In Situ TEM Observation of Structural Changes in Rapidly Solidified bcc Solid Solution Phase in Ti Cr Alloy Focusing on Spontaneous Vitrification (SV)

In Situ

TEM Observation of Structural Changes in Rapidly

Solidi

fi

ed bcc Solid-Solution Phase in Ti

­

Cr Alloy

Focusing on Spontaneous Vitri

fi

cation (SV)

Takeshi Nagase

, Satoshi Anada

, Keita Kobayashi

, Hidehiro Yasuda

and Hirotaro Mori

1_{Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki 567-0047, Japan}

2_{Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita 565-0871, Japan}

Changes in the structure of the rapidly solidified body-centered-cubic (bcc) solid-solution phase in Ti­Cr alloy were investigated in order to determine whether or not the unique heat-induced amorphization (i.e., SV) actually occurs. Annealing-induced amorphization of the single solid-solution phase was not observed during eitherex situisothermal annealing at 873 K orin situannealing experiments in HVEM. The previously reported SV may not correspond to the formation of an amorphous phase but could actually be due to the formation of a finely grained polycrystalline structure during the decomposition of the bcc solid-solution phase. [doi:10.2320/matertrans.MD201306]

(Received September 3, 2013; Accepted November 5, 2013; Published December 13, 2013)

Keywords: in situ transmission electron microscopy (TEM), high-voltage electron microscopy (HVEM), solid-state amorphization (SSA), phase transition, rapid solidification, irradiation

1. Introduction

Blatter et al.1­4) reported novel amorphization known as

spontaneous vitrification (SV) in various alloys. Compared

with conventional solid-state amorphization (SSA) such

as interdiffusion-induced amorphization in multilayers,5,6)

hydrogen-induced amorphization,7,8)

mechanical-milling-induced amorphization,9,10) _{irradiation-induced}

amorphiza-tion11­16)_{and so on, SV is novel because the starting material}

is a single solid-solution crystal phase rather than a single phase of intermetallic compound or a mixture of two crystalline phases. Several researchers have conducted experiments to determine whether or not SV actually occurs.

Although Kim et al.17) and Yan et al.18) reported that SV

had occurred, Prasad et al.,19) Sinkler et al.,20) and Hirata

et al.21,22) reported that it had not. Ranganathan and

Ramachandra have recently suggested a new thermodynamic

model for SV,23) while Prasad and Greer concluded that

SV was thermodynamically impossible.24) Although many

researchers have been trying to understand SV, there is little agreement as to the annealing-induced formation of an amorphous phase from a solid solution. In other words, it remains an unsettled question about the occurrence of the unique heat-induced amorphization (i.e., SV). As mentioned in detail in the followings, these must be considerable doubt as to whether the SV actually occurs in metallic materials. The main objective of the present study is to apply X-ray diffraction (XRD), thermal analysis, and transmission

electron microscopy (TEM) to a rapidly solidified

body-centered-cubic (bcc) solid-solution phase in order to determine whether or not the SV actually occurs in a Ti60Cr40 alloy.

2. Materials and Methods

Master ingots of Ti60Cr40 (at%) alloy were prepared by

arc-melting Cr and Ti in a water-cooled Cu hearth under a

purified Ar atmosphere. The composition of the alloy was the

same as that of alloys used by Blatteret al.1,2) and by Kim

et al.17) A single-roller melt-spinning method was used to

produce rapidly quenched ribbons from the master ingots. A quenching apparatus consisting of a 200-mm-diameter copper roller was operated at 4000 rpm in an Ar atmosphere;

therefore, the roller-surface velocity was about 42 ms¹1_.

A 14-mm-diameter quartz nozzle with a 0.5-mm-diameter

orifice was used. The gap between the nozzle bottom and the

roller surface was maintained within 0.4 mm.

The melt-spun ribbons wereex situ thermally annealed at

873 K in an evacuated silica tube containing a Ti oxygen getter. The structures of the melt-spun and annealed ribbons

were examined using XRD with Cu­K¡radiation and using

TEM. Twin-jet polishing was used to prepare thin foils from the ribbons for TEM observation. The thin foils were

polished in a solution of CH3OH/C2H5OH/HClO4 (45/45/

10) at 213 K. TEM and high resolution TEM (HRTEM) observations were performed using Hitachi H-800 and JEOL JEM-ARM200F/UHR microscopes, respectively, both oper-ated at 200 kV.

