Abstract
Understanding the coarsening mechanisms and the role of solute atoms during recovery annealing of ultrafine lamellar-structured alloys produced by high strain deformation is crucial to tailor their microstructures and mechanical properties. In the present work, a lamellar-structured Al–0.3%Cu alloy with a boundary spacing of 200 nm was prepared by cold rolling to a von Mises strain of 4.5 (a thickness reduction of 98%), featuring Cu segregation to high angle lamellar boundaries. During recovery annealing in the temperature range of 100–175 °C, precipitation of fine Al2Cu particles occurred preferentially at lamellar boundaries. Recovery kinetics was analyzed based on measurements of lamellar boundary spacings in the annealed samples, showing an increase in the apparent activation energy from 77 kJ/mol at the beginning to 106 kJ/mol at the end of recovery. In situ observations of annealing in a transmission electron microscope revealed that the dominant coarsening process is the motion of Y-junctions formed by lamellar boundaries, which is subjected to various degrees of pinning from dislocations, dislocation boundaries and particles. Furthermore, it was found that this local pinning effect can be reinforced with the increase of misorientation angles of the attached dislocation boundaries, the coarsening of Al2Cu particles and the combined effect of interconnecting boundaries and particles. The results underpinned the importance of alloying elements in stabilizing finely spaced lamellar structures during deformation and annealing, providing guidelines for tailoring stable ultrafine structured alloys.
Highlights
After plastic deformation to high and ultra-high strain, the microstructures of metals often exhibit lamellar morphology and have lamellar boundary spacings in the sub-micrometer and nanometer range
To study the recovery kinetics in heavily rolled Al–0.3%Cu, a number of small samples were cut from the as-rolled plate, and subjected to isothermal annealing at temperatures of 100, 125, 150 and 175 °C over time intervals ranging from 4 min to 4096 min since our previous studies [21,23] showed that initiation of recrystallization began after annealing at 175 °C for 1 h
The coarsening kinetics and the role of solute atoms during recovery annealing of an ultrafine lamellar-structured Al–0.3%Cu alloy produced by 98% cold rolling have been studied by scanning electron microscope (SEM)-electron channeling contrast (ECC) characterizations of bulk samples and in situ transmission electron microscopy (TEM) observations of foil samples
Summary
After plastic deformation to high and ultra-high strain, the microstructures of metals often exhibit lamellar morphology and have lamellar boundary spacings in the sub-micrometer and nanometer range. The energy stored in this kind of deformation induced ultrafine structures is very high, providing a large driving force for recovery and recrystallization These ultrafine lamellar-structured metals usually lack thermal stability [6,7,8]. In the process of thermal treatment and during service at an elevated temperature, these stabilization factors themselves may change dynamically, such as precipitation and dissolution of second phase particles [13,14]. These changes are affected by the high densities of grain boundaries and dislocations. This means that the high density of grain boundary, boundary segregation and precipitation would interact with each other during annealing, resulting in a complex annealing phenomenon
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