Antiphase Boundary-Like Structure of B19 Martensitein Ti-Ni-Pd Shape Memory Alloy

Mitsuhiro Matsuda1,+, Masatoshi Mitsuhara2, Kazuki Takashima1 and Minoru Nishida2

1Department of Materials Science and Engineering, Kumamoto University, Kumamoto 860-8555, Japan2Department of Engineering Sciences for Electronics and Materials, Kyushu University, Kasuga 816-8580, Japan

The antiphase boundary (APB)-like structure of B19 martensite in the Ti-Ni-Pd alloy was investigated by means of conventionaltransmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. Some APB-like structures withwide and curved contrast exhibited shifts along the (010)B19 and (001)B19 planes; that is, it exhibited facets composed of those planes at theatomic level. This atomic displacement reflects the atomic movement stemming from not only martensitic but also a kind of R-phasetransformations. [doi:10.2320/matertrans.MB201505]

(Received September 3, 2015; Accepted October 20, 2015; Published December 4, 2015)

Keywords: antiphase boundary, martensitic transformation, displacement vector, high-angle annular dark-field scanning transmission electronmicroscopy, titanium-nickel-palladium alloy

1. Introduction

Near equiatomic Ti-Ni and Ti-Pd alloys undergo thermo-elastic martensitic transformation from the B2 to B19A(monoclinic) and B19 (orthorhombic) structures upon cool-ing, respectively. The former alloy is a technologicallyimportant material with superior shape memory effect andsuperelasticity.1) The latter alloy is a candidate for high-temperature shape memory material because its transforma-tion temperature is about 800K.2) We have so far analyzed anantiphase boundary (APB)-like contrast of the martensite inTi-Ni and Ti-Pd shape memory alloys.3,4) We characterizedthose APB-like structures as a kind of stacking fault with anAPB-like morphology induced by the martensitic trans-formation. It has been reported that the atomic displacementof APB-like structure is influenced by not only martensiticbut also pre-martensitic transformations. Concerning thepre-martensitic transformations, it is well known that theathermal ½-phase forms during quenching of ¢-Ti alloys,and that R-phase transformation in Ti-Ni-based alloys occurs.For example, Inamura et al.5) discovered an APB-likestructure in ¡AA-martensite in a Ti-Nb-Al alloy. The displace-ment vector was determined as a transformation-inducedAPB, with an additional small displacement stemming froma specific variant of the pre-existing athermal ½-phase. Wealso investigated an APB-like structure of the B19A martensitevia an R-phase transformation in a Ti-Ni-Fe alloy.6) It isconcluded that the atomic displacement of an APB-likestructure in a Ti-Ni-Fe alloy reflects the atomic movementstemming from the R-phase transformation itself, in additionto the displacement due to the atomic shuffling during thetransformation directly from the B2 parent phase to B19Amartensite.

It is pointed out that the transition from B2 to B2(½)structure before a martensitic transformation, meaning a pre-martensite, is caused by a substitution of Ni for Pd in thebinary Ti-Pd alloy.7,8) Based on the increase of electricalresistivity during cooling process in the Ti-35.5 at% Ni-

15.5 at% Pd alloy,8) the transition from B2 to B2(½) isgenerally regarded as a kind of R-phase transformation. Wetherefore expect that the characteristics of APB-like structureof B19 martensite in the Ti-Ni-Pd alloy via a kind of R-phasetransformation is different from those in Ti-Pd binary alloywithout the pre-martensitic transformation. It is aim of thepresent paper is to clarify the crystallography and morphol-ogy of the APB-like structure of the B19 martensite in theTi-Ni-Pd alloy by conventional transmission electron mi-croscopy (CTEM) and high-angle annular dark-field scanningtransmission electron microscopy (HAADF-STEM). Theobtained results are discussed on the basis of the localheterogeneity of atomic movements during not onlymartensitic but also pre-martensitic transformations.