In addition to these ex situ annealing experiments,in situ

annealing experiments were also conducted using H-800 and Hitachi H-3000 high voltage electron microscope (HVEM) operated at 2 MV. In these experiments by using HVEM, TEM specimens were irradiated with 2 MeV electrons prior to the annealing experiments, aiming to introduce defects beforehand. The irradiation was carried out at 298 K. The

dose rate wasfixed at 5.0©1024m¹2s¹1. The vacuum of the

specimen chamber in the H-3000 was <2.0©10¹5Pa. The

changes in the brightfield (BF) image and the corresponding

selected-area diffraction (SAD) patterns were observed

in situ during the irradiation and the subsequent annealing

experiments.

3. Results

Figure 1 shows the outer appearance (a), XRD pattern (b),

TEM-BF images (c), (d), TEM-SAD patterns (cA), (dA),

+_{Corresponding author, E-mail: t-nagase@uhvem.osaka-u.ac.jp}

Special Issue onIn SituTEM Observation of High Energy Beam Irradiation

HREM image (e), and fast Fourier transform (FFT) pattern

obtained from the HREM image (eA) for the as-melt-spun

ribbon. A rapidly solidifiedflake-like specimen whose

cross-section was about 2.0 mm wide and 0.04 mm thick was obtained (see Fig. 1(a)). The XRD pattern (b) shows sharp diffraction peaks. The strong peaks indicated by the open

circles are identified as the bcc solid-solution phase whose

lattice constant is 0.312 nm. This value is similar to that

predicted by the Vegard’s law for the Ti60Cr40 (at%) alloy.

Some of the low-intensity peaks do not correspond to the bcc

phase, and these peaks are identified as the Cr2Ti compound

(indicated by thefilled circles). The XRD pattern, therefore,

indicates the formation of a bcc solid-solution phase as the

main constituent phase and the Cr2Ti compound as the minor

phase. The TEM-BF image (Fig. 1(c)) is a low magnification

photo of the sample, showing the formation of a polycrystal-line structure whose grain size is on the order of tens of

micrometers. It is noted that numerous nodules with fine

lamella-like structures were formed along grain-boundaries (One of such nodules is arrowed in Fig. 1(c)). Figure 1(d) is

an enlargement of a grain-boundary region and Fig. 1(dA) is

the corresponding SAD. Although the matrix was identified

as the bcc solid-solution phase from the SAD pattern

(Fig. 1(cA)), the SAD pattern obtained from the

grain-boundary region (Fig. 1(dA)) shows numerous diffraction

spots, which do not correspond to the bcc solid-solution phase. The compositions of the matrix and the grain-boundary region were evaluated using TEM energy-dispersive X-ray

spectroscopy (TEM-EDS). The matrix consisted of 61.6 at%

Ti and 38.4 at% Cr, and the boundary region consisted of

41.7 at% Ti and 58.3 at% Cr, where the composition is the

average of the analysis of 7 different positions in the same specimen. The composition of the bcc solid-solution matrix is

similar to that of the master ingot of Ti60Cr40 alloy. The

concentration of Cr is higher in the boundary region than in

the master Ti60Cr40 alloy, indicating the formation of Cr2Ti,

which is the only one intermediate phase in the Ti­Cr

system.25)_{The formation of a single bcc solid-solution matrix}

without any crystalline precipitates can be confirmed in the

HREM image ((Figs. 1(e) and 1(eA)). The results of the XRD,

TEM, TEM-EDS, and HREM show that the main constituent phase was the bcc solid-solution phase and that a small amount of Cr2Ti was formed in the grain-boundary region of the bcc grain in the as-melt-spun specimen.

