Materials Science and Engineering A 494 (2008) 436–444 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Heat-treating below recrystallization temperature to enhance compressive failure strain and work of fracture of magnesium M. Paramsothy a , N. Srikanth b , S.F. Hassan c , M. Gupta a,∗ a Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore b Centre for Management of Science and Technology, Department of Industrial & Systems Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574, Singapore c Setsco Services Pte Ltd., 18 Teban Gardens Crescent, Singapore 608925, Singapore article info Article history: Received 28 February 2008 Received in revised form 16 April 2008 Accepted 21 April 2008 Keywords: Magnesium Aluminium Macrocomposite Stressed interface Compressive properties abstract New bimetal magnesium/aluminium macrocomposite containing millimeter-scale Al core reinforcement was fabricated using solidification processing followed by hot coextrusion. Microstructural characteri- sation revealed increased grain size, Mg texture change and unbalanced interfacial interdiffusion of Mg and Al into each other. Stress at the bimetal interface was attributed to solid solution formation, thermal expansion mismatch, unbalanced Kirkendall strain, lattice misfit strain, and strain localization effects, these being interface localized strengthening phenomena. Compressive testing revealed that presence of Al core decreased 0.2% YS (−23%) and ultimate compressive strength (UCS) (−11%), but significantly increased failure strain (+134%) and work of fracture (+60%) of Mg in the as-extruded macrocomposite. Also, interfacial relaxation during heat treating significantly increased failure strain (+17%) and work of fracture (+17%) of Mg/Al macrocomposite without compromising 0.2% YS and UCS. The effects of presence of millimeter-scale Al core as well as interfacial relaxation on the compressive properties of the bimetal macrocomposite are investigated in this paper. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Magnesium has been used in many compressive loaded parts in weight-critical automotive and aerospace structural applica- tions. Wheels and engine/transmission housings are good examples of such parts [1]. Bayerische Motoren Werke (BMW) GmbH has recently manufactured a new Mg/Al bimetal composite crank case for its straight-six cylinder Otto engine by pressure die-casting liquid Mg alloy around a solid Al alloy insert and almost fully encasing it [2]. Mg is about 35% lighter and has superior vibra- tion damping characteristics than Al. Both metals have similar melting points. However, the ductility of Mg is limited compared to Al. This can be attributed to limited number of active (basal) slip systems in its HCP structure. Traditional alloying has been used to improve compressive properties of Mg [3]. Additionally, compressive properties of Mg have been improved beyond the limits of alloying with the use of reinforcement [4,5]. Currently, research literature on compressive testing of Mg-based materials is limited. Compressive deformation has been used for studying: (a) acoustic emission response [6], (b) crystallographic textur- ∗ Corresponding author. Tel.: +65 6516 6358; fax: +65 6779 1459. E-mail address: mpegm@nus.edu.sg (M. Gupta). ing [7], and (c) extension twinning [8,9] in Mg-based materials. Limited studies have been done on millimeter length scale inte- gration of bimetals including bimetal rolling of cladded sheet and extrusion of cladded rod, involving the Mg–Al [10], Al–Cu [11–15], Cu–steel [12–15], Al–steel [16,17], brass–steel [18], Al–Zn [19], Al–Sn [19], Al–Pb [19,20] and Ni–Ti [21] bimetal material systems. Relevant findings include different intermetallic com- pound growth rates in bimetal composites processed differently [11], lower crack propagation resistance at the bimetal interface [16], sub-critically thick intermetallic layer and parallel-oriented interface leading to bond strength increase [20], lower compressive stress of the separate metals compared to the bimetal compos- ite [21] and strain localization at the bimetal interface [15]. Such relevant findings suggest the existence of stressed bimetal inter- face. The results of literature search indicate that no attempt has been made to improve the overall compressive behavior of Mg via integration with Al in millimeter length scale using a lower cost solidification processing methodology and bimetal interfacial relaxation. Accordingly, the primary aim of this study was to assess the compressive properties of Mg-based macrocomposite containing Al as reinforcement in millimeter length scale, inclusive of bimetal interfacial relaxation effects. The Mg/Al bimetal macrocomposite was formed using combination of an innovative disintegrated melt 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.04.049