Microstructure evolution and strengthening mechanisms in commercial-purity titanium subjected to equal-channel angular pressing

Abstract

High-resolution electron backscatter diffraction (EBSD) was applied to examine grain refinement in commercial-purity titanium Grade 4 subjected to equal-channel angular pressing (ECAP) via the Conform technique. This approach enables the production of long-length billets and thus has the potential for commercial application. Microstructure evolution was found to be a relatively-complex process which included several stages. At relatively-low accumulated strains, microstructure changes were markedly influenced by mechanical twinning. However, the concomitant grain refinement suppressed this mechanism, and subsequent microstructure development was dictated by the evolution of deformation-induced boundaries which developed preferentially near the original grain boundaries. The final material produced after an effective strain of ~ 8.4 was characterized by a mean grain size of 0.3 µm, high-angle boundary fraction of 55 pct., a texture of moderate strength, and a yield strength of ~ 1050 MPa. Based on the detailed microstructural analysis, the contributions of various strengthening mechanisms were quantified. The rapid material strengthening during the early stages of ECAP was explained in the terms of a major increase in dislocation density and the extensive formation of the deformation-induced boundaries. With further increments in accumulated strain, however, the dislocation as well as grain-boundary density reached a saturation, thus reducing the hardening efficiency of ECAP at high strains.

Introduction

The evolution of microstructure during severe plastic deformation (SPD) and the properties of materials thus processed have received considerable attention in the literature [1], [2], [3]. To date, a great number of results has been obtained, and the fundamental principles related to the formation of an ultrafine-grain (UFG) structure and the enhancement of mechanical properties in metals and alloys via SPD have been formulated [1], [2], [3]. In particular, it has been shown recently that the formation of a UFG structure in titanium leads to substantial strengthening, thereby making this material attractive for medical applications [4], [5].

Most of the research dedicated to SPD of titanium and resulting mechanical properties has been focused on microstructure evolution at high accumulated strains; microstructure development at low and intermediate strains has received less attention.

From a broad perspective, it has been demonstrated that microstructure evolution during equal channel angular pressing (ECAP) of titanium is relatively complex and comprises several stages. At relatively low strains (~ 1 ECAP pass), microstructural changes are markedly influenced by mechanical twinning which provides rapid grain refinement [6], [7], [8], [9], [10], [11], [12]. $\{10\overline{1}1\}$ twins are often reported to predominate [6], [7], [8], [9], [10], [11], [12], but the twinning mode is expected to vary depending on material purity, deformation temperature, and initial texture. Concomitant grain refinement suppresses twin activity [7], [10], [12], [13], and microstructure evolution at higher strains is presumably governed by other mechanism(s). In particular, the dislocation density increases markedly, reaching ~ 10¹⁵ m⁻² [14]. However, a distinct cell structure does not develop [7], and microstructure is characterized by large orientation gradients [13]. The mean grain size estimated from transmission electron microscopy (TEM) observations approaches 0.2 µm [14]. Microstructure evolution at this stage has been hypothesized to be governed by continuous recrystallization [13] but the details of this process are unclear. The results clearly demonstrate, however, that the severely-deformed microstructure is relatively complex and contains dislocation substructure. If so, the strengthening effect presumably cannot be explained simply in the terms of the Hall-Petch relation. Indeed, a number of studies have shown that substructure hardening may also play a significant role [15], [16], [17], [18]. Despite recent progress in this field, however, the microstructure-strength relationship in heavily-deformed titanium remains poorly understood.

To obtain a comprehensive idea of microstructure evolution and strengthening mechanisms, a large amount of information on boundary density, types, misorientations as well as crystallographic texture must be obtained. Transmission electron microscopy (ТЕМ), which is typically applied to study substructure, cannot provide sufficient quantitative data. By contrast, high-resolution electron backscatter diffraction (EBSD) appears to be a better choice. Despite its limited angular accuracy (~ 2°), this technique enables quantification of microstructural features over a much larger scale than TEM and thus provide insights that may be more significant from a statistical point of view. In recent years, EBSD has developed markedly, making it a very popular technique. This paper demonstrates the feasibility of high-accuracy EBSD for severely-deformed UFG titanium. The overall objective of the present work was to establish the interrelation between microstructure and strength in SPD-processed titanium. A thorough analysis of grain boundary evolution with increasing strain was made. The results of these measurements were used to elucidate the contributions of various hardening mechanisms to overall material strength.

A recent modification of ECAP technique (the so-called ECAP-Conform process [19]) was used to impart SPD in the present work. This approach enables production of long-length billets and has higher productivity than “classical” (batch-mode) ECAP. Moreover, the application of the Conform scheme allows ECAP of titanium at relatively-low temperatures (~ 200 °C), thus enhancing the strengthening effect.