Characterisation of Nimonic 90 by the use of miniaturised multiproperty mechanical and physical tests

An innovative testing system, capable of testing miniature testpieces at elevated temperatures, has been used to characterise appropriate samples of Nimonic 90, a nickel alloy. This alloy is being used for extensive validation testing to underpin a new European Code of Practice for TMF (thermomechanical fatigue) tests. This paper provides additional characterisation information using the miniature electro-thermomechanical test system (the ETMT) that has been developed at the National Physical Laboratory to obtain multiproperty data over a range of temperatures. The system uses DC electric current to heat rectangular cross-section testpieces. It has water cooling for the testpiece grips, a facility for inert gas supply to prevent sample oxidation, and full computer-control and digital data acquisition. In the present arrangement, strain is deduced by monitoring changes in resistance. Both thermal and mechanical loads can be cycled and a special program was written for conducting either IP or OP TMF tests. The system was used to obtain data on the variation of electrical resistivity with temperature and this data provided insight on the phase changes that occur in Nimonic 90 as a function of temperature. Uniaxial mechanical tests were performed at 400 and 850 °C, the temperature limits of the specified TMF cycle. Preliminary TMF tests were performed for comparison with the European interlaboratory testing exercise on macroscopic testpieces. The repeatability of miniature tests was good and the fracture mechanism appeared to be representative of deformation observed in macroscopic tests on the alloy.