The melt-spun ribbon was thermally analyzed to better understand the phase-transition behavior during thermal annealing. Figure 2 shows the results of the differential

thermal analysis (DTA) performed at 0.33 K s¹1on the

melt-spun ribbon under an Ar atmosphere. The DTA curve shows

an exothermic peak indicated by “A” and an endothermic

peak indicated by“M”. The peak indicated by A corresponds

to heat released when the bcc phase transitioned to another phase whose Gibbs free energy is lower than that of the bcc phase, and the peak indicated by M corresponds to the melting of the alloy. As will be mentioned in the followings, the exothermic peak A corresponds to the phase transition from the bcc solid-solution phase to a mixture containing a hexagonal closed packed (hcp) solid-solution phase and a Cr2Ti intermetallic compound. The onset temperature at which the bcc solid-solution phase decomposed was

measured as³930 K.

The melt-spun ribbon was then subjected to ex situ

isothermal annealing experiments to investigate whether or not an amorphous phase formed through thermal-annealing

as envisaged in SV. The annealing temperature wasfixed at

873 K on the basis of the results of the above thermal analysis and of previous reports. The formation of an amorphous

phase through SV was previously reported for Ti60Cr40 thin

film annealed at 873 K,1) _{for water-quenched Ti}

60Cr40 alloy

100 nm

boundary

(c’)matrix

110 011

1 μm (c)

20 mm (a)

(b) (d) (d’)

(e) (e’)

1 nm

X-ray intensity (Arbitrary unit)

100 80

60 40

2 theta, 2θ / degree bcc, 110

200 211

220 Cr2Ti

bcc Cr2Ti

Fig. 1 Outer view and microstructure of rapidly-solidified melt-spun ribbon in Ti60Cr40 alloy. (a) Outer appearance, (b) XRD, (c) TEM-BF

image, (cA) TEM-SAD pattern obtained from a matrix region away from the grain-boundary region, (d) magnified TEM-BF image of a grain boundary region, (dA) TEM-SAD pattern obtained from a grain-boundary region (i.e., from nodules), (e) HREM image obtained from matrix, and (eA) FFT pattern obtained from (e). Compositions of matrix and grain boundary are as follows: (matrix) 61.6 at%Ti and 38.4 at%Cr, (grain boundary) 41.7 at% Ti and 58.3 at% Cr. In (c) and (d), a nodule is indicated by a white arrow.

T_onset=930 K

DTA, 0.33 Ks−1

Exothermic (Arbitrary unit)

1600 1200

800 400

Temperature, T / K A

M

annealed at 873 K for 1.8©104_s,2)_{for mechanically alloyed}

Ti60Cr40 (obtained from an elemental powder blend of

Ti60Cr40) annealed at 823 K for 1.2©103s,17) and for

mechanically alloyed Ti50Cr50 annealed at 850 K for

1.8©103_s,18)_{indicating that ex situ annealing at 873 K for}

a period on the order of 104_{s was a suitable condition for the}

current experiment to determine whether SV actually occurs. The changes in the microstructure of the alloy were investigated using XRD and TEM, where the alloy was

ex situannealed at 873 K for various periods, and the results

are shown in Fig. 3. Figure 3(a) shows the XRD patterns for the ribbon annealed at 873 K for various periods. The intensity of the peaks in the XRD patterns is represented by the logarithmic scale. Intense peaks corresponding to the hcp

phase and the Cr2Ti compound appeared in the XRD pattern

after annealing for 3.6©103_{s. With continued annealing, the}

intensities of the peaks corresponding to the Cr2Ti compound

and the hcp phase increased, while the intensities of the peaks corresponding to the bcc phase decreased. After

annealing for 1.8©104s, the peaks corresponding to the bcc

phase disappeared, and only the peaks corresponding to the

hcp and Cr2Ti phases remained in the XRD pattern. The

broad peak typical of the formation of an amorphous phase did not appear in the XRD pattern. After annealing for

