**3. Results**

#### *3.1. Structure and Phase Composition of the Alloy before HPT*

Al-Zn-Mg-Fe-Ni alloy specimens of the same composition but different histories were the initial materials for severe plastic deformation (SPD). Batch I specimens (nickalin 1) were cut from the ingot and annealed in two stages at 450 and 540 ◦C with a 3 h holding at each stage. Their structure and properties were described in detail in [32]. Heat treatment forms a structure consisting of an Al matrix and Al9FeNi eutectic aluminides located on the grain boundaries and dendritic cells. The grain size of the Al matrix is 380 μm and that of the eutectic aluminides is 2 to 3.5 μm. Besides, inside the grains of the Al solid solution there are precipitates of the secondary T-phase (Al2Zn3Mg3) resulting from ingot cooling after homogenization.

A 24-mm-diameter rod produced by RSR at 480 ◦C after two passes was an object for HPT of batch II specimens (nickalin 2). Figure 1 shows the rod cross-section structure.

**Figure 1.** Microstructure in cross-section of the nickalin 2 rod after RSR (OM).

Three zones with a gradient structure typical for this RSR pattern are clearly visible. The width of the SPD zone on the rod periphery is ~2.5 to 3.0 mm; the middle zone at half the radius and the central zone have a width of 4.0 to 4.5 mm each. The gradient structure is characterized by different hardness values. The mid-radius zone has a hardness of 105 HB (105 HV), whereas the hardness in the central zone decreases to 97 HB (97 HV). In the peripheral zone, there is a dynamically recrystallized structure with the grain size increasing from 3 mm at the surface to 8 mm at the boundary with the second zone formed by the hot-deformed structure (Figure 2a). In the peripheral zone, the intermetallic particles become fragmented due to deformation and arrange themselves along the direction of rolling (Figure 2b). Elements of the cast structure of the matrix and eutectic conglomerates are preserved in the central part, the shape and size of aluminides being changed only slightly from the as-cast condition (Figure 2c,d). The EDS element analysis of these crystals has shown that they contain Fe and Ni, i.e., they belong to the Al9FeNi phase (Figure 2e).

**Figure 2.** SEM microstructure showing the dynamically recrystallized structure (**a**) and the intermetallic particles (**b**,**d**) in peripheral zone and the intermetallic particles in the center zone (**c**) of cross-section of the nickalin 2 rod after RSR. The EDS element analysis of the Al9FeNi phase (**e**).

The TEM results testify that, in the deformed region, the rod has a well-developed subgrain structure resulting from polygonization. The subgrains are mainly equiaxial and sized 2 to 6 μm. Along the subgrain boundaries there are disk- or stick-shaped (up to 1–5 μm) Al9FeNi intermetallics of the crystallization origin. Globular and stick-shaped secondary phases have been found inside the subgrains. According to electron diffraction pattern, equiaxial nanoparticles 30 to 75 nm in diameter are the metastable Al3Zr phase with an ordered cubic lattice L12 (Figure 3a,b). These dispersoids result from hot deformation. Sticks with a diameter of 5 nm and a length ranging between 50 and 100 nm (separate large particles reach a diameter of 20 nm and a length of 250 nm (Figure 3c,d)), belong to the MgZn2 phase precipitating from the supersaturated Al solid solution during cooling after RSR.

**Figure 3.** TEM dark-field images (**<sup>a</sup>**,**<sup>c</sup>**) and electron diffraction pattern (**b**,**d**) of intermetallic particles corresponding to the Al3Zr (**<sup>a</sup>**,**b**) and MgZn2 (**<sup>c</sup>**,**d**) phases in the nickalin 2 rod after RSR.

#### *3.2. Structure and Phase Composition after HPT*

The Al-Zn-Mg-Fe-Ni alloy in two structural states (nickalin 1 and nickalin 2) discussed in Section 3.1 was taken as macrocrystalline analogs for HPT specimens. The evolution of the structural and phase transformations in nickalin 1 and nickalin 2 was studied on

specimens after HPT with different strains by varying the number of anvil revolutions *n* = 5, 10, and 15 (*ε* = 6.0, 6.7, and 7.1 respectively). Microhardness was measured in order to characterize the uniformity of their structure along the specimen radius. It follows from the measurements that microhardness variation from the edge to the center does not exceed 150 MPa. However, in order to compare correctly the structure and phase composition of the specimens produced with different numbers of revolutions, all the TEM studies were performed on foils cut from the mid-radius region.

