Unique microstructure evolution of HPT-processed (α plus γ) two-phase stainless steel

A high-pressure torsion ( H P T) processed Fe-2 1 Cr-5 Ni-2 M o (mass %) two-phase stainless steel was used t o study the morphology and crystallographic features of a ust eni te (& gamma;) precipitated from ferrite (& alpha;) during aging in the (& alpha; + & gamma;) two-phase region. The starting material was a gas-atomized powder with a completely ferriti c structure. The HPT process was carried out t o produce a fully dense compact under 6 GPa for 5 revolutions. The compact was given a n equivalent strain of about 130. After the HPT process, the matrix ferrite formed a pancake-l ike nano gra i ne d structure with a strong texture, i.e. ND (Normal Direction) / / <110= & alpha;. By annealing at 1173 K for 3.6 k, a n ultra fine (& alpha; + & gamma;) micro duplex structure with high-angle grain boundaries was formed. In addition, the strong texture formation of {110} & alpha; / {111} & gamma; / ND plane was formed in the & alpha; and the & gamma; grain duplex structure. The & alpha; and & gamma; phases had average grain sizes of 2.1 & mu;m and 1.6 & mu;m, respectively. The area fraction of the & gamma; phase was 37.2%, which exceeded that of a cold-pressed compact, 6.7%. Both ultra fi ne grain refinement and & gamma; precipitation were accelerated by the HPT process. In other words, the application of the H PT process t o the two-phase alloys enables the formation of the ultra fi n e micro dupl ex structure. The Kurd jum ov-Sac hs (K-S) orientation relationship between & alpha; and & gamma; phases i s usually observed in the alloy, however, the K-S orientation relationship was not dominant except for the close packing plane parallel orientation relationship, {110} & alpha; / {111} & gamma;, i n the HPT-pr oces se d material.