2. Experimental Procedure

A Ti-25.0 at% Ni-25.0 at% Pd alloy was prepared from99.7% Ti, 99.9% Ni, and 99.8% Pd (mass%) by arc meltingin an argon atmosphere. The ingots were homogenized invacuum at 1273K for 3.6 ks. Subsequently, the samples weresolution-treated in vacuum at 1273K for 3.6 ks, and thenquenched into ice water. Differential scanning calorimetry(DSC) measurements were performed using a calorimeter(NETZSCH DSC-3500 Sirius) at a cooling and heating rateof 0.083K/s. The TEM specimens were electropolishedusing a twin jet method in an electrolyte solution consistingof 20% H2SO4 and 80% CH3OH by volume at around 253K.CTEM observation was carried out with a JEM-2000FXmicroscope (JEOL, LTd., Tokyo) at an accelerating voltageof 200 kV. HAADF-STEM observations were performedusing a JEM-ARM200F microscope (Cs-corrected 200 kVSTEM; JEOL). The electron probe measured 0.1 nm and thecurrent was approximately 20 pA. To obtain the HAADF-STEM images, the annular detector was set to collectelectrons scattered at angles between 90 and 170mrad. TheHAADF-STEM allowed us to identify columns of relativelyheavy atoms by their bright contrast in the image resultingfrom the atomic number (Z) contrast.9,10) The informationthat we obtained directly represented the structural image+Corresponding author, E-mail: matsuda@alpha.msre.kumamoto-u.ac.jp

Materials Transactions, Vol. 57, No. 3 (2016) pp. 250 to 256Special Issue on New Aspects of Martensitic Transformations©2015 The Japan Institute of Metals and Materials

corresponding to the actual atomic columns. HAADF-STEMis, therefore, a very powerful and useful technique forproviding direct information on local structures at the atomiclevel. The crystal structure of the B19 martensite is of theAuCd-type with an orthorhombic cell. The lattice corre-spondence between the B2 parent phase and the B19martensite is illustrated in Fig. 1.1) The dashed and solidlines indicated B2 and B19 structures, respectively. Thefollowing lattice parameters were used for the analysis:aB19 = 0.278 nm, bB19 = 0.445 nm, and cB19 = 0.471 nm.11)

3. Results and Discussion

Figure 2 shows DSC curves for the solution-treated Ti-25.0 at% Ni-25.0 at% Pd alloy. There is an exothermic peakcorresponding to the B2 to B19 martensitic transformationduring cooling. The martensitic transformation start (Ms)and martensitic transformation finish (Mf ) temperatures aredetermined to be 425.4K and 409.1K, respectively. Duringheating an endothermic peak is observed. The reversetransformation start (As) and reverse transformation finish(Af ) temperatures are determined to be 432.8K and 442.4K,respectively. These transformation temperatures are almostagreement with previous results.12) The present alloy seemsto undergo only martensitic transformation on the basis ofeach exothermic and endothermic peak.

Figures 3(a) and (b) show the typical bright field imageand the electron diffraction pattern taken from the areamarked B in (a), respectively, in the Ti-25.0 at% Ni-25.0 at%Pd alloy. This alloy is composed of B19 martensite as well asthe near-equiatomic Ti-Pd alloy. The martensitic plates in thisalloy are wider and larger than those of binary Ti-Ni and Ti-Pd alloys. The electron diffraction pattern shown in Fig. 3(b)consists of two sets of reflections, that are in mirror symmetrywith respect to the ð1�11ÞB19 plane, from the [121]B19 directionof a B19 structure. The trace of boundaries in region B isparallel to the ð1�11ÞB19 plane. This fact indicates that the twosets of reflections show a ð1�11ÞB19 twin pattern, and theplatelet is a ð1�11ÞB19 type I twin, which is a lattice invariantshear of the martensitic transformation from the B2 to theB19 structures in the Ti-Ni-Pd alloy. These observations

are consistent with the results provided in the previousreports.13,14) Note that many lined and curved contrasts areobserved within the martensitic platelets, as indicated by thearrows in Fig. 3(a). In order to analyze these contrasts,CTEM observations are carried out from the [100]B19direction. HAADF-STEM observations were also performedalong this direction, described below, because the atomiccolumns of Ti and Ni and/or Pd in the B19 structure caneasily be distinguished.

Figures 4(a) and (b) show the bright-field image andcorresponding electron diffraction pattern of [100]B19 direc-tion of a B19 structure, respectively. Some APB-likestructures with wide and curved contrasts along both the(001)B19 basal plane and (010)B19 planes are observed inmartensitic plates of the Ti-25.0 at% Ni-25.0 at% Pd alloy, asindicated by the one and double arrows in Fig. 4(a),respectively, whereas APB-like structures with thin linearcontrasts along the basal plane are observed in the B19martensitic plates of the equiatomic Ti-Pd alloy, as discussedin our previous reports.4) Figure 4(c), (d) and (e) showsdark-field images taken using 001B19, 002B19, and 003B19reflections, in which APB-like contrasts are observed. It has

cB2

[110]B2 // cB19

TiNi, Pd

aB19

aB2[110]B2 // bB19

bB2

Fig. 1 Lattice correspondence between the B2 parent phase and the B19martensite. Dashed and solid lines indicate the B2 and B19 structures,respectively.

