Martensitic Characterization of the Ti<sub>45.3</sub>Ni<sub>54.7</sub> Melt Spun Alloy

doi:10.4236/msa.2011.29169

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Materials Sciences and Applicatio ns, 2011, 2, 1256-1259

doi:10.4236/msa.2011.29169 Published Online September 2011 (http://www.SciRP.org/journal/msa)

Martensitic Characterization of the Ti45.3Ni54.7 Melt

Spun Alloy

Geroge C. S. Anselmo, Walman Benicio de Castro*, Carlos J. de Araújo

UAEM, Universidade Federal de Campina Grande, Aprígio Veloso, Brazil.

Email: georgeanse[email protected], *[email protected], [email protected]

Received February 4th, 2011; revised July 8th, 2011; accepted July 25th, 2011.

ABSTRACT

The ribbons of the Ti45.3Ni54.7 shape memory alloy were prepared through the melt spinning technique. The study was

focused on investiga ting the effect o f the rapid solidifica tion and gra in size at characteristic sta rt martensitic (Ms), final

martensitic (Mf), start austenite (As) and final austenite (Af) transformation temperatures. Changes on martensitic

transformation temperatures in Ti45Ni55 melt spun ribbons were observed as grain size is reduced. Results of optical

microscopy and differential scanning calorimetry (DSC) were used to associate grain size with transformation tem-

peratures.

Keywords: Shape Memory Ni-Ti Alloy, Rapid Solidificatio n, Martensitic Transformation

1. Introduction

Shape memory alloys (SMAs) represent a unique class of

materials that undergo a reversible phase transformation

(martensitic transformation) allowing these materials to

display dramatic pseudoelastic stress-induced deforma-

tions and recoverable temperature-induced shape mem-

ory deformations. These materials are used as smart ma-

terials in a variety of aerospace, biomechanical, and mi-

croelectronics applications. Among the known shape

memory alloys, Ni-Ti is the most commonly used be-

cause of its excellent mechanical properties, corrosion

resistance and biocompatibility [1]. In the recent past, the

field of microsystems has been subject to growing atten-

tion from both the industry and the research community.

Microsystems have been recognized as having the poten-

tial to revolutionize the performance of a wide range of

products by merging silicon-based microelectronics with

micromachining technologies, thus enabling complete

systems-on-a-chip to be developed and allowing novel

functionalities at reduced costs. In this context, the ap-

plication of shape memory alloys for actuation of mi-

cropneumatic devices might bring a relevant technologi-

cal breakthrough. SMA materials exhibit the highest en-

ergy density amongst current micro-electromechanical

systems MEMS compatible materials and, more impor-

tantly, as size is reduced towards the micro-scale, they

benefit from improved heat transport, which increases

their response speed [2].

Many studies have been undertaken to find a method

to control the martensitic transformation. According to

previous studies, it is significantly affected by the alloy

composition, crystallographic defects such as disloca-

tions, precipitation and grain size [3]. Grain boundaries

are believed to strengthen parent phases, and therefore

Ms decreases with decreasing grain size. In Ti-Ni-based

alloys, the critical austenite grain size for martensitic

transformation is known to be 50 nm, below which

martensitic transformation does not occur.

The aim of this work is to study the relationship be-

tween grain size and martensitic transformation tem-

perature in the Ti45.3Ni54.7 alloy. In order to attain a wide

range of cooling rate and grain size, the melt spinning

technique is therefore considered a suitable preparation

route for this alloy. Melting spinning is an important

method for producing metals with improved mechanical

and/or physical properties. This technique employs a

very high cooling rate (up to 106˚C/s). In general, such a

high cooling rate has the advantage of refinement of

grain sizes [4].

