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.     

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.     

A new investigated method on hot tearing behavior in aluminum alloys

A new investigated method based on the applied forces for assessment on hot tearing behavior in aluminum alloys is introduced in the paper. In this method, molten metal is cast in the rod-shaped mold cavity. One side of the casting specimen is hooked by a steel bolt which restrains its free contraction and transfers the tensile forces during solidification. A steel threaded rod connected to a load cell which records the realtime measurement of the tensile forces during every experiment. Thermal history is monitored by k-type thermocouple. The data of the temperature and tensile forces are acquired by a data acquisition system. Through the use of this method, it is possible to estimate the initiation of hot tearing, its propagation and cracking during solidification. It is also obtained the critical tensile stress for hot tearing initiated and fractured. Experiment is conducted with A356 alloys to investigate the accuracy of the apparatus and modify its operating parameter. Accordingly, the tensile forces curves, the temperature curves and the microstructure of the test specimen are obtained. This data provide useful information about hot tearing formation and solidification characteristics, from which their quantitative relations are derived. In this manner, the hot tearing behavior in aluminum alloys can be studied.     

An analysis of cavitation occurring in near-&gg titanium aluminide during superplastic deformation

In order to understand the cavitation behavior of near-&gg titanium aluminide alloys under superplastic forming conditions, the uniaxial hot-tension behavior of a Ti-45.5Al-2Cr-2Nb (at. pct) rolled sheet material containing a microduplex structure was determined. Three initial microstructures were examined: as-rolled, and two coarser-grained rolled-and-heat-treated conditions (1177 °C/4 h or 1238 °C/ 2 h). The cavitation behavior was analyzed after isothermal constant-strain-rate tests were conducted at temperatures between 900 °C and 1200 °C and strain rates in the range of 10⁻⁴ to 10⁻² s⁻¹. Interrupted tests and strain-to-failure tests were conducted in order to track cavity growth with time. After testing at a given temperature and strain rate, as-rolled specimens developed fewer large-size cavities than heat-treated specimens, possibly due to the finer grain size in the as-rolled material. Cavity growth was found to be plasticity controlled; the largest cavity size and density of cavities increased with increasing strain or strain rate and decreasing temperature. Since the number of finest-sized cavities examined did not decrease with strain, it is believed that continuous cavity nucleation occurred. For all three initial microstructures, the optimum sheet-forming temperature in the regime examined was identified as 1200 °C, at which the lowest cavity growth rates and highest ductilities were observed.     

Evaporation behavior of aluminum during the cold crucible induction skull melting of titanium aluminum alloys

Taking the Ti-Al binary alloy as an example, this article studied the evaporation behavior of Al during the cold crucible induction skull melting (ISM) process of titanium alloys. A formula was deduced to predict the activity of Al in a molten Ti-Al binary system. The calculated activity of Al negatively deviates from an ideal solution. A model was established to judge the evaporation controlling mode and, on this basis, several conclusions were obtained. (1) The evaporation controlling mode of Al in molten Ti-Al transfers from the evaporation reaction controlling mode to the double controlling mode (diffusion and evaporation reaction) with increasing melt temperature (T _ms) and/or Al content (x _Al) and/or decreasing pressure (P) in the melting chamber. (2) The expression P≤P _crit (P _crit≈0.44 P _e(Al)) is a criterion used to judge whether the evaporation is in the state of free evaporation. (3) The term P _impe (P _impe=(3.5 to 4) P _e(Al)) is a critical value which impedes the evaporation loss. Almost all of common used ternary additions could enhance the activity of Al in molten Ti-Al and, accordingly, aggravate the evaporation of Al, except for Zr. The enhancing sequence is Y, Ni, Nb, Mn, V, Fe, Cr, Mo, Cu, Si, W, Mg, B, and Sn. The Al evaporation mass-transfer losses, measured from the melting experiments of several titanium aluminum alloys, were in reasonable agreement with the calculated results.     

Plastic deformation behavior of aluminum casting alloys A356/357

The plastic deformation behavior of aluminum casting alloys A356 and A357 has been investigated at various solidification rates with or without Sr modification using monotonic tensile and multi-loop tensile and compression testing. The results indicate that at low plastic strains, the eutectic particle aspect ratio and matrix strength dominate the work hardening, while at large plastic strains, the hardening rate depends on secondary dendrite arm spacing (SDAS). For the alloys studied, the average internal stresses increase very rapidly at small plastic strains and gradually saturate at large plastic strains. Elongated eutectic particles, small SDAS, or high matrix strength result in a high saturation value. The difference in the internal stresses, due to different microstructural features, determines the rate of eutectic particle cracking and, in turn, the tensile instability of the alloys. The higher the internal stresses, the higher the damage rate of particle cracking and then the lower the Young’s modulus. The fracture strain of alloys A356/357 corresponds to the critical amount of damage by particle cracking locally or globally, irrespective of the fineness of the microstructure. In the coarse structure (large SDAS), this critical amount of damage is easily reached, due to the clusters of large and elongated particles, leading to alloy fracture before global necking. However, in the alloy with the small SDAS, the critical amount of damage is postponed until global necking takes place due to the small and round particles. Current models for dispersion hardening can be used to calculate the stresses induced in the particles. The calculations agree well with the results inferred from the experimental results. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.