Assessing thin films as predictors of bulk properties in high-throughput alloy design

Thin film deposition offers a high-throughput route for alloy screening, but its ability to predict bulk alloy behavior remains uncertain. In this study, we compare the microstructure and mechanical properties of thin-film and bulk forms of a five-element NbMoTaTiV refractory high-entropy alloy (RHEA). Fifteen thin-film compositions were fabricated via magnetron sputtering, and five representative compositions were synthesized in bulk by arc melting. Both forms exhibited a single-phase BCC structure, confirming that thin films can capture phase formation. However, significant differences were observed: thin films exhibited ultrafine columnar grains (∼100 nm) with chemical homogeneity at micron length scale, while bulk alloys showed coarse equiaxed grains (∼100–150 μm) and stable elemental segregation even after high-temperature heat treatment. Nanohardness trends in thin films generally correlated with bulk nanoindentation and Vickers hardness, but compressive testing revealed that hardness alone could not predict yield strength (YS). In particular, the alloy with the highest hardness showed the lowest YS due to segregation and defects. These findings highlight that thin films provide valuable insights into intrinsic compositional effects but must be paired with microstructural characterization and bulk validation.