Technical Report High temperature tensile properties of modified Mg/Mg 2 Si in situ composite Farshid Mirshahi ⇑ , Mahmoud Meratian Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran article info Article history: Received 17 January 2011 Accepted 3 May 2011 Available online 6 May 2011 abstract Non-reactive in situ process is one of the newly emerged routes for metal matrix composite manufactur- ing, in which recycling of their scraps has been found to be advantageous. In the present work, the Mg/ Mg 2 Si composite was synthesized using non-reactive in situ method. Improving coarse and dendritic structure of primary Mg 2 Si precipitates was a reason for modification of primary Mg 2 Si particles. By adding 0.5 wt.% bismuth to the melt, the primary Mg 2 Si particles were formed in polygonal morphologies while their average size decreased. Tensile tests in different temperatures were conducted on non- modified and modified composite specimens. Tensile properties of the modified composites in different temperatures were enhanced compared to those of non-modified ones. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Magnesium and its alloys, owing to their attractive physical properties, have been potential candidates for light-weight struc- tural materials. However, magnesium alloys have been limited for use in high performance applications, due to their low mechan- ical properties [1,2]. Magnesium matrix composites can combine the ductility, toughness, and damping capacity of the magnesium matrix along with higher strength and modulus characteristics of the ceramic reinforcements [3,4]. However, improving the elevated tempera- ture properties has become a critical issue for possible applications of magnesium alloys in hot environment [5]. In recent years, the in situ manufacturing route has been exten- sively studied for aluminum matrix composites. But, for magne- sium matrix composites, this technique is still relatively new [3,6]. The Mg–Mg 2 Si system is probably the first and thus the most important magnesium matrix composite fabricated by the non- reactive in situ synthesis. According to Mg–Si phase diagram [7,8], the maximum solubility of silicon in magnesium reaches 0.003 at.% (Fig. 1); therefore, extra silicon reacts with magnesium and forms Mg 2 Si intermetalic compound [8–10]. The physical properties of Mg 2 Si such as high melting temperature (1085 °C), low density (1.99  10 3 kg m 3 ), high hardness (4.5  10 3 Nm 2 ), a low thermal expansion coefficient (7.5  10 6 K 1 ) and a reason- ably high elastic modulus (120 GPa) [11]; have made it a desirable candidate for reinforcement. Once the cooling rate is high enough, solidification occurs in non-equilibrium path as can be seen in the following equation [12]. L ! L 1 þ Mg 2 Si P ! L 2 þ Mg 2 Si P þ Mg P ! Mg 2 Si P þ Mg P þðMg þ Mg 2 SiÞ E ð1Þ where the subscripts P and E represents the primary particles, the eutectic crystals respectively. In this case, primary Mg 2 Si solidifies dendritically and coarsely, which decreases mechanical properties and castability undesir- ably. Modification effect of some elements and compounds on primary Mg 2 Si phase in Mg–Si alloys has been studied in recent years. (e.g. lanthanum, yttrium, strontium, K 2 TiF 6 , KBF 4 and KBF 4 +K 2 TiF 6 , Sb, Ca, P) [13–18]. In current work the effect of bismuth addition on primary Mg 2 Si morphology in Mg–Si alloys was investigated. It was found that 0.5% amount of Bi added as chemical modifier to the alloy, was suf- ficient in refining the Mg–5 wt.% Si alloy microstructure. The size of primary Mg 2 Si was also significantly decreased and its morphology changed from coarse dendrites to polyhedral shape. The aim of this work was to study tensile properties and frac- ture behavior of modified and non-modified Mg/Mg 2 Si composite in different temperatures. 2. Experimental Commercially pure magnesium (99.6 wt.% purity) and silicon (99.4 wt.% purity) were used as starting materials to prepare Mg– 5 wt.% Si alloy. The charge was melted inside a low carbon steel (st 37) crucible by an electric resistance furnace under protective argon gas atmosphere. 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.05.001 ⇑ Corresponding author. Tel.: +98 935 8713587; fax: +98 551 2226498. E-mail addresses: F.mirshahi@ma.iut.ac.ir (F. Mirshahi), Meratian@cc.iut.ac.ir (M. Meratian). Materials and Design 33 (2012) 557–562 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes