Showing papers in "Metallurgical transactions. A, Physical metallurgy and materials science in 1991"

On the rate of dendrite arm coarsening

Abstract: Approximate but simple expressions for the rate of isothermal coarsening of secondary dendrite arms are presented. Their derivation differs from previous work in that (a) focus is placed on growing dendrite arms, which simplifies the derivation, and (b) the effect of a finite volume fraction solid is taken into account. Expressions for isothermal coarsening are then integrated to predict the final dendrite arm spacing in a cast dendritic microstructure. Coarsening rates predicted for Al-Cu alloys, succinonitrile, and Fe-26 wt pct Ni agree with published experimental data within a factor of 2 which is within the range of variation of experimental data.

The effect of copper, chromium, and zirconium on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloys

Abstract: The present study evaluates the effect of the systematic variation of copper, chromium, and zirconium contents on the microstructure and mechanical properties of a 7000-type aluminum alloy. Fracture toughness and tensile properties are evaluated for each alloy in both the peak aging, T8, and the overaging, T73, conditions. Results show that dimpled rupture essentially characterize the fracture process in these alloys. In the T8 condition, a significant loss of toughness is observed for alloys containing 2.5 pct Cu due to the increase in the quantity of Al-Cu-Mg-rich S-phase particles. An examination of T8 alloys at constant Cu levels shows that Zr-bearing alloys exhibit higher strength and toughness than the Cr-bearing alloys. In the T73 condition, Cr-bearing alloys are inherently tougher than Zr-bearing alloys. A void nucleation and growth mechanism accounts for the loss of toughness in these alloys with increasing copper content.

Lorentz Force Infiltration Of Fibrous Preforms

Abstract: A new process for the production of metal matrix composites, whereby molten metal is forced into the interstices of a fibrous preform using electromagnetic body forces, is presented. These forces are created by subjecting the molten matrix to a concentrated transient magnetic field which, in turn, induces intense eddy currents in the melt. This gives rise to Lorentz forces which propel the metal into the preform. Equations governing the mechanics of Lorentz force infiltration of an axisymmetric preform surrounded by molten metal are solved numerically. A finite difference algorithm is applied to solve Maxwell's equation of electromagnetic field propagation and to determine the flux density as a function of radial position. The resulting Lorentz force is then calculated and balanced with the inertial, fluid friction and capillary forces, taking preform compression into account, to predict infiltration velocity and cumulative infiltration distance. Apparatuses were designed and constructed to infiltrate cylindrical preforms of 24 vol pct 3-μm-diameter chopped alumina fiber preforms with commercial purity aluminum. Two capacitor banks were charged from 1 to 4 kV and rapidly discharged to produce magnetic pulses of up to 4 tesla peak, at frequencies of 2 to 3 kHz in the infiltrating furnace. A commercial MAGNEFORM unit was also used to produce fields of up to 5 tesla at 5.6 kHz.-Sound composite samples were produced, to a depth of 1.8 mm into the preforms, with little or no breakage of fibers. Good agreement between theoretical model predictions and experimentally measured infiltration depths was demonstrated. Primary process variables for a given matrix-preform system, were the number of discharges, the magnetic pulse intensity and frequency, and the melt ring thickness. The model predicts a pulse frequency below which infiltration does not occur and an optimum frequency for maximum infiltration depth. Successive pulses are predicted to produce only slightly decreasing increments in infiltration depth with the parameters explored, indicating that the process allows greater infiltration depths than were attained with preforms and apparatuses used in this work.

Solidification of highly undercooled Fe-P alloys

Abstract: Rapid solidification behavior of highly undercooled iron-phosphorus alloys was investigated by using a high-speed optical temperature measurement system. The experimental results on solidification rate as a function of bulk undercooling agree well with a model which includes a treatment of nonequilibrium effects during the solidification process. The model is based on an earlier analysis by Boettinger, Coriell, and Trivedi[81] (BCT) and employs temperature-dependent values of equilibrium liquidus slope, equilibrium solute distribution coefficient, and solute interdiffusion coefficient. Values of the kinetic parameters,a0 andV0, in the analysis which best fit the experimental results are 5 x 10-10 m and 600 m/s, respectively. Comparison of experimental results and theory suggests that a transition from local equilibrium to nonequilibrium solidification takes place with increasing undercooling and that interface kinetic effects become predominant at higher undercooling (or growth velocity).

