stabilizing the ultrafine-grained austenite with a high strength in the microstructure of an fe-9ni-6mn alloy under high-pressure torsion

Severe plastic deformation (spd) techniques are excellent procedures for fabricating ultrafine-grained (ufg) structures by imposing significant magnitudes of plastic strain in various metals and alloys. high-pressure torsion (hpt) is one of the spd methods in which a large compressive hydrostatic stress is introduced in the material. therefore, it is the most effective method for processing difficult-to-work alloys comparing to the others spd techniques. in this process, a thin disk-shaped specimen is compressed between two rigid anvils and subjected to torsional shear straining. as hpt processing has considerable characteristics such as significant grain refinement, a high density of lattice defects, high strength and hardness, superplastic properties and reasonable thermal stability therefore, it has attracted many researchers in two recent decades. the fe-9ni-6mn (wt.%) alloy is classified as an ultra-high strength steel which exhibits adequate ductility in the solution-annealed (sa) condition. however, this excellent age-hardenable steel suffers from post-aging intergranular embrittlement along prior austenite grain boundaries. recent efforts have focused on the effects of adding alloying elements, intercritical annealing, and also employing various spd methods to improve the alloy ductility after aging. accordingly, this research was conducted to examine the effects of the hpt processing by 5 turns on the ufg austenite formation in the microstructure and mechanical properties of an fe-9ni-6mn martensitic steel. x-ray diffraction (xrd) patterns and electron backscatter diffraction (ebsd) analyses showed that the initial microstructure of the alloy in the sa condition was a fully lath α′-martensite which partially transformed to a strain-induced austenite with an ultrafine grain size by hpt processing. this indicated that the hpt processing could supply the driving force needed for occurring the reverse transformation of the martensite to the austenite under deformation and stabilizing the austenite phase after removing the pressure. the hpt processing significantly decreased the grain size in the present alloy from  5.2 µm in the sa sample to ~ 265 nm for the martensite and  45 nm for the reversed austenite after hpt. also, the hpt processing led to a significant increase in the microhardness value from ~ 270 hv in the sa sample to ~ 630 hv after 5 turns hpt due to a high density of dislocations and the associated grain refinement of the microstructure. the tensile testing raveled that the hpt processing increases the ultimate tensile strength to ~ 1950mpa, but the ductility is decreased from ~ 15.4% in the initial sample to ~ 8.2% after 5 turns. in addition, the fracture mode changed from a fully ductile nature in the sa sample to a combination of ductile and brittle nature after applying the hpt.