In Situ TEM Study of Interaction between Dislocations and a Single

Letter www.acsami.org

In Situ TEM Study of Interaction between Dislocations and a Single Nanotwin under Nanoindentation Bo Wang,†,‡ Zhenyu Zhang,*,† Junfeng Cui,†,‡ Nan Jiang,‡ Jilei Lyu,‡ Guoxin Chen,‡ Jia Wang,§ Zhiduo Liu,⊥,|| Jinhong Yu,‡ Chengte Lin,‡ Fei Ye,# and Dongming Guo† †

Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education, and #School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China ‡ Key laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China § Department of Microelectronic Science and Engineering, Ningbo University, Ningbo 315211, China ⊥ State Key Laboratory of Integrated Optoelectronics, CAS Center for Excellence in Brain Science and Intelligence Technology, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China || University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Nanotwinned (nt) materials exhibit excellent mechanical properties, and have been attracting much more attention of late. Nevertheless, the fundamental mechanism of interaction between dislocations and a single nanotwin is not understood. In this study, in situ transmission electron microscopy (TEM) nanoindentation is performed, on a specimen of a nickel (Ni) alloy containing a single nanotwin of 89 nm in thickness. The specimen is prepared using focused ion beam (FIB) technique from an nt surface, which is formed by a novel approach under indentation using a developed diamond panel with tips array. The stiffness of the specimen is ten times that of the pristine counterparts during loading. The ultrahigh stiffness is attributed to the generation of nanotwins and the impediment of the single twin to the dislocations. Two peak loads are induced by the activation of a new slip system and the penetration of dislocations over the single nanotwin, respectively. One slip band is parallel to the single nanotwin, indicating the slip of dislocations along the nanotwin. In situ TEM observation of nanoindentation reveals a new insight for the interaction between dislocations and a single nanotwin. This paves the way for design and preparation of high-performance nt surfaces of Ni alloys used for aircraft engines, gas turbines, turbocharger components, ducts, and absorbers. KEYWORDS: in situ TEM, Ni alloy, nanoindentation, single nanotwin, slip

T

single elemental materials, and they are widely used in structure materials and engineering. A specimen containing a single nanotwin is essential to investigate the fundamental mechanisms of interaction between dislocations and a single nanotwin. To obtain the specimen, an nt surface of an alloy is necessary. To perform the in situ TEM nanoindentation, a single nanotwin with a length up to several micrometers is required, in terms of the size limitation of the indenter and instrument. This demands that the specimen has both a length and width up to several micrometers. It is a challenge for the traditional fabrication means of nt materials, because of the random orientations of nanotwins in grains with sizes of several hundreds of nanometers.1 The conventional methods for fabrication nt materials consist of electro-

ensile strength of nanotwinned (nt) copper (Cu) is ten times that of pristine coarse-grained counterparts, remaining ductile.1 Nt materials have attracted much attention,2−6 in light of their unique physical and mechanical properties. Nevertheless, the fundamental mechanisms between the dislocations and a single nanotwin are not understood. In situ transmission electron microscopy (TEM) mechanical testing is an effective way for the direct observation at nanoscale.7−15 It is widely used for the discovery of fundamental mechanisms of mechanical deformation at nanoscale, as well as the direct observation of experimental evidence. Nonetheless, in situ TEM mechanical testing focuses on compression,7,8,13,15 tension,10,12,14,15 and bending.10 Moreover, the specimens of in situ TEM testing are mainly single elemental materials, such as magnesium (Mg),10,14 silicon (Si),8 platinum (Pt),12 gold (Au),13 aluminum (Al),15 etc. Little has been reported on the in situ TEM nanoindentation for an alloy. Alloys have superior mechanical properties compared with © XXXX American Chemical Society

Received: July 27, 2017 Accepted: August 22, 2017 Published: August 22, 2017 A

DOI: 10.1021/acsami.7b11103 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. Characterization of a developed diamond panel with tips array. (a) Photograph, (b) Raman, (c) XRD, (d) photograph of reflection of an incident red laser, and SEM images at (e) low and (f) high magnifications of the developed diamond panel with tip array.

