Atomistic study of hydrogen embrittlement of grain boundaries in nickel: I. Fracture
Atomistic study of hydrogen embrittlement of grain boundaries in nickel: I. Fracture
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DOI：
10.1016/j.jmps.2017.01.020
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发表时间：
2017
影响因子：
5.3
通讯作者：
A. Tehranchi;W. Curtin
中科院分区：
文献类型：
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作者：
A. Tehranchi;W. Curtin
氢进入金属是脆化的持久原因。断裂表面通常是沿晶的，这表明有利的解理裂纹沿沿着晶界（GB）生长是脆化的一个驱动因素。在这里，原子模拟被用来研究偏析氢的行为上的裂纹沿着各种对称倾斜晶界的fcc镍的影响。首先，根据新的量子水平计算的H在Ni <$5（120）<$100 <$GB内特定位置的能量，重新校准了Ni-H的原子势。计算了H原子与Ni <$3（111）（孪晶）、Ni <$5（120）<$100 <$、Ni <$99（557）<$110 <$和Ni <$9（221）<$110 <$GB中不同原子位置的结合能，以及将这些GB分成两个可能的断裂面所产生的不同表面的结合能，并用于确定Ni中典型脆化的体H浓度下的平衡H浓度。然后研究了I型断裂行为，检查H在改变晶间裂纹的位错发射（裂纹钝化;“韧性”行为）和解理断裂（“脆性”行为）之间的竞争中的影响。模拟结果进行了比较与理论预测（格里菲斯理论裂解，水稻理论的排放）使用计算的表面能。然而，GB处的变形行为通常是复杂的，并且不像尖锐裂纹尖端处的解理或发射那样简单，由于GB结构的复杂性，这并不出乎意料。在预测从裂纹尖端发射位错的情况下，H原子的存在降低了位错发射的临界载荷，并且没有发现解理。在预测解理的情况下，H原子的存在降低了解理应力强度，并且使解理更容易，包括Ni <$9（221）<$110 <$H，其在不存在H的情况下发射位错。除了一个不寻常的Ni沿着GB的裂纹，H没有倾向于导致韧性到脆性的转变，无论是根据理论或模拟的初始平衡H偏析和没有，或有限的，H扩散附近的新产生的断裂面。实验发现，Ni-113（111）孪晶界根本不吸收H，这表明在具有较高比例的这种孪晶界的材料中脆化更困难。因此，实验观察到的解理样故障可能是由涉及H扩散或动态裂纹扩展的机制。
Hydrogen ingress into a metal is a persistent source of embrittlement. Fracture surfaces are often intergranular, suggesting favorable cleave crack growth along grain boundaries (GBs) as one driver for embrittlement. Here, atomistic simulations are used to investigate the effects of segregated hydrogen on the behavior of cracks along various symmetric tilt grain boundaries in fcc Nickel. An atomistic potential for Ni–H is first recalibrated against new quantum level computations of the energy of H in specific sites within the NiΣ5(120)⟨100⟩ GB. The binding energy of H atoms to various atomic sites in the NiΣ3(111) (twin), NiΣ5(120)⟨100⟩, NiΣ99(557)⟨110⟩, and NiΣ9(221)⟨110⟩ GBs, and to various surfaces created by separating these GBs into two possible fracture surfaces, are computed and used to determine equilibrium H concentrations at bulk H concentrations typical of embrittlement in Ni. Mode I fracture behavior is then studied, examining the influence of H in altering the competition between dislocation emission (crack blunting; “ductile” behavior) and cleavage fracture (“brittle” behavior) for intergranular cracks. Simulation results are compared with theoretical predictions (Griffith theory for cleavage; Rice theory for emission) using the computed surface energies. The deformation behavior at the GBs is, however, generally complex and not as simple as cleavage or emission at a sharp crack tip, which is not unexpected due to the complexity of the GB structures. In cases predicted to emit dislocations from the crack tip, the presence of H atoms reduces the critical load for emission of the dislocations and no cleavage is found. In the cases predicted to cleave, the presence of H atoms reduces the cleavage stress intensity and makes cleavage easier, including NiΣ9(221)⟨110⟩ which emits dislocations in the absence of H. Aside from the one unusual NiΣ9(221)⟨110⟩ case, no tendency is found for H to cause a ductile-to-brittle transformation for cracks along GBs in Ni, either according to theory or simulation for initial equilibrium H segregation and with no, or limited, H diffusion near the newly-created fracture surfaces. The NiΣ3(111) twin boundary does not absorb H at all, suggesting that embrittlement is more difficult in materials with higher fraction of such twin boundaries, as found experimentally. Experimental observations of cleavage-like failure are thus presumably caused by mechanisms involving H diffusion or dynamic crack growth.