Effect of V on the microstructure and mechanical properties of Mg–10Er–2Cu alloy with a long period stacking ordered structure

Effect of V on the microstructure and mechanical properties of Mg–10Er–2Cu alloy with a long period stacking ordered structure

Materials Letters 122 (2014) 312–314 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet E...

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Materials Letters 122 (2014) 312–314

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Effect of V on the microstructure and mechanical properties of Mg–10Er–2Cu alloy with a long period stacking ordered structure X.H. Du a, G.S. Duan b, M. Hong a, D.P. Wang a, B.L. Wu a,n, Y.D. Zhang c,d, C. Esling c,d a

Sheyang Aerospace University, School of Materials Science and Engineering, Shenyang 110316, China Key laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, 110819 Shenyang, China c LEM3UMR CNRS 7239, Université de Lorraine, 57045 Metz, France d Laboratory of Excellence on Design of Alloy Metals for Low-mass Structures (DAMAS), Université de Lorraine, 57045 Metz, France b

art ic l e i nf o

a b s t r a c t

Article history: Received 12 December 2013 Accepted 15 February 2014 Available online 25 February 2014

In this study, the effects of a trace amount of vanadium (V) on the long-period stacking ordered (LPSO) phase and the associated mechanical properties of ternary Mg–Er–Cu alloy were investigated. The addition of V has modified the morphology of 18R-type LPSO phase from coarse block-shaped into finer lamellar-shaped. The as-extruded Mg–Er–Cu (V) alloy presented more promising mechanical performance with the tensile stress of 430 MPa and apparent elongation of 11% at room temperature. Simple interpretation based on thermodynamics was also given. & 2014 Elsevier B.V. All rights reserved.

Keywords: Magnesium alloys Long period stacking order phase Phase transformation Microstructure Vanadium element Thermodynamics

1. Introduction During the past decades, Mg and Mg base alloys have attracted intensive researching activities due to their low densities, high specific strength, well shielding characteristics and easy recycle [1]. One of these activities focuses on enhancing their servicing capability applied as structural materials, especially the strength in order to make them to be comparable with the aluminum alloys. In such a case, the long period stacking ordered (LPSO) phase found in some Mg alloys has received more attention due to its excellent strengthening and toughening effects [2–5]. So far, a variety of LPSO structure composing of 6H, 10H, 14H, 18R and 24R has been observed in the Mg–RE–TM (RE¼ rare elements: Y, Gd, Tb, Dy, Ho, Er, Tm; TM¼ transition elements: Zn, Cu, Ni, Co) systems and Mg–Al–Gd alloys [6–8]. Recently, several efforts have indicated that the Mg alloys could be strengthened further if the morphology of LPSO phase was modified to finer scale by adding minor alloying elements, such as Zr and Sr [9,10]. It has provided a novel and effective way to improve the strength of Mg alloys with LPSO phase. In this study, the refractory element vanadium which might have the potential to modify the LPSO

n

Corresponding author. Tel.: þ 862489728701; fax: þ 862489728729. E-mail addresses: [email protected], [email protected] (B.L. Wu).

http://dx.doi.org/10.1016/j.matlet.2014.02.056 0167-577X & 2014 Elsevier B.V. All rights reserved.

phase of Mg alloys was chosen to investigate its function on the resultant microstructure and mechanical properties.

2. Materials and experimental Ternary Mg–10Er–2Cu alloys with and without 0.3 wt% V was prepared from Mg–Er, Cu–V master alloys and high-purity Cu element. Here, we select 0.3 wt% V to investigate its effect on the precipitated phase of Mg alloys. More works on the effect of various amounts of V on the precipitated phase of Mg alloys are ongoing. Alloy making was conducted in an induction melting furnace under a partial pressure of Ar as the protective gas during both melting and casting. The alloy melts were occasionally stirred mechanically, in addition to the flux produced by the electromagnetic induction. The alloys were cast into a permanent mould made of mild steel, homogenized and subsequently hot-extruded. The homogenized treatment was performed at 450 1C for 24 h followed by air cooling. Direct extrusion was carried out 430 1C with an extrusion ratio of 1:14. The microstructures of the cast and extruded samples were observed by optical microscopy (OM).The microstructural morphologies of the alloys were determined using a VegaIILMU SEM equipped with an EDS. The detailed microstructures of the samples were further examined under a TEM (Zeiss Libra 200FE) operated at an accelerating voltage of 200 kV.

X.H. Du et al. / Materials Letters 122 (2014) 312–314

Tensile testing at ambient temperatures was performed on a Shimadzu CMT-5105 material testing machine at a stretching rate of 6.6  10  3 s  1. 3. Results and discussion Fig. 1(a) shows the optical micrograph of the homogenized Mg– 10Er–2Cu alloy. Block-shaped phases were sparsely distributed in the grains or at the grain boundaries. Fig. 1(b) shows the optical micrograph of the homogenized Mg–10Er–2Cu (V) sample. Different from the Mg–Er–Cu samples, the addition of V has made more lamellar-shaped phases precipitated throughout all the grains. The volume fraction of precipitated phases of as-homogenized Mg– 10Er–2Cu and Mg–10Er–2Cu(V) alloys was about 10% and 30%, respectively. After extrusion, the block-shaped phase of the homogenized Mg–10Er–2Cu alloy was elongated along the extrusion direction, as shown in Fig. 1(c). And after extrusion, long strip-like phase which was rearranged along the extrusion direction for Mg–10Er–2Cu (V) alloy could be observed, as shown in Fig. 1(d). Fig. 2(a) and (b) show the SEM micrographs of the extruded Mg–10Er–2Cu and Mg–10Er–2Cu–(V) alloys, the rectangular regions of the main precipitated phases were analyzed by EDS. It is found

Fig. 1. The optical microscope observations (a) homogenized Mg–10Er–2Cu alloy; (b) homogenized Mg–10Er–2Cu–(V) alloy; (c) extruded Mg–10Er–2Cu–(V) alloy; (d) extruded Mg–10Er–2Cu–(V) alloy.

