Effect of Heat Treatment on the Microstructure of Mg-4Al-Nd Alloys

technologies

Article Effect of Heat Treatment on the Microstructure of Mg-4Al-Nd Alloys

Sibani Mishra 1, Anil Chaubey 2,* and Animesh Mandal 1 1 School of Minerals, Metallurgical and Materials Engineering, Indian Institute of Technology Bhubaneswar, Bhubaneswar 751007, Odisha, India; [email protected] (S.M.); [email protected] (A.M.) 2 CSIR (Council of Scientiﬁc and Industrial Research)-Institute of Minerals and Materials Technology, Bhubaneswar 751013, Odisha, India * Correspondence: [email protected]; Tel.: +91-674-237-9204

Academic Editor: Manoj Gupta Received: 27 March 2017; Accepted: 18 May 2017; Published: 23 May 2017

Abstract: In the present work, Mg-4Al-xNd (x = 0, 1, 4 wt.%) alloys were prepared by a stir casting method, and the effect of the addition of Neodymium (Nd) as-cast and of heat-treated microstructures was studied. The addition of 1 wt.% Nd preferentially formed the Al2Nd phase and completely suppressed the formation of the intermetallic Mg17Al12 (γ) phase, which was initially present in the base alloy (Mg-4Al alloys). On increasing the Nd percentage from 1 to 4 wt.% in the base alloy, two intermetallic phases, Al2Nd and Al11Nd3, were observed in the microstructure, as higher levels of Nd led to a peritectic reaction between Al and the Al2Nd phase, and part of the Al2Nd transformed into the Al11Nd3 phase. The hardness of the as-cast alloy increased with the Nd content. Thus, the hardness increased from 57.1 ± 4.1 Hv of Mg-4Al to 66.5 ± 2.6 Hv of Mg-4Al-4Nd. It was also found that solutionizing and isothermal aging of alloys containing Nd at 180 ◦C for 96 h led to the size reduction of Al- and Nd-containing intermetallics without altering their morphologies. Further, it was found that Nd does not have any effect on the aging kinetics of the alloys because all of the alloys with and without Nd attained peak hardness at 24 h of aging time.

Keywords: magnesium alloys; microstructure; Neodymium; intermetallics; Vickers micro hardness

1. Introduction Magnesium alloys possess an attractive combination of properties such as outstanding speciﬁc strength and stiffness, high damping capacity, excellent dimensional stability, and recyclability [1]. The most widely used magnesium alloys are based on the Mg-Al system, where Al is added to improve strength and castability. In addition, Mg-Al alloys have a wide freezing range, and as a result, in the process of cooling from a liquid state to a solid state, a precipitation of γ-Mg17Al12 intermetallic phase occurs at the grain boundary and is considered to be the main strengthening phase at room temperature [2]. However, the use of Mg-Al alloys is restricted to non-critical parts due to their poor hardness and creep properties at temperatures beyond 120 ◦C[3–5]. This is attributed to the discontinuous grain boundary precipitation of a low melting point γ phase from the supersaturated solid solution of α-Mg along with the coarsening of γ in the interdendritic eutectic region at elevated temperatures [6]. In order to circumvent the deleterious effect of the γ phase, several alloying elements such as Ca, Sr and rare earth elements that are preferentially reactive to Al over Mg have been added to Mg-Al alloys [7–9]. Commonly, rare earth elements in Mg-Al alloys suppress the formation of the Mg17Al12 phase during solidiﬁcation due to the preferential reaction of Al and rare earth (RE) to form Al-RE intermetallic phases. The addition of rare earth elements to an Mg-4Al alloy leads to formation of Al2RE and Al11RE3 phases. However, the thermodynamic stability of the Al2RE phase is higher

Technologies 2017, 5, 23; doi:10.3390/technologies5020023 www.mdpi.com/journal/technologies Technologies 2017, 5, 23 2 of 8

