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AGING EFFECTS AND PHASE TRANSFORMATIONS
IN A T149._5N148Cr2.5 ALLOY

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Chulsoo Kim

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L/p 7/ A” .

AGING EFFECTS AND PHASE TRANSFORMATIONS
IN A T149.5 N148 C122.5 ALLOY

BY

Chulsoo Kim

A THESIS
Submitted to
Michigan State University
in partial fufillment of the requirements
for the degree of
MASTER OF SCIENCE

Department of Metallurgy, Mechanics and Materials Science

1986

ABSTRACT

AGING EFFECTS AND PHASE TRANSFORMATIONS
IN A T149.5N148 Crz‘5 ALLOY

BY

Chulsoo Kim

The transformation behavior and the effects of aging on
the transformation temperatures in a Tili‘ngiz‘BCrZ.5 alloy
have been studied using electrical resistance measurements
between room temperature and -196'C.

Transmission electron microscopy and electron
diffraction studies were performed at ambient temperature
and in-situ cooling experiments were carried out in a
scanning transmission electron microsc0pe between room
temperature and -l36°C. Based on the results obtained, the
Ti495Ni48chS alloy exhibits the charge density wave (CDW)
phenomena similar to TiNiFe and TiNiAl alloys.

The aging treatments decrease the Ms, As, and Af
temperatures up to a certain period of aging and then
increase them after that. This effect was attributed to the
formation of Ti-rich precipitates.

The alloy was cycled thermally and was little affected
even after 50 cycles. The Ms temperature was increased by
cold-rolling and returned to the as-annealed value after

aging treatment.

 

ACKNOWLEDGEMENTS

I should express my sincerest thanks to my parents
for their never ending love and support. I wish to thank
to Dr. C. M. Hwang for his advice and guidance. I am also
indebted to Mr. Shull Vivon, from Physics department, who
helped in using the STEM. Thanks are due to Ms. Patricia
Dodge for her careful reading and valuable correction.
Finally, special thanks to Raychem Co. for their supply

of the material.

ii

TABLE OF CONTENTS

Page

LIST OF TABLES . . . . . . . . . . . . . . . . iv
LIST OF FIGURES . . . . . . . . . . . . . . . . v
I. INTRODUCTION . . . . . . . . . . . . . . . 1
II. EXPERIMENTAL PROCEDURE . . . . . . . . . . 8
III. RESULTS AND DISCUSSION . . . . . . . . . . 10
IV. SUMMARY . . . . . . . . . . . . . . . . . . 67
LIST OF REFERENCES . . . . . . . . . . . . . . . 68

iii

LIST OF TABLES

Table Page

I.Transformation temperatures of the as—
annealed and thermal-cycled Ti49.5Ni48
Cr2.5 alloy . . . . . . . . . . . . . . . . . 14

II.The effect of aging on the transformation

temperatures of the Ti49 SNi48Cr2 5 alloy . . 23

III.Transformation temperatures of the Ti49 5
N148Cr2 5 alloy, cold—rolled and cold—rolled
followed by aging . . . . . . . . . . . . . . 61

iv

LIST OF FIGURES

Figure Page

1.

Electrical resistance vs. temperature curve

for the as—annealed Ti4905Ni48Cr2.5alloy . . . . . 11

Electrical resistance vs. temperature curve
for the 50th full thermal—cycled specimen . . . . .12

Transmission electron micrograph and corresponding
diffraction patterns of the parent phase of the
Ti4905Ni48Cr205alloy taken at room temperature.
(a) Bright field image (x50,000); (b) [111]

zone diffraction pattern; (c) [110]B2 zone

B2

diffraction pattern . . . . . . . . . . . . . . . .15

Transmission electron micrographs and corresponding
diffraction pattern of the TiagosNi48Cr2.5alloy
taken at room temperature. (a) Bright field image
(x40,000); (b) [111]B2
Dark field image (x40,000) . . . . . . . . . . . . 18

zone diffraction pattern; (c)

Scanning transmission electron micrograph taken at
—136°C and corresponding diffraction patterns taken
at —136°C and room temperature. (a) Bright field
image (x50,000); (b) Selected area diffraction
pattern taken at -136 C; (c) [110]B2

pattern taken at room temperature . . . . . . . . .20

zone diffraction

Electrical resistance vs. temperature curve for

the Ti49osNi48Cr2.5alloy aged at 400°C. (a) aged

for 5 min.; (b) aged for 30 min.; (c) aged for 1

hr.; (d) aged for 4 hrs.; (e) aged for 12 hrs.;

(f) aged for 24 hrs. . . . . . . . . . . . . . . . 24

. Transmission electron micrographs of the Ti49 5

Ni48Cr2 5alloy aged at 400°C. (a) Bright field
image (x60,000), aged for 1 hr.; (b) Bright field
image (x60,000), aged for 4 hrs.; (c) Bright field

image (x60,000), aged for 12 hrs. . . . . . . . . .31

V

LIST OF FIGURES (Continued)

Figure Page

8.

10.

11.

12.

Electrical resistance vs. temperature curve

for the Ti49OSNi48Cr2.5alloy aged at 500°C.

