Abstract
While nanocrystalline (NC) metals exhibit superior strength to conventional microcrystalline metals, their thermal instability has hampered their application at high temperatures. Herein, two-dimensional (2D) boron nitride nanosheets (BNNS) are proposed as reinforcement to enhance the strength as well as the thermal stability of NC Al. The strength of pure Al was increased from 80 to 468 MPa by refining its grains from ~600 to ~40 nm, and it was further enhanced to 685 MPa by incorporating 2 vol% of BNNS. Moreover, the small amount of BNNS was found to effectively suppress grain growth of NC Al at 580 °C (~0.9 Tm, where Tm is the melting point of Al), which prevented a strength drop at high temperature. Finally, the Zener pinning model in conjunction with phase-field simulations was utilized to qualitatively analyze the effect of the BNNS on the grain boundary pinning as a function of volume, shape, and orientation of the reinforcement. The model demonstrated that the pinning force of 2D reinforcements is much higher than that of spherical particles. Hence, 2D BNNS offer the possibility of developing Al-matrix nanocomposites for high-temperature structural applications.
Highlights
According to the Hall–Petch relationship[1,2], a reduction in grain size (D) results in an increase in the yield strength with a D−1/2 dependency; when the grain size is relatively large, greater stress can be concentrated near the adjacent grains due to the pile-up of multiple dislocations, leading to decreased yield strength
We have investigated the effect of 2D boron nitride (BN) nanosheets (BNNS), which are mechanically exfoliated from h-BN, on grain growth suppressing behaviors at high temperatures in Al-based nanocomposites
According to the investigation involving grain size evolution (Fig. 3), the grain sizes of monolithic Al and the Al/boron nitride nanosheets (BNNS) composite increased from 47.3 and 39.4 nm to 104.9 and 56.8 nm after heat treatment for 12 h, respectively, and the grain sizes increased further to 112.5 and 72.8 nm after 72 h. These results demonstrate that the grain growth rate decreased with increasing heat treatment time; the corresponding rates for monolithic Al and the Al/ BNNS composite are 4.8 and 1.45 nm/h for 12 h, whereas those for the range between 12 and 72 h are reduced to 0.13 and 0.27 nm/h
Summary
According to the Hall–Petch relationship[1,2], a reduction in grain size (D) results in an increase in the yield strength with a D−1/2 dependency; when the grain size is relatively large, greater stress can be concentrated near the adjacent grains due to the pile-up of multiple dislocations, leading to decreased yield strength. When grain size reduces to within the nanocrystalline (NC, grain sizes below 100 nm) regime, the activities of lattice dislocations become less significant and the yield stress starts to deviate from the Hall–Petch relationship[3,4,5,6,7]. Grain growth can be suppressed by reducing the driving force of grain boundary migration (thermodynamic stabilization) and/or by increasing the activation energy for grain boundary migration (kinetic stabilization)[11,12,13,14]. Segregation of solute atoms at the grain boundaries is an effective method of reducing grain boundary energy, as illustrated by the following equation: www.nature.com/scientificreports/. Kinetic stabilization operates according to the ideal grain growth kinetic model during isothermal heat treatment, as expressed in the following equation: Dtn − D0n = kgg t (2).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
