C.G. McKamey

A review of recent developments in Fe3Al-based alloys

Fe{sub 3}Al-based iron aluminides have been of interest for many years because of their excellent oxidation and sulfidation resistance. However limited room temperature ductility ({lt}5%) and a sharp drop in strength above 600 {degree}C have limited their consideration for use as structural materials. Recent improvements in tensile properties, especially improvements in ductility produced through control of composition and microstructure, and advances in the understanding of environmental embrittlement in intermetallics, including iron aluminides, have resulted in renewed interest in this system for structural applications. The purpose of this paper is to summarize recent developments concerning Fe{sub 3}Al-based aluminides, including alloy development efforts and environmental embrittlement studies. This report will concentrate on literature published since about 1980, and will review studies of fabrication, mechanical properties, and corrosion resistance that have been conducted since that time.

Effect of chromium on properties of Fe 3 Al

The effects of the addition of chromium on several properties of Fe{sub 3}Al, including tensile strength and ductility, fracture behavior, and slip and dislocation characteristics, were studied. Alloying with up to 6 at.% chromium results in an increase in room temperature ductility from approximately 4% to 8--10%. Along with this increase in ductility, the addition of chromium produces a change in fracture mode from transgranular cleavage to a mixed mode of intergranular-transgranular cleavage, and a change in slip behavior from coarse straight slip to fine wavy slip. These phenomena are discussed in terms of the effect of chromium on the antiphase boundary energies and dislocation characteristics.

Physical Metallurgy and Mechanical Properties of Transition-Metal Laves Phase Alloys

Abstract This paper provides a comprehensive review of the recent research on the phase stability, point defects, and fracture toughness of AB 2 Laves phases, and on the alloy design of dual-phase alloys based on a soft Cr solid solution reinforced with hard X Cr 2 second phases (where X =Nb, Ta and Zr). Anti-site defects were detected on both sides of the stoichiometric composition of NbCr 2 , NbCo 2 , and NbFe 2 , while they were observed only on the Co-rich side of ZrCo 2 . Only thermal vacancies were detected in the Laves phase alloys quenched from high temperatures. The room-temperature fracture toughness cannot be effectively improved by increasing thermal vacancy or reducing stacking fault energy through control of phase stability. Microstructures, mechanical properties, and oxidation resistance of dual-phase alloys based on Cr–NbCr 2 , Cr–TaCr 2 , and Cr–ZrCr 2 were studied as functions of heat treatment and test temperature at temperatures to 1200°C. Among the three alloy systems, Cr–TaCr 2 alloys possess the best combination of mechanical and metallurgical properties for structural use at elevated temperatures.

Effect of chromium on room temperature ductility and fracture mode in Fe3Al

The unique electrical, magnetic, and corrosion resistance properties of iron-aluminides with compositions near that of Fe/sub 3/Al have long stimulated the interest of material scientists. Combined with low cost of iron and aluminum, lower density compared with steels, and adequate strength at temperatures below 600/sup 0/C, iron aluminides are attractive candidates for structural applications in many industries. However, the lack of ductility at ambient temperatures and a sharp drop in strength above 600/sup 0/C have been major obstacles to their development as structural materials. The authors studies, conducted using a base alloy of Fe-28 at.% Al, have indicated that the room temperature ductility can be improved by a factor of at least two by additions of chromium of up to 6 at.%. It has been found that, although the room temperature strengths are decreased slightly, the elongation to fracture is increased from below 4% with no chromium to 8-10% for additions of 2-6 at.% chromium. Elongations of up to 13% were produced by varying the rolling schedule and/or a post-rolling heat treatment.

Effect of recrystallization on room temperature tensile properties of an Fe3Al-Based alloy☆

The purpose of the present study was to determine whether recrystallization alone had an effect on the room temperature tensile properties of a Fe[sub 3]Al-based alloy. The best room temperature tensile strength and ductility were attained in specimens which had been heat treated to relieve stresses produced by the fabrication process but that had a minimum number of recrystallized grains. The exact mechanism for this improvement is unclear, but could involve texturing effects or the enhancement of dislocation mobilities. Also the elongated grain structure characteristic of as-rolled material provides a minimum of transverse cleavage planes (as well as a minimum of grain boundaries), and could simply be disrupting the path of atomic hydrogen entering the specimen during tensile stressing.

