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

Abstract

The temporal dependence of $$\gamma '$$ γ ′ 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 $$\gamma $$ γ phase up to the $$\gamma /\gamma '$$ γ / γ ′ interface. It is demonstrated that dissolution is affected by heating rate as well as the precipitate size. Below a threshold heating rate of $$\sim $$ ∼ 0.1 $${^\circ }$$ ∘ C s$$^{-1}$$ - 1 , 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 $$\gamma $$ γ phase at the $$\gamma /\gamma '$$ γ / γ ′ 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 $$\gamma '$$ γ ′ 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 $$\gamma '$$ γ ′ solvus temperature is shifted to progressively higher temperatures. Unless complete dissolution of $$\gamma '$$ γ ′ occurs prior to subsequent cooling, erroneous interpretations of nucleation undercooling can arise.

In-Situ Monitoring of Phase Transition and Microstructure Evolution in Ni-Based Superalloys by Electrical Resistivity: Direct Comparison With Differential Scanning Calorimetry and Application to Case Studies

“…[18][19][20][21] In the case of Ni-based superalloys, it can be used to characterize grain growth, [22] volume fraction of the c¢ phase, [23,24] and dissolution and precipitation kinetics of c¢ phase. [24,25] However, electrical current can accelerate recovery and recrystallization, and it can, in principle, affect the phase transformation process in metallic materials. [26][27][28] Several studies have made attempts to extract microstructural information from Ni-based superalloys through electrical resistivity measurement.…”

“…[26][27][28] Several studies have made attempts to extract microstructural information from Ni-based superalloys through electrical resistivity measurement. [22,23,25,[29][30][31][32][33] Other studies directly compared DSC and resistivity for analyses of physical properties, but data on superalloys are particularly limited. [19][20][21]34,35] Overall, the analyses so far have been limited to specific and fundamental uses, and there is a need for comprehensive understanding to make the best use of this method.…”

An In Situ Resistance-Based Method for Tracking the Temporal Evolution of Recovery and Recrystallization in Ni-Base Single-Crystal Superalloy at Super-Solvus Temperatures

“…Specifically, it has been observed that precipitate coarsening at sub-solvus temperatures leads to decreasing resistance, owing to a reduction in c/c¢ interfacial area resulting in reduced electron scattering at interfacial sites. [18] A more potent case exists in the case of additively built microstructures, where precipitate sizes of the order of ~5 nm [29] result in a markedly higher resistivity (~100 to 150 nXm) in the as-built condition, compared with the case where the re-precipitated c¢ following heat treatment is characterized by a precipitate size of ~100 nm. [30] However, this effect can be negated in the present experiments, since the isothermal hold is above the local solvus, as indicated by the normalized resistance curve, as in Figures 3 and 12.…”

“…Electrothermal mechanical testing (ETMT) is a technique developed at the National Physical Laboratory (NPL) capable of measuring a wide range of mechanical and physical properties in miniature test specimens. [17][18][19][20] The ETMT equipment used for these experiments is fitted with a ± 0.5 kN load cell and provides resistance heating to test specimens via direct current (DC) across the specimen. Temperature is controlled via a Type-R Pt/Pt-13 pct Rh thermocouple spot welded along the center of the gage length ~(2.75 to 3 mm) where the temperature is uniform within ± 1 K. [17] Resistance changes are measured between two Pt/ Pt-13 pct Rh wires attached within the isothermal gage length of the specimen.…”

“…[13][14][15][16] Specifically, this was conducted using electrothermal mechanical testing (ETMT) capable of measuring a wide range of mechanical and physical properties. [17][18][19][20] It was proposed that not only was the history dependence of the strain path crucial, but it also pointed to the existence of a critical temperature range to delineate the work hardening portion of the plastic strain from the creep strain; the former being associated with an increasing dislocation density leading to recrystallization. [17,21] In polycrystalline alloys, improved experimental data have led to the development of crystal plasticity finite element (CPFE) models which are capable of simulating loading and dynamic recrystallization; however, experimental data to validate such models for deformation during directional solidification are sparse, with most studies instead exploring isothermal, cold deformation.…”