Improvement of the strength of a metastable austenitic stainless steel by cyclic deformation-induced martensitic transformation at 103 K

Abstract

Specimens of a metastable austenitic stainless steel were cyclically deformed at 103 K under constant plastic strain control with constant plastic strain ranges Δε pl = 1% and 2%. The tests were performed both with a symmetrical plastic strain amplitude (mean plastic strain ε pl = 0 ) and with an asymetrical plastic strain amplitude ( ε pl > 0 ) with the minimum plastic strain equal to zero. The cyclic deformation behaviour was investigated, especially in the stage of cyclic hardening, and correlated with the deformation-induced formation of martensitic phases which could be detected by transmission electron microscopy. The effect of the transformed martensite on the mechanical properties of the steel was quantified by subsequent tensile tests at room temperature with the cyclically deformed specimens. In contrast to results of earlier studies at room temperature, no cumulative plastic “incubation” strain was necessary at 103 K to trigger the deformation-induced martensitic transformation. At Δε pl = 2% and ε pl = 0 the maximum cyclic range was attained after only 18 cycles. In the reference test with an asymmetrical plastic strain amplitude ( ε pl = 1% ) 23 cycles were required to reach the maximum cyclic stress range. Tension tests revealed an increase of up to 200% in the 1% offset yield stress and a 75% increase in the tensile strength at room temperature after cycling the specimens to the maximum stress range with either symmetrical or asymmetrical plastic strain amplitudes. The ductility and the toughness of the steel remained surprisingly high with an elongation after failure of 45%.