LIU Linlin, ZHENG Yang, LIU Cuixiu, SUN Wei

(Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China)

Abstract: A novel Fe-enriched lamella sandwich phase (χ-phase) has been found to precipitate along the basal (0001)Mg planes in the heat-treated Mg-Gd-Fe alloy and its structure is clearly revealed by means of atomic-resolution transmission electron microscopy. The layered χ-phase only has a thickness of mono-unit-cell and consists invariably of ten atomic layers stacking along the [0001]Mg direction, of which the outermost atomic layers had larger in-plane atom-pillar spacing than the inner layers. Fe/Gd atoms are mainly enriched in the outer four atomic layers in the χ-phase, forming two structurally unsymmetrical four-layer shells to sandwich the middle two Mg layers. An atomic model has been proposed for this layered sandwich-structured χ-phase.

Key words: Mg-Gd-Fe alloys; microstructure; Fe-enriched precipitate; lamella sandwich phase

1 Introduction

The layered precipitation can play an essential role in improving the ambient and elevated temperature mechanical performance of the Mg-RE-TM (RE: rare earth, TM: transition metal) based alloys[1-8]. The layered precipitation behaviors and structures have attracted much attention since the first discovery of the layered phase with long-period stacking ordered (LPSO)structure in the Mg97Y2Zn1(at%) alloy[1,2]. Revealing the structural features and formation mechanisms for various layered precipitates is of directive significance for achieving the desired properties of Mg alloys by tailoring the precipitation structures.

The formation of layered precipitation structures is closely related to the TM-addition in the Mg-REbased alloys. As an empirical rule, it is generally suggested that only the TM which has an atomic radius smaller than that of Mg (0.16 nm) and also has a strong affinity with RE (i e, has more negative mixing enthalpycan promotethe formation of layered structures in Mg alloys[9-11]. To date, various types of layered precipitation structures have been observed in the Mg-RE-based alloy systems after adding Al, Zn,Cu, Ni, Co, and Ag, respectively, allowing for these TM elements all meeting the above empirical rule (see those listed in Table 1[12]). In the Mg-RE-TM (Al, Zn,Cu, Ni) alloys, the layered structures formed coherently along the Mg basal plane are mainly LPSO precipitates of 10H, 12R, 14H, 18R, and 24H types[3,5,13-19]. These LPSO structures can be described as stacking of ABCA-type building blocks (also called asγ′ phases[8]) in different sequences, where the RE/TM elements are distributed randomly in their middle two atomic layers[13,20]or distributed in the form of L12-type TM6RE8clusters in their four atomic layers[14,18,19,21]. Some new types of layered LPSO precipitates with 12H, 15R, 21R, 29H,51R, 60H, 72R, 102R, 192R and 654R type structures are observed in the Mg-Y-Co alloys[11,22,23], of which the 12H, 15R, 21R type LPSO structures are composed of ABC-type building blocks with Y/Co elements distributing randomly in B atomic layers[22]. Also, some other structurally-irregular layered precipitates can be formed containing both ABC- and ABCA-type building blocks with Y/Co elements distributing randomly in B and BC atomic layers, respectively[11,23]. In the Mg-RE-Ag alloys, the layered basal-plane precipitates can be formed as mono-layered GP zones and three-layeredγ′′′ orγ′′phases[6,24,25]. The GP zones have a hexagonal in-plane arrangement of RE/Ag atoms, and theγ′′′ phases have an ABA-type stacking sequence of which each layer resembles the mono-layered GP zone[6]. Theγ′′ phases show an ADA-type stacking sequence, of which the outer two layers have an in-plane ordering similar to that of theγ′′′ phases, while the inner D layer locates at new lattice position different from that of A, B, C layers[6,24]. Summarily, the empirical rules seem to provide a specific basis for realizing various layered precipitations in Mg-RE-TM alloys almost without exception.

In this study, we will report on the finding of a novel Fe-enriched layered precipitate formed due to the Fe-addition to a Mg-Gd alloy. Fe is a common TM element with an atomic radius (0.124 nm) smaller than that of Mg. However, Fe has almost zero solid solubility in the Mg matrix and it only has a weak affinity with most of RE elements (e g,= -1 kJ/mol[12]),which does not strictly conform to the empirical rules for the formation of layered structures. Nevertheless,a kind of unique sandwich-layered precipitate that is structurally different from any of those previously-reported Mg-RE-TM layered phases can be formed in the heat-treated Mg-1.89Gd-0.15Fe (at%) alloy.Atomic-resolution HAADF-STEM technique has been applied to characterize this novel Fe-enriched layered phase, and on the basis of which an atomic structural model has been proposed for it.