(a) aged for 5 min.; (b) aged for 30 min.;

(c) aged for 1 hr.; (d) aged for 4 hrs.; (e)

aged for 12 hrs.; (f) aged for 24 hrs. . . . . . .34

. Transmission electron micrographs of the Ti49 5

Ni48Cr2.5alloy aged at 500°C. (3) Bright field

image (x150,000), aged for 1 hr.; (b) Bright

field image (x150,000), aged for 4 hrs.; (c)

Bright field image (x150,000), aged for 12 hrs. . .41

Electrical resistance vs. temperature curve

for the Ti49.5Ni48Cr2.5alloy aged at 600°C.

(a) aged for 5 min.; (b) aged for 30 min.;

(c) aged for 1 hr.; (d) aged for 4 hrs.; (e)

aged for 12 hrs.; (f) aged for 24 hrs. . . . . . . 43

Transmission electron micrographs and selected

area diffraction pattern of the T149.5N148cr2,5

alloy aged at 600°C. (a) Bright field image
(x150,000),aged for 1 hr.; (b) Bright field

image (x150,000), aged for 4 hrs.; (c) Bright

field image (x150,000), aged for 24 hrs.; (d)

[211]B2 zone diffraction pattern; (e) Dark field
image (x150,000), aged for 24 hrs. . . . . . . . . 49

Variation of the transformation temperatures vs.
aging time at various temperatures. (a) Variation
of Ms temperature vs. aging time; (b) Variation of
As temperature vs. aging time; (c) Variation of Af
temperature vs. aging time; (d) Variation of Tp
temperature vs. aging time; (e) Variation of Td

temperature vs. aging time . . . . . . . . . . . . 53

vi

LIST OF FIGURES (Continued)

Figure Page

13.

14.

15.

16.

Electrical resistance vs. temperature curve

for the Ti49OSNi48Cr2.Salloy, 20% cold—rolled. . . 59

Electrical resistance vs. temperature curve
for the Ti49.5Ni48Cr2.5alloy, 204 cold-rolled
followed by aging at 500'C for 4 hrs. . . . . . . .60

Transmission electron micrographs and corresponding
diffraction patterns of T149.5Ni48Cr2.5alloy,

20% cold-rolled. (a) Bright field image (x50,000);
(b) [122]B2
and [T1I]B19, zone diffraction pattern; (d) Dark
field image (x60,000) . . . . . . . . . . . . . . .62

zone diffraction pattern; (0) [122]B2

Transmission electron micrographs of the Ti49.5
Ni48Cr2.5alloy, 20% cold—rolled followed by aging

at 500°C for 4 hrs. (a) Bright field image (x20,000)
; (b) Bright field image (X60,000) . . . . . . . . 66

vii

I. Introduction

TiNi with nearly equiatomic composition has received
considerable attention because of its peculiar shape memory
effect and premartensitic anomalous phenomena. The shape
memory effect can be described as follows. An object in the
low temperature martensitic condition, ( usually below Mf )
when it is deformed, and the stress then removed, will
regain its original shape when heated above the As
temperature. The process of regaining the original shape is
associated with the reverse transformation of the deformed
martensitic phase.

The number of materials exhibiting the shape memory
effects is now extensive. The lists includes the binary TiNi
alloys ( Nitinol ), ternary TiNiX alloys, many Cu-based
alloys, noble metal ( Au, Ag )-based alloys, Ni-Al alloys
and Fe-Pt alloys. Among these alloys that exhibit shape
memory effect TiNi type alloys were reported to have the
best combination of physical and mechanical properties.

Wayman and Shimizu (1) first suggested the necessary
conditions for the shape memory effect as follows. The
martensites formed thermoelastically, are either internally
twinned or internally faulted as a consequence of the
inhomogeneous shear process, and the parent phase is
ordered.

A thermoelastic martensitic transformation is defined

as one in which the martensite forms and grows continously

1

as the temperature is lowered, and shrinks and vanishes
continuously as the temperature is raised.

It has been reported that the martensitic
transformation in TiNi exhibits 'premartensitic phenomena'
immediately above the martensitic start temperature. These
include an electrical resistivity increase, streaks in the
electron diffraction patterns, and 1/3 spots in both
electron and X-ray diffraction patterns. Other anomalies in
physical and mechanical properties have also been reported
such as an unusual high damping capacity , internal friction
peaks, a decrease in sound velocity, a softening of certain
elastic moduli and specific heat peaks. These premartensitic
phenomena were interpreted earlier as precursory effects
closely related with the martensitic phase transformation.
However, after many investigations by a number of workers,
it was shown that TiNi alloys undergo a characteristic
premartensitic transition in addition to the martensitic
transformation. The crystal structure of the phase produced
by the premartensitic transformation was reported to be
rhombohedral by Dautovich and Purdy (2). Ling and Kaplow (3)
referred the phase associated with premartensitic behavoir
as the 'R phase' .

Many investigators have also discussed the
premartensitic phenomena. Chandra and Purdy (4) observed the
diffuse scattering of electrons in TiNi foils and they
interpreted in terms of kinematical scattering from large

amplitude, short wavelength phonons. Sandrock et al. (5)

noticed the formation of [111} planes of diffuse intensity
in reciprocal space well above the Ms temperature. They
discussed the premartensitic phenomena in terms of lattice
vibrations reflecting an incipient mechanical instability of
the 82 lattice and proposed that the origin of the
instability presumably lies in bonding ( electronic )
changes.

