Cyclic oxidation of β-NiAl with various reactive element dopants at 1200 °C

Abstract

Cyclic oxidation of β-NiAl alloys doped with different reactive elements (REs) each of which had an optimized concentration was investigated. All of REs effectively suppressed growth of voids beneath oxide scales formed on the alloys. The La-doped alloy suffered from severe internal oxidation. The Hf or Zr-doped alloys exhibited lower oxidation rate, while the Dy-doped alloy showed less scale rumpling. The effect mechanisms associated with solid solubility and ion size of RE dopants on cyclic oxidation were discussed. Co-doping of REs, such as Dy and Hf or Zr, could be anticipated to provide better cyclic oxidation performance.

Introduction

The high melting point, low density and good isothermal oxidation resistance of β-NiAl intermetallic compound has been widely documented over the past decades [1], [2], [3]. Recently, β-NiAl has been considered as a potential candidate for high-temperature protection of underlying superalloy substrates and suitable bond coat in thermal barrier coating (TBC) system due to these desirable properties [4], [5], [6], [7], [8], [9], [10]. However, the alumina scale formed on NiAl spalls readily during high-temperature cyclic oxidation, especially above 1200 °C.

To improve oxide scale adhesion to NiAl alloy or coating, RE additions of Hf, Zr, Y and La as well as their oxides dispersions in NiAl were systematically studied [11], [12], [13], [14], [15]. Numerous attempts have been made to clarify the beneficial RE effects on cyclic oxidation performance, but the mechanical explanation is still in debate. For alumina-forming alloys, a continuous flux of RE ions diffusing from the alloy to the scale/alloy interface and then to the scale grain boundaries is essential to realize the beneficial RE effects, which is recognized as dynamic segregation theory (DST) [16], [17]. It reveals that the reduction in oxide scale growth rate by RE doping is due to the slow outward diffusion of RE ions inhibiting the outward diffusion of Al ions. Several hypotheses are also proposed concerning the enhancement on oxide scale adhesion by RE additions [18], [19], [20], [21], [22], [23], [24]. The formation of voids beneath the oxide scale weakens the interfacial bonding and impurity elements such as sulfur in alloys segregates to the scale/alloy interface and promotes the interfacial void growth. RE ions are considered reacting and tying up sulfur due to their strong affinity for sulfur [18], [19], [25], [26]. Apart from sulfur, other impurities such as C, O and P in alloys might be also detrimental to the oxidation performance [27], [28], [29]. Recent studies have indicated that mechanical interlocking of the oxide scale by “pegs” formed at the oxide/metal interface plays a dominate role in improving scale adhesion, while the excellent oxidation resistance of oxide dispersion strengthened alumina-forming alloys reveals that this pegging mechanism is sufficient but not necessary for optimizing the oxidation performance [14], [17], [23], [24].

Dysprosium (Dy) is also a typical reactive element, but its benefit in improving oxidation resistance of alumina-formers has been just reported in recent years. The addition of Dy in NiAl significantly improved cyclic oxidation performance in mechanisms similar to other RE additions did [30], [31], [32], [33]. However, no efforts are made to compare the capabilities of improving oxide scale adhesion between Dy and other reactive elements as quantifying the beneficial RE effects is rather difficult. In attempting to do this, identifying the differences in solubility, reactivity and diffusion kinetics between these reactive elements in Ni–Al system are necessary.

Early results suggested that similar mechanisms occurred when the RE was doped in NiAl as either an oxide dispersion or an alloy addition, but the addition of RE as an oxide dispersion which strengthens the alloy substrate might deteriorate oxide scale adhesion as a weaker substrate is better able to dissipate strain energy by deforming [14], [17], [34], [35]. Besides, ion implantation is less effective in improving the performance of alumina-formers than chromia-formers [36], [37], [38]. Therefore, arc-melting was chosen for specimen preparation in the present work.

In this work, a comparative research is carried out on different RE-doped NiAl alloys to investigate into the roles of different reactive elements in affecting the oxide scale growth rate and scale adhesion, to better understand those factors affecting the strength of the RE effects.