Temperature, T/K350

Hea

t flo

w (a

rb. u

nit)

400 450 500

Cooling

Heating

MsMf

As A

Fig. 2 DSC curves for a Ti-25.0 at% Ni-25.0 at% Pd alloy. There areexothermic and endothermic peaks corresponding to the martensitictransformation.

(a) (b)

500nm

B

000

EB//[121]B19

111M, T

101M

012T

Fig. 3 (a) Bright-field image of a Ti-25.0 at% Ni-25.0 at% Pd alloy and(b) Electron diffraction pattern taken from the area B in (a). This alloy iscomposed of B19 martensite.

Antiphase Boundary-Like Structure of B19 Martensite in Ti-Ni-Pd Shape Memory Alloy 251

been widely recognized that ³ contrast of APB induced byorder-disorder transformations can be observed using super-lattice reflections for the ordered structure, whereas nocontrast is observed using fundamental reflections. Thepresent APB-like contrast is observed when both the 001B19and 003B19 superlattice and 002B19 fundamental reflectionsfor the B19 structure are used. These facts mean that theAPB-like contrast does not correspond to ³ contrast: they do,however, correspond to stacking faults with an APB-likemorphology induced by the martensitic transformation, asexpected from Ti-Ni and Ti-Pd alloys.3,4) The 020B19reflection is used because the 010B19 reflection is forbiddenaccording to the extinction rule of the B19 structure.15) Nocontrast is observed when the 020B19 reflection is used, asshown in Fig. 4(f ).

The HAADF-STEM technique using Z contrast wasperformed to determine the positions of the atomic columnsat the interface in APB-like contrasts. Figure 5(a) shows anHAADF-STEM image of the APB-like structure taken alongthe [100]B19 direction of a B19 structure. Figure 5(b) showsthe image intensity profile taken along the white line X-Y in

Fig. 5(a). It has been reported that Pd atoms can be locatedby electron channeling enhanced microanalysis at the Niatom sites preferentially in the Ti-Ni-Pd alloy.16) Similarly, inthe present case of Ti-25.0 at% Ni-25.0 at% Pd alloy Pdatoms are considered to be located at the Ni atom sitespreferentially. Therefore, due to the nature of Z-contrast,the higher intensity profile shows Ni (Z = 28) and/or Pd(Z = 46) columns, whereas the lower intensity profile showsTi (Z = 22) columns. The bright and dim spots in Fig. 5(a),therefore, correspond to the Ni(Pd) and Ti atomic columns,respectively. The Fourier filter-processed image of Fig. 5(a)is presented in Fig. 5(c), and the atomic arrangements inFig. 5(c) are schematically illustrated in Fig. 5(d). The openand solid circles indicate Ni(Pd) and Ti atom columns,respectively. We can see the atomic shifts along both the(010)B19 and (001)B19 planes by the arrows of Fig. 5(d). Thatis to say, the APB-like structure consists of facets composedof both (010)B19 and (001)B19 planes at an atomic level, asindicated by the line between the arrows of Fig. 5(d).

Based on the results of CTEM of Fig. 4 and HAADF-STEM of Fig. 5, we discuss the atomic displacement of the

000

EB//[100]B19

001020

200nm

(a) (b)

(c) (d)

g

(e) (f)

200nm

200nm200nm

g

g g

Fig. 4 (a) Bright-field image and (b) corresponding electron diffraction pattern of a Ti-25.0 at% Ni-25.0 at% Pd alloy along the [100]B19direction of a B19 structure. (c)­(f ) Dark-field images of the area shown in (a); APB-like structure in contrast: g = (c) 001, (d) 002 and(e) 003; no APB-like structure in contrast: g = (f ) 020 with EB near [100]B19.

