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Acta Metall Sin  2016, Vol. 52 Issue (4): 491-496    DOI: 10.11900/0412.1961.2015.00503
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DETWINNING BEHAVIOR INDUCED BY LOCAL SHEAR STRAIN IN NANOTWINNED Cu
Jingsheng BAI,Qiuhong LU,Lei LU()
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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Abstract  

Nanotwinned materials have attracted widespread attention due to their superior mechanical properties, such as high strength, good ductility and work hardening. Experimental and molecular dynamics (MD) simulation results had indicated that there are three distinctly different dislocation-mediated deformation mechanisms in nanotwinned metals, namely dislocation pile-up against and slip transfer across twin boundaries (TBs), Shockley partials gliding on twin boundaries leading to twin boundary migration, and threading dislocations slip confined by neighboring twin boundaries. However, most of the previous studies are focused on the homogenous plastic deformation under tension and compression tests, the non-homogenous deformation and its deformation mechanism, especially under low strain and complex stress condition/confined condition, of nanotwinned metals are still not explored so far. In this study, the electrodeposited bulk Cu samples with preferentially oriented nanotwins were cold rolled with the normal of the rolling plane parallel to the growth direction (ND//GD) to strain of 15% at room temperature. The microstructure features of as-rolled Cu were investigated by SEM and TEM. Microstructure evolution indicates that many detwinning bands appeared in the direction about 30°~45° with respect to the rolling direction, which is the direction with the largest shearing stress. The twin lamellae in the detwinning bands coarsened obviously. Based on calculation of the local shear strain and strain gradient of TBs in a selected detwinning band, it indicates that the maximum shear strain occurs in the middle of the deformation bands, and its detwinning mechanism is directly related the localized shear strains (γ). The twin lamellae in the detwinning bands were coarsened obviously. When 0.3<γ<0.8, the detwinning process via producing amount of Shockley dislocations on twin boundaries dominates the deformation. After detwinning, Shockley partial dislocations stored at the area with the maximum strain gradient and formed incoherent twin boundaries (ITBs). The present investigation indicates detwinning process dominates the plastic deformation and sustains the local shearing strain in nanotwinned Cu at small strains under cold rolling.

Key words:  Cu      nanoscale twin      cold rolling      detwinning      shear strain     
Received:  30 September 2015     
Fund: Supported by National Basic Research Program of China (No.2012CB932202) and National Natural Science Foundation of China (Nos.51420105001, 51371171 and 51471172)

Cite this article: 

Jingsheng BAI,Qiuhong LU,Lei LU. DETWINNING BEHAVIOR INDUCED BY LOCAL SHEAR STRAIN IN NANOTWINNED Cu. Acta Metall Sin, 2016, 52(4): 491-496.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00503     OR     https://www.ams.org.cn/EN/Y2016/V52/I4/491

Fig.1  Longitudinal-section SEM-BSE image (a) and TEM bright-field image (b) of the as-deposited columnar-nanotwinned (nt) Cu sample, and the corresponding selected area electron diffraction (SAED) pattern (inset) (GD—growth direction of the electro-deposited Cu)
Fig.2  Longitudinal-section SEM-BSE image of cold rolled nt-Cu sample showing the deformation bands along the shear direction indicated by arrows (ND—normal direction of cold rolling, RD—rolling direction)
Fig.3  Longitudinal-section TEM image of cold rolled nt-Cu sample, showing a detwinning band (a), SAED patterns corresponding to the regions labeled by b (b) and c (c) in Fig.3a, distributions of twin thickness in as-deposited sample (d) and detwinning bands of the as rolled sample (e)
Fig.4  Enlarged TEM image of the square region in Fig.3a (a), schematic of the morphology of the twin boundaries (TBs) in Fig.4a (b), geometric relation for calculating the shear strain (θ0—angle of shear direction (AA') to the direction of original TB (BB'); θ—angle of the shear direction (AA') to the direction of curved TB (CC'); δ—width (transverse to the shear direction) of a unit segment; d—shear displacement of a unit segment) (c) and distributions of shear strain and strain gradient along the TB marked by the dashed line in Fig.4b (d) (Arrows pointed to the areas with γmax of the twin lamellae, γmax—maximum value of shear strain)
Fig.5  Schematics of the detwinning process assisted by dislocation-TB interaction, where the arrows indicate the compression stress along ND and the tension stress along RD during cold rolling
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