Fabrication and characterization of nanostructured immiscible Cu–Ta alloys processed by high-pressure torsion
Introduction
Nanostructured alloys have attracted significant interest due to their unique sets of properties which are not achievable in coarse-grained polycrystalline materials [1,2]. Non-equilibrium solid solutions have also drawn attention as they can evolve as nanoscale microstructures through processing and then form well-dispersed nanoscale composites upon annealing at elevated temperatures [3]. Both simulations and experiments indicate remarkable physical and mechanical properties for this new class of material. The Cu–Ta system is one of these immiscible systems with almost zero solubility of Cu(Ta) in Ta(Cu) at room temperature (RT) [4].
Several processing methods are available for producing nanocrystalline microstructures consisting of solid solutions having different elements. For example, processing through the introduction of severe plastic deformation (SPD) has been used successfully over the last two decades to generate non-equilibrium microstructures in a range of metals and alloys [[5], [6], [7]]. Potential SPD techniques, including equal-channel angular pressing (ECAP) [8,9], accumulative roll bonding (ARB) [10] and high-pressure torsion (HPT) [[11], [12], [13]], have the potential of not only producing nanocrystalline microstructures but also of generating alloys of immiscible systems due to the extra crystalline defects and dislocations that are introduced into the matrix through the SPD processing. This excess of defects will thereby facilitate diffusion in these systems leading to the production of non-equilibrium solid solutions.
In general, the HPT process is usually considered the most effective procedure for achieving exceptional grain refinement, typically to the nanometer range in many systems, and this has become an established processing method for studying nanocrystalline and non-equilibrium solid solutions. In HPT processing, grain refinement may be achieved without cracking due to the imposed hydrostatic pressure which effectively prevents the propagation of fracture during the torsional straining [14]. Recently, FEM modelling [[15], [16], [17], [18]] and experimental observations on two phase materials such as duplex stainless steel [[19], [20], [21], [22], [23], [24]] and Cu–Ag alloys [25] demonstrated that, in addition to the in-plane shear strain, there is also mass transfer within the samples in HPT due to the development of turbulent eddy flows within the sample cross-sections during the processing [26]. It is evident that this will assist in the redistribution of metal components in an immiscible system.
The synthesis of novel nanostructured alloys formed from forced solid solutions, especially for the non-equilibrium solid solutions such as Cu and immiscible solute species, has been the focus of some recent studies [27,28]. For example, mechanical alloying has been used to generate Cu-based powders composed of immiscible alloying elements such as Ta [29] and Mo [30] where these systems have shown moderate to extraordinary high microstructural stability at elevated temperatures. There are also recent reports on the thermal stability and microstructure of high strength nanocrystalline Cu–Ta alloys [29]. For example, isothermal annealing of a Cu–10% Ta powder produced via high-energy cryogenic mechanical alloying led to a nanocrystalline, two-phase composite structure of spheroidal Ta particles and nanolamellar Ta dispersed in a Cu-rich Cu–2% Ta alloyed matrix [29]. It is important to note also that the consolidation of nanostructured powder into a bulk form is often very challenging because of limitations associated with the processing methods that are not easily scalable or due to uncontrolled grain growth of the non-equilibrium structures during processing. Despite the excellent properties achieved in these Cu–Ta alloys via mechanical alloying, there is only a limited report on the fabrication of nanostructure Cu–Ta alloys through processing a mixture of bulk Cu and bulk Ta, where the initial disc samples were a Cu50Ta50 system consisting of 19 Cu foils and 18 Ta foils assembled alternately in a single stack and successfully processed by HPT to 10, 30, 50, 100 and 150 turns under a pressure of 4.0 GPa [31]. It is noted that the HPT-processed 150 turns Cu–Ta alloy showed a superior thermal stability even after annealing at 1000 °C for 1 h [31].
The HPT processing of the Cu50Ta50 system [31] has limitations in its practical fabrication due to the difficulty and time required to prepare Cu foils and Ta foils, where the foil thickness is only ∼24.3 μm, in order to make the 0.9 mm thick disc sample of stacked 19 Cu foils and 18 Ta foils before HPT processing. To improve the efficiency in materials preparation and processing, the present research was initiated to process immiscible Cu–Ta alloys using bulk Cu discs and Ta discs (thickness 0.8 mm for each Cu disc and Ta disc) which were packed in a sandwich-like structure of Cu/Ta/Cu for HPT processing. The microstructures and mechanical properties of the HPT-processed Cu–Ta alloys were subsequently investigated in detail. As will be demonstrated, the use of HPT processing produces a material having exceptionally high hardness and excellent tensile strength.
Section snippets
Experimental materials and procedures
The experiments were conducted using rods of oxygen-free Cu (99.95 wt%) and Ta (99.9 wt%). The Cu rod was first annealed for 1 h at 673 K whereas the Ta rod was received in an annealed state. Both the annealed Cu rod and the as-received Ta rod were cut into discs with diameters of 10 mm and thicknesses of 1.1 mm and this was followed by grinding to a thickness of 0.8 mm. Then a Ta disc was placed between two Cu discs in a sandwich-like configuration and the piled discs were processed by HPT at
Initial microstructural characteristics after HPT processing
The cross-sections of the HPT-processed samples were first observed using an optical microscope (OM) to examine the evolution of the stacked Cu–Ta–Cu layers. As shown in Fig. 1, after 0.25 HPT turn the bulk Ta layer is clearly visible between the two Cu bulk layers and there are sharp and distinct Cu–Ta interfaces. After 5 and 10 turns of HPT processing, in some areas the Cu–Ta boundaries disappear but in some regions the Cu–Ta interfaces are distinct but with significant curvature. When the
Discussion
An evaluation of the microstructure and the phase evolution of the Cu–Ta sample through the HPT processing shows that in the early stages the Ta layer within the stack becomes thinner as a result of the HPT shear strain and then gradually these thin Ta layers are broken to form small flakes. The size of these Ta-rich flakes decreases as the HPT process continues and ultimately there is essentially a uniform dispersion of flakes throughout the Cu matrix. Since the edge area of the disc
Summary and conclusions
- 1.
Nanostructured Cu–Ta alloys were successfully fabricated from Cu/Ta/Cu stacked discs by HPT processing at room temperature. A uniform two-phase layered microstructure was developed by increasing the numbers of HPT turns where these two phases included a Cu-rich layer with a composition of about Cu81Ta19 and a Ta-rich layer with a composition of about Ta78Cu22.
- 2.
Significant microstructural refinement was achieved in the Cu–Ta alloy with average crystallite sizes in the range of ∼35–45 nm after 150
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.
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.
Acknowledgements
This work was supported in part by the European Research Council under grant agreement no. 267464-SPDMETALS and in part by the National Science Centre, Poland, within the project SONATINA 1 “Synthesis of novel hybrid materials using High-Pressure Torsion”, under Grant Agreement No.2017/24/C/ST8/00145. One of the authors (YH) thanks the QR fund from Bournemouth University.
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