M. Matsuda, M. Mitsuhara, K. Takashima and M. Nishida252

APB-like structure in the present Ti-25.0 at% Ni-25.0 at% Pdalloy. We previously reported that the displacement vector ofthe APB-like structure on the B19 martensite in equiatomicTi-Pd alloy can be expressed as R = ©0¹1/2 1/3ªB19, wherethe lattice correspondence of B19 martensite for B2 parentphase in equiatomic Ti-Pd alloy is in accordance with that inthe present Ti-25.0 at% Ni-25.0 at% Pd alloy. The value of Rin equiatomic Ti-Pd alloy is equal to the displacement causedby atomic shuffling alone during the martensitic trans-formation to maintain the B19 structure between domains.Therefore, if the atomic displacement of the APB-likestructure in the B19 martensite of Ti-25.0 at% Ni-25.0 at%Pd alloy is equal to the displacement caused by atomicshuffling alone during the martensitic transformation as wellas the equiatomic Ti-Pd alloy, the value of R in the presentTi-25.0 at% Ni-25.0 at% Pd alloy should be expressed asR = ©0¹1/2 1/3ªB19, meaning that no APB-like contrast wasobserved using each g = 0 2n 0, and 0 0 3n, where n is aninteger, by the relationship between the phase angle and R.17)

As seen in Fig. 4(f ), no APB-like contrast is observed usingg = 020B19. HAADF-STEM observation of Fig. 5(c) alsosupports that the atomic displacement along b-axis of APB-

like contrast in the present Ti-Ni-Pd alloy is 1/2 by thepositions of the atomic columns at the interface in APB-likecontrasts. However, APB-like contrast is observed usingg = 003B19 shown in Fig. 4(e), meaning that the atomicdisplacement along c-axis in the APB-like contrast is not 1/3,which is not consistent with the R of equiatomic Ti-Pd alloy.

Furthermore, to confirm the atomic displacement alongthe a-axis in APB-like structure, CTEM observations werecarried out along the [001]B19 direction; these revealed thedisplacement along the a-axis on the a-b plane. Figures 6(a)and (b) shows a bright-field image and correspondingelectron diffraction patterns of [001]B19 direction of a B19structure, respectively. Some APB-like structures with curvedcontrasts are observed in martensitic plates as well as thosealong [100]B19 direction of a B19 structure in Fig. 4.Figures 6(c) and (d) shows dark-field images taken using100B19 and 020B19 reflections, respectively. The APB-likecontrast is observed when the 100B19 reflections, indicatingthat the APB-like contrast shows a displacement in its a-axiscomponent. No contrast is observed when using the 020B19reflection, as shown in Fig. 6(d). Figure 7(a) shows anHAADF-STEM image of the APB-like structure along the

(010)

(001

)

1nm

Inte

nsity

(arb

it. u

nit)

X Y1nm

Ti

Ni, Pd

Ti

Ni, Pd

Ti

Ni, Pd

(a) (b)

(c)

X YNi, Pd

Ni, PdTi

(d)

(010)

(001

)

1nm

Fig. 5 (a) A HAADF-STEM image of the APB-like structure in a Ti-25.0 at% Ni-25.0 at% Pd alloy along the [100]B19 direction of a B19structure. (b) Image intensity profile taken along the white line X-Y in (a). (c) Fourier filter-processed HAADF-STEM image around theAPB-like interface. The open circles indicate the Ni and/or Pd atomic columns. (d) Schematic illustration of the atomic arrangementsin (c).

Antiphase Boundary-Like Structure of B19 Martensite in Ti-Ni-Pd Shape Memory Alloy 253

[001]B19 direction of a B19 structure. The Fourier filter-processed image of white framed area indicating APB-likecontrast in Fig. 7(a) is presented in Fig. 7(b). The bright anddim spots in Fig. 7(b) correspond to the Ni(Pd) and Ti atomiccolumns, respectively, because of the nature of Z contrast, aswas found in Fig. 4(a). As indicated by the area between thearrows in Fig. 7(b) the contrasts of atomic columns on the(100)B19 planes becomes obscure. To enhance the visibility ofthe atomic column, a Fourier filtered images using only{100}B19 and {200}B19 reflections marked in the diffracto-gram Fig. 7(d) is presented in Fig. 7(c), indicating that thedistortion of atomic columns on the (100)B19 planes isobserved around APB-like structure. On the other hand, noatomic displacement on the (010)B19 planes is observed inFig. 7(e), which is a Fourier filtered images using only{020}B19 reflections marked in the diffractogram Fig. 7(f ).This proves that APB-like contrast in the present Ti-25.0 at%Ni-25.0 at% Pd alloy has a displacement along a-axiscomponent in contrast to that in equiatomic Ti-Pd alloy.That is to say, the R of APB-like structure in both alloys isdifferent although the crystal structure of both alloys is thesame as B19 martensite at room temperature. These meanthat the atomic displacement of the APB-like structure in thepresent Ti-25.0 at% Ni-25.0 at% Pd alloy is not only due tothe atomic shuffling during the martensitic transformationdirectly from the B2 phase to B19 structure, which isdifferent from the APB-like structure in equiatomic Ti-Pdbinary alloy.

