Texture evolution and anisotropy in the thermo-mechanical response of UFG Ti processed via equal channel angular pressing

Abstract

The texture evolution and thermo-mechanical response of grade 1 titanium after equal channel angular pressing (ECAP) at different strain rates and temperatures were measured. As-received Ti was processed via ECAP at 275 °C for up to four passes using route B_C. The uniform microstructure from the as-received condition, goes from long, coarse bands with some grains relatively undeformed after a single pass to mostly uniform fine and ultrafine grains after four passes. The texture of the as-received Ti was of a typical cold-rolled plate. The basal plane normal direction stayed perpendicular to, and rotated about, the longitudinal direction of the billet during all numbers of passes. The mechanical response of UFG Ti, subjected to four passes, was determined at −196, 22 and 375 °C, and at strain rates from 10⁻⁴ to 2000 s⁻¹ in the different loading directions. It was found that the yield strength and flow stress were different in all three loading directions, at all strain rates and testing temperatures. Macroscopic shear band failure was observed in the samples subjected to dynamic loading, and the onset was dependent on the loading direction and testing temperature. Finally, the anisotropy shows up in the yield strength between tension and compression loading, where it is greater in tension.

Introduction

Equal channel angular pressing (ECAP) is a technique using severe plastic deformation to produce ultrafine grain (UFG) sizes in the range of hundreds of nanometers to bulk course grained materials (Segal et al., 1981, Valiev et al., 1993, Segal, 1995, Furukawa et al., 1998). ECAP is performed by pressing a billet of material through a die that has two channels which intersect at an angle. The billet experiences simple shear deformation, at the intersection, without any precipitous change in the cross sectional area because the die does not allow for lateral expansion. A single pass with channels 90° to each other, induces approximately 1.15 equivalent strain in the billet, so the billet can be pressed more than once.

Titanium subjected to a single pass of ECAP showed $\{10\overline{1}1\}$ twinning as the dominant deformation mechanism (Kim et al., 2001, Kim et al., 2003, Alexandrov et al., 2008). After two passes, Shin et al. (2003) concluded for route B the dominant mechanism was type a slip. There are several papers that report on the texture of titanium after ECAP, but data are only given in one state – after a single pass (Shin et al., 2003), during pass 3/4 (Chen et al., 2010), and eight passes (Alexandrov et al., 2004, Gubicza et al., 2003). Finally, Suwas et al. (2011) reports on the texture as a function of the number of passes (up to three) and for different processing routes.

For ECAP Ti, Zhernakov et al. (2001) notes that route B_C results in higher strength and ductility, another study by Stolyarov et al. (2001) reported that route B_C is most effective at producing equiaxed grains and reducing their size. Tabachnikova et al. (2005) reported that the strength in compression is noticeably greater than tension down to 4.2 K. Anisotropy in the mechanical behavior and texture of coarse grained pure titanium is well known (Nixon et al., 2010a, Nixon et al., 2010b), but only a couple of papers have reported on the anisotropy of titanium following ECAP (Korshunov et al., 2008, Sabirov et al., 2011). Korshunov et al. found that for two different shipments the yield strength from highest to lowest was in general in the z, y, and x loading directions (see Fig. 1), respectively, for up to four passes and at both 22 and 450 °C testing temperatures.

Two studies on CP Ti processed by ECAP warrant a review since they perform experiments at dynamic strain rates. The first by Jia et al. (2001), ECAP is done at 450 °C and for eight passes, and compression experiments are performed at room temperature. The UFG Ti is essentially perfectly plastic for both quasi-static and dynamic strain rates, and only the dynamic specimens fail. Later Wang et al. (2007) prepared UFG Ti similarly to Jia et al. but received different flow stresses. The UFG Ti, at quasi-static strain rates, showed perfectly plastic behavior until about 12% true strain when the strain hardening increased, with failure at about 35%. In the dynamic regime, the work hardening is positive from the beginning of the flow stress, but at about 12%, the work hardening greatly increases, and failure occurs at about 23%. A number of high strain rate experiments on UFG and nanocrystalline materials subjected to different processing methods, in addition to ECAP, have been performed by Gray et al., 1997, Jia et al., 2001, Khan and Meredith, 2010, Farrokh and Khan, 2009, among others.

In this study, grade 1 titanium specimens were processed by ECAP for up to four passes at 275 °C, using route B_C. The microstructure was observed (optically) without ECAP and at one, two, three and four passes, and the crystallographic texture was examined to determine the evolution due to the ECAP process. Next, the temperature and strain rate (quasi-static to dynamic) sensitivities of the material under uniaxial compressive loadings were investigated to elucidate their changes in different sample loading directions, and the dynamic failure of the material was explored. Finally, the tension/compression asymmetry is presented.