As a result of deformation, coarse eutectic aluminides in nickalin 1 become fragmented, they break into particles sized ~1–2 μm. The Al matrix for *n* = 5 and 10 is represented by a fragmented structure with grain/subgrain diffusion boundaries and a nonuniform contrast inside the grains/subgrains, which is caused by a high level of internal stresses (Figure 4a). Fragmentation increases with the amount of strain and, according to the results of the quantitative analysis of the EDS element analysis, the average size of the grain/subgrain ranges from 120 to 160 nm at *n* = 10, with only 20% of them being larger than 200 nm. At *n* = 15, mainly high-angle boundaries (HABs) are formed in the structure, this being evidenced by the annular electron diffraction pattern with numerous discretely located point-like reflections (Figure 4b). The main array of grains/subgrains retains the strain-induced contrast, but in triple joints there are small (20 to 30 μm) dislocation-free grains formed through dynamic recrystallization. This mixed two-level structure decreases the average grain/subgrain size to the nanoscale (~100 nm).

**Figure 4.** TEM microstructure showing the deformation structure of the nickalin 1 after HPT, *n* =5(**a**) and *n* = 15 (**b**).

The phase composition of nickalin 1 changes with the amount of strain. When *n* = 5 and 10, the volume fraction of the particles of the secondary T-phase decreases due to their deformation dissolution, and when *n* = 15, there appears a strain-induced Al solid solution supersaturated with zinc and magnesium.

The five-revolution deformation nickalin 2 results in significant fragmentation of the polygonized structure of the alloy. When *ε* = 6.0, there appear variously sized subgrains, irregularly shaped or elongated. Most of them (~80%) have a size ranging from 50 to 250 nm, with separate large subgrains having a size of up to 500 nm (Figure 5a,b). The annular character of the electron diffraction patterns with diffuse reflexes and the analysis of the dark-field images of the microstructure demonstrate that the fragments are separated by both high- and low-angle boundaries (LABs), Figure 5c. Deformation changes the morphology and average size of intermetallic particles. Fragmentation of Al9FeNi particles occurs, their characteristic size decreases to 0.6–3.5 μm, and the diameter of the Al3Zr dispersoid precipitates also decreases to 30–40 nm, possibly, due to partial dissolution during HPT, as was noted in [30].

**Figure 5.** TEM bright-field image (**a**), dark-field images (**b**,**d**) and electron diffraction pattern (**c**) of deformation structure (**<sup>a</sup>**–**<sup>c</sup>**) and intermetallic particles corresponding to the T (**d**,**<sup>e</sup>**) phase in the nickalin 2 after HPT, *n* = 5. (**e**) HRTEM image of T-phase (d = 0.3229 nm) particle. (**f**) An enlarged region of the particle shown in (**<sup>e</sup>**,**f**) and intensity histogram.

The analysis of the bright-field and dark-field images has shown that five-revolution deformation causes a complete dissolution of the zinc-magnesium phases, with the appearance of a strain-induced Al solid solution supersaturated with zinc and magnesium. On the other hand, the HPT process causes the decomposition the supersaturated solid solution and the formation of dispersed particles sized up to 20 nm along the boundaries of the matrix fragments (Figure 5d). The high dispersion and low volume fraction of these precipitates prevent them from being identified by electron microdiffraction patterns.

The calculation of the interplanar distances by the direct resolution of the particle lattice allows us to conclude that they are T-phase precipitates (d (420)T = 0.3222 nm, Figure 5e,f).

A characteristic feature of the structural state for *n* = 10 is the formation of a banded structure consisting of 40–120 nm grains/subgrains separated by low-angle and high-angle boundaries (Figure 6a–c).

**Figure 6.** TEM bright-field image (**<sup>a</sup>**,**d**), dark-field images (**b**,**<sup>e</sup>**), and electron diffraction pattern (**c**) deformation structure (**<sup>a</sup>**–**<sup>c</sup>**) and intermetallic particles corresponding to the T and η (**d**–**g**) phases in the nickalin 2 after HPT, *n* = 10. HRTEM images of T (d = 0.5882 nm) (**f**,**g**) and η (d = 0.4276 nm) (**h**,**i**) phases particles. (**g**,**i**) Enlarged regions of the particle shown in (**f**,**h**) and intensity histograms.

