Materials Science and Engineering A326 (2002) 288 – 296 Interfacial phenomena and microstructure evolution during solidification of binary and ternary Al–Mg–Si alloys cast with titanium carbonitride George Levi *, Wayne D. Kaplan, Menachem Bamberger Department of Materials Engineering, Technion -Israel Institute of Technology, 32000 Haifa, Israel Received 11 January 2001; received in revised form 16 May 2001 Abstract In this work the microstructural evolution of Al–Mg 2 Si alloys cast with titanium carbonitride (TiCN) nano-powder was studied, based on Al(Mg) – TiCN and Mg(Si) – TiCN model systems. From the investigation of the model systems, it is concluded that the presence of TiCN leads to the formation of a Mg-rich (or pure Mg) bulk liquid layer in the alloy at the melt–TiCN interface, before solidification begins. The driving force for the formation of the interfacial Mg-rich layer is the reduction of the total interface energy. It is postulated that this layer forms due to the interaction between Mg segregated at the alloy – TiCN interface with Mg-rich Al – Mg-clusters and Si-rich Mg – Si-clusters from the molten alloys. It is concluded that a Mg-rich bulk layer also forms at the Al(Mg,Si)–TiCN interface in cast Al–Mg 2 Si alloys, which prevents nucleation of Mg 2 Si on the TiCN particles. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Interfacial phenomenon; Microstructure evolution; TiCN; Al – Mg – Si www.elsevier.com/locate/msea 1. Introduction Mechanically alloyed Al – Mg 2 Si alloys exhibit good thermo-mechanical properties at elevated temperatures [1–3]. However, there is a significant decrease in the mechanical strength at elevated temperatures of cast Al–Mg 2 Si, due to the relatively coarse microstructure of Mg 2 Si grains in castings ( 150 m Mg 2 Si mean grain size) compared to mechanically alloyed material ( 1 m Mg 2 Si mean grain size) [1,4]. Thus, to ap- proach the properties of mechanically alloyed Al– Mg 2 Si using casting technology, it is necessary to refine the mean grain size of Mg 2 Si grains from 150 to 1 m. Grain refinement of castings is possible by several methods, but inoculation is the most practical [5]. Inoc- ulation consists of the addition to the melt of foreign particles (inoculants) which themselves form or lead to the formation of a large number of sites for heteroge- neous nucleation. The more grains nucleated in the unit volume, the finer the final microstructure, and hence the inoculation is more effective. For Mg 2 Si there were some partially successful at- tempts at inoculation with Nd and with Mg 3 (PO 4 ) 2 [1], but the goal of refining Mg 2 Si to an average grain size of only 1 m has not been achieved. Based on previous studies, it was proposed to use stable nano-sized parti- cles as heterogeneous nucleation sites [6], and TiCN (titanium carbonitride) [7] was chosen as a candidate material. The inoculation process is strongly related to interfa- cial phenomena at the liquid – nucleant interface, and therefore it is important to study these phenomena. However, for complicated systems such as (Al– Mg 2 Si)–TiCN, this is difficult due to the multitude of varying parameters. In order to simplify the results and interpretation it was decided to adopt a work strategy of model experiments on less complicated systems re- sembling the (Al–Mg 2 Si)–TiCN system. In these model experiments, designed to trace the phenomena occur- ring during solidification of an Al–Mg 2 Si alloy cast * Corresponding author. Tel.: +972-4-8284-290; fax: +972-4- 8321-978. E-mail address: glevi@tx.technion.ac.il (G. Levi). 0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0921-5093(01)01485-X