Bernard Patrick Bewlay

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.

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.

Thermodynamic modeling of the Nb-Hf-Si ternary system

Abstract A thermodynamic description of the Nb–Si–Ti ternary system is developed by modeling the Gibbs energies of the individual phases in the system. Due to insufficient experimental data, the ternary description is obtained primarily by extrapolation based on the descriptions of the constituent binary systems, i.e. Nb–Si obtained in this study, Ti–Si and Nb–Ti available in the literature. Nevertheless, the calculated ternary phase equilibria using the thermodynamic description so obtained agree very well with available experimental data, which suggests that the ternary description is reasonable. Although there might be large uncertainties involved in this description for alloy compositions far away from the Nb–Ti boundary binary, it can be very useful, in the absence of experimental data, as a guide for alloy design and processing development. Moreover, it can be used with great advantages in planning phase equilibrium measurements in the future and thus reduce the effort to be expended.

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.

TiAl alloys in commercial aircraft engines

The present article will describe aspects of the science and technology of titanium aluminide (TiAl) alloy system and summarise the low and high temperature mechanical and environmental properties exhibited by different alloy generations. In terms of processing developments, conventional gravity casting and near net shape casting would be discussed in detail. Also newer and non-conventional forging and additive manufacturing routes would be briefly highlighted. Extensive investigations of TiAl alloys have enabled their commercial implementation in aerospace and automotive industries. The GEnx™ engine is the first commercial aircraft engine that used TiAl (alloy 48–2–2) for their low pressure turbine blades. Among non GE engines, recently, new β-stabilised TiAl alloy (TNM) is being used to manufacture LPT blades for PW1100G™ engines. TiAl materials and design processes can reduce engine weight and improve engine performance.

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.

Superplastic roll forming of Ti alloys

Abstract The high cost of aerospace alloys and their components makes them prime candidates for net-shape manufacturing techniques. Conventional processes for manufacturing disk components include hammer, hot die, and isothermal forging. This paper will examine the potential of a revolutionary approach for the manufacture of aircraft engine disks, superplastic roll forming. The process of superplastic roll forming, developed at the Institute for Metals Superplasticity Problems, Ufa, employs pairs of small opposed rollers to shape a cylindrical workpiece into a complex axisymmetric shape by simultaneously adjusting the roll gap and by moving the rolls radially outward on the workpiece while it is rotated about its axis of symmetry. Both the workpiece and the rolls are maintained at temperatures close to the beta transus. This paper will describe metallurgical evaluations of superplastically roll formed disks of alloy VT25. The evaluations of the disks included microstructure, crystallographic texture, heat treatment response, tensile strength, stress rupture resistance, and ultrasonic characteristics. The disk microstructures were found to be uniform and without any strongly textured colonies. Mechanical properties of the roll formed VT 25 were compared with those of Ti-6242S, IMI834, and conventionally forged VT25. The RF VT25 disk was found to possess low ultrasonic noise and high inspectability, which provided an increase in signal to noise for synthetic flaws.

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.