A 316L stainless steel was processed by high-pressure torsion (HPT) to evaluate the grain refinement and phase transformation. The initial material was essentially a single phase γ-austenite with a coarse-grained microstructure of ~42 µm but the grain size was reduced to ~45 nm after 10 turns of HPT. In addition, there was a phase transformation and the initial γ-austenite transformed initially to ε-martensite and finally to α'-martensite with increasing strain. The dislocation density increased to an exceptionally high value, of the order of ~10 16 m -2 , in the main α'-martensite phase after 10 HPT revolutions. The formation of the multiphase nanocrystalline microstructure yielded a four-fold increase in hardness to reach an ultimate value of ~6000 MPa. The Hall-Petch behavior of the HPT-processed alloy is compared directly with coarse-grained materials.

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Differential scanning calorimetry (DSC) was used to study the thermal stability of the microstructure and the phase composition in nanocrystalline 316L stainless steel processed by high-pressure torsion (HPT) for ¼ and 10 turns. The DSC thermograms showed two characteristic peaks which were investigated by examining the dislocation densities, grain sizes and phase compositions after annealing at different temperatures. The first DSC peak was exothermic and was related to recovery of the dislocation structure without changing the phase composition and grain size. The activation energies for recovery after processing by ¼ and 10 turns were ~163 and ~106 kJ mol -1 , respectively, suggesting control by diffusion along grain boundaries and dislocations. The second DSC peak was endothermic and was caused by a reverse transformation of α'-martensite to γ-austenite. The hardness of annealed samples was determined primarily by the grain size and followed the Hall-Petch relationship.Nanocrystalline 316L steel processed by HPT exhibited good thermal stability with a grain size of ~200 nm after annealing at 1000 K and a very high hardness of ~4900 MPa.

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An investigation was conducted to evaluate the effect of annealing at different temperatures on the tensile properties of ultrafine-grained 316L stainless steel processed by high-pressure torsion (HPT). A "moderate-temperature" annealing at 740 K resulted in reduced strength and elongation due to the annihilation of mobile dislocations. A "high-temperature" annealing at 1000 K yielded a remarkably good combination of yield strength (~1330 MPa) and elongation to failure (~43%) which can be attributed to the almost full reversion of α'martensite formed during HPT into γ-austenite while the grain size remained very fine with a value of about 200 nm.

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