The effect of fatigue deformation on microstructural evolution in a superplastic aluminum-zinc eutectoid alloy

The microstructure of a superplastic aluminum-zinc eutectoid alloy that had been fatigue tested at 100 °C and 200 °C was examined. At 100 °C, in the aluminum-rich phase, precipitate-free zones (PFZs) formed beside (Al)/(Zn) interphase boundaries because of interphase boundary migration. Interphase boundary migration was due to phase growth, which proceeded more rapidly during fatigue deformation than during annealing. At 100 °C and 200 °C, PFZs beside (Al)/(Al) grain boundaries were asymmetrical owing to grain boundary migration. The precipitation of the equilibrium zinc-rich phase in the aluminum-rich phase proceeded more rapidly during fatigue deformation than during annealing. J. W. BOWDEN, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON.     

Microstructural characterization of a rapidly solidified Al-Mg-Mn powder alloy

A detailed investigation concerning the microstructural characteristics of a rapidly solidified Al-Mg-Mn powder alloy has been carried out. A range of microstructures was observed in the atomized powder: dendritic, cellular, and featureless morphologies. Variations in structure have been related to cooling rate, nucleation undercooling, and recalescence during solidification. A metastable Al-Mn phase was created during atomization and was present in two distinct morphologies. Decomposition behavior during thermal and thermomechanical processing has been studied. An interesting feature was the retention of the metastable phase through the initial consolidation stages.     

Microstructures of rapidly solidified aluminum alloy submicron powders

The microstructures of electron transparent submicron aluminum alloy powders produced by an electrohydrodynamic process are described. The observations are coupled with thermodynamic, kinetic, and heat flow concepts to deduce the thermal history and solidification mode of the powders. The range of microstructures observed includes: homogeneous plane-front solidified single crystals; cellular crystals; and powders containing blocky segregates, multiple grains, and twins. In general, a decrease in the size of the submicron particles decreases the amount of segregation. This observation appears to be caused by an increase in undercooling prior to nucleation which leads to higher liquid/solid interface velocities. On the other hand, the same trend promotes multiple nucleation and increases the incidence of twin crystals. Liquid/solid interface breakdown from planar to cellular in the middle of the powders is correlated with solute “trapping” during recalescence and rejection of solute ahead of the solidification front after recalescence. The latter manifests itself as a solute depleted zone in the plane-front to cellular transition region. Finally, some of the microstructural observations lend credence to the recent extension of the morphological stability theory to rapid solidification. Formerly a Research Associate at the University of Illinois.     

Explosive consolidation of rapidly solidified aluminum alloy powders

Dynamic consolidation by explosive detonation was investigated for rapidly solidified Al-Cu-Li-Mg alloy powder. The densification, extent of interparticle bonding, microstructural modifications, and tensile properties were determined. The Al-alloy powders were prepared by vacuum atomization with cooling rates of 10³ to 10⁵ K per second. The powders were packed in steel cylinders, which were then evacuated to 1 to 7 Pa (1 to 5 × 10⁻² torr) for several hours and then sealed. Compaction experiments were conducted with the explosive surrounding the powder pack and detonated longitudinally. Variations in compaction pressure and detonation velocity were obtained by using different explosives and packing densities. The densification and interparticle bonding were correlated with explosive and powder parameters.     

Processing and microstructural characterization of Al-Cu alloys produced from rapidly solidified powders

This paper concerns the processing of Al-Cu alloys via a novel powder-metallurgy route. The specific technique used for powder processing involves the rapid solidification of coarse, molten droplets following impulse atomization. This produces a fine, homogeneous, dendritic microstructure within the alloy granules. Following consolidation via hot pressing, the microstructure consists mostly of an Al matrix with fine CuAl₂ particles and partially recrystallized dendrites. Further heat treatment and/or thermomechanical processing completes the spheroidization process in the CuAl₂ phase. Blending powders with different Cu has been used to make materials with a bimodal distribution of the local particle-volume-fraction content. The high temperature (773 K) strength of these materials decreases with increasing CuAl₂ content. This can be explained using a flow model based on superplastic deformation, controlled by diffusion-accommodated sliding at Al grain boundaries. This mechanism may also explain the deformation-enhanced particle coarsening observed during channel-die forging operations.     