◦ than that of the Al11RE3 phase. At temperatures above 150 C, the Al11RE3 phase decomposes to form Al2RE and Al. This Al comes back into the solid-solution after phase transformation and forms a γ phase, as a result of which the properties deteriorate at elevated temperatures [5,10]. Hence, there is a need for an alternative element that could augment the properties of Mg-4Al at higher temperatures. For this, Neodymium (Nd) could be an effective strengthening element in Mg for elevated temperature applications [11,12]. Nd forms more stable intermetallics with Al and thus restricts the γ phase formation. However, limited work has been reported on the effect of Nd in Mg-Al alloys [12–15]. Zheng et al. [15] studied the effect of aging time on Vickers hardness of Mg-4Al-4Nd-0.5Zn-0.3Mn alloy and reported that the morphologies of the Al2Nd and Al11Nd3 phases change from large polyhedral and long rod-like to granular and short rod-like precipitates, respectively, at T6 conditions. This reﬁnement of precipitates after heat treatment improves the hardness and yield strength of the alloy both at room temperature and at elevated temperatures. Reports indicate that T6 heat treatment offers several advantages in Mg-Al alloys such as improved ductility, strength, hardness, and resistance to dynamic loads [11,16]. Therefore, the present work aims to study the effect of Nd on the microstructure and mechanical properties of Mg-4Al alloy in cast as well as under heat-treated conditions.

2. Materials and Methods In this study, Mg-4Al-xNd (x = 0, 1, 4 wt.%) alloys were prepared by the stir casting method ◦ in a protective atmosphere of SF6 (1%, v/v) and N2 (Bal.) at a temperature of 720 C. In order to ensure homogeneous mixing, the melt was stirred with the help of a mild steel mechanical stirrer at a speed of 25 rpm for 5 min. Nd addition to the abovementioned alloy was achieved by using a Al-12Nd master alloy, which was prepared separately by melting commercially pure Al and Nd in a graphite crucible using an electric resistance furnace at a temperature of 750 ◦C, and was then held for 45 min for the complete dissolution of Nd, after which the melt was poured into a steel mold. For the Mg-4Al-xNd (x = 0, 1, 4 wt.%) alloy preparation, commercially pure Mg and Al were melted in a graphite crucible using an induction furnace. Thereafter, the requisite amount of Al-12Nd master alloy was added to the melt at 720 ◦C and held for 10 min with continuous stirring. The melt was then poured by bottom pouring arrangements into a preheated (200 ◦C) mild steel mold with a diameter of 7 cm and a height of 10 cm. Cast samples were sectioned from the middle of the ingot and prepared as per standard metallographic techniques. The samples were etched in a freshly prepared etchant (100 mL ethanol, 20 mL distilled water, 6 mL acetic acid and 12 g picric acid). A field emission scanning electron microscope (FESEM) (Carl Zeiss, Merlin, Germany) equipped with an energy dispersive X-ray spectrometer (EDS) was employed for microstructural characterization. The phase analysis of the alloys were conducted by the X-ray diffraction method (XRD) (Bruker, Model: D8 Discover, Canton, MA, USA) using a Siemens diffractometer operating at 40 kV and 40 mA with Cu Kα radiation (λ = 1.54 Å). The image analysis was carried out using Image J software (Version: k 1.45). The as-cast Mg-4Al-xNd (x = 0, 1, 4 wt.%) alloys were solution treated at 400 ± 1 ◦C for 8 h to obtain a homogeneous solid solution, and were subsequently quenched in water at room temperature. The alloys were then artificially aged in an air circulated muffle furnace (KANTHAL, Sandvik Group, Sweden) at 180 ± 1 ◦C[16,17] up to 96 h in the interval of 24 h followed by air cooling. Vickers micro hardness tests were carried out to investigate the aging behavior of the alloys. Hardness measurements were carried out using a Micro Hardness Tester (Model: LM-247; Make: LECO, Saint Joseph, MI, USA) at a load of 10 gf with a dwell time of 13 s. All hardness values of as-cast and heat-treated alloys are expressed as an average of at least 10 symmetrical indentations.

