Superplasticity in a new dispersion strengthened zinc alloy

A new dispersion strengthened zinc alloy has been developed which exhibits both high yield strengths at room temperature and superplasticity at elevated temperatures. The alloy has a nominal composition of Zn-0.10 Ni-0.04 Mg. A dispersion of submicron size precipitates was readily produced within this alloy by rapid solidification to form a supersaturated solid solution followed by thermal mechanical processing. The precipitates suppressed superplastic deformation at room temperature and stabilized the grains at elevated temperatures, reducing the grain growth rate of zinc to 4.1 × 10⁻² μ/h at 203°C. The deformation characteristics of the alloy are investigated with special emphasis on the influence of the precipitates on the tensile properties. It was found that the stress increased with a decrease in precipitate size and spacing for strain rates less than those at maximum rate sensitivity at 151°C and also for nonsuperplastic strain rates at 24°C. The slope of the Hall-Petch plot was found to be sensitive to both the grain diameter and strain rate at 24°C. The change of sign of the slope of the Hall-Petch plot at slow strain rates and small grain diameters was found to coincide with a rapid increase in the strain rate sensitivity. An equation of the form σ =A d ^ψ exp(Q/RT) ε^ηε^mhas been developed to describe the deformation characteristics of this alloy at elevated temperatures and presuperplastic strain rates. Formerly Graduate Student in Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada,     

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     

An experimental study on the alignment of cavities in a superplastic commercial copper alloy

A detailed study of cavitation was conducted in a commercial copper alloy deformed superplastically at a strain rate of 1.3 × 10^-3 s^-1. Cavities are observed to form in stringers parallel to the tensile axis due to cavity nucleation around aligned stringers of large cobalt-rich particles present in the as-received alloy. The increase in the number density of cavities and the range of cavity sizes observed indicates that cavities nucleate continuously during superplastic deformation. At large elongations, the interlinkage of cavities in a direction perpendicular to the tensile axis tends to mask the alignment of cavity stringers. It is demonstrated that the present results can rationalize satisfactorily the previous observations of cavity alignment in the superplastic copper alloy. Formerly with the Department of Materials Science, University of Southern California, Los Angeles, CA     

Superplasticity and residual tensile properties of a microduplex copper-nickel-zinc alloy

Structural superplasticity in two phase alloys of the copper-nickel-zinc system (nominal composition in wt. pct Cu-15Ni-38 Zn-0.2 Mn) occurs over a wide range of strain rates in the temperature range 850 to 1050∮F (454 to 565°). The upper temperature limit for super-plastic behavior in this system is determined by the reversion of the fine-grained two-phase structure to a single phase structure in which extensive grain growth is possible. Residual room temperature tensile properties and microstructure of the microduplex alloy after superplastic straining have been studied as a function of test temperature and total super-plastic strain. At test temperatures sufficiently removed from the phase transformation temperature, the high tensile properties and fine microstructure of the starting material are essentially retained after superplastic strains approaching 200 pct. In the immediate vicinity of the phase transformation temperature, rapid degradation of the microduplex structure occurs during superplastic deformation with a consequent severe degradation of the residual room temperature tensile properties. Formerly with The International Nickel Company, is now with Gulf Energy and Environmental Systems, Materials Science Department, P. O. Box 608, San Diego, Calif. 92112.     

Superplasticity of an aluminum-lithium 1420 alloy in various structural states

The superplasticity of Al-Li-Mg alloy 1420 with a grain size of 0.3–20 μm has been studied in various structural states. A nonlinear dependence of the superplastic elongation on the grain size is revealed. The optimum temperatures and strain rates are determined for nanocrystalline and submicrocrystalline alloys: they have large elongations (1200–1500%), high strain-rate sensitivity coefficients (higher than 0.45), and low activation energies (60–70 kJ/mol). The existence of an optimal grain size for reaching the maximum strain under structural superplasticity conditions is explained using a theoretical model proposed in this work.     

Superplasticity and internal friction of a superplastic Zn-22% Al eutectoid alloy

Temperature spectrums of internal friction, in other words, specific points have been investigated and discussed in the wide temperature range from room temperature to equilibrium eutectoid isotherm for Zn-22% Al eutectoid alloy in order to measure grain boundary peaks. Three large grain boundary peaks of Pα, Pαβ and Pβ which are associated with superplasticity have been observed over the range of temperatures from 447K (174°C) to 525K (252°C). The Activation energies of Pα, Pαβ and Pβ have been calculated to be 109, 93 and 62 kJ·mol⁻¹ respectively in internal friction measurements. These peaks seem due to the grain boundary sliding which can be accommodated by the diffusive flux on a boundary between like phases, α/α, for Pα and β/β for Pβ, and on an interphase boundary, α/β for Pαβ. Furthermore, it has been indicated that a utilization of Pα as the damping alloys would be possible.     

