Table 2.

Comparison of thin films and SPD-processed bulk materials.

Characteristic	Thin Films	SPD-Processed Bulk Materials
Purity	High purity (vacuum environment, clean surfaces) [82]	Presence of impurities [74,86,87]
Achievable Grain Size	<10 nm [82]	HPT: 10–100 nm [76] Mechanical milling: 5–50 nm [84,85] Other SPD processes: <1 μm [76,86]
Grain Structure	Primarily columnar grains [82,83] Lattice strain from rapid cooling [82]	Equiaxed grains [86] High-angle grain boundaries [86,87]
Grain Structure Control	Grain size can be controlled via substrate temperature [82,83]	Grain boundary misorientation evolves with strain [74] Flash annealing to control grain size
Scalability	Limited by vacuum/clean environment	HPT: size limited (samples typically 10 mm in diameter and 1 mm thick) [76] Other processes: scalable
Limitation	Specimen thickness effect [64]	Difficult to control grain boundary character Impurity level (mechanical milling)