Detwinning-twinning behavior during compression of face-centered cubic twin-matrix layered microstructure

Highlights

• The conjugated role of FCC detwinning and twinning mechanisms.

• Plastic deformation of FCC twin-matrix system dominated by detwinning.

• Detwinning and twinning mechanism contributing to the texture evolution.

Abstract

In this paper, we have examined the mechanical response of a face-centered cubic layered twin – matrix system, which has been generated in Cu–Al alloy single crystal. This kind of microstructure was obtained by specially designed uniaxial tension experiments of Cu-9at.%Al single crystals oriented for “easy” glide. It has been shown, that detwinning system, which operates along the twinning plane, may at specific crystallographic conditions totally dominate the process of plastic deformation. The detwinning system was activated in a confined area that creates a localized deformation regions in the form of kink bands that were found to be hierarchical in formation (i.e. higher order kinking within already existing kink bands). This mode of plastic deformation generates lattice rotation that was identified to occur around normal to the plane of shear of the active detwinning system. Hence, the paper gives direct experimental prove of the importance of detwinning systems on the mechanical properties and deformation texture analysis of face-centered cubic materials.

Introduction

Detwinning of face-centered cubic (FCC) deformation twins was recently extensively studied both on theoretical and experimental basis. It has been shown, that this mechanism may be involved during uniaxial tension and compression tests of pre-twinned Cu–Al and Au single crystals, as well as different poly- and nanocrystalline materials [[1], [2], [3], [4]]. More recent studies indicate the importance of detwinning by introducing a so-called “soft path” during shear band formation of FCC materials [5]. Other important experimental work, based on electron backscattered diffraction technique, were recently conducted by McCormack et al. and Saleh et al. studying twin-induced plasticity (TWIP) steel, showing direct prove of detwinning of a deformation twin [6,7]. The behavior of cyclic loading of nanocrystalline TWIP steel was very recently also modeled highlighting a major role of detwinning [8]. It was also found, that this mechanism plays an important role during plastic deformation of other engineering materials (e.g. duplex and stainless steels), which gives further evidence of the importance of detwinning mechanism of FCC materials from the practical point of view [[9], [10], [11], [12], [13]]. More basic properties of detwinning were evaluated by studying Cu–Al single crystals. For example, it has been experimentally shown, that the critical resolved shear stress (CRSS) to initiate detwinning was sufficiently lower, than that of the primary deformation twinning responsible for introducing the fine twin – matrix layered system [1,3]. Quasi-static tension experiments of Cu–9%Al single crystals at room temperature have shown, that the CRSS of twinning equal to about 90 MPa. Whereas, during subsequent compression, one may activate a detwinning system at a stress level of ~55 MPa only. It has been demonstrated, that twin boundaries may act as effective barriers for other slip or twin systems that are forced to penetrate them [14]. Taking this into account, it is reasonable to assume, that detwinning mechanism, that possesses low CRSS, may play a substantial role during deformation of the initially twinned material. It is important to emphasize, the difference between a layered twin – matrix system generated by deformation twinning and those created other ways leading to growth or annealing twins. In the latter case, the matrix and twin regions are fairly undistinguishable from each other. One must then arbitrarily choose the twin region that has eventually undergone detwinning by external loading [[15], [16], [17], [18], [19], [20], [21], [22], [23]]. Deformation twins, unlike growth or annealing ones, have also additional physical factors, which allow to distinguish from the untwinned region, the matrix. One of important differences concern some features of dislocation substructure. For instance, the sessile a[001] cubic dislocation of the (100) plane, observed within a deformation twin, as a product of deformation twinning itself, does not occur within the matrix region. This specific difference as well as others were previously described in the work of Basinski et al. [24]. Similar theoretical analysis of dislocation changes induced by detwinning were described in Ref. [25]. Recently, some moderate activity of detwinning was also observed during unloading [26,27]. However, this is not the case considered in this paper, which reports on the detwinning mode activated only after necessary re-loading.

In the current literature of the subject, the experimental studies were mainly focused on the analysis of FCC detwinning mechanism and its role during plastic straining/cycling of polycrystalline materials. On the other hand, the single crystalline experiments have been focused on the evaluation of basic properties such as: twinning stress, dislocation interactions etc. In this paper, using large single crystalline Cu–Al alloy, we were able to track the evolution of microstructure during plastic deformation beyond the activity of “primary” detwinning and observe more complex situation induced by plastic deformation. Thus, we have observed secondary detwinning (detwinning inside a region that has been previously deformed by primary detwinning). Another interesting feature observed at higher strains was the activity of twinning operating together with secondary detwinning, as an accommodation mechanism (Fig. 1). Thus, in this paper using single crystalline materials, we have shown more complex deformation modes in which detwinning participates.

Particularly it is shown that detwinning, because of relatively low CRSS, may at some specific conditions totally dominate the process of plastic deformation of pre-twinned FCC materials characterized by a layered twin – matrix system. In this paper, we will focus on microstructural changes induced by uniaxial compression experiments of Cu–Al twin – matrix system and the associated crystallographic texture evolution.