Dissolution of hydrogen from the gas in steel at room temperature

The purpose of this paper is to show that embrittlement of steel deformed in a hydrogen atmosphere is caused by hydrogen entering the crystal lattice. Tensile tests are made under pure compressed hydrogen gas. It is shown that the hydrogen penetrates steel before any fissures develop. The penetration depends on the time of exposure of fresh surface produced by straining but does not depend directly on the strain rate. The hydrogen distribution vs depth was measured. It agrees with a transient distribution calculated with an apparent solubility (1 cu cm per 100 g at 150 kp per sq cm) and an apparent diffusivity (1.3·10^?7 sq cm per sec).     

Lattice changes of iron-nitrogen martensite on aging at room temperature

The aging behavior of iron-nitrogen martensite (5.5 at. pct N ≙5.8N/100Fe) at about 297 K was investigated by employing X-ray diffractometry, thereby following, in particular, the changes in the {002} and {200} line profiles. Martensitic specimens were prepared by gaseous nitriding of pure iron in a mixture of NH₃ and H₂, followed by quenching in brine and subsequently in liquid nitrogen. The aging process can be divided into two stages. First, a redistribution of nitrogen atoms in the martensite matrix occurs (aging time < about 40 hours) in three ways: segregation of nitrogen to lattice defects (about 0.07N/100Fe), transfer of a small amount of nitrogen (about 0.06N/100Fe) fromalb- toc-type octahedral interstices, and local enrichment in an ordered way of the majority of the nitrogen atoms (coherent α′’-Fe₁₆N₂ precipitates). Second, formation of incoherent α″-Fe₁₆N₂ takes place (aging time > about 40 hours). Within the range of aging times employed (up to 670 hours), the diffraction by the residual austenite did not change.     

Study on notch fracture of TiAl alloys at room temperature

In-situ observations of fracture processes combined with one-to-one observations of fracture surfaces and finite-element method (FEM) calculations are carried out on notched tensile specimens of two-phase polycrystalline TiAl alloys. The results reveal that most cracks are initiated and propagated along the interfaces between lamellae before plastic deformation. The driving force for the fracture process is the tensile stress, which is consistent with a previous study.^[1] In specimens with a slit notch, most cracks are initiated directly from the notch root and extended along lamellar interfaces. The main crack can be stopped or deflected into a delamination mode by a barrier grain with a lamellar interface orientation deviated from the direction of crack propagation. In this case, new cracks are nucleated along lamellar interfaces of grains with favorable orientation ahead of the barrier grain. The main crack and a new crack are then linked by the translamellar cleavage fracture of the barrier grain with increasing applied load. In order to extend the main crack, further increases of the applied load are needed to move the high stress region into the ligament until catastrophic fracture. The FEM calculations reveal that the strength along lamellar interfaces (interlamellar fracture) is as low as 50 MPa and appreciably lower than the strength perpendicular to the lamellae (translamellar fracture), which shows a value higher than 120 MPa. This explains the reason why cracks nucleate and preferably extend along the lamellar interfaces.