The interaction of hydrogen with the interface of AI 2 O 3 particles in iron
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I.
INTRODUCTION
EMBRITTLEMENT of iron and steel by hydrogen is well established and has been the subject of many investigations. ~-s Darken and Smith 9 found that the amount of hydrogen absorbed in cold worked steel from an acid solution at room temperature was much greater than that absorbed in hot worked steel. They suggested that hydrogen was trapped in the many lattice defects produced by cold working, thus increasing the amount of dissolved hydrogen in the steel. Following this suggestion, it has been found that dislocations, ro microvoids, H grain boundaries, f2,13TiC/lattice interface, r4'15 and MnS/lattice interface j6 act as trapping sites for hydrogen. The trapping effect on hydrogen transport in steels has been applied to reduce the detrimental effects of hydrogen such as the reduction of failure stress and fracture strain. Hydrogen transport toward the crack tip area can be suppressed by dispersing fine strong trap sites like TiC particles. 3 This indicates that hydrogen trapping is important to understand fully the hydrogen embrittlement phenomena of steels. Recently Pressouyre and Zmudzinski ~7 reported that the hydrogen induced cracks are nucleated at A1203 particles/ matrix interfaces by the precipitation of hydrogen molecules during cathodic charging of hydrogen. They presumed that the A1203/lattice interface is an irreversible trapping site for hydrogen. In spite of this finding, the trapping capability of the interface of A1203-matrix for hydrogen has never been quantitatively established. This work is focused on the determination of trapping capability of the interface of A1203-matrix for hydrogen, namely, the trap binding energy of hydrogen.
II. THEORETICAL MODEL ON THE DETERMINATION OF TRAP BINDING ENERGY OF HYDROGEN When the energy level of hydrogen around the trapping site is assumed as in Figure 1, the variation of hydrogen occupancy fraction of the trap site with time (dn/dt), suggested by McNabb and Foster,~S is expressed as the difference between the rates of trapping and detrapping.
On at
kCL(I - n)
METALLURGICAL TRANSACTIONS A
[1]
where k: the transition probability for hydrogen transport from a lattice site to a trapping site [= v~ exp(-Es/RT)] p: the transition probability for hydrogen transport from a trapping site to a lattice site [= t, zNL exp(-(Es + EB)/RT)] n: the trap occupancy fraction CL: the number of hydrogen atoms dissolved in interstitial sites of a normal lattice per gram of Fe u~: the vibrational frequency of hydrogen at a normal lattice site u2: the vibrational frequency of hydrogen at a trapping site R: the gas constant T: the absolute temperature t: time NL: the number of a normal lattice interstitial site per gram of Fe When equilibrium between hydrogen atoms in trapping sites and those in interstitial sites of the normal lattice is achieved at charging temperatures during gaseous hydrogen charging, On/Or = 0 and Eq. [1] is written as Eq. [2].
kCL p + kCL
n - - JONG-LAM LEE, formerly Graduate Student, Korea Advanced Institute of Science and Technology. is
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