Elsevier

Scripta Materialia

Volume 54, Issue 1, January 2006, Pages 115-119
Scripta Materialia

Saturated solid-solution hardening behavior of Ir–Hf–Nb refractory superalloys for ultra-high temperature applications

https://doi.org/10.1016/j.scriptamat.2005.08.038Get rights and content

Abstract

The principles of multi-component alloying and saturated solid-solution hardening were combined for the design and development of Ir-based alloys for ultra-high temperature applications. It is found that as a solute, the transition metal Hf combined with Nb exerts its full potential for solid-solution hardening on the solvent Ir at room temperature and higher. With a monolithic saturated face-centered cubic structure, the Ir–3Hf–5Nb alloy has a 0.2% yield strength of as high as 140 MPa even at 1950 °C.

Introduction

To develop new generation high temperature structural materials for application at temperatures in excess even of 2000 °C, Ir-based refractory superalloys have been investigated. With regard to applications at elevated temperatures, the high-temperature strength and the creep resistance are the primary issues on which to focus. Attempts to improve the high-temperature strength of Ir have involved solid-solution hardening and precipitate hardening [1], [2], [3], [4].

Ir, with a melting point of about 2447 °C, equilibrates with most of the transition metals, such as Ti, Nb, Hf, Zr, Ta, and V, and forms a face-centered cubic (fcc) and L12 two-phase structure [5]. Most of the previous work [1], [2] has focused on those Ir-based binaries with an fcc/L12 two-phase structure, where the L12 phase maintains a coherent relationship with the fcc phase. With an fcc/L12 two-phase structure, Ir-based alloys, such as Ir–Hf, Ir–Zr, and Ir–Nb binaries, have a high-compressive strength of over 800 MPa at 1200 °C. In addition, the creep resistance of the binary alloys have shown promise for them to be used as structural materials at 1500 °C. The minimum creep rates of the binary alloys were about 10−8 s−1 at 137 MPa [3], [4]. With regard to the creep lifetime, the secondary creep continued for 300 h, while the creep strain remained at 2%. However, the high-temperature strength of the Ir-based alloys drastically decreased at temperatures above 1500 °C. At the same time, the minimum creep rate at 1650 °C became one or two orders higher than that at 1500 °C, and the tertiary creep appeared after 10 h at 1650 °C and a few hours at 1800 °C. Recently, a primary comparison of the responses of the high-temperature strength in the fcc and L12 phase was made [6], and the results revealed that the strength of the saturated Ir solid solution of the fcc-dominated microstructure was higher than that of the single L12- or L12-dominated microstructure at room temperature and higher. It was also found that, during creep at 1800 °C, the coherent cuboidal L12 precipitates coarsened and became semi-coherent precipitates [3], [4]. This caused a coarse fcc channel and a lower resistance for dislocation movement.

Although the L12 phase can induce coherent hardening of the bulk materials, the role of the L12 phase in the improvement of the bulk strength of Ir-based alloys needs to be reexamined for the reasons mentioned above, in particular at ultra-high temperatures. The search for a new strengthening phase or a design for a saturated monolithic Ir solid solution has been attempted to investigate the possibility of achieving Ir-based alloys of superior strength [6], [7]. A series of Ir-based binary alloys with a single Ir solid solution containing a different concentration of solute have been successfully designed. The designed alloy has a clear tendency to extra-high strength at elevated temperatures [7]. Alloying elements that have a larger atomic size misfit parameter with Ir and a limited solid solubility in Ir, such as Hf and Zr, showed the strongest solid-solution hardening effect on Ir. At 1950 °C, for example, the compressive strength of the pure Ir was about 20 MPa, and that of monolithic Ir solid solution, which solidified 3 mol% Zr or 3 mol% Hf, was increased up to 80 MPa, while their size misfit parameters were as high as 0.16 and 0.17, respectively. These findings suggest that the monolithic fcc phase has more potential for improving the high-temperature strength by sufficient solid-solution hardening. Based on the size misfit parameter and solubility limitation of the transition metals to Ir, the idea of multi-component alloying and saturated solid solution was used for the design of monolithic fcc Ir alloys in order to probe the strength limitations of Ir-based alloys at ultra-high temperature.

Section snippets

Experimental procedures

According to the Ir–Hf and Ir–Nb binary phase diagrams [5], the solubility limitations of Hf and Nb in Ir are 5 mol% and 12 mol%, respectively, at 1950 °C. Due to the unavailability of an Ir–Hf–Nb ternary phase diagram, the nominal composition of Ir–3Hf binary was carefully chosen as the base and 1, 3, and 5 mol% Nb were respectively added as the second solute to obtain the saturated monolithic fcc solid solution at and above 1950 °C. Elements with 99.9 mass% or higher purity were used. Button ingots

Phase and microstructure

The X-ray diffraction pattern of the sample Ir–3Hf–5Nb heat-treated at 2000 °C for 24 h is shown in Fig. 1. All the peaks detected on the X-ray diffraction pattern correspond exactly to the Ir fcc phase, and no evident L12 peaks were observed. This means that the samples with a nominal composition of Ir–3Hf–(1–5)Nb should have a monolithic fcc microstructure. In fact, in the SEM observation, less than 1% volume fraction L12 particles with size up to 5 μm were found on the fcc matrix. Precipitation

Conclusions

Ir–Hf–Nb ternary alloys were developed according to the principles of multi-component alloying and saturated solid-solution hardening, and the following conclusions were reached.

  • (1)

    The solubility limitation of the solutes Hf and Nb on the Ir solid solution in Ir–3Hf–xNb alloys is close to Ir–3Hf–5Nb.

  • (2)

    The solid-solution hardening efficiency on Ir at room and high temperatures is perfectly carried out. The Vickers hardness and Young’s modulus are as high as 871 and 212 GPa, respectively. At 1950 °C,

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