In a new report on Science Advances, Fenghui Duan and a research team in China detailed continuous strengthening in nanotwinned pure Nickel materials. The material recorded an unprecedented strength of 4.0 GPa at extremely fine twin thickness, 12 times stronger than that of conventional coarse-grained Nickel. Theories suggest diverse mechanisms of softening nano-grained metals. Continuous strengthening can occur in nanotwinned metals with extremely fine twin thickness to realize ultrahigh strength.
It is challenging to experimentally verify this hypothesis while regulating the synthesis of nanotwinned metals with a thickness below 10 nm. In this work, the team developed columnar-grained nanotwinned nickel with twin thickness ranging from 2.9 to 81 nm, using direct current electrodeposition to show the process of continuous strengthening. Duan et al. used transmission electron microscopy (TEM) to reveal the attributes of strengthening and credited the outcomes to the fine-spaced architecture of the material.
The bulk nickel specimens maintained a high purity and contained a high density of nanoscale twin lamellae embedded with nanoscale columnar grains synthesized using direct current electrodeposition in a citrate bath. The team regulated the nickel and citrate ion contents in the electrolyte to refine the average twin thickness. The material showed 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 transmission electron microscopy results.
The scientists then used electrodeposition as a non-equilibrium process for the widespread formation of pure nickel. The stress-relaxed nanotwinned metals were energetically more stable than the highly stressed deposits. The lower concentration ratio of citrate and nickel ion resulted in higher internal tensile stress. The team also added hydrogen to promote twin nucleation. To understand the mechanical properties of the material, they conducted uniaxial compression tests on micropillars with a diameter of 1.3 microns in scale. The stress-strain curves indicated the material with a smaller twin thickness to be stronger, showing that the strengthening behavior is still functional even with a refined twin thickness.
To understand the mechanisms that are responsible for continuous strengthening, Duan et al. characterized the microstructure of the material. To accomplish this, they used a three percent plastic strain on the material region and noted the consistently high density of the nano twins despite deformation, similar to its structure prior to inducing plastic strain. This indicated the high stability of nano twins in the material, a characteristic that arose from the suppressed activity of twinning partial dislocations. The high stacking energy of the material, therefore, played an important role to hinder the detwinning process of the material. Duan et al. further studied the interactions using transmission electron microscopy and confirmed the strengthening mechanisms of the nanotwinned nickel material, as well as the secondary nano twins inherent to the material, which provided it additional strength.
In this way, Fenghui Duan and colleagues showed how secondary nano twins or hierarchical nano twins can be formed in metals or alloys. Researchers had previously developed nucleation and growth of secondary twins and calculated the critical yield stress for twin nucleation in the specimen. Based on the model, they found the existence of a transition in the strengthening mechanism of nanotwinned Nickel at an extremely fine twin thickness. The team showed how the nanotwinned nickel achieved via direct current electrodeposition with its extremely fine twin thickness, exhibited a strength greater than those of pure nickel, derived from continuously strengthening the twin thickness.
Journal Information: Duan F. et al, Extremely nanotwinned pure nickel with extremely fine twin thickness, Science Advances, DOI: 10.1126/sciadv.abg5113 Yip S. et al, The strongest size. Nature, DOI: 10.1038/35254 Wang J. et al, Near-ideal theoretical strength in gold nanowires containing angstrom scale twins. Nature Communications, DOI: 10.1038/ncomms2768