Y. Ro

Development of Ir-base refractory superalloys

In the present study the authors propose a new class of superalloys: refractory superalloys. This new concept is defined as alloys with {gamma}-fcc and {gamma}{prime}-L1{sub 2} phases coherent structures similar to Ni-base superalloys, and yet with considerably higher melting points. Ir, having a melting point of 2,443 C, is selected as the base metal for the refractory superalloys since Ir has {gamma}-fcc structure and can be equilibrated with {gamma}{prime}-L1{sub 2} structure according to binary phase diagrams, e.g., in Ir-Nb, Ir-Ti, Ir-Ta, Ir-Hf, Ir-Zr and Ir-V systems. Hence the authors designed Ir-base refractory superalloys and tested their mechanical properties and oxidation resistance at up to 1,800 C. The Ir-Nb and Ir-Ti alloys are reported in this paper.

Rh-base refractory superalloys for ultra-high temperature use

In the previous paper, the authors proposed a new class of superalloys, namely, refractory superalloys. This new concept is defined as alloys with fcc and L1{sub 2} coherent two phase structures similar to Ni-base superalloys, and yet with considerably higher melting temperatures. In this paper, Rh-Nb and Rh-Ti systems were selected to compare with Ir-Nb and Ir-Ti systems which were shown in the previous paper. Rh-Ta system was also selected because of its highest melting temperatures among above binary systems. The microstructure evolution and high temperature strengths of these Rh-base alloys were investigated.

Microstructure dependence of strength of Ir-base refractory superalloys

Abstract Precipitation hardening was investigated in Ir–Nb and Ir–Zr alloys with a two-phase structure consisting of the fcc matrix and L1 2 coherent precipitate phases, similar to that in Ni-base superalloys. Cuboidal L1 2 precipitates and plate-like L1 2 precipitates were formed with coherent interfaces in the fcc matrix in the Ir–Nb and Ir–Zr alloys, respectively. Effects of precipitate shape and coherency strains on precipitation hardening are discussed in terms of lattice misfit. Plate-like precipitates forming a 3-dimensional maze structure in the Ir–Zr alloys were profitable to precipitation hardening in both factors, that is precipitate shape and coherency strains.

NiTi-base intermetallic alloys strengthened by Al substitution

Abstract A series of NiTi-base alloys with Al additions substituting the Ti were designed and evaluated in terms of the microstructure and mechanical properties. It was found that the compression strength is improved drastically by the Al additions, especially when the Al amount is high enough to precipitate Ni 2 TiAl (Heuslar compound) phase which is coherent to the NiTi(B2) phase matrix; an alloy with 8.4 mol.% Al showed compressive yield strengths as high as 2300 and 200 MPa at room and high (1000°C) temperatures, respectively. When the Al content exceeds 11 mol.%, however, Ni 2 TiAl phase started to deposit in a dendritic manner to reduce the strength. Although compressive ductility declined with the increase in Al content, 5.2% deformation was achieved with the 8.4 mol.% Al-containing alloy at room temperature.

High temperature tensile properties of a series of nickel-base superalloys on a γ/γ′ tie line

Abstract A series of nickel-base superalloys with γ′ fractions changed from 0 to 100 mol% and with γ and γ′ compositions kept constant were designed and examined in terms of high temperature tensile properties. In the temperature range from 900 to 1000°C, 0.2% proof stress shows a maximum at about 85 mol% in a corrected amount of γ′ , whereas at 1100°C, it shows maximum at about 65 mol%. The maximum shift of proof stress from the law of mixture in strength was constant for these temperatures. In the strain rate range from 10 −8 to 10 −2 /s at 1000°C, the maximum shift of proof stress increased with the strain rate. The proportion of the shift stress to the proof stress becomes larger as the temperature increases.

Precipitation hardening of Ir-Nb and Ir-Zr alloys

The authors previously developed refractory superalloys based on platinum-group metals with fcc and L1{sub 2} two-phase coherent structures. The refractory superalloys are potentially useful at ultra-high temperature, at which Ni-based superalloys can not be used. From among the platinum-group metals, they selected Ir as the base material because its melting temperature (2447 C) is higher than that of Ni (1455 C) and because Ir, with an fcc structure, can be equilibrated with the L1{sub 2} structure phase. Preliminary results demonstrated the superior strength of Ir-based refractory superalloys above 1200 C. Those results also revealed that precipitation hardening occurs and is affected by the shape of the precipitate. The precipitate shape of Ir-based alloys heat-treated at 1200 C is affected by the lattice misfits between the matrix and precipitates. In this study, the authors investigated the precipitation hardening mechanism by observing dislocation structures in deformed samples of Ir-Nb and Ir-Zr alloys using bright-field imaging and dark-field weak-beam imaging techniques with a transmission electron microscope (TEM).

Temperature dependence of the flow stress of Ir-based L12 intermetallics

The temperature dependence of compressive strength of Ir3Zr, Ir3Hf, and Ir3Ti with an L12 structure was investigated. Ir3Zr showed weak anomalous behavior; in other words, the strength increased up to a peak strength with increasing temperature at intermediate temperatures but decreased slightly with increasing temperature below room temperature. Ir3Ti and Ir3Hf showed normal strength behavior. Their strength behavior including Ir3Nb is discussed in terms of the phase stability of the L12 structure with respect to other related structures. The lower phase stability of Ir3Ti with respect to the D024 phase suggests stable SISF-bounding dislocations, which cause the normal strength behavior to occur. The lower phase stability of Ir3Nb with respect to the D0a causes anomalous behavior at intermediate temperature.

New developed quaternary refractory superalloys

Abstract The novel idea for designing quaternary superalloys by two kinds of binary alloys with coherent structure was applied to Ir–Nb–Ni–Al superalloys. Eight alloys were prepared by combining two sorts of binary alloys, Ir–Nb and Ni–Al, in different proportions. The effects of the mole fraction of Ir-based alloy or the L1 2 alloy on the microstructure and the 0.2% flow stress of quaternary alloys were investigated. Two sorts of coherent structure, fcc/L1 2 –Ir 3 Nb and fcc/L1 2 –Ni 3 Al, were observed in the quaternary superalloys. The new developed quaternary Ir-based superalloys are of the advantages of Ir-based alloys and Ni-based superalloys. The 0.2% flow stress of Ir–Nb–Ni–Al at 1200°C could reach up to 350 MPa and the compressive strains were improved greatly compared with Ir-based alloys.

Platinum group metals base refractory superalloys

Ir- and Rh-base refractory superalloys wit h an fcc and L1{sub 2} two phase structure similar to Ni-base superalloys, yet with considerably higher melting temperatures have been proposed. Fcc and L1{sub 2} two phases were observed in these alloys by transmission electron microscopy and X-ray powder diffractometry. The compression tests of these alloys showed that the strengths of several alloys were about 200 MPa at 1,800 C and these alloys have potential to become ultra-high temperature materials for use in power engineering field.

Development of Cr-base alloys and their compressive properties

Abstract Four Cr-base alloys were designed and produced from commercial raw materials. Their compressive and oxidation properties were measured at several temperatures. Most of the developed alloys were ductile in compression. Compared to existing Ni-base superalloys, some of these alloys showed superior compressive strength and oxidation resistance.