The investigation of influence factors on differential scanning calorimetry (DSC) analysis of a Ni-base superalloy: heating/cooling rate and sampling position

Abstract:

The differential scanning calorimetry (DSC) experiments with different heating/cooling rates in the range of 5~40℃/min were performed on a directionally solidified (DS) Ni-base superalloy DZ22. The equilibrium transformation temperatures (zero-heating/cooling rate) of the alloy were obtained by linear extrapolating the different heating/cooling rates or averaging specific peak temperatures of both heating and cooling DSC curves. The DSC experiments with 10℃/min heating/cooling rate were carried out on samples cut from different position of DS testing bar. The results indicated that the heating/cooling rate and sampling position both had obvious effect on the values of phase transformation temperatures of DSC curves. (1) The heating and cooling rate had effect on the DSC results, including the phase transformation temperatures of liquidus, MC carbides, solidus, eutectic (γ+γ′) and secondary γ′. As the heating and cooling rate increased, the peak of transformation temperature on the heating DSC curve shifted to high temperature direction, whereas the cooling curve tended to deviate to lower temperature. The peak height increase accompanied by the heating/cooling rate increase. However, the average values of heating and cooling curves corresponding to the phase transformation temperature points are consistent. The equilibrium transformation temperatures of alloy acquired by linear extrapolating the different heating/cooling rates will result in some differences for the result, whereas to average specific peak temperature of both heating and cooling DSC curves is an effective method to determine the equilibrium phase transformation temperatures of superalloys. The influence of different heating and cooling rate on the phase transformation temperature results in DSC test has reference significance for the control of the actual process parameters in the process of heat treatment and solidification cooling of superalloy. (2) The sampling position had obvious effect on the eutectic (γ+γ′) dissolve and solidus temperatures of heating DSC curve in relative low temperature range, there is a 17℃ and 20℃ gap respectively for eutectic and solidus temperatures between samples cut from the top and bottom part of the same directionally solidified test bar due to the micro-segregation and microstructure difference. However, this difference is absent in liquidus and MC carbides dissolve temperatures in high temperature range. Upon cooling, the sampling position has minor effect on phase transformation temperature of DSC curve because the similar microstructure of the different sampling parts of the alloy formed in the following solidification cooling process after heating to a full liquid state and phase transformation temperature tends to be consistent. For a superalloy with the same composition, the DSC test results are only meaningful as the microstructure of the sample is similar. In General, the heating/cooling rate and sampling position had obvious effect on the phase transformation temperatures of DSC curves and these influence factors should be considered for selecting the process parameters of Ni-base superalloys.

Key words:Ni-base superalloy; DSC; heating/ cooling rate; sampling position; DZ22; phase transformation; microstructure