Abstract
High-pressure torsion (HPT) is known as an effective severe plastic deformation (SPD) technique to produce bulk ultrafine-grained (UFG) metals and alloys by the application of combined compressive force and torsional shear strains on thin disk samples. In this study, the microstructures and microhardness evolution of an additively manufactured (AM) 316L stainless steel (316L SS) processed through 5 HPT revolutions are evaluated at the central disk area, where the effective shear strains are relatively low compared to the peripheral regions. Scanning electron microscopy (SEM) analysis showed that the cellular network sub-structures in AM 316L SS were destroyed after 5 HPT revolutions. Transmission electron microscopy (TEM) observations revealed non-equilibrium ultrafine grained (UFG) microstructures (average grain size: ~115 nm) after 5 revolutions. Furthermore, energy dispersive x-ray spectroscopy (EDX) analysis suggested that spherical Cr-based nano-silicates are also found in the as-received condition, which are retained even after HPT processing. Vickers microhardness (HV) measurements indicated significant increase in average hardness values from ~220 HV before HPT processing to ~560 HV after 5 revolutions. Quantitative X-ray diffraction (XRD) patterns exhibit a considerable increase in dislocation density from ~0.7 × 1013 m−2 to ~1.04 × 1015 m−2. The super-high average hardness increment after 5 HPT revolutions is predicted to be attributed to the UFG grain refinement, significant increase in dislocation densities and the presence of the Cr-based nano-silicates, according to the model established based on the linear additive theory.
Highlights
High-pressure torsion (HPT) is a well-known severe plastic deformation (SPD) technique
A radial dependency in the torsional strain values imposed where r is the distance from the disk centre, N is the number of HPT revolutions, and h is throughout the disk during HPT processing is expected, in which the peripheral regions the disk thickness
A 316L stainless steel (316L SS) alloy was fabricated by laser powder bed fusion (L-PBF) additively manufactured (AM) technique and processed through 5 HPT revolutions
Summary
High-pressure torsion (HPT) is a well-known severe plastic deformation (SPD) technique. 316L stainless steel (316L SS) is a popular alloy that is widely used in marine, petrochemical, nuclear power, oil and gas, food and beverage, and biomedical industries due to its exceptional resistance to corrosion, minimum rate of absorption of neutron radiation, and good ductility [7,8]. This alloy can not be strengthened by heat treatment procedures due to its very low carbon content, the only suitable means to strengthen 316L SS is by reducing the grain size, i.e., the grain refinement route (Hall–Petch mechanism) through cold working. Numerous studies have demonstrated favourable mechanical (hardness, strength, and ductility) and functional (thermal stability, corrosion, and tribological) properties of wrought and cast 316L SS after HPT processing [9,10,11,12,13,14,15,16,17,18,19]
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