Effect of Hydrogen in Advanced High Strength Alloys Revisited via Small Scale- In-Situ Techniques
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This thesis demonstrates a comprehensive study on the interaction between hydrogen and plasticity by using in-situ fine scale testing methods. It contains small-scale tensile tests with in-situ SEM observation as well as nanoindentation tests with in-situ electrochemical hydrogen charging and in-situ SPM imaging. There are two groups of materials studied in this PhD thesis. One is high- Mn TWIP steel, the other one is HEAs with stable and metastable states. The main object of this study is to gain understanding of hydrogen degradation mechanism by revealing the interaction between hydrogen and crystal defects. First of all, a proper glycerol-based electrolyte was developed to protect specimen surface from corrosion through long-time cathodic hydrogen charging and testing, easing the following in-situ observation and characterization without further surface preparation. The deleterious effect of hydrogen on mechanical properties was demonstrated by tensile tests with in-situ SEM observation on pre-charged TWIP steel. During the tests, both fracture elongation and strength showed an accumulative degradation with amount of charged hydrogen indicated by the charging time. The charged hydrogen also induced secondary cracks on the gauge surface and a ductile-to-brittle fracture transition. This transition was triggered by sufficient hydrogen concentration with a threshold, which was quantitively determined by using the thermal desorption spectroscopy testing. In addition, the initiation and propagation of intergranular secondary cracks were comprehensively studied, and they were proposed as the result of hydrogen intrinsic detrimental effect on grain boundaries in combination with the stress concentration at intersecting points between deformation twin and grain boundary. To get a deeper insight into the hydrogen detrimental effect, nanoindentation tests with in-situ hydrogen charging and in-situ SPM imaging were carried out. This technique can detect the effect of hydrogen on dislocation nucleation events, nanomechanical properties, and sample surface degradation. During the hydrogen charging, no hydrogen-induced surface degradation was observed on TWIP steel, while hydrogen-induced surface slip line formation and martensitic phase transformation were detected on HEAs with stable and metastable states, respectively. In the case of TWIP steel, introducing hydrogen results in a reduced pop-in load and an increased hardness. During the egression of hydrogen, both pop-in load and hardness showed recovery behavior but with different recovery rates. The hydrogen reduced pop-in load indicates the hydrogen-enhanced homogeneous dislocation nucleation, which was fulfilled by reducing dislocation line energy or stacking fault energy. Tabor relation-based models were applied to analyze the hardness increment, and this was proposed due to the hydrogen-enhanced lattice friction. Moreover, the pop-in load and hardness have different affecting depths, which contain different amount of residual hydrogen during egression process lead to their different recovery rate. In the case of HEAs, a severe hydrogen charging results in the formation of irreversible surface slip lines or martensitic phase transformation. The formation of surface lines is related to the plastic deformation caused by hydrogen-enhanced dislocation nucleation together with the hydrogen induced internal stress, which was quantitively determined in this study. The formed surface lines irreversibly reduced the elastic modulus and pop-in load due to stress concentration and heterogeneous dislocation nucleation. The hardness, by contrast, was not influenced by the surface lines due to its deeper affecting depth. Martensitic phase transformation was detected on metastable HEA upon hydrogen charging, leading to an irreversible hardness increment. The hydrogen-induced internal stress together with the intrinsic hydrogen effect on SFE reduction and superabundant vacancies formation are proposed as the reasons. In addition, a critical amount of hydrogen is needed to trigger this phase transformation, and this threshold hydrogen content was determined in this study.
Has partsPaper 1: Wang, Dong; Lu, Xu; Wan, Di; Guo, Xiaofei; Johnsen, Roy. Effect of hydrogen on the embrittlement susceptibility of Fe-22Mn-0.6C TWIP steel revealed by in-situ tensile tests. The final published version is available in Materials Science & Engineering: A 2021 ;Volum 802 https://doi.org/10.1016/j.msea.2020.140638
Paper 2: Wang, Dong; Lu, Xu; Deng, Yun; Guo, Xiaofei; Barnoush, Afrooz. Effect of hydrogen on nanomechanical properties in Fe-22Mn-0.6C TWIP steel revealed by in-situ electrochemical nanoindentation. Acta Materialia 2019 ;Volum 166. s. 618-629 https://doi.org/10.1016/j.actamat.2018.12.055
Paper 3: Wang, Dong; Lu, Xu; Wan, Di; Li, Zhiming; Barnoush, Afrooz. In-situ observation of martensitic transformation in an interstitial metastable high-entropy alloy during cathodic hydrogen charging. Scripta Materialia 2019 ;Volum 173. s. 56-60 https://doi.org/10.1016/j.scriptamat.2019.07.042
Paper 4: Wang, Dong; Lu, Xu; Deng, Yun; Wan, Di; Li, Zhiming; Barnoush, Afrooz. Effect of hydrogen-induced surface steps on the nanomechanical behavior of a CoCrFeMnNi high-entropy alloy revealed by in-situ electrochemical nanoindentation. Intermetallics 2019 ;Volum 114. s. 1-11 https://doi.org/10.1016/j.intermet.2019.106605