Superplastic Deformation of Titanium Alloy with Different Structure and Phase Composition
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CONDENSED-STATE PHYSICS SUPERPLASTIC DEFORMATION OF TITANIUM ALLOY WITH DIFFERENT STRUCTURE AND PHASE COMPOSITION I. V. Ratochka, I. P. Mishin, O. N. Lykova, and E. V. Naidenkin
UDC 539.214:539.382.2
The α+β titanium alloy Ti-6Al-4V having different structure and phase composition is investigated in this paper under the tensile deformation in the temperature range from 773 to 1223 K. The transmission and scanning electron microscopy observations show that superplastic deformation of the Ti-6Al-4V alloy with the ultra-fine grain structure produces the formation of the secondary phase particles along the grain boundaries. It is identified that this process is connected with the vanadium redistribution during deformation. The obtained temperature dependence of the fracture elongation of the alloy samples is nonmonotonic and exceeds 700% at 973 K. It is supposed that the reduction in the fracture elongation within 1073–1173 K range is associated with the deformation-induced micron-sized grains and the formation of lamellar β- and α"-phases along the grain boundaries. Keywords: titanium alloy, severe plastic deformation, ultra-fine grain structure, phase transformation, superplastic deformation.
INTRODUCTION Nowadays, titanium alloys find wide application in various industries due to their high specific strength and corrosion resistance [1–5]. According to [6–8], during the formation of a globular fine structure with a grain size less than 0.5Tm) and strain rates below 10–3 s–1. The initial structure and phase composition of alloys can have a significant effect on plastic deformation and the fracture elongation [6–13]. For example, the grain refinement by severe plastic deformation of these alloys leads to a shift of the temperature and speed intervals of the superplasticity development to the lower temperature region and/or higher strain rates [7–16]. The superplastic properties are determined not only by the grain size, structure and phase composition of the alloy, but also the high nonequilibrium state and, as a consequence, increased diffusion permeability of the interface [8–10]. As compared to fine grain materials, superplastic deformation of the ultra-fine grain (submicro- and nanocrystalline) structure of materials has a spectrum of properties that today cannot be explained unambiguously. The optimum structure and phase composition of the ultra-fine grain structure is still a debatable question for realization of superplastic deformation. Thus, according to our early research [17, 18], in contrast with the fine grain, the ultra-fine grain structure demonstrates a higher superplasticity at a certain optimum (intermediate) grain size. It is also found that the temperature dependence of the fracture elongation of the ultra-fine grain structure can have two maxima. Moreover,
The Institute of Strength Physics and Materials Science of the Siberian Branch of the Russian Academy of Sciences, Tomsk, Russia, e-mail: [email protected]; [email protected]; [email protected]; [email protected]. Translated from Izvestiya Vysshik
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