Figure 2. Fabrication and characterization of an nt surface of the Ni alloy. (a) Surface roughness and (b) schematic diagram of fabrication for an nt surface on the polished Ni alloy, (c) cold field-emission SEM image, cross-sectional TEM images at (d) low magnification and (e) enlarged images, (f) marked by a white square and circle, respectively, in d of an nt surface of the Ni alloy. Inset in d shows the corresponding SAED pattern.

deposition,1 magnetron sputtering,16 surface mechanical attrition treatment,17 and dynamic plastic deformation.18 These methods are difficult to form uniform nt surfaces, due to the difficulty to control the preparation conditions on nt surfaces at microscale.1,16−18 Furthermore, they employ the toxic solution1 or extreme conditions.17,18 The density of nanotwins is very high in the nt materials prepared by conventional approaches,1 resulting in it being difficult to acquire the specimen at microscale with a single nanotwin.16−18 Figure 1 illustrates the photograph, Raman, XRD, photograph of reflection of an incident red laser, and SEM images at

low and high magnifications of a developed diamond panel with a tip array. The diamond panel is black, as pictured in Figure 1a. It has a length of 11.5 mm, a width of 10 mm, and a thickness of 0.5 mm. The diamond panel consists of nanocrystals, microcrystals, and a graphitic phase, displaying the black color. Six peaks are identified at 1130, 1192, 1332, 1350, 1458, and 1548 cm−1 (Figure 1b) after the Lorentz peak-fit processing from the measured Raman curve. The characteristics of diamond is present at 1332 cm−1.17,19 Three peaks at 1130, 1192, and 1458 cm−1 derive from the nanocrystals,19,20 which is induced by the C−H bonds at grain boundaries of nanocrystals. B


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Figure 3. In situ TEM nanoindentation on the specimen containing a single nanotwin. (a) SEM and (b) TEM images of the specimen, (c) schematic diagram of nanoindentation and (d) the loading−unloading curve of in situ TEM nanoindentation on the specimen containing a single nanotwin. Inset in b shows the corresponding SAED pattern. (d) For a comparison, the loading−unloading curve of in situ TEM indentation on a pristine counterpart of the Ni alloy is illustrated.

Two peaks at 1350 and 1548 cm−1 are D and G bands, respectively.19,20 Sharp diamond peaks of {111} and {200} are confirmed by XRD in Figure 1c. A beautiful red rainbow is illustrated in Figure 1d, which is reflected by an incident red laser on the diamond panel with tips array. This exhibits the character of tips array. An array are composed of uniform pyramidal tips on the surface of the diamond panel, as diagramed in Figure 1e. The distance between adjacent two diamond tips is 30 μm. Each pyramidal diamond tip is 30 × 30 μm2 at the bottom and 21.2 μm in height. The included angle of 70° of a diamond tip is marked in Figure 1f with a green color. Arithmetic average surface roughness Ra and peak-to-valley (PV) values are 0.65 and 7.1 nm, respectively on the polished surface of the Ni alloy, as observed in Figure 2a. A schematic diagram of fabrication for an nt surface is illustrated in Figure 2b, in which the developed diamond panel is employed to indent the polished surface and generate an nt surface. Stripes of nanotwins are found on the nt surface, as shown in Figure 2c. All the nanotwins are along the same orientation, reaching a length to several micrometers. This is different from the nt Cu prepared by electrodeposition, where the orientations of nanotwins are random distributed in grains with sizes of hundreds of nanometers.1,24 A specimen containing a single twin is cut by FIB technique, which is selected from a stripe on the nt surface, different from the dense stripes in Figure 2c. This is a novel approach to obtain a specimen including a single nanotwin, using for the investigation of fundamental interaction mechanisms between dislocations and a single nanotwin. High density nanotwins are observed, which is determined by the regular double-dot pattern of selected area electron diffraction (SAED) in the inset of Figure 2d. Two orientations of nanotwins with an angle of 70.53° are obvious along the {111}

planes in Figure 2d. In Figure 2e, there are eight twin boundaries, marked by white arrows, forming seven nanotwins. The thicknesses of seven nanotwins are 1.71, 0.96, 6.58, 2.09, 1.63, 2.94, and 1.55 nm from the top left to the bottom right corners. The average thickness of seven nanotwins is 2.51 nm. The critical twinning stress is calculated as follows21

τc =

2αμbp λ

+

γ bp

(1)

where α is a constant, μ is the shear modulus, bp is the magnitude of the Burgers vector of the Shockley partial dislocation, λ is the thickness between two adjacent twin boundaries, and γ is the stacking fault energy (SFE). For C2000, α, γ, and μ are 0.5, 1.22 mJ/m2, and 64.5 GPa.22−24 The values of bp and λ are 0.149 nm23 and 2.51 nm, respectively. Thus, τc is 3.86 GPa, which is consistent with previous reports.25,26 Two nanotwins with thicknesses of 2.1 and 1.05 nm are shown in Figure 2f, and clearer crystal lattice of nanotwins is characterized by an aberration corrected TEM. The specimen containing a single nanotwin is transparent in Figure 3a, because of the thickness of the specimen is 150 nm. The single nanotwin has a thickness of 89 nm, marked in Figure 3b. SAED pattern diagrams the regular double-dot pattern in the inset of Figure 3b, showing the characteristics of the twin. A schematic diagram of nanoindentation is drawn in Figure 3c. The curvature radius of the tip of indenter is 50 nm. Considering the transmitted depth of electrons and convenience of nanoindentation, the thickness of the specimen is set at 150 nm in Figure 3a. Loading−sunloading curves of in situ TEM nanoindentations are pictured in Figure 3d. In situ TEM nanoindentations consist of loading and unloading, without the dwelling operation. There are two stages during loading for the C


