Dynamic recrystallization during hot compression of as-cast and homogenized noncombustible Mg–9Al–1Zn–1Ca (in mass%) alloys

Abstract

As-cast and homogenized Mg–9Al–1Zn–1Ca (in mass%) alloys were hot-compressed. Dynamic recrystallization (DRX) occurred locally around the insoluble second-phase particles during hot compression of the as-cast sample. Solute Al segregated around the particles activated basal slip operation through the c/a ratio increase in hcp lattice, which led to the DRX localization.

Introduction

Mg-based alloys have drawn much industrial and scientific interest for their low density, high specific strength and damping capacity [1], [2], [3]. One of the disadvantages of Mg alloys is its combustibility due to its high reactivity with oxygen in air; thus the casting operation of Mg alloys generally requires much attention. It has been reported that the addition of Ca as an alloying element significantly inhibits the oxidation and combustion of Mg [4]. Ca addition is currently being utilized to render the casting of Mg easier than before.

When Ca is added to conventional Mg–Al–Zn alloys, Ca and Al form insoluble second phases mainly composed of (Mg,Al)₂Ca [5], [6]. These second-phase particles are usually harmful to deformation; however, proper conditions of deformation enable good workability [7], [8], [9], [10]. The particles also offer additional nucleation sites for dynamic recrystallization (DRX) [11] and increase the fraction of the recrystallized volume [12]. Grain refinement is an effective way of strengthening Mg alloys by means of Hall–Petch effect [13]; therefore, hot forging inducing DRX is expected to be proper for producing Mg alloy parts with high strength.

As-cast Mg alloys often exhibit segregation of solutes around the second-phase particles; this segregation disappears upon subsequent homogenization annealing [14]. Solutes in the hexagonal close-packed (hcp) crystal lattice of the Mg matrix often affect the c/a axis ratio [15], [16]. When the c/a ratio is high, $(0001)〈11\overline{2}0〉$ basal slip is predominant during the plastic deformation of the Mg alloy. The high c/a ratio generally results in poor workability because the critical resolved shear stress (CRSS) for basal slip is much lower than that for nonbasal (prismatic and pyramidal) slip [17], [18]. On the other hand, a reduction in the c/a ratio seems to activate nonbasal slip [19], [20], although there is much room for study on the relationship between c/a ratio and workability of hcp metals.

The activities of basal and nonbasal slips may be closely related to the stress concentration, stress relaxation and DRX in hcp metals. In the case of face-centered-cubic (fcc) metals, DRX is more likely to occur in the metals with lower stacking fault energy; that is, dislocations easily extend into relatively large stacking faults, which hinders the climb and/or cross-slip of dislocations, so that a sufficient dislocation density can accumulate to trigger DRX for such metals deformed at elevated temperatures [21], [22]. In hcp metals such as Mg, DRX may be documented in terms of the c/a ratio because of the large difference in CRSS between basal and nonbasal slips. In this study, as-cast and homogenized Ca-containing Mg alloys were compressed at 573 K, and DRX during hot compression was examined. The effect of the segregation of solute Al on DRX is focused upon.