3. Results

3.1. Phase Analysis and Microstructure Figure1 shows the X-ray diffraction pattern of Mg-4Al, Mg-4Al-1Nd and Mg-4Al-4Nd cast alloys. Mg-4Al alloy is characterized by α-Mg and γ (Mg17Al12) peaks. The addition of 1 wt.% Nd to Mg-4Al Technologies 2017, 5, 23 3 of 8 Technologies 2017, 5, 23 3 of 8 Figure 1 shows the X-ray diffraction pattern of Mg-4Al, Mg-4Al-1Nd and Mg-4Al-4Nd cast alloys. Mg-4Al alloy is characterized by α-Mg and γ (Mg17Al12) peaks. The addition of 1 wt.% Nd to Mg-4Alalloy resulted alloy resulted in the complete in the complete suppression suppression of the γ phase, of the and γ phase, peaks ofandα-Mg peaks and of Al α2-MgNd phasesand Al were2Nd phasesobserved. were When observed. the Nd When content the wasNd content increased was in theincreased alloy toin 4the wt.%, alloy peaks to 4 ofwt.%, new peaks intermetallics of new intermetallicsAl11Nd3 along Al with11Nd3 the along existing with theα-Mg existing and Alα-Mg2Nd and peaks Al2Nd were peaks seen were in the seen pattern. in the pattern. Zou et al.Zou also et al.reported also reported Al2Nd Al and2Nd Al and11Nd Al3 11intermetallicsNd3 intermetallics phases phases in ZA52 in ZA52 alloy alloy after after the addition the addition of Nd of [Nd18]. [18]. The Theformation formation of intermetallics of intermetallics could could be explainedbe explained by by noting noting electronegativity electronegativity of of the the elements, elements, as as it it is isknown known that that the probabilitythe probability of compound of compound formation formation between between two elements two dependselements on depends the difference on the in differenceelectronegativity. in electronegativity. The values of The electronegativity values of elec fortronegativity Mg, Al and for Nd Mg, are 1.31,Al and 1.61 Nd and are 1.14, 1.31, respectively. 1.61 and 1.14,Here, respectively. it is clear that Here, Nd and it Alis haveclear athat higher Nd electronegativity and Al have a difference higher electronegativity compared to that difference of Mg-Nd comparedand Mg-Al, to hence that of it Mg-Nd led to the and formation Mg-Al, hence of Al-Nd it led intermetallics to the formation [19,20 of]. Al-Nd In the solidiﬁcationintermetallics sequence, [19,20]. Inthe the Al-Nd solidification phases formed sequence, prior tothe the Al-Ndγ phase phases because formed the intermetallics prior to the of Alγ phase and Nd because formed the at a intermetallicsmuch highertemperature of Al and Nd compared formed toat thea muchγ phase higher [13, 18temperature]. Al2Nd is acompared proeutectic to phasethe γ whilephase Al[13,18].11Nd3 Alis2 theNd resultis a proeutectic of the peritectic phase reaction while betweenAl11Nd3 is Al the2Nd result particles of the and peritectic liquid Al, reaction because between high Nd contentAl2Nd particlespromotes and the liquid rate of Al, the because peritectic high reaction. Nd content promotes the rate of the peritectic reaction.

FigureFigure 1. 1. XRDXRD pattern pattern of of as-cast as-cast alloys: alloys: ( (AA)) Mg-4Al, Mg-4Al, ( (BB)) Mg-4Al-1Nd, Mg-4Al-1Nd, and and ( (CC)) Mg-4Al-4Nd. Mg-4Al-4Nd.

Figure 2 shows the FESEM micrograph of the as-cast alloys. The presence of α-Mg and the Figure2 shows the FESEM micrograph of the as-cast alloys. The presence of α-Mg and the discontinuously precipitated γ (Mg17Al12) phase along the grain boundaries is observed in Mg-4Al discontinuously precipitated γ (Mg17Al12) phase along the grain boundaries is observed in Mg-4Al alloy (Figure 2a). The α-Mg grains are mostly equiaxed and have a rosette morphology. In contrast, alloy (Figure2a). The α-Mg grains are mostly equiaxed and have a rosette morphology. In contrast, Mg-4Al-1Nd alloy consists of small, irregularly shaped Al2Nd homogeneously distributed Mg-4Al-1Nd alloy consists of small, irregularly shaped Al2Nd homogeneously distributed throughout throughout the matrix (Figure 2b), while both Al2Nd and Al11Nd3 particles are seen in Mg-4Al-4Nd the matrix (Figure2b), while both Al 2Nd and Al11Nd3 particles are seen in Mg-4Al-4Nd alloy alloy (Figure 2c). Due to Nd in the Mg-4Al alloy, the γ (Mg17Al12) phase was completely suppressed (Figure2c). Due to Nd in the Mg-4Al alloy, the γ (Mg17Al12) phase was completely suppressed and two additional intermetallic (Al2Nd and Al11Nd3) phases evolved. and two additional intermetallic (Al2Nd and Al11Nd3) phases evolved. The histogram in Figure3 shows the particle size distribution of Al 2Nd particles in the as-cast Mg-4Al-1Nd alloy (Figure3a) and after 24 h of aging at 180 ◦C (Figure3b). In the as-cast alloy, a major fraction of Al2Nd particles are in the range of 2–4 µm and with an average particle size of 2.9 µm. Meanwhile, in the heat-treated alloy, the majority of Al2Nd particles are in the size range of 1–2 µm.