Chemical stability of zirconia-stabilized alumina fibers during pressure infiltration by aluminum

Abstract: Metal matrix composites composed of high-purity aluminum and Du Pont PRD-166 continuous zirconia-stabilized polycrystalline alumina fibers are fabricated by liquid metal infiltration using three different casting procedures. The microstructure of the composites is analyzed using optical and electron microscopy, including analytical electron microscopy. It is found that discrete faceted particles of ZrAl3 form at the interface and grow into the matrix of samples processed above the melting point of the matrix for 13 minutes or more. The formation of this compound is in agreement with thermodynamic stability calculations. It is also found that there is a reaction between solid aluminum and the fibers at 913 K, yielding a reaction product which has the same morphology as that observed with molten aluminum. When the fibers are infiltrated with an initial preform temperature below the metal melting point and a solidifination time below 1 minute, no reaction products were visible in the composite using the scanning electron microscope (SEM). This leads to the conclusion that aluminum matrix composites can be cast with no apparent interfacial reaction product using these fibers provided that adequate processing parameters are chosen.

Growth-speed dependence of primary arm spacings in directionally solidified Pb-10 wt pct Sn

Abstract: The dependence of primary arm spacings on growth speed has been investigated for cellular and dendritic arrays in directionally solidified Pb-10 wt pct Sn. The spatial arrangements of cells and dendrites, as given by their coordination number, are not very different from each other. The primary arm spacing maxima and the cell-to-dendrite transition appear to be strongly influenced by the magnitude of the solute partition coefficient (k). The planar-to-cellular transition in Pb-Sn (k = 0.50) is supercritical as compared to the subcritical behavior reported in Al-Cu (k = 0.14) and succinonitrile-acetone (k = 0.1).

A critical-evaluation of the stress-corrosion cracking mechanism in high-strength aluminum-alloys

Abstract: Attempts have been made to elucidate the mechanism of stress-corrosion cracking (SCC) in high-strength Al-Zn-Mg and Al-Li-Zr alloys exposed to aqueous environments by considering the temperature dependence of SCC susceptibility based upon the anodic dissolution and hydrogen embrittlement models. A quantitative correlation which involves the change of threshold stress intensity,KISCC, with temperature on the basis of anodic dissolution has been developed with the aid of linear elastic fracture mechanics. From the derived correlation, it is concluded that the threshold stress intensity decreases as the test temperature increases. This suggestion is inconsistent with that predicted on the basis of hydrogen embrittlement. It is experimentally observed from the Al-Zn-Mg and Al-Li-Zr alloys that the threshold stress intensity,K,ISCC, decreases and the crack propagation rate,da/dt, over the stress intensity increases with increasing test temperature. From considering the change in SCC susceptibility with temperature, it is suggested that a gradual transition in the mechanism for the stress-corrosion crack propagation occurs from anodic dissolution in stage I, where the crack propagation rate increases sharply with stress intensity, to hydrogen embrittlement in stage II, where the crack propagation rate is independent of stress intensity.

Containerless processing and rapid solidification of Nb-Si alloys of hypereutectic composition

Abstract: A combination of bulk undercooling in an electromagnetic levitation apparatus and splat quenching between two copper plates is used to process Nb-Si alloys in order to maximize rapid solidification conditions and minimize the effects of recalescence, with emphasis on the solidification of characteristics of alloys in the 21 to 27 at. pct Si range of composition. SEM and TEM as well as X-ray diffraction are used to characterize the microstructures of the processed samples. In the range of compositions studied, the splat-quenched drops always formed the tetragonal Nb3Si phase directly from the liquid. Drops solidified in the coil were characterized by the presence of the primary intermetallic Nb5Si3 and the absence of both peritectic Nb3Si and the equilibrium eutectic. In these cases, a metastable alpha-Nb + beta-Nb5Si3 eutectic formed. The results are discussed in terms of possible metastable configurations of the Nb-Si phase diagram as well as concepts of nucleation and growth kinetics applied to the Nb3Si and Nb5Si3 intermetallics.

