Microstructural evolution and mechanical properties of Mg–Cu–Zn ultrafine eutectic composites Gi A. Song, Wonhee Lee, Nae S. Lee, and Ki B. Kim a) Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Gwnagjin-gu, Seoul 143-747, Korea Jin M. Park, Do H. Kim, and Do H. Kim Center for Non-crystalline Materials, Department of Metallurgical Engineering, Yonsei University, Seodaemungu, Seoul 120-749, Korea Jaeseoul Lee Korea Institute of Industrial Technology (KITECH), Buk-gu, Gwangju 500-480, Korea Jun-Sik Park Division of Advanced Materials Engineering, Hanbat National University, Daejoen 305-719, Korea (Received 12 November 2008; accepted 26 February 2009) Novel ultrafine eutectic composites containing structural and spatial heterogeneities have been systematically developed in an Mg–Cu–Zn ternary system. Microstructural investigations of the ultrafine eutectic composites revealed that the bimodal eutectic structure consists of a mixture of cellular-type fine (a-Mg + MgZn 2 ) and anomalous-type coarse (a-Mg + MgZn 2 + MgCuZn) eutectic structures. An Mg 72 Cu 5 Zn 23 alloy composed of the bimodal eutectic structure without micron-scale a-Mg dendrites presents a strong improvement of yield strength up to 455 MPa with a decent plastic strain of 5%. The rotation of the bimodal eutectic colony along the interfaces is considered to be an effective way to dissipate the stress localization thus enhancing the macroscopic plasticity. I. INTRODUCTION Development of ultrafine eutectic composites consist- ing of micron-scale dendrites and nano-/ultrafine eutec- tic matrix has been highlighted in several alloys to achieve high-strength alloys combined with enhanced plasticity at room temperature. 1–15 For example, a series of Ti-based ultrafine eutectic composite consisting of micron-scale b-Ti dendrites and nano-/ultrafine eutectic matrix revealed outstanding mechanical properties, i.e., ultimate compressive strength of 2.4 GPa and com- pressive plasticity of 14%. 1 Further systematic investi- gations on the deformation mechanisms of Ti-based ultrafine eutectic composite have shown that the strength and plasticity of the material are strongly related to the occurrence of the slip/shear bands in the micron-scale dendrites and the propagation of primary and secondary shear bands in the ultrafine eutectic matrix. 1,7 Hence, it is believed that the strength and plasticity of the ultrafine eutectic composite can be optimized by controlling the volume fraction, length-scale of the microstructure, and phase selections. On the other hand, Ti- and Fe-based ultrafine eutectic alloys even without micron-scale den- drites can also exhibit large plastic strains of 7–12% under room-temperature compression. 16,17 The detailed investigations on such interesting ultrafine eutectic alloys point out that the spatial heterogeneity formed by additional elements can be an origin to dissipate the localized stress by a rotation of the eutectic colonies. 16,17 In this scenario, it is considered that the plasticity of the ultrafine eutectic alloys and composites can be controlled dominantly by the spatial heterogeneity of samples. Recently, there was an interesting finding that bimod- al eutectic composites in a series of (Ti 70.5 Fe 29.5 ) 100–x Sn x alloys with x = 5, 7, and 9 consisting of a mixture of micron-scale Ti 3 Sn dendrite phase, and coarse (b-Ti + Ti 3 Sn) and fine eutectic (b-Ti + TiFe) in matrix can also present a good combination of a strength of 1.3 GPa and plasticity of 15.7%. 18 The investigations on micro- structural evolution of the bimodal eutectic composite in Ti–Fe–Sn alloys reveal that the bimodal eutectic struc- ture in matrix forms possibly due to a class II four-phase equilibrium L + Ti 3 Sn ! b-Ti + TiFe forming the spa- tial, i.e., length-scale difference in the bimodal eutectic structures and structural heterogeneities; i.e., differences in phase formation. On the basis of this understanding, it is feasible to suggest that the existence of class II four- phase equilibrium can be a condition to form the bimod- al eutectic composites. Along the line to develop the a) Address all correspondence to this author. e-mail: kbkim@sejong.ac.kr DOI: 10.1557/JMR.2009.0330 J. Mater. Res., Vol. 24, No. 9, Sep 2009 © 2009 Materials Research Society 2892