Yutaka Koizumi

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.

Partitioning behavior of platinum group metals on the γ and γ' phases of Ni-base superalloys at high temperatures

Abstract The partitioning behavior of platinum group metals (PGMs) on the γ and γ′ phases of Ni-base superalloys has been investigated with alloys containing 5 mass% Re and 1 at.% PGMs. It was shown that Ru partitions preferably into the γ, while Rh, Pd, and Pt partition into the γ′, and Ir partitions almost equally. The partitioning behavior is not affected by the 1 at.% PGM addition.

Dependence of creep strength on the interfacial dislocations in a fourth generation SC superalloy TMS-138

Abstract There is an important relation between the γ/γ ′ interfacial dislocations and the minimum creep rate. During creep (1100 °C, 137 MPa), the finer the interfacial dislocation networks are, the smaller the minimum creep rate is. This is attributed to the interaction between the γ/γ ′ interfacial dislocations and slip dislocations in the γ matrix.

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.

Slip geometry of dislocations related to cutting of the γ′ phase in a new generation single-crystal superalloy

Abstract The superdislocations in the γ′ phase of a fourth-generation single-crystal TMS-138 superalloy have been characterized after creep rupture at a stress of 137 MPa under different temperatures. The number of slip dislocations increases with the increase of creep temperatures from 1100 to 1150 °C. The dominant cutting mode of the γ′ phase is dislocation pairs coupled by an antiphase boundary (APB). The popular superdislocations in the γ′ phase show screw character with Burgers vectors or . Also, observed in the γ′ phase are 60° or screw superdislocations in either [1 1 0] or [ 1 1 0 ] directions, and [0 0 1] edge superdislocations. Stacking faults are seen from the dissociation of superdislocations. These superdislocations are related to the damage of the rafted structure in the superalloy.

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.

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.

TERTIARY CREEP BEHAVIOUR OF A NEW SINGLE CRYSTAL SUPERALLOY AT 900°C

*National Research Institute for Metals, 1-2-1 Sengen Tsukuba-shi, Ibaraki, Japan**CNR—TEMPE, Via R. Cozzi 53, 20125 Milano, Italy(Received April 10, 2000)(Accepted in revised form May 22, 2000)Keywords: Nickel alloy; Creep; MicrostructureIntroductionFor temperatures/stresses that are relevant for the service, the shape of creep curves in nickel basesuperalloys is dominated by an extensive tertiary behaviour over most of the life, rather than a dominantsteady state. To describe such accelerated creep behaviour a strain, rather than a time–softening,approach [1] appears to be more appropriated. For single crystal or, in general, directionally solidifiednickel base superalloys, the following linear strain softening relationship is often utilized [2–4]:e˙5 e˙8~1 1 Ce! (1)where e˙ is the instantaneous strain rate, e˙° is the strain rate at the beginning of tertiary creep,eis thetertiary creep strain and C is a damage coefficient, generally depending on stress and temperature,which leads to a progressive increase in the creep rate with the accumulated creep strain.The Eq. 1 is also utilized in the uconcept methodology, in fact its analytical solution is identical tothe portion of the uconcept function [5] describing the tertiary creep with u