Visualization of the hydrogen desorption process from ferrite, pearlite, and graphite by secondary ion mass spectrometry
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THE analyses of hydrogen diffusion, trapping states, and visualization, mainly in steels and titanium alloys, have been attempted in recent years.[1–10] The diffusion coefficient and the content of hydrogen in high-strength steels have been measured using the electrochemical permeation technique[11,12] to determine the hydrogen-trapping capacity of each steel. The trap activation energy of hydrogen has been also calculated using thermal desorption spectroscopy (TDS), and the hydrogen-trapping sites have been discussed.[13,14,15] However, direct observation methods are required for identifying the trapping sites of hydrogen. Various methods have been used to observe the hydrogentrapping sites. Tritium autoradiography has revealed that the grain boundaries and the interface between the precipitation and the matrix are trapping sites.[16,17,18] This method, however, is troublesome, since it requires use of a radioactive isotope. On the other hand, in secondary ion mass spectrometry (SIMS), which is widely used in analyzing semiconductors, it is not necessary to use a radioactive isotope. Depth analyses by SIMS have been used to investigate hydrogenand deuterium-trapping sites in various metals.[19–22] Although secondary ion image analyses by SIMS were used to observe the distribution of heavy water in Zr-Nb alloys,[23] only few attempts have, so far, been made on the visualization of hydrogen by means of the secondary-ion image analyses. The authors have recently visualized trapping sites of K. TAKAI, Associate Professor, Y. CHIBA and K. NOGUCHI, Graduate Students, and A. NOZUE, Professor, are with the Department of Mechanical Engineering, Sophia University, Tokyo 102-8554, Japan. Contact e-mail: [email protected] Manuscript submitted September 5, 2001 METALLURGICAL AND MATERIALS TRANSACTIONS A
hydrogen in high-strength steels using the secondary-ion image analyses.[24,25] Assuming that the behavior of hydrogen desorption, local hydrogen concentration, and binding energy between the hydrogen and trapping sites can be clarified by the secondary-ion image analyses using SIMS, this information will be very important in discussing the delayed fracture mechanism, in order to develop steels and cast iron with a high resistance to delayed fracture. Based on these viewpoints, four principal aspects are presented here: (1) visualizing the hydrogen distribution, (2) visualizing the hydrogen desorption process from each metallurgical microstructure under various holding times at 25 ⬚C, (3) visualizing the hydrogen desorption process during heating, and (4) establishing the correspondence between TDS and SIMS results. II. EXPERIMENTAL A. Materials Spheroidal graphite cast iron was used for visualizing the hydrogen distribution, since it consists of basic microstructures of steels such as ferrite and pearlite. In addition, the size of each microstructure is appropriate for the SIMS resolution. The cast iron was made using a vacuum melting furnace. The metallurgical microstructures are spheroidal graphite, the ferrite around
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