School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ, 85287, USA.
US Army Research Laboratory, Aberdeen Proving Ground, Adelphi, MD, 21005, USA.
Nat Commun. 2018 Jul 12;9(1):2699. doi: 10.1038/s41467-018-05027-5.
Fundamentally, material flow stress increases exponentially at deformation rates exceeding, typically, 10 s, resulting in brittle failure. The origin of such behavior derives from the dislocation motion causing non-Arrhenius deformation at higher strain rates due to drag forces from phonon interactions. Here, we discover that this assumption is prevented from manifesting when microstructural length is stabilized at an extremely fine size (nanoscale regime). This divergent strain-rate-insensitive behavior is attributed to a unique microstructure that alters the average dislocation velocity, and distance traveled, preventing/delaying dislocation interaction with phonons until higher strain rates than observed in known systems; thus enabling constant flow-stress response even at extreme conditions. Previously, these extreme loading conditions were unattainable in nanocrystalline materials due to thermal and mechanical instability of their microstructures; thus, these anomalies have never been observed in any other material. Finally, the unique stability leads to high-temperature strength maintained up to 80% of the melting point (1356 K).
从根本上说，当变形速率超过典型的约 10s-1 时，材料流动应力会呈指数级增加，导致脆性失效。这种行为的起源源于位错运动，由于声子相互作用的阻力，在较高应变速率下导致非阿雷尼乌斯变形。在这里，我们发现，当微观结构长度稳定在极细尺寸（纳米尺度）时，这种假设就不会表现出来。这种发散的应变速率不敏感行为归因于一种独特的微观结构，它改变了平均位错速度和移动距离，从而阻止/延迟位错与声子相互作用，直到达到比已知系统更高的应变速率；因此，即使在极端条件下，也能保持恒定的流动应力响应。以前，由于其微观结构的热和机械不稳定性，纳米晶材料无法达到这些极端的加载条件；因此，在任何其他材料中都从未观察到这些异常现象。最后，这种独特的稳定性导致高温强度保持在熔点的 80%左右（约 1356K）。
