A model for subgrain superplastic flow in aluminum alloys

Aluminum alloys that contain low angle boundaries exhibit different superplastic behavior than alloys consisting of high angle boundaries. On a relative basis, the low angle boundaries increase the flow stress, but impart a greater resistance to cavitation; the strain-rate sensitivity of this material is generally smaller and the change in the strain-rate sensitivity with strain rate shows a minimum instead of a maximum as observed in the large angle boundary materials. As a result, the subgrain material can be deformed to large tensile strains at fast strain rates. A kinetic model for subgrain superplasticity that invokes a balance between the arrival and emission rates of dislocations at low angle boundaries is presented. It explains several features of subgrain superplasticity. It also explains why ultrafine dispersoids of intermetallics appear to stabilize the subgrain structure in aluminum. Early work on the correlation between flow stress and the subgrain size in dynamic recrystallization of metals may also be consistent with the model.     

Effect of hydrostatic pressure on cavitation in superplastic aluminium alloys

Cavitation during superplastic flow has been examined in 3 aluminium alloys (Supral 220, Al-7475E and AlCuLi alloy) strained in uni-axial and bi-axial tension with varying superimposed hydrostatic pressures. Measurements of volume fractions, population densities and size distributions of cavities have been made using precision density and/or quantitative metallographic procedures. It has been found that the rate of increase of the volume fraction of cavities with strain can be decreased by increasing the superimposed pressure. The experimental observation that cavity growth is controlled primarily by plastic flow and that cavitation should be greatly reduced by pressures equal to approximately half of the effective flow stress of the material is in broad agreement with theoretical predictions. The difference between the magnitude of the experimentally determined cavity growth rate factors and the smaller predicted values is believed to arise from the combined effects of non-equilibrium cavity growth, coalescence and the decrease in strain rate sensitivity with strain. It is envisaged that superimposed pressures substantially greater than the flow stress of the material would be required to eliminate cavitation completely.     

Cavitation and cavity growth during superplastic flow in microduplex Cu-Zn-Ni alloys

A study has been made of cavity growth during superplastic tensile deformation of two microduplex α/β nickel-silvers, one a Cu-Zn-Ni alloy and the other a Cu-Zn-Ni-Mn alloy. For cavities with radii of >0.5 /gmm, measured growth rates were found to be in good agreement with values calculated on the assumption that cavity growth was controlled by viscous flow of the matrix. For smaller cavity sizes a diffusional growth mechanism could predominate. Metallography revealed that cavity morphology changed with strain in a manner consistent with diffusion-controlled growth at small sizes, and matrix deformation controlled growth at intermediate and large cavity sizes. Density studies showed that the overall level of cavitation was independent of both strain rate and temperature, and was influenced only by strain.     

Simulation of cavitation processes in superplastic deformation

Cavitation behavior during superplastic deformation is simulated by developing a three-dimensional model which incorporates the continuous nucleation, plastic growth, and coalescence of cavities. The cavity growth rate is determined by using an empirical relationship between the Poisson’s ratio and the cavity volume fraction, and cavities after coalescence are represented as overlapped spheres. The volumetric cavity growth-rate parameter (2.0 to 2.5) obtained from the simulation is consistent with the range of experimental observation. Comparison of the simulation with a modified Pilling’s model for cavity coalescence shows that the growth rates of the average cavity volume are consistent with each other at small strains, whereas they are higher in the former than in the latter at large strains. This is because multiple coalescence, rather than the pairwise coalescence assumed in the Pilling’s model, becomes predominant at large strains in the simulation. Between the simulation and experiments, close agreement is also found in the cavity-size distribution normalized with a maximum cavity size.     

Superplastic behavior and cavitation in high-strain-rate superplastic Si3N4p /Al-Mg-Si composites

High-strain-rate superplastic behavior has been investigated for Si₃N_4p /Al-Mg-Si (6061) composites with a V _f =20 and 30 pct, respectively, where V _f is the volume fraction of reinforcements. A maximum elongation was attained at a temperature close to the onset temperature for melting for both composites. The maximum elongation for the 30 vol pct composite was larger than that for the 20 vol pct composite. Development of cavities transverse to the tensile direction is responsible for the lower maximum elongation of the 20 vol pct composite. However, development of the transverse cavities was limited to the optimum superplastic temperature for the 30 vol pct composite. The differential scanning calorimetry (DSC) investigation showed that a sharp endothermic peak appeared for the 30 vol pct composite, indicating that sufficient partial melting occurs. It is, therefore, likely that the stress concentrations are sufficiently relaxed by a liquid phase and that the development of transverse cavities is limited for the 30 vol pct composite.     

