An experimental investigation of the superplastic forming behavior of a commercial al-bronze

The superplastic behavior of thermomechanically processed complex commercial Al-bronze (Cu-10Al-4.5Fe-6Ni-2Mn) sheet material has been investigated under biaxial bulge-forming conditions using constant gas pressures at different temperatures. The results of bulge testing were systematically analyzed usingm values determined from uniaxial tensile tests on the same sheet. The variations of dome height and thickness were examined relative to forming time. The rate of change of dome height was observed to be comparable to the corresponding apex strain rate, and both dome height and m value were seen to influence the thickness gradient. It was also demonstrated that the geometry of a dome was not constant during bulge formation. It could be flatter than, equivalent to, or sharper than a spherical shape. The grain size was relatively small (2 to 3 μm@#@) and very stable, and this is likely to be responsible for the large superplastic strains and relatively low levels of cavitation observed for the alloy. formerly with the Manchester Materials Science Centre formerly Visiting Scientist, Manchester Materials Science Centre     

Superplasticity in a commercial copper dispersion alloy

Abstract

The high-temperature ductility of a fine-grained commercial copper dispersion alloy (CDA 638 - Cu - 2.8% Al - 1.8% Si - 0.4%. Co) has been investigated in the temperature range 400°–800°C at strain rates of 10-2 to 1 min-I. Ductility was markedly temperature and strain rate dependent with a maximum of 320 % at 550°C and 3.9 × 10^?2 min^?1. The strain rate sensitivity of the flow stress was about 0.5 under these conditions and the behaviour can be termed ‘superplastic’. Extensive intergranular cavitation was observed which restricts the ductility; to optimize ductility in this alloy the factors of both cavitation and strain rate sensitivity must be evaluated.

Résumé

La ductilite d'un alliage commercial de cuivre à particules dispersées (CDA 638 – Cu: 2.8% Al, 1.8% Si, 0.4% Co) a été étudiée dans l'intervalle de température 400–800°C pour des vitesses de déformation entre 10^?2 et 1 min^?1. La ductilityé dépendant fortement de la température et de la vitesse de déformation avec une valeur maximale de 320% à 500°C pour une vitesse de 3.9 × 10^?2 min^?1. Ce comportement est considéré comme superplastique dans le mesure où l'exposant reliant la contrainte et la vitesse de déformation a une valeur de ～0.5 dans ces conditions. La ductilité est limitée par une cavitation intergranulaire importante et pour l'augmenter il faut tenir compte de ce facteur aussi.     

Creep cavities in copper: An ultrasonic-velocity and composite-modeling study

This study considered cavities produced in polycrystalline copper by high-temperature tensile creep. Experimentally, we measured longitudinal sound-wave velocities, both parallel and perpendicular to the stress axis. Theoretically, we used a scattered-plane-wave ensemble-average model to predict the effects of voids on sound velocity. The model contains several void-geometry variables: volume fraction, shape, orientation. Comparison of model and velocity measurements shows: (a) voids are nonspherical, (b) voids are oblate spheroidal, (c) voids do not orient randomly, (d) disc-shaped voids tend to orient perpendicular to stress axis. All model predictions agree with metallography. The sound velocities showed also that the copper contains texture and that the texture changes during heating. Neutron pole figures confirm the ultrasonic-velocity texture prediction.     

Plastic anisotropy in a superplastic Al-Li-Mg-Cu alloy

The plastic anisotropy of AA8090 Al-Li-Cu-Mg alloy sheet has been evaluated by tensile testing and by deep drawing at temperatures in the range 200 °C to 525 °C. At temperatures of about 500 °C and strain rates of about 10^-3 s^-1, this material exhibits a high strain-rate sensitivity of flow stress which reduces any tendency to strain localization in stretching and allows so-called superplastic forming of the sheet. Most models of the material behavior in this regime require highly inhomogeneous deformation on the scale of the material’s grain size. The plastic anisotropy measured in the superplastic regime was similar in form, though of reduced magnitude, to that measured under conditions associated with a much smaller strain-rate sensitivity. Homogeneous slip models predict the correct form of anisotropy, and inclusion of slip-rate senitivity can reduce the magnitude of anisotropy predicted but not sufficiently to give good correlation with the experimental results unless very high values are used. The development of the preferred crystallographic orientation in deep drawing has also been examined. The predictions of homogeneous slip models correlate quite well with experimental results at low temperatures, but the situation is more complex in the superplastic regime where, although there is some evidence of texture changes as predicted, there is a general reduction in the intensity of preferred orientation with deformation. However, the results indicate that a greater contribution of homoeneous slip deformation is involved in superplastic deformation than is assumed in some models of superplasticity.     

