Microstructure and strengthening mechanisms in Cu/Fe multilayers

Nanostructured Cu/Fe multilayers on Si (1 1 0) and Si (1 0 0) substrates were prepared by magnetron sputtering, with individual layer thicknesses h varying from 0.75 to 200 nm. The growth orientation relationships between Cu and Fe at the interfaces were determined to be of the Kurdjumov–Sachs and Nishiyama–Wasserman type. Nanoscale columnar grains in Fe, with an average grain size of 11–23 nm, played a dominant role in the strengthening mechanism when h ⩾ 50 nm. At smaller h the hardness of Cu/Fe multilayers with (1 0 0) texture approached a peak value, followed by softening due to the formation of fully coherent interfaces. However, abundant twins were observed in Cu/Fe films with (1 1 1) texture when h = 0.75 nm, which led to the retention of high hardness in the multilayers.