Effect of Lamellar Spacing on Creep Strength of ¡-Mg/C14­Mg2Ca Eutectic Alloy+1

Koji Oishi+2, Satoshi Araki+3 and Yoshihiro Terada+4

Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology,Yokohama 226-8502, Japan

In tensile tests, ¡-Mg/C14­Mg2Ca eutectic alloy with a lamellar structure is plastically deformed above 473K but ruptures before yieldingat temperatures below 423K. This study investigates the effect of the ¡/C14 interface on the creep strength of ¡-Mg/C14­Mg2Ca eutectic alloyat 473K under 40MPa stress. The creep curves of the alloy exhibited three stages: a normal transient creep stage, minimum creep-rate stage, andaccelerating stage. The minimum creep rate was proportional to the lamellar spacing, indicating that the ¡/C14 lamellar interface plays a creep-strengthening role. In high-resolution transmission electron microscope observations of the specimens after the creep test, a-dislocationsappeared within the ¡-Mg lamellae and were randomly distributed on the ¡/C14 interface. It was deduced that the ¡/C14 interface presents abarrier to dislocation glide and does not annihilate and/or rearrange the dislocations caused by the creep test.[doi:10.2320/matertrans.MT-M2021101]

(Received June 3, 2021; Accepted June 16, 2021; Published July 26, 2021)

Keywords: magnesium­calcium alloy, lamellar spacing, interface, creep, dislocation

1. Introduction

Magnesium alloys are the lightest of structural metallicmaterials. These alloys are increasingly used in automotiveand aerospace industries, where they are instrumental inincreasing fuel efficiency and thus minimizing carbon dioxideemissions.1) The widespread application of the magnesiumalloys to high-temperature components, not only to room-temperature components, is essential for designing transportequipment with low weight and high fuel efficiency.2,3) PureMg has low strength at high temperatures,4,5) and thesolubility of alloying elements in Mg is quite limited.6) Thethermally stable intermetallic phases, which are utilized asthe precipitation­dispersion phase7,8) or covering phase,9,10)

are indispensable to improve the high-temperature strengthof heat-resistant Mg alloys.11,12)

Another method for improving the high-temperaturestrength by using intermetallic phases is to control themicrostructure so that the matrix and intermetallic phases arelayered via a eutectic reaction.13­16) The Mg­Al­Ca ternarysystem without rare-earth elements, which usually exhibitsexcellent nonflammability, is a promising alloy system fordeveloping highly versatile heat-resistant magnesium alloyswith excellent cost performance.17­21) When the Mg­Al­Caternary alloys in the Mg-rich composition region are meltedand cast, three kinds of eutectic reactions are possible duringsolidification depending on the [Ca]/[Al] ratio: (i) L¼¡-Mg + A12­Mg17Al12, (ii) L¼ ¡-Mg + C36­(Mg,Al)2Ca,and (iii) L¼ ¡-Mg + C14­Mg2Ca.22) Only third eutecticreaction occurs in the composition region with [Ca]/[Al] > 1.5, and the resultant ¡/C14 eutectic structure isextremely fine compared to the ¡/A12 and ¡/C36 eutecticstructures.23,24)

When a binary Mg­Ca hypoeutectic alloy with acomposition close to the eutectic composition (Mg­

16.2mass% Ca) is melted and then cast with a mild steelmold, the resultant ¡/C14 eutectic structure has a lamellarstructure with a curved ¡/C14 interface. The lamellar spacingis a submicron size less than 1 µm.25) In a previous work, thecrystal orientation relationship between ¡-Mg lamellae andthe ¡/C14 interface in the ¡/C14 lamellar structure wasexamined, and the temperature region in which the ¡/C14lamellar structure is stable in morphology was determined.25)

The results showed that (1) the primary slip plane (0001)¡of the ¡-Mg lamellae was oriented toward the ¡/C14interface, and (2) the ¡/C14 lamellar structure becameincreasingly coarse at temperatures above 573K. Thequantitative relationship between lamellar spacing (­) andaging time (t) was obtained: ­2 ¹ ­0

2 = kTt, where ­0 is the¡/C14 lamellar spacing for the as-cast specimen, and kT is aconstant depending on aging temperature.