2 Experimental

A small alloy ingot (about 10 g) with the actual composition of Mg-1.89Gd-0.15Fe (at%) was obtained after high-purity (>99.99%) Mg, Gd and Fe were repeatedly melted in a high-frequency induction under the protection of pure argon gas. The ingot was sealed in an evacuated quartz tube and then heat-treated at 480 ℃ for 15 hrs, followed by naturally cooling to room temperature. Samples for TEM were prepared by twin-jet polished in a solution of 15% nitric and 75%methanol at -20 ℃, and subsequently their surfaces were cleaned by low-energy ion beam using precision ion polishing system (PIPS). TEM and HAADF-STEM observations were carried out using an FEI Tecnai F30 TEM operated at 300 kV. Electron diffraction pattern(EDP) and HAADF-STEM image simulations were executed using SingleCrystal and MacTempas softwares,respectively.

3 Results and discussion

After the as-cast Mg-1.89Gd-0.15Fe (at%) alloy was heat-treated at 480 ℃ for 15 hrs, a few sparsely dispersed precipitation colonies were observed (see Figs.1(a, d)) in which many thin layered precipitates exclusively parallel to basal (0001)Mgplane were found to be formed (see Figs.1(b, e)). Each of these layered precipitates had a thickness of about 2.6 nm and appeared to be sandwiched by two shell-layers exhibiting dark-contrast (as indicated by the arrows in Figs.1(b,e)). Notice that the layered sandwich precipitates were arranged in irregular stacking spacing along the [0001]Mgdirection, and this will result in pronounced long diffraction streaks crossing the (0000)Mgand diffraction spots along the [0001]Mgdirection as seen in Figs.1(c,f)). Also, extra diffraction streaks along the [0001]Mgdirection were observed crossingpositions(n= 1, 2) in the pattern andpositions (m= 1 - 8) in thepattern, as indicated by white arrows in Figs.1(c, f). Note that these extra streaks are positionally different from those similar diffraction streaks observed in the previously-reported LPSO,γ′,γ′′ andγ′′′ lamella phases in Mg-RE-TM(Al,Zn, Ni, Cu, Ag) alloys[6-8,13,14,18,20,21]. Since these extra diffraction streaks are also closely associated with the structural and chemical ordering states in the closepacked planes besides their stacking sequence, it thus can be said that the arrangements of Gd/Fe atoms in the basal planes for the sandwich precipitates are likely to be different from those of the LPSO,γ′,γ′′ andγ′′′layered phases. As a new layered precipitation product,the lamella sandwich precipitate formed in the Mg-Gd-Fe alloy is hereinafter referred to as χ-phase for short.

Figs.2(a, b) are HAADF-STEM images clearly revealing that the χ-phase can be formed in a precipitation colony densely and that its sandwich structure has two-layered shells exhibiting brightZ-contrasts,which indicated that Gd/Fe solute atoms are mainly distributed in these coupled shells. Figs.2(c-f) are energy-dispersive X-ray spectroscopy (EDXS) mapping results obtained from a χ precipitation region, which can directly confirm that the χ-phases are enriched in both Fe and Gd elements but contain more Fe than Gd.A quantitative EDXS analysis showed that theχ-phase had an average composition of Mg77.3Gd7.8Fe14.9(at.%),with the Gd/Fe ratio being close to 1:2. It is of signifi-cance to note that theχ-phase does not match with any of the currently identified intermetallic compounds in the Pearson′s Handbook containing RE and Fe or even O or H together[26], and thus should be treated as a novel compound.

Table 1 Atomic radii of TM elements and RE-TM mixing enthalpy values (TM = Al, Zn, Cu, Ni, Co, Ag; RE = Gd, Y)

Table 1 Atomic radii of TM elements and RE-TM mixing enthalpy values (TM = Al, Zn, Cu, Ni, Co, Ag; RE = Gd, Y)

TM Al Zn Cu Ni Co Ag Atomic radius/nm 0.143 0.133 0128 0.125 0.125 0.144/(kJ/mol) -39 -31 -22 -31 -22 -29/(kJ/mol) -38 -31 -22 -31 -22 -29

Fig.1 TEM characterization of layered precipitation structures formed in the heat-treated Mg-1.89Gd-0.15Fe (at%) alloy: (a) Low- and (b)high-magnification bright-field (BF) images and (c) electron diffraction pattern (EDP) taken from the precipitation region along theaxis; (d) low- and (e) high-magnification BF images and (f) EDP along the axis

Fig.2 HAADF-STEM characterization along the direction and compositional analysis for the layered χ-phases formed in the heat-treated Mg-Gd-Fe alloy: (a) a precipitation colony consisting of layered χ-phases; (b) irregularly distributed χ-phases with a sandwich structure as indicated in the inset; (c)-(f) χ-phase imaging and the corresponding element mapping for Fe, Gd, and Mg, respectively

Fig.3 Atomic-scaled HAADF-STEM images of the χ-phase taken along the axis, showing (a) χ sandwich structure and (b) the atomic arrangements in it, where the circles represent Gd-rich and Fe-rich and Mg atomic pillars, respectively; (c) proposed atomic structure of χ-phase and (d) its simulated HAADF-STEM image (inset) overlapping with the observed image; (e) simulated EDPs of the χ-phase viewed along the and axes, respectively

