Unravelling the Size‐Dependent Mechanical Properties of Nanocrystalline Face‐Centered‐Cubic Metals: From the Dislocation Point of View

Abstract

The grain size dependence of strain rate sensitivity, apparent activation volume, and yield stress of nanocrystalline (NC) face‐centered‐cubic metals (FCC) are modeled based on the bow‐out model of single dislocation. It is found that the grain size limitation on the dislocation length gives rise to the grain size effects on the mechanical properties. The model gives predictions that when temperature is at or below room temperature and the strain rate is in the quasistatic range, dislocation‐mediated mechanisms play a dominant role in the plastic deformations of FCC NC metals even for the grain size of several nanometers, which is consistent with known experimental and simulation results. Based on the model, abnormal mechanical phenomena of nanostructured metals, such as the appearance of the inverse Hall–Petch relationship, the existence of the critical grain size, and its strain rate dependence, are well predicted and interpreted.

Theoretical Characterization of the Temperature and Grain Size‐Dependent Yield Strength of Fine‐Grained Metals

“…According to Wang et al, [30] mode-І fracture toughness at different temperatures can be formulated as (7) where K ІC ðT 0 Þ is mode-І fracture toughness at reference temperature T 0 .…”

“…Qiu [ 26 ] also established a grain size‐dependent YS model through the microshear‐band model, but the model did not quantitatively characterize the effect of temperature. Based on the bow‐out model [ 27 ] of single dislocation, Chen [ 7 ] established a grain size‐dependent YS model of face‐centered‐cubic metals; the model can predict the abnormal phenomena of nanostructured metals. However, this model can only give predictions when the temperature is less than or equal to room temperature.…”

“…Yield strength (YS) is one of the key parameters, which determines the safety and reliability of materials. It increases substantially as grain size decreases, [3][4][5][6][7][8][9] so refining grain size is one of the common measures to improve the YS of fine-grained metals. Besides, they are inevitably exposed to different temperature environments during their services, and the YS degenerates dramatically at elevated temperatures.…”

Grain Distribution for Optimal Strength in Homogeneous and Gradient Nano‐Grained Coppers

“…And compared with the wholly ordered structure, the O-R-O structure's PRX contributes to the accumulation of dislocations in the large-grained region, resulting in enhanced resistance to plastic deformation. [48,49] Moreover, PRX causes the formation of many small grains inside the large-grained region of the O-R-O structure. This transforms the O-R-O structure's original unidirectional gradient distribution of "small grains-large grains-small grains" to a multidirectional gradient distribution consisting of alternating small and large grains, which increases the strain strength gradient in the material's plastic deformation, further strengthening the HDI effect.…”