The strength of metallic materials with lamellar micro-structures is known to increase with decreasing lamellarspacing at room temperature.25­29) In contrast, the effect of ­on high-temperature creep strength has not been elucidatedfor Mg alloys. The objectives of this study were to clarify thefollowing three points with regard to the binary Mg­Caeutectic alloy with the ¡/C14 lamellar microstructure: (1) Toclarify the temperature range in which the plastic deformationoccurs and decide the temperature for the creep test; (2) toclarify quantitatively the correlation between creep strengthand ­; (3) to clarify the role of the ¡/C14 lamellar interfaceon dislocation glide during creep. To clarify the first point,tensile tests were performed for the as-cast specimen undera wide range of temperature and strain rate conditions. Toclarify the second point, creep tests were performed for thealloys whose ­ was controlled by the aging treatment. Toclarify the third point, the dislocation substructure of thecreep specimens was examined.

2. Experimental

A binary Mg­13.8mass% Ca hypoeutectic alloy was usedin this study. The Ca content of the alloy was reduced by2.4mass% from the eutectic composition (Mg­16.2mass%Ca) to avoid precipitation of the brittle primary C14­Mg2Ca

+1This Paper was Originally Published in Japanese in J. Japan Inst. Met.Mater. 85 (2021) 223­228.

+2Graduate Student, Tokyo Institute of Technology. Present address: JFESteel Corporation, Kawasaki 210-0868, Japan

+3Graduate Student, Tokyo Institute of Technology+4Corresponding author, E-mail: terada.y.ab@m.titech.ac.jp

Materials Transactions, Vol. 62, No. 9 (2021) pp. 1414 to 1419©2021 The Japan Institute of Metals and Materials

phase. A block of the alloy of dimensions 100 © 160 ©20mm was gravity-cast at a casting temperature of 1053K bythe permanent mold-casting method with a mild steel moldin argon atmosphere using easily available starting materialsof the highest purity. The as-cast specimen showed a mixedmicrostructure of primary ¡-Mg phase and ¡-Mg/C14­Mg2Ca eutectic lamellae.25) According to the Mg­Ca binaryphase diagram,6) the weight ratios of the primary ¡-Mg phaseand ¡-Mg/C14­Mg2Ca lamellar were estimated as 16%and 84%, respectively, based on the lever rule. The ¡/C14lamellar spacing, ­, in the ¡/C14 lamellar region for theas-cast specimen was 0.9 µm.25) The microstructure of thealloy was almost occupied by the ¡/C14 lamellar structure.Therefore, this alloy is termed the ¡-Mg/C14­Mg2Caeutectic alloy hereafter in this study, and the microstructureobservation was performed in the ¡/C14 lamellar region.

Tensile-test specimens with a gage length of 13.2mm anda cross-sectional area of 3 © 1mm were prepared by electric-discharge machining and mechanical polishing from therectangular ingot manufactured by permanent mold-castingas mentioned earlier. Tensile tests were performed at fourtemperatures between 298 and 473K at strain rates between1.2 © 10¹5 and 1.2 © 10¹4 s¹1. Creep test specimens with agage length of 28mm and a cross-sectional area of 6 © 3mmwere prepared from 3-mm-thick plates cut out from therectangular ingot.30,31) The creep specimens were subjectedto aging treatment at 673K for 3.6 © 104 s (10 h) and 3.6 ©105 s (100 h) to achieve ­ = 1.9 µm and ­ = 5.3 µm.25) Acreep test specimen that had undergone the aging treatment at723K for 3.6 © 103 s (1 h) to achieve ­ = 1.3 µm was alsoprepared. Creep tests under tension were performed in air at473K under a stress of 40MPa. The current of the creepfurnace was designed to stop at creep rupture to minimize theexposure time at high temperatures for the creep-rupturedspecimen. During creep interruption, the sample was rapidlycooled under a load by using compressed air followed bywater quenching, to preserve the dislocation substructure.