3.6©104_{s, the spectrum essentially remained unchanged.}

These results suggest that the bcc phase had transformed

into a mixture of the hcp solid-solution and Cr2Ti phases by

annealing. The annealed specimens were observed using TEM to investigate in detail the possibility of the formation of an amorphous phase in localized areas of the specimens. Figures 3(b), 3(c), and 3(d) show the BF image of the

specimens annealed at 873 K for 3.6©103, 1.8©104, and

3.6©104s, respectively. Figures 3(bA), 3(cA), and 3(dA) inset,

are the corresponding SAD patterns. Plate-like precipitates were embedded in the bcc matrix phase, as observed in

the BF image of the specimen annealed for 3.6©103s

(Fig. 3(b)). Hirata et al. previously investigated in detail

the annealing-induced structural change in the bcc

solid-solution phase in the Ti70Cr30 alloy.23,24) They found that

the structural change occurred as follows: bcc¼bcc+

Zone I¼bcc+Zone II¼bcc+Zone II+hcp¼bcc+

hcp+C15¼hcp+C15. The plate-like precipitates formed

in the current study are similar to those that Hirata et al.

described in their previous reports. With further annealing,

the matrix changed into a fine polycrystalline structure as

seen from Figs. 3(c) and 3(d), and correspondingly there appeared discontinuous Debye rings in the SAD patterns

(Figs. 3(cA) and 3(dA)). Since the discontinuous Debye rings

are consistently indexed as those of the hcp solid-solution

and the Cr2Ti phases (see also Fig. 8(cA)), the polycrystalline

structure is identified as a mixture of the two phases. Namely,

the bcc phase had decomposed into the hcp solid-solution

phase and the Cr2Ti intermetallic compound under ex situ

thermal annealing at 873 K, resulting in the formation of a

fine polycrystalline structure (Figs. 3(c) and 3(d)). Thermal

1 μm (a)

100 nm

100 nm (b) 3.6x103_s

(1 h)

(b’)

(c) 1.8x104_s (5 h)

(d) _3.6x104_s

(10 h)

(d’) (c’)

(c”)

hcp 1011

C15-Cr₂Ti 311 222 400

X-ray intensity (log scale,

Arbitrary unit)

80 70 60 50 40 30

2 theta, 2θ / degree Cr2Ti

bcc

3.6x103s (1 h) 3.6x104s (10 h)

bcc

hcp Cr2Ti A_B

C

hcp

1.8x104s (5 h)

(d”)

hcp C15-Cr2Ti

1011 311 222 400

111B C

A

Fig. 3 Structural change in rapidly solidified bcc solid-solution phase in Ti60Cr40 alloy observed duringex situ annealing. (a) XRD patterns for

melt-spun ribbon thermally annealed at 873 K. Intensity of X-ray peaks is represented by logarithmic scale. The index A, B, and C correspond to the peaks of hcpð1011Þ, Cr2Ti (311) and Cr2Ti (222), respectively. (b)(bA) BF

and SAD of a specimen taken from Ti60Cr40 alloy ex situ thermally

annealed at 873 K for 3.6©103_{s (1 h), and (c)(cA) BF and SAD of a}

specimenex situthermally annealed at 873 K for 1.8©104_{s (5 h), (d)(dA)}

BF and SAD of a specimen ex situ thermally annealed at 873 K for 3.6©104_{s (10 h). (c″) Magnified SAD of (cA). (d″) Magnified SAD of}

(dA).

110 011

(a) 298 K

868 K

(b)

1 μm 1 μm

(a’)

(b’)

Fig. 4 BF images (a), (b) and SAD patterns (aA), (bA) for rapidly solidified bcc solid solution in Ti60Cr40 alloy taken during thermal annealing

experiment in conventional TEM. (a)(aA) at room temperature before annealing, (b)(bA) at 868 K.

annealing induced structural changes in the rapidly-solidified

alloy were also investigated using in situTEM observation;

samples were heated in the range 298­863 K. Figure 4 shows

the change in the BF images and SAD patterns of the

melt-spun ribbon in a Ti60Cr40alloy. The significant change in the

bcc matrix and grain boundary region was not observed during annealing. Halo rings due to solid-state amorphization did not appear in the SAD patterns. SV in the bcc

solid-solution phase was neither detected during the in situ TEM

annealing nor detected during the ex-situ annealing

experi-ments.

