Effect of Solution-Hardening on the Evolution of Nanostructure in Ni-Fe alloys during Severe Plastic

Introduction

Overview

The average grain size calculated by the line intersection method is 8μm, 7μm and 5μm for Ni-10%Fe, Ni-20%Fe and Ni-30%Fe, respectively. Even after N=1 rotation, the grain size in the central region is quite large ~855 nm and the fraction of HAGB is low ~0.35. The variation in grain size and HAGB fraction with strain at different radial locations in Ni-10%Fe alloy is summarized in Fig.4.5.

The variation in grain size and HAGB fraction at different radial locations in Ni-20% Fe is summarized in Fig.4.7. This is accompanied by sharp reduction in uniform grain size from ~1um in the central region to 165 nm in the center. The grain size decreases continuously with a simultaneous increase in the HAGB fraction in the central region with increasing number of rotations.

However, in the middle and edge regions, a slight increase in grain size is observed with increasing strain beyond N=5 rotations. The variation in grain size and HAGB fraction at different radial locations in Ni-30%Fe is summarized in Fig.4.9. As the number of rotations increases, the grain size decreases, however, in all three alloys the grain size is minimal after one rotation.

Then the grain size increases slightly, as can be seen in Table 5.1. It can be observed that the behaviors of Ni-20%Fe and Ni-30%Fe are almost similar, so that the HAGB fraction and grain size are uniform across the disk. In Ni-Fe alloys, the minimum grain size is reached much earlier (i.e. after N=1 rotation) compared to the Ni-Co alloy series.

Ni-30%Fe clearly shows a smaller grain size compared to the other two Ni-Fe alloys after N=1 rotation. However, the difference in grain size is not as significant as in the Ni-Co alloy series. This grain size variation can also be observed in Table 5.1, which shows that the grain size of Ni-Fe and Ni-Co alloys is in the boundary region at different strain rates.

The final grain size obtained after N=10 rotations in Ni-30%Fe and Ni-60%Co is almost identical, indicating that solute hardening is also equally effective in achieving ultrafine grain size. The grain size is smaller at the edge area than in the central area at lower number of revolutions in the three alloys. Extreme homogeneity in terms of grain size is achieved in Ni-20%Fe and Ni-30%Fe is at N=10.

Hardness values compared to low SFE alloys such as Ni-Co are significantly higher in Ni-Fe alloys despite having almost similar grain sizes, implying that the increase in hardness is for due to solidification of the solution.

Fig. 3.1.Schematic illustration of microhardness test measurements

Objective of study

Literature Review

Experimental Procedure

Sample Preparation for HPT processing

Samples for HPT processing in the form of 10 mm diameter discs were prepared from the annealed plates using a wire-cut Electric Discharge Machine (EazycutTM, Electronica). The discs were further ground with sandpapers with grit size 1000 to 2000 to a final thickness of 0.85 mm.

High Pressure torsion Processing

Characterization

Microhardness Measurements
Microstructure and texture characterization

For the Ni-10%Fe alloy, the average grain size decreases drastically from 8.4 µm in the central region to 390 nm and 270 nm in the middle and edge regions, respectively, after N=1/2 rolling. The grain size in the middle and edge regions after N=10 rotations is slightly larger than that observed after N=5 rotations. In the middle, the grain size increases from 155 nm after N=5 spins to 177 nm after N=10 spins, while for the edge region the grain size increases from 175 nm after N=5 spins to 185 nm after N=10 spins.

At N=1, grain size decreases significantly from 1.53 µm in the center region to 181 nm at the center region with accompanying increase in the HAGB from 22% at the center region to 65% at the center region. Reduction in grain size with increasing solute concentration can be observed, except for N=1/12 which corresponds to a very low strain value so that the formation of deformation induced grains is not complete and only fragmented structure with a large fraction of LAGB observed can be . Steady state grain size is reached during SPD when dislocation accumulation balances dislocation destruction and grain boundary movement.