The dislocation substructure of the creep specimens wasobserved by high-resolution transmission electron microsco-py (HRTEM). Thin foils were cut from the creep gage portionof the sample and machined to disks with a diameter of3mm. In the case of the creep-ruptured specimen, the foilswere cut out from a distance of 5mm from the rupturedportion. The disks were further thinned down to 90 µm bymechanical polishing, followed by dimple grinding and ion-milling, to perforate the center portion of the disks. Theperforated disks were examined using a Cs-correctedscanning transmission electron microscope FEI Titan3 G260-300 operated at 300 kV.

3. Results and Discussion

3.1 Stress­strain curveFigure 1 shows the stress­strain curves of the ¡-Mg/C14­

Mg2Ca eutectic alloy at temperatures between 373 and 473K.For a strain rate of 1.2 © 10¹5 s¹1, the specimen rupturedbefore yielding, and plastic deformation did not occur at 373and 423K. In contrast, the stress increased continuously withincreasing strain after yielding at 473K, and a plastic strainexceeding 5% was evident after the maximum stress at

132MPa. When the strain rate was doubled to 2.4 © 10¹5 s¹1

at 473K, the stress increased drastically with increasingstrain after yielding, and the specimen ruptured at themaximum stress of 143MPa.

Table 1 summarizes the tensile properties of the ¡-Mg/C14­Mg2Ca eutectic alloy against temperature and strainrate. In Table 1, the tensile-test conditions under whichrupture occurred before yielding are denoted as ©; theconditions under which rupture occurred before themaximum stress after yielding are marked by ; theconditions under which rupture occurred at the maximumstress and after the maximum stress are denoted by and ,respectively. Under six types of tensile-test conditions in thetemperature range below 423K, the specimens rupturedbefore yielding, and there was no plastic deformation. Incontrast, plastic deformation was evident at any strain ratesexamined at 473K. That is, when the strain rate was as highas 1.2 © 10¹4 s¹1 and 6.0 © 10¹5 s¹1, the specimens rupturedafter yielding before exhibiting the maximum stress. Therupture strain increased with decreasing strain rate, and thespecimen ruptured after the maximum stress at the loweststrain rate of 1.2 © 10¹5 s¹1. Thus, the above results showthat for the ¡-Mg/C14­Mg2Ca eutectic alloy, plastic

Nominal strain, (%)

Nom

inal

stre

ss,

/ M

Pa

0 5 10 150

50

100

150

200

423K473K

473K

Black: = 1.2 10−5 s−1Red: = 2.4 10−5 s−1

..

373K

Fig. 1 Stress­strain curves at temperatures between 373 and 473K for theas-cast ¡-Mg/C14­Mg2Ca eutectic alloy.

Table 1 Summary of the tensile property for the as-cast ¡-Mg/C14­Mg2Caeutectic alloy.

Effect of Lamellar Spacing on Creep Strength of ¡-Mg/C14­Mg2Ca Eutectic Alloy 1415

deformation occurred at temperatures higher than 473K, andthe lower strain rate enhanced plastic deformability.

3.2 Creep strength and lamellar spacingFrom the results of the tensile tests shown in the previous

section, the creep tests were performed at 473K. Plasticdeformation of the ¡-Mg/C14­Mg2Ca eutectic alloy wasevident at this temperature. From the stress­strain curves at astrain rate of 1.2 © 10¹5 s¹1 (Fig. 1), the 0.2% proof stressfor the alloy was evaluated as 84MPa at 473K. Therefore, inthis study, the applied stress of the creep tests was adopted as40MPa, which is less than a half of the 0.2% proof stress forthe alloy at 473K. This value was adopted to ensure excellentplastic deformability in the evaluation of the long-term creepcharacteristics.