To confirm whether or not the SV actually occurs in bcc

solid-solution, the following experiments were performed by using HVEM: (1) the melt-spun specimen was irradiated at room temperature for the introduction of defects in the bcc solid-solution, and (2) the specimen with the irradiation

defects wasin situannealed from room temperature to 873 K

according to a stepwise temperature-increase program (a temperature vs. time diagram is shown in Fig. 5). The change in microstructure of bcc solid-solution phase was investigated

in situ using HVEM. Above mentioned in situ experiment

has advantages compared with that using conventional TEM; (1) the enhancement of thermal diffusion via introduced defects may stimulate SV at the temperature lower than 873 K. An amorphous phase is not a thermal equilibrium phase, and changes to crystalline phases at the crystallization temperature. The lower the onset temperature of SV is, the longer the lifetime of an amorphous phase is. The enhance-ment of atomic diffusion at temperatures lower than 873 K

may be effective for detecting SV by in situTEM if SV is

realistic. (2) Blatter et al. pointed out that SV ability of

the Ti60Cr40 alloy was strongly related with the sample

preparation method;4) _{the strongly distorted and}

anisotropi-cally strained bcc solid-solutions had a high SV tendency.

These deficiencies indicated the presence of a high defect

concentration, providing a large number of incoherent sites as

nucleation centres for the amorphous phase. The irradiation

may enhance the SV ability in rapidly solidified bcc

solid-solution. Figure 5 shows a diagram of the employed

annealing temperature as a function of time for the in situ

experiment in HVEM. Index A; the irradiated specimen was quickly heated at the desired temperature (i.e., 373, 473, 573, 673, 737, 873 and 973 K) and was maintained at that temperature for 600 s. Index B; the irradiated specimen was quickly heated at 873 K, and kept at 873 K.

Figures 6(a)­6(c), 6(a″), 6(b″) are BF images showing

successive stages of defect introduction into the bcc

solid-solution phase in the Ti60Cr40 alloy under 2.0 MeV electron

irradiation at 5©1024_m¹2_s¹1 _{at 298 K, along with the}

corresponding SAD patterns (aA)­(cA). The BF images

depicted in Figs. 6(a″) and 6(b″) are a magnified image of

Figs. 6(a) and 6(b), respectively. Black spot defects appeared in the specimen after irradiation for 30 s as indicated by the

white arrow in Figs. 6(b) and 6(b″), and the size and the

number density of the defects increased with increasing total dose of electrons (Fig. 6(c)). In contrast, the irradiation-induced structural changes cannot be detected from the

analysis of the SAD patterns shown in Figs. 6(bA) and 6(cA).

Figures 6(c″) and 6(cAAA) show an HREM image and the

corresponding FFT pattern respectively for the specimen irradiated for 600 s, and they strongly support that a single bcc-solid solution phase was kept after irradiation for 600 s; the black spot defect is not due to the precipitation of other phases but is due to the accumulation of irradiation-produced point defects, indicating that the irradiation can introduce

Fig. 5 Schematic of temperature vs. time employed inin situannealing experiment in HVEM. Index A; the irradiated specimen was quickly heated at the desired temperature (i.e., 373, 473, 573, 673, 737, 873 and 973 K) and was maintained at that temperature for 600 s. Index B; the irradiated specimen was quickly heated at 873 K, and kept at 873 K. In the present study, the temperature deviation from the desired temperature and temperature overshoot from the desired temperature during heating were within 5 K at 373 and 473 K, and 2 K at the temperature range between 573 and 973 K, respectively. The time needed in heating to each desired temperature is shorter than 30 s.

(a)0 s

110 011

100 nm

100 nm

100 nm (b)30 s,

1.5x1026 _m-2

(a’)

(b’)

(c’)

1 nm

(c”’) (c”)

20 nm (b”)

20 nm (a”)