The characteristics of cavitation in superplastic metals and ceramics

It is now well established that cavities are often formed during superplastic deformation. However, experimental investigations suggest important differences in the nature of the cavitation in typical superplastic metals and ceramics. These differences are demonstrated with reference to a superplastic Cu-based alloy and yttria-stabilized tetragonal zirconia (Y-TZP). By using a quantitative metallo-graphic procedure and scanning video images, measurements are presented showing the size, shape, and configuration of internal cavities in these two materials after deformation at high temperatures. This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium as part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

Cavitation at grain and phase boundaries during superplastic flow of an aluminum bronze

Cavities have been observed to form at grain and phase boundaries under certain strain rate conditions during superplastic tensile deformation of a Cu-9.5 pct Al-4 pct Fe aluminum-bronze. The cavities form preferentially at α-β interfaces or triple junctions involving both phases. The process of cavitation is associated with grain boundary sliding and cavity nucleation probably occurs at points of stress concentration in the sliding interfaces. The ductility is not markedly impaired by the cavities because the high strain-rate sensitivity of the material inhibits the interlinkage of cavities at high strains. A range of strains and strain rates for superplastic forming processes has been determined at which the volume fraction of cavities present was tolerable.     

Mechanical behavior and properties of mechanically alloyed aluminum alloys

The fracture and deformation behaviors of several product forms produced from mechanically alloyed (MA) aluminum alloys 9052 and 905XL were studied. The main operative strengthening mechanism is strengthening due to the submicron grain size. Ductility and toughness were found to be controlled by the morphology of the prior particle boundaries. We propose that the work-hardening behavior of these MA alloys is similar to the behavior exhibited by a deformed fcc alloy that (a) contains rigid barriers to dislocation motion, (b) deforms by wavy slip, and (c) forms a cell substructure upon deformation.     

Dynamic fracture behavior of SiC whisker-reinforced aluminum alloys

This paper presents a study of dynamic fracture initiation behavior of 2124-T6 aluminum matrix composites containing 0, 5.2, and 13.2 vol pct SiC whiskers. In the experiment, an explosive charge is detonated to produce a tensile stress wave to initiate the fracture in a modified Kolsky bar (split Hopkinson bar). This stress wave loading provided a stress intensity rate, K_I,, of about 2 × 10⁶ MPa√m/s. The recorded data are then analyzed to calculate the critical dynamic stress intensity factor,K _Id, of the composite, and the values obtained are compared with the corresponding quasi-static values. The test temperatures in this experiment ranged from −196 °C to 100°C, within which range the fracture initiation mode was found to be mostly ductile in nature. The micromechanical processes involved in void and microcrack formation were investigated using metallographic techniques. As a general trend, experimental results show a lower toughness as the volume fraction of the SiC whisker reinforcement increases. The results also show a higher toughness under dynamic than under static loading. These results are interpreted using a simple dynamic fracture initiation model based on the basic assumption that crack extension initiates at a certain critical strain developed over some microstructurally significant distance. This model enables us to correlate tensile properties and microstructural parameters, as, for instance, the interspacing of the SiC whiskers with the plane strain fracture toughness.     

Modeling high-temperature stress-strain behavior of cast aluminum alloys

A modified two-state-variable unified constitutive model is presented to model the high-temperature stress-strain behavior of a 319 cast aluminum alloy with a T7 heat treatment. A systematic method is outlined, with which one can determine the material parameters used in the experimentally based model. The microstructural processes affecting the material behavior were identified using transmission electron microscopy and were consequently correlated to the model parameters. The stress-strain behavior was found to be dominated by the decomposition of the metastable θ′ precipitates within the dendrites and the subsequent coarsening of the θ phase, which was manifested through remarkable softening with cycling and time. The model was found to accurately simulate experimental stress-strain behavior such as strain-rate sensitivity, cyclic softening, aging effects, transient material behavior, and stress relaxation, in addition to capturing the main deformation mechanisms and microstructural changes as a function of temperature and inelastic strain rate.     

