In-Situ Neutron Diffraction Study on Tensile Behavior of LPSO Mg­Zn­Y Alloys

Kazuya Aizawa1,+, Wu Gong1, Stefanus Harjo1, Jun Abe1, Takaaki Iwahashi1 and Takashi Kamiyama2

1J-PARC Center, Japan Atomic Energy Agency, Naka-gun, Ibaraki 319-1195, Japan2Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan

Tensile behavior of single-phase 18R-Long period stacking ordered structure (LPSO) Mg­Zn­Y alloys and two-phase Mg­Zn­Y alloyswhich consist of Mg matrix and 18R-LPSO phase were studied by in-situ pulsed-neutron diffraction technique under tensile stress. Anisotropicbehavior between the a-axis and c-axis in the 18R-LPSO structure was observed for the single-phase LPSO Mg­Zn­Y casted alloy from theelastic region under tensile stress. On the other hand, two-phase Mg­Zn­Y alloys of low 18R-LPSO phase concentration, which were producedby extrusion, behave isotropic until the yield point under tensile stress, and the 18R-LPSO phase acts as a strengthening phase in the plasticregion. [doi:10.2320/matertrans.MI201214]

(Received December 25, 2012; Accepted April 1, 2013; Published May 11, 2013)

Keywords: long period stacking ordered structure, magnesium alloy, neutron diffraction, tensile stress

1. Introduction

New Mg­Zn­Y alloys with the high strength, whichconsist of Mg matrix and long period stacking ordered(LPSO) structure phase, has been developed1) and the originof the high strength is thought due to the unique LPSOstructure.

It is well known that excellent mechanical properties ofLPSO alloys appear by extrusion. There have been manystudies on the crystal structure, microstructure and mechan-ical properties of LPSO alloys until now.2­9) Those studieswere performed mainly using the electron microscope andstandard mechanical test method.

It is important to obtain the bulk structural informationunder stress to understand the high strength of LPSO alloys.The neutron diffraction technique is adequate to obtainthe average microscopic structural information of bulkspecimen.

Therefore, in order to clarify the relationship between themicrostructure and the high strength property of bulk LPSOalloys, in-situ neutron diffraction measurements of singlephase LPSO Mg­Zn­Y alloy and two phase LPSO Mg­Zn­Y alloys, which were produced by casting or extrusion withthe different LPSO phase volume fraction were performedunder tensile stresses.

2. Experimental Procedure

We used a single-phase 18R-LPSO Mg85Zn6Y9 castedalloy and a two-phase 18R-LPSO Mg97Zn1Y2 casted alloy10)

in this study. Two-phase LPSO Mg97Zn1Y2 and Mg89Zn4Y7

extruded alloys are also used. The extrusion ratio is 10, andthe extrusion temperatures are 623 and 723K, respectively.To observe the microstructure of these alloys, specimenswere sectioned and polished by a conventional metallo-graphic technique. 2% nital etchant was used to revealmicrostructure. The central area of the specimen wasobserved by scanning electron microscopy (SEM) using aHitachi 4300 microscope. SEM images of these specimens

are shown in Fig. 1. Cylindrical rods of LPSO alloys weremachined into JIS No. 4 test pieces for tensile tests to usein-situ neutron diffraction measurements under tensile stress.The test piece dimension was º3mm © 47.8mm in allspecimens.

In-situ neutron diffraction experiments were performed atbeamline 19 ‘TAKUMI’11) in Materials and Life ScienceExperimental Facilities at J-PARC. TAKUMI is a pulsed-neutron diffractometer dedicated engineering issues such asstrain analysis in bulk materials. There are two detector bankswith scattering angle of «90 degree at its central position.Therefore strain analyses of two orthogonal axes of aspecimen are available by one measurement. The cylindricaltest piece was attached to the load frame. The axial directionwas in the 45 degree direction to the incident neutron beam.The +90 degree detector-bank detects the diffracted neutronto the axial direction and the ¹90 degree detector-bankdetects the diffracted neutron to the radial direction of thespecimen.

Fig. 1 SEM microstructure of LPSO Mg­Zn­Y alloys: (a) Mg97Zn1Y2

casted alloy, (b) Mg85Zn6Y9 casted alloy, (c) Mg97Zn1Y2 extruded alloyand (d) Mg89Zn4Y7 extruded alloy. In (a), (c) and (d) the darker area isMg-matrix and the brighter area is the LPSO phase. RD: radial direction,ED: Extrusion direction.

