Mathias Göken

Abstract

Heterostructured Lamellar Materials and their Back-Stress Strengthening Potentia

Reaching high strength in metals is straightforward; preserving ductility is the true challenge. Heterostructured materials, in which soft and hard zones with a strength contrast of a factor of two or more are coupled across well-defined interfaces, offer a general route out of this strength–ductility trade-off. When the soft zones yield first, strain compatibility at the hetero-interfaces is maintained by geometrically necessary dislocations (GNDs) that accumulate as pile-ups and networks, generating a long-range back stress in the soft zone and a balancing forward stress in the hard zone. The resulting hetero-deformation-induced (HDI) stress raises strength and strain hardening at the same time and thereby delays necking.
Heterostructrued composites produced by accumulative roll bonding (ARB) are ideal lamellar systems to investigate HDI strengthening, because the interface density can be tuned systematically by repeated stacking and rolling cycles. Our new ARB+ route adds a final foil-rolling step and, for the first time, allows one and the same material system to be processed from the micrometre scale down to layer thicknesses of only 1–10 nm. Immiscible fcc/bcc combinations such as Cu/Nb or Fe/Al form thermally stable “quasi-alloys”, while Al of different purities develops graded microstructures. In CuCr/Nb-alloy laminates, the strength increases continously down to a layer thickness of only some nanometers and the uniform elongation still stays constant.
To quantify the back stress, we combine refined loading–unloading–reloading (LUR) tensile tests with a new spherical LUR nanoindentation method that maps the back strain locally. In Al5754/Al1050 laminates it peaks at the hetero-interfaces – direct evidence that interface GND structures, not grain boundaries per se, govern the back stress. These results pave the way to a predictive design of a new class of graded heterostructured laminates for structural and functional applications.