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Figure 4. In situ TEM nanoindentation process on the specimen containing a single nanotwin of the Ni alloy. Snapshots of in situ TEM nanoindentation at indention depths of (a) 280, (b) 410, (c) 450, (e) 556 and (f) 580 nm during loading, and TEM images (h), (i), (j) marked by a black rectangle, circle and square respectively in (f) after unloading. (d) The corresponding SAED pattern in (a) marked by a black square. Inset in (i) showing the corresponding SAED pattern marked by a black circle. (b) and (e) corresponding to the Peak I and Peak II in Figure 3(d) respectively.

dislocations induced by the indentation. This contributes for the ultrahigh stiffness. Moreover, double-dot pattern displays the characteristics of twins in Figure 4d. This is also responsible for the ultrahigh stiffness. The formation of nanotwins is related to the SFE and critical twinning stress τc. The SFE and τc for the Ni alloy of C-2000 are 1.22 mJ/m2 and 3.86 GPa, respectively.22−24 The SFE is very low, which is easy to slip under stress induced by the indention. With increasing the indentation depth, the stress reaches the τc, resulting in the production of twins. The curvature radius of the tip of indenter is 50 nm, leading to high stress generated under the tip and facilitating the formation of twins. It is similar to the developed diamond panel with tips array, in which the tips are sharp, as shown in Figure 1e, f, resulting in the fabrication of an nt surface (Figure 2d). On the other hand, the critical twin nucleus thickness, λc, i.e., the thickness of a twin embryo is presented29

in situ TEM indentation on the specimen containing a single nanotwin. Stages I and II constitute the loading process, and unloading process is composed of an elastic−plastic curve. Load increases sharply in tage I, and two peaks appear in stage II. This is different from the two approximately linear curves present in the nanoindentation on the pristine counterpart of the Ni alloy. At stage I, the stiffness is expressed as27,28 S=

dP dh

(2)

where h is the indentation depth. With increasing depths from 268 to 296 nm, loads increase 59.17 and 5.97 μN for the specimen containing a single nanotwin and pristine counterpart, respectively, meaning that the stiffness of the former is 9.9 times that of the latter. It is interesting that the stiffness of the specimen is ten times that of the pristine counterpart of the Ni alloy. It is intriguing for the ultrahigh stiffness during loading on the specimen containing a single nanotwin. To investigate the fundamental mechanism of this phenomenon, in situ TEM nanoindentation is illustrated in Figure 4 and Movie S1. A snapshot is pictured in Figure 4a at an indentation depth of 280 nm, falling in the depth increasing range for the happening of ultrahigh stiffness. Plastic deformation zone (PDZ) arrives at the single nanotwin in panel a, impeding the transfer of

λc =

5π ρμγTB 2 σ2

(3)

where ρ is the ratio of thickness to diameter of the twin embryo (a constant), σ is the driving stress of twin nucleation, and γTB is the twin boundary energy. The shear modulus, μ, of C-2000 is 64.5 GPa.22,24 The critical twin nucleus thickness, λc, is proportional to γTB, and reversely proportional to σ2. The σ is D


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and the penetration of dislocations over the single nanotwin leads to the second drop of load. After unloading, dislocations are stopped at the loading direction at the right side of the impression. A slip band is parallel to the single nanotwin after unloading, revealing the slip of dislocations along the nanotwin. Another slip band is rebounded by the nanotwin, displaying the impediment of the nanotwin to the dislocations. In situ TEM observation of nanoindentation reveals the evolution of deformation at nanoscale, which is vital for the understanding of the interaction between the dislocations and nanotwins.

