, the same has also been studied on the as-cast AZ91D alloy without any addition. The results indicate that both individual and combined additions refine the grain size and β-Mg 17 Al 12 phase, which is more pronounced with combined addition. Besides α-Mg and β- Mg 17 Al 12 phases, a new rod-shaped Mg 3 Sb 2 and an irregular shaped Al 4 Sr phases are formed following individual additions of Sb and Sr in the AZ91D alloy. With combined additions, both Mg 3 Sb 2 and Al 4 Sr phases are formed in the AZ91D alloy. All the alloys have been creep tested at a temperature of 175 °C and at an initial stress of 70 MPa. All the modified AZ91D alloys exhibit superior creep resistance as compared to the base AZ91D alloy. Individual addition of Sb is more effective as compared to the individual addition of Sr in improving creep resistance of the AZ91D alloy owing to the higher thermal stability of the Mg 3 Sb 2 phase. Among the modified alloys, the best creep resistance is obtained in the AZ91D alloy pertaining combined addition of both Sb and Sr owing to the reduced amount of β-Mg 17 Al 12 phase and increased amount of high melting point Mg 3 Sb 2 and Al 4 Sr intermetallic phases. 1. Introduction There is a significant increase in the consumption of magnesium (Mg) alloys in automotive sector in the past few years as the require- ment of lightweight to improve fuel efficiency and reduce emissions have become the foremost criteria. The potential development areas of Mg alloys are automotive powertrain components that require superior creep resistance in the range 150–300 °C [1,2]. The most common low cost Mg alloys, namely, AZ91D, AM50A and AM60B exhibited a superior blend of ambient temperature strength and ductility, corro- sion resistance and excellent die-castability. Unfortunately, these alloys do not possess essential elevated temperature performance. Amongst the AZ91 alloy is the maximum utilized in Mg cast products, however, its utilization is restricted to interior parts in the form of die-cast [3]. The creep resistance of the AZ91 alloy deteriorates very fast above 130 °C due to softening of the β-Mg 17 Al 12 phase present at grain boundaries [4] and thus, rendering it unsuitable for promising power- train applications. Mg alloy series i.e. AS (containing Mg, Al and Si) as well as AE (containing Mg, Al and RE) with superior creep resistance were developed to avoid creation of undesired β-Mg 17 Al 12 phase. However, the improvement in creep resistance of the AS series alloys was only marginal and AE series alloys were not economic owing to the costly rare earth (RE) addition. Further, their use was restricted to less complex cast component because of poor castability. Over the years, creep resistant Ca and/or Sr containing Mg-Al alloys (i.e., Mg-Al-Ca, Mg-Al-Sr, Mg-Al-Ca-Sr) were developed. However, they were not commercialised for their poor die-castability. In recent years, there were many attempts to enhance creep resistance of the AZ91 alloy through modification of microstructure via trace element addition [5]. The addition of alloying element results in thermally stable interme- tallics that impede dislocation movement and avoid elevated tempera- ture grain boundary sliding. A summary of trace element additions to the AZ91 alloy concluded the single addition of Sb effectively enhanced its creep resistance by forming thermally stable high melting point Mg 3 Sb 2 (1228 °C) inter- metallic phase [6–8]. To the best of authors’ knowledge, there is no investigation especially on creep behaviour of the AZ91 alloy following the single addition of Sr, although it was added with Ca and reported to improve creep resistance of the AZ91 alloy by forming thermally stable high melting point Al 4 Sr (1040 °C) intermetallic phase [9]. In addition, the combined additions of some elements were more effective for http://dx.doi.org/10.1016/j.msea.2016.12.006 Received 28 September 2016; Received in revised form 28 November 2016; Accepted 1 December 2016 ⁎ Corresponding author. E-mail addresses: mondala@nitrkl.ac.in, ashok.mondal@gmail.com (A.K. Mondal). Materials Science & Engineering A 683 (2017) 37–45 0921-5093/ © 2016 Elsevier B.V. All rights reserved. Available online 02 December 2016 crossmark