2. Experimental Procedure

The Ti45.3Ni54.7 ingot with 19 mm in diameter by 100 mm

long was prepared using the conventional vacuum arc-

remelting (VAR) method. High purity Ti and Ni raw

materials were repeatedly melted six times in an argon

atmosphere for homogenization. Then they were cut into

Martensitic Characterization of the TiNi Melt Spun Alloy1257

45.3 54.7

small pieces, each of which has a weight in the range of

10 to 30 grams, induction-melted in an argon atmosphere

in a quartz crucible at 1250˚C supplied into a single-

roller melt-spinning machine and subsequently ejected

by high pressurized argon out of a 0.4 mm orifice onto a

200 mm diameter copper roller with at different tangen-

tial speeds between 30 and 50 m/s. The final ribbons of

the melt-spinning process ranged between 30 µm and 41

µm in thickness and 1 mm in width. Transformation

temperatures and enthalpies of as-spun ribbons were de-

termined by differential scanning calorimetry TA Q20

DSC with 10 ˚C/min heating and cooling rate and the

temperature scanning range was from −30 to +200˚C.

The software for DSC for data acquisition, storage and

evaluation under MS WINDOWS. It has multitasking

with simultaneous operation of several thermal analysis

systems and simultaneous evaluation graphical user sur-

face, integrated detailed HELP system. Microstructure of

cross-section of ribbons was examined by Nikon

FX-35DX optical microscope (OM). For optical mi-

croscopy, the specimens were polished using conven-

tional procedures and etched in a solution composed of

HF: HNO3: H2O = 4: 5: 10 (in volume). The etching time

was about 10 - 15 s. From the OM images under 1000×

magnification, the average grain size of ribbons was es-

timated by the linear intercept method [5]. The numbers

of intercepted grains were at least 10 and 50 for large and

small grains, respectively.

3. Results and Discussion

The wheel velocity was changed from 30 to 50 m/s while

the melt spinning temperature was fixed at 1350˚C. Fig-

ure 1 shows the thickness of as-spun Ti45Ni55 alloy rib-

bons as a function of the melt spinning wheel velocity.

The increase of the wheel velocity from 30 to 50 m/s

results in a decrease of the ribbon thickness from 41 m

to 30 m. As the increase of the wheel velocity leads to a

Figure 1. Relationship between ribbon thickness and wheel

velocity.

reduced ribbon thickness, the heat transfer coefficient at

the quenching wheel-ribbon interface is enhanced and the

cooling rate increases. Therefore, this result clearly indi-

cates that variations in the melt spinning temperature and

wheel velocity allow the effective control of the cooling

rates in the melt spinning process.

Figure 2 shows optical micrographs of as-spun rib-

bons fabricated at two different wheel velocities of 30

and 50 m/s. As seen in Figure 2, as-spun Ti45.3Ni54.7 rib-

bons are fully crystallized and most of the columnar and

small grains are located at the free surface and copper

roller surface of the ribbons, respectively.

Refined grain structure is the hypothesis to the de-

crease in the transformation temperature, as shown in

Figure 2. When the ribbon is produced at a higher wheel

velocity in melt spinning, the degree of undercooling

becomes high because of its thinner thickness and the

amount of crystalline layer decreases with wheel velocity.

Variations in grain size are often accompanied by

changes in dislocation structure and precipitation, which

also greatly affect the transformation temperatures.

Therefore, it seems to be difficult to investigate the effect

of grain size on the transformation temperatures in Ti-Ni

alloys. Grain refinement has been reported in literature to

decrease the transformation temperatures [6].

The transformation temperature decreased when the

velocity of wheel was changed from 30 to 50 m/s, as

shown the Table 1.

(a)

(b)

Figure 2. Optical micrographs of the ribbons (light brown)

fabricated at the wheel velocities of (a) 30 m/s and (b) 50

m/s.

Martensitic Characterization of the TiNi Melt Spun Alloy

1258 45.3 54.7

Table 1. Transformation temperatures as cast and cooled in

melting spinning.

Cooling

Conditions MS

(˚C) MF

(˚C) AS

(˚C) AF

(˚C) Thickness

(m)

As casting 49.7 34.9 58.0 77.9 -

30 m/s 42.1 29.0 52.0 73.2 41

50 m/s 28.7 22.5 44.2 53.3 30

Figure 3 shows the DSC curve of as-spun grain-size

mixed Ti45.3Ni54.7 ribbons. As can be seen, to ribbons that

were fabricated with wheel velocities of 30 m/s the

transformation peaks are short with large transformation

enthalpies and for the ribbons that were fabricated with

wheel velocities of 50 m/s the transformation peaks are

broad with short transformation enthalpies. This is be-

cause as-spun Ti45Ni55 ribbons fabricated with wheel

velocities of 50 m/s contain greater amount of defects

and residual stress. At the same time, the grain size in-

herent in ribbons is finer than the ribbons that were fab-

ricated with wheel velocities of 30 m/s, as shown in Fig-

ure 2. The grain boundaries and defects can act as barri-

ers to the martensitic transformation as a result of the

extra energy required during transformation [7]. Thus

fine-grain ribbons which have lots of grain boundaries

would be expected to have lower transformation tem-

peratures and smaller transformation enthalpies, as

shown in Figure 3 and Tabl e 1.