High temperature deformation of a commercial aluminum alloy

A study of high temperature deformation of a commercial aluminum alloy has been undertaken through tensile tests at strain rates ranging from 5.6×10⁻⁵ s⁻¹ to 5.6×10⁻² s⁻¹ and load relaxation testing in the temperature range 473 to 873 K. Experiments have established that maximum ductility is reached at about 623 K and at maximum strain rates. Maximum fracture ductility corresponds to minimum uniform elongation. The deformation and fracture mechanisms operating in the temperature range 473 to 573 K seem to differ from those between 623 K and 823 K; different strain rate sensitivities are also observed. Dynamic recovery is the dominant softening mechanism in high temperature plastic deformation—that is, a thermally activated process whose kinetics can be suitably described by an empirical power relation.     

High temperature deformation of a commercial aluminum alloy

A study of high temperature deformation of a commercial aluminum alloy has been undertaken through tensile tests at strain rates ranging from 5.6 × 10_-5 s_-1 to 5.6 × 10_-2 s_-1 and load relaxation testing in the temperature range 473 to 873 K. Experiments have established that maximum ductility is reached at about 623 K and at maximum strain rates. Maximum fracture ductility corresponds to minimum uniform elongation. The deformation and fracture mechanisms operating in the temperature range 473 to 573 K seem to differ from those between 623 K and 823 K; different strain rate sensitivities are also observed. Dynamic recovery is the dominant softening mechanism in high temperature plastic deformation—that is, a thermally activated process whose kinetics can be suitably described by an empirical power relation.     

The micromechanisms of fatigue crack growth in a commercial Al-Zn-Mg-Cu alloy

A detailed fractographic and microstructural study, using combined scanning and transmission electron microscopy has been made of the fracture surfaces produced by fatigue of a commercial aluminium alloy 7010:T76 in moist air, dry argon and dry oxygen. In the dry environments fracture is entirely ductile: the fracture surface is non-crystallographic and essentially striation free. In moist air fracture occurs by cleavage on {110} coupled with plastic deformation during the blunting and closing of the crack: this results in the formation of well defined surface striations. Striation topography is controlled by the local orientation of the cleavage plane to the direction of maximum tensile stress and to the slip systems of highest local shear stress. In consequence a range of striation profiles are developed. It is suggested that the cleavage component of growth is the result of hydrogen embrittlement of the matrix ahead of the crack and that dislocation transport is necessary to introduce this hydrogen.     

Superplasticity in a high strength powder aluminum

The superplastic properties of a rapidly solidified, high strength P/M Al alloy and the same alloy reinforced with SiC particulates (SiC _p ) have been studied. To prepare superplastic test materials, a matrix alloy powder of composition 7.2Zn-2.4Mg-2Cu-0.2Zr-0.12Cr-0.2Co (Kaiser PM-64) and the powder mixed with 10 to 20 vol pct SiC _p (~5 μm diameter) were thermomechanically processed to very fine equiaxed grain structures of ~6 μm and ~8 μm, respectively. Superplasticity in these materials was evaluated by characterizing (1) high temperature stability, (2) dynamic grain growth, (3) strain rate sensitivity, (4) flow stress behavior, (5) cavitation and cavitation control, and (6) total superplastic strain. It was observed that the PM-64 alloy could achieve a total elongation of over 800 pct, while the SiC_p reinforced alloy could attain an elongation greater than 500 pct before failure. Also, it was shown that with the use of hydrostatic pressure during superplastic flow, cavitation could be controlled. Observations were made of the effect SiC _p reinforcement particles had on the superplastic flow stress behavior. Interpretations are proposed to explain the role of particulates during superplastic straining.     

Precipitation effects during hot deformation of a copper alloy

Hot compression tests were performed on a previously solution-treated Cu-3Ni-lSi-O.8Cr-O.1 Mg alloy below the solvus temperature. The effects of precipitation occurring during hot deformation and the accompanying flow stresses were analyzed on the basis of microstructural evolution using optical, scanning, and transmission electron microscopy, and microhardness measurements. It was found that the hardening stage was followed by strain-induced localized Ni₂Si-precipitate coarsening at the temperature related to the most effective dynamic precipi-tation. Intensive coarsening of precipitates began at grain boundaries. Very fine Ni₂Si precip-itates were transformed into elongated particles at grain boundaries, producing flow localization, softening, and finally sample fracture. Formerly Graduate Student, Academy of Mining and Metallurgy, Cracow, Poland