Otsuka et al.(6) using electron diffraction found the
1/3 reflections and they speculated that the transition may
be either an electronic ordering or a lattice modulation due
to periodic displacements of atoms. Several years ago, Moine
et a1. (7) also studied the premartensitic effects in TiNi
by using electron diffraction and various transmission
electron microscopy imaging techniques. They observed an
extensive array of the extra spots in the electron
diffraction patterns and interpreted it as lattice
displacement waves ( LDW ) in the structure.

Most recently, using ternary TiNiFe and TiNiAl alloys,
it was proposed that three-dimensional charge density wave
(CDW) phenomena and associated phase transitions are
involved in the premartensitic behavoir of the TiNi type
alloys (8-16). Two premartensitic transitions are suggested.
The first premartensitic transition is a second order '
normal-to-incommensurate'transition and in this stage the
positions of the 1/3 superlattice reflections in the
electron diffraction patterns are deviated slightly from the

exact 1/3 positions. Antiphase-like microdomains ( APD's )

were revealed in dark-field images using the deviated 1/3
reflections.

A second premartensitic transition, which occurs at a
lower temperature (Td ), is a first order ‘
incommensurate-to-commensurate' transformation and the
positions of 1/3 supperlattice reflections are at exact 1/3
positions. At below Td, needle domains were observed.

Thus, the sequence of transformations ( upon cooling )
in the TiNiFe and TiNiAl alloys is as follows:

parent phase ( BZ ) - incommensurate phase ( distorted
cubic ) - commensurate phase ( rhombohedral ) -
martensitic phase ( monoclinic ).

A charge density wave ( CDW ) is a static modulation of
conduction electrons and is a Fermi-surface driven
phenomenon usually accompanied by a periodic lattice
distortion. Numerous examples of a CDW phase change have
been found in quasi-one-dimensional organic conductors (e.g.
TTF—TCNQ) and quasi-two-dimensional layered compounds ( e.g.
transition-metal dichalcogenides ) (16—18).

A favorable Fermi surface geometry is necessary for the
formation of a CDW which will most likely occur when the
shape of the Fermi surface permits a connection by the same
wave vector Q, ie. Q = 2 Kf. This modulation with wave
vector Q will modify the Fermi surface by creating gaps at
these nested positions. If the nested portion of the Fermi
surface is significant the energy gain by creating energy

gaps may overcome the energy cost arising from the periodic

lattice distortions, thus allowing the formation of a CDW.

Another requirement for forming a CDW is a strong
electron-phonon coupling. This permits ion displacements to
reduce the prohibitive coulomb energy. Precursor phenomena,
such as a soft phonon mode, might also occur above the
transition temperature to assist the CDW instability.

Since a CDW is accompanied by a lattice distortion,
diffraction techniques ( electron, neutron, X-ray ) can be
used to reveal satellite reflections appearing near the
Bragg reflections of the parent phase as a consequence of
the formation of the CDW. These reflections are seperated
from the associated Bragg reflections by a reciprocal
lattice vector determined by the CDW wave vector.

Honma et al. (19) studied the effects of 3d transition
elements ( V, Cr, Mn, Fe and Co ) on the phase
transformation in TiNi alloy and concluded that Ms point
decreases as the valence electron concentration ( e/a )
deviates from the seven in those ternary alloys. It was also
reported that the decreasing rate of the Ms point was larger
than that of the premartensitic transition temperature.

Thus the ternary T1495Nl48 alloy, which was used

Crzv5
in this present study, is expected to be ideal for studying
the premartensitic transition. As previously mentioned, the
premartensitic transition in the TiNiFe and TiNiAl alloys
are believed to be associated with the formation of

three-dimensional charge density wave. Therefore, one of the

main purpose of the present investigation is to see whether

 

the TiAQSNi48 Crzs alloy undergoes the charge density wave
transition.

In a practical point a View, it seems to be important
to investigate the effect of thermo-mechanical treatments on
the transformation behavior and the shape memory effects in
the various shape memory alloys.

Saburi et al. (20) investigated the aging effects on
the transformation temperatures and the deformation behavior
of Ni-rich Ti-Ni alloys with different Ni contents. They
found that Ms temperature and mechanical behavior of Ni-rich
off-stoichiometric Ti-Ni alloys were sensitive to
heat-treatment, while those of a near-stoichiometric alloys
were not.

Recently, Nishida and Honma (21) discovered an
excessive reversible shape memory effect , which was dubbed
as an 'all-round shape memory effect (ARSME)' , in Ni-rich
TiNi alloys by constrained aging treatment and proposed that
this phenomena was ascribed to the formation of
widmanstatten lenticular precipitates with two variants.
Further studies were carried out by Nishida et al.(22) using
transmission electron microscopy and EDX microanalytical
system. It was confirmed that the composition of the
precipitate was TillNil4 and the structure of the TillNi14
can be explained as monoclinic.

In general, the Ms temperature depends on the chemical
composition, hydrostatic stress, lattice defects ( vacancy,

dislocation, precipitate and grain boundary ) and external

 

 

shear stress (23). In the case of TiNi type alloys , it has
been reported that the Ms temperature is strongly dependent
on the alloy composition.