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Figure 2. FESEM micrographs of the as-cast alloys showing the distribution of different intermetallic particles in the Mg-Al matrix: (a) Mg-4Al, (b) Mg-4Al-1Nd, and (c) Mg-4Al-4Nd.

The histogram in Figure 3 shows the particle size distribution of Al2Nd particles in the as-cast Mg-4Al-1Nd alloy (Figure 3a) and after 24 h of aging at 180 °C (Figure 3b). In the as-cast alloy, a majorFigure fraction 2. FESEMof Al2Nd micrographs particles are of the in theas-cast range alloys of 2–4showin showing μmg andthe distributionwith an average of different particle intermetallic size of 2.9 μm. Meanwhile,particles in in the the heat-treatedMg-Al matrix: alloy, ( a) Mg-4Al, the majority ( b) Mg-4Al-1Nd, of Al2Nd and particles ( c) Mg-4Al-4Nd. are in the size range of 1–2 μm.

The histogram in Figure 3 shows the particle size distribution of Al2Nd particles in the as-cast Mg-4Al-1Nd alloy (Figure 3a) and after 24 h of aging at 180 °C (Figure 3b). In the as-cast alloy, a major fraction of Al2Nd particles are in the range of 2–4 μm and with an average particle size of 2.9 μm. Meanwhile, in the heat-treated alloy, the majority of Al2Nd particles are in the size range of 1–2 μm.

FigureFigure 3. 3. ParticleParticle size size distribution distribution of Al 2NdNd particles in Mg-4Al-1Nd alloyalloy ((aa)) as-castas-cast condition,condition, and and ◦ (b(b) )aged aged at at 180 180 °CC for for 24 24 h. h.

In Mg-4Al-4Nd alloy, four different morphologies of Al and Nd intermetallics were identified: In Mg-4Al-4Nd alloy, four different morphologies of Al and Nd intermetallics were identified: largelarge quadrangle, quadrangle, small small polygonal, polygonal, rod-like rod-like and dendritic (labelled(labelled inin FigureFigure4 ).4). Large Large quadrangular quadrangular andand small smallFigure polygon polygon3. Particle particles particles size distribution are are identified identified of Al2Nd as particles Al22Nd in while Mg-4Al-1Nd thethe particlesparticles alloy with(witha) as-cast dendriticdendritic condition, morphologymorphology and areare identified identified(b) aged at asas 180 Al Al °C1111Nd forNd3 24.3 .The h. The rod-shaped rod-shaped particles particles have have a a length length of about 8.78.7 µμmm areare identifiedidentified as as AlAl1111NdNd3 3particles.particles. Al Al2Nd2Nd particles particles are coarsercoarser as as they they were were observed observed in the in firstthe phasefirst phase to solidify, to solidify, while In Mg-4Al-4Nd alloy, four different morphologies of Al and Nd intermetallics were identified: whileAl11Nd Al311particlesNd3 particles are fine are as fine they as were they formedwere formed at later at stage later of stage solidification. of solidification. The formation The formation of the fine of large quadrangle, small polygonal, rod-like and dendritic (labelled in Figure 4). Large quadrangular theAl 11fineNd 3Alphase11Nd3 couldphase be could the result be the of result the dissolution of the dissolution of coarse Alof 2coarseNd particles Al2Nd dueparticles to the due peritectic to the Technologiesand small 2017 polygon, 5, 23 particles are identified as Al2Nd while the particles with dendritic morphology5 of 8 peritecticreaction withreaction Al. with Al. are identified as Al11Nd3. The rod-shaped particles have a length of about 8.7 μm are identified as Al11Nd3 particles. Al2Nd particles are coarser as they were observed in the first phase to solidify, while Al11Nd3 particles are fine as they were formed at later stage of solidification. The formation of the fine Al11Nd3 phase could be the result of the dissolution of coarse Al2Nd particles due to the peritectic reaction with Al.

