PSI - Issue 64

Maryam Mohri et al. / Procedia Structural Integrity 64 (2024) 376–383 M.Mohri et al./ Structural Integrity Procedia 00 (2019) 000–000

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This improvement was attributed to the formation of stacking faults and the reversible movement of correlated partial dislocations (Baruj et al. (2010)). The effects of thermo-mechanical processing on both the microstructure and functional properties of a Fe–17Mn– 5Si–10Cr–4Ni-1(V, C) shape memory alloy was systematically investigated (Mohri et al. (2022), Mohri et al. (2023)). Golrang et al. (2024) reported that when this alloy was subjected to 25% cold rolling followed by recrystallization at 925°C, along with single or double aging treatments, transmission electron microscopy analysis revealed the formation of ε - martensite, annealing twin boundaries, and specific orientation relationships between ε - martensite and γ -austenite in the double aged specimen. Cyclic tensile testing showed that the recrystallized and double aged alloy exhibited excellent SE, achieving high peak stress and minimal residual strain over 50 cycles. This stability was attributed to precipitation strengthening and interactions between martensite and refined microstructural features. Moreover, the recrystallized and double aged sample demonstrated the highest recovery stress of 450 MPa upon heating after pre straining, due to its high yield strength inhibiting new martensite formation during cooling. High-resolution transmission electron microscopy revealed a non-Shoji-Nishiyama orientation relationship between stress- induced ε martensite and γ -austenite, leading to additional irrecoverable strain and higher recovery stress. This study aims to explore the impact of double-aging and thermomechanical training on the SME and SE behavior of the Fe–17Mn– The Fe-SMA examined in this research was a rebar measuring 50 mm in diameter, possessing a chemical composition of Fe–17Mn–5Si–10Cr–4Ni-1(V, C) (wt. %), manufactured by voestalpine Boehler Edelstahl GmbH & Co. KG, Austria. Dog-bone-shaped specimens were fabricated via electrical discharge machining, with their dimensions and geometry depicted in Fig.1. The as-received samples were then subjected to a two-step heat treatment (aged at 600 °C for 20 h followed by aging at 680 °C for 8 h) followed by air cooling. The heat-treated samples underwent a thermomechanical treatment comprising quasi-static cyclic strain-controlled tensile loading-unloading up to 2% prestrain at room temperature and subsequent annealing at 200 °C for 30 min. The thermomechanical cycling was repeated for three times to ensure complete forward and reverse transformation and rearrangement of martensite (training). Tensile tests were conducted using a universal tensile testing machine (Z020, Zwick/Roell). During these tests, the specimens underwent loading up to a strain of 4% and subsequent unloading to a constant force of 10 N, all at a steady displacement rate of 0.5 mm/min. The strain recovered due to SE (ε se ) was determined by subtracting the elastic strain (ε el ) from the total recovery strain variation after unloading (ε ul ). The SME was evaluated in samples pre strained to 4%. Maintaining at a constant strain, the samples underwent heating to 200 °C and subsequent cooling to room temperature at a rate of 2 °C/min. The stress changes during the thermal cycle were monitored, and the stress measured at the conclusion of the heating/cooling cycle was designated as the recovery stress. The specimens underwent microstructural analysis using scanning electron microscopy (SEM; FEI NanoSEM230). Prior to microstructural analysis, the specimen surfaces were prepared according to standard metallography procedures (ASTM E3), involving grinding and polishing. 5Si–10Cr–4Ni–1(V, C) alloy. 2. Experimental procedure

Fig.1. Dimensions and geometry of the tensile dog-bone-shaped specimens (dimensions in mm).