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     

Transition of dominant diffusion process during superplastic deformation in AZ61 magnesium alloys

The superplastic behavior of the AZ61 magnesium alloy sheet, processed by one-step hot extrusion and possessing medium grain sizes of ∼12 μm, has been investigated over the temperature range of 523 to 673 K. The highest superplastic elongation of 920 pct was obtained at 623 K and a deformation rate of 1×10⁻⁴ s⁻¹. In the lower and higher strain rate regimes, with apparent m values of ∼0.45 and ∼0.25, respectively, grain-boundary sliding (GBS) and dislocation creep appeared to dominate the deformation, consistent with the scanning electron microscopy (SEM) examination. The SEM examination also revealed that individual GBS started to operate from the very initial deformation stage in the strain rate range with m∼0.45, which was attributed to the relatively high fraction (88 pct) of high-angle boundaries. The analyses of the superplastic data over 523 to 673 K and 5×10⁻⁵ to 1×10⁻³ s⁻¹ revealed a true stress exponent of ∼2, and the activation energy was close to that for grain-boundary and lattice diffusion of magnesium at 523 to 573 K and 573 to 673 K, respectively. The transition temperature of activation energy is ∼573 K, which is attributed to the change in the dominant diffusion process from grain-boundary diffusion to lattice diffusion. It is demonstrated that the effective diffusion coefficient is a valid parameter to characterize the superplastic behavior and the dominant diffusion process.     

The effect of grain size on the high temperature plastic deformation of nb3sn

A study of high temperature plastic deformation has been undertaken on 10, 20, and 60 μm grain size Nb₃Sn. The materials were produced by the hot isostatic pressing of powder blends. The 20 and 60 μm grain size material involved a stoichiometric blend of Nb and Sn powder, whereas the 10 μm grain size material involved a blend of 30.2 wt pct Sn powder and 69.8 wt pct Nb-1 Zr powder. The ZrO₂ formed during processing limits grain size and NbO formation. Through compression testing and load relaxation testing, deformation has been studied over a strain rate range from 10^-6 to 10^-2 per second and a temperature range from 1150 to 1650 °C. “Power law creep” was generally observed, although stress exponent reduction at the higher temperatures and lower strain rates suggests substructural coarsening. Analysis of stress-strain rate-temperature data projected an activation energy for creep of 400 to 500 kJ/mol. Grain size refinement clearly strengthened the polycrystals. Assuming a Hall-Petch relationship, “lattice friction stresses” and “unpinning constants” were calculated, both increasing with decreased temperature and increased strain rate. Grain size refinement from 60 to 10 μm lowered the ductile-to-brittle transition temperature for simple compression by the order of 125 °C.     

Effect of friction stir processing on the kinetics of superplastic deformation in an Al-Mg-Zr alloy

The effect of friction stir processing on the superplastic behavior of extruded Al-4Mg-1Zr was examined at 350 °C to 600 °C and at initial strain rates of 1×10⁻³ to 1 s⁻¹. A combination of a fine grain size of 1.5 μm and high-angle grain boundaries in the friction stir-processed (FSP) alloy led to considerably enhanced superplastic ductility, much-reduced flow stress, and a shift to a higher optimum strain rate and lower optimum temperature. The as-extruded alloy exhibited the highest superplastic ductility of 1015 pct at 580 °C and an initial strain rate of 1×10⁻²s⁻¹, whereas a maximum elongation of 1280 pct was obtained at 525 °C and an initial strain rate of 1×10⁻¹s⁻¹ for the FSP alloy. The FSP alloy exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive relationship for superplasticity in fine-grained aluminum alloys. A possible origin for enhanced superplastic deformation kinetics in the FSP condition is proposed.     