We further discuss about the R of APB-like structure onthe B19 martensite in the present Ti-25.0 at% Ni-25.0 at% Pdalloy. As described above, it has been so far reported that theatomic displacement of APB-like structure is influenced by

not only martensitic but also pre-martensitic transforma-tions.5,6) In the case of the present Ti-Ni-Pd alloy, there seemsto be no clearly additional peak indicating the R-phasetransformation except for martensitic transformation shownin DSC curves of Fig. 2. However, the existence of a kind ofR-phase transformation as a pre-martensite is pointed out onthe basis of the increase of electrical resistivity during coolingprocess in ternary Ti-Ni-Pd alloy containing much Pd.7,8) TheR-phase lattice can be described by stretching the B2 parentlattice along the ©111ªB2 diagonal directions. Since there arefour ©111ªB2 orientations in the B2 parent lattice, four latticecorrespondences are possible between the B2 parent latticeand the R-phase lattice.18­20) These suggest that the displace-ment along ©111ªB2 direction of the B2 parent phasestemming from the R-phase transformation corresponds toeither the ©101ªB19 direction on the (010)B19 or the ©110ªB19direction on the (001)B19 plane for the B19 martensitic lattice,as shown in Fig. 1. That is to say, these displacements can beexpressed as Rxz = ©¤x 0 ¤zªB19 and Rxy = ©¤x ¤y 0ªB19, respec-tively. Therefore, the displacement adding the Rxz to thedisplacement vector caused by atomic shuffling during themartensitic transformation; that is, R = ©0¹1/2 1/3ªB19 +©¤x 0 ¤zªB19 = ©¤x¹1/2 1/3 + ¤zªB19, can explain the APB-likecontrasts using each g = 100B19 and 003B19. The Rxy

associated with the APB-like contrasts using g = 020B19 isnow under study. From the relationship of g·R in the presentAPB-like contrasts described above, it is concluded thatatomic displacement of the APB-like structure of themartensite via a kind of R-phase in the Ti-Ni-Pd alloyreflects the atomic movement stemming from a kind of R-phase transformation, meaning that the APB-like structure isaffected by the atomic movements during not only marten-

000

EB//[001]B19

100020

200nm

(a) (b)

(c) (d)

g g

200nm 200nm

Fig. 6 (a) Bright-field image and (b) corresponding electron diffraction pattern of a Ti-25.0 at% Ni-25.0 at% Pd alloy along the [001]B19direction of a B19 structure. (c) and (d) Dark-field images of the area shown in (a); (c) APB-like structure in contrast: g = 100; and (d) noAPB-like structure in contrast: g = 020 with EB near [001]B19.

M. Matsuda, M. Mitsuhara, K. Takashima and M. Nishida254

sitic but also a kind of R-phase transformations. As a matterof concern, the crystallography and exact transformationtemperature region of a kind of R-phase transformation itselfin this alloy should be investigated by in-situ TEMobservations before and after the transformation and measur-ing the electrical resistivity. The present obtained resultsprovide that it is important to characterize the APB-likestructure for the elucidation of the atomic movements duringpre-martensitic and martensitic transformations, resulting in afurther understanding of the shape memory behavior.

4. Conclusion

An APB-like structure of the B19 martensite in the Ti-Ni-Pd alloy was investigated by means of CTEM and HAADF-STEM. Some APB-like structures with wide and curvedcontrast exhibited shifts along the (010)B19 and (001)B19planes; that is, it exhibited facets composed of those planesat the atomic level. The atomic displacement on the APB-likestructure of the B19 martensitein Ti-Ni-Pd alloy reflectsthe atomic movement stemming from a kind of R-phasetransformation, in addition to the displacement due to the

atomic shuffling during the transformation directly from theB2 parent phase to B19 martensite.

Acknowledgments

The authors would like to express their sincere gratitudeto Professor S. Tsurekawa and Professor Y. Morizono ofKumamoto University for their supports in the arc meltingand for their valuable comments.

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