A Thermodynamic Approach to Guide Reactive Element Doping: Hf Additions to NiCrAl

Abstract

A method based on thermodynamic modeling was developed to determine optimal amounts of Hf additions to Al 2 O 3-forming, γ-γ NiCrAl alloys. The alloy ability to maintain Hf in solution was set by the Hf concentration required to form HfO 2 at the oxygen activity defined by the alloy/Al 2 O 3 equilibrium. This Hf tolerance decreased with increasing temperature and increased with increasing γ fraction. The latter was due to the higher solubility of Hf in γ , compared to γ. The validity of the procedure was evaluated by oxidizing a series of NiCrAl-Hf alloys in dry air at 1000-1200 • C. The experimental results followed the predicted trends, although the Hf tolerance tended to be overestimated. The applicability of the criterion, and potential routes for improved predictability, were discussed by considering the influence of the compositional changes occurring at the metal surface during the transient and steady-state stages of the oxidation process.

The Effect of Various Additions on the Oxidation Behavior of the γ/γ′ Ni-Based Alloy

“…It should be noted that, both qualitatively and quantitatively, the results obtained are consistent with those reported by other authors in terms of the following: (1) The k p values of Si-containing alloy were close to that observed for Ni15Cr5Al + Si (1 wt%) [17]; (2) Si additions resulted in reaching the steady-state oxidation earlier (shortened transient oxidation stages) [17]; (3) Hf additions, if exceeding the solubility limit (over-doping), form oxide precipitates in the underlying substrates as well as in the scales [6,7,14,15,19]. Comparison of the results obtained with those in an extensive study of the effect of Hf, Y and Si additions on the oxidation behavior of Ni20Al5Cr alloy and René N5 superalloy reported elsewhere [11] led to some discrepancies that need further investigation involving the effect of Cr and the content of the additions.…”

“…The two-phase γ/γ′ Ni-Al-based alloys, mostly γ/γ′-type ones, alloys that usually contain also Pt, are considered as promising high-temperature coating materials [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Their oxidation resistance can be improved by additions of reactive elements * Jerzy Jedliński jedlinsk@agh.edu.pl Extended author information available on the last page of the article (e.g., Hf, Y), due to the well-evidenced, so-called, reactive-element-effect, reviewed elsewhere [21][22][23], while the effect of silicon additions is still under investigation [11,14,17]. However, in order to optimize the beneficial effect of the reactive elements in qualitative and quantitative terms, as well as to examine the effect of Si, the oxidation behavior should be elucidated as a function of the additions (or their combinations), their amounts and exposure conditions.…”

“…The contents of the additions were chosen in order to: (1) investigate the effectiveness of the beneficial effect of reactive elements in terms of lower and higher Hf contents and combined Hf + Y additions, (2) determine whether additions of Si can enhance the oxidation resistance of the alloy containing higher amounts of Hf. The lower content of Hf was chosen taking into account results reported by Gheno et al [14] in order to meet the so-called Hf-tolerance criterion related to the solubility limit of Hf in the alloy and the propensity to HfO 2 precipitation in the alloy, while higher content exceeded significantly this threshold. The contents of Hf and Y added together were based on the co-doping approach proposed by Pint [24] and investigated more in detail by Kim et al [18].…”

Modes of Deposit-Induced Accelerated Attack of MCrAlY Systems at 1100 °C

“…The link between primary reaction modes identified using unary or binary de- Figure 13: Map showing the effect of sulfate-oxide deposits on the oxidation of NiCoCrAlY alloys at 1100 • C. The boundaries were determined from work by Giggins and Pettit [36] and our work [38] on NiCrAl alloys. "NiO" denotes alloy compositions which form external NiO and internal Al 2 O 3 and Cr 2 O 3 ; "Cr 2 O 3 " denotes compositions which form external Cr 2 O 3 and internal Al 2 O 3 ; "Al 2 O 3 " denotes compositions which form external Al 2 O 3 exclusively; "transition" denotes compositions subject to localized internal oxidation of Al.…”

CHAPTER 4 High Temperature Oxidation of Stainless Steels

“…4.31. where R is the gas constant, T is the absolute temperature, Ni-Cr-Al alloys. They further suggested that the level of hafnium content varies due to structural changing of alloy subsurface region by the selective oxidation of aluminium because  dissolution in the substrate [64]. Thus, the factors which may control the activities of aluminium and reactive elements are expected to affect the optimised concentration of reactive elements.…”