Modelling of the microsegregation in CMSX-4 superalloy and its homogenisation during heat treatment

Abstract

Microsegregation in the single crystal superalloy CMSX-4 has been studied using electron probe microanalysis, both in the as-cast condition and after solution heat treatment. In order to establish the solidification path, a statistical treatment of the data is proposed, which is based upon the local value of the quantity Cra - CR=. Using differential scanning calorimetry and by appealing to a database of thermodynamic parameters, it is shown that the solidification path cannot be explained without acknowledging that backdiffusion occurs. Analysis of the microsegregation remaining after progressive heat treatment reveals that the dendritic and interdendritic regions homogenise at different rates, owing to the presence of the eutectic mixture; there is evidence of up-hill diffusion of solutes in the eutectic region during homogenisation. Simple expressions based upon a sinusoidal variation of composition are inadequate for the estimation of homogenisation times. A coupled thermodynamic/kinetic theory is able to explain most of the effects which occur, i.e. incipient melting, enrichment of residual y’ by Ta, MO and subsequent 7’ dissolution. Although more work needs to be carried out to better establish the thermodynamic and kinetic parameters required by the model, it is at this stage already useful for the assessment of new superalloy compositions and for the design of optimal heat treatments.