Dynamic instantaneous dissolution of the precipitates in aged Mg-Zn-Zr alloy at high strain rate

Abstract

The commercial ZK60 magnesium alloy with extruded state experienced aging heat treatment (T6) was dynamically loaded at strain rate of 3000 s−1 by means of the split Hopkinson pressure bar (SHPB) in this paper. Transmission electron microscopy (TEM) observations showed that the precipitated β′1 phases partially dissolved (spheroidized) with blurred interfaces within 160 μs at 3000 s−1. The average length and diameter of the rod-shaped β′1 phase particles were 48.5 and 9.8 nm after the T6 heat treatment; while the average diameter of the spherical β′1 phases changed to 8.8 nm after loading. The deformed β′1 phase generated larger lattice distortion energy than Mg matrix under high strain rate loading. Therefore, the difference of free energy (the driving force of dissolution) between the β′1 phase and the matrix increased, making the instantaneous dissolution of the β′1 phase thermodynamically feasible. The dissolution (spheroidization) of the β′1 phase particles was kinetically promoted because the diffusion rate of the solute Zn atoms was accelerated by combined actions of adiabatic temperature rise, high density of dislocations (vacancies) and high deviatoric stresses during high strain rate loading. The increase in hardness of ZK60-T6 alloy could be attributed to solid solution strengthening, dislocation strengthening and second phase particle strengthening.

1. Introduction

Magnesium (Mg) stands out as one of the lightest structural metals globally, boasting a high specific strength. When compared at equal volumes, it is approximately 30% lighter than aluminum and a remarkable 75% lighter than steel [1, 2]. These unique properties render Mg a preferred material for applications in both the automotive and aerospace industries [3−5]. The research and application of magnesium alloys have garnered extensive attention across various sectors and disciplines [6, 7]. Nowadays, Mg alloys are widely used in aerospace, transportation and biomedical fields with the continuous development of lightweight alloys [8−10].

ZK60 alloy is a classic wrought magnesium alloy with extensive applications. Numerous studies have been conducted on the static precipitation of second-phase particles during the aging process of ZK60 magnesium alloy. The precipitation sequence is well-documented [11−13] as follows: SSSS → G.P. zones → β′1 → β′2 → β, where SSSS denotes the supersaturated solid solution, G.P. zones are disk-shaped and semi-coherent with the matrix, β′1 adopts a rod-like morphology, and β′2 is disk-shaped. The mechanical properties of ZK60 magnesium alloy can be enhanced through heat treatment. The β′1 phase serves as the primary phase for aging strengthening [14] and exerts a significant influence on the mechanical properties of the alloy. Several studies [15, 16] have indicated that during the extrusion process, the second phase particles underwent spheroidization and partial dissolution within the matrix, leading to an enhancement in strength and a reduction in plasticity of the ZK60 alloy. However, the majority of current research efforts have focused on phase transformations under static or quasi-static loading conditions, with limited studies exploring the behavior under high strain rate loadings.

In practical engineering applications, Mg alloys are inevitably exposed to impact loads [17]. The deformations of pure metals and their alloys are complicated and highly nonlinear under high strain rate loadings (strain rate > 10^2 s⁻¹).