An investigation of the stored energy and thermal stability in a Cu–Ni–Si alloy processed by high-pressure torsion

ABSTRACT

The stored energy and activation energy for recrystallization were investigated for a Cu-Ni-Si alloy after high-pressure torsion processing for N = ½, 1, 5 and 10 turns at room temperature. The contributions of geometrically necessary dislocations (GNDs), statistically stored dislocations (SSDs) and vacancies to the stored energy were calculated through the Vickers microhardness measurements, kernel average misorientation (KAM) measurements and an analysis by differential scanning calorimetry (DSC). The results show that the total stored energy decreases rapidly after equivalent strain of ε_eq ∼ 9 and then saturates through ε_eq ∼ 86 at ∼70 J/mol. Concurrently, the local stored energy in GNDs and SSDs was found to depend strongly on the radial distance from the centre of the disc and increase with increasing equivalent strain at ε_eq ∼ 16 and saturate with further straining. Accordingly, the results indicate that the GNDs and vacancies are responsible for the high stored energy in the initial stage of deformation at equivalent strain range of ε_eq = 8.6–16 and thereafter their contribution decreases slightly due to the occurrence of dynamic recrystallization and the formation of fine grains. At the same time, the contribution of the SSDs is similar to that of the GNDs only in high strain deformation as at ε_eq = 49.3 to accommodate the deformation process. An activation energy for recrystallization was estimated in the range of ∼ 89.7–98.7 kJ/mol, thereby suggesting poor thermal stability.

KEYWORDS:

• Copper alloy
• DSC
• hardness
• recrystallization
• severe plastic deformation
• stored energy

Acknowledgements

The authors wish to thank Mr. Hacen Hadj Larbi from CLAL-FRANCE for kindly providing the Cu–Ni–Si alloy. One of the authors (YIB) wishes to heartily thank Prof. Jose Maria Cabrera from Polytecnia ETSEIB, Universidad Politécnica de Cataluña (UPC), for an invitation to UPC and for help during the scientific research and to the staff of the Biomaterials research group of UPC for their support in the DSC analysis.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Megumi Kawasaki http://orcid.org/0000-0003-0028-3007

Terence G. Langdon https://orcid.org/0000-0003-3541-9250

Additional information

Funding

This study was supported in part by the National Science Foundation of the United States under Grant No. DMR-1810343 (MK) and in part by the European Research Council under ERC Grant Agreement No. 267464-SPDMETALS (TGL).