Microstructural evolution of cryomilled Ti/Al mixture during high-pressure torsion
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Jittraporn Wongsa-Ngam Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand
Terence G. Langdon Departments of Aerospace and Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-1453; and Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, United Kingdom
Enrique J. Lavernia Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 (Received 29 August 2013; accepted 30 December 2013)
To provide insight into the influence of the length scale on the kinetics of phase evolution during severe plastic deformation, we studied the microstructure evolution of cryomilled Al and Ti mixture, which is further subjected to high-pressure torsion (HPT). The cryomilled microstructure consisted of elemental Al and Ti, and the subsequent HPT deformation at ambient temperature led to the solid state formation of Al-rich intermetallics. X-ray diffraction peaks originating from TiAl2 and TiAl3 were observed after one revolution of HPT, suggesting a shear strain-assisted formation of the intermetallics. A high resolution transmission electron microscope confirmed the formation of TiAl2 following HPT for one revolution. Further HPT straining led to microstructure refinement and a mixing of the Ti and Al, as well as of any phases formed initially. The solid state formation of the intermetallics and the overall evolution of the microstructure are discussed based on the generation of a high density of lattice defects that evolve under the strain conditions present during HPT.
I. INTRODUCTION
Severe plastic deformation (SPD) routes, such as high energy ball milling,1 high-pressure torsion (HPT),2 and equal channel angular pressing (ECAP),3,4 have been used in research related to intermetallic compounds that are generally hard and brittle at room temperature. The formation of intermetallics typically involves elevated temperature processing due to slow diffusion in these ordered structures. Therefore, attempts have been focused on increasing the processing temperature to facilitate the diffusion of atomic species in the microstructure. For example, it is reported that HPT at an elevated temperature leads to the formation of Al3Ni2 in the Ni–Al system5 and TiAl3, TiAl2, and TiAl in the Ti–Al system.6 In this respect, combined SPD techniques were proven to provide interesting and unique microstructures due to the different deformation paths involved in each process. In related studies, a combination of ECAP 1 rolling7 and ECAP 1 HPT8 was utilized in an effort to tailor the microstructures of Cu–30%Zn and of Cu–0.1%Zr alloys, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.18 578
J. Mater. Res., Vol. 29, No. 4, Feb 28, 2014
http://journals.cambridge.org
Downloaded: 04 Aug 2014
respectively. Furthermore, ball milling 1 ECAP and ball milling
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