An example of the effect of hydrogen trapping on hydrogen embrittlement
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IN a series of recent papers ~-5 we have analyzed and modelled the ways in which hydrogen can interact with lattice and interface heterogeneities, denoted by the term traps, and how such trapping can affect and control the nature, extent and kinetics of hydrogen embrittlement. In particular, the trap theory of hydrogen embrittlement ( T T H E ) 5 describes various situations where three groups of parameters are important: 1) the means by which hydrogen diffuses into the material (as dislocation atmospheres, or by interstitial diffusion, or along high diffusivity paths), 2) the location of hydrogen prior to the test (internal hydrogen vs external hydrogen), 3) the character of traps (irreversible vs reversible traps). The extent of embrittlement will vary according to what combination of these parameters exists in a given test. One case that was experimentally studied 3 was of interstitial diffusion into a hydrogen free material (iron-titanium-carbon alloys) containing both reversible (Ti atoms) 1,2,4and irreversible traps (TiC particles)? ,2,4In this case, all traps acted as sinks obeying T T H E predicted behaviors. The hydrogen induced damage, which took the form of planar inter or transgranular cracks was very dependent on trap densities and character. We wish to now extend such considerations to the case where a metallic specimen (iron-titanium-carbon alloy) containing a macroscopically homogeneous concentration of dissolved hydrogen is subjected to an external stress, as in a tensile test. In this experiment, the combination of the previously discussed parameters is the following: 1) transport of hydrogen as dislocations atmospheres, 9.~0 2) internal hydrogen, 3) reversible and irreversible traps. G. M. PRESSOUYRE is with Creusot-Loire Research Center, 71208 Le Creusot, France. I. M. BERNSTEIN is with the Department of Metallurgy and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted January 16, 1980. METALLURGICAL TRANSACTIONS A
The T T H E predicts that, in such a situation, reversible traps such as Ti atoms should act as hydrogen s o u r c e s for moving dislocations that will then release hydrogen into nonsaturable irreversible traps. In the present development we will monitor any observed hydrogen embrittlement as the change in the reduction in area, after final fracture, between identical specimens with and without hydrogen. As discussed in detail elsewhere, 6,7 this can be accompanied with or without a change in fracture mode. Thus, in the general sense, this is a macroscopic monitoring of hydrogen embrittlement and does not directly identify the precise microscopic or submicroscopic changes brought about by the presence of hydrogen which promoted the premature failure. Such changes, which relate to the precise mechanism or mechanisms by which hydrogen embrittles, will be considered so far as possible in the Discussion section of the paper. We will first examine the precursor stage to fracture, namely how during deformation a small, uniform, hydrogen concentration
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