The structure-property relations in natural materials always inspire material scientists to seek ways to enhance the mechanical properties of engineering structural materials.

Bamboo stems possess a gradient structure that comprises a decreasing density of vascular bundles from their exterior inwards the center, leading to enhanced flexibility yet overall strength and stiffness. Attempts have been made to replicate the gradient structure to create structural metallic materials with similar properties.

In such gradient structures, strength and ductility can be achieved, but it is a challenge to guarantee high fracture resistance, as it is local microstructure that affects the fracture toughness of the materials.

Recently, Prof. LI Yi’s team from Institute of Metal Research of the Chinese Academy of Sciences, collaborating with researchers from UC Berkeley, evaluated the fracture properties and crack propagation process in gradient structured (GS) nickel, involving grain size gradients from ~30 nm to 4 μm. Their finding was published in Materials Today.

The researchers found that an optimized combination of high strength and high toughness can be achieved in the gradient structured material, compared with the ultrahigh-strength nano-grained (NG) and low-strength coarse-grained (CG) uniformed grain-sized structures.

CG→NG gradient structure where a pre-existing crack initiates from CG zone and propagates into NG zone displays the best combination of strength and toughness properties, with largest degree of R-curve toughening behavior, similar to the CG material. Once crack extension approaches the end of gradient structure, however, unstable brittle fracture can occur as the crack encounters the nano-sized grains. Therefore, the CG→NG gradient structure represents a characteristic transition from ductile fracture to brittle fracture.

On the other side, the NG→CG gradient structure where a pre-existing crack initiates from NG zone and propagates into CG zone exhibits a degree of R-curve toughening in excess of the NG structure, but less than that of the CG→NG gradient structure. However, it is less susceptible to outright fracture as the propagation of brittle cracks in the nano-grains of the early part of the gradient region become arrested once they reach the coarser-grained regions due to excessive crack-tip blunting. Therefore, the NG→CG gradient structure represents a characteristic transition from brittle fracture to ductile fracture, which might be preferred for some safety-critical applications.

This study not only develops a mechanistic understanding of the fracture behavior of GS materials, but also provides practical guidelines for the use of such materials in safety–critical applications.