Revealing the maximum strength in nanotwinned copper

Area of Science:

• Materials Science
• Nanotechnology
• Mechanical Engineering

Background:

• Polycrystalline material strength typically increases as grain size decreases.
• Atomistic simulations suggest a critical grain size below which softening may occur.
• The optimal size for maximum strength is hypothesized to be at the transition between lattice dislocation and grain boundary deformation processes.

Purpose of the Study:

• To investigate the relationship between twin thickness and the maximum strength in nanotwinned copper.
• To identify the critical twin thickness at which copper strength transitions from strengthening to softening.
• To understand the underlying deformation mechanisms responsible for strength variations at the nanoscale.

Main Methods:

• Fabrication of nanotwinned copper samples with varying twin thicknesses.
• Mechanical testing to determine tensile strength and ductility.
• Microscopic analysis to observe deformation mechanisms and dislocation activities.

Main Results:

• Material strength increased with decreasing twin thickness, reaching a maximum at 15 nanometers.
• A softening phenomenon was observed for twin thicknesses below 15 nanometers.
• Enhanced strain hardening and tensile ductility were noted at smaller twin thicknesses, correlating with softening.

Conclusions:

• The maximum strength in nanotwinned copper is achieved at a specific twin thickness (15 nm).
• A transition in yielding mechanism from slip transfer across twin boundaries to dislocation source activity underlies the observed strength-softening behavior.
• Nanotwinned structures offer a pathway to optimize material strength while potentially improving ductility.