SOLID-SOLUTION HARDENING AND SOFTENING BY FE ADDITIONS TO NIAL

Solid-solution hardening in the case of a ternary alloy addition to a B2 compound with the triple defect structure has been investigated. Th e fact that the ternary element may occupy either of two sublattices o r may affect the concentration of other types of point defects present in the material makes this a very interesting problem to consider. Ni -rich (40 at% Al), stoichiometric (50 at% Al), and Al-rich (52 at% Al) alloys were doped with up to 12 at% Fe. Lattice parameter, bulk densi ty and hardness measurements were performed on samples quenched from 1 000 degrees C. It was found that solid-solution softening actually occ urs in the Ni-rich alloys, while hardening was observed in the stoichi ometric and Al-rich alloys. The vacancy concentration was determined f rom the experimental data, and the site occupancies of the Fe atoms we re estimated from a thermodynamic model. Through careful consideration of all point defect concentrations the solid-solution hardening and s oftening behaviors could be effectively rationalized. (C) 1997 Elsevie r Science Limited.