Mechanical properties of crystal-amorphous composites as a function of mean grain size and grain boundary thickness

Abstract

Mechanical properties of polycrystals have been intensively studied as a function of mean grain diameter D, but rarely as a function of (D, l) where l is the mean grain-boundary thickness. Here we measure the mechanical properties in the (D, l) space by simulation for polycrystals with thick grain boundaries which are often called as crystalline-amorphous composites. The strength σ_y of a polycrystal increases with D at D ≳ 50 atoms (i.e., the famous Hall-Petch (HP) behavior) and decreases at D ≲ 50 (i.e., the inverse Hall-Petch (IHP) behavior). These behaviors generally hold for all kinds of polycrystals and have not been generalized to a function of other structural parameters before. Our simulations generalize the HP and IHP behaviors of σ_y(D) to σ_y(D, l). We demonstrate that increasing l and decreasing D have similar effects on reducing dislocation motions and promoting GB deformations. Consequently, the classical HP and IHP behaviors and our generalized HP-IHP behaviors share similar mechanisms and can be unified as σ_y (A_GB∕A_tot), where A_GB∕A_tot is the fraction of the amorphous region. In 2D, we only observe the IHP behavior because dislocations are rare in 2D. In 3D, we observed the HP and IHP behaviors in systems with various particle compositions and interactions. The maximum strength is robustly reached at (D, l) ≃ (50, 5) particles for single-component face-centered cubic solids and at (D, l) ≃ (50, 2) for bidispersed or body-centered cubic solids due to their different activation stresses for dislocation motions. The deformation mechanism changes from dislocation-dominated to grain-boundary-dominated as l increases. The results explain the recent alloy experiments and provide a route to exceed the maximum strength of polycrystals. Besides σ_y, the ductility and elastic moduli are similarly measured in the broad (D, l) space. The ductile and brittle regimes in the (D, l) space exhibit distinct fracture morphologies. Properly thickening grain boundaries can avoid the strength-ductility trade-off. These novel results in the (D, l) space can guide the fabrication of crystalline-amorphous composites with outstanding mechanical properties.

Date of Award	2024
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Yilong HAN (Supervisor) & Qingping SUN (Supervisor)