Identification of precipitate phases in a mechanically alloyed rapidly solidified Al-Fe-Ce alloy

The stable and metastable precipitate phases which form during mechanical alloying of a rapidly solidified (RS) Al-8.4Fe-3.4Ce alloy have been unambiguously identified using X-ray diffraction, transmission electron microscopy (TEM), and energy dispersive spectroscopy techniques. The metastable Al₁₀Fe₂Ce and stable Al₁₃Fe₃Ce and Al₁₃Fe₄ intermetallic phases, with crystal structures and lattice parameters as reported in the literature, have been identified. It is shown that the metastable Al₁₀Fe₂Ce intermetallic phase particles have elongated shapes and their sizes range between 100 and 200 ran and are free of any localized faults, whereas the equilibrium Al₁₃Fe₃Ce and Al₁₃Fe₄ intermetallic phases are equiaxed in shape and have particle sizes ranging from 200 to 500 nm. It is suggested that the presence of the metastable Al₁₀Fe₂Ce in this material is due to its incomplete transformation to the equilibrium Al₁₃Fe₃Ce phase.     

Deformation mechanisms of a rapidly solidified Al-8.8Fe-3.7Ce alloy

The mechanisms of deformation of a rapidly solidified and compacted Al-8.8Fe-3.7Ce (wt pct) alloy were investigated in the stress range 20 to 115 MPa and temperature range 523 to 623 K. The stress dependence of the steady state strain rates indicated a transition from diffusional creep to power law creep, the transition stress decreasing with increasing temperature from 70 MPa (σ/G = 3.1 × 10^-3) at 523 K to 40 MPa (σ/G = 1.9 × 10^-3) at 623 K. The activation energy in the power law creep regime was close to that of bulk self-diffusion in aluminum, while the activation energy in the diffusional creep regime was close to that of grain boundary self-diffusion in Al. The creep strain rates in the power law creep regime were found to be predicted much better by the substructure-invariant creep law (Sherby, 1981) than by the semi-empirical Dorn equation for Al, with the inclusion of a “threshold” stress. In the Coble creep regime, it was found that the cell/subgrain boundaries are inefficient vacancy sources/sinks and that their contribution to Coble creep is totally suppressed in this alloy. The Coble creep rates could be explained by using the average diameter of the powder particles as the effective grain size in the Coble creep equation.     

On the embrittlement of a rapidly solidified Al-Fe-V-Si alloy after high-Temperature exposure

The effects of high-temperature exposure on the mechanical properties and the microstructure of a rapidly solidified Al-Fe-V-Si alloy were examined in order to identify the critical factors controlling the thermal stability of the alloy, particularly at temperatures above 400 °C. Room-temperature (RT) tensile tests were conducted on specimens exposed to temperatures ranging from 150 °C to 482 °C for 100 hours. The microstructure of the extrusion is characterized by a banded structure consisting of a layer containing fine silicide dispersoids and a layer containing coarse silicide dispersoids, which is a replication of the microstructure of the melt-spun ribbon. The alloy did not show any significant change in tensile properties after 100 hours exposure up to 427 °C due to the stability of the microstructure. However, after exposure above 427 °C, tensile elongation decreased significantly and the brittle cleavage regions were observed on the fracture surface. The occurrence of brittle cleavage fracture is due to the presence of coarse equilibrium Al₁₃Fe₄ or Al₃Fe phase, which forms by the transformation of the coarse silicide dispersoids above 427 °C.     