The present study was undertaken to investigate the
transformation behavior and the effects of aging on the
transformation temperatures in a Ti4QSNi48 chS alloy. The
effects of thermal cycling and cold-rolling on the

transformation temperatures were also examined.

II. Experimental procedure

The alloy composition used for the present study was
49.5 at% Ti, 48 at% Ni and 2.5 at% Cr. The rod, which was
approximately 14.7 mm in diameter, was cut into the 0.3 mm
thick slices using a diamond saw. The samples were
mechanically polished to a thickness of 0.1 mm by using SiC
paper, and the discs, 3 mm in diameter, were sectioned for
the transmission electron microscopy investigation.

The strip samples for electrical resistance
measurements were also cut using a diamond saw and the final
dimensions of the strips were approximately 34.8 mm in
length, 1.5 mm in width and 0.4 mm in thickness.

All the samples were annealed at 950°C for 2hrs. in a
evacuated quartz tube and then quenched into cold water,
breaking the quartz tube. They were sealed again and aged at
400'C, 500°C and 600°C for different aging times between 5
min. to 24 hrs., and then water-quenched, breaking the
quartz tube. Some specimens were 20% cold-rolled after the
annealing treatment and some were aged at 500°C for 4hrs.,
then water quenched.

Thin foils for TEM observations were made by a
Tenupol-Z twin jet polishing machine equipped with the
constant voltage supply ( Polipower ) and photo cell. An
electrolyte, containing 92% glacial acetic acid and 8%
perchrolic acid ( 70% conc. ) by volume, was used at 15°C

2 under an applied potential of about 20 Volts. Electron

diffraction and TEM observations were carried out in a
Hitachi H-800 microscope at room temperature, operated at
200 KV and equipped with a 1 60°tilting stage. In-situ
cooling experiment was performed in a V.G. HB 501 scanning
transmission electron microscope between room temperature
and -136°C, operated at 100 KV. The samples for electrical
resistance measurements were electropolished after heat
teatment to remove the oxidized surface. The same
electrolyte was used at about 25 Volts.

A continuous measurement of electrical resistance as a
function of temperature was achieved by means of an
omnigraphic 200 X-Y recorder with a fast response time (24).

The liquid nitrogen vapor in a deep, wide mouth Dewar
Vessel was used for a cooling medium. The cooling, or
heating, rate of the specimen was approximately 0.6°C/sec.
Four copper leads were spot welded to the ends of the sample
; outer two for current supply and inner two for potential
measurement. A copper—constantan thermocouple was spot
welded to the center of the specimen.

A regulated d.c. power supply served as a current
source. One hundred and forty Volts was applied across the
specimen with a current of approximately 140 mA. The
specimen was lowered into the cooling bath by engaging a

stepping motor, and raised by activation of a microswitch.

 

III. Results and Discussion

Transformation behavior

 

The electrical resistance versus temperature curve,
obtained during a full thermal cycle between room
temperature and liquid nitrogen temperature, for the
as-annealed T1495Ni48cr25 alloy is shown in Fig.1. It can
be seen that the overall shape of the curve is analogous to
that of TiNiFe and TiNiAl alloys which undergo the charge
density wave transition.

Upon cooling the resistance starts to increase at Tp,
which corresponds to the onset of the first premartensitic
transition. The second premartensitic transformation begins
at Td where the curve seems to show an inflection point. At
the Ms temperature the resistance start to decrease until
liquid nitrogen temperature, which is approximately the Mf
temperature.

Upon heating, the resistance begins to increase slowly
and then rises abruptly at the austenite start temperature
(As). A small hysterisis can be found between Af and Tp.

In Fig.2, the electrical resistance vs. temperature
plot for the 50th full thermal cycled specimen is shown. It
can be considered that the effects of thermal cycling on
both the transformation temperatures and the shape of the
electrical resistance vs. temperature curve are not

significant in the Ti alloy. Transformation

495N14scrz5
temperatures of the as-annealed and the thermally-cycled

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specimen are given in Table I.

Fig.3 shows a transmission electron micrograph and
corresponding diffraction patterns of the parent phase of
the T1495Ni48cr25 alloy taken at room temperature. The
Bright field image in Fig.3(a) shows a mottling,'tweed-like'
structure as reported earlier in TiNi alloys (25). In
Fig.3(b), the diffraction pattern shows the weak 1/3
superlattice reflections. By measuring the distance between
the fundamental spots and the superlattice spots, it is
found that the positions of the superlattice reflections are
deviated slightly from the exact 1/3 position. In addition
to the weak 1/3 superlattice reflections, the diffraction
patterns in Fig.3(b) and (c) shows the diffuse streaks, arcs
and interlocking rings.

These diffraction patterns seems to be quite similar to
the diffraction patterns taken from the incommensurate phase
in the TiSSJNi3ZSAl38 alloy. The diffraction patterns were
explained as the intersections of the Ewald sphere with
non-spherical, curved surfaces in the reciprocal space and
possibly related to the Fermi surface geometry of this alloy
(14)..

From the results obtained from the electrical
resistance vs. temperature measurement, the Tp temperature
of the bulk specimen is about 7°C, close to the room
temperature. The transformation temperature of the thin foil
could be slightly different from that of the bulk material.

By considering the above facts, it might be thought that a

14

 

TABLE I. Transformation temperatures of the as—annealed

and the thermal—cycled Ti4 Ni Cr alloy.

