Enhanced strength and ductility of AZ80 Mg alloys by spray forming and ECAP

Abstract

The relatively low strength and poor ductility of conventional AZ80 Mg alloys have been attributed to the limited number of independent slip systems, in combination with the formation of fragile eutectic β-Mg₁₇Al₁₂ networks at grain boundaries. In an effort to overcome these limitations, spray forming followed by equal channel angular pressing (ECAP) was employed to obtain a unique bi-modal microstructure: coarse grains were separated and surrounded by deformation networks consisting of ultrafine-grained Mg with an average grain size of 0.6 µm and ellipsoidal shaped β-Mg₁₇Al₁₂ particles with sizes of 200–300 nm. Tensile tests revealed the advantage of this structure: a yield strength of 235 MPa combined with an elongation to failure of 14%; the values are significantly higher than those of their conventional counterparts (100 MPa-12%, and 140 MPa-5%). The underlying strengthening and deformation mechanisms of this particular microstructure are discussed and analyzed.

Introduction

Mg alloys have potential for wide industrial applications, including those in the automobile and aviation industries as well as in 3C products (computer, communication and consumer electronic), because of their low density, high specific strength, high specific stiffness along with good machinability, recyclability and damping capacity. However, there are still several issues that limit their widespread industrial applications [1], [2], one of which is their limited plasticity. Their hexagonal close-packed (hcp) crystal structure determines their limited independent slip systems, which is a major reason for their low plasticity. Other microstructural features may also significantly affect the plasticity. For example, in the AZ series of Mg alloys, which contain Al and Zn as solutes, continuous β-Mg₁₇Al₁₂ precipitates usually form networks at grain boundaries (GBs) during casting. These β-Mg₁₇Al₁₂ networks are fragile and tend to initiate cracks during subsequent deformation [3], which further lowers the limited plasticity. In addition, the eutectic β-Mg₁₇Al₁₂ phase also behaves as a cathodic phase, which accelerates the corrosion of α-Mg matrix [4].

It is well known that rapid solidification techniques such as spray forming can overcome some of the problems associated with conventional casting techniques. For examples, previous studies on aluminum and iron-based alloys prepared by spray forming [5], [6], [7] revealed significant reduction in grain size and micro-segregation, which consequently led to enhancement of mechanical properties. However, review of published studies [8], [9] also shows that spray formed materials frequently contain a high volume fraction of pores which limit their ductility. Approaches to solve the porosity problem include extrusion, hot/cold roll or iso-static pressing, heat treatment, and forging following spray forming [10], [11], [12], [13]. In reference [14], AZ91 prepared by spray forming and extrusion exhibited outstanding combinations of mechanical properties with a tensile ultimate strength (UTS) of 435 MPa, and YS of 360 MPa, and elongation to failure (EF) of 9.2%, whereas the hot-rolled AZ91 alloy [15] exhibited a UTS of 345 MPa and a YS of 297 MPa.

Severe plastic deformation (SPD) techniques, such as equal channel angular pressing (ECAP), have been effectively used to refine the overall microstructure of copper [16], [17], aluminum [18], titanium [19], nickel [20], Mg [21] and other metals [22], [23]. In the case of hcp metals such as Mg, ECAP processing has been reported to have the following effects. First, grain refinement; this is noteworthy because it is difficult to refine Mg grains to sub-micron level via traditional processing, such as rolling, forging and extrusion. However, ECAP processing has been used to effectively produce homogeneous ultrafine grained (UFG) microstructures in a number of commercial Mg alloys including AZ31, AZ61 and AZ91 [24], [25], [26]. It is known that the extent of grain refinement increases with the number of ECAP passes. Moreover, to successfully process pure Mg by ECAP, temperature of 400 °C is required; in the case of Mg-0.9% Al the required temperature is 200 °C [27]. These deformation temperatures are well above the recrystallization temperature (T_deformation>0.4T_M), which inevitably leads to dynamic recrystallization (DRX). It has been reported [28], [29], [30] that the reduced grain size of Mg alloys is a consequence of accumulated large strain and DRX.

Second, SPD techniques can break down secondary phases. For Mg alloys with high levels of Al, such as AZ80 and AZ91, a large volume fraction of β-Mg₁₇Al₁₂ precipitates continuously and grows directly on the Mg base plane, which belongs to the space group of I$\bar{4}$3m and has long-plate morphology, thus resulting in ineffective age hardening. During ECAP, the distribution, morphology and size of the precipitates can be optimized for better performance [31], [32], [33]. Specifically, the breakdown and redistribution of the precipitates by ECAP processing can reduce fracture initiation during deformation, and thereby enhance plasticity. For example, the YS, UTS and EF of the AZ91 alloy [34] were remarkably increased to 290, 417 MPa and 8.45%, respectively, after a two-step ECAP processing, mainly due to the refinement of grain and Mg₁₇Al₁₂ precipitates at GBs. Interestingly, several studies on Mg alloys [35], [36], [37] reported the presence of a bi-modal microstructure, that is CGs surrounded by a deformation layers near the original grain boundary that are composed of UFGs and a large numbers of second phase particles. Such a bi-modal microstructure has been reported to improve mechanical properties. For example [38], an ZK60 alloy processed by ECAP for 6 passes shows superplastic behavior with an elongation of 2040% at the tensile temperature of 473 K, which was attributed to the bi-modal structure with an area fraction of ∼20% of large grains (20–50 µm) and ∼80% of UFGs (~1 µm). Finally, SPD processing may modify the texture, which plays an important role in mechanical behavior of hcp metals. For example, ECAP processing and subsequent annealing have been reported to decrease the yield strength of an extruded AZ31 alloy due to texture modification [39]. A subsequent study by Lin et al. [40] found that the strength decrease was caused by the change of the Schmid factor due to tensile testing direction. The strong texture formation during the processing of Mg alloys may produce a strong anisotropy in mechanical properties [41].

A temperature step-down approach has been reported for ECAP processing of Mg alloys, in which lower processing temperature was used with increasing ECAP passes [42], [43], [44]. The advantage of this processing approach is that any DRX that occurs at higher temperatures effectively randomizes grain orientation and improves the plasticity for subsequent passes, while the lower temperatures at later passes generates a high defect density and fine grain sizes, which are beneficial to strength.

On the basis of the above published results, we hypothesize that it should be possible to implement a combination of ECAP and spray forming to simultaneously enhance the strength and ductility of Mg alloys. To verify this hypothesis, we processed an AZ80 Mg alloy by casting and spray forming and then ECAP processing via route Bc at various temperatures. Systematic microstructure studies on the extrusion direction (ED, X plane), flow direction (FD, Y plane) and longitudinal direction (LD, Z plane) were carried out. The Vickers microhardness and tensile properties were also determined. These experimental data were used to elucidate the underlying mechanisms that were responsible for the observed increase in strength and ductility.