Growth crystallography and lamellar to rod transition in directionally solidified Nb-Nb2C eutectic composites

The crystallographic relationship displayed by the niobium and niobium carbide <Nb₂C> phases in an aligned eutectic sample with a lamellar carbide morphology is lamellar interface ∥ {110}_NB ∥ (001)_{Nb 2C} growth direction ∥<112>_NB ∥ [010]Nb₂C or [1-20]_{Nb 2}C and for the rod-like carbide morphology rod interface (major axis) ∥{110}_Nb ∥ (001)_{Nb 2}C growth direction 11(H2)Nb II l010]Nb.,c or [210]NB₂C.     

The morphology and crystallography of directionally solidified pb-sn eutectic alloys

Directional solidification of Pb-Sn eutectic alloys at high temperature gradients has shown that two distinct eutectic morphologies occur, a regular lamellar structure and a wavy lamellar structure, termed degenerate. An electron channelling technique was used to characterize the crystallography of the two morphologies. The degenerate grains grow in advance of the regular grains and are the preferred growth morphologies in spite of the fact that coarsening experiments revealed that the degenerate grains possessed a higher lamellar interface energy. This result conflicts with the Zener model of eutectic growth. A possible rationalization of this conflict based on the anisotropy of thermal conductivity in Sn is presented.     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Metallurgical and Materials Transactions A - Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature...     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology, as well as the behavior of the eutectic spacing and undercooling, suggest that the resultant microstructure is governed by two different growth processes. That is, at low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, λ²V = constant, where λ is the eutectic spacing andV is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The experimental findings are interpreted in the light of recently developed models for the columnar to equiaxed transition and for irregular eutectic growth.     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology, as well as the behavior of the eutectic spacing and undercooling, suggest that the resultant microstructure is governed by two different growth processes. That is, at low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, λ²V = constant, where λ is the eutectic spacing andV is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The experimental findings are interpreted in the light of recently developed models for the columnar to equiaxed transition and for irregular eutectic growth.     

The thermal expansion of the directionally solidified AI-CuAI2 eutectic

Alloys of Al-CuAl₂ eutectic composition were prepared from 99.999 pct pure materials and directionally solidified in a temperature gradient of about 45 °C/cm at different growth ratesR. The λ²R= constant relation was verified and lamellar spacings of 7.5, 3.5, 2.6, 1.8 and 1.4 μm were obtained. Dilatometer specimens were machined with axes aligned in the principal lamellae coordinate directions. Thermal expansion was measured by standard dilatometry (Cu standard) using a set point program cycling between room temperature and 500 °C. Thermal expansion of the directionally solidified Al-CuAl eutectic is greatest in the growth direction (in the plane of the lamellae), least in the tranverse direction (orthogonal to the growth direction in the plane of the lamellae) and intermediate in the direction normal to the lamellae. The most significant finding of the study is that the thermal expansion increases with decreasing lamellar spacing between limits defined approximately by the thermal expansion of the CuAl₂ phase alone and the predicted thermal expansion of an isotropic elastic model of the composite.     

Monotonic and fatigue deformation of Ni-W directionally solidified eutectic

Unlike many eutectic composites, the Ni-W eutectic exhibits extensive ductility by slip. Furthermore, its properties may be greatly varied by proper heat treatments. Here results of studies of deformation in both monotonic and fatigue loading are reported. During monotonie deformation the fiber /matrix interface acts as a source of dislocations at low strains and an obstacle to matrix slip at higher strains. Deforming the quenched-plus-aged eutectic causes planar matrix slip, with the result that matrix slip bands create stress concentrations in the fibers at low strains. The aged eutectic reaches generally higher stress levels for comparable strains than does the as-quenched eutectic, and the failure strains decrease with increasing aging times. For the composites tested in fatigue, the aged eutectic has better high-stress fatigue resistance than the as-quenched material, but for low-stress, high-cycle fatigue their cycles to failure are nearly the same. However, both crack initiation and crack propagation are different in the two conditions, so the coincidence in high-cycle fatigue is probably fortuitous. The effect of matrix strength on composite performance is not simple, since changes in strength may be accompanied by alterations in slip modes and failure processes.     

Monotonic and fatigue deformation of Ni-W directionally solidified eutectic

Unlike many eutectic composites, the Ni-W eutectic exhibits extensive ductility by slip. Furthermore, its properties may be greatly varied by proper heat treatments. Here results of studies of deformation in both monotonic and fatigue loading are reported. During monotonie deformation the fiber /matrix interface acts as a source of dislocations at low strains and an obstacle to matrix slip at higher strains. Deforming the quenched-plus-aged eutectic causes planar matrix slip, with the result that matrix slip bands create stress concentrations in the fibers at low strains. The aged eutectic reaches generally higher stress levels for comparable strains than does the as-quenched eutectic, and the failure strains decrease with increasing aging times. For the composites tested in fatigue, the aged eutectic has better high-stress fatigue resistance than the as-quenched material, but for low-stress, high-cycle fatigue their cycles to failure are nearly the same. However, both crack initiation and crack propagation are different in the two conditions, so the coincidence in high-cycle fatigue is probably fortuitous. The effect of matrix strength on composite performance is not simple, since changes in strength may be accompanied by alterations in slip modes and failure processes.     