9.5 48 2.5

 

(°C) M5 M5 As A Tm T;
J. l.

l‘h
(‘

 

As—annealed —108 <9196 —92 —49 7 -33

50th cycle -110 (E196 —93 —51 5 —33

 

 

15

 

Fig.3 Transmission electron micrograph and corresponding
diffraction patterns of the parent phase of the Tia9.5
NiZ‘BCrZ.5 alloy taken at room temperature. (a) Bright
field image (x50,000); (b) [111] zone diffraction

B2
pattern.

16

 

(Continued). (c) [110]F” zone diffraction p

,.
C.

ttern.

 

17

thin foil is undergoing the very early stage of the
parent-to-incommensurate transition.

Fig.4 shows another transmission electron micrograph and
the corresponding diffraction pattern taken at room
temperature. In Fig.4(a), small 'needle-like' structures are
shown. A corresponding diffraction pattern in Fig.4(b) shows
that the intensity of some of inner 1/3 (110) type
reflections is increased and the additional superlattice
reflections, which seems to be connected by small
interlocking rings, appear. Fig.4(c) is a dark field image
of Fig.4(a) taken by using three superlattice reflections
forming a small interlocking ring. The dark field image
shows one variant of needle-like structures. Thus, it can be
considered that one variant of needle-like structures might
create thoes interlockings.

The exact nature and the origin of these 'needle-like'
structures is not clear at this moment. However, these are
not the same as the 'needle' domains of the commensurate
phase in TiNiFe and TiNiAl alloys, considering that the thin
foil is in the very early stage of the parent-to-
incommensurate phase transition.

Fig.5 shows a scanning transmission electron micrograph
taken at -136'C and corresponding diffraction patterns of
the Ti4g5Ni48Cr25 alloy taken at -136‘C and room
temperature. In FigS(a), Two variants of 'needle' domains of

the commensurate phase are shown. Although the temperature

is below the Ms temperature of a bulk material, no

 

18

 

Fig.4 Transmission electron micrographs and corresponding

‘ ”r i r »C *1 Ti '1 l ‘ “ 1'
diffraction pattern CL tie 1 49.5N 48Cr2.5alloy taken at
room temperature. (a) Bright field image (x40,000)

(b) [111] zone diffraction pattern.

B2

 

Ila-I} . I .. 111...: . . u.
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Fig.4 (Continued). (c) Dark field image (x40,000).

20

 

Fig.5 Scanning transmission electron micrograph taken
at —136°C and corresponding diffraction patterns taken
at —136‘C and room temperature. (a) Bright field image
(x50,000) ; (b) Selected area diffraction pattern taken
at —l36'C

“is.” -<

 

21

 

Fig.5 (Continued). (c) [110]quone diffraction pattern taken

at room temperature.

22

martensite phase can be seen. It might be ascribed to the
thin foil effect. It has been reported that the martensite
plates begin to form at thicker regions of a thin foil.

A corresponding selected area diffraction pattern in
Fig.5(b), taken from one of two variants, clearly shows the
charateristic 1/3 (111) type superlattice reflections at the
exact 1/3 positions. Fig.5(c) shows the diffuse [110132 zone
diffraction pattern taken at room temperature. No 1/3
superlattice reflections can be seen. Thus, it is clear that
Ti495Ni48Cr25 alloy does undergo the second premartensitic
transformation.

From the results obtained, it can be considered that Ti495
Ni48Cr25 alloy exhibits similar CDW transition phenomena
observed in TiNiFe and TiNiAl alloys.

Aging effects

 

The effect of aging on the transformation temperatures
of the T1495Ni48cr25 alloy is given in Table II.
Fig.6(a)-(f) shows the electrical resistance vs. temperatue
curves of Ti495Ni48Cr25 alloy aged at 400°C with varing the
aging time from 5 min. to 24 hrs. In general, the Ms
temperature of specimens aged at 400°C continues to decrease
with a longer aging time. It can also be seen that the shape
of the curve is obviously changed and the decreasing rate of
Ms temperature is increased after aging for 12 hrs. The
difference of resistance between the cooling and heating
curve becomes narrow after aging for 12 hrs. The electrical

resistance of specimens around the Mf temperature is found

 

 

 

TABLE II. The effect of aging on the transformation

temperatures of the Ti49,5NiasCr2,5 alloy.

 

 

 

 

Ms Mf As Af T Td

400°C 5 min. ~118 <<~196 ~100 ~59 2 ~36
30 min. ~117 <~196 ~89 ~57 0 ~36

1 hr. ~121 <}196 ~97 ~59 ~7 ~45

4 hrs. ~123 <}196 ~113 ~70 -2 ~45

12 hrs. ~132 <3196 ~78 ~2 ~46

24 hrs. ~142 <3196 ~79 —2 ~48

500°C 5 min. ~108 <}196 ~98 ~55 7 ~37
30 min. ~109 <3196 ~89 ~49 6 ~33

1 hr. ~‘12 <3196 ~97 ~56 ~1 ~35

4 hrs. ~135 <3196 ~115 ~75 ~1 ~46

12 hrs. ~113 <3196 ~103 ~57 —1 ~38

24 hrs. ~100 <3196 ~98 ~45 7 ~34

600°C 5 min. ~‘12 <3196 ~104 ~56 3 ~37
30 min. ~106 <&196 ~89 ~46 ~3 ~31

1 hr. ~108 <}196 ~93 ~49 ~3 ~36

4 hrs. ~113 <3196 ~109 ~57 ~3 ~37

12 hrs. ~111 <3196 ~100 ~61 ~3 ~42

24 hrs. ~90 ~190 ~94 ~38 7 ~30

 

 

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30

to be increased with increasing aging time. It has been
reported that the martensite phase decreases the electrical
resistance in TiNi type alloys.

