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.     

Microstructural studies of a Ni-W directionally solidified eutectic composite

The influence of heat treatment on the microstructure of a Ni-45.5 wt pct W directionally solidified eutectic composite has been studied. Four types of microstructural conditions have been examined using light and thin foil electron microscopy techniques. The as-grown composite consists of W fibers in a matrix which contains some WNi₄ precipitates. Quenching the composite from above the peritectoid temperature results in W fibers in a Ni-W solid solution matrix. Subsequent aging results in precipitation of coherent WNi₄. Long time aging at temperatures just under the peritectoid temperature promotes the peritectoid reaction leading to envelopment of the W fibers by a layer of WNi₄. The envelopment reaction is accompanied by a change from faceted to round fiber morphology. The presence of W fibers in the Ni-W solid solution influences aging in two ways when compared to a solid solution alloy. First, the fiber/matrix interfaces act as dislocation sources on quenching; these dislocations are absorbed into the WNi₄ precipitate/matrix interfaces leading to early loss of coherency. Second, the WNi₄ precipitates undergo a preferential coarsening due to elastic interaction between the fibers and the WNi₄ precipitates; this leads to a mosaic structure, the occurrence of which is uniquely dependent on the presence of W fibers.     

Microstructural studies of a Ni-W directionally solidified eutectic composite

The influence of heat treatment on the microstructure of a Ni-45.5 wt pct W directionally solidified eutectic composite has been studied. Four types of microstructural conditions have been examined using light and thin foil electron microscopy techniques. The as-grown composite consists of W fibers in a matrix which contains some WNi₄ precipitates. Quenching the composite from above the peritectoid temperature results in W fibers in a Ni-W solid solution matrix. Subsequent aging results in precipitation of coherent WNi₄. Long time aging at temperatures just under the peritectoid temperature promotes the peritectoid reaction leading to envelopment of the W fibers by a layer of WNi₄. The envelopment reaction is accompanied by a change from faceted to round fiber morphology. The presence of W fibers in the Ni-W solid solution influences aging in two ways when compared to a solid solution alloy. First, the fiber/matrix interfaces act as dislocation sources on quenching; these dislocations are absorbed into the WNi₄ precipitate/matrix interfaces leading to early loss of coherency. Second, the WNi₄ precipitates undergo a preferential coarsening due to elastic interaction between the fibers and the WNi₄ precipitates; this leads to a mosaic structure, the occurrence of which is uniquely dependent on the presence of W fibers.     

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     

Tensile properties of directionally solidified Al-Si eutectic

Al-Si eutectic alloys have been directionally solidified in a horizontal resistance heated furnace. The temperature gradient, G, ahead of the solid/liquid interface was kept fairly constant at 80°c/cm, while the growth rate,R, was varied between 0.28 and 131 μm/s. Microstructural studies show a definite alignment of the rod-like Si at low growth rates. At growth rates higher than 14 /μm/s the microstructures appear irregular, although some preferential orientation of the Si rods parallel to the growth direction can be observed. Tensile tests show higher values in both yield and ultimate strengths than was found in previous investigations, most likely due to the careful sample preparation prior to testing in the present work. The yield strength increases with the growth rate up to about 14 /μm/s, and only a slight increase is observed at higher rates. The ultimate strength also increases with the growth rate, but shows less tendency toward saturation. Superposition of hardness and yield data show excellent correlation, while comparison between hardness and ultimate strength shows higher hardness than ultimate values with decreasing growth rates. Formerly Metallurgist with Materials and Molecular Research Division, Lawrence Berkeley Laboratory.     

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.     

The mechanical properties of directionally solidified aluminum-based eutectic castings

Abstract

The mechanical properties for directionally solidified Al-Al₃Ni, Al-Si, and Al-Al₂Cu eutectic castings have been determined and it is shown that the properties of castings (solidified at approximately 80 cm/hour) compare favourably to those obtained for slowly solidified eutectics (solidified at approximately 2 cm/hour). The strengths of Al-Al₂Cu and Al-Si eutectic castings are higher than those obtained for slowly solidified alloys while the strengths for the Al-Al ₃Ni eutectic castings are somewhat lower than for the slowly solidified alloys. The Al-Al₃Ni and Al-Al₂Cu cast eutectics essentially maintain their yield strength up to 300°C whereas the yield strength of the Al-Si eutectic decreases continuously with increasing temperature. It is concluded that the high strengths reported result from a combination of the fine micro-structure obtained from the relatively rapid solidification rate and by strengthening of the intermetallic phase.

Réesumé

Les auteurs ont déterminé les propriétés mécaniques d'alliages eutectiques Al-Al₃Ni, Al-Si et Al-Al₂;Cu solidifiésdirec-tionnellement. Ils montrent que les propriétés des coulées (solidi-fiées à environ 80 cm/h) se comparent favorablement à celles d'eutectiques solidifiés lentement (à environ 2 cm/h). Les résistances mécaniques des coulées eutectiques de Al-Al ₂Cu et Al-Si sont plus élevées que celles obtenues pour des alliages solidifiés lentement tandis que les résistances mécaniques des coulées eutectiques Al-Al₃Ni sont légèrement plus basses que celles des alliages solidifiés lentement. Les limites d'élasticité des alliages eutectiques coulés d'Al-Al₃Ni et Al-Al₂;Cu demeurent élevées jusqu 'à des tempéraptures de 300°C, tandis que lalimite d'élasticité de l'alliage eutec tique Al-Si diminue continuellement quand la température croft. Les auteurs conc1uent que les bonnes propriété s mécaniques obtenues ré sultent de la combinaison de l'effet de la microstructure fine produite par le taux de solidification relativement élevéet de l'effet de fibres produit par la phase intermétallique.     

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.     

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.     

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

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.     

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.     

Creep regimes for directionally solidified Al-Al3Ni eutectic composite

Creep characteristics of Al-Al₃Ni eutectic composites directionally solidified at 2.2 × 10^-2 mm/s were determined over a wide range of stress and temperature. Four distinct regions of creep were observed. The rate controlling mechanisms for the four regions appear to be high-temperature dislocation climb in the Al matrix, low-temperature climb in the Al matrix, boundary sliding, and a mechanism involving deformation of the Al₃Ni fibers. Creep rates of the Al-Al₃Ni composite are several orders of magnitude smaller than for pure Al, and apparently, in the regions where deformation of the Al matrix is rate controlling, only a very small fraction of the matrix is deforming during creep of the composite. Formerly Graduate Student, Department of Mechanical and Industrial Engineering, University of Manitoba     

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.     

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     

Anisotropic yielding behavior of a fiber-reinforced,directionally solidified eutectic alloy

A simple experimental procedure is outlined which is capable of providing information re-quired to describe multiaxial yielding and flow behavior anisotropic materials that have a rotational symmetry. By applying the anisotropic plasticity theory that has been de-veloped recently, it is shown that a total of four simple tension and compression tests is sufficient to describe the yield surface of the highly anisotropic material. The experi-mental data obtained with a directionally solidified (DS), nickel-based, tantalum carbide reinforced eutectic alloy is shown to be in excellent agreement with the theory. Calcu-lated internal parameters based on the plasticity theory are used to interpret the role of microstructural features that contribute to anisotropic yielding behavior.     

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.