Abstract
A study on the role of deformation temperature on a twin-assisted refinement of austenitic structure and phase transformations in high-pressure torsion of high-Mn Hadfield steel single crystals (Fe-13Mn-1.3C, in mass. %) has been carried out. In high pressure-torsion, twinning has been experimentally confirmed as a high-temperature deformation mechanism and has been observed at the temperature 400 °C. An increase in deformation temperature of up to 400 °C decreases the activity of mechanical twinning but does not fully suppress it. A dense net of twin boundaries, which has been produced in cold deformation by high-pressure torsion at room temperature, possesses high thermal stability and stays untransformed after post-deformation annealing at a temperature of 400 °C. In high-pressure torsion at a temperature of 400 °C, the complex effect of high temperature and severe plastic deformation on the strengthening of high-carbon Fe-13Mn-1.3C steel has been observed. A synergetic effect of severe plastic deformation and elevated temperature stimulates a nucleation of nanoscale precipitates (carbides and ferrite) along with deformation-induced defects in austenitic structure. These fine precipitates are homogeneously distributed in the bulk of the material and assist high values of microhardness in high-pressure torsion-processed specimens, which is similar to twin-assisted microstructure.
Highlights
Grain boundary engineering (GBE) proposed by T
A Comparison of the Microstructures Produced by high-pressure torsion (HPT)-Processing at Room Temperature and 400 ◦ C
During the HPT deformation of single-crystalline Fe-13Mn-1.3C steel at room temperature, multiple twinning is realized in defective structure saturated by local barriers, which prevent separate twin lamellae to growth both in width and length—substitutional and interstitial atoms, Mn–C pairs in the early stages of the plastic flow, slip and twinning dislocations, twin boundaries and dislocation-assisted complexes, etc. in the stages of developed plastic deformation
Summary
Grain boundary engineering (GBE) proposed by T. A high-pressure torsion (HPT) has been successfully applied for obtaining ultrafine-grained and nanograined structures in a variety of metallic materials [9,10,11]. In this way, ultrafine-grained materials look very attractive due to superior strength properties but, having a high-volume fraction of intergranular boundaries, they simultaneously possess limited plasticity and thermal stability compared to coarse-grained counterparts [8,9,12]. GBE could be the possible way to make the ultrafine-grained materials more stable against recrystallization during post-deformation heat treatment, via producing of low-energy coherent special boundaries, for instance, twin-assisted Σ3n boundaries [13,14,15,16]. For nanotwinned 330SS (stainless steel), Zhang and Misra [13] demonstrated a superior thermal stability of coherent twin boundaries, as Metals 2020, 10, 493; doi:10.3390/met10040493 www.mdpi.com/journal/metals
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