The mechanical properties of in-situ composites intended for use as gas turbine materials are reviewed. Particular emphasis is placed on the current understanding of strengthening and fracture mechanisms of these materials. The need to modify current methods of data representation, that have been developed for more conventional materials, in order to improve property extrapolation and design procedures are highlighted.

Superalloy eutectics strengthened with monocarbide whiskers are highly anisotropic in strength. Mechanical behavior transverse to the growth axis is governed largely by the fracture resistance of the matrix grain boundaries. While the matrices of these eutectics are essentially columnar grained superalloys, there are significant differences between d.s. superalloys and superalloy-monocarbide eutectics. These differences arise, in part from different solidification processes and from the alloying limitations in eutectic systems.

A challenge exists for wider application of low cost iron-base alloys in the extreme conditions of high temperature, hot corrosion, and high stress experienced in gas turbine engines. In response to this challenge the constitution of the quaternary, Fe-Cr-Mn-C, and to a lesser extent the quinary, Fe-Cr-Mn-Al-C, systems were examined for in situ composite alloy candidates. Multivariant eutectic compositions, determined from phase equilibria, were directionally solidified to produce aligned composites consisting of M7C3 carbides within a gamma iron matrix. The composition and lattice parameters of the carbide and matrix phases were determined to establish their respective stabilities. Stress rupture tests in the direction of phase alignment for certain compositions (e.g., Fe-20 wt% Cr-lO wt% Mn-3.2 wt% C) indicated strength values comparable to cast nickel-base superalloys and exceeded those of the strongest iron-nickel superalloys developed for automotive turbines. Results of cyclic sulfidation testing at 9000 indicated a balance of Cr and Al content to be important to the achievement of outstanding surface stability.

This paper reports a study of off-axis tensile properties of γ/γ'-αMo Ds eutectic and the dynamic fracture process directly observed with a scanning electron microscope. If the loading direction deviates from the Mo fiber growth direction, the tensile strength and ductility decrease gradually. During the loading process, two types of cracking are observed: one is cracking of the fiber, and the other is delamination along the fiber-matrix interface. In plastic deformation, slip can pass through both the matrix and the fiber. As a rule, the thicker fibers are cracked earlier than the thinner ones, because they have less defects and better ductility. Delamination is the main source of final fracture.

The low cycle fatigue behavior at 25°C and 825°C of three advanced nickel-base eutectics is described. Fatigue lives are shown to obey a linear relation with plastic strain range (Coffin-Manson relation) but lives are much lower than are observed for conventional metals and alloys. Cyclic hardening and softening were observed in each alloy at 25 °C; however, this behavior differs from the classical saturation behavior observed with isotropic materials.

A study of fatigue crack growth behavior in four directionally solidified eutectic composites has been carried out in air at room temperature and at 870°C (1600 °F). The alloys investigated included two cobalt-base alloys, CoTaC and CoTaC+Cr, and two nickel base alloys, γ′-Mo and γ/γ′-δ. Fatigue crack growth was studied in directions parallel and perpendicular to the solidification direction. It was found that the addition of chromium to the CoTaC alloy strengthened the interface and promoted trans-fiber crack growth. In addition, the effect of interlamellar spacing on the rate of fatigue crack growth was examined for the γ/γ′-δ alloy. A reduction in interlamellar spacing reduced the crack growth rate. The growth rates are related to the corresponding stress intensity factors and are discussed in the light of prior investigations reported in the literature.

A nickel base-TaC eutectic alloy consisting of 53 wt % Ni, 11 W, 3 Re, 5 Co, 3 Mo, 2 Al, 21.67 Ta and 1.33 C was directionally solidified at rates from 0.32 to 3.8 cm/hr. Samples were taken from the regions of aligned TaC fibers and tested in tension at 1000°C and in stress-rupture. The fiber density increased from 1×106 fibers/cm2 at 0.32 cm/hr to 27×106 fibers/cm2 at 3.8 cm/hr. The stress-rupture life at 927°C and 275.6 MPa ranged from 4 hrs for samples solidified at 0.32 cm/hr to 4187 hrs for the sample solidified at 2.54 cm/hr at which time the test was terminated. Examination, by transmission electron microscopy, of the samples tested at 1000°C in tension, to just the beginning of fiber breakage, revealed a change in the mode of deformation of the nickel-base alloy matrix with increasing fiber density. It was also seen that the TaC fibers deformed by two mechanisms; by slip and by the formation of stacking faults.

It appears that tailoring the alloy composition to allow for higher solidification rates may be a more effective strengthener than alloying the matrix for high strength.

For the Ni-base TaC eutectics, it has been shown previously that carbide volume fraction is a strong function of Ta/C ratio. Now in a number of Ni,Cr-TaC and Ni,Cr,Al-TaC alloys, Cr has been observed to have only a small influence on volume fraction of carbide, while Al has a major influence acting to decrease the volume fraction. The Al present in the matrix causes the phase equilibrium to approach that of a much greater Ta/C ratio. This analysis has been extended to more complex NiTaC alloys as well. For the simple Ni-Cr-Ta-C alloys, creep behavior has been studied as a function of volume fraction of carbide.