+Corresponding author, E-mail: aizawa.kazuya@jaea.go.jp

Materials Transactions, Vol. 54, No. 7 (2013) pp. 1083 to 1086©2013 The Japan Institute of Metals and Materials

3. Results and Discussions

Figure 2 shows the stress and strain curves of 18R-LPSOalloys during in-situ neutron diffraction experiments. Tooptimize corresponding neutron diffraction intensities forstress of interest, several controls were used. In the elasticregion, the constant-speed load control was used. It repeatsconstant-speed load and keeping load with short time. On theother hand, in the plastic region, the displacement control wasused. That is either constant-speed small displacementcontrol or repeated control of constant-speed displacementand keeping displacement with long time. In the case of thelatter control, the stress varied within keeping time. Tensilestrength of the extruded specimen becomes higher than thatof the casted specimen in the case of Mg97Zn1Y2. SingleLPSO-phase Mg85Zn6Y9 casted alloy is brittle and theelastic region is unclear. In the case of two-phase extrudedspecimens, tensile strength of high volume fraction of LPSOphase specimen is higher than that of low volume fraction ofLPSO phase specimen.

Figure 3 shows a typical neutron diffraction pattern of thesingle-phase 18R-LPSO Mg85Zn6Y9 casted alloy beforestress loading. Neutron diffraction patterns were analyzedusing the crystal structure model (Space group P3212,a = 1.11 nm, c = 4.69 nm) proposed by Egusa12) by the Z-Rietveld code.13)

The tensile behavior of the single-phase 18R-LPSOMg85Zn6Y9 casted alloy in the axial and radial directionsto the applied stress is shown in Fig. 4. Figure 4(a) showsthe tensile behavior of the averaged internal lattice strains forthe a-axis and the c-axis directions, which were evaluatedby the change of the lattice parameter obtained by Rietveldanalysis. Figure 4(b) shows the tensile behavior of theinternal strains for ð60�60Þ and (00018) planes which havestrong diffraction intensities among planes in the parallel tothe a-axis and the c-axis directions, respectively. The tensilebehavior of the internal strains for ð4�2�28Þ and ð4�2�2 �10Þ planeswhich have strong diffraction intensities (see Fig. 3) amongplanes in the non-parallel to both the a-axis and c-axis arealso shown in Fig. 4(b). Internal strains of individual latticeplanes were evaluated by the diffraction peak shift of theindividual lattice plane. Elastic anisotropy between the a-axisand c-axis of 18R-LPSO structure was observed in bothaxial and radial directions. Internal strains of both axes andindividual lattice planes seem to be nearly linear relation tothe macroscopic stress until fracture. Evaluated diffractionelastic constants are shown in Table 1. The internal strains ofthe planes with not being parallel to the a-axis or c-axis takeintermediate values between the value of the (00018) planeand the value of the ð60�60Þ plane.

The tensile behavior of the individual lattice planes ofthe Mg matrix and the 18R-LPSO phase of the two-phaseMg89Zn4Y7 extruded alloy, which is high 18R-LPSOconcentration, in the axial direction to the applied stressis shown in Fig. 5. The behaviors of lattice planes for the

Fig. 3 The neutron diffraction pattern of the single-phase 18R-LPSOMg85Zn6Y9 casted alloy before applying tensile stress in the radial andaxial directions.

Fig. 4 The tensile behavior of the single-phase 18R-LPSO Mg85Zn6Y9

casted alloy in the radial and axial directions. (a) averaged internal latticestrains evaluated by the lattice parameter change (b) internal strains ofindividual lattice planes, which were evaluated by the diffraction peakshift of the individual lattice plane.

Fig. 2 Stress and stran curves of LPSO Mg­Zn­Y alloys.

K. Aizawa et al.1084

18R-LPSO phase, which have same Miller indices selectedin Fig. 4, excepting (00018) planes of which diffractionpeak position is overlapped with the position of the (0002)diffraction peak of the Mg matrix, are plotted. The behaviorsof main lattice planes of the Mg matrix, which diffract withstrong intensities, are also plotted.

Most planes of the 18R-LPSO phase keep elasticity aftermacroscopic yield stress and the elastic anisotropy ofindividual lattice plane is small. On the other hand, aftermacroscopic yield stress, some lattice planes of Mg matrixdo not change the internal strains to the applied stress, thisbehavior implies those planes deform plastically. Similartensile behavior, i.e., the plastic deformation of the Mgmatrix, was observed in the radial direction.