From the above results, it can be considered that the
changes in the transformation temperatures and electrical
resistance are due to the changes in the microstructure of
the specimens, possibly the precipitates.

Fig.7 shows the tansmission electron micrographs of the
specimens aged at 400°C for 1 hr, 4 hr. and 12 hrs. The
extremely fine precipitates are shown and they appear to be
coherent to the matrix. The size of the precipitates seems
to be unchanged as the aging time increases.

According to the Ti-Ni phase diagram, TiNi phase in a
~Ti-rich alloy will decompose into 'TiZNi ' phase (26). The
crystal structure of TizNi phase was reported to be fcc
structure (27). Although the exact composition of the
precipitates in Ti495N148CrZ5 alloy is not known, it is
suspected that the composition of the precipitates will be
near ‘TiZNi '.

The decrease in Ms temperature due to the precipitates
can be explained by the two factors, which was proposed by
Horbogen (28). The first factor is the compositional
dependence of the Ms temperature and the second factor is
the strengthening mechanism.

It is well-known that Ms temperature of the TiNi alloy
decreases with increasing nickel content from the

stoichiometic composition (29). The Ti-rich precipitates in

31

   

Fig.7 Transmission electron micrographs of the Ti49.5Ni48Cr2.5

alloy aged at 400°C. (3) Bright field image (x60,000), aged for
1 hr. ; (b) Bright field image (x60,000), aged for 4hrs..

 

 

32

   

Fig.7 (Continued). (c) Bright field image (x60,000), aged

for 12 hrs..

33

this alloy will increase the nickel content of the matrix
phase, depressing the Ms temperature. The precipitates are
extremely fine and coherent to the matrix. Thus, these
precipitates can increase the strength of matrix phase and
the initiation of the martensitic transformation will be
more difficult.

The electrical resistance vs. temperature curves of the
T1495Ni48cr25 alloy aged at 500°C are shown in
Fig.8(a)-(f). It can be found that the electrical resistance
is increasing up to the aging for 4 hrs, which corresponding
to the minimum Ms temperature. Aging for a longer time after
4 hrs decreases the electrical resistance and increases the
Ms temperature.

The variation of Ms temperature vs. aging time for
specimens aged at 500°C is quite different from that of
specimens aged at 400°C , as can be seen in Fig.12(a). This
can be explained as follows. The Ms temperature is almost
unchanged up to the aging for 30 min. In case of 1 hr. aging
treatment, Ms temperature is slightly decreased, which can
be recognized as the onset of the precipitation. Ms
temperature is futher decreased up to aging for 4 hrs. and
stops to decrease, corresponding to the finishing point of
the precipitation. Aging for a longer time increases the Ms
temperature. It might be the overaging effect due to the
coarsening of the precipitates. The mechanism of the Ms
drops due to the precipitates is the same as in case of 400

t aging treatment. In addition, the coarse precipitates

 

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40

might decrease the strength of the parent phase and , as a
result, Ms temperature can be increased for specimens aged
for a longer time.

Fig.9 shows the transmission electron micrographs of
specimens aged at 500°C for 1 hr., 4 hrs. and 12 hrs.
Comparing the Fig.9(c) and 9(b), it is clear that the size
of the precipitates in Fig.9(c) is larger than that of the
precipitates in Fig.9(b).

The electrical resistance vs. temperature curves of the
T1495Ni48cr25 alloy aged at 600°C are in Fig.10(a)-(f). In
case of the 600°C aging treatment, the general shape of°
curves is not changed greatly as in the case of the 500°C
aging treatment. In general, the effect of aging at 600°C is
not as obvious as that of aging at 500°C. The electrical
resistance increases for specimens aged for 4 hrs and 12
hrs, corresponding to a low Ms temperature. The electrical
resistance of a specimen aged for 24 hrs. is markedly
decreased and Ms temperature of this specimen is high as can
be expected.

Fig.11 shows the transmission electron micrographs and
selected area diffraction pattern of specimens aged at 600
C. From Fig.11(a) to Fig.11(c), it can be seen that the
precipitates are coarsening to a larger size. In Fig.11(c),
the coherent strain contrasts around the precipitates are
shown. The corresponding diffraction pattern in Fig.11(d)
exhibits relatively strong streaks at the diffracted

reflections. Figll(e) is the dark field image taken by

 

41

 

 

er 5

L48

(a) Bright field image (x150,000),aged for

(b) Bright field image (x150,000), aged for 4 hrs..

.sN’

tne T149

C
A.

Fig.9 Transmission electron micrographs o-

loy aged at 500°C.

a1

3

1 hr.

 

42

 

Fig.9 (Continued). (c) Bright field image (x150,000), aged
for 12 hrs..

