**1. Introduction**

At present, Al-based composite materials are known to become increasingly important, for they have essential advantages over conventional commercial alloys due to a favorable combination of practical design characteristics and functional properties [1–4]. For example, aluminum-matrix composite materials reinforced with silicon carbide are used in automobile industry, aerospace engineering and other industries owing to high wear resistance, high specific strength, and heat conductivity [5–7]. It was shown that silicon carbide reinforced aluminum matrix composites are effectively additionally alloyed with the chromium [8]. The authors have found that the aluminum matrix composites with a varying weight percent of chromium (0–3 wt%) and a fixed percentage of silicon carbide (10%) were formed through the vortex casting. It was found that the chromium additive in the Al-SiC composites significantly improved the hardness, friction coefficient, and mechanical properties (strength, abrasion resistance, and wear resistance). Borides, nitrides, and oxides of refractory metals are often used as reinforcement particles for the Al matrix; the disperse phases of these particles reinforce the plastic Al matrix, thus creating

**Citation:** Brodova, I.; Rasposienko, D.; Shirinkina, I.; Petrova, A.; Akopyan, T.; Bobruk, E. Effect of Severe Plastic Deformation on Structure Refinement and Mechanical Properties of the Al-Zn-Mg-Fe-Ni Alloy. *Metals* **2021**, *11*, 296. https:// doi.org/10.3390/met11020296

Academic Editor: Christine Borchers

Received: 28 December 2020 Accepted: 31 January 2021 Published: 9 February 2021

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barriers for the migration of dislocations and grain boundaries [1–3]. Besides artificial composites, which are produced by mechanical alloying or by powder metallurgy, there is a big class of natural aluminum-based eutectic composite materials. Typical examples of such alloys are represented by Al-Si alloys (silumins). New advanced alloys based on Al-Fe, Al-Ni, and Al-Fe-Ni eutectics have lately been developed and termed nickalins [9–11]. Well-known compositions of the 7xxx series—i.e., alloys belonging to the Al-Zn-Mg-Cu system—were proposed as the Al matrix of these alloys. Economical Al-Zn-Mg alloys with a total Fe and Ni content of 1 wt% are highly technological due to the eutectic constituent (Al + Al9FeNi) and have high strength characteristics resulting from a dispersion-hardened Al matrix [10,11].

The state-of-the-art trend in improving the structure and properties of commercial alloys is the application of methods of severe plastic deformation, the best-studied ones being equal-channel angular pressing (ECAP) [12–14], equal channel angular extrusion (ECAE) [15–18], and torsion in Bridgman anvils (HPT) [19–21]. New SPD methods have been developed in recent years. For example, there are an accumulative roll bonding (ARB) [22,23] and a differential velocity sideways extrusion (DVSE) [24,25]. DVSE is a novel method for directly forming curved profiles from billets in one extrusion operation using two opposing punches. Modeling the process, authors [24] identified that a curvature of extrudate can be controlled and varied using a difference between the velocities of the two punches, defined by velocity ratio, as well as an extrusion ratio. The effectiveness of the new SPD method was tested during manufacturing a curved AA1050 bar widely used in the transport industry [25]. Due to the severe strains arising in the DVSE process, (greater than that for conventional pass of equal channel angular extrusion), significant grain refinement in the curved bar (grain size ~3 μm) from the original billet (grain size ~357 μm) was achieved in one extrusion operation. On the basis of structural studies, the authors found that grain refinement in the DVSE process is due mainly to continuous dynamic recrystallization. The refinement of the structure led to an increase in mechanical properties. Compared with the billet, the hardness, yield strength, and ultimate tensile strength of the formed curved bar increase by 134.8%, 354.0%, and 116.8% respectively, although the elongation to fracture was decreased by 60.0%. DVSE method includes hot extrusion (DVSE-HE), welding extrusion (DVSE-WE), and coextrusion (DVSE-CE). DVSE-CE technology used for a forming curved Al/Mg sandwich bar was described in [26].

According to [14,17,18,27–29] submicrocrystalline and nanocrystalline alloys of 7XXX series exhibit a good combination of strength and plastic properties resulting from thermal deformation processing under different conditions.

The analysis of the available results on the design of high-strength Al-Zn-Mg-alloybased composites raises a question of how the shape, size, and amount of Al9FeNi eutectic aluminides change and how this affects the structure, phase composition, and properties of nickalins as a whole. The first studies along this line have been conducted in the last three years [30–32]. The effect of thermal pretreatment on the deformation behavior of nickalin under high-pressure torsion (HPT) was established [30], the sequence and kinetics of postdeformation processes under heating to 400 ◦C were determined [31], and the dynamic properties of an Al-Zn-Mg-Fe-Ni alloy before and after HPT in a wide range of strain rates, 10<sup>3</sup> to 10<sup>5</sup> s<sup>−</sup>1, were measured [32]. The results have shown that the principal mechanisms of structure formation in a 7xxx series alloy under HPT remain the same, but the presence of Al9FeNi eutectic aluminides in the composite structure affects the morphology and kinetics of the precipitation of secondary phases and increases the thermal stability of the composite and its strength.

