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 μm. 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 behavior 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 crack-growth toughnesses exceeding 200 MPa.m½.

中文翻译：

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关于梯度金属材料的超常损伤容限

摘要 描述了与金属镍中裂纹萌生和扩展相关的断裂韧性和微观机制的实验研究，其中包含显着的晶粒尺寸梯度，范围从~30 nm 到~4 μm。具体而言，裂纹在梯度结构 (GS) 镍中生长，晶粒尺寸梯度范围从粗宏观尺度到纳米尺度 (CG → NG)，反之亦然 (NG → CG)，测量抗裂性 R - 曲线与均匀纳米晶粒 (NG) 和粗晶粒 (CG) 材料中的相应行为相比。发现与均匀晶粒尺寸的材料相比，梯度结构显示出大大改善的高强度和韧性组合。然而，基于梯度材料中的 J 积分测量，对于沿粗晶粒到纳米晶粒梯度方向生长的裂纹，裂纹起始韧性要高得多，反之亦然，我们主要将这一结果归因于粗晶粒显微组织中过度的裂纹尖端钝化。然而，这两种梯度结构都显示出显着上升的 R 曲线行为，具有超过 200 MPa.m½ 的卓越裂纹扩展韧性。