The Role of Grain Orientation and Local Heating in the Strain-Induced Martensitic Transformation : In-situ and Ex-situ Studies
Pun, Lalit (2025)
Tampere University
2025
Teknisten tieteiden tohtoriohjelma - Doctoral Programme in Engineering Sciences
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Väitöspäivä
2025-03-28
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-03-3802-2
https://urn.fi/URN:ISBN:978-952-03-3802-2
Tiivistelmä
Metastable austenitic stainless steels, like AISI 301LN, with tuneable mechanical properties and excellent corrosion resistance, are used across a wide range of industries. Good mechanical properties, including high strain hardening, result primarily from strain-induced martensitic transformation. The deformation occurs during manufacturing of components and during their service. The phase transformation characteristics are generally influenced by various factors such as strain rate, deformation temperature, and grain orientation. This present study aims at enhancing the understanding of the effects of these factors and the relationship between the microstructure evolution and the mechanical response in a single grade of 301LN steel. Concrete knowledge of deformation behavior is necessary for further advancements in the mechanical performance of these steels. In this work, the mechanical response of the studied material was examined on several length scales at different strain rates, ranging from 2×10–4 s–1 to 1 s–1 using incremental and continuous tensile tests. The strategically interrupted tensile tests were performed to minimize macroscopic adiabatic heating and understand the effects of strain rate on the martensitic transformation. Microstructural evolution was characterized on the same microstructural region throughout the deformation process using the electron backscatter diffraction (EBSD) analysis. A digital image correlation (DIC) study of microlevel strain distribution was also carried out. Furthermore, continuous tensile experiments were conducted at various strain rates with simultaneous in-situ high-energy synchrotron X-ray diffraction (XRD) measurements to study microstructural changes in real-time without disturbing the loading history. The methodology allows high-frequency data acquisition, maintaining high XRD data quality and excellent signal-to-noise ratio. These experiments aimed to study the effects of adiabatic heating and austenite grain orientation on phase transformations, providing insights into the bulk material behavior.
The micro-DIC results show a non-uniform distribution of plastic strain at the grain level, whereas the EBSD data from the same area highlights the formation of α’-martensite particles in the regions with high local strains. The EBSD microstructural studies further indicate that the number of α′-martensite nucleation sites is unaffected by strain rate. However, the strain rate significantly influences the transformation behavior around newly formed α′-martensite particles. At lower strain rates, repeated nucleation and coalescence lead to substantial growth of α′-martensite particles. In contrast, deformation at higher strain rates results in the formation of smaller and isolated α′-martensite particles with minimal growth despite low macroscopic temperature increase at both low and high strain rates. This behavior can be at least partly attributed to localized microstructural heating caused by plastic deformation and exothermic phase transformation, which inhibits further growth of α′-martensite particles in their vicinity, reducing the overall transformation rate with increase in strain rate. The synchrotron XRD data show that the parent austenite (γ) grains undergo rotations at the early stages of deformation. At higher plastic strains, two major orientations, <111>γ and <100>γ double fiber parallel to the loading axis, develop in the austenite phase. The results show that at low strain rate, <100>γ fiber-oriented grains transform more readily into α’-martensite at a true strain of approximately 0.10, while <111>γ fiber-oriented grains transform only at high strain level of 0.20. The phase transformation rate of both <111> and <100> fiber-oriented austenite grains is suppressed with increasing strain rate. A theoretical model including the effects of applied stress and stacking fault energy was used to calculate the critical tensile stress required for the formation of extended stacking faults, which are suitable nucleation sites for martensite particles. This model provides estimates for the start of the austenite-to-α’-martensite phase transformation. The model predicts the observed phase transformation behavior of <111>γ fiber orientations at all strain rates but does not predict the reduced transformation rate of <100>γ fiber-oriented grains at higher strain rates, suggesting the presence of other effects, such as possible changes in the dislocation structures with increase in strain rate.
The micro-DIC results show a non-uniform distribution of plastic strain at the grain level, whereas the EBSD data from the same area highlights the formation of α’-martensite particles in the regions with high local strains. The EBSD microstructural studies further indicate that the number of α′-martensite nucleation sites is unaffected by strain rate. However, the strain rate significantly influences the transformation behavior around newly formed α′-martensite particles. At lower strain rates, repeated nucleation and coalescence lead to substantial growth of α′-martensite particles. In contrast, deformation at higher strain rates results in the formation of smaller and isolated α′-martensite particles with minimal growth despite low macroscopic temperature increase at both low and high strain rates. This behavior can be at least partly attributed to localized microstructural heating caused by plastic deformation and exothermic phase transformation, which inhibits further growth of α′-martensite particles in their vicinity, reducing the overall transformation rate with increase in strain rate. The synchrotron XRD data show that the parent austenite (γ) grains undergo rotations at the early stages of deformation. At higher plastic strains, two major orientations, <111>γ and <100>γ double fiber parallel to the loading axis, develop in the austenite phase. The results show that at low strain rate, <100>γ fiber-oriented grains transform more readily into α’-martensite at a true strain of approximately 0.10, while <111>γ fiber-oriented grains transform only at high strain level of 0.20. The phase transformation rate of both <111> and <100> fiber-oriented austenite grains is suppressed with increasing strain rate. A theoretical model including the effects of applied stress and stacking fault energy was used to calculate the critical tensile stress required for the formation of extended stacking faults, which are suitable nucleation sites for martensite particles. This model provides estimates for the start of the austenite-to-α’-martensite phase transformation. The model predicts the observed phase transformation behavior of <111>γ fiber orientations at all strain rates but does not predict the reduced transformation rate of <100>γ fiber-oriented grains at higher strain rates, suggesting the presence of other effects, such as possible changes in the dislocation structures with increase in strain rate.
Kokoelmat
- Väitöskirjat [4963]