It was reported in [33] that radial-shear rolling is an effective method for increasing the mechanical properties of nickalin through creating a microscale gradient structure and a uniform distribution of strengthening phases in it.

The purpose of this paper is to continue the study aimed at creating a composite material based on an Al-Zn-Mg-Fe-Ni alloy by combining thermal and deformation processing techniques. Particularly, the effect of complex deformation processing including RSR and HPT on the structural and phase transformations of the Al-Zn-Mg-Fe-Ni alloy and its mechanical properties is discussed. In view of the complex chemical composition of the alloy, it is obvious that the intermetallic phases of different origins make a grea<sup>t</sup> contribution to the formation of its structure and properties; therefore, the paper focuses on studying the evolution of their formation mechanisms under HPT with the application of high-resolution TEM methods.

#### **2. Materials and Methods**

A high-strength Al-Zn-Mg-Fe-Ni alloy (nickalin) was studied. The chemical composition of the experimental alloy is given in Table 1.


**Table 1.** Chemical composition (wt%) of the experimental alloy.

The ingot production conditions, and the raw material composition are described in detail in [11,33]. The melting was carried out in an electric resistance furnace in graphite -chased crucible based on high purity Al (99.99%). Zn and Mg were introduced into Al melt in the form of pure metals, whereas Fe, Ni and Zr in the form of binary master alloys (Al-10% Fe, Al-20% Ni, Al-1.5% Zr). The casting was carried out in a metal mold at a temperature of 780 ◦C (cooling rate during solidification was ~10 K/s). The ingots were subjected to two-stage homogenizing annealing at 450 ◦C and 540 ◦C with holding for three hours in each stage. The alloy was studied in the form of ingots, calibrated rods and disks produced by HPT at room temperature.

The ingots were turned to a diameter of 40 mm, heated in an oven to 480 ◦C, and subjected to two-pass radial shear rolling (RSR) in MiSiS 100 T and 14–40 mills [33]. The total rod elongation ratio was 2.78, the diameter of the initial billet was 31 mm, and the diameter of the resulting rod was 24 mm.

Severe plastic deformation by HPT was performed on 20-mm-diameter and 1-mmthick specimens cut from an ingot (nickalin 1) and a rod after RSR (nickalin 2). The number of anvil revolutions was *n* = 5, 10, 15, for the true accumulated strains (with allowance for upsetting) *ε* = 6.0, 6.7, and 7.1, respectively. The mechanical properties were determined on flat microspecimens (5.7 mm long, 2 mm wide, and 1 mm thick) in a Shimadzu AGX-50 Plus (Kyoto, Japan) universal testing machine. To fix the microspecimens in the grips of the testing machine, we used a special clamping device described in detail in [34]. The strain rate was 5 × 10−<sup>4</sup> s<sup>−</sup>1.

The mechanical properties were estimated from the values of tensile yield strength YS, ultimate tensile strength UTS, and percentage elongation δ. Microhardness Hv was determined at a load of 0.2 N in a PMT-3 device (Moscow, Russia). The measurement error was at most 10%. Hardness was measured by the Brinell method, with a 5 mm ball, at a load of 250 kg. The specimen structure was studied at different scale levels by means of a Neophot-32 optical microscope (OM, Carl Zeiss, Jena, Germany), a Quanta 200 scanning electron microscope (SEM, FEI Company, Hillsboro, OR, USA), and a Tecnai G<sup>2</sup> 30 Twin transmission electron microscope (TEM, FEI Company, Oregon, USA) with an accelerating voltage of 300 kV. X-ray diffraction (XRD) analysis was carried out using an Empyrean diffractometer (PANanalytical, Almelo, The Netherlands) at room temperature and Cu K α radiation. Based on the X-ray diffraction data the lattice parameter of an Al-matrix was calculated. The specimens for *a*analysis were made with the use of mechanical polishing on a diamond suspension. Final smoothing was performed on a suspension of colloidal silicon dioxide. The specimens for thin structure examination were made by jet polishing in a Tenupol-5 (Struers, Denmark) in a 20% nitric acid solution and 80% methanol at a temperature of −25 ◦C and a voltage of 15 to 20 V. The size of the structural constituents

was quantitatively analyzed from dark-field images with the use of the SIAMS 700 image analysis program (Russia).