An exploratory study of a zirconium-modified,precipitation-strengthened nickel-30 copper alloy

A precipitation-strengthened alloy has been produced through minor additions of zirconium to a base Ni-30 Cu alloy. The results of this exploratory study indicate that thermomechanical processing of a solution-treated Ni-30Cu-0.2 Zr alloy produced a dispersion of precipitates. The precipitates have been tentatively identified as a Ni₅Zr compound. Comparison of the mechanical properties, as determined by testing in air, of the Zr-modified alloy to those of a Ni-30 Cu alloy reveals that the precipitation-strengthened alloy has improved tensile properties to 1200 K and improved stress-rupture properties to 1100K. The oxidation characteristics of the modified alloy appeared to be equivalent to those of the base Ni-30 Cu alloy.     

Precision forging of a high strength superplastic zinc-aluminum alloy

Precision forgings can be produced in a high strength zinc aluminum alloy exhibiting superplasticity by means of an isothermal forging cycle which combines an approach at constant speed followed by a dwell at constant load. This study shows that integral threads, sharp corners, and very thin webs can be formed directly in the forging operation. A pressure compensated time parameter is developed which permits forging operations over a range of different conditions to be correlated. This parameter may be used to predict the result of actual precision forging processes from scale model tests. Formerly with St. Joe Mineral Corp.     

An investigation of the thermal stability of a commercial Ni-Cr-Fe-Mo alloy (hastelloy alloy X)

The changes in hardness and room temperature impact toughness of Hastelloy ∗ Alloy X∗Hastelloy is a registered trademark of Cabot Corporation. after aging at 1000, 1200, 1400 and 1600°F (538, 649, 760 and 871°) for times up to 10,000 h were investigated. The alloy exhibits age-hardening at 1200 and 1400°F (649 and 760°).’A slight hardness increase was observed at 1600°F (871°) followed by overaging after 4000 h. No age-hardening was observed at 1000°F (538°) up to 10,000 h. Aging at all temperatures resulted in a substantial drop in room temperature impact toughness. The microstructure after aging was characterized by optical metallography and X-ray diffraction, while fracture mode was characterized by scanning electron microscopy. The results suggest that the toughness degradation is primarily associated with carbide precipitation. M₆C’ type carbides are believed to be the major phase precipitated during aging at all temperatures, although σ and μphases were also detected after 10,000 h at 1400 and 1600°F (760 and 871°), respectively.     

Effect of cavitation on post-deformation tensile properties of a superplastic copper-base alloy

The effects of cavitation, introduced during superplastic tensile flow, on post-deformation tensile properties have been studied for a microduplex Cu-Zn-Ni alloy. Increasing levels of cavitation of up to 4 percent by volume caused a progressive reduction in tensile strength, while ductility was decreased to a substantially greater extent. Increasing the volume fraction of cavities also changed the appearance of the fracture surface from cup and cone to macroscopically brittle. A post deformation annealing treatment reduced the level of cavitation and caused an increase in room temperature ductility, but the proportion of cavities removed during annealing became progressively less as cavitation was increased.     

Atmospheric stress corrosion cracking of a superplastic 7475 aluminum alloy

The influence of different heat treatments upon the atmospheric stress corrosion cracking (SCC) of fine-grained 7475 Al-alloy plates has been investigated. The small size of the matrix precipitates and grain-boundary precipitates (GBPs) was found to be the main cause of atmospheric SCC suscepti-bility. Increasing the size of the matrix precipitates and GBPs by increasing the degree of aging could improve the atmospheric SCC resistance. The size of the matrix precipitates was the major factor affecting the atmospheric SCC resistance when GBPs were larger than a critical size that could nucleate hydrogen bubbles. However, if the size of the GBPs was smaller than this critical size, the improvement of atmospheric SCC resistance due to grain refinement, resulting from a more homo-geneous slip mode, could not be obtained because hydrogen embrittlement became serious. By meas-uring the electrical conductivity, the influence of matrix precipitates, but not that of GBPs, on SCC susceptibility could be obtained. Retrogression and reaging (RRA) treatment could effectively im-prove the atmospheric SCC resistance of T6 temper because RRA temper could produce larger sizes of both the matrix precipitates and GBPs than could T6 tempered condition.     

