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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to 2% pre-straining followed by annealing at 200 °C for 30 min. The elastic modulus for the as-received, heat-treated and trained samples are 140 and 125, and 135 GPa, respectively. Variations in elastic modulus may be attributed to differences in the amount of martensite and austenite phases present in the as-received and heat-treated specimens. It should be noted that due to the pronounced nonlinear behavior of Fe-SMAs, the measurement of elastic modulus lacks precision. Therefore, it is recommended to utilize the slope of the tangent line to the loading stress–strain curve below 100 MPa for elastic modulus determination. After the heat treatment, there is a decrease in the 0.01% yield stress, indicating a higher propensity for stress-induced martensite formation. Specifically, the 0.01% yield stresses of the as-received and heat-treated samples are 390, 136 and 330 MPa, respectively. The strengths at 4% strain (σ 4% ) are 619, 806 and 900 MPa for the as-received, heat-treated, and trained samples, respectively. Higher strength of heat treated and trained samples is owning to work-hardening. The tensile tests indicate a reduction in yield strength following heat treatment. The yield strength of these alloys is influenced by the martensite transformation, which is further bolstered by precipitation. Consequently, the decline in yield strength is attributed to the formation of precipitates and the advancement of martensite transformation. Mohri et al. (2022) reported that the formation of semi-coherent VC precipitates after aging and their lattice constants mismatch results in the creation of stress fields around the precipitates. Additionally, the austenite phase serving as the matrix material and the carbide precipitates exhibit different thermal coefficients during cooling, leading to thermal stresses and the formation of stress fields in the local vicinity of the precipitates. These stress fields influence the martensitic transformation temperature. According to the Clausius-Clapeyron equation, these stresses cause a shift in the phase transformation temperatures towards higher temperatures, aiding in the formation of stress induced martensite. Consequently, ε -martensite plates are relatively easily formed during the aging process. In close proximity to these primary ε plates, the energy barrier for greater phase transformation decreases. Meng et al. (2006) noted that stress fields surrounding the formed martensitic plate, along with crystal defects such as dislocations, could lower the martensite nucleation barrier. Furthermore, the interaction of carbide particles and stacking faults (SFs) in austenite could enhance the shape memory properties (Baruj et al. (2004)). The findings revealed an increase in yield stress following training, as illustrated in Fig. 3b and Table 1. In aged samples, a sharp rise in yield stress occurred after the initial cycle, with subsequent cycles showing a less pronounced increase. This upsurge was attributed to the work-hardening effect observed in each cycle. The results suggest a reduction in the minimum stress required to induce martensite (σ y0.01% ) across all trained samples. The decline in the slope of the tensile curves indicates a decrease in work-hardening and facilitates the formation of stress-induced martensite. Consequently, the thermomechanical treatment led to an expansion of the stress differential between the transformation stress (σ y0.01% ) and slip stress (σ y0.2% ). The ε se for the as-received sample is 0.30%, rising to 0.52% after heat treatment (Fig. 3a). Contrasting with the as received specimen, the ε se of the trained specimen elevates to 0.63%. Additionally, the increased strain-hardening rate observed in the heat- treated and trained samples is a result of increased ε -martensite volume fraction and dislocation density induced by aging and training (Lai et al. (2018)). Previous studies have shown that there is an elastic strain field around the precipitates which act as a spring back for reverse transformation and leading to an improved SME and SE. Moreover, that the creation of SFs and reversible motion of correlated partial dislocations after training enhanced the SE (Baruj et al. (2010), Mohri et al. (2022)). The decrease in Young's modulus post-training might be attributed to the abundance of stacking faults (SFs). The presence of carbide precipitates in the austenite alongside SFs contributes to heightened dislocation density within the microstructure. As suggested by Chen et al. (2016) and Over et al. (1982), the decline in Young's modulus is associated with alterations in the interatomic bonding structure at SFs and subsequent dislocation movement, respectively.

Table 1. Mechanical properties of as-received, heat-treated and thermomechanical trained samples (E: Young’s modulus , 0 . 01% : Yield stress, ε se : pseudo-elastic strain) Specimens name E (GPa) . % ( ) ε se % σ 4% (MPa) As-received 140 390 0.30 619 Heat-treated 125 137 0.52 809 Trained 135 330 0.63 900