Microstructural and nuclear magnetic resonance studies of solid-state amorphization in Al–Ti–Si composites prepared by mechanical alloying I. Manna a, * , P. Nandi a , B. Bandyopadhyay b , K. Ghoshray b , A. Ghoshray b a Metallurgical and Materials Engineering Department, I.I.T., Kharagpur 721 302, India b Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India Received 28 December 2003; received in revised form 14 May 2004; accepted 17 May 2004 Available online 17 June 2004 Abstract Three Al 30 Ti 70  x Si x (x ¼ 10, 20, 30), along with an Al-rich (Al 50 Ti 40 Si 10 ) and an Al-lean (Al 10 Ti 60 Si 30 ) elemental powder blends were subjected to mechanical alloying by high-energy planetary ball milling to yield a composite microstructure with varying proportions of amorphous and nanocrystalline intermetallic phases. Microstructural characterization at different stages of milling was carried out by X-ray diffraction, high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Furthermore, 27 Al nuclear magnetic resonance (NMR) studies were undertaken to probe the mechanism of solid- state amorphization. Ball milling leads to alloying, nanocrystallization and partial solid-state amorphization followed/accompanied by strain-induced nucleation of nanocrystalline intermetallic phases from an amorphous solid solution. Both these amorphous and nano-intermetallic phases are associated with characteristic NMR peaks at lower frequencies (than that of pure Al). Thus, me- chanical alloying of Al–Ti–Si appears a suitable technique for developing nanocrystalline intermetallic phase/compound dispersed amorphous matrix composites. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Mechanical alloying; Nuclear magnetic resonance; Aluminum alloy; Amorphous; Nanocrystalline 1. Introduction Al-alloys are the most widely used material for high- specific strength structural applications in aviation and transportation industry. While age hardenable Al-alloys can attain a maximum strength level of 500–600 MPa, it is reported that significant improvement in compressive strength up to 1200–1400 MPa is possible in Al-alloys in amorphous or nanocrystal dispersed amorphous condi- tion [1–3]. For this reason, development of amorphous Al-alloys, particularly Al-based bulk amorphous alloys by solid-state processing has received considerable re- search attention in the recent past [4–6]. Mechanical alloying is a versatile solid-state synthesis route to develop partially or completely amorphous/ glassy microstructure from elemental powder blend, al- loys or intermetallic phases/compounds [7–9]. Besides mechanical attrition, solid-state amorphization is also feasible by sandwich/pack rolling [10], heavy-duty wear/ erosion [11] and equi-channel angular pressing [12] that involves severe plastic deformation at slow strain rate. In contrast, solid-state amorphization by mechanical alloying involves deformation at much higher strain rate, cold welding, fragmentation and dynamic recrys- tallization [9]. Mass transport accompanying mechani- cal alloying occurs at a much faster effective diffusion rate equivalent to volume diffusivity at an effective ele- vated temperature [13]. Solid-state amorphization is usually favored in systems with large negative heat of mixing and/or large difference in diffusivity of the con- stituents at the reference temperature [14]. While the * Corresponding author. Tel.: +91-3222-283266; fax: +91-3222- 282280. E-mail address: imanna@metal.iitkgp.ernet.in (I. Manna). 1359-6454/$30.00 Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2004.05.026 Acta Materialia 52 (2004) 4133–4142 www.actamat-journals.com