Figs.3(a, b) show the atomic-scaled HAADFSTEM images of theχ-phase viewed along theaxis, in which the brightest and second-brightestZ-contrast imaging dots are supposed to correspond to Gd-rich and Fe-rich pillars, respectively,whereas the weak bright dots of majority represent Mg pillars. As can be seen in Fig.3(a), each individual lamellaχ-phase is completely coherent with the Mg matrix in the (0001)Mghabit plane and its inner two Mg layers are sandwiched by two solute-rich shells with a thickness of about 1.02 nm. In particular, each shell consists of four atomic layers and its outermost one is mainly occupied by Gd-rich pillars (see Fig.3(b)). This unique shell structure can also be described instead by using parallelogram-shaped motifs with an interior angle of about 80º and their linear arrangement along thedirection (see Fig.3(a)). The Gd and Fe atoms are mainly located on the four sides of each motif, and their projection positions were marked with circles in Fig.3(b), respectively.

As clearly revealed in Fig.3(b), theχ-phase invariably contains a total of ten atomic layers stacking parallel to the (0001)Mgplane, but its two Gd-rich outermost layers (coherent with adjacent Mg layers) have an obviously different atomic-pillar spacing along thedirection (d1) in comparison with the other eight inner ones, which is 4/3 times larger than the most inner layer of each solute-rich shells (d2) (i e,d1= (4/3)d2). Thus the relevant atomic-pillar spacings in each shell should have to displace so as to accommodate this difference. Due to the overlapping of atomic layers including the atom arrangement with varied spacing on them, it is practically impossible to clarify the in-plane atomic structure of the χ-phase directly from the [0001]Mgdirection. Nevertheless, from the understanding of the structural features of the layered precipitates in Mg-RE-TM alloys, it can be inferred that each atomic layer for constructing theχ-phase basically has a hexagonal-closely-packed atomic arrangement resembling the Mg basal plane. With respect to the Mg matrix with a stacking sequence of BCBC.. for the basal-planes, the stacking sequence in theχ-phase can be described as(see Fig.3(b)). Here, letters A,B, C represent the closely-packed planes at different stacking positions, whereas letters A′, B′, C′ represent theχinner planes. The underlined bold letters in the sequence represent the solutes-enriched atomic layers that construct two shells for the χ-phase with a sandwich structure. For two adjacent atomic layers having the same position (C′C′) in the structurally unsymmetricalshells should be the result of accommodation between the outermost and inner atomic layers with different atomic spacing.

According to the above results and analysis, an atomic model was proposed for theχ-phase as shown in Fig.3(c). The model had unit cell parameters ofa=b= 1.61 nm,c= 2.61 nm,α=β= 90º, andγ= 120ºand each individual stacking atomic layer had a hexagonal symmetry (here Mg cell parameters are taken to bea=b= 0.321 nm,c= 0.521 nm as reference). The orientation relationship between theχ-phase and the Mg matrix follows [110]χ//, [001]χ//[0001]Mgand. The chemical composition of the proposed model is Mg376Gd20Fe42, whose Gd/Fe ratio is also close to 1:2, coinciding well with the EDXS analysis result. Based on the proposed model,the simulations of HAADF-STEM image and EDPs were performed. The simulated HAADF-STEM image matches well with the observed image (see the inset in Fig.3(d)). Also in the simulatedpatterns (see Fig.3(e)), diffraction spots rows appear to cross the(n= 1, 2) andm/9(m= 1 - 8) positions (as denoted by arrows), indicating that the in-plane atomic arrangement in the proposed model can reproduce the main characteristic of the observed EDPs. However, the observedχ-phases appear to be highly faulted in that displacive or fluctuant atomic arrangements could occur due to the strain accommodating in their inner stacking planes, especially in C′C′ layers, which can cause blurredZ-contrast for atomic imaging (see Fig.3(d)). For the same reason, the recorded EDPs in Figs.1(c, f) only appear extra diffraction streaks instead of the spots as seen in the patterns simulated from the ideal atomic model. This difference between the experimental and simulated EDPs of theχ-phases is similar to the case for the faulted LPSO structure with a short range of stacking order in Mg-Gd-Al alloy[14]. Since the layered sandwichχ-phase has to allow accommodating strain existing in its inner stacking layers, it is formed by stacking only in shortrange and therefore has just mono-unit-cell in thickness.

4 Conclusions

In summary, we had found a novel Fe-enriched lamella sandwichχ-phase (Gd/Fe ratio (at%) close to 1:2), which invariably precipitated coherently along the (0001)Mghabit plane and with only mono-unit-cell thickness. Theχ-phase consists of ten atomic layers parallel to the (0001)Mgplane, of which the outermost atomic layers had larger in-plane atom-pillar spacing than the inner layers. As a layered sandwich phase, theχ-phase has a solute-enriched shell structure which can be described by the linear arrangement of parallelogram-motifs along thedirection, where the Fe/Gd atoms were mainly located on the four sides of each parallelogram-motif.