The influence of SiC particulates on fatigue crack propagation in a rapidly solidified Al-Fe-V-Si alloy

The fatigue crack propagation properties of a rapidly solidified aluminum alloy are compared with those of a metal matrix composite (MMC) made of the same base alloy with the addition of 11.5 vol pct SiC particulate. The high-temperature base material, alloy 8009 produced by Allied-Signal, Inc. (Morristown, NJ), is solidified and processed using powder metallurgy techniques; these techniques yield a fine-grained, nonequilibrium microstructure. A direct comparison between the fatigue crack propagation properties of the reinforced and unreinforced materials is possible, because alloy 8009 requires no postprocessing heat treatment. As a consequence, this comparison reflects the influence of the SiC particulate and not differences in microstructure that could arise during processing and aging. The experimental data demonstrate that the SiC-reinforced material exhibits modestly superior fatigue crack propagation properties: slower crack growth rates for a given ΔK, at near-threshold crack growth rates. Even when the data are corrected for crack closure using an effective stress intensity factor, ΔK_eff, the composite exhibits lower crack propagation rates than the unreinforced matrix alloy. Microscopic evidence shows a rougher fracture surface and a more tortuous crack path in the composite than in the base alloy. It is argued that the lower crack growth rates and higher intrinsic threshold stress intensity factor observed in the composite are associated with crack deflection around SiC particles. Formerly Graduate Research Assistant, University of California-Davis     

Effect of cerium content on the deformation behavior of rapidly solidified Al-Fe-Ce alloys

The deformation behavior of a rapidly solidified Al-8.9Fe-6.9Ce (wt pct) alloy was studied in the temperature range of 250 °C to 350 °C and stress range of 20 to 175 MPa. The stress exponents and activation energies suggest that the alloy exhibits a pronounced diffusional creep regime with a transition to power law creep behavior at stresses beyond 60 MPa. Comparing these data with those obtained earlier for an Al-8.8Fe-3.7Ce alloy, it was found that in the diffusional creep regime, the Ce content had no effect on the creep rate. However, in the power law creep regime, a strong dependence on the precipitate spacing, as predicted by the structureinvariant creep law,^[5] was observed. The higher volume fraction of precipitates in the Al-8.9Fe6.9Ce alloy causes a decrease in the power law creep rates by a factor of 5. Formerly Graduate Student. Formerly Assistant Professor, University of Illinois at Urbana-Champaign.     

Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet

Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were observed over a strain rate range of 3 × 10⁻⁵ s⁻¹ to 1 × 10⁻² s⁻¹, with a maximum value of 0.65 at 5 × 10⁻⁴ s⁻¹ and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation, which produced a tensile elongation of 671 pct at 525 °C. R. VERMA, formerly Visiting Scientist, Department of Materials Science and Engineering, University of Michigan     

Indentation characteristics of rapidly solidified Al-Cu-V and Al-Cu-Ti alloys

We report indentation characteristics of metallic glasses and nano-composites in Al-Cu-V as well as in Al-Cu-Ti systems. The melt spun ribbons of these alloys are employed to study the surface hardness characteristics of the specimen containing microstructural features at various length scales. The characterization of these materials has been done with the help of transmission electron microscope, scanning electron microscope and X-ray diffractometer. The surface hardness characteristics of melt spun ribbons with and without crystallization have been studied using a micro-hardness tester. We shall discuss the indentation behavior of ribbons in relation to their structures and microstructures.     

The role of matrix dislocations in the superplastic deformation of a copper alloy

It is demonstrated that the densities of dislocations trapped in coherent twin boundaries may be used to provide a direct and quantitative comparison of the extent of intragranular slip in the three regions of behaviour associated with superplasticity. Measurements on a superplastic copper alloy, Coronze CDA 638, show that there is very little movement of matrix dislocations at low strain rates in region I, the movement of matrix dislocations increases with increasing strain rate in region II, and there are large numbers of mobile matrix dislocations at high strain rates in region III. Deformation in region II is considered to be controlled by grain boundary sliding occurring by the movement of grain boundary dislocations, while control of flow in region I is attributed to the rate at which grain boundary dislocations can bypass interfacial obstacles.