Dynamic embrittlement in metallic alloys

Dafni Bika, University of Pennsylvania

Abstract

Dynamic embrittlement is called the reduction in cohesive strength resulting from stress-driven diffusion of an embrittling element from a free surface into the grain boundaries of a solid, in a manner analogous to Hull-Rimmer creep-cavity growth. A free surface can be contaminated by an embrittling element originating from a liquid, solid, or gaseous environment, or from the bulk of the solid itself. Because grain boundaries are paths of rapid diffusion, the phenomenon is usually manifested by intergranular cracking in polycrystals at elevated temperatures. The objective of this thesis is to study dynamic embrittlement in metallic alloys, when the embrittling element segregates to the free surface of the alloy from solid solution. On the experimental side, the mode and the rate of cracking were examined in two alloy systems: A MnMoCrNi steel and a Cu-8%Sn alloy, in which sulfur and tin are the surface-active elements, respectively. Compact-tension specimens were dead-loaded in near-UHV conditions and the crack-growth rate was measured using a potential-drop technique. The nature of crack initiation and propagation was further explored using notched cylindrical tensile bars. Detailed microscopic analysis of the fracture surfaces and of metallographic sections of specimens from interrupted tensile tests was applied to determine the microscopic cracking mechanism. Finally, a kinetic model for dynamic embrittlement was developed based on a mass balance of the diffusing embrittling species coupled with an empirically based fracture criterion. In both alloys, the cracking mode was found to be brittle intergranular in the early stages of crack growth, but it shifted to rupture as the crack length increased. In low-alloy steels, the crack velocity oscillated in the range 0.1 to 1$\mu$m/s at 550$\sp\circ$C due to discontinuous brittle cracking which caused striations on the fracture surface. In bronze, intermittent cracking occurred with a maximum macroscopic velocity of 0.1$\mu$m/s at 265$\sp\circ$C. Load drops were observed in a typical load vs. time curve, resulting in striations on the crack faces due to crack blunting after each load drop. The kinetic model determined the functional dependence of the microscopic crack-growth rate on temperature, interfacial mobility of the penetrating element, and the local stress raised to a power of ${(n-1)}\over{3}$ + 1, where n is the creep exponent. The calculated microscopic crack-growth rates due to dynamic embrittlement compared well with the macroscopic measured ones, for both alloy systems. This supports the thesis that dynamic embrittlement is the rate-controlling step during the early stages of brittle cracking, in the temperature range and microstructural conditions studied for the two alloys.

Subject Area

Materials science|Mechanical engineering|Metallurgy

Recommended Citation

Bika, Dafni, "Dynamic embrittlement in metallic alloys" (1992). Dissertations available from ProQuest. AAI9308534.
https://repository.upenn.edu/dissertations/AAI9308534

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