Annealing treatments to enhance thermal and mechanical stability of ultrafine-grained metals produced by severe plastic deformation

Abstract

Ultrafine-grained (UFG) metals produced by techniques of severe plastic deformation, such as equal channel angular pressing (ECAP), exhibit extraordinary strength properties. However, in the as-ECAP-processed state, the heavily deformed microstructure of such UFG metals is rather unstable and is prone to undergo grain coarsening (recrystallization) at moderate temperatures. This microstructural instability is enhanced in the presence of modest mechanical stressing as, for example, in cyclic deformation. Thus, all measures to enhance the thermal stability are also considered as beneficial for the improvement of the mechanical stability.One main objective of the present work is to analyse the thermal and mechanical stability of ECAP-processed metals during specific annealing and cyclic deformation tests. As a by-product, some conclusions relating to the separate effects of dislocation density, grain size (in the UFG regime) and internal stresses on the (micro)yielding behaviour will be drawn. Another goal is to explore the potential of different annealing treatments with respect to the stabilization of the microstructure and the optimization of the mechanical properties of ECAP-processed UFG metals in terms of an optimal combination of strength and ductility. In order to demonstrate the potential and the limitations of this approach, experimental work performed on UFG copper, aluminium and α-brass produced by ECAP will be reported and discussed. The results presented indicate strongly that a heat treatment leading to a bimodal grain size distribution provides the best compromise between strength and ductility.

A Portrait of Copper Processed by Equal Channel Angular Pressing

“…9) This was also shown to be beneficial with regard to the fatigue behaviour. 10) To accomplish this goal, the thermal stability of copper after ECAP needs to be known. In order to obtain these data, isochronal annealing (10 minutes) was performed on specimens after 8 ECAP passes in a temperature range of 100 C to 200 C. Hardness tests were carried out to characterize thermal stability.…”

Effect of Pre-Fatigue on the Monotonic Deformation Behavior of a Coplanar Double-Slip-Oriented Cu Single Crystal

“…It should be mentioned that the term "ultrafine-grained", widely used by the ECAP community, sometimes (e.g., UFG pure Cu) represents a microstructure comprising very fine, highly misoriented dislocation cells or subgrains [16][17][18][19]. Such ultrafine-celled (the cell diameter is around some hundred nanometers) microstructures produced by the ECAP severe plastic deformation technique normally exhibit an extraordinarily high strength but are followed by a great loss of ductility, although a subsequent, controlled short-term annealing treatment might cause a joint enhancement of strength and ductility [20]. In contrast, the cell structure developed under fatigue deformation is well recognized to be a typical type of heterogeneous dislocation distribution, and high-symmetry orientations, low friction stress, large deformation and the easy operation of cross slip favor the formation of dislocation cell structures [21].…”

Recent Findings in Superior Strength and Ductility of Ultrafine-Grained Materials

“…Another approach to enhance ductility is based on introduction of a bimodal distribution of grain sizes [60,69]. In [60], nanostructured Cu was produced through a combination of ECAP and subsequent rolling at the liquid nitrogen temperature prior to heating to a temperature of ~450 K. This processing route resulted in the formation of a bimodal structure of micrometer-sized grains, with a volume fraction of around 25%, embedded in a matrix of nanocrystalline grains.…”

“…The reason for this behavior is that while the nanocrystalline grains provide strength, the embedded larger grains stabilize the tensile deformation of the material. Other evidences for the importance of grain size distribution come from investigations on Zn [70], Cu [69], and Al alloys [71]. Furthermore, the investigation of Cu [69] showed that bimodal structures may increase the ductility not only during tensile testing but also during cyclic deformation.…”

“…Other evidences for the importance of grain size distribution come from investigations on Zn [70], Cu [69], and Al alloys [71]. Furthermore, the investigation of Cu [69] showed that bimodal structures may increase the ductility not only during tensile testing but also during cyclic deformation. This observation is also important in improving the fatigue properties of materials.…”