Other carbide systems have been evaluated as well, including TiC and NbC. A general understanding of volume fraction differences between these systems and TaC can be reached by consideration of ideal solution behavior and the appropriate phase diagrams.

Groundwater circulation is in the long term probably the most important mechanism for the transport of radionuclides from deep underground repositories to the biosphere. The fate of leached long-lived nuclides, such as the actinides, is of particular interest to repository safety analysis. Both experimental and theoretical studies should lead to a comprehensive description of their behaviour in the geosphere.

Elastic and plastic properties of in situ Cu-based composites with Nb, V, and Fe filaments are reviewed. The evidence is presented for a pronounced size dependence of both the ultimate tensile strength and the Young's moduli. In composites with the smallest filaments (d∼50–200Å) and filament densities as high as 1010/cm2 dislocation density reaches values of 1013 cm/cm3. The yield stress of these samples increases dramatically over the predictions based on the “rule of mixtures” and their ultimate tensile strength approaches the estimated theoretical strength of the material (∼2.7GPa). The observed decrease of Young's modulus as a function of inverse wire diameter in the as-drawn composites is attributed to lattice softening due to high density of extended lattice defects. Upon annealing, Young's modulus increases by as much as 100% and exceeds the maximum values calculated from bulk elastic constants. Possible mechanisms leading to modulus enhancement and to related changes in magnetic and superconducting behavior of in situ composites are discussed.

Deformation by creep or by stress-relaxation involve similar thermally activated phenomena. In particular our microstructural observations on the Al-Al2Cu lamellar composite illustrated the importance of the interphase accommodation mechanisms, specially movements of linear defects at the phase boundaries.

Our experimental results have shown that the asymptotic stress measured in stress-relaxation tests takes the same value as the threshold stress limiting the steady state creep domain. From this finding we develop a simple Bingham rheological model in which a temperature dependent threshold stress is combined with a generalized Dorn viscosity law. The model correlates satisfactorily the results of both steady state creep tests and the results of stress-relaxation tests.

The calculated activation energy and stress exponent are compared with the values deduced from physical models of deformation accommodation through grain boundary sliding. The preexponential factor of the viscosity law is discussed with reference to its dependence on the dislocation substructure.

The paper summarizes the results of a series of investigations carried out for a γ/γ'-α(Ni-Al-Mo) eutectic. The subjects include a thermal analysis of the composite growth in a high temperature gradient provided by a fluidized bed cooling, a study of defect formation such as the banding structure at unstable thermal conditions, Tem observation and phase identification for micro-phases existed in an as-growth state as well as phases produced after various quench and aging treatments, and an evaluation of the composite thermal stability in terms of the microstructures and the high temperature strength. The fluidized bed placed beneath a melting furnace was proved to establish a temperature gradient equivalent to the liquid metal cooling, and to promote a stable steady growth of the composites without bandings. Quenching the eutectic above a peritectoid temperature diminished the blocky γ'. The subsequent aging treatments induced small Mo platelets above 830°C and blow the temperature a metastable Ni2Mo particle precipitation in the γ phase causing appreciable composite hardening. The creep rupture strength, however, was reduced by the heat treatment due to δ-NiMo phases precipitated during the test in consumption of the Mo fibers.

In the aligned Ni-MC eutectic alloys being developed for jet engine blade applications, the fibers are considered to be Tac, Tic, etc. In fact, these fibers will contain Cr, Mo, W and possibly other elements as substitutions for the “M” constituent, and often will not be stoichiometric. Quantitative chemical analysis of fibers is difficult because of their size. However, lattice parameter measurements can be used to learn much a out the carbide chemistry and stoichiometry. Results will be described in detail for fibers extracted from simple Ni-TaC, Ni,Cr-TaC and Ni,Cr,AI-Tac systems. In addition, experiments on mixed (Ta,V)C and (Ta,Ti)C carbides will be discussed. Carbide lattice parameters can be understood in terms of the free energies of formation of the various carbides.

The effects of various alloy additions to the microstructures and properties of the γ-γ'-Cr3C2 in-situ composite have been investigated. Combinations of platinum and Yttrium prevent the formation of an additional phase that occurs when Yttrium alone is added; the phase morphology is unaffected by rhenium and ruthenium dopants.

Combined quenching studies and differential thermal analysis show that phase alignment at high growth rates in this alloy is not due to plane front solidification but to highly branched carbide growth ahead of the melt/ matrix interface. This mechanism allows good phase alignment to be achieved over a wide composition range. Tests on six alloy casts to commercial specifications indicate retention of, and often improvement to, the microstructures and properties obtained with laboratory control.

