Deformation-induced nanocrystallization: A comparison of two amorphous Al-based alloys
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F.E. Pinkerton General Motors R & D Center, Warren, Michigan 48090
M. Atzmon Department of Nuclear Engineering and Radiological Sciences, and Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109 (Received 23 October 2004; accepted 10 December 2004)
Using conventional and high-resolution transmission electron microscopy (HRTEM), the effects of rolling at room temperature on the microstructures of amorphous Al90Fe5Gd5 and Al86.8Ni3.7Y9.5 were compared. In rolled Al90Fe5Gd5, nanocrystallites were observed at shear bands, whereas none were observed in rolled Al86.8Ni3.7Y9.5. When HRTEM was combined with with Fourier transform filtering, nanoscale, low-density defects were imaged. In Al90Fe5Gd5, the shear bands contain few defects, which are concentrated at the boundary zone between the shear bands and undeformed region, whereas in Al86.8Ni3.7Y9.5, the shear bands contain a uniform distribution of defects with a density higher than the undeformed region. The preferential precipitation of nanocrystallites in rolled Al90Fe5Gd5 is attributed to a kinetic effect due to uniformly-distributed excess free volume in the shear bands.
I. INTRODUCTION
Recently, mechanically induced nanocrystallization in amorphous alloys, achieved by ball milling,1–3 bending,4,5 rolling,6 tension,7,8 and nanoindentation,9,10 has received much attention. The process of mechanically induced nanocrystallization is of potential practical significance for the synthesis of amorphous-nanocrystallite composites and their processing and service. In addition, the mechanisms of atomic displacement and formation of crystalline phases under plastic deformation are of significant fundamental interest. To date, no consensus concerning the microscopic mechanism of mechanically induced nanocrystallization has been reached. Since atomic mobility is very sluggish, it has been suggested that a temperature rise may play a crucial role in their formation.11 Recently, we combined nanoindentation with transmission electron microscopy (TEM) to study the effect of deformation at a low strain rate on amorphous Al90Fe5Gd5 alloy, to rule out a temperature rise during deformation. The results indicated that mechanical deformation at or near room temperature led to the precipitation of nanocrystalline Al.10 Furthermore, DOI: 10.1557/JMR.2005.0090 696
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 3, Mar 2005 Downloaded: 20 Mar 2015
our work also demonstrated that the stress state is an important factor in mechanically induced nanocrystallization of an amorphous alloy. Nanocrystallization occurred only in the predominantly compressive region of a sample bent at room temperature.12 Mechanically induced nanocrystallization appears to be a general phenomenon, since it has been observed in Al-, Zr-, and Fe-based alloys.1–12 However, this process is composition dependent; it may vary between otherwise similar alloys.1,4 Mechanically induced nanocrystallization occurs exclusively at shear bands, which are the main micros
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