Considering the alloying effect in two alloy systems, it can be noted that with increasing alloy content from 20%Co to 60%Co the grain size is drastically reduced in Ni-Co system after N=10 rotations indicating that additional grain refinement operates in low SFE alloys, such as Ni-60%Co. Thus, when the contribution of solution hardening is not significant, the SFE plays a significant role in obtaining minimum ultrafine grain size. Regardless of the SFE, the steady state grain size is similar indicating that SFE is not the only dominant parameter responsible for grain refinement [18].

However, the interesting point is that in all the alloys where there is clear structural coarsening at different stages of deformation, only the Ni-60%Co shows a consistent decrease in grain size up to N=10 rotations. Even though solute hardening is effective in achieving similar ultrafine grain size as demonstrated in Ni-Fe alloy series, structural coarsening is evident as SFE is not lowered by alloying. This increase in hardness is due to solid solution hardening as well as due to effect of decrease in grain size.

Recently, a study on several FCC alloys deformed by HPT was carried out and it was reported that grain size reduction is a major hardening mechanism in single-phase alloys, while effect of solid solution hardening is less than 15% of total hardening [ 18]. It can be observed from Fig.5.8(a) that volume fraction of this component also increases with increasing alloy content in Ni-Fe alloys. Although there is no significant change in SFE when alloying Ni with Fe, but the grain size achieved is comparable to the alloys that have very low SFE, this suggests that solute effect is as effective in grain refinement as SFE.

Experimental Results

Starting materials characterization

X-Ray diffraction pattern
Microstructure
Microhardness measurements
Microstructure characterization of HPT processed Ni-10%Fe

It can be observed that the structural evolution is faster in the Ni-Fe alloy series. Fig. 5.6. Volume fractions of texture components with increasing number of HPT spins Three machined Ni-Fe alloys: (a) A/A-, (b) A*-.

Fig 4.4.Grain boundary maps of HPT processed Ni-10%Fe alloy at center (r ~ 0), middle (r ~ 2.5) and edge (r ~5 mm) regions obtained after different

Microtexture evolution in HPT processes Ni-Fe alloys

Microhardness measurements of Ni-Fe disks after HPT processing

The center-to-edge microhardness profile of Ni-20%Fe and Ni-30%Fe is quite similar, and significant inhomogeneity of microhardness between the center and edge regions is observed even up to the highest strain level. In this study, different Ni-Fe alloys (Ni-10%Fe, Ni-20%Fe, and Ni-30%Fe) were deformed by HPT to different strain rates using an applied load of ~5 GPa. However, at N = 10, a homogeneous microstructure can be seen across the disc in Ni-20%Fe and Ni-30%Fe.

Remarkable microstructure homogeneity is achieved in both Ni-20%Fe and Ni-30%Fe alloys after N =10 turns (Fig 4.7a and Fig 4.9a). To better understand the factors affecting grain refinement, we compare two alloy systems, namely Ni-Fe and Ni-Co [19]. While in the Ni-Fe alloy system the change in SFE is negligible as already stated, in the Ni-Co system the SFE decreases systematically with increasing alloys without significant solution strengthening[7].

It can also be observed that there are not many differences in the hardness values of Ni-20%Fe and Ni-30%Fe, but homogeneity in hardness is achieved only in Ni-30%Fe and Ni-60%Co. These higher strengths can be attributed to the solution hardening effect, which is pronounced in Ni-Fe alloys but not in Ni-Co alloys. Since alloying Ni with Fe has no significant change in SFE, thus, we can assume that the texture development of Ni-Fe alloys can be very similar to pure Ni.

Texture development in Ni-Fe alloys treated by HPT shows the presence of all conventional texture components during simple shearing [22,23]. The trend followed by different texture components with different strain values remains almost similar with increasing alloy content in Ni-Fe alloys, i.e. in Ni-Co alloys, the volume fraction of different texture components is different at a certain strain value in different Ni-Co alloys, as shown in Figure 5.7.

Comparing Ni-Fe alloys with Ni-Co alloys, it can be observed that A, A*, B and C are prominent textural components in both alloys. The C component decreases with increasing Co content and increasing strain, and in Ni-Fe alloys it increases up to the same strain. Figure 5.7. Volume fractions of textural components with increasing number of revolutions of HPT Processed three Ni-Co alloys: (a) A/A-, (b) A*-.