On the Rate Dependence of Precipitate Formation and Dissolution in a Nickel-Base Superalloy

Abstract

Abstract The temporal dependence of $$\gamma '$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>γ</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:math> dissolution in the polycrystalline Ni-base superalloy RR1000 has been studied with implications to thermo-mechanical processing. A resistivity-based method using an electro-thermal mechanical testing (ETMT), which overcomes the drawbacks associated with other approaches, such as calorimetry, dilatometry, and diffraction, has been used to explore the effect of transient and isothermal thermal cycles. This is supplemented by DICTRA numerical models that simulate the diffusion within the $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>γ</mml:mi></mml:math> phase up to the $$\gamma /\gamma '$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>γ</mml:mi><mml:mo>/</mml:mo><mml:msup><mml:mi>γ</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:mrow></mml:math> interface. It is demonstrated that dissolution is affected by heating rate as well as the precipitate size. Below a threshold heating rate of $$\sim $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>∼</mml:mo></mml:math> 0.1 $${^\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow/><mml:mo>∘</mml:mo></mml:msup></mml:math> C s $$^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow/><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> , the dissolution kinetics are marginally affected, however, is sensitive to microstructure. The role of precipitate size during dissolution is governed by diffusion flux in the $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>γ</mml:mi></mml:math> phase at the $$\gamma /\gamma '$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>γ</mml:mi><mml:mo>/</mml:mo><mml:msup><mml:mi>γ</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:mrow></mml:math> interface, which is inversely proportional to size. It is argued that numerical simulations that predict constitutional liquation during rapid heating by altering the width of the computation domain to match the average precipitate size of the $$\gamma '$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>γ</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:math> population will yield inaccurate predictions. The influence of the heating rate on the nucleation undercooling, during subsequent cooling, has also been addressed. With increasing heating rates, the local $$\gamma '$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>γ</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:math> solvus temperature is shifted to progressively higher temperatures. Unless complete dissolution of $$\gamma '$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>γ</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:math> occurs prior to subsequent cooling, erroneous interpretations of nucleation undercooling can arise.

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