LOCAL LATTICE-PARAMETER CHANGES IN MONOCRYSTALLINE TURBINE-BLADES SUBJECTED TO SERVICE-LIKE CONDITIONS

Two helicopter turbine blades of the monocrystalline nickel-base super alloy CMSX-6 which had been used in test engines under service-like co nditions were investigated. While the microstructure was investigated by conventional methods of metallography and scanning electron microsc opy (SEM), a special high-resolution X-ray double-crystal diffractomet er was used in the present work in order to determine the local lattic e parameter distributions. In some parts of the turbine blades directi onal coarsening of the originally cuboidal precipitates was found, sim ilar to that known from the investigation of creep-deformed laboratory samples. The measurement of profiles of X-ray Bragg peaks at differen t points within the turbine blades yielded local changes of the lattic e parameter distributions, induced by the exposure to operation condit ions. These lattice parameter changes are connected with internal stre sses. The estimated effects are compared with results obtained on samp les creep-deformed under defined conditions. The results are important for the understanding of the stress distributions within the turbine blades during service.