A planar simple shear test and flow behavior in a superplastic Al-Mg alloy

Superplasticity is generally studied by performing tensile and gas-pressure-bulge tests. In formed parts, however, a variety of strain states, including in-plane shear, are encountered. The understanding of the mechanical response in shear is helpful in the study of superplastic metal forming. In this study, a device for a planar simple shear test was designed and used to perform tests on a superplastic Al-Mg alloy sheet at the elevated temperatures of 500 °C (773K) and 550 °C (823K). In such a test, the incremental rotation of the principal strain axes and specimen-end effects during deformation can complicate the determination of true mechanical response. The possible approximations regarding the strain state in the specimen gage have been investigated. The σ _e -ε _e curves developed based on a simple-shear assumption show a lower flow stress than that under uniaxial tension, and strain hardening is related to dynamic grain growth. The rate of strain hardening at a fixed _e level is essentially the same for both uniaxial tension and shear, but the difference in the effective stress between uniaxial tension and shear depends upon strain rate and temperature. This study marks the first known attempt to characterize large strain response for superplastic metals under conditions of simple shear.     

Fatigue behavior and failure mechanisms of modified 7075 aluminum alloys

The effects of purity level and dispersoid type on the fatigue behavior of 7000 series alloys were investigated. Ten different compositions based on the 7075 alloy were produced with five levels of Fe + Si and either Cr or Zr dispersoids. Notched axial fatigue specimens were tested at room temperature and the fatigue life did not correlate with either purity level or dispersoid type. Specimens failed by three macroscopic modes designated as: slant, vee, or flat fracture. Sectioning analysis showed that the slant, vee, and flat fractures resulted from single, double and multiple initiation, respectively. Both initiation and propagation in all three modes of failures were dominated by slip related fracture on the {111} planes inclined at 35 deg to the tensile axis of the textured material. The same failure mechanisms were observed in smooth fatigue specimens. formerly with the Metals and Ceramics Division, Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH 45433     

Microstructural observations of superplastic cavitation in fine grained 7475-Ai

Intergranular cavitation is an important consideration in the successful development of a commercially viable superplastic forming process for the high strength aluminum alloy, 7475. This work examined the microstructural features involved in the initiation stages of cavity formation. The observations suggest that, with the optimum superplastic deformation conditions, cavity nucleation is generally the rate determining step in the overall development of cavitation with strain. Cavities do not generally form at even the largest of the common single phase inclusion particles unless forming conditions are such that the flow stress significantly increases. It appears that, as well as local stress concentrations, additional effects are required, such as temperature induced particle decohesion and internal gas evolution, in order that cavities may grow to stable sizes. Such conditions may exist at certain two phase inclusion particles in the 7475 Al alloy. Suitable modifications to the standard alloy processing may therefore be devised which result in even lower rates of cavitation at the optimum superplastic forming conditions. C.C. BAMPTON, formerly at the Rockwell International Science Center J.W. EDINGTON, formerly at the University of Delaware     

Deformation behavior of submicrocrystalline aluminum alloys during dynamic loading

The structure and the mechanical properties of aluminum V95 and AMts alloys with various grain sizes (from micron to submicron) are studied in a wide range of strain rates (from 10^–3 to 10⁵ s^–1). Submicrocrystalline (200–600 nm) materials are formed by dynamic channel-angular pressing at a strain rate of 10⁵ s^–1 using a pulsed power source.     

The kinetics of transformation in Zn-Ai superplastic alloys

We report herein on the kinetics of transformation of a eutectoid Zn-AI alloy containing additions of Cu, Mg and Ca. The alloy possesses excellent superplasticity at elevated temperatures, and it has a relatively high strength at ambient temperature (∼345 MPa). TTT curves for the alloy are presented, and the corresponding microstructures obtained at the various transformation temperatures are reported. Also, the results of Jominy endquenched tests are reported and the corresponding continuous cooling kinetics are compared to the isothermal kinetics. The alloy was observed to decompose by two distinct mechanisms, depending upon the degree of supercooling. At temperatures just below the eutectoid, it decomposes into a lamellar microstructure, whereas at larger undercooling it decomposes into a coherent two-phase mixture. The interlamellar spacing and colony size are reported as a function of transformation temperature, and shown to follow expected trends. Since neither the lamellar nor coherent microstructure is superplastic, thermomechanical methods of producing a superplastic structure are discussed.     

