A recent article in Materials & Design presented an in-situ study of compression creep and stress relaxation in a Fe-based shape memory alloy (Fe-SMA; Fe-17Mn-5Si-10Cr-4Ni-1(V,C)) using high-energy X-ray diffraction (HEXRD). The time-dependent properties of the Fe-SMA were evaluated under various stress levels relative to its yield strength at room temperature.
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Background
Shape memory alloys (SMAs) are materials that exhibit the shape memory effect (SME) and pseudoelasticity, properties valuable for various technical applications. Among SMAs, Fe-based SMAs are used as reinforcements in concrete.
Fe-SMAs have two primary phases with distinct crystal structures and characteristics: the high-temperature γ-austenite phase, which is face-centered cubic, and the low-temperature ε-martensite phase, which is hexagonal close-packed. The SME in Fe-SMAs results from the reversible transformation between austenite and martensite.
In civil engineering, Fe-SMAs enable effective pre-stressing without requiring traditional components like anchorage, ducts, or hydraulic jacks, and they reduce pre-stress force losses due to friction. Given the typical lifespan of reinforced concrete structures (50 to 100 years), the time-dependent properties of reinforcement materials, such as creep and stress relaxation, are essential for assessing the durability of these structures.
This study applied in-situ testing with synchrotron radiation to observe the real-time behavior of Fe-SMA under these conditions.
Methods
This study used commercially prepared Fe-SMA rods with a diameter of 18 mm. A length-to-diameter ratio of two was chosen to prevent buckling during the experiments. Cylindrical samples were cut from the center using electrical discharge machining for microstructural analysis with electron backscatter diffraction both before and after the creep tests.
Compression creep and stress relaxation tests were performed for one hour using a HEXRD setup at room temperature (23 °C). Strain information was recorded using a pushrod connected to a linear variable differential transformer (LVDT).
For the creep tests, samples were subjected to targeted load values aligned with yield strength: (0.7 σYS = 337 MPa, 0.9 σYS = 433 MPa, 1.3 σYS = 625 MPa, 1.6 σYS = 770 MPa). The applied stress varied ± 3 MPa around each target value. The samples were loaded at a strain rate of 8×10-3 s−1 to reach the target stresses quickly and minimize creep or relaxation during the loading phase. The same loading protocol was used for stress relaxation tests, with stress drop monitored over time.
Diffraction peaks were indexed using the Crystallography Open Database in Xpert HighScore software, and individual peaks were fitted using the PseudoVoigt function in Python for integrated intensity and peak position. Rietveld refinement was applied to the HEXRD spectra for detailed analysis.
Results and Discussion
The creep behavior of Fe-SMA showed a clear dependence on applied stress. Creep strain increased with higher stress levels over the one-hour holding period, with the rate of creep gradually decreasing.
At stresses below 0.7 σYS, where no stress-induced martensite was present, creep was minimal. In contrast, rapid creep occurred at stress levels above 1.6 σYS and 1.3 σYS due to martensitic transformation. Stress near 0.9 σYS did not produce rapid creep, but creep strain did increase due to phase transformation.
The stacking fault probability increased over time, indicating more stacking faults, which served as nucleation sites for martensitic transformation. Evidence of transformation was seen as the γ (200) peak intensity decreased in the loading and transverse directions for most γ austenite grain families, with the γ (200) peak showing the greatest intensity decrease in the loading direction without overlapping with martensite peaks.
Compressive strain in elastically stiff grain families, γ (311) and γ (200), decreased with time in the loading direction, while it increased for the more compliant grain families, γ (111) and γ (220), indicating load redistribution.
Stress relaxation of Fe-SMA increased with increasing stress levels, saturating at higher stress levels. The integrated intensity evolution of the γ (200) peak at different stress levels was negligible with time, suggesting limited transformation. Additionally, the empirical logarithmic models fitted well with the relaxation behavior of Fe-SMA.
Conclusion and Future Prospects
This study investigated in-situ compression creep and stress relaxation in Fe-SMA under varying stress levels over a one-hour holding period at room temperature (23 °C) using HEXRD. Evaluation of Debye-Scherrer rings showed the behavior of {hkl} families over time, while Rietveld refinement of HEXRD spectra provided phase volume fractions for each phase.
Researchers plan to explore different materials, test conditions, and tensile stress states. Future ex-situ experiments are also planned to examine the long-term creep behavior of Fe-SMA.
Journal Reference
Oza, M. J., Stark, A., Polatidis, E., Vila, PB., Shahverdi, M., Leinenbach, C. (2024). Characterization of low-temperature creep and stress relaxation of an iron-based shape memory alloy (Fe-SMA) using in-situ synchrotron diffraction. Materials & Design. DOI: 10.1016/j.matdes.2024.113378, https://www.sciencedirect.com/science/article/pii/S0264127524007536
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