Microstructural and Textural Investigation of an Mg-Zn-Al-Ca Alloy after Hot Plane Strain Compression

Abstract

The formability of magnesium alloys can be significantly improved by Ca as an alloying addition. Compared to conventional alloy sheets such as AZ31, texture modification can be found in rolled Mg-Ca sheets, which reveal a randomized orientation distribution. The hot deformation behavior of a twin-roll cast and homogenized Mg-2Zn-1Al-0.3Ca (ZAX210) alloy was characterized during hot compression at a temperature of 350 °C and strain rates of 0.1 s−1 and 10 s−1. Electron backscatter diffraction (EBSD) analysis was performed in order to describe the microstructural and texture evolution. The ZAX210 alloy exhibits a pronounced dynamic recrystallization (DRX) behavior during compression at high strain rates, while at lower strain rates DRX hardly occurred. This effect can be attributed to different DRX mechanisms that take place as a function of strain rate. At low strain rates, DRX occurred locally at the grain boundaries of the original microstructure, forming a so-called necklace structure. Increasing strain rate results in an increased fraction of recrystallized grains from 18% (0.1 s−1) to 39% (10 s−1). The microstructure revealed that twin boundaries act as nucleation sites for the DRX (TDRX). The recrystallized areas exhibit a weaker texture compared to the deformed microstructure.

Cold Formability of Twin-Roll Cast, Rolled and Annealed Mg Strips

“…As written in Kittner et al (2022) [33] recrystallisation due to PSN can be excluded in the current ZAX210 strip. Rather, this strongly weakened texture, in which the c-axis of the crystallites is tilted from the normal direction (ND) to the RD-direction primarily, is caused by continuous or twin-induced dynamic recrystallisation at double and compression twins.…”

“…Particlestimulated nucleation (PSN), recrystallisation at deformation and shear bands, changes in grain boundary mobility brought on by secondary phases, and recrystallisation at double and compression twins are possible mechanisms for the weakening of textures [1,2,8,21]. As written in Kittner et al (2022) [33] recrystallisation due to PSN can be excluded in the current ZAX210 strip. Rather, this strongly weakened texture, in which the c-axis of the crystallites is tilted from the normal direction (ND) to the RD-direction primarily, is caused by continuous or twin-induced dynamic recrystallisation at double and compression twins.…”

“…The texture softening in ZAX210, resulting from the weakening of basal texture intensity and the splitting of basal poles towards the rolling direction (RD), contributed to its enhanced formability. Recrystallisation at double and compression twins, along with the potential grain boundary segregation of calcium, were identified as mechanisms for the weakened texture in ZAX210 [33]. The ZAX210 alloy offers an outstanding combination of strength, ductility and formability, which is comparable to those of AA6xxx aluminium alloy sheets.…”

“…The Mg 17 (Al,Zn) 12 phases are dissolved according to [7]. The ZAX210 strips show Mg 2 Ca-, MgZn-and Ca 2 Mg 6 Zn 3 -phases in α-Mg, based on [33]. The influence of the alloying elements on the texture can be clearly seen in the IPFs and pole figures (Figure 3).…”

“…Additionally, it is possible that Ca, which segregates at grain boundaries, limits their mobility. As a result, the intensity of the resulting texture is affected, and the preferential growth of grains is inhibited [33]. In Wang et al (2021) [1], the weak off-basal texture after extrusion or rolling-annealing procedures is attributed to a homogeneous fine-grained microstructure as a result of dynamic and/or static recrystallisation associated with the activity of the non-basal slip systems and the chance of stacking fault energy.…”