PSI - Issue 42

Liese Vandewalle et al. / Procedia Structural Integrity 42 (2022) 1428–1435 Vandewalle et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Nomenclature Λ

total dislocation length

E k IF

kink migration energy

m

τ

relaxation time

internal friction

C d hydrogen concentration at dislocation D H,dils diffusivity of hydrogen at dislocation

H-CW hydrogen cold work peak

HE

hydrogen embrittlement

D k E a E B E k

kink diffusivity activation energy

l 0 Q Q T

mean free dislocation segment length

internal friction coefficient

-1

binding energy (of hydrogen to dislocation)

-1 max relaxation peak intensity

kink formation energy

temperature

E k,eff

effective kink formation energy hydrogen migration in iron lattice

T C

critical temperature

E m

TDS

thermal desorption spectroscopy

H

E H m,disl hydrogen migration energy at dislocation

While thermal desorption spectroscopy (TDS) is one of the most used methods to study the interaction of H with microstructural features (Depover and Verbeken (2018)), it only provides indirect information. Moreover, interpretation of the TDS spectra is proven to be rather difficult, especially in the case of complicated microstructures containing various defects interacting with H (Drexler et al. (2021)). Therefore, the use of other, complementary techniques is necessary. In this regard, the internal friction (IF) technique is very interesting as it is able to provide valuable information on the distribution of small interstitial atoms in steels, by the presence of Snoek relaxation peaks, as well as on their interaction with microstructural defects, based on dislocation and Snoek-Köster peaks, which are often denoted as cold work (CW) peaks. Of particular interest is the so called H-cold work (H-CW) peak since it may provide useful information on the H-defect interactions. The H-CW peak is only observed in iron-alloys that are subjected to both H doping and cold work. Activation energies (E a ) reported for the H-CW peak in literature range between 20 kJ/mol and 40 kJ/mol (Gibala (1967), Sakamoto and Shimada (1981), San Juan et al. (1985), Sturges and Miodownik (1969), Takita and Sakamoto (1976)). Based on the dependency of the H-CW peak height and peak temperature on the amount of cold work and on the H content, it was concluded that the corresponding relaxation process should be related to H-dislocation interactions. However, discussion still exists on the exact nature of these interactions and various models describing the relaxation process have been proposed. While some of these models give a good basic description of the peak, none of them are able to capture its full complexity. Consequently, many open questions remain, especially regarding the responsible dislocation types, relation to dislocation structure and interstitial concentration, and the influence of the presence of other interstitial solutes. In a previous study by Vandewalle et al. (2022), the IF and TDS spectra of cold rolled ultra-low carbon (ULC) steel after various annealing stages have been evaluated together with the microstructural evolution. It was found that the H-CW peak could be related to metastable dislocation segments which do not contribute to the hardness, e.g. kinks. Moreover, the H-CW peak could be correlated to the dislocation peak in the TDS spectra. Hence, the dislocation trapping could involve the same metastable dislocation segments. Further microstructural characterization by IF and positron annihilation spectroscopy, revealed that cold rolling introduced various C-vacancy clusters, which gradually dissolved upon low temperature annealing. As a result, additional C was provided to the dislocations. Hence, the decrease of the H-CW in IF and the dislocation trapping peak in TDS could be related to the additional C segregation to dislocations, expelling H due to the repulsive forces, and/or to annihilation of specific dislocation segments. Clearly, the study of the H-CW peak has a strong potential to provide new insights regarding the H-material interactions, especially when combined with TDS. However, uncertainties regarding the corresponding relaxation mechanism hinder the interpretation of the results and hence more in-depth studies are necessary. Therefore, the goal of this work is to evaluate the H-CW peak more in-depth, focusing on revealing the physical mechanism as well as its relation to H trapping observed by TDS. For this purpose, ULC steel is subjected to various thermo-mechanical treatments and the resulting microstructures are evaluated in addition to the evolution in H trapping and H-CW peak.