Research ArticleImproving toughness of a Mg2Ca-containing Mg-Al-Ca-Mn alloy via refinement and uniform dispersion of Mg2Ca particles
Introduction
In recent years, developing high strength and high ductility magnesium alloys has become an important research issue owing to the urgent demand for weight reduction in new energy automobile and aerospace applications [[1], [2], [3], [4]]. However, most of the high strength wrought magnesium alloys developed by now belong to Mg-RE (rare earth elements) based alloys [5,6]. Owing to a large number of RE additions, these alloys usually exhibit excellent mechanical properties, but together with high cost and high density, which restrict their further industrial applications [[7], [8], [9]].
Recent researches on low cost non-RE Mg alloys have suggested that Mg-Al-Ca-Mn alloys possess great potential as one of promising high strength and low cost wrought alloys [[10], [11], [12], [13], [14], [15], [16]]. High tensile yield strength above 400 MPa has already been reported in several as-extruded Mg-Al-Ca-Mn alloys [[10], [11], [12]]. The high strength of these alloys mainly originates from the formation of three Laves phases with high melting points, i.e., Mg2Ca (C14), Al2Ca (C15) and (Mg,Al)2Ca (C36) [13]. The formation of these Laves phases is dependent on the Ca/Al mass ratio of Mg-Al-Ca-Mn alloys, and it follows the sequence of Al2Ca → (Mg,Al)2Ca → Mg2Ca with the increase of Ca/Al mass ratio [14,15]. According to the studies reported so far, it is universally acknowledged that alloys with high strength exceeding 400 MPa mainly contain Mg2Ca phase or Mg2Ca + (Mg,Al)2Ca phases [[10], [11], [12]], while Al2Ca-containing alloys usually exhibit strength lower than 300 MPa [[17], [18], [19]]. Therefore, the Mg2Ca-containing Mg-Al-Ca-Mn alloys are attractive in engineering applications.
Generally, tensile strength alone is not enough to guarantee the safety of metallic components to avoid the low stress brittle fracture under practical engineering environments, especially for those components with large dimensions or used at low temperatures [20,21]. In addition to strength, toughness is also a key mechanical property index for materials design, selection and evaluation, which could reflect the capacity of materials to impede the initiation and propagation of cracks under loading [22,23]. Therefore, toughness sometimes is a more important index than strength. With regard to high strength Mg2Ca-containing alloys, a fatal problem could be their poor toughness, as the fracture elongations of these as-extruded alloys are usually lower than 5% [10,12]. By analyzing and comparing the microstructure of recently developed high strength as-extruded Mg-Al-Ca-Mn alloys, the Mg2Ca second phase was refined into fine particles (lower than 2 μm), and these particles were always aggregated and aligned in lines along the extrusion direction. These Mg2Ca gathered regions contain high density of crystal defects, and even precracks [10]. Consequently, microcracks are inclined to generate within the Mg2Ca particle regions and grow rapidly, causing early failure of the alloys and impairing their ductility and toughness. Therefore, to improve the toughness of Mg2Ca-containing alloys, it is preferable to tailor the distribution of Mg2Ca particles in an uniform pattern, as inspired by recent progress of high-performance metallic materials or metal matrix composite [[24], [25], [26]]. Unfortunately, the uniform dispersion of Mg2Ca particles has not been developed and reported yet.
In this study, we prepared two Mg2Ca-containing wrought alloys with the same chemical composition but with different morphology and distribution of Mg2Ca particles. One is the as-extruded alloy with typical aggregated and aligned Mg2Ca particles, the other one is an ECAP alloy with dispersedly distributed Mg2Ca particles. By exploring the differences in microstructure and mechanical properties of two wrought alloys, we mainly discussed the strengthening and toughening mechanisms of the Mg2Ca-containing alloys, which could provide insight for future alloy design and preparation of high performance magnesium alloys.
Section snippets
Experimental
To ensure the second phases in this studied Mg-Al-Ca-Mn alloy are all Mg2Ca phase, we set the designed alloy composition as Mg-3.5Al-4.0Ca-0.4 Mn (wt%) with Ca/Al ratio higher than 1 [12,15]. The alloy was prepared form pure Mg, pure Al, Mg-20Ca (wt%) and Mg-10 Mn (wt%) master alloys by the semi-continuous casting method. Ingot with diameter of 90 mm was obtained, and its actual chemical composition was analyzed to be Mg-3.66Al-4.25Ca-0.43 Mn by an inductively coupled plasma atomic emission
Morphology and distribution of Mg2Ca phase in three alloys
Fig. 1 shows the optical images of as-cast, as-extruded and ECAP alloys. It is apparent that Mg2Ca phase exhibits obviously different morphologies in three alloys. Mg2Ca phase (or Mg2Ca + α-Mg eutectic structure) forms continuous network at intergranular regions in as-cast alloy (Fig. 1(a)). Although both microstructures were obviously refined after two different processing routes, great differences can be identified. The network-shaped Mg2Ca phase is broken and aligned along the extrusion
Conclusions
In the present study, the microstructure and mechanical properties of two deformed Mg-3.66Al-4.25Ca-0.43 Mn (wt%) alloys with different Mg2Ca distributions were comparatively investigated. The following conclusions can be drawn:
(1) Banded microstructure is obtained for as-extruded alloy. Network-shaped Mg2Ca phase in as-cast alloy is broken into fine particles, which are aggregated and aligned in lines along extrusion direction. The Mg2Ca second phase bands are alternately arranged with α-Mg
Acknowledgements
This work was supported financially by the Fundamental Research Funds for the Central Universities (B200202131), the National Natural Science Foundation of China (Nos. 51901068, 51979099 and 51774109), the Nantong Science and Technology Project (No. JC2018109) and the Key Research and Development Project of Jiangsu Province (No. BE2017148).
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