Strengthening during heat treatment of HPT processed copper and nickel

Significance 

Materials are susceptible to failures which compromise their properties and functionalities. For instance, they undergo deformations that results in the lattice defects due to dislocations of the grains and microstructures especially when exposed to severe plastic deformation. Metals like copper and nickel have their grains sizes decreased and lattice defects density increased when deformed.

Recently, annealing behavior of the defects induced in face-centered cubic metals through severe plastic deformation has attracted significant attention of researchers. The effects of the homologous temperature that depend on the alloy purity, on the annealing process have been showed. Furthermore, the temperature ranges have been used to describe various classical annealing stages

In a recently published literature, specific annealing behavior at different temperatures together with vaccenic density and dislocation density in high-pressure torsion were determined using both electrical resistometry and X-ray line profile analysis. Vacancy type defects annealed mostly at the first annealing stage i.e. stage A while dislocation as a result of the induced deformation were observed in stage B of the annealing process. Regardless of the important insights, changes in the mechanical strength during the annealing of the defects induced by severe plastic deformation in different metals have not been fully explored.

Recently, a group of researchers at the University of Vienna: Peter Cengeri, Dr. Michael Kerber, Dr. Erhard Schafler, Dr. Michael Zehetbauer, and Dr. Daria Setman investigated the changes in the mechanical strength during annealing of the defects induced by severe plastic deformation at various levels. In particular, the isochronal annealing behavior of copper and nickel metals subjected to high-pressure torsion was examined. The strength changes were measured using the Vickers microhardness while in-situ synchrotron X-ray Bragg profile analysis was used to determine the changes in the microstructure, dislocation density, and grain sizes. They purposed to understand the relationship between the changes in the microhardness and the various types of lattice defects. Additionally, the dependence of hardening on the distances between the grain boundaries was also confirmed. Eventually, they compared the obtained results to those available in the literature. Their work is currently published in the journal, Materials Science and Engineering A.

The authors observed that the microhardness measurements exhibited two annealing stages: stage A and stage B that corresponded to the theoretical stages III and V respectively. The former indicated the unusual anneal hardening process that is not affected by the changes in the dislocation density or decrease in the crystallite sizes while the latter showed characteristics of the primary recrystallization. Alternatively, the increase in the strength was attributed to agglomeration and annihilation of vacancies at the homologous temperatures while the decrease in the strength was due to the increase in the crystallite size and annealing of the dislocations. From the results, the authors confirmed the interpretation of the stages taking into consideration the specific annealing effects. Altogether, the study can be extended to the strengthening of various metals which will ensure design and fabrication of high-performance materials.

Reference

Cengeri, P., Kerber, M., Schafler, E., Zehetbauer, M., & Setman, D. (2019). Strengthening during heat treatment of HPT processed copper and nickel. Materials Science and Engineering: A, 742, 124-131.

Go To Materials Science and Engineering: A

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