Effect of combined thermomechanical treatment on structure, mechanical properties, electrochemical behavior and functional corrosion fatigue of biodegradable Fe-30Mn-5Si alloy

Abstract

In present study, the combined thermomechanical treatment (TMT), including the radial-shear rolling (RSR) at 900 °C followed by longitudinal rolling (RSR+LR) at 700 °C, was applied to biodegradable Fe-30Mn-5Si (wt%) alloy. The effect of these TMTs on the microstructure, phase composition, mechanical properties (ultimate tensile strength UTS, apparent yield stress σ0.2, relative elongation to failure δ, Young's modulus E), electrochemical behavior and functional corrosion-fatigue behavior in Hanks’ solution were studied. It was found that RSR leads to the formation of a statically recrystallized γ-austenite structure with an average grain size of 10±3 and 20±3 µm in the peripheral and central zones, respectively. The RSR+LR results in a mixture of dynamically recrystallized and dynamically polygonized structures with the average grain size of 5±1 µm. Moreover, RSR and RSR+LR lead to the formation of a single-phase FCC γ-austenite state compared to reference heat treatment (RHT), where microstructure is characterized by a two-phase state of FCC γ-austenite and HCP ε-martensite with an average grain size of 200–300 µm. Based on static mechanical tensile tests to failure, it was established that RSR and RSR+LR lead to a significant increase of UTS up to 770±21 and 935±55 MPa, σ0.2 up to 300±38 and 490±43 MPa, and δ up to 28±2 and 19±10 %, while maintaining acceptable of E value about 165±29 and 150±8 GPa, respectively as compared to 460±41 MPa, 210±91 MPa, 11±3 %, and 140±14 GPa after RHT. The electrochemical studies showed that single-phase structure (FCC γ-austenite) formed after RSR and RSR+LR leads to the non-critical biodegradation rate lowering as compared to the two-phase state (γ-austenite and ε-martensite) after RHT. Besides RSR and RSR+LR lead to significant improvement of functional corrosion-fatigue life in Hanks’ solution.