(c)600 s, 3.0x1027 _m-2

Fig. 6 BF images (a)­(c), (a″), (b″) and SAD patterns (aA)­(cA) for rapidly solidified bcc solid-solution phase in Ti60Cr40 alloy during in situ

irradiation at 5©1024_m¹2_s¹1 _{at 298 K in HVEM, and HREM image}

(c″) and the corresponding FFT pattern (cAAA) taken by JEM-ARM200F/ UHR. (a)(aA)(a″); before irradiated, (b)(bA)(b″); irradiated for 30 s to a dose of 1.5©1026_m¹2_{, and (c)(cA)(c″)(cAAA); irradiated for 600 s to a dose of}

defects in the specimen without changing the structure of the bcc phase. Figure 7 shows the BF images (a)­(d) and SAD

patterns (aA)­(dA) obtained for the electron-irradiated bcc

solid-solution phase before (a), (aA) and during thermal

annealing (annealing profile A in Fig. 5) at 673 K (b), (bA),

873 K (c), (cA) and 973 K (d), (dA). It is noted that the intensity

of diffuse scattering around each spot increased with increasing annealing temperature, as can be seen from the

SADs (Figs. 7(bA), 7(cA) and 7(dA)), while no essential changes

can be detected in the corresponding BF images (Figs. 7(b), 7(c) and 7(d)). The diffuse scattering observed in the present study is similar to those reported in the previously

reports,2,4,19­22) _{and can be attributed to the mechanical}

instability and/or the ½ transition of the bcc phase.4,20,26,27)

The evolution of diffuse scattering during annealing indicates the decrease in the stability of the irradiated bcc

solid-solution phase by annealing, however, the occurrence of SV cannot be seen. Figure 8 shows the change in BF images (a)­

(d) and SAD patterns (aA)­(dA) of irradiated bcc solid solution

in a Ti60Cr40 alloy during thermal annealing at 873 K

(annealing profile B in Fig. 5). Annealing induced structural

change by just heating up to 873 K from room temperature

was not observed (Figs. 8(a) and 8(aA)). After annealing for

600 s, the appearance of diffraction spot indicated by the

white arrow was observed in SAD pattern (Fig. 8(bA)), and

this can be considered to be corresponding to thefirst stage of

the annealing induced structural change.21,22)The appearance

of diffuse streak with the h110i direction was also detected

in SAD pattern of the specimen annealed at 873 K in SAD

pattern (Fig. 8(bA)). The intensity of diffuse streak increased

with the increase in the annealing time, as shown in the SAD

pattern of the specimen annealed for 3.6©103_{s (Fig. 8(cA))}

110 011 (a) 298 K

after irradiation

100 nm (b) 673 K

100 nm

(c) 873 K

100 nm

(a’)

(b’)

(c’)

(d) 973 K

100 nm

(d’)

Fig. 7 BF images (a)­(d) and SAD patterns (aA)­(dA) for rapidly solidified bcc solid solution in Ti60Cr40 alloy taken during thermal annealing

experiment in HVEM. (a)(aA) at room temperature before annealing, (b)(bA) at 773 K, (c)(cA) at 873 K, and (d)(dA) at 973 K. The specimen was, prior to annealing experiment, irradiated with 2 MeV electron, at 5© 1024_m¹2_s¹1 _{and at 298 K for 600 s. A schematic diagram showing}

the temperature vs. time program employed in the in situ annealing experiment is presented by the index A in Fig. 5.

0 s

after irradiation

600 s

3.6x103_s

110 011

100 nm

100 nm

100 nm

(a) (a’)

(b’) (b)

(c’) (c)

1.8x104_s (d’)

(d)

100 nm

Fig. 8 BF images (a)­(d) and SAD patterns (aA)­(dA) for rapidly solidified bcc solid solution in Ti60Cr40 alloy taken during thermal annealing

experiment at 873 K in HVEM. (a)(aA) for 0 s at 873 K, (b)(bA) for 600 s at 873 K, (c)(cA) for 3.6©103_{s at 873 K, and (d)(dA) for 1.8©10}4_{s at 873 K.}

The specimen was, prior to annealing experiment, irradiated with 2 MeV electron, at 5©1024_m¹2_s¹1_{and at 298 K for 600 s. A schematic diagram}

showing the temperature vs. time program employed in the in situ

annealing experiment is presented by the index B in Fig. 5.

and 1.8©104s (Fig. 8(dA)). This phenomenon was similar to

the previous report.4)The change in diffuse scattering during

annealing corresponding to the decrease in the stability of the irradiated bcc solid-solution phase by annealing was observed, however, it is emphasized here that the evidence

for SV cannot be obtained fromin situannealing experiment

in HVEM.