relatively large under the tips of the indenter and diamond panel with tips array. γTB is proportional to the SFE. The SFE of C-2000 is very low, bringing about the low λc that gives rise to the formation of nanotwins, as found in Figures 4d and 2e, f. Because of the impediment of the single nanotwin, the dislocations slip along the naontwin rightward. With increasing the indentation depth to 410 nm (Figure 4b), another slip system is activated, generating the drop of load, corresponding to peak I in Figure 3d. The PDZ is enlarged in Figure 4b compared to that in Figure 4(a). An angle between two {111} slip planes is 70.53°, as marked in Figure 4b. When the indentation depth reaches 450 nm in Figure 4c, the dislocations expand forward and rightward under the tip. Nevertheless, the dislocations are blocked and stopped by the single nanotwin at the lower side. With increasing the indentation depth to 556 nm (Figure 4e), PDZ is further enlarged, arriving at the nanotwin and starting to penetrate the single nanotwin. This results in the drop of load, corresponding to peak II in Figure 3d. Increasing the indentation depth to 580 nm (Figure 4f), the PDZ penetrates and reaches another side of the single nanotwin. Prior to the indentation depth of 556 nm, the PDZ slips along the single nanotwin, leading to the enlargement of PDZ at the right side under the tip. After 556 nm, dislocations penetrate the single nanotwin, forming a new PDZ. After unloading, the penetrated dislocations are left, as illustrated in Figure 4h, locating vertically under the tip of indenter, as marked by a black rectangle in Figure 4f. Highdensity dislocations are found below the single nanotwin, indicating the impediment of the single nanotwin to the dislocations. Two slip bands are observed in Figure 4i, which is located at the right side of the tip of indenter. Slip band I is parallel to the single nanotwin, meaning the slip of dislocations along the nanotwin. Slip band II is tilted to the single nanotwin, which is hindered and rebounded by the single nanotwin. This verifies the impediment of the single nanotwin to the dislocations. Nanotwins are formed in the slip bands, which is confirmed by the double-dot pattern of SAED in the inset of Figure 4i. High density dislocations are completely stopped by the single nanotwin at the loading side (Figure 4j), which is situated at the right side of slip band II. The penetration of PDZ is attributed to the interaction between the dislocations and the single nanotwin. In the interaction, an incident 60° dislocation dissociates into another 60° dislocation and a Shockley partial dislocation. The produced 60° dislocation penetrates through the single nanotwin, leaving the Shockley partial dislocation at the single nanotwin. The interaction is proposed as follows30 1 1 1 [101] = [101]T + [ 1̅ 1̅ 2] 2 2 2

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11103. Detailed experimental section, additional characterizations and performance tests, and caption for the in situ TEM nanoindentation on a specimen of Ni alloy containing a single twin (PDF) Video S1 (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhenyu Zhang: 0000-0002-2393-520X Author Contributions

Z.Y.Z., N.J., and D.M.G. conceived the projects. B.W., G.X.C,. and J.F.C. performed the in situ TEM nanoindentations. B.W. and J.L.L. deposited the diamond panel with tips array. Z.Y.Z. wrote the paper. B.W., Z.Y.Z., J.W., Z.D.L., J.H.Y., C.T.L., N.J., and F.Y. analyzed the ultrahigh stiffness happened during the loading process. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.W. is grateful for the valuable discussion with Prof. Mingzhi Dai at NIMTE, CAS, for the deformation mechanism of nt structure. The authors acknowledge financial support from the Excellent Young Scientists Fund of NSFC (51422502), Integrated Program for Major Research Plan of NSFC (91323302), Science Fund for Creative Research Groups of NSFC (51621064), Changjiang Scholars Program of Ministry of Education of China, the Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF14A03), Tsinghua University, the Science Fund of the State Key Laboratory of Metastable Materials Science and Technology (201501), Yanshan University, the Xinghai Science Funds for Distinguished Young Scholars and Thousand Youth Talents at Dalian University of Technology, the Natural Science Foundation of Jiangsu Province (BK20151190), Distinguished Young Scholars for Science and Technology of Dalian City (2016RJ05), the Science Fund of Key laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (2017K02), and the Collaborative Innovation Center of Major Machine Manufacturing in Liaoning.

(4)

In the interaction process, (111) is the assumed twin boundary (TB), and the dissociated 60° dislocation is gliding in (11̅ 1̅ ) plane. The absorption of dislocation in the TB plays an important role to maintain the plasticity of the single nanotwin. In summary, a novel approach is proposed to fabricate an nt surface using a developed diamond panel with tips array. A specimen containing a single nanotwin is obtained, which is cut from the nt surface after CMP. In situ TEM nanoindentation is conducted on the specimen. Ultrahigh stiffness is observed during loading on the specimen. This is because of the formation of nanotwins under the tip of indenter and the impediment of the single nanotwin to the dislocations. The activation of a new slip system results in the first drop of load, E


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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on August 23, 2017, with errors in Figure 1, panels e and f. The corrected version was reposted on August 23, 2017.

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In Situ TEM Study of Interaction between Dislocations and a Single

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