Microstructure and crystallography of unidirectionally solidified Ni-W eutectic alloy

A Ni-W eutectic alloy was subjected to a process of unidirectional solidification (UDS) by the Bridgman-Stockbarger technique. Three phases were identified by transmission electron microscopy, namely: W fibers, a solid solution matrix of W in Ni, and Ni₄W precipitates of the Dla structure in the matrix. The growth axis of the W-fibers was found to be <111> and the orientation relationship between them and the Ni(N) matrix was identified as the Bain type, so that (100)_bcc ‖ (100)_fcc. The shape of the Ni₄W precipitates varies from equiaxial at high solidification rates to elongated plates at low rates. The orientation relationship between the precipitates and the matrix is the same for all solidification rates. The microstructure of specimens subjected to creep deformation was studied and the deformation modes were identified. These include dislocations and microtwins that originate mainly at the boundary between the Ni(W) matrix and the W-fibers. In some specimens the creep test was carried out after solution treatment at 1030 °C followed by quenching, which resulted in a Ni(W) matrix reinforced with W-fibers without Ni₄W precipitates. The microstructural changes during this creep process and the fracture surface were studied by SEM and TEM.     

Carbide reinforcement in two directionally solidified alloyed nickel eutectic alloys

Two directionally solidified carbide-reinforced alloyed nicel eutectics were evaluated; an alloy consisting of monocarbide fibers in a single phase matrix and one containing monocarbide fibers in a two-phase γ-γ′ matrix. The mechanical properties and microstructures of these alloys are compared to those of two directionally solified alloys having the same nominal matrix compositions, but not containing carbide fibers. The calculated strengths of the monocarbide fibers in the γ′-containing eutectic alloy are 1,400,000 psi (9650 mn/m²) and 243,000 psi (1680 mn/m²) at room temperature and 1000°C, respectively, while those in the single phase γ matrix eutectic at the same temperatures are 590,000 psi (4060 mn/m²) and 298,000 psi (2050 mn/m²). At room temperature, the lower strength of fibers from the γ matrix alloy is believed to result from stress concentrations induced by the presence of growth facets on the fibers. The lower apparent strength at 1000°C of fibers from the γ′-containing eutectic alloy is related to nucleation of needles believed to be M₂₃C₆ on the monocarbide fibers during deformation. These needles appear to act as stress raisers and cause early failure of fibers.     

Structure and mechanical properties of the directionally solidified Al-Cu-Mg eutectic

The ternary eutectic is located at Al-33.1 pct Cu-6.25 pet Mg and consists of an aluminum-rich phase, CuAl₂, and CuMgAl₂. In this work its morphology, crystallography, interfacial dislocation arrangements, and mechanical properties have been studied. The scale of the phases is proportional to the negative one-half power of the growth rate. The phases have preferred growth directions and, in many cases, preferred interfacial planes. Interfaces between the aluminum-rich and the CuMgAl₂ lamellar phases are semicoherent, having a/2 〈110〉 {111} aluminum misfit dislocations spaced 300Å apart. The observed dislocations are imaged only in the aluminum and appear to be aluminum-phase slip dislocations. The deformation of the composite to failure in tension is macroscopically elastic, and the failure strength depends on growth rate. It is shown that Griffith brittle fracture theory may be applied to the failure process, with cracks developed in the CuMgAl₂ phase during loading acting as the required Griffith cracks. A specific failure mechanism is proposed and related to observations of the fracture surface.     

The anisotropy of deformation and fracture in a directionally solidified Ni/Ni3Al-Ni3Cb lamellar eutectic alloy

The orientation dependence of deformation and fracture modes was investigated for a directionally-solidified Ni−Ni₃Al−Ni₃Cb lamellar eutectic alloy (Ni-20 wt pct Cb-2.5 wt pct al-6.0 wt pct Cr) using optical and transmission microscopy to examine tensile and compression specimens tested at temperatures below the softening point of the δ (Ni₃Cb) reinforcing phase (∼1050 K). In this temperature range there is a large difference between longitudinal and transverse tensile ductibility (>5 pct longitudinalvs<1 pct transverse). No single preferred fracture path (such as interfacial delamination) could be found to account for the low transverse tensile ductility. Analysis of the δ twinning geometry, however, indicated that the twinning strains for twins of the type {211}, which operate copiously in longitudinal tension, are negative in most transverse orientations, with Schmid factors being very low (<0.013) in the limited range of transverse orientations where {211} twin strains are positive. Examination of transverse tension test specimens broken at 1033 K confirm the absence of {211} twins, with only limited {011} twinning being found in selected grains, leading to the conclusion that the relatively low transverse tensile ductility of the eutectic results from the very limited number of deformation systems which operate in the δ reinforcing phase below the softening temperature.     

