The vast majority of our understanding about the deformation mechanisms in nanocrystalline materials is limited to information gained from experimental and theoretical characterization of FCC materials. Related behavior in nanocrystalline BCC materials is not as frequently studied, and thus outstanding questions remain regarding deformation regimes and Hall-Petch trends. Using in situ TEM, we investigate the deformation mechanisms of nanocrystalline BCC iron films with an average grain size of 35 nm produced by physical vapor deposition. The tensile experiments showed that fracture resulted after strains of about 5%. Crack propagation occurred primarily by separation of grain boundaries at the crack tip, which was accompanied by localized intragranular ductile (often superplastic) fracture. Deformation at the crack tip was accommodated by dislocation motion, grain rotation, and grain growth. No evidence was observed of twinning in nanocrystalline BCC iron. The concurrent nature of the grain rotation and dislocation motion indicates that grain rotation occurs at fairly large grain sizes and there is no sharp transition from dislocation-mediated to grain boundary sliding mechanisms as grain size is decreased in BCC iron.

我们对纳米晶体材料变形机理的绝大多数了解仅限于从FCC材料的实验和理论表征中获得的信息。纳米晶BCC材料中的相关行为并未得到广泛研究，因此关于变形机制和Hall-Petch趋势仍然存在悬而未决的问题。原位使用TEM，我们研究了物理气相沉积产生的平均晶粒尺寸为35 nm的纳米BCC铁膜的变形机理。拉伸实验表明，在约5％的应变后导致断裂。裂纹扩展主要是通过裂纹尖端处晶界的分离而发生的，伴有局部的颗粒内延性（通常是超塑性）断裂。通过位错运动，晶粒旋转和晶粒生长来适应裂纹尖端处的变形。没有观察到在纳米晶BCC铁中孪生的证据。晶粒旋转和位错运动的同时性表明晶粒旋转发生在相当大的晶粒尺寸上，并且随着BCC铁中晶粒尺寸的减小，没有从位错介导到晶粒边界滑动机制的急剧转变。