Thermal stability of highly nanotwinned copper: The role of grain boundaries and texture
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To study the effect of nanotwins on thermal stability, a comprehensive characterization study was performed on two types of ultrafine grained (UFG) copper samples, with and without nanotwins. The two samples were sequentially heat-treated at elevated temperatures, and the grain size, grain boundary character, and texture were characterized after each heat treatment. The as-prepared nanotwinned (nt) copper foil had an average columnar grain size of ;700 nm with a high density of coherent twin boundaries (CTBs) (twin thickness, ;40 nm), which remained stable up to 300 °C. In contrast, the other UFG sample had few CTBs, and rapid grain growth was observed at 200 °C. The thermal stability of nt copper is discussed with respect to the presence of the low energy nanotwins, triple junctions between the twins and columnar grains, texture and grain growth.
I. INTRODUCTION
Highly nanotwinned (nt) Cu has been the subject of many recent studies, as it has shown ultrahigh strength, decent ductility, and stable microstructure during deformation as compared to ultrafine grained (UFG, grain size 100 nm to 1 lm) and nanocrystalline (grain size ,100 nm) Cu.1–5 However, the processing conditions for nt Cu vary, which leads to different microstructures (grain size and twin thickness) with distinct behaviors. Specifically, a study by Lu et al.2 compared the mechanical response of nt Cu samples with various twin thicknesses ranging from 4 to 96 nm and demonstrated a drastic mechanical behavior change as twin thickness decreased below 15 nm. To date, there are two major processing techniques to synthesize highly nt Cu: electrodeposition and magnetron sputtering. Samples synthesized using electrodeposition generally have equiaxed grains (grain size 400–600 nm) with varying nt thicknesses ranging from 4 to 96 nm.1–3 For the samples synthesized by magnetron sputtering, the grains are typically columnar with grain sizes ranging from UFG (600–800 nm) to nanocrystalline (;40 nm) and twin thicknesses ranging from 4 to 40 nm.4–7 While the mechanical behavior of all these samples has been well characterized, the effect of twin thickness on the thermal stability must still be explored. A recent study by Anderoglu et al.6 on nt Cu (twin thickness 4 nm, grain size 40 nm) at 800 °C reported modest nt growth and an order of magnitude grain growth, although their sample hardness decreased from 3.5 to 2.2 GPa. However, as pointed out by Zhang and Misra,7 the superior thermal a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.376 J. Mater. Res., Vol. 27, No. 24, Dec 28, 2012
stability of nt materials requires further study and must include additional factors such as sample texture. The uniqueness of this study is that we evaluate the thermal stability of highly nt Cu in the perspective of grain boundary (GB) character and texture. Specifically, since the GB character affects GB migration, this aspect becomes more critical with the presence of nanotwins. Some studies6–10 have shown that the presence of low energy co
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