An experimental and theoretical study of cementite dissolution in an Fe-Cr-C alloy

The dissolution of cementite at 910 °C in an Fe-2.06Cr-3.91C (at. pct) alloy is investigated experimentally. The Cr concentration profiles in austenite and cementite are measured by means of the scanning transmission electron microscopy/energy dispersive spectrometry (STEM/EDS) technique at different dissolution times. The measurements show the Cr enrichment in the cementite during the dissolution process. The measurements suggest that the main part of the reaction for this alloy is controlled by Cr diffusion in the cementite or in the austenite matrix. This observation is in agreement with predictions of the local equilibrium hypothesis. The carbide fraction and average particle diameter are evaluated as functions of dissolution time. The Cr enrichment of the cementite results in a supersaturation and a possible decomposition of the cementite. Microstructural evidence for such a decomposition is found by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A new program package called DICTRA,_[11] which is suitable for the simulation of diffusional reactions in multicomponent alloys, has been applied to the present case. The simulation is compared with the experimental data, and a good agreement between the two is found.     

An experimental study of carbide/austenite equilibria in the high-speed steel alloy system

Carbide/austenite equilibria were studied experimentally in 55 alloys at 1200 °C and 19 alloys at 1100 °C. The alloys contained different amounts of C, Cr, Mo, W, V, Si, and Co, Fe being the base. The chemical composition of the individual phases, austenite, M₆C, M₂C, and MC carbides, were measured with a microprobe. Carbon activity, volume fraction of carbides, and their lattice parameters were also measured. The results are presented vs the carbon content of the austenite.     

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.     

An experimental study of deformation of a columnar dendritic mushy zone using a transparent succinonitrile-acetone alloy

The deformation of a directionally solidified columnar dendritic mushy zone in a transparent succinonitrile-acetone (SCN-ACE) alloy has been studied expermentally. In addition to solidifying dendritically like a metal alloy, this alloy also has mechanical properties that are similar to those of metals near the melting point. The experiments are relevant, for example, to the deformation of a partially solidified strand during continuous casting of steel slabs. A test cell was designed which allows for directional solidification of the alloy and controlled compression of the solid-liquid mush which forms. Measurements during solidification and deformation include temperatures, interface positions, local displacements of the solid skeleton in the mush, and liquid concentrations. Results are presented for a range of initial test-cell thicknesses, deformation amounts, and deformation start times. The measurements are suitable for validation of future models.     

The growth of creep cavities in a low alloy steel

Experimental observations of cavity sizes and spacings, after creep of a low alloy steel, are used to assess the applicability of some theoretical models for cavity growth. It is suggested that, for this type of material, various factors may control the growth of cavities, dependent on their size and the test temperature. There also exists a need for accurate diffusion data under these testing conditions. C. D. HAMM, formerly at University of Manchester     

Effects of microstructural evolution on superplastic deformation characteristics of a rapidly solidified Al-Li alloy

This study is concerned with the effects of microstructural modification on superplastic deformation characteristics of a rapidly solidified (RS) Al-3Li-1Cu-0.5Mg-0.5Zr (wt pct) alloy. This Al-Li alloy has a very fine grain structure desirable for improved superplasticity. The results of superplastic deformation indicated that the alloy exhibited a high superplastic ductility, e.g., elongation of approximately 800 pct, when deformed at temperatures above 500 °C and at the strain rates of 10⁻²/s to 10⁻¹/s. Such a high strain rate is quite advantageous for the practical superplastic forming application of the alloy. Stress-strain rate curves were obtained by performing a series of load relaxation tests in the temperature range from 460 °C to 520 °C in order to examine the superplastic deformation behavior and to establish its mechanisms. The stress-strain rate curves could be separated into two parts according to their respective physical mechanisms, i.e., grain matrix deformation and grain boundary sliding, as was proposed in a new superplasticity theory based on internal deformation variables. The microstructural evolution during superplastic deformation was also analyzed by using transmission electron microscopy. During superplastic deformation, grains were kept fine and changed into equiaxed ones due to the presence of fine secondary phase particles and the continuous recrystallization due to the development of subgrains. Consequently, the rapidly solidified (RS) alloy showed much improved superplasticity compared to the conventional ingot cast 8090 alloy.