 

 

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Fig.11 Transmission electron micrographs and selected area
diffraction pattern of the 'l‘i49.51\1i48Cr2.5 alloy aged at 600°C.
(a) Bright field image (x150,000), aged for 1 hr.

field image (x150,000), aged for 4 hrs..

; (b) Bright

.3551:-

50

 

(c) Bright field image (x150,000), aged

[211]nq zone diffraction pattern.

Fig.11 (Continued).

DL

(d)

24 hrs. ;

for

 

51

 

Fig.11 (Continued). (e) Dark field image (x150,000), aged for
24 hrs..

 

 

52

including the diffracted reflections and streaks. Thus, the
precipitates as well as the matrix phase are in contrast.

Fig.12(a)-(e) shows the variation of the transformation
temperatures ( Ms, As, Af, Tp and Td ) vs. aging time at
various temperatures. It can be seen that the variation of
Ms, As and Af temperature is quite similar each other. The
variation of Ms temperature is mentioned earlier. Thus, the
variation mode of As and Af temperature can be explained. It
is noticed that Tp and Td are not significantly changed by
the aging treatments.

Considering the fact that Tp and Td are associated with
the CDW transition and the CDW transition is electronic in
origin, the effect of aging on these temperatures is
expected to be small. The variation of Td is larger than
that of Tp. This might be ascribed to the difference in the
order of transformation, that is, Td represents the first
order transformation temperature, whereas Tp represents the
second order transition temperature.

Due to the lack of the additional diffraction spots
from the precipitates, the crystal structure of this
precipitate can not be obtained. By considering the fact
that the streaks can be originated from the precipitates,
the morphology of this precipitate is assumed to be the
disc-shape. Further studies of larger precipitates by using
X-rays, EDX and transmission electron microscopy will be
necessary to obtained the exact crystal structure,

composition and morphology of the precipitates in this alloy.

 

 

 

 

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58

Cold—rolling and cold-rolling plus aging effect

The electrical resistance vs. temperature curves of
specimens, which were 20% cold—rolled and 20% cold-rolled
followed by aging at 500°C for 4 hrs., are given in Fig.13
and Fig.14, respectively. Table III shows the transformation
temperatures of these specimens. The electrical resistance
of the cold-rolled specimen is markedly decreased and ,as a
result, the shape of the curve is also changed greatly when
comparing with that of the as-annealed one. The
transformation temperatures except Td are increased and
especially, A temperature is markedly increased by 20%
cold-rolling.

The transmission electron micrographs and corresponding
diffraction patterns of a 20% cold-rolled specimen are shown
in Fig.15. In Fig.15(a), a bright field image clearly shows
two variants of the stress—induced martensite plates and the
high density of dislocations. No particular internal
structures of martensite are found in these stress-induced
martensite plates.

A selected area diffraction pattern in Fig.15(b) is
obtained from the parent phase area without a martensite
plate. On the other hand, the selected area diffraction
pattern in Fig.15(c) is obtained from the area including the
martensite plates and the parent phase. The extra spots,
which come from the stress-induced martensite plates, can be
seen. This diffraction pattern is indexed by comparing the

results of monoclinic ( Bl9' ) martensite in TiNi alloy,

 

39

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61

 

TABLE III. Transformation temperatures of the Ti49 SNi48

Cr2 5 alloy, cold—rolled and cold—rolled
followed by aging.

 

(°C) Ms Mf As Af Tp Td

 

20% cold—rolled —97 —187 — —19 —2 -13

20% cold—rolled
+

Aging at 500°C -109‘<:196 — -52 10 —32
for 4 hrs.

 

 

 

62

 

(b)

Fig.15 Transmission electron micrographs and corresponding
diffraction patterns of Ti49,5Niagcr2_5 alloy, 20% cold—
rolled. (a) Bright field image (x50,000) ; (b) [122]B2 zone

diffraction pattern.

 

 

63

 

and [l

1
pattern ; (d) Dark field image (x0 00

Fig.15 (Continued). (c) [122]

B2 l] 319, zone diffraction
).

 

 

64

which were reported earlier (6). The corresponding dark
field image is obtained by using one of the reflections from
the martensite plates and two variants of the stress-induced
martensite plates are clearly shown.

From the above results, it can be considered that the
decrease in the electrical resistance is due to the
formation of stress—induced martensite plates. The increase
in Ms temperature by cold-rolling might be analyzed as
follows. The driving force necessary for transformation is
reduced by a portion of the mechanical work performed by
cold-rolling , and the stress-induced martensite plate
produces stress fields in the surrounding parent phase and
this can enhance the formation of the martensite by the
autocatalystic effect.

The dislocations produced by cold-rolling and the
interface between the parent and stress-induced martensite
plates can provide the favorable nucleation sites for the
martensitic transformation. The marked increase of the Af
temperature can be attributed to the stresss-induced
martensite plates, which can impede the reverse
martensite-to-parent phase transformation.

The exact effects of cold-rolling on Tp and Td are not
fully understood. However, the decrease in Tp might be due
to the changes in the geometry of Fermi-surface. If the
deformation of the parent phase by cold-rolling can
introduce the changes in the reciprocal lattice , the

geometry of the Fermi-surface will change. This , in turn ,

 

 

65

will change the premartensitic transition behavoir and
temperatures.

