Melvin Robert Jackson

The balance of mechanical and environmental properties of a multielement niobium-niobium silicide-basedIn Situ composite

This article describes room-temperature and high-temperature mechanical properties, as well as oxidation behavior, of a niobium-niobium silicide basedin situ composite directionally solidified from a Nb-Ti-Hf-Cr-Al-Si alloy. Room-temperature fracture toughness, high-temperature tensile strength (up to 1200 °C), and tensile creep rupture (1100 °C) data are described. The composite shows an excellent balance of high- and low-temperature mechanical properties with promising environmental resistance at temperatures above 1000 °C. The composite microstructures and phase chemistries are also described. Samples were prepared using directional solidification in order to generate an aligned composite of a Nb-based solid solution with Nb3Si- and Nb5Si3-type silicides. The high-temperature mechanical properties and oxidation behavior are also compared with the most recent Ni-based superalloys. This composite represents an excellent basis for the development of advanced Nb-based intermetallic matrix composites that offer improved properties over Ni-based superalloys at temperatures in excess of 1000 °C.

High-temperature refractory metal-intermetallic composites

In this article, toughness, oxidation, and rupture behaviors of present-generation refractory metal-intermetallic composites are compared to the performance requisites necessary to make these materials a competitive choice for the jet engine turbine environment of the future.

The aluminization of platinum and platinum-coated IN-738

The chemistry and morphology of aluminide coatings formed on platinum and platinum-coated IN-738 have been studied. Most of the aluminide coatings evaluated were applied using the pack cementation process. For aluminized platinum a series of intermetallic Pt-Al compounds form. The stoichiometries of these compounds are essentially in agreement with those that would be predicted based upon phase diagram considerations. For aluminized, platinum-coated IN-738, the coating morphology and chemistry are highly dependent upon the thickness of the platinum layer. A relatively thick platinum layer (∼25 microns) confines the initial reaction for the aluminizing conditions used so that refractory metal elements from the substrate are excluded from the outer regions of the coating. Thinner platinum layers only partially confine the reaction and do not exclude the refractory metals from the coating. Microstructures that develop are related to the appropriate phase diagrams.

Solidification processing of high temperature intermetallic eutectic-based alloys

Abstract This paper describes the role of solidification processing of high-temperature eutectic alloys in the development of high-temperature structural materials. Particular emphasis is placed on directional solidification of eutectic-based alloys using cold crucible Czochralski crystal growth. Alloys with melting temperatures up to 2250 °C were directionally solidified by this technique for study of the microstructure, phase equilibria and mechanical properties. The attributes and limitations of the Czochralski technique are compared with those of other methods for solidification of high-temperature alloys. The microstructures and fracture toughness data of directionally solidified in situ composites generated from NbSi and CrNb alloy systems are presented. Property enhancements that can be achieved by directional solidification of high-temperature materials are discussed.

Ductility in boron-doped, nickel-base L12 alloys processed by rapid solidification

Etude de la ductilite dans les composes intermetalliques de type Ni 3 X (ou X=Al, Ga, Si, Ge) de structure ordonnee L1 2 en fonction de la teneur en bore (0,1 a 1% at.)

The Nb-Ti-Si ternary phase diagram : Evaluation of liquid-solid phase equilibria in Nb- and Ti-rich alloys

Phase equilibria in ternary Nb-Ti-Si alloys have been investigated using scanning electron micros-copy, XRD, and electron beam microprobe analysis. Alloys containing up to four phases, Nb(Ti) 5 Si 3 , Ti(Nb) 5 Si 3 , (Nb,Ti) 3 Si, and (Nb,Ti,Si) were directionally solidified using cold crucible Czochralski crystal growth. A broad range of Nb and Ti compositions were investigated for Si concentrations up to 35.0%. Microstructural and microchemical evidence provided a clear definition of the Nb-Ti-Si liquidus surface and indicate that the metal-rich end of the ternary phase diagram possesses two transition reactions: L + Nb(Ti) 5 Si 3 → (Nb,Ti) 3 Si + Ti(Nb) 5 Si 3 L + (Nb,Ti) 3 Si → (Nb,Ti,Si) + Ti(Nb) 5 Si 3 The first of these transition reactions occurs at a composition of approximately Nb-66Ti-19Si, and probably at a temperature between 1600 and 1650 ‡C. The second of these transition reactions oc-curs at a composition of approximately Nb-76Ti-13.5Si and a temperature of approximately 1350 ‡C. No ternary eutectic was observed in the compositions that were investigated.

