Nanocrystalline and microcrystalline metals have demonstrated many improvements in important technological and engineering properties such as high strength and increased wear resistance. Recent studies have underscored the complexity in quantifying the interplay of different mechanisms in accommodating strain and accompanying microstructural evolution. Moreover, it has been established that severe plastic deformation at ambient temperature is a means to generate ultrafine grains through formation of a stationary sub grain structure with a fraction of high-angle boundaries. Current research has also confirmed the stationary character of the sub grain structure depending on the deformation mode. Therefore, it is imperative that microcrystalline copper produced by equal channel angular pressing and static recrystallization be assessed.
Recently, Professor Wolfgang Blum University of Erlangen-Nürnberg in Germany in collaboration with Czech Academy of Sciences scientists: Dr. Jiri Dvorak, Dr. Petr Král and Professor Vaclav Sklenička investigated the microstructure-strength relation of equal channel angular pressing (ECAP)-processed copper for the highest test temperature 573K 0.42 = Tm. In particular, they utilized the research program on creep of pure, ECAP-processed ultrafine-grained copper at Brno. They focused on using the distinction between sub grain-free and sub grain-containing grains as clue for better comprehension. Their work is currently published in the research journal, Materials Science & Engineering A.
In brief, the research method employed commenced with the utilization of a scanning electron microscope equipped with electron back scatter diffraction facility to generate orientation image micrographs and a transmission electron microscope, to investigate the microstructures. Next, the researchers performed creep tests, both uniaxial and compression, in an environment of purified argon with the testing temperatures maintained constant. They then took measurements on the evolution of the deformation resistance with strain using various techniques.
The authors observed severe pre-deformation at certain conditions that generated microcrystalline grains of specific mean size through grain refinement and static recrystallization. They also noted that the grains coarsened discontinuously during creep. Oscillations of strength indicated that the dynamic grain coarsening (dDGC) occurred in successive waves. Moreover, it was seen that the ductility at test temperature was significantly improved by equal channel angular pressing processing. It was also established that the deformation strength at the first maximum was controlled by the (sub) grain-free grains. Furthermore, dDGC was seen to be responsible for the hardening of the material through he generation of (sub) grain-containing grains.
In summary, the study demonstrated dynamic grain coarsening in creep of pure copper at 0.42 Tm after pre-deformation by equal channel angular pressing. In general, it was seen that discontinuous coarsening of grains led to dramatic changes of creep rate. These attributes reflected general hardening as the grains coarsened, superimposed by transient net softening when grain boundary migration activity peaked.
W. Blum, J. Dvořák, P. Král, V. Sklenička. Dynamic grain coarsening in creep of pure Cu at 0.42 Tm after pre-deformation by ECAP. Materials Science & Engineering A, volume 731 (2018) page 520–529.Go To Materials Science & Engineering A