Superplasticity in a thermomechanically processed High-Mg,Al-Mg alloy

Superplastic elongations in excess of 400 pct have been observed in tension testing at 573 K (300 °C) and strain rate έ= 2 × 10^-3 s^-1 for a thermomechanically processed Al-10.2 pct Mg-0.52 pct Mn alloy. The thermomechanical processing consists of solution treatment and hot working, followed by extensive warm rolling at 573 K (300 °C), a temperature below the solvus for Mg in the alloy. This processing results in a fine subgrain structure in conjunction with refined and homogeneously distributed β(Al₈Mg₅) and MnAl₆ precipitates. This structure does not statically recrystallize when annealed at 573 K (300 °C) but appears to recrystallize continuously during deformation at such a temperature and the resulting fine grain structure deforms with minimal cavitation. At temperatures above the Mg-solvus,e.g., 673 K (400 °C), recrystallization and growth occur readily resulting in rela tively coarser structures which deform superplastically but with extensive grain boundary sliding and cavitation. Formerly in Materials Group, Mechanical Engineering, Naval Postgraduate School Formerly Graduate Student in Mechanical Engineering, Naval Postgraduate School     

Effects of fatigue on the GP zones in Al-Zn alloys

Small-angle X-ray scattering has been employed to study the stability of GP zones in two Al-Zn alloys during fatigue, in order to provide statistically sound information on this process. In a 5.3 at. pct Zn alloy containing l0²⁵ zones per m³ with an average diameter of 18 Å, the normally sluggish coarsening was accelerated by fatigue at room temperature by factors of l0⁶ to l0⁷. No coarsening occurred for larger zones, except near a fatigue crack. Reverted samples aged rapidly during fatigue at room temperature, but in a reverted sample of Al-3.5 at. pct Zn cycled at 77 K no appreciable zone growth occurred. Upon warming this sample to room temperature (without any load) rapid clustering took place. These results imply that a vacancy fraction of 10^-5 to l0^-6 was produced by the fatigue, and this excess vacancy concentration appears to cause the growth during cycling at room temperature.     

Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining

A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total strain of about 16 resulted in an average grain size of about 1 μm. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in the wide temperature range 250 °C to 500 °C at strain rates higher than 10⁻² s⁻¹. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹ with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10⁻¹ s⁻¹, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast, at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication of high-volume components.     

Superplastic behavior of spray-deposited 5083 Al

A 5083 Al alloy was synthesized using spray deposition processing with N₂ as the atomization gas. It was noted that the grains that were present in as-spray-deposited 5083 Al were equiaxed with an average size of 15.2 μm. The matrix of the material was supersaturated with Mg and Mn. The asspray-deposited microstructure contained irregular pores with porosity in the range of 0.1 to 5.4 vol pct, depending on spatial location in the preform. The spray-deposited alloy was thermomechanically processed using extrusion and multiple-pass warm rolling to reduce grain size and close porosity. It was observed that spray-deposited 5083 Al exhibited superplasticity following thermomechanical processing by extrusion followed by rolling. Superplasticity was observed in the 500 °C to 550 °C temperature range and 3 × 10⁻⁵ to 3 × 10⁻³ s⁻¹ strain rate range. The corresponding strain-rate-sensitivity factors were in the 0.25 to 0.5 range and increased with decreasing strain rate. A maximum elongation of 465 pct was noted at 550 °C and 3 × 10⁻⁵ s⁻¹. The spray-deposited 5083 Al, thermomechanically processed by direct rolling, exhibited superplasticity in the same temperature and strain rate ranges as those for the extruded and rolled materials. The superplastic elongation of the spray-formed and rolled material was relatively low, being in the range of 250 to 300 pct. The deformation behavior is discussed in light of the presence of porosity in the microstructure.     