FigureFigure 4. 4. FESEMFESEM micrograph micrograph of of Mg-4Al-4Nd Mg-4Al-4Nd at at higher higher magnifications. magniﬁcations.

Figure 5 shows the microstructure of Mg-4Al-1Nd and Mg-4Al-4Nd after solutionizing at 400 °C for 8 h followed by artificial aging at 180 °C for 24 h. The Al2Nd particles in Mg-4Al-1Nd alloy show marginal size reduction (1–2 μm) with irregular morphology persisting throughout the microstructure, similar to those found in the as-cast alloys (2.86 μm). Likewise, the morphology of Al2Nd and Al11Nd3 particles in Mg-4Al-4Nd alloy after heat treatment was not altered. The rod-shaped Al11Nd3 particles in Mg-4Al-4Nd alloy are found to be 6.5 μm as compared to the cast alloys (8.7 μm). Also, it was found that T6 treatment resulted in a more homogeneous distribution of phases in Nd-containing alloys compared to as-cast condition.

Figure 5. FESEM micrograph after 24 h aging: (a) Mg-4Al-1Nd alloy and (b) Mg-4Al-4Nd alloy.

The ratio of phases in different alloys after 24 h of aging was estimated from the XRD pattern (using the relative intensities of the maximum peak height of phases), as shown in Figure 6. The fraction of the γ (Mg17Al12) phase in Mg-4Al alloys is 0.022, while in Mg-4Al-1Nd alloy, the ratio of the α-Mg and Al2Nd phases is 0.92:0.08. The fraction of the α-Mg, Al11Nd3 and Al2Nd phases in Mg-4Al-4Nd alloy are respectively 0.788, 0.075 and 0.137. Thus, it can be inferred that the alloys containing Nd show higher fractions of Al2Nd phases, almost 2–3 times as much as the cast alloys, formed after aging for 24 h. This could be one of the reasons for the increase in hardness of Nd-containing alloys at peak aged conditions. Further, the XRD also confirmed the complete suppression of the γ phase in Nd-containing alloys.

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Technologies 2017, 5, 23 5 of 8 Figure 4. FESEM micrograph of Mg-4Al-4Nd at higher magnifications. Figure5 shows the microstructure of Mg-4Al-1Nd and Mg-4Al-4Nd after solutionizing at 400 ◦C Figure 5 shows the microstructure of◦ Mg-4Al-1Nd and Mg-4Al-4Nd after solutionizing at 400 °C for 8 h followed by artiﬁcial aging at 180 C for 24 h. The Al2Nd particles in Mg-4Al-1Nd alloy show for 8 h followed by artificial aging at 180 °C for 24 h. The Al2Nd particles in Mg-4Al-1Nd alloy show marginal size reduction (1–2 µm) with irregular morphology persisting throughout the microstructure, marginal size reduction (1–2 μm) with irregular morphology persisting throughout the similar to those found in the as-cast alloys (2.86 µm). Likewise, the morphology of Al2Nd and Al11Nd3 microstructure, similar to those found in the as-cast alloys (2.86 μm). Likewise, the morphology of particles in Mg-4Al-4Nd alloy after heat treatment was not altered. The rod-shaped Al11Nd3 particles Al2Nd and Al11Nd3 particles in Mg-4Al-4Nd alloy after heat treatment was not altered. The in Mg-4Al-4Nd alloy are found to be 6.5 µm as compared to the cast alloys (8.7 µm). Also, it was found rod-shaped Al11Nd3 particles in Mg-4Al-4Nd alloy are found to be 6.5 μm as compared to the cast that T6 treatment resulted in a more homogeneous distribution of phases in Nd-containing alloys alloys (8.7 μm). Also, it was found that T6 treatment resulted in a more homogeneous distribution of compared to as-cast condition. phases in Nd-containing alloys compared to as-cast condition.

