Modeling of Precipitation Kinetics of TCP-Phases in Single Crystal Nickel-Base Superalloys

Abstract

Abstract. The precipitation of brittle so-called TCP-phases is critical for the application of Recontaining single crystal superalloys. In this work a fully multicomponent precipitation model is presented, which is capable of simulating the precipitation process of the TCP-phases in superalloys considering complex precipitation sequences with several metastable phases. The model is coupled to multicomponent thermodynamic CALPHAD calculations and relies on multicomponent diffusion models based on the TC-API interface of the software DICTRA. The required mobility database has been newly developed and covers all relevant alloying elements of the Ni-base superalloys including rhenium (Re) and ruthenium (Ru). It is well known that adding Ru strongly reduces TCP-phase precipitation. Based on the developed precipitation model, possible mechanisms are investigated to explain this effect and it is concluded that Ru mostly influences the nucleation rate by a combined influence on interface energy, "reverse partitioning" and γ'-phase fraction. IntroductionNickel-base superalloys are widely applied for high temperature applications, especially for turbine blades in the hottest sections of industrial and aero-engine gas turbines [1-2]. Reduction of fuel consumption and engine emissions are very important design goals today. The efficiency of the gas turbine is determined by the burning temperature. Obviously, the material capability of the turbine blades limits this temperature. Therefore improvements in creep strength through material development as well as single-crystalline casting and application of thermal barrier coatings increase the turbine efficiency [3]. Nickel-base superalloys contain typically more than eight alloying elements. Although they have been under investigation for decades due to the complexity there is still space for improvements [2,4]. In the last decade, major improvements in creep strength could be achieved by the development of the 3 rd and 4 th generation single crystalline superalloys, which contain up to 6 wt-% Re and 6 wt-% Ru [1]. Rhenium is known to be a very good solid solution hardener [5], nevertheless it strongly increases the susceptibility to brittle topologically-close packed (TCP) phase precipitation, which limits the acceptable content of rhenium [6][7]. One answer to this problem was to add ruthenium in the 4 th generation superalloys. This element very effectively reduces the TCP-phase precipitation and increases the creep strength [8][9]. The mechanism of this effect is still not well understood [5,7,8,10]. Both elements, Re as well as Ru are extremely expensive and there are doubts on the availability [11]. In order to optimize the alloy composition, the mechanism of the Ru-effect is investigated in this work with numerical methods. Modeling precipitation in superalloys is challenging because of the high number of alloying elements. Most of the multicomponent models developed in the past were applied to high temperature steels. The first, still simple models benefiting from the ...

Effect of ruthenium on the precipitation of topologically close packed phases in Ni-based superalloys of 3rd and 4th generation

“…Several possible mechanisms for this effect were proposed based on experiments [10,[17][18][19][20] and simulations [21,22]. Among the solubility related mechanisms, 'reverse partitioning' [18][19][20] or increased solubility of Re and W within the c-matrix phase [10,17] was suggested.…”

“…Alternatively, a change in TCP/matrix interface energy [21,22] was considered as a potent factor influenced by Ru. However, as TCP phases exhibit very small sizes in the early stages of precipitation and are very sensitive to alloy chemistry, it was difficult to draw the final conclusions without advanced high-resolution characterization and quantitative description of phase transformation.…”

The effect of Ru on precipitation of topologically close packed phases in Re – containing Ni base superalloys: Quantitative FIB-SEM investigation and 3D image modeling

“…Because of the high cost of both elements, it is of particular importance to optimize their content in the alloy and to understand the exact way they influence precipitation of TCP phases. Since the so called "reverse partitioning" effect [14,15] cannot be the only mechanism [1,7,13,14] responsible for the Ru influence a Corresponding author: kamil.matuszewski@ww.unierlangen.de on the TCP phase precipitation, the question of the exact nature of this phenomenon remains open.…”

Creep Rupture Strength of Re and Ru Containing Experimental Nickel-Base Superalloys

“…Unfortunately, the mechanisms of the key benefits of Re and Ru additions are still not well understood, so that the optimization of the desired Re and Ru amount is challenging. The present article is part of an extensive investigation of a newly designed alloy series, with the aim to systematically determine the element influence on solidification properties [12,13], thermophysical properties and casting conditions [14], γ/γ'-microstructure changes [15], creep resistance, and phase stability [16][17][18]. The alloys are based on the commercial alloy CMSX-4 and contain different contents of Re and Ru which were added in well-defined atomic percent contents to guarantee a systematic investigation of the element influence.…”