Evaluating the Superplastic Flow of a Magnesium AZ31 Alloy Processed by Equal-Channel Angular Pressing

Experiments show that the magnesium AZ31 (Mg-3 pct Al-1 pct Zn) alloy exhibits excellent superplastic properties at 623 K (350 °C) after processing by equal-channel angular pressing using a die with a channel angle of 135 deg and a range of decreasing processing temperatures from 473 K to 413 K (200 °C to 140 °C). A maximum elongation to failure of ~1200 pct was achieved in this alloy at a tensile strain rate of 1.0 × 10^?4 s^?1. Microstructural inspection showed evidence for cavity formation and grain growth during tensile testing with the grain growth leading to significant strain hardening. An examination of the experimental data shows that grain boundary sliding is dominant during superplastic flow. Furthermore, a comprehensive review of the present results and extensive published data for the AZ31 alloy shows the exponent of the inverse grain size is given by p ≈ 2 which is consistent with grain boundary sliding as the rate-controlling flow mechanism.     

Low-temperature superplasticity of ultra-fine-grained Ti-6Al-4V processed by equal-channel angular pressing

The low-temperature superplasticity of ultra-fine-grained (UFG) Ti-6Al-4V was established as a function of temperature and strain rate. The equiaxed-alpha grain size of the starting material was reduced from 11 to 0.3 μm (without a change in volume fraction) by imposing an effective strain of ∼4 via isothermal, equal-channel angular pressing (ECAP) at 873 K. The ultrafine microstructure so produced was relatively stable during annealing at temperatures up to 873 K. Uniaxial tension and load-relaxation tests were conducted for both the starting (coarse-grained (CG)) and UFG materials at temperatures of 873 to 973 K and strain rates of 5 × 10⁻⁵ to 10⁻² s⁻¹. The tension tests revealed that the UFG structure exhibited considerably higher elongations compared to those of the CG specimens at the same temperature and strain rate. A total elongation of 474 pct was obtained for the UFG alloy at 973 K and 10⁻⁴ s⁻¹. This fact strongly indicated that low-temperature superplasticity could be achieved using an UFG structure through an enhancement of grain-boundary sliding in addition to strain hardening. The deformation mechanisms underlying the low-temperature superplasticity of UFG Ti-6Al-4V were also elucidated by the load-relaxation tests and accompanying interpretation based on inelastic deformation theory.     

Strain-Rate Sensitivity and Failure Peculiarities in Compression of the Nanocrystalline Ni-20 Pct Fe Alloy at Low Temperatures

The strain-rate sensitivity of the applied stress was measured, and the values of the activation volume of the process of the plastic deformation V along the deformation curve in compression of the nanocrystalline (NC) Ni-20 pct Fe alloy (with average grain size ~22 nm) in the temperature range of 300 to 77 K (27 to −196 °C) were calculated. It was found that the decrease of the temperature from 300 to 77 K leads to a decrease of the value of V from 20 to 8 b ³, and values of V do not depend on the strain. Fractographic features of the failure surfaces were studied in the temperature range 300 to 4.2 K (27 to −269 °C). Observed traces of melting on these surfaces at temperatures below 300 K indicate the intense heating in the catastrophic shear band of the alloy in the moment of failure. Causes of low-temperature decohesion along grain boundaries are discussed in terms of the sulfur segregation influence.     

An investigation of superplasticity in a thermomechanically processed U-2Mo (α + δ) alloy

A uranium-2 molybdenum (U-2Mo) alloy was shown to exhibit superplastic behavior over the β + γ two-phase field temperature regime and over a limited temperature span in the α + γ field. At Oak Ridge, two distinct processes were developed that evolved microstructures conducive to superplasticity. These microstructures were shown to exhibit superplasticity (elongations >500 pct) over a broad range of strain rates, from 2.5 × 10^-4 to 1 × 10^-2 s^-1. A maximum value of 700 pct elongation was reached at 695 °C and a true constant strain rate of 2.5 × 10^-3 s^-1. This study details the processing sequences, microstructures, strain-rate sensitivity, and maximum elongation data generated to characterize the superplastic U-2Mo alloy. In addition, the fracture and cavitation analyses conducted on constant strain-rate tensile test specimens are discussed.     

