Room Temperature Superplasticity in Fine/Ultrafine-Grained Zn-Al Alloys with Different Phase Compositions

Three Zn-Al alloys, namely Zn-22Al, Zn-5Al and Zn-0.3Al, were subjected to equal-channel angular pressing (ECAP), and the effect of ECAP on their microstructure and room temperature (RT) superplastic behavior were investigated in detail referring to previous studies reported by the authors of the current study. ECAP remarkably refined the microstructures of three alloys as compared to their pre-processed conditions. While the lowest grain size was achieved in Zn-22Al alloy as 200 nm, the grain sizes of Zn-5Al and Zn-0.3Al alloys were ~540 nm and 2 µm, respectively, after ECAP. After the formation of fine/ultrafine-grained (F/UFG) microstructures, all Zn-Al alloys exhibited superplastic behavior at RT and high strain rates. The maximum superplastic elongations were 400%, 520% and 1000% for Zn-22Al, Zn-5Al and Zn-0.3Al alloys, respectively. It is interesting to point out that the highest RT superplastic elongation was obtained in Zn-0.3Al alloy with the largest grain size, while Zn-22Al alloy having the lowest grain size showed the minimum superplastic elongation. This paradox was attributed to the different phase compositions of these alloys. The formation of Al-rich α/α phase boundaries, where grain boundary sliding is minimum comparing to Zn-rich η/η and η/α phase boundaries of Zn-Al alloys, is the lowest level in Zn-0.3Al alloy among all the alloys. Therefore, it can be concluded that if it is desired to achieve high superplastic elongation in Zn-Al alloys at RT, keeping Al content at a possibly minimum level seems to be the most suitable way.

Effect of Porosity on Tensile and Compressive Deformations of Superplastic Zn-22Al Alloy Foam

Closed-cell Zn-22Al superplastic alloy foams were manufactured through the melt foaming process. The Zn-22Al foams were produced with varying porosity of 51-71%. The tensile and compressive properties of the Zn-22Al foams were investigated at 523 K. The compressive specimen has m-value of 0.55 in the low strain rate region. This is because of the superplastic deformation induced by the fine microstructure of the cell wall. Though the superplastic elongation was not obtained in high temperature tensile test, the elongation was higher than that of conventional aluminum foams.

Effect of ECAP Hot Processing on Grain Refinement and Superplasticity of 1933 Aluminium Alloy

Cold and hot Equal-Channel Angular Pressing (ECAP) is an effective method to refine metallic grains. In this paper, superplastic properties of 1933 aluminium alloy were evaluated and the effect of hot ECAP on grain refinement and superplasticity was investigated. The testing results show that the refinement of grains can not be infinitely increased with the increasing of ECAP passes (or total strain). Under the isothermal ECAP conditions of the present study, optimum ECAP passes for 1933 alloy are 4 passes. The grain size of 1933 alloy was refined from 20~50μm to 7~12μm by means of ECAP for 4 passes at 300°C (route Bc), so its superplasticity was improved. Compared with original samples annealed at 400°C, the superplastic elongation of samples processed by ECAP for 4 passes increases by a factor of 130% about, and the range of superplastic temperature varies from 140°C to 210°C. The optimal superplastic temperature and initial strain rate is 510°C and 3.3×10-4s-1 individually, at which the elongation reaches 262% and the flow stress is 7.8MPa only. In a word, 1933 aluminium alloy can present more excellent superplasticity in wide range of superplastic temperature and strain rate.

Tensile fracture during transformation superplasticity of Ti–6Al–4V

During thermal cycling through the α–β phase transformation under the action of a small external biasing stress, Ti alloys exhibit an average deformation stress exponent of unity and achieve superplastic strains. We report tensile experiments on Ti–6Al–4V with an applied stress of 4.5 MPa, aimed at understanding the failure processes during transformation superplasticity. The development of cavities was assessed as a function of superplastic elongation, and macroscopic neck formation was quantified at several levels of elongation by digital imaging techniques. The effects of thermal inhomogeneity on neck initiation and propagation were also elucidated experimentally. Tensile ductility during transformation superplasticity is compared with that during isothermal creep at the average, effective cycling temperature, and a numerical model is used to show the effect of thermal gradients in limiting superplastic elongation.

Assessing Failure Mechanisms During Transformation Superplasticity of Ti-6Al-4V

During thermal cycling through the α/β phase transformation under the action of a small external biasing stress, Ti alloys exhibit an average deformation stress exponent of unity and achieve superplastic strains. We report tensile experiments on Ti-6Al-4V with an applied stress of 4.5 MPa, aimed at understanding the failure processes during transformation superplasticity. The development of cavities is assessed as a function of superplastic elongation, and macroscopic neck formation is quantified at several levels of elongation by digital imaging techniques. The effects of thermal inhomogeneity on neck initiation and propagation are also elucidated experimentally.