Grain size effect on deformation twin thickness in a nanocrystalline metal with low stacking-fault energy
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FOCUS ISSUE
INTRINSIC AND EXTRINSIC SIZE EFFECTS IN MATERIALS
Grain size effect on deformation twin thickness in a nanocrystalline metal with low stacking-fault energy Yusheng Li1
, Liangjuan Dai1, Yang Cao1, Yonghao Zhao1,a), Yuntian Zhu2
1
Nano and Heterogeneous Materials Center, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China 2 Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, USA a) Address all correspondence to this author. e-mail: [email protected] Received: 29 November 2018; accepted: 20 May 2019
Grain size effect on twin thickness has been rarely investigated, especially when the grain size is less than 1000 nm. In our previous work (Mater. Sci. Eng. A527, 3942, 2010), different severe plastic deformation techniques were used to achieve a wide range of grain sizes from about 3 lm to 70 nm in a Cu–30% Zn alloy. Transmission electron microscopy (TEM) revealed a gradual decrease in the deformation twin thickness with decreasing grain size. In the present work, high-resolution TEM was used to further identify deformation twins and measure their thickness, especially for grain sizes below 70 nm. The twin thickness was found to gradually reduce with decreasing grain size, until a critical size (20 nm), below which only stacking faults were observed. Interestingly, the relationship between twin thickness and grain size in the ultrafine/nanocrystalline regime is found similar to that in the coarse-grained regime, despite the differences in their twinning mechanisms. This work provides a large set of data for setting up a model to predict the twin thickness in ultrafine-grained and nanocrystalline face-centered cubic materials.
Introduction Deformation twinning in metals and alloys has been investigated extensively because it is one of the few mechanisms
KTB rTB ¼ pffi t
;
ð1Þ
that can increase strength and ductility simultaneously [1, 2,
where KTB is a constant value and t is the average twin thickness. It is obvious that, when the twin thickness is larger than a critical
3]. It is well known that the activation of deformation
value, the strengthening effect of twin boundaries is inversely
twinning is affected by intrinsic physical and structural
proportional to the twin thickness [10]. Apparently, besides
characteristics of materials [e.g., stacking-fault energy (SFE)
twinning density, twin thickness is another important micro-
and grain size] [4] and extrinsic deformation conditions (e.g.,
structural feature, and increasing interests have been put on
strain, strain rate and temperature) [5, 6]. Among these
nanoscale twinned materials with unprecedented strengths [1,
factors, the grain size effect is less explored because its
11]. Twin thickness has been reported to be affected by SFE [12]
significance was mainly noticed after the discovery of nano-
and extrinsic deformation conditions such as strain rate and
crystalline (NC) materials. The grain size effect on def
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