The results of an exploratory study of eutectics based on Ni-25Cr–15Ti (wt %)and Co-10Cr-32Mo compositions will be presented. These alloys were selected from a total of ten different eutectic systems, all having melting points above 1200°C, a specific gravity less than 9,000 kg/m3, and a parabolic oxidation rate below 10−7 g2 · cm−4 · s−1 at 1150°C as reported by Haour, Mollard, Lux, and Wright on work done at Battelle-Geneva Research Centre. The Ni-25Cr-15Ti eutectic appeared to have a high volume fraction of ductile chromium fibers in a brittle Ni3Ti matrix. This alloy although the strongest of the six alloys that have been tested from the group reported by Battelle is still not competitive with advanced superalloys. Its tensile strength at 1093°C was 227 MPa2 and its elongation was 14%. The Co-10Cr-32Mo eutectic had the highest melting point (1340°C) of the alloys screened by Battelle. It appeared to solidify in a basically bladelike or lamellar structure, consisting of the intermetallic μ phase in a metallic Co (Cr) matrix. The strongest of the Co-Cr-Mo eutectics tested had a tensile strength of 164 Mpa and an elongation of 15% at 1093°C. The correlation observed between the strengths of the Ni-Cr-Ti and Co-Cr-Mo alloys, compositional variations, and their microstructures will be discussed.

Attempts were made to directionally solidify lanthanum hexaboride as uniformly spaced rods, less than one micron in diameter, in a boron matrix. Lanthanum hexaboride and boron powders were combined in a near eutectic mixture, formed into right circular cylinders and melted by RF induction heating in a pure hydrogen atmosphere using a modified internal zone melting technique.

Ball milling, uniaxial cold pressing, isostatic pressing and hot pressing were investigated as possible means of increasing the sample density before melting in the RF induction furnace. RF coupling was achieved directly in the hot pressed pellet, and indirectly through the use of a molybdenum preheater in the uniaxially cold pressed pellets. Internal zone melting and conventional crucible melting in various materials were tried.

Directional growth was observed in the pellets that were uniaxially cold pressed and preheated with a molybdenum tube sleeve. Lanthanum hexaboride fibers with an ℓ/d ratio of about ten to one and other forms of aligned microstructures were observed in selected areas of the pellet.

Thin slices of directionally solidified, fibrous composite, monotectic alloy samples have been selectively etched to remove the fibrous phase, so to produce fine sieves of filters having pore diameters ≲10μm. Alloys in systems Al-In, Al-Bi and CuAl-Pb have been examined using as etchants aqueous nitric acid (Al-In, Al-Bi) and aqueous citric acid (CuAl-Pb) without applied anode voltages. These alloys and etchants illustrate examples in which fiber and matrix are attacked to different extents (Al-In), the phase interface is selectively attacked (Al-Bi) or only the fibers are attacked (CuAl-Pb). Hole sizes and perfection are compared for etching times, concentrations and anode voltage.

Laser melting of the surface of CuAl-Pb samples has been used to screen or frame areas of the composite microstructure and also shows how local melting of the fibrous Pb phase can be used to assess the heat affected zone.

As the minimum dimension of electronic circuits is reduced below one micron, there has been increased interest in developing techniques for fabricating submicron structures. Pb-Sn, Cd-Pb and Al-Al 2Cu eutectic thin films were directionally solidified at rates between .0015 cm/sec and .15 cm/sec with both a lamp and a laser heat source to fabricate fault free submicron periodic structures. A transition from lamellar to cellular structures with increasing solidification rate was observed with a thermal gradient of 200°C/cm applied with the lamp heat source. Cells did not form with the increased thermal gradient produced by the laser heat source, 8000° C/cm. Spacings as fine as .1 micron were observed in the Cd-Pb eutectic. A maximum theoretical growth rate was estimated which corresponds to a spacing of 80A.

Glasses in the potassium silicate system were directionally crystallized in a temperature gradient furnace. A composition of approximately 40 wt % K2O and 60 wt % SiO2 was used. Quenched glass samples were placed in alumina crucibles with K2O·2SiO2 seed crystals on the bottom surface. The crucible was translated from the hot zone to the cold zone of the furnace. A thermal gradient of 14°C/mm was maintained. Sample height was about 6 mm and translation rates were about 5 mm/h, 1.33 mm/h and 0.83 mm/h. Well-aligned crystals were observed to grow from the bottom of the crucible. Individual crystals were about 0.07 mm wide and extended the length of the crucible parallel to the thermal gradient. The degree of alignment was found to depend on initial temperature and translation rate. The samples have been studied using primarily optical microscopy.

A heating stage and an advanced video recording system were used in the 1 MeV electron microscope of CEN Grenoble. This allowed us to observe various aspects of eutectic solidification. These observations include regular lamellar solidification with faceted or non faceted solid-liquid interfaces, change of lamellar spacing with solidification rate, non reciprocal heterogeneous nucleation of eutectic solidification.

We discuss them with respect to the classical theories and introduce a new eutectic nucleation mechanism based on surface dendrites.