Research On Behaviors Of Hydrogen In BCC Iron Using First-principle Calculation Method

In this thesis first-principle calculation method based on Density FunctionTheory (DFT) is used to study the stability of hydrogen (H) atom in16Fe-atomdefect-free (2×2×2) bcc iron supercell and in a monovacancy of15Fe-atom (2×2×2)bcc iron supercell. We have also investigated double H atom interaction in16Fe-atom defect-free (2×2×2) bcc iron supercell, electronic and elastic propertiesof16Fe-atom (2×2×2) bcc iron supercells in the absence and presence of H atomsabsorbed at octahedral interstitial site (OIS) and tetrahedral interstitial site (TIS).The migration mechanisms of H atom from an intrinsic region of bcc iron into amonovacancy has been explored together with the effect of H atom presence on theself-diffusion characteristic of bcc iron atoms. The effect of H-vacancy (VA)interactions on the ductility and plasticity of bcc iron has been investigatedthrough calculations of stability, electronic and elastic properties of mH+VAcomplexes in a monovacancy of bcc iron where m=1-6is the number of trapped Hatoms. These first-principles calculations are backed up experimental worksfocused on investigating the effects of pH of HCl acid solution on the mechanicalproperties of40Cr steel.H atoms are predicted to prefer being absorbed at TIS with H atom solutionenergy Esol(H) of~0.0812eV/H compared to OIS (Esol(H)~0.2177eV/H) inintrinsic regions of bcc iron. Inside a monovacancy of bcc iron, H atoms favorsbeing absorbed at a distorted OIS’ with Esol(H)~-0.135eV over a near octahedralinterstitial site (NOIS). The distorted OIS’ inside a monovacancy of bcc iron isdisplaced by~0.218towards the vacancy-center (VAcenter). Double H atominteraction in defect-free (2×2×2) bcc iron supercell reveals that two H atoms areincapable of spontaneous self-trapping to form a H2molecule in bcc iron. The H-Hpair equilibrium separation distance of~2.02, is much larger than the H2molecule bond length~0.75. The calculated electronic and elastic properties of(2×2×2) bcc iron supercell with a H atom absorbed at TIS or OIS reveals creationof new H-Fe bonds in bcc iron at the expense of weakened metallic Fe-Fe bonds.However, bcc iron still remains ductile and shows good strength against shearing.There is no evidence of hydrogen embrittlement (HE) in bcc iron due to absorptionof a H atom in16Fe-atom defect-free (2×2×2) bcc iron supercell.A H atom migrating from an intrinsic region of bcc iron into a monovacancy isfound to favor the TISâ†'TIS diffusion path via an adjacent TIS (diffusionactivation energy, Q=0.28eV and0.30eV) compared to TISâ†'OISâ†'TISdiffusion path (Q=0.51eV) outside the monovacancy. Inside a monovacancy of bcc iron, the H atom favors the OIS’â†'OIS’ diffusion path via an adjacent OIS’(Q=0.26eV) over the OIS’â†'VAcenterâ†'OIS’ diffusion path (Q=0.79eV). A H atomjumping into a monovacancy from a1NN TIS from the monovacancy is predictedto overcome an energy barrier of0.39eV and thereafter loses0.87eV. Thepresence of H atoms in monovacancy of bcc iron is shown to inhibit self-diffusionof iron atom by increasing the Fe atom vacancy migration energy. However, Hatom presence in bcc iron is found to lower the formation energy of vacancy thuspromotes the formation of vacancies in bcc iron.The5H+VA complex is identified as the most stable mH+VA complex in amonovacancy of bcc iron owing to its minimum values of H atom solution energyEsol(VA,mH) and H-vacancy complex formation energy Ef(VA, mH) of-1.761eVand0.278eV respectively. The2H+VA complex though not the most stable isidentified as a predominant complex in the lower H atom concentration regime asit possesses minimum values of binding energy Ebind(VA,mH) and trapping energyEtrap(VA,mH) of-0.255eV/H and-0.644eV/H respectively. The formation ofmH+VA complexes in monovacancy of bcc iron is shown to lead to a reduction inductility and plasticity of bcc iron. The transition of bcc iron from ductile to brittlematerial commences with the formation of4H+VA complex and is fully realizedwith the formation of the5H+VA complex. Thus the5H+VA complex is identifiedas the mH+VA complex responsible for hydrogen embrittlement of bcc iron. Theembrittlement of bcc iron due to5H+VA complex is a very energetically favorableprocess, thus once ductility of bcc iron is lost due to contact with high H atomsconcentration from the environment, it is impossible to restore ductility.High concentration of H atoms as obtained in HCl acid solution with pH valueof1when in contact with40Cr steel leads to hydrogen embrittlement of the steel.The ductile and fracture properties of40Cr steel are degraded, with the fracturesurfaces of40Cr steel changing from being irregular and fibrous to being relativelyflat. The H atoms change the fracture mechanism of40Cr steel from ductilefracture to brittle fracture. Microstructural analysis of40Cr steel exposed to HClacid solution of pH1shows a reduction in the number and depth of dimplescoupled with extensive shearing and coalescence of dimples and microvoids priorto final fracture of40Cr steel test pieces. The amount of plastic deformation40Crsteel undergoes before it fails is significantly reduced hence conclusion that high Hatom concentrations in40Cr steel is the cause the embrittlement of40Cr steel.