The Role of Grain Boundary Sliding in Microstructural Evolution during Superplastic Deformation of a 7055 Aluminum Alloy

Abstract

The microstructure evolution in a 7055 aluminum alloy subjected to thermomechanical processing (TMP) was studied at 450 • C anḋ ε = 1.7 × 10 −3 s −1 at which the material exhibits superplastic behavior with a total elongation of 720% and the coefficient m = 0.58. Partially recrystallized initial structure of the as-processed 7055 Al consisted of bands of recrystallized grains with a mean size of 11 µm alternating with bands of recovered subgrains with a mean size of 2 µm. The true stress-true strain curve exhibits a well-defined peak stress, followed by gradual strain softening. The coefficient of strain rate sensitivity, m, remains unchanged at ε ≤ 1 and tends to decrease with strain at ε > 1. The initial microstructure persists near the peak strain. Following strain leads to evolution of initial partially recrystallized structure into uniform fully recrystallized structure due to occurrence of continuous dynamic reactions, i.e. continuous dynamic recrystallization (CDRX). The data of microstructural observation and misorientation analysis show that low-angle boundaries (LAB) gradually convert to high-angle boundaries (HAB) resulting in an extensive flow softening. It was shown that grain boundary sliding (GBS) provides superplastic flow at all strains. Concurrently, GBS plays an important role in the dynamic evolution of new grains facilitating conversion of LABs to HABs.

Feasibility of attaining uniform grain structure and enhanced ductility in aluminum alloy by employing a beveled punch in equal-channel angular pressing

“…It is shown that the superplastic behavior of aluminum alloys in the unrecrystallized structure could be attributed to grain boundary sliding along high-angle grain boundaries. This grain boundary sliding along separate high-angle grain boundaries induces rotation in subgrains and facilitates transformation of low-angle grain boundaries to high-angle grain boundaries [25]. Therefore, uniform subgrain structure seems to provide enhanced grain boundary sliding and further transformation of low-angle grain boundaries to high-angle grain boundaries which leads to achieving higher elongations than banded subgrain structure.…”

EBSD characterization of repetitive grain refinement in AZ31 magnesium alloy

“…This is related to the fact that the HABs can efficiently absorb the dynamically generated dislocations [30,31]. The simulation of microstructural evolution during CDRX has denoted that at such large strains the dynamic recovery alone is unable to justify the restoration of microstructure [13] and that misorientations are mainly increased by grain boundary sliding [32]. The latter analogy may be held for SPD of magnesium alloys to very large strain at such a high temperatures (330 C).…”

A Study on Microstructure Evolution of 7055 Aluminum Alloy Based Under Extreme Environment

“…When the feed rate is 0. 5 mm, 1 mm and 1.5 mm, the minimum grain size is 0.75 µm, 0.83 µm and 0.67 µm at low temperature, respectively; and the minimum grain size is 0.54 µm, 0.75 µm and 0.55 µm at the high temperature environment, respectively [11]. At the same feed rate, the dislocation density at low temperature is higher than that of high temperature, and the grain size at low temperature is higher than that of high temperature too.…”