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Extremely strong nano-twinned pure nickel with an extremely thin twinning thickness

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Microstructure typical of NT-Ni as deposited with extremely thin binoculars thickness. (A) Three-dimensional structure of NT-Ni composed of bright-field TEM images in plan view and cross-section. (B) Double thickness and (C) Column width distributions measured from TEM and HRTEM images of the deposited specimen NT-2.9. (D) Upper cross-sectional TEM image of sample NT-2.9. (E) HRTEM image taken along the [011] axis of the zones. The inset in (E) shows the electron diffraction pattern corresponding to the selected area. (F) XRD model showing the dominant orientation (111) present in the NT-2.9 sample. au, arbitrary units. Credit: Scientists progress, doi: 10.1126 / sciadv.abg5113

In a new report on Scientists progress, Fenghui Duan and a research team in China detailed the continuous strengthening of nano-twin pure nickel materials. The material recorded an unprecedented 4.0 GPa strength at an extremely thin match thickness, 12 times stronger than that of conventional coarse-grained nickel. Theories suggest various softening mechanisms for nanograined metals. Continuous reinforcement can occur in nano-twin metals with extremely thin binoculars thickness to achieve ultra-high strength. It is difficult to experimentally verify this hypothesis while regulating the synthesis of nanotextiles with a thickness of less than 10 nm. In this work, the team developed nano-twinned columnar-grained nickel with a twin thickness ranging from 2.9 to 81nm, using direct current electroplating to show the continuous strengthening process. Duan et al. used transmission electron microscopy (TEM) to reveal the attributes of the reinforcement and attributed the results to the fine-spaced architecture of the material.

Microstructure of nano-twinned nickel developed

The bulk nickel specimens maintained high purity and contained a high density of nanoscale twin lamellae encrusted with nanoscale columnar grains synthesized by direct current electrodeposition in a citrate bath. The team regulated the levels of nickel and citrate ions in the electrolyte to refine the average thickness of the twins. The material exhibited a narrow distribution ranging from 0.5 to 15 mm. The researchers used magnified micrographs to observe details of the materials and using X-ray diffraction patterns, they noted an out-of-plane crystallographic texture, consistent with the results of transmission electron microscopy.

Mechanisms for developing and strengthening materials.

Scientists then used electroplating as a non-equilibrium process for the widespread formation of pure nickel. The relaxed nanotextiles were energetically more stable than the heavily stressed deposits. The lower concentration ratio of citrate and nickel ion resulted in higher internal tensile stress. The team also added hydrogen to promote nucleation of the twins. To understand the mechanical properties of the material, they performed uniaxial compression tests on micropillars with a diameter of 1.3 microns to scale. The stress-strain curves indicated that the material with a smaller nipple thickness was stronger, showing that the reinforcement behavior is still functional even with a refined nipple thickness.

  • Extremely strong nano-twinned pure nickel with an extremely thin twinning thickness.

    Mechanical properties of NT-Ni abutments. True uniaxial stress-strain curves for the abutments showing that the flow stress at 2% plastic strain in samples NT-2.9 and NT-6.4 is 4.0 and 2.9 GPa, respectively. The true stress-strain curves for NG- and CG-Ni from (22) are also presented for comparison. The red square, orange circle, and blue and black triangles indicate the 2% plastic strain flow stresses for the four samples. The inset shows a diagram of the compression test which was carried out on NT-Ni test pieces with a diameter of 1.3 µm. Credit: Scientists progress, doi: 10.1126 / sciadv.abg5113

  • Extremely strong nano-twinned pure nickel with an extremely thin twinning thickness.

    Continuous reinforcement in NT-Ni. Yield strength variation with mean grain size or double thickness for Ni and Mo microalloy NT-Ni (1.3 at.%), As well as literature data directly obtained from tensile tests and compression for the Ni, Ni, ED pillars deposited by electrolysis (ED) NT-Ni (22, 24-33, 53, 54) and NT-Cu (2). Continuous reinforcement behavior extending to twin thicknesses of 2.9 and 1.9 nm is observed in NT-Ni samples as deposited and NT-Ni microallied with Mo, respectively. Conversely, a softening behavior, i.e. a decreasing yield strength with decreasing grain size or thickness, is observed in NT-Cu as deposited when the average doubling thickness is less than 10 to 15 nm. Credit: Scientists progress, doi: 10.1126 / sciadv.abg5113

  • Extremely strong nano-twinned pure nickel with an extremely thin twinning thickness.

    Deformation mechanisms in NT-Ni with λ = 2.9 nm. (A) Bright field postmortem image, showing the shear band and columnar grains in the sample. The inset shows the morphology of the abutment after uniaxial compression at ~ 3% plastic deformation. (B) An enlarged TEM image over the R1 box in (A) showing the nanotriminal structure preserved in the deformed regions. (C) A typical HRTEM image and (D) its corresponding GPA deformation map (rigid body rotation in the plane, ωxy) in the deformed region, showing that a partial dislocation has slipped with a tilted direction towards twin planes, leaving behind a stacking fault. Credit: Scientists progress, doi: 10.1126 / sciadv.abg5113

The evolution of microstructure and reinforcement mechanisms.

To understand the mechanisms responsible for continuous reinforcement, Duan et al. characterizes the microstructure of the material. To do this, they used a three percent plastic stress on the region of the material and noted the high and constant density of nanotwins despite deformation, similar to its structure before inducing plastic stress. This indicated a high stability of nanotwins in the material, a characteristic which resulted from the suppression of the activity of partial twinning dislocations. The high stacking energy of the material therefore played an important role in hampering the process of de-mating the material. Duan et al. then studied the interactions in transmission electron microscopy and confirmed the reinforcement mechanisms of the nanopatched nickel material, as well as the secondary nanotwins inherent in the material, which give it additional resistance.

Nickel pur nano-jumelé extrêmement résistant avec une épaisseur de jumelage extrêmement fine.

Formation of a secondary nanotumeau in a distorted NT-2.9 sample. (A) HRTEM image of box R2 in Figure 4A showing secondary nanotwins (marked with yellow arrows) crossing the initial TBs formed inside the NT-Ni columnar grains during deformation. (B and C) Enlarged HRTEM images from boxes B and C in (A) respectively showing nucleation and termination of secondary nanometers. (D) Corresponding GPA deformation map (rotation of the rigid body in the plane, xy) for the HRTEM image (C). Credit: Scientists progress, doi: 10.1126 / sciadv.abg5113

Perspectives in materials chemistry

In this way, Fenghui Duan and his colleagues showed how secondary nanotwins or hierarchical nanotwins can be formed in metals or alloys. The researchers had previously developed the nucleation and growth of secondary twins and calculated the critical yield strength for the nucleation of twins in the specimen. Based on the model, they discovered the existence of a transition in the nano-paired nickel strengthening mechanism to an extremely fine match thickness. The team showed how nano-twinned nickel obtained by direct current electrodeposition with its extremely thin twin thickness, exhibited higher strength than pure nickel, derived from the continuous strengthening of the twin thickness.

Hybrid technique to produce stronger nickel for automotive, medical, manufacturing

More information:
Duan F. et al, Extremely nano-twinned pure nickel with extremely fine twinning thickness, Scientists progress, DOI: 10.1126 / sciadv.abg5113

Yip S. et al, The largest size. Nature, DOI: 10.1038 / 35254

Wang J. et al, Theoretical near-ideal force in gold nanowires containing angstrom-scale twins. Nature Communication, DOI: 10.1038 / ncomms2768

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