Combined size and texture-dependent deformation and strengthening mechanisms in Zr/Nb nano-multilayers

Abstract

A combination of transmission electron microscopy analyses and nanomechanical measurements was performed in this study to reveal deformation and strengthening mechanisms occurring in sputtered Zr/Nb nanoscale metallic multilayers (NMMs) with a periodicity (L) in the range 6 -167 nm. Electron diffraction analyses revealed a change in the crystallographic orientation of α-Zr when L ≤ 27 nm, while Nb structure retained the same orientations regardless of L. For L > 60 nm, the strengthening mechanism is well described by the Hall-Petch model, while for 27 < L < 60 nm the refined CLS model comes into picture. A decrease in strength is found for L < 27 nm, which could not be simply explained by considering only misfit and Koehler stresses. For L ≤ 27 nm, plastic strain measured across compressed NMMs revealed a change in the plastic behaviour of α-Zr, which experienced a hard-to-soft transition. At these length scales, the combination of two structural factors were found to affect the strength. These relate to the formation of weaker interfaces which extend the effective distance between strong barriers against dislocation transmission, thus producing a softening effect. 1The second effect relates to the crystallographic orientation change exhibited by α-Zr for L < 27 nm with a consequent change of the dominant slip system. The actual strength at these smaller length scales was effectively quantified by taking these structural aspects into account in the interface barrier strength model.

Effect of layer thickness on the mechanical behaviour of oxidation-strengthened Zr/Nb nanoscale multilayers

“…The modulation periodicity had little effect on the room temperature hardness (indicated as "as deposited" in Fig. 7), as has been shown before and explained based on changes in the crystalline orientation of the Zr layers with layer thickness [45]. However, the hardness decreased more rapidly with temperature for L = 10 nm.…”

“…They are slightly larger than the one given by the rule of mixtures from the hardness of the Zr and Nb constituents (H Zr = 6.1 GPa and H Nb = 6.7 GPa) and this strengthening is typically found on metallic NMs with incoherent interfaces[1]. The actual deformation mechanisms of Zr/Nb NMs have been analysed previously[45,49] and it has been reported that the strength of the NMM with L = 75 nm follows the confined layer slip (CLS) model whereas the strength is controlled by dislocation transmission across interfaces for L = 10 nm.The annealing treatment at 350°C of the Zr/Nb NMMs and the monolithic Zr and Nb films led to a large increase in hardness(Fig. 7b).…”

Effect of Proton Irradiation on the Defect Evolution of Zr/Nb Nanoscale Multilayers

“…Recently, NMCs based on hcp/bcc systems have begun to attract attention: Mg/Nb [8], Zr/Nb [17][18][19], Co/Mo [9], Mg/Ti [10], Cu/Zr [20], etc. Most studies of hcp-based NMCs are focused on the relationship between the structural and strength characteristics of as-deposited nanoscale layers.…”

Competing mechanisms on the strength of ion-irradiated Zr/Nb nanoscale multilayers: Interface strength versus radiation hardening

“…The mechanical behaviour of interfaces is a crucial aspect to be assessed as this significantly affects overall material properties. Recently, hcp-based NMMs started to attract some attention and cases of hcp-based NMMs are: Mg/Nb [18], Zr/Nb [19,20], Co/Mo [21], Mg/Ti [22], Cu/Zr [23], etc.…”

“…Most of the studies on hcp-based NMMs focused on the structure-strength relationship of pristine NMMs, while no relevant investigations were performed to assess their behaviour after ion irradiation. To fill the gap, the Zr/Nb system, whose structure-strength relationship [19] and thermal stability [24,25] are reported in detail in our previous studies, was used to investigate and correlate its structural and mechanical stability when subjected to irradiation by energetic particles.…”