Abstract
Achieving high fracture toughness and maintaining high strength at the same time are main goals in materials science. In this work, scale-bridging fracture experiments on ultrafine-grained chromium (UFG, Cr) are performed at different length scales, starting from the macroscale over the microscale (in situ SEM) down to the nanoscale (in situ TEM). A quantitative assessment of the fracture toughness yields values of ∼3 MPa m1/2 in the frame of linear elastic fracture mechanics (LEFM) for the macrosamples. The in situ TEM tests reveal explicitly the occurrence of dislocation emission processes involved in energy dissipation and crack tip blunting serving as toughening mechanisms before intercrystalline fracture in UFG body-centered cubic (bcc) metals. In relation to coarse-grained Cr, in situ TEM tests, in this work, demonstrate the importance of strengthening grain boundaries as promising strategy in promoting further ductility and toughening in UFG bcc metals.
Highlights
The conflict between the increase of the polycrystals material’s strength by different strengthening mechanisms and the frequently observed simultaneous decrease of toughness and ductility is an everlasting problem in materials science [1]
This is clearly seen in the overall fracture surface, the load–displacement data presented in Fig. 3(a), and a comparison with literature data [21], for example, considering the CG state of the same material [9]
Fracture experiments on UFG Cr were performed on the same material at different scales from macroscale down to nanoscale
Summary
The conflict between the increase of the polycrystals material’s strength by different strengthening mechanisms and the frequently observed simultaneous decrease of toughness and ductility is an everlasting problem in materials science [1]. Decreasing grain size offers strength increases, recent studies show a simultaneous dramatic decrease of the uniform elongation strain from ;30% to ;0–3% for different bcc and fcc UFG and NC metals [4]. This is explained by the higher proportion of grain boundaries (GBs) existing in UFG and NC materials compared to coarse-grained (CG) materials. Higher probabilities exist for cracks to initiate and propagate along GBs in very fine-grained materials, causing this ductility decrease and the lower fracture toughness of UFG and NC materials Because of this behavior, lower strength alloys are often selected for engineering applications, as their higher fracture toughness is decisive for the safety of structural applications [1]
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