It has been said that ‘God invented plasticity, but the Devil invented fracture!’ Both mechanisms represent the two prime modes of structural failure, respectively, plastic collapse and the rupture/breaking of a component, but the concept of developing materials with enhanced resistance to fracture can be difficult. This is because fracture resistance invariably involves a compromise—between strength and ductility, between strength and toughness—fundamentally leading to a ‘conflict’ between nano-/micro-structural damage and the mechanisms of toughening. Here, we examine the two major classes of such toughening: (i) intrinsic toughening, which occurs ahead of a crack tip and is motivated by plasticity—this is the principal mode of fracture resistance in ductile materials, and (ii) extrinsic toughening, which occurs at, or in the wake of, a crack tip and is associated with crack-tip shielding—this is generally the sole mode of fracture resistance in brittle materials. We briefly examine how these distinct mechanistic processes have been used to toughen synthetic materials—intrinsically in gradient materials and in multiple principal-element metallic alloys with the example of metallic glasses and high-entropy alloys, and extrinsically in ceramics with the example of ceramic-matrix composites—in comparison to Nature which has been especially adept in creating biological/natural materials which are toughened by one or both mechanistic classes, despite often consisting of constituents with meagre mechanical properties. The success of Nature has been driven by its ability to cultivate the development of materials with multiple length-scale hierarchical structures that display ingenious gradients and structural adaptability, a philosophy which we need to emulate and more importantly learn to synthesize to make structural materials of the future with unprecedented combinations of mechanical properties.

This article is part of a discussion meeting issue ‘A cracking approach to inventing new tough materials: fracture stranger than friction’.

有人说“上帝发明了可塑性，但魔鬼发明了断裂！” 这两种机制分别代表了结构失效的两种主要模式，即塑性坍塌和部件的破裂/断裂，但开发具有增强抗断裂性的材料的概念可能很困难。这是因为抗断裂性总是涉及强度和延展性之间、强度和韧性之间的折衷，从根本上导致纳米/微观结构损伤与增韧机制之间的“冲突”。在这里，我们研究了这种增韧的两大类：（i） 内在增韧，它发生在裂纹尖端之前，由塑性驱动——这是韧性材料中抗断裂性的主要模式，以及（ii）外在强化，它发生在裂纹尖端或裂纹尖端之后，并与裂纹尖端屏蔽有关 - 这通常是脆性材料中唯一的抗断裂模式。我们简要地研究了这些不同的机械过程是如何被用来增韧合成材料的——以金属玻璃和高熵合金为例，在梯度材料和多主元素金属合金中，以及在陶瓷中，以陶瓷为例——基质复合材料——与 Nature 相比，Nature 尤其擅长创造生物/天然材料，这些材料通过一种或两种机械类别进行增韧，尽管通常由机械性能较差的成分组成。

这篇文章是讨论会问题“发明新的坚韧材料的破解方法：断裂比摩擦更奇怪”的一部分。