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that the compositions of the precipitated phase were similar to that of the 18R LPSO structure observed in the Mg97Y2Zn1 (at%) prepared by the Cu-mold casting method (Mg–6 at%Y–3 at%Zn) and induction melting method (Mg–6 at%Y–4 at%Zn) [11]. Zhang et al. also presented similar observation on the Mg–Y–Er–Zn extruded alloy [12]. Fig. 3(a and b) shows the TEM micrograph and the selected area diffraction (SAD) pattern of the precipitated phase of extruded Mg– 10Er–2Cu and Mg–10Er–2Cu–(V) alloys, respectively. The SAD patterns of 〈11–20〉 clearly showed that some extra diffracting spots has happened at the 71/3(0 0 0 1)α and 72/3(0 0 0 1)α positions, which is the evidence commonly used in previous study to prove the existence of 18R structure [11,13,14]. The above results show that the addition of V with a traced amount has modified the LPSO phase of homogenized and extruded Mg–10Er–2Cu alloy from coarse block-shaped into finer lamellar-shaped. Here, a simple interpretation on the modified effect of V to the LPSO phase was made base on thermodynamic analysis. It is known that the LPSO phase is a precipitated phase having a (0 0 0 1) basal plane which is the same as that in Mg (2H, Ramsdell notation), but its stacking periodicity was lengthened 18-fold (18R) or 14-fold (14H) along the c-axis [15]. The crystallographic orientation relationship of the LPSO phase with α-Mg matrix can be characterized by (0 0 0 1)LPSO//(0 0 0 1)α and 〈0–110〉LPSO//〈 1–120〉α [16]. Hence, it is assumed that if a distribution of heterogeneities on the Mg lattice is formed, they may promote the nucleation of the LPSO phase, as argued by the study [17]. Based on this, in choosing of alloying elements, a resultant driving force for heterogeneity formation, i.e. a clustering of the alloying elements upon thermal process should be taken into account. From thermodynamic viewpoint, these alloying elements should exhibit repulsive trend with the basic constituents of the Mg alloys [18]. In the present study, the LPSO-formed elements Mg, Er, Cu were the basic constituents of the alloying systems. Hence, the choosing of the alloying addition X should be based on that the phase diagrams of the Mg–X, Cu–X, Er–X exhibiting a large miscibility gap topological feature. Several elements V, Hf, Nb, Mo, Ti based on the binary phase diagrams could be selected [19]. Namely, the addition of V, Hf, Nb, Mo, Ti elements could provide thermodynamic drive force to modify LPSO phase by promoting nucleation. The current authors also discovered that the minor addition of Hf exhibited similar effect as V in modification of LPSO phase of Mg–10Er–2Cu alloy. Table 1 lists the tensile mechanical properties of the extruded Mg– 10Er–2Cu and Mg–10Er–2Cu–(V) alloy tested at room temperature.

Fig. 3. TEM image and SAED pattern of precipitated phase of (a) extruded Mg–Er– Cu and (b) Mg–Er–Cu–(V) alloys.

Table 1 The mechanical properties of the extruded Mg–10Er–2Cu–(V) alloys at room temperature.

Fig. 2. SEM micrographs as well as EDS results of the extruded samples of Mg–10Er–2Cu (a) and Mg–10Er–2Cu–(V) alloy (b).

Mg–10Er–2Cu Mg–10Er–2Cu–V

TYS (MPa)

UTS (MPa)

ε (%)

320 370

380 430

15 11

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The tensile yield stress (TYS), ultimate tensile stress (UTS) and apparent elongation (ε) of the V-traced extruded alloy were 370 MPa, 430 MPa and 11% at room temperature, respectively. This means that the finer lamellar-shaped LPSO phase could enhance effectively the strength but not benefit the ductility of the extruded Mg–Er–Cu alloys. This could be explained by using the short-fibre reinforcement mechanism, in which the LPSO phases could become the sites of crack nucleation [20,21]. 4. Conclusions The effect of traced V on the long period stacking ordered (LPSO) phase and mechanical properties of Mg–Er–Cu alloys processed by hot extrusion were investigated. The following conclusions could be derived: (1) The addition of trace amount of V has modified the morphology of 18R long period stacking order (LPSO) phase of the asextruded alloy from coarse block-shaped into finer lamellarshaped. (2) The modification of LPSO phase stemmed from the resultant thermodynamic drive force to promote heterogeneous nucleation due to the addition of V element. (3) The modified LPSO phase could enhance effectively the strength but not benefit the ductility of the extruded Mg–10Er–2Cu alloy.

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