Atomistic insights into hydrogen-assisted fracture in a polycrystalline Al matrix model for 7075 aluminum alloy

Hydrogen embrittlement reduces the fracture resistance of high-strength aluminum alloys. Using an Al-H 2NN-MEAM potential, molecular dynamics simulations at a strain rate of 2×109 s-1 examined deformation and crack propagation in polycrystalline Al-matrix models representative of 7075 alloy. Grain sizes of 9.71, 7.70, and 6.11 nm, hydrogen concentrations of 0, 1, and 2%, and intergranular, intragranular, and transgranular cracks were considered. Within the 6.11-9.71 nm range, decreasing grain size reduced the elastic modulus and peak stress, consistent with inverse Hall-Petch-like softening; the transition size remains undetermined. Hydrogen lowered the modulus and accumulated at vacancies, dislocations, grain boundaries, and crack surfaces. Intergranular cracks showed pronounced boundary-controlled propagation, intragranular cracks deflected toward grain boundaries, and transgranular cracks promoted void nucleation and coalescence. Increasing hydrogen to 2% raised crack-tip stress and accelerated crack propagation, while, in the cracked models, Shockley partial-dislocation activity and the HCP-like CNA fraction decreased. These findings link hydrogen trapping, grain-boundary-mediated deformation, and crack geometry to atomistic cracking trends; they should not be interpreted as direct engineering-scale predictions.