In-situ study on tensile deformation and damage evolution of metastable β titanium alloy with lamellar microstructure

Abstract

Tensile deformation and damage evolution of a metastable β titanium alloy (Ti–5Cr–4Al–4Zr–3Mo–2W–0.8Fe) with lamellar microstructure are studied by in-situ tensile test under scanning electron microscopy. In the tensile process, the main deformation modes include the dislocation slip, phase interface shear and grain boundary shear, and the geometrical orientation of α lamellae determines the activation of different slip systems and whether the interface shear is involved in deformation. The α lamellae may kink and even fragment under severe deformation. Slip transfer is prone to occur between α lamella and β interlayer that satisfy the Burgers orientation relationship, and the slip lines will deflect and bifurcate as more slip systems are activated and grains rotate. Due to the localized stress concentration and inhomogeneous deformation, the grain boundaries, phase interfaces, tips of β interlayers, junctions of colonies composed of α lamellae and shear bands are all the positions where are easy to generate microvoids. The crack propagates along a zigzag path on account of the synergistic reaction mechanism of critical resolved shear stress, activated slip system, shear band and grain boundary shear, resulting in an intergranular and transgranular mixed fracture mode.

Introduction

Due to high specific strength, excellent crack resistance and satisfactory corrosion resistance, metastable β titanium alloys have been widely used in the fields of aerospace and marine [[1], [2], [3]]. Various microstructures of metastable β titanium alloys can be purposefully tailored through proper thermomechanical and heat treatment processes. For example, the typical bimodal and lamellar microstructures are expected to achieve excellent comprehensive mechanical properties of β titanium alloy, which is applied to the structural components [4,5]. Specifically, the alloys with lamellar microstructure have moderate strength, good crack propagation resistance, but limited plasticity. Therefore, an in-depth understanding of the deformation behavior and damage evolution of the alloys with lamellar microstructure is of great significance to fully tap the formability of alloys and effectively improve the service life of components [6].

With respect to the titanium alloys with lamellar microstructure, the plastic deformation behavior is mainly affected by the geometrical orientation of α lamella [7]. Due to the obvious anisotropy, the activated slip systems in the lamellae with different orientations are not the same [[8], [9], [10]]. Previous studies have focused mainly on the tensile property and embrittlement of alloys [11,12], but relatively little attention has been paid to the dependence of deformation mechanism on activated slip systems in lamellae. In addition to dislocation slip, the deformation of phase interface and grain boundary, as another important deformation mode, also deserves attention [13,14]. Moreover, due to local stress concentration, the phase interface and grain boundary are usually the preferred positions for dislocation accumulation and damage generation. As an effective deformation coordination mode, the deformation twins may appear in the lamellae with specific orientation [[15], [16], [17]]. It can be seen that the actions of various deformation modes of titanium alloy with lamellar microstructure and the responses of lamella itself in the deformation process are worth further exploring.

The complex microstructures in titanium alloys with lamellar microstructure, such as α lamella, β interlayer, phase interface and grain boundary, often lead to the whole deformation process being affected interactively by multiple deformation modes. It is necessary to observe the dynamic deformation behavior in real time when studying the damage evolution process, so as to decouple the interactive deformation modes. Recently, for these purposes, many studies have been carried out on titanium alloys from elastic deformation to ultimate fracture using in-situ tensile tests under scanning electron microscopy (SEM). Ren et al. [18] observed the shear bands along the α/β interface and the slip bands in the α-laths in Ti–6Al–4V alloy with basket-weave microstructure, and considered that their existence is the main reason for the zigzag crack propagation. Li et al. [19] found that the slip system activation and the grain rotation in Ti–6Al–4V alloy with bimodal microstructure have coordinated effects on the overall deformation. Hémery et al. [20] studied that the occurrence of slip transfer in high probability is due to the high resolved shear stress on the outgoing slip system and low angle grain boundary. Wang et al. [21] observed the complete deformation process and damage evolution of a novel metastable β titanium alloy with single β phase during in-situ tensile tests, and described in detail the slip transfer and crack propagation in equiaxed grain microstructure and their dependence on crystallographic orientation. The lamellar microstructure absent from the above studies has numerous interfaces and strong anisotropy, which will make the initiation and propagation of crack in the damage process more complicated.

In this work, a new metastable β titanium alloy Ti–5Cr–4Al–4Zr–3Mo–2W–0.8Fe (Ti-54432) designed for good compatibility between strength and plasticity was used as the experimental material. To systematically investigate the slip transfer and crack propagation in complex lamellar microstructures, including α lamella, β interlayer, α/β phase interface and grain boundary, the whole processes of tensile deformation and damage evolution of the alloy during in-situ tensile test are observed and analyzed under SEM combined with electron backscatter diffraction (EBSD). The Schmid factor (SF) and geometric compatibility factor (m') related to the slip system activation in α/β phases were calculated. The local microstructure near the crack tip was characterized in detail to explore the dependences of the initiation and propagation of crack on the α/β phases crystallographic orientation, phase interface, grain boundary and slip system.