An investigation of the limits of grain refinement after processing by a combination of severe plastic deformation techniques: A comparison of Al and Mg alloys

Abstract

An Al-7075 aluminum alloy and a ZK60 magnesium alloy were processed by a combination of equal-channel angular pressing (ECAP) for 4 passes and high-pressure torsion (HPT) through total numbers of up to 20 turns. Processing by ECAP and HPT were performed at 473 K and room temperature, respectively. Mechanical testing showed an increase in the hardness value of the Al-7075 alloy after a combination of ECAP and HPT whereas in the ZK60 alloy the hardness was reduced. Microstructural images of the Al-7075 alloy revealed very significant grain refinement after a combination of ECAP + HPT compared to the individual processing techniques. By contrast, and consistent with the hardness measurements, the average grain size of the ZK60 alloy was larger after processing by the two-step SPD technique. These results are examined and an explanation is presented based on the available microstructural evidence.

Electrochemical behavior of a magnesium ZK60 alloy processed by high-pressure torsion

“…For the extruded sample with conventional grain size, it is reasonable to expect that the majority of corrosion pathways are the grain boundaries, due to similarity and size of grains and boundaries between Figure 1 and Figure 12 for extruded samples. In the HPT-processed samples with 1, 2 and 5 turns, these paths expand but are not uniform due to incomplete grain refinement as shown in Figure 1 and were extensively discussed in a previous publication [27,29]. Consequently, a micro-galvanic corrosion between the interiors of the large grains and non-distorted areas as cathode with cluster of distorted grains with high boundary densities as anode can form [5,13].…”

“…Earlier investigations of authors on the ZK60 Mg alloy showed that HPT induced an inhomogeneous strain along the radius of samples by torsional straining and, as a result, the average grain size and the mechanical properties tended to vary along the radius of the samples but an almost homogeneous microhardness distribution with finer grain size and reasonable uniformity was attained after higher numbers of HPT turns [27][28][29]. The mechanical and thermal stability behavior of ZK60 after HPT was evaluated recently by the current authors [30,31] but it is imperative to study the electrochemical behavior of the refined ZK60 alloy.…”

Diffusive and Displacive Phase Transformations Under High Pressure Torsion

“…If one starts to deform the material with grain size of several millimetres, it quickly decreases down to few hundreds of nanometers. Then, the grain size reaches the steady-state value, stabilises and does not decrease anymore [17,[26][27][28][29]. In the same material, the steady-state grain size depends first of all on the SPD mode.…”

ECAP-treated aluminium alloy AA2030: microstructure and mechanical properties

“…In recent decades, there has been a growing interest in ultrafine-grained (UFG) materials obtained with severe plastic deformation (SPD). [1][2][3] The SPD processes provide for an accumulation of high-degree deformation, which contributes to the formation of high-angle grain boundaries. [4][5][6] UFG materials are characterized by an increased complexity of mechanical, physical and operational characteristics.…”