Improving the mechanical properties of metallic materials via combined structural and compositional gradients

Abstract

Overcoming the strength‑ductility trade‑off remains a central challenge in structural alloys. We demonstrate a scalable dual‑gradient structure in Cu–Al alloys that couples a compositional gradient with a near‑surface structural gradient. Wire arc additive manufacturing forms the compositional gradient; cold rolling, annealing and rotationally accelerated shot peening generate nano‑/ultrafine‑grained layers, so both gradients act in concert within one component.

Characterisation and modelling show melt‑pool heat, flow and solute fields control layer morphology and compositional steps: Cu‑rich layers are narrow and deep, whereas Al‑rich layers are shallow and wide; a columnar‑to‑equiaxed transition develops along the build direction. The Al‑rich domain undergoes intermittent discontinuous dynamic recrystallisation, while the Cu‑rich domain exhibits continuous dynamic recrystallisation.

During rolling, low stacking fault energy promotes shear bands and twinning. After annealing, grain size decreases with increasing Al content, and transition regions display a bimodal grain‑size distribution.

Full‑field digital image correlation combined with finite‑element analysis reveals distinct mechanical roles: a sole structural gradient maintains a stable strain gradient, whereas a sole compositional gradient transfers load from surface to core. Acting together, the dual gradient provides parallel plasticity channels and interface pinning, building a 2D crack‑deflection network that suppresses localisation and increases fracture‑energy dissipation.

At room temperature the dual‑gradient specimen shows yield strength ~385 MPa, UTS ~495 MPa, uniform elongation ~38% and fracture strain ~49%, delivering a strength‑ductility index >140 MJ m^-3 and markedly outperforming homogeneous and single‑gradient counterparts. These results establish a cross‑scale gradient‑interface co‑design guideline and a general route beyond the strength‑ductility trade‑off.