N.S. Stoloff

Iron aluminides : present status and future prospects

Abstract This paper constitutes a broad survey of the physical, mechanical and corrosion properties of Fe3Al alloys, as well as a review of principal processing methods. This class of alloys, once thought to be inherently brittle, is shown to possess moderate ductility, provided that mechanical testing is carried out in an inert environment. Methods to improve mechanical properties by alloying and microstructural control are described. The influence of alloying elements on corrosion and stress corrosion resistance and weldability also is reviewed.

An overview of powder processing of silicides and their composites

Abstract The use of powders to prepare fully dense silicides and their composites is reviewed. Since most of the techniques are well known, we concentrate on recent developments in this field, including the effects of processing techniques on composition, microstructure and mechanical properties. The use of hybrid processes to produce silicides is described, as are hybrid materials resulting from these processes. Included in the latter are functionally graded materials (FGM). The paper concludes with a brief discussion of in-situ melt processing as a competitive means to produce aligned composite structures

Fabrication, structure and properties of MoSi2-base composites

Several techniques have been employed to consolidate MoSi2 composites reinforced with either Al2O3 or niobium. Powder injection molding was used to prepare MoSi2 with aligned short FP Al2O3 fibers. Plasma spray was used to produce lamellar MoSi2Al2O3. Niobium particles, random short fibers and continuous fibers were incorporated into MoSi2 by hot isostatic pressing. Microstructures and mechanical properties of the various composites are reported. Significant microscopic toughening was obtained with continuous niobium fibers, but at the expense of creep resistance.

Powder processing of intermetallic alloys and intermetallic matrix composites

Abstract This presentation covers the use of powder metallurgy for the formation of monolithic intermetallics and intermetallic matrix composites. A notable development has been the fabrication of homogeneous high density compacts from elemental powders by reactive sintering. A variant process involving simulataneous pressurization in a hot isostatic press, termed reactive hot isostatic pressing, is applicable to those compounds that prove difficult to consolidate by pressureless reactive sintering. This paper describes the effects of various processing factors on fabrication of compounds including Ni 3 Al, NiAl, TaAl 3 , MoSi 2 and their composites. A key concern is with processing effects on microstructure, selection of compatible ceramic reinforcing phases, and whisker alignment through injection molding.

The influence of environment on fatigue crack growth of an Fe3Al, Cr alloy

Iron aluminides are currently being studied for use in applications where their low cost, relatively low weight, and excellent resistance are desired. The main disadvantage of the alloys is low room temperature ductility in air due to moisture which causes hydrogen embrittlement (1-2). This paper examines changes in fatigue crack growth resistance of an Fe{sub 3}Al type intermetallic compound in various test environments. The alloys was studied in two ordered states, the DO{sub 3} and the B2, both of which are BCC type structures composed of interpenetrating cubic lattices. The DO{sub 3} is a highly ordered superlattice, while the B2 is a superlattice of imperfect order.

Tensile properties of a SiCf/SiC composite

Abstract This work examines the mechanical behavior of a 2D woven, 0–90 SiC fiber-reinforced SiC matrix composite. Tensile experiments show that the short-term behavior is largely independent of test temperature below 1000 °C. Microscopic examination reveals that the extent of fiber pull-out and the integrity of the remaining material are also independent of temperature in this range. Conversely, at 1200 °C, the material retains much of its low-temperature stiffness and proportional limit, while the strength increases substantially. Micrographs of these specimens reveal little individual fiber pull-out and a higher density of matrix microcracks. Room-temperature tensile data show that the mechanical behavior is rate-dependent; higher strain rates lead to a lower Youngs Modulus, higher proportional limit and higher ultimate strength. In-plane shear experiments demonstrate that the unreinforced matrix strength is approximately 10% of the composite tensile strength.

The influence of composition on the environmental embrittlement of Fe3Al alloys

Abstract This paper reviews recent research on embrittlement of iron aluminides at room temperature brought about by exposure to moisture or hydrogen. The tensile and fatigue crack growth behavior of several Fe–Al alloys, ranging in aluminum content from 16–35 at.%, are described. Some alloys also contain small amounts of Nb, Zr or C. It will be shown that tensile ductility and fatigue crack growth behavior are dependent on composition, type and degree of long range order, environment, humidity level and frequency. Environments studied include vacuum, oxygen, hydrogen gas and moist air. All cases of embrittlement are ultimately traceable to the interaction of hydrogen with the lattice.

Hydrogen embrittlement of Fe3Al alloys

Abstract The tensile and fatigue crack growth behavior of two FeAl alloys, FA-129 and FAP-Y, are described. FA-129 is an ordered intermetallic while FAP-Y is disordered. The intermetallic is embrittled by both laboratory air and by hydrogen gas, while FAP-Y is embrittled only by hydrogen gas. Fractographic features as well as dislocation substructures for both alloys are described. Oxygen is found to provide the highest ductility for FA-129 over a range of test temperatures. The fatigue crack growth rate of FA-129 is influenced by both the type of long-range order and by the test temperature, with the maximum growth rate noted at room temperature. The mechanism of embrittlement by various environments is discussed.

Effects of heat treatment on microstructure and texture in Ti24at%Al-11at%Nb

Abstract Hot cross-rolled sheets of Ti-24at.% -11at.%Nb are subject to heat treatments in the α 2 + β phase field and in the β phase field. Changes in microstructure are monitored by metallography while crystallographic texture is evaluated by pole figure analysis. Annealing in the α 2 + β regime produces equiaxed α 2 grains and intergranular β with little modification to the starting texture. Annealing in the β regime results in a Widmanstatten structure whose lamellar spacing depends on the cooling rate. A much sharper texture in the α 2 phase is observed. The sharpness of the β phase texture correlated with the sharpness of the α 2 texture. Hardness measurements indicate that the highest strength occurs in the fine Widmanstatten structure where rapid cooling from the β regime produces the greatest amount of microstructural refinement.

Dislocation substructure in NiAl single crystals cyclically deformed at ambient temperature

Abstract Dislocation structures have been characterised in cyclically deformed NiAl single crystals tested at room temperature. The primary slip system {110} was found to be in operation for the ‘soft’ orientation. The dislocation structure was found to have a high density of dipoles and point defect clusters. Dislocation cross grids that accommodate the misfit between PSBs (persistent slip bands) and the matrix were observed. Possible cyclic hardening mechanisms at room temperature are discussed. The density of the dipoles was not uniform and was found to be modulated on the slip plane, unlike Ni 3 Al which shows a homogeneous distribution. Computer simulation of the microstructure has been carried out, based on the mechanism of dipole diffusion in a stress gradient.