To clarify the effect of the ¡/C14 interface on the creepstrength, creep tests were performed at 473K under a stressof 40MPa for the ¡-Mg/C14­Mg2Ca eutectic alloy aged at673K and 723K to increase ­. The ¡/C14 lamellar structureof the alloy had stable morphology at 473K.25) Figure 2shows the creep rate­time curves for the alloy aged at 673Kfor 10 and 100 h, together with the data for the as-castspecimen. The overall creep rate­time curve for everyspecimen shows a downward curvature from stressapplication until creep rupture. That is, after stress wasapplied, a normal transient creep was detected, and the creeprate decreased continuously with creep time. Subsequently,there was a gradual increase in the creep rate in theaccelerating region that led to the creep rupture of thespecimen. Awell-defined steady state was barely evident. Forthe as-cast specimen, the creep rate decreased by more thantwo orders of magnitude in the transient region, and theminimum creep rate (6.6 © 10¹9 s¹1) was observed at a creeptime of 8.0 © 105 s (222 h); subsequently, creep ruptureoccurred at 3.6 © 106 s (1006 h). The creep rate­time curvesof the specimens aged at 673K for 10 and 100 h are similar tothat for the as-cast specimen; meanwhile, the minimum creep

rate increased, and the decrease in creep rate during thetransient stage becomes less significant with longer agingtime at 673K.

The minimum creep rate (_¾min), rupture life (trup), andrupture strain (¾rup), and ­, for the as-cast and aged ¡-Mg/C14­Mg2Ca eutectic alloys are summarized in Table 2.These values were obtained from creep tests performed at473K under 40MPa stress. The creep test of the specimenaged at 723K was interrupted immediately after showing theminimum creep rate; the results of this specimen too areincluded in the table. The data in Table 2 show that _¾min

continuously increased and trup continuously decreased withincreased aging time at 673K, and ­ increased from 0.9 to5.3 µm. In addition, ¾rup was maximum at 20.6% for thespecimen aged at 673K for 10 h, and it decreased when ­

was changed from the value of 1.9 µm.Morphologically, the interface area included in a unit

volume of the two-phase lamellar microstructure decreases to1/N, when ­ of the alloy increases by a factor of N.32)

Therefore, the quantitative correlation between _¾min and ­ forthe ¡-Mg/C14­Mg2Ca eutectic alloy can be evaluated byusing a power approximation, not a linear approximation.The _¾min of the ¡-Mg/C14­Mg2Ca eutectic alloy thatunderwent creep at 473K under 40MPa stress is plottedagainst ­ in double logarithmic coordinates in Fig. 3. In thefigure, _¾min increases continuously with increasing ­, and allthe four data points are on a straight line with a slope of unity.It is evident that the correlation between _¾min and ­ can beexpressed by using a power approximation. Note that thegradient of the _¾min­­ curve was evaluated as 0.89 by themethod of least squares. Since the _¾min decreases continu-ously with decreasing ­, it is inferred that the ¡/C14 interfaceenhances the creep strength, i.e., the ¡/C14 interface acts asa creep strengthener. Hence, introducing the interface into amicrostructure by utilizing the fine lamellar structure can bean effective way for enhancing the high-temperature creepstrength of Mg alloys. In addition, a novel high-temperaturestrengthening mechanism was identified in this study;according to this mechanism, the interface enhances thehigh-temperature strength, and this phenomenon is termedinterface strengthening (IFS).

The phenomenological creep equation for the ¡-Mg/C14­Mg2Ca eutectic alloy should include microstructure parame-ters, such as colony size (d) and ­, in addition to the creeptesting temperature (T ) and applied stress (·).33,34) The _¾min isexpressed as a function of ·, T, d, and ­ for the alloy, asshown in eq. (1).

