H induced decohesion of an Al grain boundary investigated with first principles: General conditions for instant breakage and local delayed fracture

Abstract

The uniaxial tensile test response of a H decorated Σ5 [100] twist grain boundary (GB) in face-centred-cubic Al has been examined with first principles. The impurity shows a strong tendency to relocate during loading. To capture these H movements, the standard model framework was extended to probe loading-unloading hysteresis. Due to the strong monotonic decrease in the H formation energy with rising GB elongation, the maximum tensile stress accepted by the H decorated GB in the slow fracture limit generally is reached before breakage becomes thermodynamically favourable. For any intact GB configuration visited upon exceeding this stress, the assumption of global chemical equilibrium is argued to be violated. Moreover, while breakage remains ensured from the slow fracture limit considerations, it may in practice require the influx of H to the GB vicinity. A quantitative analysis of GB destabilisation in this scenario requires multi-scale modelling even in the slow fracture limit.

Simulating intergranular hydrogen enhanced decohesion in aluminium using density functional theory

“…The concept of dynamic embrittlement, applying to slow crack propagation scenarios, was initially explored in Al by Van der Ven and Ceder [13], but has more recently been applied by Ehlers et al [12]. The idea behind this process is that during separation, Γ H is not fixed, and the thermodynamic potential to be minimised in the quasi-static regime is now the grand potential Ω = F − μ H Γ H , or, more appropriately, the grand force potential.…”

“…This in turn would lower the traction required for further separation, as highlighted in figure 7. Ehlers et al stress the importance of finite temperature effects [12], especially when considering the relocation of H into more preferable sites during separation. At T ∼ 300 K segregation of H does not readily occur in Al, but small deformations like the small displacements shown in figure 9 can dramatically alter the heat of segregation.…”

“…Modern computational capabilities have spawned efforts to model HE using an ab initio approach, where embrittlement mechanisms are explored by circumventing the challenges associated with experimental measurements of H in Al alloys [10]. A thermodynamic framework has been developed by Rice and Wang to quantify this embrittlement using a cohesive zone fracture model (CZFM) [11], which has been explored for Al at the atomistic scale using density functional theory (DFT) based ab initio tensile tests (AbTTs) [12][13][14][15]. Typically these tests make use of the rigid grain shift (RGS) method following the work on universal binding energy relationships introduced by Rose et al [16], however Van der Ven et al adopted an elongation approach whereby their computational cell in an atomistic model is uniaxially homogeneously strained [13]; this method notably recovered the behaviour of Rose's universal binding for large elongations steps, but provides a phenomenologically different result for smaller elongation steps as the strain becomes less localised to one plane or interface.…”

“…Ab initio methods enable incomparable control over system conditions relative to experimentation. The addition of segregant elements, such as alloying elements, and their interaction with GB strength and energetics has been used to investigate Al GB embrittlement [13,17,18], and more recently to H embrittlement where simulations show promising results in support of the HEDE hypothesis [12,15]. Ab initio calculations from Zhang et al [17] revealed the nature of Al GB embrittlement by Zn segregation, showing how Al-Zn bonds were weaker than Al-Al.…”

Hydrogen at symmetric tilt grain boundaries in aluminum: segregation energies and structural features

“…Besides the strong interactions with vacancies, H-atoms interact also strongly with dislocations in Al and can accumulate at other lattice defects such as GBs 21 , 31 , 32 . An increased concentration of H-atoms at GBs can have severe detrimental effects in the physical–chemical properties of the material, most importantly in the decohesion of grains for example, which is a serious problem if the material is to be used in high pressure vessels 33 – 35 .…”