Thickness effect on fracture behavior of columnar-grained Cu with preferentially oriented nanoscale twins
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sheng You Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China
Lei Lua),b) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China (Received 1 May 2017; accepted 12 July 2017)
The effect of specimen thickness on fracture toughness and fracture mechanism was investigated in bulk columnar-grained Cu with preferentially oriented nanoscale growth twins. Below a critical specimen thickness of ;1.0 mm, plane stress state prevailed ahead of the crack tip and the fracture initiation toughness JC decreased with decreasing thickness. Above the critical thickness, JC decreased with increasing thickness until approaching an intrinsic thickness-independent value when the crack front was mainly under plane strain condition. Under plane strain condition, threading dislocations were majorly activated to glide along the nanotwin channels and to produce severe stress concentrations when they piled-up against grain boundaries (GBs). As a result, intergranular cracking mediated the failure of the nanotwinned Cu. On the contrary, under plane stress condition, dislocations slipping-transfer across twin boundaries (TBs) or partial dislocations gliding at TBs were activated to accommodate the plastic deformation. Consequently, stress intensification at GBs was plastically relaxed through enhanced detwinning and shear banding, which suppressed the intergranular fracture and promoted transgranular shear fracture.
I. INTRODUCTION
Nanotwinned materials, with a high density of nanoscale twins embedded within micro- or submicro-sized grains, have attracted worldwide interest over the past decade, due to their unique mechanical performances and physical properties, such as high strength, considerable tensile ductility, superior fatigue resistance, and excellent electronic conductivity.1–6 These properties are promising for many engineering applications. However, to ascertain the safety and reliability of the potential applications, in-depth investigations of the fracture behavior and underlying failure mechanisms associated with the nanotwinned structures are still highly needed. Previous studies appear to evidence considerable tolerance of nanoscale coherent twin boundaries (TBs) Contributing Editor: Yuntian Zhu a) Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/editor-manuscripts/. DOI: 10.1557/jmr.2017.309
to damage nucleation and propagation.7–10 Molecular dynamics simulations demonstrated that interactions between TBs and different types of dislocations emanated from crack tip could relieve stress concentration and enhance the plastic strain accommodation.11–13 In situ transmission electron microscope (TEM) observations revealed that the crack was perio
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