: 673 K/10 h aged: as cast

: 673 K/100 h aged

Time, t / s102 103 104 105 106 107 10810 9

10 8

10 7

10 6

10 5

Cre

ep ra

te,

/ s−

1

: creep rupture

Fig. 2 Creep rate vs. time curves at 473K under a stress of 40MPa for the¡-Mg/C14­Mg2Ca eutectic alloys aged at 673K for 10 and 100 h,together with that for the as-cast specimen.

Table 2 Summary of minimum creep rate (_¾min), rupture life (trup), andrupture strain (¾rup) for the as-cast and aged ¡-Mg/C14­Mg2Ca eutecticalloys, together with the lamellar spacing (­). The creep tests were carriedout at 473K under a stress of 40MPa.

K. Oishi, S. Araki and Y. Terada1416

_¾min ¼ Að·=GÞnðb=dÞmð­=bÞp expð�Qc=RT Þ ð1Þwhere A is the material constant; G is the shear modulus; b isthe length of the Burgers vector in the ¡-Mg lamellae; R is thegas constant; Qc is the activation energy for creep; and n, m,and p are constants. The gradient of the _¾min­­ curve in Fig. 3corresponds to p in eq. (1), and the results shown in Fig. 3suggest p = 1 for the ¡-Mg/C14­Mg2Ca eutectic alloy.

3.3 Dislocation substructure of creep specimensThe dislocation substructure of the ¡-Mg/C14­Mg2Ca

eutectic alloy that underwent creep at 473K under 40MPastress was investigated to clarify the role of the ¡/C14interface on dislocation glide during creep. The HRTEMimage of the as-cast specimen that underwent creep ruptureis shown in Fig. 4(a), where the incident beam direction isB ¼ ½01�11�¡ for the ¡-Mg lamellae under the multiple-beamdiffraction condition. Many dislocations are observed withinthe ¡-Mg lamellae. Most dislocations are distributeduniformly inside the ¡-Mg lamellae, while some dislocationsare aligned through the ¡-Mg lamellae, as indicated by whitearrowheads in Fig. 4(a). In contrast, dislocations are scarcewithin the C14­Mg2Ca lamellae. From these observation, itis deduced that the ¡/C14 interface limits the dislocationswithin the ¡-Mg lamellae.

The g·b values with an incident beam direction B ¼½01�11�¡ were calculated for a reciprocal lattice vector g ¼0�112¡ (Table 3) to identify whether perfect a, a + c, andc-dislocations in the hcp structure with the Burgers vector(b) are visible. Further, a-dislocations with b ¼ 1=3½11�20�and b ¼ 1=3½1�210�, a + c dislocations, and c-dislocationsare visible, whereas no a-dislocations with b ¼ 1=3½�2110�are visible in the case of g ¼ 0�112¡. Figure 4(b) shows theHRTEM image of the same field as that shown in Fig. 4(a),taken under the two-beam diffraction condition with g ¼0�112¡. The visible dislocations within the ¡-Mg lamellaeunder the multiple-beam diffraction condition in Fig. 4(a) arescarcely detected under the two-beam diffraction condition,

as shown in Fig. 4(b). The result indicates that mostdislocations operating in the ¡-Mg lamellae during creepfor the ¡-Mg/C14­Mg2Ca eutectic alloy are a-dislocationswith b ¼ 1=3½�2110�.

The HRTEM image of the ¡/C14 interface for thespecimen with ­ = 1.3 µm, obtained by aging at 723K for1 h and exhibiting creep at 473K under 40MPa stress, isshown in Fig. 5. The creep test was interrupted at a strain of

(b)

(a)

C14

200nm

01120000

C14

g=0112

Fig. 4 HRTEM image, taken with B ¼ ½01�11�¡, of the as-cast ¡-Mg/C14­Mg2Ca eutectic alloy creep-ruptured at 473K under a stress of40MPa under (a) multiple and (b) two-beam diffraction conditions. Thedislocation alignment is indicated by white arrowheads in (a).

Table 3 The g·b invisibility criterion for perfect dislocations in thehexagonal close-packed crystals close to the ½01�11� zone axis.

