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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Fig. 3. Stress–strain curves of (a) As-received, heat-treated and thermomechanical trained, (b) cyclic thermomechanical trained samples (each cycle of training consists of 2% pre-straining followed by annealing at 200 °C for 30 min, N: number of training cycle).

3.3. Thermomechanical behavior To assess the recovery stress resulting from the SME, all samples underwent a 4% strain at room temperature, followed by an activation procedure involving heating to 200°C and subsequent cooling to room temperature. Figure 4 depicts the temperature-dependent recovery stress following the 4% strain for the as-received, heat-treated, and trained samples. The SME, which generates recovery stress, in Fe-based SMAs arises from the stress-induced martensite transformation. This transformation occurs from a parent γ -austenite phase (face- centered cubic, fcc) to an ε martensite phase (hexagonal close-packed, hcp) at room temperature. Upon heating beyond the transformation temperature, a reverse transformation from ε - to γ -phase occurs. As illustrated in Fig. 4, the activation process involves three distinct steps. Initially, during the heating cycle (step I), the SMA undergoes thermal expansion. At the austenite start temperature, A s , the transformation from hcp to fcc starts, leading to the development of tensile stresses due to the SME in the SMA (step II). Subsequently, during cooling, further tensile stresses arise due to the thermal contraction of the SMA (step III). (a) (b)

Fig. 4. Recovery stress versus temperature curves of the as-receive, heat treated and trained samples subjected to 4.0% prestrain.

Fig. 5. Recovery stress versus temperature curves of the trained samples after each cycle of training.

Figure 5 presents the recovery stress-temperature curves of the heat-treated sample following each training cycle. Both Figs, 4 and 5 indicate a slight increase in the A s as a result of heat treatment and training due to the changes in matrix composition after the precipitation. The A f temperatures for all samples surpass 200°C. Additionally, the findings suggest that both heat-treated and trained samples exhibit higher recovery stresses compared to the as received sample. Specifically, the heat-treated sample demonstrates a higher recovery stress (490 MPa and 515 MPa)

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