Institute of Metals Division - The Effect of Precipitation on Fracture Path in a Mercury-Embrittled Cu-4 Pct Ag Alloy

The fracture path in a Cu-4 pct Ag alloy wet with mercury was found to he dependent on the heat treatment used prior to tensile testing. Both quenched and slow-cooled alloys were embrittled by the mercury.. For quenched alloys, which were single -phase, the crack path was intergranular. Slowly cooled alloys, which contained precipitate platelets, exhibited transgranular fracture. By comparing stress-strain data for these alloys with similar data on mercury -embyittled solid-solution alloys, it was found that the role of the precipitate and that of the grain boundary in the fracture process were equivalent . THE fracture path which results when metals are strained in a liquid environment appears to be dependent upon the nature of the specific system. Mixed transgranular and intergranular cracking has been observed in the 2024 T4 aluminum alloy when wet with a Hg-Zn amalgam.' Transgranular fracture was induced in an A1-4 pct Cu alloy by masking off the grain boundaries from the liquid Hg-Zn amalgam.' The embrittlement of solid-solution alloys of copper in mercury results in intergranular fracture.' The effect of a precipitated phase on the fracture path in copper alloys is the subject of the present investigation. Cu-4 pct Ag was chosen so that its behavior could be compared with previous work on embrittlement of copper alloys. PROCEDURE High-purity copper (99.999 pct) was used as a base metal, along with 99.999 pct purity silver. Melting was carried out in graphite crucibles with manual agitation to get adequate mixing. The ingots were homogenized by means of vacuum annealing, chemically cleaned, and cold-rolled with intermediate vacuum anneals, to the desired thickness of 0.025 in. The rolled sheets were sheared to the final specimen size of 1 by 0.025 by 0.1 in. The specimens were then electropolished in phosphoric acid (concentrated) and heat-treated in an evacuated capsule at the solution temperature of 770°C. One batch was slow-cooled and the other quenched in water. The thermal etching which occurred during heat treatment was sufficient to permit grain boundary and precipitate delineation. Tensile tests were carried out in air and in mercury on a Table-Model Instron at a pulling speed of 0.2 in. per min. Wetting was accomplished by electro-polishing in phosphoric acid, rinsing in alcohol, and immersing in a pool of mercury. Once wet the coating was stable throughout the mechanical tests. In our studies it was found that wetting the surface or total immersion in mercury produced identical results. For convenience, the wetting technique was used for the experiments described. The crack pattern was observed directly on the surface by removing the mercury after testing. De-wetting was accomplished by flame heating the samples in a vacuum after testing and before observation of the surface. No etchant or polishing was used so as not to obscure the crack pattern on the surface. The mercury removal, however, resulted in a pitted surface but otherwise left the pertinent features visible. Photomicrographs were taken in this condition so as not to disturb the structure associated with the crack pattern. The average precipitate spacing was measured previous to coating the sample with mercury by use of an optical eyepiece micrometer. RESULTS The microstructure of the quenched specimens was single-phase while that of the slow-cooled specimens contained thin platelets of a second