Figure 5. FESEM micrograph after 24 h aging: ( a) Mg-4Al-1Nd alloy and (b) Mg-4Al-4Nd alloy.

The ratio of phases in different alloys after 24 h of aging was estimated from the XRD pattern The ratio of phases in different alloys after 24 h of aging was estimated from the XRD pattern (using the relative intensities of the maximum peak height of phases), as shown in Figure 6. The (using the relative intensities of the maximum peak height of phases), as shown in Figure6. The fraction fraction of the γ (Mg17Al12) phase in Mg-4Al alloys is 0.022, while in Mg-4Al-1Nd alloy, the ratio of of the γ (Mg17Al12) phase in Mg-4Al alloys is 0.022, while in Mg-4Al-1Nd alloy, the ratio of the α-Mg the α-Mg and Al2Nd phases is 0.92:0.08. The fraction of the α-Mg, Al11Nd3 and Al2Nd phases in and Al Nd phases is 0.92:0.08. The fraction of the α-Mg, Al Nd and Al Nd phases in Mg-4Al-4Nd Mg-4Al-4Nd2 alloy are respectively 0.788, 0.075 and 0.137.11 Thus,3 it can2 be inferred that the alloys alloy are respectively 0.788, 0.075 and 0.137. Thus, it can be inferred that the alloys containing Nd containing Nd show higher fractions of Al2Nd phases, almost 2–3 times as much as the cast alloys, show higher fractions of Al Nd phases, almost 2–3 times as much as the cast alloys, formed after formed after aging for 24 2h. This could be one of the reasons for the increase in hardness of aging for 24 h. This could be one of the reasons for the increase in hardness of Nd-containing alloys at Nd-containing alloys at peak aged conditions. Further, the XRD also confirmed the complete peak aged conditions. Further, the XRD also conﬁrmed the complete suppression of the γ phase in suppression of the γ phase in Nd-containing alloys. TechnologiesNd-containing 2017, 5 alloys., 23 6 of 8

Figure 6. XRD pattern of T6-treated alloys in peak aged conditions.

3.2. Hardness The average hardness of as-cast Mg-4Al, Mg-4Al-1Nd and Mg-4Al-4Nd alloys are 57.157.1 ± 4.14.1 Hv, Hv, 62.6 ± 4.84.8 Hv, Hv, and 66.5 ± 2.62.6 Hv, Hv, respectively. As As expected, expected, the the hardness hardness of of the as-cast alloys increased as a function of Nd content due to the increase in the volume fraction of Al2Nd and Al11Nd3 intermetallics, depending on the alloy composition. Also, it can be noted that the standard deviation in Mg-4Al-4Nd alloy is the maximum which suggests the inhomogeneous distribution of phases in the microstructure. Figure 7 represents the Vickers micro hardness of Mg-4Al, Mg-4Al-1Nd and Mg-4Al-4Nd alloys isothermally aged at 180 °C up to 96 h [16,17]. It is interesting to note that the hardness of all of the alloys reaches peak value after 24 h of aging. This warrants a further detailed investigation of the microstructure of peak aged samples.

Figure 7. Vickers micro hardness of heat-treated alloys.

The hardness of the relatively coarse Al2Nd particles in Mg-4Al-1Nd and Mg-4Al-4Nd alloys is 120.8 Hv and 144.1 Hv, respectively. However, the hardness of Al11Nd3 particles could not be measured due to their finer size as compared to Al2Nd particles. These intermetallic particles have significantly higher hardness than that of the matrix, and hence contribute to the overall hardness of the alloy by dispersion hardening. It can be seen that in the initial stage of aging up to 24 h, the hardness increases steeply and thereafter the hardness decreases slowly after attaining the

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Figure 6. XRD pattern of T6-treated alloys in peak aged conditions.

