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

Abstract

In the present study, 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 εeq ~ 9 (N = 1 turn) and then saturates through εeq ~ 86 (N = 10 HPT turns) 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 εeq ~ 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 εeq = 8.6–16 (N = ½–1 turn) 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 εeq = 49.3 (N = 5 turns) to accommodate the deformation process. The recrystallization peak was detected in the range of 157–194 °C depending on the number of HPT turns and heating rate. An activation energy for recrystallization was estimated in the range of ~ 89.7–98.7 kJ/mol, thereby suggesting a poor thermal stability.

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

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