By aging at 500°C for 4 hrs, the electrical resistance
of a 20% cold-rolled specimen rises as can be seen in
Fig.14. This might be attributed to the dissapearance of the
stress-induced martaensite plates. The values of the
transformation temperatures are almost the same as that of
the as—annealed one ( Table III ). Thus, it might be assumed
that the effect of 20% cold-rolling is compensated by the
effect of aging at 500°C for 4 hrs.

Fig.16 shows the transformation electron micrographs of
a cold-rolled and aged specimen. It can be seen that the
stress-induced martensite plates are almost dissapeared with
leaving the dislocation arrays at the interfaces. The

precipitates can also be observed in Fig.16(b).

66

 

Fig.16 Transmission electron micrographs of the Ti49 5Ni48

Cr2 5 alloy, 20% cold—rolled followed by aging at 500°C
for 4 hrs. (a) Bright field image (x20,000) ; (b) Bright
field image (x60,000).

 

 

IV. Summary

The electrical resistance vs. temperature measurements,
and the electron diffraction and micrograph study show that
the T1495N148Cr25 alloy exhibits the CDW transition
phenomena, which was observed earlier in TiNiFe and TiNiAl
alloys.

In general, the effect of aging on Ms, As and Af
temperature is more prominent than that of Tp and Td. The
aging effects are due to the Ti-rich precipitates. In the
case of aging at 400°C, the Ms temperature continues to
decrease up to an aging time of 24 hrs.

The variation of Ms temperature vs. aging time for
specimens aged at 500°C shows a characteristic aging process
including the starting and finishing of the precipitation.
The effect of aging at 600°C is not as obvious as that of
aging at 500°C.

The cycling effect on the transformation temperatures
is not important in a Ti4g5Ni48Cr15 alloy. Twenty percents
cold-rolling treatment increases Ms temperature and
decreases the electrical resistance of this alloy. These
effects are due to the formation of stress-induced
martensite plates. Aging at 500°C for 4 hrs after 20%
cold-rolling changes the transformation temperatures of a

cold-rolled specimen to the values of as-annealed one.

67

LIST OF REFERENCES

1. C.M. Wayman and K. Shimizu, Met. Sci. J., 6 , 175 (1972)

2. D.P. Dautovich and G.R. Purdy, Can. Metall. Q., 4 , 129
(1965)

3. H.C.Ling and R. Kaplow, Met. Trans. A, 11A , 77 (1980)

4. K. Chandra and G.R. Purdy, J. Appl. Phys., 39 , 2176
(1968)

5. G.D. Sandrock, A.J. Perkins and R.F. Heheman, Metall.
Trans., 2 , 2769 (1971)

6. K. Otsuka, T. Sawamura and K. Shimizu, Phs. Stat. Sol.,
5 , 457 (1971)

7. P. Moine, G.M. Michal and R. Sinclair, Acta Metall.,
30 , 109 (1982)

8. C.M. Hwang, M. Meichle, M.B. Salamon and C.M. Wayman,
Phil. Mag. A, 47 , No.1, 9 (1983)

9. C.M. Hwang, M. Meichle, M.B. Salamon and C.M. Wayman,
Ibid., 47 , No.1, 31 (1983)

10. C.M. Hwang, M. Meichle, M.B. Salamon and C.M. Wayman,
Ibid., 47 , No.2, 177 (1983)

11. C.M. Hwang and C.M. Wayman, Scripta Met., 17 , 385
(1983)

12. C.M. Hwang and C.M. Wayman, Ibid., 17 , 1345 (1983)
13. C.M. Hwang and C.M. Wayman, Ibid., 17 , 1449 (1983)

14. C.M. Hwang and C.M. Wayman, Acta Metall., 32 , 183
(1984)

15. C.M. Hwang and C.M. Wayman, Metall. Trans. A, 15A ,
1155 (1984)

16. P. Bak and V.J. Emery, Phys. Rev. Lett., 36 , 978
(1976)

17. G.J. Tatlock, Inst. Phys. Conf., No.36, 161 (1977)

18. J.A. Wilson, F.J. DiSalvo and S. Mahajan, Adv. Phys.,
24 , 117 (1975)

68

 

 

 

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

69

T. Honma, M. Matsumoto, Y. Shugo, M. Nishida and
I. Yamazaki, Titanium Science and Technology,
R. Jaffe ed., 3, 1455 (1980)

T. Suburi, T. Tatsumi and S. Nenno, 'ICOMAT 82',
261 (1982)

M. Nishida and T. Honma, Scripta Met., 18, 1293
(1984)

M. Nishida and C.M. Wayman, Ibid., 19, 983 (1985)

Z. Nishiyama, 'Martenstic Transformation',
M.E. Fine, Meshii, and C.M. Wayman ed., 263. (1978)

K.A. Thornburg, D.P. Dunne and C.M. Wayman,
Met. Trans., 2, 2302 (1971)

P. Moine, E. Goo and R. Sinclair, 'ICOMAT 82',
243 (1982)

G. R. Purdy and J. Gordon Parr, TMS-AIME Trans.,
221, 636 (1965)

P. Duwez and J. L. Taylor, Trans. AIME., 188, 1173
(1950)

B. Hornbogen, Acta Metall., 33, 595 (1985)

J. E. Hanlon, S.R. Butler and R.J. Wasilewski,
TMS-AIME Trans., 239, 1323 (1967)

 

   

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