Results of copper-silver rail materials tests

The goal of this phase of our investigation of better rail materials was to assess the multiple shot performances of copper-silver alloy (Cu-24% Ag) test coupons. The focus of these tests was on the assessment of changes in material properties and microstructure of test coupons. The primary metric used was the change in hardness. Material surface deformations and microstructure changes were evaluated by optical microscopy and scanning electron microscopy. Hardness gradually increased as the number of tests increased. Test coupons in four states were studied: pre-test; after one pulse test; after three pulse tests; and after eight pulse tests. The changes in hardness are related to the initial state of strain and dislocation distributions in the test specimens. The hardness is changed by the nucleation of new strain-free crystallites within the heavily worked, dislocation-dense grain structure. An annealing, recrystallization, and re-straining model is proposed to predict the bandwidth within which the hardness will fluctuate. The retained hardness of the copper-silver alloy test coupons has an average value that corresponds to a tensile strength greater than 650 MPa (100 ksi). This is well above the 500-MPa strength of the usual contacting aluminum alloy armature. Based on these test data, it is concluded that copper-silver remains an attractive material for use as a strong and thermally stable conductor.