Solid state SiC/Ni alloy reaction

The solid state reaction between silicon carbide and a model superalloy consisting of 70 at. pct Ni, 20 at. pct Cr, and 10 at. pct Al was studied between 700 °C and 1150 °C for times ranging from “0” hours to 330 hours. Reaction couples consisting of SiC/Ni, SiC/Cr, and SiC/NiCr were also studied. The reactions were carried out in air with the materials, in the shape of discs, maintained in contact under a pressure of 7 MPa. A reaction was detected with SiC and the model alloy at all temperatures studied, and the reaction was diffusion controlled with an activation energy of 184 kJ/mole. In the ceramic the reaction was dominated by the diffusion of Ni into the ceramic forming a banded structure consisting of alternating layers of δ-Ni2Si and a two phase mixture of graphite and δ. On the metal side, the reaction was very dependent on the presence of alloying elements, with pure Ni reacting to the greatest extent, followed by the binary NiCr alloy, and finally by NiCrAl. The growth and presence of the phases detected in these reactions is consistent with phase equilibria concepts.

The Nb-Ti-Si ternary phase diagram: Determination of solid-state phase equilibria in Nb- and Ti-rich alloys

Ternary Nb-Ti-Si phase equilibria have been investigated for the metal-rich end of the ternary phase diagram using scanning electron microscopy, XRD, and electron beam microprobe analysis. Microstructural and microchemical observations have been used to define isothermal sections at temperatures of 1500 and 1340 °C. These temperatures delineate regimes defined by liquid-solid reactions. Additional isothermal sections were estimated at 1350,1320, and 1150 °C using these data. A broad range of Nb and Ti compositions were investigated for Si concentrations up to 35.0 %. Alloys containing up to four phases, Nb(Ti)5Si3, Ti(Nb)5Si3, (Nb,Ti)3Si, and (Nb,Ti,Si) were studied. The effect of Ti on the eutectoid reaction, (Nb,Ti)3Si → (Nb,Ti,Si) + Nb(Ti)5Si3 was investigated, as was the effect of Nb on the peritectoid reaction, Ti(Nb)5Si3 + (Nb,Ti,Si) → (Nb,Ti)3Si.

Microstructural and crystallographic relationships in directionally solidified NbCr2Nb and CrCr2Nb eutectics

The present paper describes processing, microstructures and phase relationships in directionally solidified Cr[sub 2]Nb-Nb and Cr[sub 2]Nb-C eutectics. Both of these eutectics, and the stoichiometric Laves phase Cr[sub 2]Nb, were directionally solidified using cold crucible Czochralski crystal growth with growth rates from 1 to 15 mm/min. Cr[sub 2]Nb-Nb had a rod/ribbon-type structure and Cr[sub 2]Nb-Cr had a lamellar structure. The crystallographic orientation relationships between both Cr and Nb and the Cr[sub 2]Nb Laves phase in the individual eutectics were characterized using transmission electron microscopy. Inter-phase orientation relationships in these eutectics were complicated by the transformation of Cr[sub 2]Nb from the C14 to the C15 crystal structure on post-solidification cooling. Twins were observed in the C15 Cr[sub 2]Nb phase in both the single phase and eutectic samples. The relationship between the twins, the C14-C15 transformation, and the eutectic morphology is also discussed.

Hf-Si binary phase diagram determination and thermodynamic modeling

A new version of the Hf-Si binary phase diagram has been constructed and it includes recent confirmation of the existence of the Hf5Si3 phase and observation of the following eutectoid reaction: Hf5Si3 ↔ Hf2Si + Hf3Si2 at 1925±25 °C. The peritectic reaction, L + Hf3Si2 ↔ Hf5Si3 at ∼2360±30 °C, was proposed with consideration of Brukl’s incipient melting results. Thermodynamic modeling of the binary Hf-Si system was performed via Thermo-Calc with existing phase diagram data, the experimental results described in this paper, and the reported enthalpies of formation for Hf silicides. A complete thermodynamic description of the Gibbs energies of all stable phases in the binary system was developed and was consistent with the majority of the phase diagram and thermochemistry data.