3.2. Hardness Technologies 2017, 5, 23 6 of 8 The average hardness of as-cast Mg-4Al, Mg-4Al-1Nd and Mg-4Al-4Nd alloys are 57.1 ± 4.1 Hv, 62.6 ± 4.8 Hv, and 66.5 ± 2.6 Hv, respectively. As expected, the hardness of the as-cast alloys increasedincreased as asa afunction function of of Nd contentcontent duedue to to the the increase increase in thein the volume volume fraction fraction of Al 2ofNd Al and2Nd Al and11Nd 3 Al11intermetallics,Nd3 intermetallics, depending depending on the on alloy the composition.alloy composition. Also, itAlso, can beit can noted be thatnoted the that standard the standard deviation deviationin Mg-4Al-4Nd in Mg-4Al-4Nd alloy is alloy the maximum is the maximum which suggestswhich suggests the inhomogeneous the inhomogeneous distribution distribution of phases of in phasesthe microstructure. in the microstructure. FigureFigure 7 represents7 represents the the Vickers Vickers micro micro hardness hardness of ofMg-4Al, Mg-4Al, Mg-4Al-1Nd Mg-4Al-1Nd and and Mg-4Al-4Nd Mg-4Al-4Nd alloys alloys ◦ isothermallyisothermally aged aged at 180 at 180 °C upC upto 96 to 96h [16,17]. h [16,17 It]. is It interesting is interesting to note to note that that the the hardness hardness of all of allof the of the alloysalloys reaches reaches peak peak value value after after 24 24h of h ofaging. aging. This This warrants warrants a further a further detailed detailed investigation investigation of the of the microstructuremicrostructure of peak of peak aged aged samples. samples.

FigureFigure 7. Vickers 7. Vickers micro micro hardness hardness of heat-treated of heat-treated alloys. alloys.

The hardness of the relatively coarse Al2Nd particles in Mg-4Al-1Nd and Mg-4Al-4Nd alloys is The hardness of the relatively coarse Al2Nd particles in Mg-4Al-1Nd and Mg-4Al-4Nd alloys 120.8 Hv and 144.1 Hv, respectively. However, the hardness of Al11Nd3 particles could not be is 120.8 Hv and 144.1 Hv, respectively. However, the hardness of Al11Nd3 particles could not be measured due to their finer size as compared to Al2Nd particles. These intermetallic particles have measured due to their finer size as compared to Al2Nd particles. These intermetallic particles have significantly higher hardness than that of the matrix, and hence contribute to the overall hardness of significantly higher hardness than that of the matrix, and hence contribute to the overall hardness the alloy by dispersion hardening. It can be seen that in the initial stage of aging up to 24 h, the Technologiesof the alloy 2017 by, 5, dispersion 23 hardening. It can be seen that in the initial stage of aging up to 24 h,7 theof 8 hardness increases steeply and thereafter the hardness decreases slowly after attaining the hardness increases steeply and thereafter the hardness decreases slowly after attaining the maximum value.maximum The increasevalue. The in hardnessincrease valuesin hardness at the peakvalues aged at the condition peak aged of Mg-4Al condition alloy of is attributedMg-4Al alloy to the is attributed to the finer and even distribution of γ precipitates throughout the microstructure. finer and even distribution of γ precipitates throughout the microstructure. It is also interesting to note that the decrease in hardness of an alloy even in the overaged conditions betweenbetween 72 72 and and 96 h96 is noth veryis not significant very significant (Figure7), (Figure which suggests 7), which that thesuggests microstructure that the microstructuredoes not undergo does significant not undergo coarsening significant (Figure coarsening8a,b). (Figure 8a,b).

Figure 8. FESEMFESEM micrograph micrograph of a alloyslloys aged for 72 h: ( aa)) Mg-4Al-1Nd Mg-4Al-1Nd and and ( (b)) Mg-4Al-4Nd Mg-4Al-4Nd alloy.

In Mg-4Al-1Nd alloy, though the microstructure reveals that there is no apparent change in the size of Al2Nd particles, a detailed particle size analysis suggested that they undergo a size reduction on prolonged holding at 180 °C. The aged sample shows particle sizes in the range of 1–2 μm. After isothermal aging, the Al2Nd particles size reduces to 1.3 μm and 0.7 μm after 24 h and 72 h, respectively. Thus, the particles seem to be significantly thermally modified in the overaged condition. However, in Mg-4Al-4Nd alloy, particle size analysis was not carried out as there were four different morphologies of two different kinds of particles in the same microstructure.

