Internal friction in deformed hydrogen-doped iron

Abstract

The internal friction spectrum of pure iron deformed and then charged with hydrogen present,s the experimentally well established hydrogen Snoek-Koster peak together with a second peak which occurs initially a t about 4 K. Like the Snoek-Koster peak, this second peak requires the presence of both, hydrogen and dislocations. It is attributed to the interaction between hydrogen and preexisting geometrical kinks on the dislocations. The absence of a hydrogen Snoek-type relaxation invalidates many of the previous determinations and estimates of the diffusion coefficient of hydrogen in iron a t low temperatures. Indirect proof of the long distance migration of hydrogen is given by dislocation unpinning experiments a t 4 K. The subsequent repinning occurs in several stages the first of which is a t 18 K.Le spectre de frottement interne du fer pur deform6 puis charge en hydrogene prksente, en plus du pic de Snoek-Koster bien connu, u n second pic qui se situe initialement a environ 4 K. De m6me que le pic de Snoek-Koster, ce second pic necessite la presence a la fois d'hydroghe e t de dislocations. I1 est attribuk Q I'interaction entre I'hydrogkne e t les dkcrochements geoni6triques preexistants sur les dislocations. L'absence d'une relaxation de type Snoek retire toute valeur aux determinations antkrieures du coefficient de diffusion de I'hydroghe dans le fer basse temperature. Des preuves indirectes btablissant la migrat.ion a longue distance de l'hydrogkne sont donnees par des expkriences de des6pinglage a 4 K. Le rkbpinglage se produit en plusieurs stades, le premier se situant Q 18 K.

The Hydrogen Cold Work Peak in BCC Iron: Revisited, with First Principles Calculations and Implications for Hydrogen Embrittlement

“…One would expect the internal friction results to show that more H atoms at screw dislocations should move the peak temperatures to lower temperatures if a H-softening, HELP-like mechanism is operative in concert with changes in the γ peak temperature. However, this is not observed, and none of the changes of the γ peak with H outgassing 31 appear to correlate with changes in magnitude and temperature of the Hcwp to the extent observed for the α peak 4,30,31 .…”

“…Thus, the H-dislocation binding energy obtained from internal friction results by Gibala 4,9 and others likely applies to mixed dislocations, such as 71º dislocations proposed by Seeger 16 , Fantozzi and Ritchie 13 , and Schultz 14 , but possibly other H-non-screw dislocation complexes as part of the sub-peak structure of the α peak 31 . The changes in Hcwp temperature with H outgassing shown in Figure 4 fit well with a Schoeck-like model for an average binding energy of 0.3 eV: qualitatively, the more H atoms at dislocations sites, the more energy required for H to move with the associated dislocations, resulting in higher peak temperatures.…”

“…Schoeck 28,29 has pointed out that experiments utilizing the effects of H outgassing on the internal friction spectra of deformed Fe-H alloys are fruitful because they allow observation of H-dislocation interaction with minimum changes in the relevant dislocation substructure. Such experiments have been done by Gibala 4,9 , Takita and Sakamoto 30 , and Ritchie et al 31 . The combined results demonstrate complementary changes in the relaxation strengths of the α peak and the Hcwp: When H is introduced into bcc Fe, the α peak decreases, those dislocations being pinned by H atoms, as the Hcwp correspondingly grows; when H is outgassed, the Hcwp decreases and the α peak recovers toward its original height prior to H charging.…”

The Potential of the Internal Friction Technique to Evaluate the Role of Vacancies and Dislocations in the Hydrogen Embrittlement of Steels

“…[42,43] Alternatively, using Hirth's interpretation, the binding of hydrogen to a screw dislocation could be estimated to be 0.2-0.3 eV based on the shift of the γ-peak toward lower temperatures, [61] which is in correspondence to binding energies obtained by DFT calculations. [41][42][43][44] But although the H-CW peak in bcc iron has been investigated quite extensively, [42,65,66,[69][70][71][72][73][74][75][76][77][78] only limited studies have been made linking this peak to the hydrogen trapping capacity and HE. Kikuta et al [79] observed a correlation with the height of the H-CW peak and the decrease of the notch tensile strength for pure iron and a high-strength steel.…”

Interaction of Geometrical Kinks With Hydrogen for Screw Dislocations in Iron

“…non-screw dislocations, disappears to give way to Snoek-KCster(H) relaxation which is constituted by two components, SKI around llOK and SK2 at about 155K (for 1Hz). The experimental characteristics of the SK(H) relaxation have been studied in various papers (1)(2)(3)(4)(5)(6)(7)(8)(9). The theoretical interpretation of these components has been reviewed in a recent paper (lo), which gives a new analysis of the SK(H) relaxation according to all experimental characteristics.…”