Unique Microstructure Evolution of HPT-Processed (α + γ) Two-Phase Stainless Steel

Abstract

A high-pressure torsion (HPT) processed Fe-21Cr-5Ni-2Mo (mass%) two-phase stainless steel was used to study the morphology and crystallographic features of austenite (γ) precipitated from ferrite (α) during aging in the (α + γ) two-phase region. The starting material was a gas-atomized powder with a completely ferritic structure. The HPT process was carried out to produce a fully dense compact under 6 GPa for 5 revolutions. The compact was given an equivalent strain of about 130. After the HPT process, the matrix ferrite formed a pancake–like nanograined structure with a strong texture, i.e. ND (Normal Direction) // {110} α. By annealing at 1173 K for 3.6 k, an ultrafine (α + γ) microduplex structure with high-angle grain boundaries was formed. In addition, the strong texture formation of {110} α / {111} γ / ND plane was formed in the α and the γ grain duplex structure. The α and γ phases had average grain sizes of 2.1 μm and 1.6 μm, respectively. The area fraction of the γ phase was 37.2%, which exceeded that of a cold-pressed compact, 6.7%. Both ultrafine grain refinement and γ precipitation were accelerated by the HPT process. In other words, the application of the HPT process to the two-phase alloys enables the formation of the ultrafine microduplex structure. The Kurdjumov-Sachs (K-S) orientation relationship between α and γ phases is usually observed in the alloy, however, the K-S orientation relationship was not dominant except for the close packing plane parallel orientation relationship, {110} α / {111} γ, in the HPT-processed material.