Effect of Strain Rate on Tensile Ductility and Fracture Behavior of Bulk Nanotwinned Copper

Abstract

A bulk columnar-grained copper with preferentially oriented nanoscale growth twins is prepared by means of direct-current electrodeposition. Tensile tests at different strain rates reveal a significant influence of strain rate on the tensile ductility and fracture behavior. The ductility, especially the postnecking elongation, reduces dramatically at low strain rates, which is associated with evident intergranular fracture. The results suggest that the grain size as well as the grain boundary microstructural evolution exert a strong influence on the intrinsic fracture mode in the nanotwinned structure.Hierarchical microstructure with nanoscale twin boundaries (TBs) incorporated into polycrystalline metals has attracted world-wide attention over the past decade, as the nanotwinned structure simultaneously exhibits several superior mechanical properties, including ultra-high strength, good ductility, and enhanced work hardening. [1][2][3][4][5][6] It has been accepted that the coherent TBs at the nanoscale play an essential role in generating the outstanding properties not only by obstructing motion of dislocations but also by providing ample space for dislocation storage. [1,7] Intrinsic microstructure characteristics, including twin length (i.e., grain size) and twin thickness, affect the mechanical properties as well as the plastic deformation mechanisms. [8][9][10] Recently, via a columnar-grained Cu with nanoscale growth TBs preferentially oriented parallel to the deposition plane, it is further instructive to detect the fact that TB orientation with respect to the loading direction could substantially affect the dominating dislocation mechanism for both monotonic and cyclic deformation. [11][12][13] For instance, if TBs are mostly aligned with tensile direction, the plastic deformation is mainly accomplished by threading dislocations nucleating from grain boundaries (GBs) and bowing out under the constraint of neighboring TBs. [12,14] Strain rate effect on plastic flow behavior is generally studied in order to reveal the intrinsic deformation mechanism of metallic materials. [15][16][17] Through systematically investigating the strain rate sensitivity of nanotwinned Cu (nt-Cu) with randomly oriented nanoscale twins, Lu et al. demonstrated an substantially elevated strain rate sensitivity contributed by strong dislocation-TB interactions as the twin thickness was reduced down to tens of nanometer. [18][19][20] For the nt-Cu with highly oriented nanoscale twins, Ye et al. demonstrated a TB orientation dependence of strain rate sensitivity, a reflection of the intrinsic plastic anisotropy. [21] On the other hand, tensile tests at different strain rates only revealed a slightly enhanced ductility with increasing strain rate. [18,22] The previous studies of strain rate effect on the mechanical response of nanotwinned structure have mainly dealt with samples with grains in the nanometer and submicron meter scale ( 500 nm). However, it has also been demonstrated that GBs can interact with impinging dislocations, lea...

Deformation Mechanisms of FCC-Structured Metallic Nanocrystal with Incoherent Twin Boundary

“…As the mechanical properties of materials are directly related to their deformation mechanisms [6][7][8][9][10][11][12][13][14], revealing the atomic-scale deformation mechanisms of materials is important for understanding their mechanical performance and realizing their desired mechanical properties [11,12,[15][16][17][18]. In recent decades, a large number of studies have been conducted on the deformation mechanisms of twin-structured metals [16][17][18][19][20][21][22][23][24][25][26][27][28]. Many molecular dynamics (MD) simulations and experimental investigations have suggested that the ultrahigh strength of twin-structured metals results from partial dislocations intersecting with coherent twin boundaries (CTBs) [10,[20][21][22].…”

Kinematic and isotropic strain hardening in copper with highly aligned nanoscale twins

“…This phenomenon is associated with plastic relaxation occurring in the vicinity of GBs once the local dislocation density or stress is larger than some critical value. Among the possible relaxation processes are dislocation reconfiguration leading to the formation of dislocation cells or sub-grains [11,14], localized destruction of growth nanotwins [14,34], and even intergranular cracking that appears to be enhanced at low strain rates [13].…”

“…There have been plenty of investigations dealing with the mechanisms how microstructural parameters, such as twin thickness, twin boundary (TB) orientation, and grain diameter (or twin length), influence the mechanical response, especially for their unique strain hardening behavior [9][10][11][12][13]. For example, in columnar-grained NT Cu with TBs in most grains highly aligned with the tensile direction, it has been demonstrated that plastic deformation tends to concentrate along grain boundaries (GBs), resulting in a strong dependence of tensile ductility on grain size [14].…”

A Review on Heterogeneous Nanostructures: A Strategy for Superior Mechanical Properties in Metals

“…Mechanical performance of metals and alloys is always sensitive to strain rates, especially for NC metals [98][99][100][101][102][103]. The SRS can be defined as: = ⁄ , where σ is the flow stress and ε is the strain rate.…”

“…ε is the strain rate. SRS generally reflects the ability of materials to resist localized deformation when deformation is unstable at higher strain rate [71,[98][99][100][101][102][103]. In general, the high SRS means higher elevation in flow stress with increasing strain rate.…”