According to the HRTEM data, the growing amount of strain initiates the decomposition of the supersaturated solid solution (Figure 6d,e); besides precipitates along the boundaries of T-phase fragments sized 15 to 40 nm (d(210)T = 0.5883 nm, Figure 6f,g), more dispersed 10 to 20 nm equiaxial particles of the η or η'-phase (d(100)η' = 0.4295 nm, d(002)η = 0.4283 nm, Figure 6h,i) are detectable inside the grains/subgrains. As the amount of strain increases, large Al9FeNi inclusions continue decreasing in size to 1–1.5 μm, with individual fragments being nanosized; the Al3Zr distribution density remains practically unchanged.

The maximum deformation with 15 revolutions causes results in a further refinement of the submicrocrystalline structure, this being clearly visible from the dark-field image (Figure 7a). The grains/subgrains size distribution becomes more uniform, with the average size reaching 30 to 90 nm, i.e., a nanostructured state is formed and grain misorientation angles increase, as follows from the annular electron diffraction pattern with numerous discrete point-like reflections (Figure 7b). Similarly to the structure of HPT nickalin 1, when *n* = 15, in the structure of HPT nickalin 2, in the triple joints and on the matrix-intermetallics interphase boundaries, there also appear recrystallized grains, i.e., two structure refinement processes occur simultaneously: fragmentation and in-situ recrystallization. On the background of the nanostructured matrix, there are intermetallic particles of different shapes and sizes, close to those found in HPT nickalin 2 after *n* = 10. In follows from the results of electron diffraction patterns and direct lattice resolution, they belong to four phases: Al9FeNi, Al3Zr, T, and η.

(**c**) 

**Figure 7.** TEM dark-field images (**<sup>a</sup>**,**<sup>c</sup>**) and electron diffraction pattern (**b**) of deformed structure in the nickalin2 after HPT, *n* = 15.

Thus, complex deformation processing, namely RSR + HPT, yields an aluminummatrix composite reinforced with dispersed intermetallics of different natures.

#### *3.3. Hardness and Mechanical Properties of Nickalin*

The size characteristics of the alloy microstructure before and after HPT are vividly demonstrated by hardness testing. It has been found that nanostructured HPT nickalin 2 has a microhardness of 1800 to 2000 MPa, which by a factor of 1.7–1.8 exceeds that measured at the mid-radius of the RSR rod. Nanocrystalline nickalin 1 produced by HPT with the same amount of strain (*n* = 15) also has double microhardness relative to the macrocrystalline analog.

The procedure discussed in Section 2 was used to measure the mechanical properties of nanocrystalline alloys that were obtained by HPT with *n* = 15. Typical stress–strain diagram is shown in Figure 8. The average values obtained by measuring the mechanical properties of two samples are shown in Table 2 in comparison with the properties of macrocrystalline analogs. Let us compare how mechanical characteristics of the cast alloy change depending on the type deformation processing. According to the Table 2, RSR increases the tensile yield strength (YS) of the cast alloy from 240 to 264 MPa. The combined treatment (RSR + HPT) causes an additional increase in YS to 628 MPa, i.e., 2.8 times relative to YS of an ingot and 1.8 times relative to ultimate tensile strength (UTS) of an ingot. It follows from the data that the strength properties of the nanostructured alloys after HPT and the combined treatment (RSR + HPT), exceed those for macrocrystalline analogs (an ingot, a rod). Tensile yield strength increases by a factor of 2.0–2.4 and ultimate tensile strength increases by a factor of 1.3–1.5. The different mechanical characteristics of nickalin 1 and nickalin 2 after HPT are determinates by the different structure, phase composition, and different structural strengthening mechanisms.

The higher values of the strength properties of HPT + RSR nickalin 2 (YS = 628 MPa, UTS = 640 MPa) then the strength properties of nickalin 1 (YS = 450 MPa, UTS = 470 MPa) are associated with a more dispersed structure and additional contribution of the strengthening from the dispersed particles of the secondary phase. The hardening mechanisms are described more detail in Section 4.

**Figure 8.** Stress–strain diagram of the Nickalin 2 after RSR + HP (*n* = 15).


**Table 2.** Uniaxial tensile tests for the nickalins after different treatments.