The tensile behavior of the individual lattice planes ofthe Mg matrix and 18R-LPSO phases in the two-phaseMg97Zn1Y2 casted alloy, which is low 18R-LPSO concen-tration, in the axial direction to the applied stress is shown inFig. 6(a).

Because the 18R-LPSO concentration is low, only ð4�2�28Þand ð4�2�2 �10Þ diffraction peaks of the 18R-LPSO phase couldbe analyzed for the peak shift with sufficient statistics. Thebehaviors of main lattice planes of the Mg matrix are alsoplotted. After yield stress, some planes of the Mg matrixphase behaved plastically. On the other hand, hardening ofthe 18R-LPSO phase occurred. Figure 6(b) shows the changeof integrated intensity of the Mg matrix in the axial directionto the applied stress. After macroscopic yield stress,significant intensity changes of every diffraction peaks

occurred. This means that Mg matrix grains rotate to theapplied stress.

The tensile behavior of the individual lattice planes ofthe Mg matrix and 18R-LPSO phase in the two-phaseMg97Zn1Y2 extruded alloy in the axial direction to theapplied stress is shown in Fig. 7. The lattice planes of the Mgmatrix analyzed for the two-phase Mg97Zn1Y2 casted alloywere used for the strain evaluation, on the other hand, onlythe ð4�2�28Þ plane was used for the 18R-LPSO phase, becausediffraction intensities of other lattice planes of the 18R-LPSOphase were insufficient to evaluate peak sifts with highaccuracy. It is clearly seen that the evolution of the internalstrain both Mg matrix and 18R-LPSO phases lies on the samestraight line. So, this alloy behaves as an isotropic material inthe elastic region. Similar result reported on Mg97Y2Zn1Gd0.5by synchrotron radiation diffraction.14) Similar to the castedspecimen, after yield stress, some planes of the Mg phasebehaved plastically, and hardening of the 18R-LPSO phaseoccurred. Also rotation of Mg matrix grains was observed.After macroscopic yield stress, significant intensity changesoccurred. This means that Mg matrix grains rotate to theapplied stress.

4. Conclusion

In-situ pulsed-neutron diffraction technique unter tensilestress applied to the LPSO Mg­Zn­Y alloys. The single-

Fig. 5 The tensile behavior of the individual lattice planes of the 18R-LPSO phase and the Mg matrix of the two-phase Mg89Zn4Y7 extrudedspecimen in the axial direction to the applied stress. Dashed line indicatesthe yield stress level.

Fig. 6 The tensile behavior of the two-phase Mg97Zn1Y2 casted specimenin the axial direction to the applied stress. (a) the evolution of theindividual lattice planes of the Mg matrix and 18R-LPSO phases. (b) thechange of integrated intensity of the Mg matrix. Dashed line indicates theyield stress level.

Table 1 Evaluated diffraction elastic constant for the a-axis, the c-axis andindividual lattice planes of the single-phase 18R-LPSO Mg85Zn6Y9 castedspecimen in the axial direction to the applied stress.

Axis/plane Diffraction elastic constant (GPa)

a-axis 49.5

c-axis 129

ð60�60Þ 36.2

ð4�2�2 �10Þ 57.5

ð4�2�28Þ 55.3

(00018) 81.2

In-Situ Neutron Diffraction Study on Tensile Behavior of LPSO Mg­Zn­Y Alloys 1085

phase 18R-LPSO Mg85Zn6Y9 casted alloy exhibited ani-sotropic internal strain behavior between the a-axis and c-axis in the 18R-LPSO structure to the tensile stress. The 18R-

LPSO phase acts as strengthening phase in the plastic regionfor two-phase LPSO Mg­Zn­Y alloys.

Acknowledgments

This work was supported by a grant-in-aid for ScientificResearch on Innovative Areas (Project: “Materials Science onSynchronized LPSO Structure ®The Evolution of theMaterial Science for Innovative Development of the Next-generation Lightweight Structure Materials®”) from theMinistry of Education, Culture, Sports, Science and Tech-nology of Japan. The authors thank Kawamura laboratory forproviding the specimens.

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Fig. 7 The tensile behavior of the two-phase Mg97Zn1Y2 extrudedspecimen in the axial direction to the applied stress. (a) the evolution ofthe individual lattice planes of the Mg matrix and 18R-LPSO phases.(b) the change of integrated intensity of the Mg matrix. Dashed lineindicates the yield stress level.

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