Fatigue crack propagation in two classes of directionally solidified eutectic alloys

Fatigue crack propagation rates were measured in two classes of directionally solidified eutectic alloys under isothermal, stress-controlled cycling at temperatures of 298 to 1311 K. Alloy 73C, a cobalt-base material reinforced by fibers of Cr₇C₃, and γ/γ′ + δ, a nickel-base alloy reinforced by lamellae (platelets) of Ni₃Cb, were grown at solidification rates of 1 and 25 cm/h to achieve significant differences in interfiber and interlamellar spacing (λ). No influence of the spacing of the reinforcing phase on crack growth rates were found for either alloy. In addition, chromium level and perfection of the microstructure had a minimal effect on propagation rates for γ/γ′ + δ. The independence of the fatigue crack growth rates on λ may be associated with the ratio of the cyclic plastic zone diameter at the crack tip to λ. In all instances, this ratio was estimated to be greater than one for the test conditions employed. At the lower temperatures, crack propagation rates in γ/γ′ + δ were up to two orders of magnitude lower than those in Alloy 73C due to crack deflection at interlamellar interfaces and grain boundaries which lowered the effective stress intensity range for opening mode cracking. Formerly of Pratt & Whitney Aircraft     

Fatigue crack propagation in two classes of directionally solidified eutectic alloys

Fatigue crack propagation rates were measured in two classes of directionally solidified eutectic alloys under isothermal, stress-controlled cycling at temperatures of 298 to 1311 K. Alloy 73C, a cobalt-base material reinforced by fibers of Cr₇C₃, and γ/γ′ + δ, a nickel-base alloy reinforced by lamellae (platelets) of Ni₃Cb, were grown at solidification rates of 1 and 25 cm/h to achieve significant differences in interfiber and interlamellar spacing (λ). No influence of the spacing of the reinforcing phase on crack growth rates were found for either alloy. In addition, chromium level and perfection of the microstructure had a minimal effect on propagation rates for γ/γ′ + δ. The independence of the fatigue crack growth rates on λ may be associated with the ratio of the cyclic plastic zone diameter at the crack tip to λ. In all instances, this ratio was estimated to be greater than one for the test conditions employed. At the lower temperatures, crack propagation rates in γ/γ′ + δ were up to two orders of magnitude lower than those in Alloy 73C due to crack deflection at interlamellar interfaces and grain boundaries which lowered the effective stress intensity range for opening mode cracking.     

Crystallography and morphology of as-grown and coarsened Al-Al3Ni directionally solidified eutectic

The structure of the rodlike Al-Al3Ni eutectic directionally solidified vertically at rates ranging from 0.8 to 36.5 cm/h was studied in as-grown and coarsened conditions. The observed crystallographic relations are growth direction ∥ [010]_A13_Ni ∥ [011]_A1or [21l]_A1 and (111)_A1 ∥ (102)_A13_Ni in the zone of the growth direction. The facet structure of as-grown rods is most pronounced at low growth rates. Elevated temperature exposure of the composite produces fiber coarsening and improved definition of the facets, both effects becoming more rapid with increasing growth rate. The facets themselves are, in many cases, of such crystallographic orientations that nearly equal atomic densities occur on both sides of the boundary. We conclude that, although faceting occurs to a marked degree, the related shape anisotropy is probably of second-order importance in calculations of coarsening rates. However, the presence of facets does preclude fiber foreshortening during coarsening.     

The processing and properties of heavily cold worked directionally solidified Ni-W eutectic alloys

Certain two phase metallic alloys display impressive strengths following extensive deformation processing. Provided an appropriate phase morphology and/or texture is developed initially, somewhat surprising combinations of metals (e.g., copper-chromium) can be so processed. Thus this scheme offers the possibility for developing high strength metal matrix composites at a comparatively low price. In the work described, we consider another material combination—the Ni-W directionally solidified eutectic—as a candidate for this interesting class of material. This alloy can be cold worked to true deformation strains of four. The tensile strengths of alloys so deformed are impressive (2470 MPa), but so are those of the cold worked nickel-tungsten solid solution which is a component of the eutectic. Based on the work-hardening behavior of tungsten and on a recently advanced model which qualitatively explains the strengths of heavily cold worked two phase metals, it is argued that further deformation processing of these alloys would lead to substantially higher strengths. Estimates on the fracture toughness of the cold worked eutectic are made from tensile properties. Estimated toughnesses are remarkably high and point to the possibility that this process can produce high strength-high toughness metallic materials to a degree not possiblevia conventional processing. D. G. KUBISCH, formerly Graduate Student, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI