Effect of Cr, Hf and temperature on interface reaction between nickel melt and silicon oxide core

Abstract

Effects of Cr, Al and Hf, varied from 0 -32 wt.%, 1 -7.5 wt.% and 0.3 -1.5 wt.% respectively, on reactions between nickel base alloy melt and silicon oxide core were investigated in this paper. Role of temperature also was studied. Morphology and structures of reaction products were analyzed by scanning electron microscope (SEM) with the energy dispersive spectrometry (EDS) and the X-ray diffraction (XRD) technique. Amount of alumina in interface increased with Cr level, and formed a layer with thickness up to several microns in alloy of 32 wt.% Cr, and 1 wt.% Al. Reaction products in alloy without Hf were mainly alumina, while in alloy with Hf were mainly hafnium oxides and alumina. More hafnium oxides were formed in alloy with 1.5 wt.%Hf and 9 wt.% Cr, while more alumina formed in alloy with 0.3 wt.%Hf and 15 wt.%Cr, a double layer of reaction products formed at the temperature of 1540 ºC, which were composed of a hafnium oxide layer near the metal and a alumina layer above it. The result shows that Cr, Hf levels in the alloy and reaction temperature have the effect of accelerating interface reaction.

Effect of C and Hf Contents on the Interface Reactions and Wettability between a Ni3Al-Based Superalloy and Ceramic Mould Material

“…The formation of these oxides is correlated with the substitution reactions at the interface. Hf is reported to react with SiO2 following the equation Hf + SiO2 (s) → HfO2 (s) + (Si) [10] . Thermodynamically, the change in the Gibbs free energy for the reaction could be written as [16,17] where Δ Θ G T is the standard Gibbs free energy, ΔG T is the Gibbs free energy at any temperature, a i is the activity of component i (i = Hf, Si), f Hf is the activity coefficient of Hf and ω Hf is the mass fraction of Hf.…”

“…The addition of Hf in superalloys aims to enlarge the solidification range and to improve the creep properties of the alloy. However, some report on the interface reactions between superalloys and ceramic mould has indicated that C, Hf as well as Cr and Al are the active elements that will cause alloy-mould interactions [10][11][12][13] . Li et al [10] has reported that Cr and Hf accelerate interface reactions at high temperature and the reaction products are mainly composed of HfO2 and Al2O3.…”

“…However, some report on the interface reactions between superalloys and ceramic mould has indicated that C, Hf as well as Cr and Al are the active elements that will cause alloy-mould interactions [10][11][12][13] . Li et al [10] has reported that Cr and Hf accelerate interface reactions at high temperature and the reaction products are mainly composed of HfO2 and Al2O3. Zheng et al [11] pointed out that Cr, Al and Ti in superalloys reacted with silicon oxide, forming Cr2O3 and inducing some metallic nodular protrusions on the surface of alloy.…”

Experimental investigations and thermodynamic calculations of the interface reactions between ceramic moulds and Ni-based single-crystal superalloys: Role of solubility of Y in the LaAlO3 phase

“…Silica-based ceramic moulds are often used as mouldmaterials thathave high bending strength and low deflection. However, during the casting process, interfacial reactions occur between the silica-based ceramic moulds and molten Ni because of the presence of reactive elements, such as Al [3][4][5][6], Cr [7][8][9], and Hf [10][11][12][13]. Interfacial reactions easily cause inclusions that are difficult to remove.…”

“…Trace amounts of REEs are often added to remove impurities (such as sulphur) and improve the performance of oxidation resistance and creep resistance [7,8]. REEs are so reactive that Xue [15] found severe interfacial reactions between yttrium and silica-based moulds during casting.…”