Interface structure in infiltrated composites of aluminum reinforced with alumina-silica fiber preforms

Abstract: Keywords: Aluminum alloys ; Amorphous materials ; Crystal lattices ; Fiber reinforced metals ; Interfaces (materials) ; Liquid metals ; Microstructure ; Silica ; Thermal effects ; Transmission electron microscopy ; Alumina silica fiber preforms ; Infiltrated composites ; Silica binder ; Metallic matrix composites Note: Massachusetts Inst of Technology, Cambridge, United States03602133 (ISSN) Reference LMM-ARTICLE-1991-004View record in Scopus Record created on 2006-10-09, modified on 2016-08-08

Effect of Fiber Strength on the Room Temperature Tensile Properties of Sic/Ti-24Al-11Nb

Abstract: SCA-6 SiC fibers of known strength were incorporated into SiC/Ti-24Al-11Nb (at. percent) composites and the effect of fiber strength variability on room temperature composite strength was investigated. Fiber was etched out of a composite fabricated by the powder cloth technique and the effect of the fabrication process on fiber strength was assessed. The strength of the composite was directly correlated with the strength of the as-received fiber. The strength of composite plates containing mixed fiber strengths was dominated by the lower strength fiber. Fabrication by the powder cloth technique resulted in only a slight degradation of fiber strength. The strength of the composite was found to be overestimated by the rule of mixtures strength calculation. Examination of failed tensile specimens revealed periodic fiber cracks and the failure mode was concluded to be cumulative. With the variation in fiber strength eliminated, the composite UTS was found to have a positive correlation with volume fraction of fiber.

Preliminary studies on NiAl/Nb2Be17 reaction and effectiveness of BeO as an interfacial reaction Barrier

Abstract: Alloys based on the ordered B2 NiAl phase are being considered as potential high-temperature structural materials One drawback for this material is its lack of high-temperature strength,[1] which can be overcome by reinforcing the alloy with high-strength fibers Like any other composite system, a suitable reinforcement material must have a matching coefficient of thermal expansion (CTE) with the matrix in addition to high-temperature strength and be chemically compatible with the matrix Although there are many high-melting ceramic materials which are thermodynamicalry stable in the NiAl matrix, [2] the high CTE of NiAl,[3] 16 X 10−6 K−1 at 1200 K, makes it difficult to find a suitable ceramic reinforcement material with a matching CTE Thus, there is a need to develop high CTE fibers for the NiAl matrix One group of materials with matching CTE[3] to NiAl are the Be-rich intermetallic compounds called beryllides (CTEs in the range of 16 to 18 × 10−6 K−1 at 1200 K) of formula M2Be17, M2Be13, or M2Be12 (where M = Ta, Nb, Ti, Zr, Hf, or Y) Matching CTEs with the NiAl matrix along with their good high-temperature strength[4] and low densities[4] make Be-rich intermetallic compounds attractive as candidate reinforcement materials for the NiAl matrix

A degenerate electron gas model for solutions of aluminum in cryolite melts

Abstract: It is shown that the measurements of Odegardet al.[1] of the solubility of Al at 1000 °C in alumina-saturated cryolite melts can be interpreted in terms of a degenerate Fermi gas of electrons in equilibrium with Na adsorbed on the Al interface. The effective mass of the electrons ism* = 0.085m°, wherem° is the rest mass of an isolated electron, and the bottom of the conduction band lies at 118 kJ (1.22 eV) below the level of electrons in pure sodium. These parameters also lead to a successful prediction of the observed depression of freezing point of NaF by Na, but only after deriving the appropriate relation to replace Raoult’s law. At NaF/AlF3 molar ratios ≤10., the treatment is inadequate; the observed solubilities are higher than predicted. This could be due to breakdown of the simplifying assumptions made or to the entry of a new factor, such as the occurrence of monovalent Al.