The Size Dependent Deformation and Strengthening Mechanisms of Nanolayered Co/Ag Micropillars
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NTRODUCTION
NANOSTRUCTURED metallic multilayers (NMMs) are the films alternately formed by two or more metal elements with a nano-scale modulated structure.[1] In recent years, the NMMs have been explored for various applications, including magnetic data storage,[2,3] hydrogen storage,[4,5] and radiation tolerance,[6–8] etc. Moreover, extensive research efforts have been devoted to the interfaces and mechanical properties of the NMMs, especially to the size effect of the individual layer thickness (hi) on the deformation and strengthening mechanisms. To date, mechanical properties of NMMs with numerous systems have been investigated, including Ag/Al,[9,10] Ag/Ni,[11–14] Ag/Cu,[15] Au/Ni,[12] and Cu/ Ni[12,16–19] of fcc/fcc systems, Al/Nb,[20] Fe/Pt,[21] Cu/ Cr,[19,22–26] Cu/Nb,[19,27,28] Cu/V,[20] and Cu/W[29,30] of fcc/bcc systems, Fe/Cr,[21] Ta/V[31] of bcc/bcc systems, Cu/Zr,[22,25,26,32–34] Co/Cu,[35] Co/Ag,[36] and Ti/Pd[37] of hcp/fcc systems, Mg/Nb,[38] Co/Mo,[39] and Ti/V[40] of hcp/bcc systems, and Mg/Ti[41] of hcp/hcp systems (fcc: face-centered cubic, bcc: body-centered cubic, hcp: hexagonal close-packed). In earlier studies, the strengthening mechanism of the NMMs was captured by the well-known Hall–Petch relationship,[42–46] but the strength of multilayers with individual layer thickness (hi) down to several tens of nanometers was CANCAN ZHAO and FUZENG REN are with the Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong, China. Contact e-mail: [email protected] RENLONG XIN is with the Joint International Laboratory for Light Alloys (MOE), College of Materials Science and Engineering, Chongqing University, Chongqing, China. Manuscript submitted May 18, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
overestimated. Moreover, the Hall–Petch relation failed to explain the saturated or decreased strength of the multilayers when hi £ 5 nm. Misra et al.[28,47] and Anderson et al.[48] proposed the deformation mechanisms active at different length scales: (1) for hi at submicron or larger scale, dislocations pile up against layer interfaces and size dependent strengthening following Hall–Petch relation; (2) for hi in the range of a few nanometers to tens of nanometers, the confined layer slip (CLS) model[28,47,48]; and (3) for hi down to several nanometers, the interface crossing model.[28] This changing strengthening mechanism provides an excellent explanation for the relationship between strength and the individual layer thickness. Among the NMMs, the Co/Ag multilayers have been widely studied for their giant magneto resistance (GMR) effects,[49–54] the mechanical properties, in particular, the size dependent deformation and strengthening mechanisms are of crucial importance for GMR-related applications. In addition, Co/Ag binary system is a typical hcp/fcc system and the Co-Ag equilibrium phase diagram[55] shows that Co and Ag are completely immiscible at room temperature, which offers an ideal model system for investigating size dependent deform
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