Accelerated coarsening behavior of the γ´-phase in CMSX-4 during non-isothermal heat treatment

Abstract

In literature, microstructural investigations can be found comparing the evolution of the well-known γ/γ ´ -phase morphology of nickel-based superalloys for long-term heat treatments with and without mechanical load, as well as for isothermal and non-isothermal creep tests. However, data of experiments conducted under non-isothermal conditions without an applied load are hard to find. To close this gap, isothermal and non-isothermal heat treatments without a mechanical load were conducted. Conductive heating was applied to samples that consisted of segments with different cross sections and allowed the simultaneous testing at different temperatures. The microstructural changes were examined by scanning electron microscopy after 500 h and 1000 h. Several established methods for the quantification of the γ ´ -phase coarsening were evaluated and employed for comparison of the two heating conditions studied. The coarsening was more distinct in samples heat-treated under non-isothermal conditions at all temperatures investigated. This was also verified by calculating the connectivity number NA, which was found to be the most useful parameter for quantification of microstructural changes observed. The γ ´ -precipitates became more connected during the heat-treatment, but no preferred direction of coarsening was observed like during rafting under an applied load. Therefore a more general particle agglomeration mechanism is assumed to be responsible for coarsening of the γ ´ -phase.