Tensile ductility of superplastic ceramics and metallic alloys

Superplastic ceramics and metallic alloys exhibit different trends in tensile ductility in the range where the strain-rate-sensitivity exponent, m, is high (m⩾0.5). The tensile ductility of superplastic metallic alloys (e.g. fine-grained zinc, aluminium, nickel and titanium alloys) is primarily a function of the strain-rate-sensitivity exponent. In contrast, the tensile ductility of superplastic ceramic materials (e.g. zirconia, alumina, zirconia-alumina composites and iron carbide) is not only a function of the strain-rate-sensitivity exponent, but also a function of the parameter ⋗e exp (Q_c/RT) where ⋗e is the steady-state strain rate and Q_c is the activation energy for superplastic flow. Superplastic ceramic materials exhibit a large decrease in tensile elongation with an increase in ⋗e exp (Q_c/RT). This trend in tensile elongation is explained based on a “fracture-mechanics” model. The model predicts that tensile ductility increases with a decrease in flow stress, a decrease in grain size and an increase in the parameter (2γ_s−γ_gb), where γ_s is the surface energy and γ_gb is the grain boundary energy. The difference in the tensile ductility behavior of superplastic ceramics and metallic alloys can be related to their different failure mechanisms. Superplastic ceramics deform without necking and fail by intergranular cracks that propagate perpendicular to the applied tensile axis. In contrast, superplastic metallic alloys commonly fail by intergranular and transgranular (shearing) mechanisms with associated void formation in the neck region.     

Material flow patterns and cavity model in friction-stir welding of aluminum alloys

Friction-stir welding (FSW) of 3-mm-thick plates of 6061 Al and LF6 Al was conducted and the materials’ flow patterns in the weld nugget along three perpendicular planes were analyzed. The onion structure viewed on any cross section normal to the travel direction is independent of weld position. The weld morphology was examined along its length by considering planes of different depths parallel to the surface. These showed semicircle streaks whose shapes depended on the depth of the observation plane. It is determined that the weld nugget is composed of a series of identical half ellipsoid regions. A tentative simplified cavity model is presented to explain the mass flow pattern and formation of defects in the weld nugget. This model is based on the assumption that only the metal between the pin surface and the last maximum circle created by the pin rotation is in a plasticized state. From this model, it is shown that the location and size of the cavity formed during the rotation of the pin changes cyclically and it is related to the position of the pin’s center. The holes or slots left in the weld nugget center or near the advancing side are directly related to the size of the cavity. The welding parameters or weld pitch affects the volume of the cavity, and consequently influence the weld defects. A large weld pitch will cause holes to be formed in the weld nugget because of the large cavity. The flow patterns, which show that the plasticized material flows from both advancing and retreating sides to the weld center behind the pin, can be easily explained with this cavity model.     

The stress-corrosion cracking behavior of high-strength aluminum powder metallurgy alloys

The susceptibility to stress-corrosion cracking (SCC) of rapidly solidified (RS) aluminum powder metallurgy (P/M) alloys 7090 and 7091, mechanically alloyed aluminum P/M alloy IN* 9052, and ingot metallurgy (I/M) alloys of similar compositions was compared using bolt-loaded double cantilever beam specimens. In addition, the effects of aging, grain size, grain boundary segregation, pre-exposure embrittlement, and loading mode on the SCC of 7091 were independently assessed. Finally, the data generated were used to elucidate the mechanisms of SCC in the three P/M alloys. The IN 9052 had the lowest SCC susceptibility of all alloys tested in the peak-strength condition, although no SCC was observed in the two RS alloys in the overaged condition. The susceptibility of the RS alloys was greater in the underaged than the peak-aged temper. We detected no significant differences in susceptibility of 7091 with grain sizes varying from 2 to 300 μm. Most of the crack advance during SCC of 7091 was by hydrogen embrittlement (HE). Furthermore, both RS alloys were found to be susceptible to preexposure embrittlement—also indicative of HE. The P/M alloys were less susceptible to SCC than the I/M alloys in all but one test.