New poly-types of LPSO structures in a non-equilibrium Mg97Zn1Y1.6Ca0.4 alloy

Abstract

In this study, a comprehensive analysis of microstructural features, morphology, crystal structures, and interface structures of long-period stacking ordered (LPSO) structures in a non-equilibrium Mg97Zn1Y1.6Ca0.4 alloy cast in a steel mold was carried out. The addition of Ca element plays an important role in the refinement of LPSO structure. The result reveals new poly-types including 20H F2F2F4, 60R (F2F3 3)3, and 66H F2F3 3F2(F6)4 featuring a 6-Mg structure, alongside the prevalent 18R and 14H LPSO structures. The incoherent interface between 20H and the Mg matrix is split into two dislocation arrays, leading to the formation of a segment of 60R1. Moreover, the superstructure 116L, designated as (F2)18F4, is formed through the ordered distribution of F4 stacking faults in 18R.

1. Introduction

Conventional magnesium alloys often encounter challenges such as poor corrosion resistance, thermal stability, and inferior mechanical properties at high temperatures [1, 2]. To address these limitations, various strategies have been employed, including chemical composition modification [3, 4], homogenization [5, 6], precipitation hardening [7, 8], texture modification [9-11], and grain refinement [12]. Recently, numerous investigations have been conducted focusing on Mg-M-RE alloys (where M comprises Al, Co, Ni, Cu, Zn, and Ga; and RE includes Y, Gd, Tb, Dy, Ho, Er, and Tm) due to their exceptional mechanical properties [13-15] and unique structural features [16, 17]. For instance, the Mg97Zn1Y2 alloy, synthesized through rapid solidification powder metallurgy, demonstrated an impressive tensile yield strength exceeding 600 MPa at room temperature and an elongation of about 5% [13, 14]. Subsequent Mg-M-RE alloys have exhibited tensile yield strengths ranging from 297 to 377 MPa [18-20]. The outstanding mechanical properties of Mg-M-RE alloys primarily originate from both grain refinement and the presence of long-period stacking ordered (LPSO) structures [13-15]. The formation of LPSO structures involves the periodic introduction of AB′C′A blocks into the hexagonal close-packed structure, with the prime-annotated letters indicating layers enriched with M/RE elements [21, 22]. These LPSO structures can exhibit both hexagonal (H) and rhombohedral (R) Bravais lattices, depending on their stacking sequence.

To comprehend the varied configurations of LPSO structures, explore their formation mechanisms, and establish inherent connections with the mechanical properties of magnesium alloys, extensive analytical studies have been undertaken, emphasizing their microstructure [23-30]. In this regard, various LPSO structures have been identified in different magnesium alloys [31-34]. In Mg-Zn-Y alloys, configurations such as 10H, 18R, 14H, and 24R have been observed, wherein 1, 2, 3, and 4 layers of magnesium atoms interspersed between every two neighboring AB′C′A blocks [18-22, 29, 35]. In Mg-Co-Y alloys, structures such as 15R, 12H, and 21R have been identified, featuring 2, 3, and 4 layers of magnesium atoms between every two neighboring AB′C blocks [32]. Moreover, Jin et al. [33, 34] discovered six additional LPSO structures, including 72R, 29H, 102R, 192R, 51R, and 60H, in a Mg92Co2Y6 alloy. These structures incorporate both AB′C′A and AB′C blocks [33]. Furthermore, an ultra-long 654R structure, derived from an ordered arrangement of 15R and 12H components, has been identified in a Mg88Co5Y7 alloy [34].

In Mg97Zn1Y2 alloys, the diameters of Mg, Zn, and Y atoms adhere to the sequence rZn(0.133 nm)