4. Discussion

Blatter et al.,4) Kim et al.,17)and Yan et al.18)previously

reported that an amorphous phase had formed through SV of the bcc solid-solution phase on the basis of the SAD and XRD patterns they obtained. Figure 9 compares the SAD patterns for the amorphous phase obtained through SV of the

bcc solid-solution phase reported by (a) Blatteret al.[Fig. 4

in Ref. 4)], (b) Kim et al. [Fig. 3(b) in Ref. 17)], and (c)

acquired from a mixture of the hcp solid-solution and the

Cr2Ti phases obtained through ex situ thermal annealing

of the bcc solid-solution phase at 873 K for 3.6©104_{s in}

the present work. The SAD pattern depicted in Fig. 9(c)

corresponds to that shown in Fig. 3(dA). The broad rings

indicated by D (Fig. 9(a)) and E (Fig. 9(b)) are clearly discontinuous and segmented, and it is reasonable to assume that these broad rings correspond to a sum of discontinuous Debye rings A, B, and C in the SAD pattern (Fig. 9(c)) that correspond to peaks A, B, and C in the XRD pattern shown in

Fig. 3(a). It is noteworthy that Kimet al.17)and Yanet al.18)

provided the BF images of an amorphous phase formed through SV and that their BF images did not exhibit the featureless contrast typical of an amorphous phase but rather

afine black-white contrast similar to the BF images shown in

Figs. 3(c) and 3(d). These results imply that the previously reported SV may not correspond to the formation of an

amorphous phase but to the formation of a finely grained

crystalline structure composed of the hcp solid-solution phase

and the Cr2Ti compound that had transformed from the bcc

solid-solution phase.

Ranganathan et al. used transformation diagrams to

develop a thermodynamic model of SV and concluded that the amorphous phase would form in the following steps: bcc

phase¼hcp phase¼amorphous phase.23) The formation

of the hcp phase as a component of a two-phase mixture decomposed from the bcc phase was observed in the present study, and this can be explained by the thermal equilibrium

phase diagram.25) In contrast, the hcp phase¼amorphous

phase transition was not detected in the present study, while the formation of the crystalline precipitates previously

reported by Hirata et al.21,22) _{and/or Cr}

2Ti was observed.

The experimental results of the present study indicate that the thermodynamic model that does not account for the simultaneous formation of crystalline precipitates and the hcp

phase21)_{does not apply to Cr­Ti alloys. It is noted that the}

experimental results of the present study are consistent with

those described in previous reports by Prasadet al.,19)_Sinkler

et al.,20)and Hirataet al.21,22)in the sense that SV could not

be experimentally observed.

5. Conclusions

The changes in the structure of the rapidly solidified bcc

solid-solution phase in the Ti60Cr40 alloy were investigated

by focusing on the formation of an amorphous phase. Our

results andfindings are summarized as follows:

(1) The previously reported SV of the bcc solid-solution phase was not observed in the present work during

eitherex situannealing at 873 K orin situannealing in

HVEM.

(2) In the in situ annealing experiment, specimens were,

prior to annealing, irradiated with 2 MeV electrons aiming to introduce a high density of defects and to enhance the diffusion, but it was in vain.

(3) The SV previously reported for Ti­Cr alloys may

actually correspond to the formation of afinely grained

polycrystalline structure through the decomposition of the bcc solid-solution phase.

(4) The bcc solid-solution phase was highly stable under MeV electron irradiation at 298 K.

Acknowledgements

This work was supported in part by Grants for Excellent Graduate Schools, funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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A B Blatter et al. [Ref. 4]

(a)

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C

(c)

Fig. 9 Comparison of SAD patterns among previously reported amorphous phases obtained through SV of bcc solid-solution phase (a), (b) and SAD pattern (c) a mixture of the hcp and Cr2Ti phases obtained throughex situ

thermal annealing of bcc solid-solution phase at 873 K for 3.6©104_{s in}

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