Enhancement of properties in cast Mg–Y–Zn rod processed by severe plastic deformation

Abstract

Ternary Mg–Y–Zn alloys have attracted considerable attention from researchers due to their excellent mechanical properties and unique microstructures, particularly from the presence of long-period stacking-order (LPSO) phases. Microstructural variations and the resulting mechanical properties can be affected by various processing routes, particularly those involving severe plastic deformation of a cast billet. The approach used in this work was based on subjecting cast Mg₉₂Y₄Zn₄ (composition in wt%) billet to severe plastic deformation by three different routes, namely equal channel angular pressing (ECAP), high pressure torsion (HPT) and ECAP followed by HPT, with the aim of refining the microstructure and improving mechanical properties. Samples processed by ECAP were annealed by post-processing and tested in compression and tension. The effect of the processing route and the process parameters on the microstructure and the hardness of the Mg–Y–Zn alloy is reported. An overall positive effect of annealing treatment on the mechanical properties of ECAP-processed alloy is demonstrated.

Introduction

Addition of rare-earth (RE) elements is known to increase the strength and to improve the creep properties of magnesium alloys at high temperatures due to the formation of intermetallic phases [1], [2], [3], [4]. Most RE-containing magnesium alloys show an excellent precipitation hardening response [1], [2], [3], [4]; however, Mg–Y–Zn ternary alloys are not precipitation hardenable [5]. Superior mechanical properties in these alloys are achieved by other techniques, for example, warm extrusion of rapidly solidified powder (strength of up to 600 MPa and 5% tensile ductility) [6], [7], or working of cast billet [8], [9], [10], which results in strengthening by grain refinement. The microstructure after such processing usually contains a mixture of nano-scale magnesium grains (100–200 nm) and intermetallic particles that exhibit a range of long-period stacking-order (LPSO) structures, among which 18R and 14H are the most common [11]. The notation used to designate the LPSO intermetallic phases is defined in Ref. [11]. A unique feature of such LPSO structures is their lamellar morphology that comprises alternating layers of magnesium and intermetallic lamellae. The accumulated experimental evidence indicates that mechanical properties of the Mg–Y–Zn alloys can be tailored by proper control of the volume fraction and size of the lamellae of the LPSO structures. It is expected that an ultrahigh strength alloy may be obtained if a nano-scale distribution of LPSO structures is achieved.

It is not possible to achieve a redistribution of LPSO structures by deformation with conventional metal forming processes, as they have a geometrical limitation on the amount of strain imparted to the material. However, by severe plastic deformation (SPD) methods [12] a billet can be subjected to virtually unlimited strain without a change in its shape and dimensions. The most well-known SPD methods are equal channel angular pressing (ECAP) [13] and high pressure torsion (HPT) [14]. The effects of SPD on the mechanical properties of Mg–Y–Zn alloys have not been explored to date. It can be expected, however, that severe plastic deformation may lead to much finer grains and potential dispersion of the LPSO structures, and therefore much higher yield strengths. A comparative study of microstructure and properties of Mg₉₂Y₄Zn₄ cast billets severely deformed by different SPD techniques has been performed in this work.