The role of dispersed particles in strengthening and fracture mechanisms in a Mo-ZrC alloy processed by mechanical alloying

The tensile properties of a ZrC particle-dispersed Mo, which was processed by spark plasma sintering with mechanically alloyed powder, were investigated at room temperature and at elevated temperatures of 1170 to 1970 K. The Mo-ZrC alloy showed much higher strength at room temperature than a fully recrystallized pure Mo. The high strength of Mo-ZrC is mainly attributed to a very small grain size (about 3 μm). The main role of the ZrC particle is not to increase strength due to the particle-dislocation interaction, but to limit grain growth during sintering and to attain the very small grain size. The elongation at room temperature of Mo-ZrC was much lower than that of pure Mo. This is probably related to the higher interstitial contents. However, Mo-ZrC showed a large elongation of 180 pct at 1970 K and 6.7×10⁻⁴ s⁻¹. It was suggested that the ZrC particles stabilized the fine-grained microstructure yet provided no cavitation sites at 1970 K; as a result, the large elongation was attained.     

Temperature and strain-rate effects on low-cycle fatigue behavior of alloy 800H

The effects of strain rate (4 × 10^-6 to 4 × 10^-3 s^-1) and temperature on the low-cycle fatigue (LCF) behavior of alloy 800H have been evaluated in the range 750 ° to 950 °. Total axial strain controlled LCF tests were conducted in air at a strain amplitude of ± 0.30 pct. Low-cycle fatigue life decreased with decreasing strain rate and increasing temperature. The cyclic stress response behavior showed a marked variation with temperature and strain rate. The time- and temperature-dependent processes which influence the cyclic stress response and life have been identified and their relative importance assessed. Dynamic strain aging, time-dependent deformation, precipitation of parallel platelets of M₂₃C₆on grain boundaries and incoherent ledges of twins, and oxidation were found to operate depending on the test conditions. The largest effect on life was shown by oxidation processes. On leave from the Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India.     

Influence of carbon content on superplastic behavior in Ti- and B-added Cr-Mo steels

Superplasticity has been investigated in various ferrous alloys and steels. However, in these materials, especially in hypoeutectoid steels below the A ₁ temperature, the relationship between the content of carbon and elongation to failure is not obvious. In the present investigation, the influence of carbon content on superplastic behavior is studied using carbon steels based on Cr-Mo steel. In order to obtain the fine grain structure, a small amount of Ti and B were added and the content of carbon was controlled to be in the range from 0.24 to 0.83 pct. The largest value of elongation to failure was 644 pct, which was obtained for a specimen containing 0.58 pct carbon. The temperature and strain rate at which the maximum value was obtained were 710 °C and 5×10⁻⁴ s⁻¹, respectively. Of all the specimens, this specimen had the minimum grain size. Moreover, the area fraction of carbide took the maximum value at the temperature where the largest elongation value was obtained. These results show that the addition of carbon has an effect on grain refinement by the formation of carbide, but excess amounts of carbon(>0.6 pct) bring about premature failure because of the resulting coarse microstructure and larger carbides.     

Environmentally assisted cracking of two-phase Fe-Mn-Al alloys in NaCl solution

Three two-phase Fe-Mn-Al alloys with nominal compositions, Fe-24Mn-9Al, Fe-27Mn-9Al-3Cr,. and Fe-27Mn-9Al-6Cr, were prepared in the solution-treated and cold-rolled conditions. The fractions of ferrite in the solution-treated condition were controlled at 46 to 60 pct, mainly by adjusting the carbon content and the relative amounts of Mn and Al. The ferrite fractions were reduced to 30 to 37 pct after 75 pct deformation by cold-rolling. Specimens were tensile tested at open circuit in aerated 3.5 pct NaCl solution at slow strain rates ranging from 4 × 10^-7 to 4 × 10^-5 s^-1 at room temperature. All of the alloys were quite susceptible to environmentally assisted cracking (EAC). The deformed specimens showed less susceptibility, presumably because the plasticity was already too limited. The EAC appeared to occur at or after the onset of plastic deformation. In this alloy system, the ferritic phase was less resistant to EAC than the austenitic phase, in contrast to the Fe-Cr-Ni stainless steels. The crack propagated preferentially through the ferrite grains or along the ferrite/austenite grain boundaries. The addition of up to 6 pct Cr did not improve the EAC resistance. Formerly Graduate Student, Department of Materials Science and Engineering, National Tsing Hua University     