4. Conclusions The microstructural evolution of Mg-4Al alloys modified by the addition of 1 wt.% Nd and 4 wt.% Nd was studied in the as-cast condition and after heat treatment. It was found that an addition of 1 wt.% Nd was sufficient to completely suppress the formation of the Mg17Al12 phase. The microstructure of Mg-4Al-1Nd alloy shows α-Mg and Al2Nd phases, but Mg-4Al-4Nd alloy contains α-Mg and two intermetallic phases—Al2Nd and Al11Nd3. The hardness of the as-cast alloy increases with the increase of Nd content, and reaches 66.5 ± 2.6 Hv for 4 wt.% Nd. It is interesting to note that peak hardness of all three alloys was attained after 24 h of aging time. The results also revealed that even after 72 h of aging at 180 °C the morphologies of Al2Nd and Al11Nd3 phases do not change as compared to the as-cast alloys. However, subsequent isothermal aging at 180 °C led to the refinement of existing Nd-containing particles by thermal modification and an increase in amount of intermetallics, which could be due to precipitation of newer phases.

Acknowledgement: The authors are grateful to CSIR-IMMT Bhubaneswar, India for providing experimental facility and technical support to carry out this work.

Author Contributions: Animesh Mandal and Anil Chaubey conceived and designed the experiments; Sibani Mishra performed the experiments; Sibani Mishra, Animesh Mandal and Anil Chaubey analyzed the data; Sibani Mishra wrote the paper. All authors have read and approved the ﬁnal manuscript.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Mordike, B.L.; Ebert, T. Magnesium Properties-applications-potential. Mater. Sci. Eng. A 2001, 302, 37–45. 2. Jing, B.; Yangshan, S.; Shan, X.; Feng, X.; Tianbai, Z. Microstructure and tensile creep behavior of Mg-4Al based magnesium alloys with alkaline-earth elements Sr and Ca additions. Mater. Sci. Eng. A 2006, 419, 181–188.

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In Mg-4Al-1Nd alloy, though the microstructure reveals that there is no apparent change in the size of Al2Nd particles, a detailed particle size analysis suggested that they undergo a size reduction on prolonged holding at 180 ◦C. The aged sample shows particle sizes in the range of 1–2 µm. After isothermal aging, the Al2Nd particles size reduces to 1.3 µm and 0.7 µm after 24 h and 72 h, respectively. Thus, the particles seem to be signiﬁcantly thermally modiﬁed in the overaged condition. However, in Mg-4Al-4Nd alloy, particle size analysis was not carried out as there were four different morphologies of two different kinds of particles in the same microstructure.

4. Conclusions The microstructural evolution of Mg-4Al alloys modified by the addition of 1 wt.% Nd and 4 wt.% Nd was studied in the as-cast condition and after heat treatment. It was found that an addition of 1 wt.% Nd was sufficient to completely suppress the formation of the Mg17Al12 phase. The microstructure of Mg-4Al-1Nd alloy shows α-Mg and Al2Nd phases, but Mg-4Al-4Nd alloy contains α-Mg and two intermetallic phases—Al2Nd and Al11Nd3. The hardness of the as-cast alloy increases with the increase of Nd content, and reaches 66.5 ± 2.6 Hv for 4 wt.% Nd. It is interesting to note that peak hardness of all three alloys was attained after 24 h of aging time. The results also revealed that even after 72 h ◦ of aging at 180 C the morphologies of Al2Nd and Al11Nd3 phases do not change as compared to the as-cast alloys. However, subsequent isothermal aging at 180 ◦C led to the refinement of existing Nd-containing particles by thermal modification and an increase in amount of intermetallics, which could be due to precipitation of newer phases.

Acknowledgments: The authors are grateful to CSIR-IMMT Bhubaneswar, India for providing experimental facility and technical support to carry out this work. Author Contributions: Animesh Mandal and Anil Chaubey conceived and designed the experiments; Sibani Mishra performed the experiments; Sibani Mishra, Animesh Mandal and Anil Chaubey analyzed the data; Sibani Mishra wrote the paper. All authors have read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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