Ultrafine-grained and nanocrystalline materials frequently show an enhanced rate sensitivity of their mechanical behavior, which is generally assumed to be the consequence of interface sliding or thermally activated dislocation processes at the boundaries. Although this has been well documented on many different materials, the underlying mechanisms and their effect on the ductility of the material are still not well understood. Therefore, here, the deformation behavior of the ultrafine-grained, heterogeneous superplastic alloy Zn-22% Al was analyzed by small-scale nanoindentation and micropillar testing. The results show a breakdown of the superplastic deformation behavior in terms of a reduced strain rate sensitivity at small scales, which has not been reported before, although grain boundary sliding is still prevalent at the nanoscale. These results suggest that grain boundary sliding does not necessarily result in a high strain rate sensitivity and high ductility. Instead, a pronounced strain rate dependent flow behavior requires grain boundary sliding to be controlled by dislocation creep.

Impact statement

Superplastic flow is shown to persist down to a small material volume corresponding to a few grains. This has far-reaching consequences for the production of small-scale devices with complex geometries by superplastic forming.

超细晶粒和纳米晶材料通常表现出对其机械性能的增强的速率敏感性，这通常被认为是边界处界面滑动或热活化位错过程的结果。尽管这已经在许多不同的材料上得到了很好的证明，但是其潜在机理及其​​对材料延展性的影响仍未得到很好的理解。因此，在这里，通过小规模纳米压痕和微柱试验分析了超细晶粒，非均相超塑性合金Zn-22％Al的变形行为。结果表明，就小尺度而言，由于应变速率敏感性的降低，超塑性变形行为发生了破裂，尽管在纳米尺度上晶界滑动仍很普遍，但迄今为止尚未见报道。这些结果表明，晶界滑动不一定导致高应变速率敏感性和高延展性。取而代之的是，取决于应变率的明显流动行为要求晶界滑动要通过位错蠕变来控制。

影响陈述

超塑性流显示持续减少到少量的材料体积，对应于一些晶粒。这对于通过超塑性成形生产具有复杂几何形状的小型装置具有深远的影响。