An HVEM study of an unusual ordering reaction in platinum-carbon

In platinum containing carbon in solid solution an ordered phase with twice the lattice parameter of the matrix has been found in regions near grain boundaries and inclusions. The ordering was observed after quench-aging treatments as well as during electron irradiation above 300 kV in the temperature range of 250 to 650 °C. Outside this temperature range disordering took place thermally at high temperatures (>650 °C) and athermally by irradiation at low temperatures (<250 °C). The threshold voltage for athermal disordering was ~380 kV, well below the 1300 kV threshold for displacement damage in Pt. The simplest ordered structure consistent with the diffraction evidence is face-centered cubic Pt₇C, similar to Pt₇Cu. An important implication of these observations is that substantial amounts of carbon can be effectively transferred from interstitial to pseudo-substitutional sites under certain conditions. This is linked to the earlier observation of a large binding energy between interstitial carbon and platinum vacancies (~0.6 eV).