Computational Simulations Of Hydrogen Embrittlement In Titanium

Hydrogen embrittlement in metals has been well known for a long time. However, the mechanism of this phenomenon is still under controversy. As titanium is usually applied to the top area such as aviation, the embrittlement in titanium always leads to a serious economic losses. Computer simulations have greater effects on the research of the mechanism than the experiments and can help people to prevent the embrittlement. The key factor in simulations is the chosen of potentials between atoms. Due to the difference of the distribution of electrons between titanium and hydrogen, it is difficult to describe the potentials correctly. The modified-embedded-atom method (MEAM) is one of the most widely used methods in the research of embrittlement. The MEAM potential can give the correct physical properties of the alloy of metals and hydrogen. The most important part of this thesis is the calculation of the MEAM potential between titanium and hydrogen.Several parameters in the MEAM potential of alloy were calculated using first principle method. This method is accurate because there isn't any empirical values using in the calculation. The relative first principle calculation in this essay was based on pseudopotentials and planewaves within density function theory (DFT). The local-density approximation (LDA) was performed as a approximation to the exchange-correlation (XC) energy function in DFT. Troullier-Martins pseudopotitials generated by A. Khein and D. C. Allan were used to represent atomic cores. The level of electronic occupancies was considered both according to the Methfessel-Paxton shceme and new-core theory of Switendick, in which the 3p eletrons of titanium atom were treated similar to band. Computational details are the P(?)1m (No.162) space group, an 240 eV planewave cutoff energy, an 0.27 eV energy searing and an 4×4×2 Monkhorst-Pack grid for k-point sampling respectively. The model of calculation was the interstitial hydrogen atom inα-Ti occupying the octahedral site. The bulk modulus B, the heat of formation△E and the atomic electron density scaling factorρ0 were calculated. The bulk modulus B and the heat of formation△E were applied to the computation of the factor of the universal energy function a and the cohesive energy Ec respectively. The atomic electron density scaling factorρ0 is one of the parameters in MEAM potentials of alloy. The elastic properties of titanium and hydrogen alloy were calculated based on the Hill approximation. The ratio between shear modulus and bulk modulus of alloy is greater than 0.5, i.e. Ghill/BHill=0.62> 0.5. Then according to the PUGH criterion, the alloy shows the property of brittle. And the alloy tends to cracking under the internal stress.Other parameters in the MEAM potential of alloy were calculated using modified-embedded-atom method. There parameters were cutoff distance, equilibrium distance, cohesive energy, factor of the universal energy function and eight screen factors. After the determination of the potential of alloy, the behavior of the interaction between the interstitial hydrogen atom and the boundary of bicrystal in titanium was simulated under the NVE (canonical) ensemble at the temperature of OK. The boundary conditions of the vertical direction (Y axis) of the boundary plane was finite while the boundary conditions of the other directions were periodic. The accomplishment of thermostasis was due to the cooperation of modifying velocities (velocity Verlet method) and a separate time integration. The time step of this MD simulation was 10-4 picoseconds and the total steps were 106. The MEAM potentials of pure elements calculated by Baskes and Lee et al were used. The MEAM potential of titanium and hydrogen alloy calculated by this thesis was adopted. The behavior of hydrogen atoms located near the boundaries of∑7(12(?)0)/[0001],∑13(11(?)0)/[0001] and∑13(10(?)1)/[1(?)10] was simulated using the molecular dynamics method. The lattice parameter, c/a ratio, bulk modulus and heat of formation were calculated to exam the MEAM potential of alloy. The results of computation were basically fit to the empirical values and other authors'results. After the comparison of the destruction degree among three boundaries, it is obvious that the∑7(12(?)0)/[0001] boundary which contains the highest miller index was destroyed the most seriously, the∑13(11(?)0)/[0001] boundary was followed and the∑13(10(?)1)/[1(?)10] boundary which contains the lowest miller index was destroyed the most lightly.Hydrogen embrittlement in titanium was simulated in this thesis. The MEAM potential of alloy was calculated using first principle method and researched preliminary. The results of this thesis provide certain theoretical basis for the further research of the mechanism of embrittlement.