Hydrogen atoms absorbed by metals in hydrogen-containing environments can lead to the premature fracture of the metal components used in load-bearing conditions. Since metals used in practice are mostly polycrystalline, grain boundaries (GBs) can play an important role in hydrogen embrittlement of metals. Here we show that the reaction of GBs with lattice dislocations is a key component in hydrogen embrittlement mechanism for polycrystalline metals. We use atomistic modeling methods to investigate the mechanical response of GBs in alpha-iron with various hydrogen concentrations.Analysis indicates that dislocations impingement and emission on the GB can provoke it to locally transform into an activated state with a more disordered atomistic structure, and introduce a local stress concentration. The activation of the GB segregated with hydrogen atoms can greatly facilitate decohesion of the GB. We propose a hydrogen embrittlement model that can give better explanation of many experimental observations.
The
repair of large bone defects poses a grand challenge in tissue
engineering. Thus, developing biocompatible scaffolds with mechanical
and structural similarity to human cancellous bone is in great demand.
Herein, we fabricated a three-dimensional (3D) porous iron (Fe) scaffold
with interconnected pores via a template-assisted electrodeposition
method. The porous Fe scaffold with a skeleton diameter of 143 μm
had the porosity >90%, an average pore size of 345 μm, and
a
yield strength of 3.5 MPa. Such structure and mechanical strength
were close to those of cancellous bone. In order to enhance the biocompatibility
of the scaffold, strontium incorporated octacalcium phosphate (Sr-OCP)
was coated on the skeletons of the porous Fe scaffold. The coated
Sr-OCP was in the form of nanowhiskers with a mean diameter of 300
nm and length of 30 μm. Such Sr-OCP coating could effectively
reduce the release rate of the Fe ions to a level which was safe for
the human body. Both in vitro cytotoxicity tests
by extraction method and direct contact assay confirmed that the Sr-OCP
coating could promote the cell adhesion and substantially enhance
the biocompatibility of the porous Fe scaffolds. Thus, the cancellous-bone-like
porous structure with compatible mechanical properties and excellent
biocompatibility enables the present Sr-OCP coated porous Fe scaffold
to be a promising candidate for bone repair and regeneration.
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