Fabrication and tensile properties of continuous-fiber reinforced Ni 3 Al–Al 2 O 3 composites

Continuous-fiber reinforced metal-matrix composites consisting of Ni{sub 3}Al alloys and Saphikon Al{sub 2}O{sub 3} single crystal fibers were fabricated by hot-pressing of fiber-foil lay-ups. Two matrix compositions were employed, namely, IC50 (Ni--22.5Al--0.5Zr--0.1B, at. %) and IC396M (Ni--15.9Al--8.0Cr--0.5Zr--1.7Mo--0.02B, at. %). Etching of the foils in aqueous FeCl{sub 3} solution prior to lay-up and hot-pressing tended to improve fiber-matrix bonding and the density-normalized room temperature yield stress. Whereas strength improvements for the IC50 matrix were only moderate, significant improvements were found for a IC396M composite reinforced with 10 vol. % of Saphikon fibers.

Creep Properties of Phosphorus+Boron-Modified Alloy 718

Creep-rupture testing of modified alloy 718 has confirmed the beneficial effect of optimum additions of P and B. Activation energies and creep exponent analyses suggest some type of pinning mechanism is involved in this strengthening. However, the exact mechanism of this improvement is still unclear.

Impurity effects on high-temperature tensile ductility of iridium alloys at high strain rate

The current study was undertaken to determine what effects, if any, larger amounts of certain impurities (Al,Cr,Fe,Ni, and Si) might have on the physical metallurgy and mechanical properties of the DOP-26 iridium alloy. This report summarizes the effects of these impurities on grain growth behavior and high-temperature high-strain-rate tensile ductility. Comparisons are made to the grain growth behavior and high-strain-rate tensile properties of the DOP-26 alloy without intentional impurity additions.

Deformation and fracture of iridium: microalloying effects

Abstract The effects of Ce and Th doping (20–50 wppm) on the mechanical properties of Ir alloys were investigated. At both low (∼10 −3 s −1 ) and high (∼10 3 s −1 ) strain rates, the Ce+Th doped alloys undergo a transition from brittle intergranular (plus some transgranular) fracture at low temperature to ductile transgranular fracture at elevated temperature. The ductile–brittle transition temperature (DBTT) is ∼400 K higher at the higher strain rate. Grain size and grain-boundary cohesion both affect the ductility of Ir alloys. Cerium and thorium, when added together, refine grain size more effectively than when Th is added by itself (especially at high temperatures). Their effect on grain-boundary cohesion is similar to that of Th by itself. At low strain rates (∼10 −3 s −1 ), the strength and ductility of alloys doped with Th were similar to those of alloys doped with both Th and Ce. The brittle–ductile transition appears to be related to a change from high work hardening rate at low temperature to low work hardening rate at elevated temperature.

Microstructural characterization of A γ-TiAl-Ni alloy produced by rapid solidification techniques

Abstract An ingot of two-phase γ-TiAl containing a small amount of nickel [composition (Ti52Al48)99.5Ni0.5] was fabricated by the following series of processes: (1) high-purity buttons were arc melted; (2) the buttons were arc-melt spun into ribbons; (3) the ribbons were crushed into alloy powder using a hammer mill; (4) the powder was consolidated into an ingot by hot pressing, followed by hot isostatic pressing; and (5) the ingot was hot forged into a 1.5 cm thick plate. Microstructures of the material from different stages of the processing were examined. Although this series of fabrication steps was successful in producing ingots, the consolidated ingots showed numerous defects including unclosed pores and contamination by molybdenum and iron. These defects undoubtedly contributed to the brittleness of the ingots and the difficulty in machining tensile specimens for mechanical testing. It is believed that these defects can be eliminated by modification of the processing parameters, including increasing the HIP pressure and temperature to close pores, increasing forging temperature and reduction, and using processing hardware that will not contaminate the alloy. The lower level of oxygen contamination found by this fabrication route compared to other powder routes is encouraging and is probably a result of the larger powder particles (and hence lower amount of surface area exposed to the environment) produced by pulverization of the melt-spun ribbons.