Achieving room-temperature superplasticity in an ultrafine-grained Zn-22% Al alloy

Abstract

The mechanisms of creep and superplasticity occurring in conventional coarse-grained materials are now understood well. However, recent study advances in the production of bulk metals with submicrometer grain sizes which provide the opportunity to demonstrate improved mechanical properties. Thermo-mechanical processing is used in conventional industrial practice to achieve substantial grain refinement in bulk metals whereas the smallest grain sizes achieved in this way are of the order of a few micrometers and generally it is not possible to achieve grain sizes within the submicrometer or nanometer range. In this report, synthesis of an ultrafine-grained Zn-22 % Al eutectoid alloy was demonstrated through solutionizing followed by thermo-mechanical processing. Microstructural investigations revealed there are stable equiaxed ultrafine grain sizes of ~0.63 µm with homogeneous distributions of Zn and Al grains. Tensile testing demonstrated the occurrence of excellent room-temperature superplasticity with a maximum elongation of 400 % at a strain rate of 1.0×10-3 s-1 where the elongation is one of the highest room-temperature superplastic elongation recorded to date in Zn-22 % Al alloy. However, the strain rate sensitivity of superplastic flow was measured as ~0.24 which is lower than the theoretical value of ~0.5 for conventional superplasticity. The present study estimates a threshold stress as one of possible reasons for lowering the strain rate sensitivity of room-temperature superplastic flow in the ultrafine-grained Zn-22 % Al alloy.

Review of Experimental Studies on Structural Superplasticity: Internal Structure Evolution of Material and Deformation Mechanisms

“…В ряде работ по исследованию СПД отмечается, что рекристаллизации не происходит, например, для двухфазного сплава Pb-62%Sn [133]; в других работах [74- 76,116], где приведены данные для того же сплава, про рост зерен не упоминается; для сплава Zn-22%Al в рассмотренных работах [69][70][71]115] об изменении размеров зерен также ничего не говорится. В другой экспериментальной работе [135] для Zn-22%Al указано, что рост зерен происходит при очень высоких температурах испытания (порядка 493К (0,75Т m )), при этом является незначительным.…”

Significance of Si impurities on exceptional room-temperature superplasticity in a high-purity Zn-22%Al alloy

“…The alloy with ultrafine grains was tested in tension at room temperature and, at an optimal strain rate of 1.0 × 10 -3 s -1 , the high-purity alloy with less Si impurity exhibited the maximum elongation of ~500% where the alloy including ~100 ppm Si recently demonstrated a lower limit of superplastic elongation of ~400% [49]. All alloy conditions demonstrated a consistent value of strain rate sensitivity of m § 0.24 and no difference was found in the flow behavior in terms of the Si content whereas there was an 9 apparent dependence upon elongations to failure where less Si impurity shows higher ductility.…”

“…Moreover, there was a visible difference in the dimple numbers and shapes depending on the Si content. Two recent reports summarizes the high m values of over 0.2 in various ultrafine-grained Zn-Al alloys tested at room temperature [48,49] and the updated table is shown in Table 3 where the list includes the present experimental data as well as other new results on several Al-Zn and Zn-Al alloys [14,[24][25][26][27]29,32,33,35,47,[49][50][51][52][53][54][55][56][57][58][59]. Thus, although almost no earlier papers carefully examined the importance of Si impurity in Zn-Al alloys, it is clear that the present high-purity Zn-22% Al alloy with less Si content recorded the maximum room-temperature superplastic elongation in Zn-Al alloy with the same composition prepared through different procedures.…”

“…A list of Al-Zn and Zn-Al alloys produced by different processing procedures showing strain rate sensitivity, m, at specific strain rate ranges at room temperature measured in different testing methods: the table updates tables published earlier[48,49].…”

“…Average grain sizes measured for all grains and for specific grains of Zn and Al and the average misorientation angles of grain boundaries for the Zn-22% Al alloy with five different Si contents.Table 3. A list of Al-Zn and Zn-Al alloys produced by different processing procedures showing strain rate sensitivity, m, at specific strain rate ranges at room temperature measured in different testing methods: the table updates tables published earlier[48,49].…”