Microstructures and superplastic behavior of eutectic Fe-C and Ni-Cr white cast irons produced by rapid solidification

Superplastic behavior of two commercial grade white cast irons, eutectic Fe-C and Ni-Cr white cast irons, was investigated at intermediate temperatures (650 to 750 °C). For this purpose, rapidly solidified powders of the cast irons were fully consolidated by compaction and rolling at about 650 °C. The volume fractions of cementite in the eutectic cast iron and in the Ni-Cr cast iron were 64 pct and 51 pct, respectively, and both cast irons consisted of fine equiaxed grains of cementite (1 to 2 μm) and ferrite (0.5 to 2 μm). The cast iron compacts exhibited high strain-rate sensitivity (strain-rate-sensitivity exponent of 0.35 to 0.46) and high tensile ductility (total elongation of 150 pct to 210 pct) at strain rates of 10^-4 to 10^-3 s^-1 and at 650 °C to 750 °C. Microstructure evaluations were made by TEM, SEM, and optical microscopy methods. The equiaxed grains in the as-compacted samples remained unchanged even after large tensile deformation. It is concluded that grain boundary sliding (e.g., along cementite grain boundaries in the case of the eutectic cast iron) is the principal mode of plastic deformation in both cast irons during superplastic testing conditions. Formerly with the Department of Materials Science and Engineering, Stanford University Formerly Visiting Scholar, Department of Materials Science and Engineering, Stanford University     

Effect of plastic deformation on the coarsening of

Particle coarsening has been studied in a rapidly solidified Al-8.8 wt pct Fe-3.7 wt pct Ce alloy subjected to isothermal annealing for various times at 425 °C. The effect of static and dynamic loading on the particle coarsening rates at the same temperature also has been examined. The dispersed particles in all specimens of the present study are the equilibrium Al¹³Fe⁴ and Al¹⁰Fe²Ce phases. They are incoherent with the matrix and constitute 23 pct of the total volume. The coarsening rate in isothermally annealed specimens is orders of magnitude greater than predicted by the modified Lifshitz-Slyozov-Wagner theory for volume diffusion controlled coarsening but can be explained using Kirchner’s model for coarsening by diffusion along grain boundaries. In the case of intragranular particles, coarsening by diffusion along dislocation cores also is likely to be significant. Creep loading is seen to cause a significant enhancement of the coarsening rate. Fatigue testing with a hold period at the maximum tensile stress also accelerates coarsening whereas continuous cycling appears initially to retard the increase in the average particle size. Dislocations connecting dispersed phase particles are observed more frequently in crept specimens and specimens fatigued with a hold period than in specimens fatigued with no hold period. The effects of plastic deformation on particle coarsening rates are discussed in terms of excess vacancy generation, short circuiting along dislocations, and fine precipitation during fatigue.     

The effect of temperature and carbon content on the cavitation behavior of a 1.5 pct Cr-0.5 pct V steel

Constant strain rate tests at 10^-6 s^-1 have been carried out at 823 K and 923 K on a vacuum melted 1 1/2 pct Cr 1/2 pct V ferritic steel containing 3 different carbon contents. After straining to various elongation values specimens were unloaded, cooled and fractured at 77 K. This gave fracture surfaces consisting almost entirely of intergranular facets, enabling a quantitative study to be made of the different stages of cavity nucleation and growth. It was found that cavity growth rates were independent of carbon content but were higher at 923 K than at 823 K. Subsequent grain boundary sliding measurements, using a surface offset technique showed that sliding increased with increasing carbon content and that cavity nucleation occurred selectively at large grain boundary carbides. Formerly of the Department of Metallurgy, University of Manchester.