Lamellar spacing, / µm

1 1010 9

10 8

10 7M

inim

um c

reep

rate

, m

in /

s−1

200.2

1

Fig. 3 Plots of minimum creep rate vs. lamellar spacing for the ¡-Mg/C14­Mg2Ca eutectic alloy, where the creep tests were carried out at 473Kunder a stress of 40MPa.

Effect of Lamellar Spacing on Creep Strength of ¡-Mg/C14­Mg2Ca Eutectic Alloy 1417

1.3%, which corresponds to the strain immediately after theminimum creep rate was observed. Many dislocations aredetected on the ¡/C14 interface, and they are not arrangedin any specific manner but randomly distributed. From theresults of the dislocation substructure for the creep speci-mens, it is inferred that the ¡/C14 interface presents a barrierto the a-dislocation glide within the ¡-Mg lamellae and doesnot annihilate and/or rearrange the dislocations caused by thecreep test.

Grain boundaries are regarded to act as a sink ofdislocations during creep deformation and diminish the creepstrength; i.e., a grain boundary weakens the creep.33) Incontrast, the ¡/C14 interface is inferred to act as a creepstrengthener because it presents a barrier to dislocation glideduring creep deformation and does not annihilate and/orrearrange the dislocations caused by the creep test. In future,the IFS may be systematically examined as a high-temperature strengthening mechanism for eutectic alloysbased on metals other than Mg.

4. Conclusions

Tensile tests were conducted for a binary Mg­13.8mass%Ca hypoeutectic alloy, whose microstructure is mostlyoccupied by the ¡/C14 lamellar structure. This alloy wastermed ¡-Mg/C14­Mg2Ca eutectic alloy in this study toclarify the temperature range in which it undergoes plasticdeformation. Creep tests were performed for the as-cast andaged specimens to evaluate the effect of the ¡/C14 interfaceon creep strength, and the dislocation substructure of thestudied specimens was observed by HRTEM. The followingresults were obtained:(1) In the tensile tests of the ¡-Mg/C14­Mg2Ca eutectic

alloy, the specimen ruptures before yielding, and noplastic deformation occurs at temperatures below423K. In contrast, plastic deformation occurs attemperatures above 473K, and a lower strain rateenhances the plastic deformability.

(2) The creep tests results of the ¡-Mg/C14­Mg2Caeutectic alloy obtained at 473K under a stress of

40MPa showed that the creep curves of the alloyexhibits three stages: a normal transient creep stage, aminimum creep-rate stage, and an accelerating stagethat is followed by creep rupture. Since the minimumcreep rate decreases continuously with decreasinglamellar spacing, the ¡/C14 interface is regarded toenhance the creep strength, i.e., the ¡/C14 interface actsas a creep strengthener.

(3) The dislocations during creep are chiefly limited withinthe ¡-Mg lamellae for the ¡-Mg/C14­Mg2Ca eutecticalloy, and most dislocations are a-dislocations with theidentical Burgers vector. The dislocations are randomlydistributed on the ¡/C14 interface. The ¡/C14 interfacepresents a barrier to the glide of a-dislocations withinthe ¡-Mg lamellae and does not annihilate and/orrearrange the a-dislocations caused by the creep test.

Acknowledgments

The alloy samples used in this study were manufacturedand provided by Mitsui Mining & Smelting Co., Ltd. Thiswork was supported by a Grant-in-Aid for ScientificResearch C (19K05054) of JSPS, Japan. One of the authors(Y. Terada) greatly appreciates the support of the Light MetalEducational Foundation. A part of this work was conductedat Hokkaido University, supported by the NanotechnologyPlatform Program of the Ministry of Education, Culture,Sports, Science and Technology (MEXT), Japan. The authorswould like to thank Mr. Kenji Ohkubo and Mr. Ryo Otaof Hokkaido University for their kind assistance with themicrostructure observation using electron microscopy.

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Effect of Lamellar Spacing on Creep Strength of ¡-Mg/C14­Mg2Ca Eutectic Alloy 1419