On the exceptional damage-tolerance of gradient metallic materials

Abstract

An experimental study is described on the fracture toughness and micro-mechanisms associated with the initiation and propagation of cracks in metallic nickel containing marked gradients in grain size, ranging from ~30 nm to ~4 m. Specifically, cracks are grown in a gradient structured (GS) nickel with grain-size gradient ranging from the coarse macro-scale to nano-scale (CG→NG) and vice versa (NG→CG), with the measured crack-resistance R-curves compared to the corresponding behaviors in uniform nano-grained (NG) and coarse-grained (CG) materials. It is found that the gradient structures display a much-improved combination of high strength and toughness compared to uniform grain-sized materials. However, based on J-integral measurements in the gradient materials, the crack-initiation toughness is far higher for cracks grown in the direction of the coarse-to-nano grained gradient than vice versa, a result which we ascribe primarily to excessive crack-tip blunting in the coarse-grained microstructure. Both gradient structures, however, display marked rising R-curve behavior with exceptional crackgrowth toughnesses exceeding 200 MPa.m ½ .

Heterostructured materials: superior properties from hetero-zone interaction

“…Most GS materials reported thus far are related to grain-size gradient [27][28][29]65,[68][69][70]. A typical gradient structure consists of nano-grained or nanostructured surface layers and a coarse-grained central layer with grain-size gradient in-between [27,29].…”

“…GS materials have been reported to exhibit synergistic strengthening [27,28], in which the global yield strength is higher than what is predicted on the basis of the rule-of-mixtures (ROM) of the strengths of constituent layers [28]. In addition, GS materials can demonstrate fracture toughness and fatigue properties that are much more superior compared to those of their homogeneous counter-parts [69,73,74].…”

“…In similar fashion, one common form of graded structures in synthetic materials has involved the use of gradients in grain size for metallic materials. As noted in Section 3.2, several GS metallic materials with grain-size gradients have been developed, generally to achieve combinations of strength and ductility [27][28][29]65,[68][69][70], although in most cases only in very small section sizes, often pertaining to a few hundred micrometers. Due to difficulties in fabricating graded structures at macro-scale dimensions, such micro-scale GS materials naturally have very limited application.…”

“…Recently, however, nominally bulk-sized GS materials of metallic nickel have been processed as centimetersized plates by a direct-current electroplating process [69,192]. Using current densities increasing from 10 to 100 mA/cm 2 (with a sodium saccharin additive), grainsize gradients in nickel, ranging from nanograins (NG) of ∼ 30 nm to coarser-grains (GC) of ∼ 4-8 μm, have been achieved.…”

“…To this end, the deformation and fracture properties of such gradient nickel have been investigated, under both quasi-static [69] and dynamic [192] loading, using such 30 nm to 4-8 μm gradients in grain size (NG-GC), and compared to corresponding behavior in uniform coarse and nano-grained nickel. Compared to the ultrahigh-strength NG and low-strength CG uniformed grain-sized structures, an optimized combination of high strength and high toughness can be achieved in the gradient structured material, combining the higher strength of the nano-grained regions with the higher toughness of the coarse grains.…”