Enhanced hardening in Cu/330 stainless steel multilayers by nanoscale twinning

Metallic multilayered composites synthesized by sputter deposition exhibit very high hardness as the bilayer period approaches a few nanometers. In the present investigation on polycrystalline Cu/austenitic 330 stainless steel multilayered films with 〈111〉 texture, we have found a high density of growth twins and stacking faults. The stainless steel layers, in particular, exhibit twinning and faults on the scale of 3–4 nm, whereas in the Cu layers, a large fraction of the grains show twins with spacings of a few tens of nanometers. A model is developed that accounts for the formation of nanoscale twins during sputter deposition in terms of twin boundary or stacking fault energy and deposition rate. The increase of hardness with decreasing layer thickness follows the Hall–Petch law for layer thickness greater than about 50 nm, but at lower layer thickness the hardness saturates to a value of about 5 GPa. Hardening mechanisms are interpreted in terms of layer thickness and twin spacing within layers.