Microstructure, phase composition and hardness evolution in 316L stainless steel processed by high-pressure torsion

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 ~1016 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 behaviour of the HPT-processed alloy is compared directly with coarse-grained materials

Stored energy in ultrafine-grained 316L stainless steel processed by high-pressure torsion

The energy stored in severely deformed ultrafine-grained (UFG) 316L stainless steel was investigated by differential scanning calorimetry (DSC). A sample was processed by high-pressure torsion (HPT) for N = 10 turns. In the DSC thermogram, two peaks were observed. The first peak was exothermic and related to the annihilation of vacancies and dislocations. During this recovery, the phase composition and the average grain size were practically unchanged. The energy stored in dislocations was calculated and compared with the heat released in the exothermic DSC peak. The difference was related to the annihilation of vacancy-like defects with a concentration of â¼5.2 Ã 10â4. The second DSC peak was endothermic which was caused by a reversion of αâ²-martensite into γ-austenite, however in this temperature range dislocation annihilation and a moderate grain growth also occurred. The specific energy of the reverse martensitic phase transformation was determined as about â11.7 J/g. Keywords: High-pressure torsion, Stored energy, Stainless steel, Phase transformation, Thermal stabilit

Effect of Heat Treatment on the Microstructure and Performance of Cu Nanofoams Processed by Dealloying

Cu nanofoams are promising materials for a variety of applications, including anodes in high-performance lithium-ion batteries. The high specific surface area of these materials supports a high capacity and porous structure that helps accommodate volume expansion which occurs as batteries are charged. One of the most efficient methods to produce Cu nanofoams is the dealloying of Cu alloy precursors. This process often yields nanofoams that have low strength, thus requiring additional heat treatment to improve the mechanical properties of Cu foams. This paper provides the effects of heat treatment on the microstructures, mechanical properties, and electrochemical performance of Cu nanofoams. Annealing was conducted under both inert and oxidizing atmospheres. These studies ultimately reveal the underlying mechanisms of ligament coarsening during heat treatment