Experimental approach to the diffusion effects near room temperature in cooper - zinc - aluminium shape memory alloys

摘要

An experimental analysis of shape memory alloys centred around the parent-to-martensite coexistence and partial cycling effects and on the room temperature effects (ageing) in the parent phase establishes that micromemory, martensite creep, and stress reduction at constant strain are a result of local changes of equilibrium temperature. The resistance time dependence in the parent phase shows a complex behaviour, with a long time tail (estimated as second-neighbouring or order L2_1 effects) coupled with a transitory effect (near-neighbouring or order B2 effects) with an exponential decay with amplitude depending on induced asymptotic change in the transformation temperature, M_s. The observed time constant in the alloy is near 3 h at 348 K, and around 1 d at 333 K. At 298 K, it would be more than 5 months. The activation energy of the process is near 1.17 eV. The M_s dependence on temperature and time explains most of the data scatter for measured transformation temperatures, as a result of diffusive processes in the material.