Effects of grain-boundary segregation and precipitation on the hydrogen susceptibility of nickel
- PDF / 2,306,104 Bytes
- 10 Pages / 594 x 774 pts Page_size
- 33 Downloads / 287 Views
I.
INTRODUCTION
THEextent of hydrogen embrittlement occurring in a metal or alloy is governed by many factors. 1 One of these is the ease of entry of hydrogen atoms into the material from the surface. In the case of an electrolytic hydrogen source at the metal surface (such as in the straining electrode experiment), Latanision and Opperhauser 2 have proposed that the segregation at grain boundaries of metalloid impurities, most of which are known as hydrogen recombination "poisons", would retard the rate of the recombination reaction, Had + Had = H2, at the intersection of the grain boundary and the free surface. As a consequence, the probability of atomic hydrogen absorption into the metal would be increased, resulting in an increased degree of hydrogen embrittlement in materials that are susceptible to embrittlement. Likewise, hydrogen absorption could be decreased through the use of species which accelerate the hydrogen recombination reaction. 3 This observation is also consistent with the above proposal. It should be emphasized that the electrochemistry of hydrogen absorption should be distinguished from the solid state chemistry of embrittlement when solute segregated grain boundaries are exposed to tensile stresses and hydrogen. 4 There is no a priori reason for expecting that solutes which stimulate hydrogen absorption must also induce disbonding at grain boundaries when in the presence of absorbed hydrogen. The first objective of the present work was to examine the nature of grainboundary segregation in three different grades of nickel (Marz Ni, Nickel 270, Nickel 200), and its role in the hydrogen embrittlement of the materials under various conditions. Another factor affecting hydrogen susceptibility is the manner in which absorbed hydrogen atoms are distributed in the microstructure. Since no real material is a perfect solid solution, the defects that exist in the matrix may interact with hydrogen atoms differently than the matrix, causing accumulation of hydrogen atoms at certain sites. Various hydrogen "traps" in metals have been documented. 5 Depending on the nature and morphology of the traps, they T. S. E LEE, formerly with Bell-Northern Research, Ottawa, ON, Canada, is with Northern Telecom, Ottawa, ON, Canada. R. M. LATANISION is Professor, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted October 17, 1985. METALLURGICAL TRANSACTIONS A
may either increase or decrease the hydrogen embrittlement susceptibility of a metal. A case in point is the work by Thompson and Wilcox 6 who have shown that thoriadispersed nickel is much more resistant to embrittlement than pure nickel. The result suggests that the finely dispersed oxide phase may trap hydrogen at discrete sites, removing it from the bulk of the nickel and thus reducing the embrittling effect of the hydrogen. Graphite is known 7 to precipitate from the Nickel 200 matrix during aging at certain temperatures. The second objective of this work, therefore, was to exami
Data Loading...
