Microstructure and mechanical properties of a Zn-0.5Cu alloy processed by high-pressure torsion
Introduction
Zinc alloys are promising biodegradable structural materials [1,2], and recent research has focused on the manufacturing, processing and characterization of new Zn-based alloys. Zinc alloys are generally processed using conventional hot rolling or extrusion sometimes followed by cold rolling [[3], [4], [5], [6]] and the application of severe plastic deformation (SPD) processing methods has received less attention. Recently, equal channel angular pressing (ECAP) and hydrostatic extrusion have been used to produce high-alloyed high-strength Zn-based alloys [7,8] while for low-alloyed Zn-alloys the same processing methods decrease the strength and may activate room-temperature (RT) superplasticity [[9], [10], [11]].
Very few reports are at present available on the processing of Zn and its alloys using high-pressure torsion (HPT), although some results are available analyzing the effect of HPT on the microstructure and mechanical properties of pure Zn [[12], [13], [14]]. The relatively low melting temperature (TM) of Zn causes very high dislocation annihilation and relatively easy recovery and recrystallization during and immediately after processing [15,16]. Thus, no substantial strengthening effect was recorded and steady-state grain sizes were measured from 5.2 to 19 μm. Only one dual-phase eutectoid Zn–22Al alloy has been investigated after HPT processing, and then the presence of a second phase leads to significant grain refinement to ~350 nm after 5 turns under a pressure of 6.0 GPa [[17], [18], [19], [20], [21]]. In contrast to pure zinc, the dual-phase Zn alloy exhibits a strain softening behavior [22]. Besides the conventional alloys, Zn–Mg hybrids produced by HPT processing have been recently reported [23,24]. Diffusional bonding allowed the joining of Zn/Mg/Zn layers into a single compact disc of Zn–Mg material; however, visible inhomogeneity in the cross-sections leads to an ambiguous determination of chemical and phase compositions considering the entire disc volume. Thus, microstructure and mechanical properties significantly vary depending on the amount of Zn, Mg or Zn–Mg phases.
The present research was conducted to provide the first experiments on a low-alloyed quasi-single-phase Zn-based alloy processing by HPT. For this investigation, a Zn-0.5Cu (wt. %) alloy was selected because earlier research using ECAP showed that this alloy provides a capability of exhibiting RT superplasticity and significant grain refinement by comparison with pure Zn [9,11]. The investigation was undertaken with two main objectives. First, to systematically analyze the evolution of microstructure and texture after processing by HPT through different numbers of turns. Second, to determine the effect of microstructural changes on the mechanical properties after processing by HPT.
Section snippets
Materials and methods
The alloy was prepared from high purity (>99.995 wt %) zinc and high-purity Cu–40%Zn brass (<0.007 wt of impurities) through melting in a graphite crucible at 650 °C in air, homogenizing for 30 min and subsequently casting into a cylindrical steel mold. The cast billets were annealed at 400 °C for 4 h to homogenize the chemical composition and subsequently water-cooled. The chemical composition of the material was analyzed using a Fischerscope® XDV-SDD X-ray fluorescence spectrometer and the
Results
The effect of torsional straining on microstructure during HPT processing depends on the distance from the disk centre and the number of rotations. Fig. 2 shows EBSD maps at positions of ~2.5 mm from the centres of the disks after a) 0, b) 1/2, c) 1, d) 2, e) 5, and f) 10 turns, respectively. After compression (Fig. 2a), there are a significant number of twins both inside larger grains and within the primary twins. After 1/2 turn (Fig. 2b), it is apparent that the severe torsional straining
Microstructure development during HPT processing
The microstructure results (Fig. 2) show that during HPT processing the initially deformed grains and twins recrystallize, producing highly oriented non-deformed grains. Earlier studies presented similar results for pure Zn [13], Cu [37] and Al [38] and many single-phase alloys [39] where lack of obstacles allows the formation of recrystallized, equiaxed grains. Based on the current results, it is assumed that twinning occurs during the initial straining and the subsequent continuous shearing
Summary and conclusions
- 1.
The effect of high pressure torsion on a Zn-0.5Cu alloy was investigated through microstructure, texture and mechanical properties analysis. With increasing numbers of rotations, a fine-grained microstructure with strong basal fiber texture developed. Grain refinement was observed up to 2 turns and after 5 turns there was significant grain growth occurred, whereas the subgrain size steadily decreased to a steady-state grain size of ~1.9 μm.
- 2.
The strain rate sensitivity increases with grain growth
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
CRediT authorship contribution statement
Wiktor Bednarczyk: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft, Visualization, Funding acquisition. Jakub Kawałko: Methodology, Formal analysis, Writing - review & editing, Supervision. Maria Wątroba: Investigation, Writing - review & editing. Nong Gao: Methodology, Resources. Marco J. Starink: Resources, Writing - review & editing. Piotr Bała: Resources, Writing - review & editing, Supervision. Terence G. Langdon: Resources, Writing - review &
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by the Polish National Science Centre [Grant number PRELUDIUM UMO-2018/31/N/ST8/01062].
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