*4.2. Accumulative HPT Procedure*

As shown in [106], the actual shear strain in samples subjected to HPT is significantly lower in comparison with the expected value. In order to achieve high strain in BMGs, the authors proposed a new method: "accumulative HPT [125]. In the accumulative HPT procedure, the sample undergoes

several cycles of HPT processing, then cutting, stacking, pressing, and subsequent HPT, as shown in Figure 18. Finally, the stacked segments are subjected to HPT with *n* ≥ 3 as a result of which the fragments are consolidated into a monolithic disk [125]. The total number of revolutions during the "accumulative HPT" processing of Vit105 BMG was *n* = 5, and the total deformation of the BMG by pressing and shear was roughly estimated as γ = 6 [125].

Some of the BMG disks were processed by conventional HPT with *n* = 5. In this case, the total deformation of the BMG could be estimated as γ = 2. These estimates are very approximate, but they show that the total deformation at the "accumulative HPT" was 2.5 times greater than during conventional HPT.

The structure of the initial BMG is amorphous, as shown by the XRD method, and the position of the amorphous halo of the BMG after HPT shifts towards lower angles (Table 5). This means an increase in *R* of the first coordination sphere (*R1*) (Table 5) [82,98]. The relative changes in Δ*V* after HPT could be estimated from *R1* according to [82]. The increase in free volume (Δ*V*) after conventional HPT *n* = 5 was Δ*V* ≈ 1% and Δ*V* after accumulative HPT was ≈ 2.5%. The Δ*V* = 2.5% is too large compared to the Δ*V* usually observed during HPT of a BMG [82]. This is due to the large error in determining Δ*V* from the XRD [82]. However, these results indicate that the structure of BMG after accumulative HPT transforms much more significantly than after conventional HPT. The increase in the FWHM after HPT (Table 5) was also explained by changes in the structure of BMG [92]. The growth of FWHM after "accumulative HPT" is greater than after conventional HPT (Table 5), which also indicates the efficiency of accumulative HPT.

**Figure 18.** Principle of the "accumulative HPT" process: (**a**) disk-shaped sample subjected to HPT *n* = 1; (**b**) sample cut into four pieces; (**c**) pieces are stacked on top of the HPT anvil and HPT *n* = 1 applied again; (**d**) BMG sample obtained according to this procedure (reprinted from [125] with permission from Elsevier).

**Table 5.** Parameters of the amorphous structure of the BMG in different states from XRD (reprinted from [125] with permission from Elsevier).

Δ