NiTi shape memory alloys (SMA) transform marten-

sitically from B2 cubic austenite into monoclinic B19’

martensite either directly or via rhombohedral R-phase

martensite. The B19’ martensite can be obtained either

by a single step transformation of B2 → B19’, or by a

two-step transformation of B2 → R-phase → B19’ [8].

Note in Figure 3 that when ribbons are fabricated with

wheel velocities of 30 m/s there are one B2  B19’

martensitic transformation peak in cooling and heating,

but when ribbons were fabricated with wheel velocities

of 50 m/s there are B2  R and R  B19’ martensitic

transformation peaks in cooling and heating and B19’ 

R and R  B2. The occurrence of the multiple marten-

sitic transformations in ribbons of Ti51Ni49 SMA is due

to the coexistence of large and small grains distributed

in the ribbons. The reason why both R → B19’ and B19’

→ B2 transformations are separated into two peaks

which correspond to large and small grains, while the B2

↔ R transformation is not separated into two peaks is

that the B2 ↔ R transformation exhibits much smaller

transformation strain than R → B19’ and B19’ → B2

transformations [6]. Figure 2 shows a fully crystallized

and most of the columnar and small grains are located at

the free surface and copper roller surface of the ribbons,

respectively. Then, the multi-stage martensitic transfor-

mation induced by the inhomogeneous grain size distri-

(a)

(b)

Figure 3. DSC curves of the ribbons fabricated at the wheel

velocities: (a) 30 m/s and (b) 50 m/s.

bution in the ribbon is observed in this study [1]. Ac-

cording to the reported studies, the R-phase appearing in

the martensitic transformation of Ti-rich TiNi thin films

is induced either by the coherent stress field around the

plate-like Guinier-Preston (GP) zones or by the semico-

herent stress field around the spherical Ti2Ni precipitates

[9-12]. Increasing wheel velocities caused the formation

of the R-phase. The as-spun Ti45Ni55 ribbon has many GP

zones, since rapid solidification produces an increase in

the number of defects [13] inducing the formation of

R-phase.

4. Conclusions

Transformation behavior of melt spun Ti45.3Ni54.7 alloy

ribbons fabricated at different cooling rates by melt spin-

ning was investigated. The increase of the wheel velocity

from 30 to 50 m/s results in a decrease of the ribbon

Martensitic Characterization of the Ti45.3Ni54.7 Melt Spun Alloy

1259

thickness. When the ribbon is produced at a higher wheel

velocity in melt spinning, the degree of undercooling

becomes high because of its thinner thickness. The grain

boundaries and defects can act as barriers to the marten-

sitic transformation as a result of the extra energy re-

quired during transformation. Thus fine-grain ribbons

which have lots of grain boundaries would be expected to

have lower transformation temperatures. The DSC result

showed that when wheel velocity increases from 30 m/s

to 50 m/s the R-phase appears because the as-spun

Ti45.3Ni54.7 ribbon has many Guinier-Preston (GP) zones,

induced by the large number of defects. According to the

partial-cycled DSC test, we conclude that the peaks of

transformations are associated with B2 → R transforma-

tion, R → B19’ transformation for large grains, R →

B19’ transformation for small grains during cooling, and

B19’ → B2 transformation for large grains and B19’ →

B2 transformation for small grains during heating. The

occurrence of the multiple martensitic transformations in

ribbons of Ti45 Ni55 SMA is due to the coexistence of

large and small grains distributed in the ribbons. The

reason why both R → B19’ and B19’ → B2 transforma-

tions are separated into two peaks which correspond to

large and small grains, while the B2 ↔ R transformation

is not separated into two peaks is that the B2 → R trans-

formation exhibits much smaller transformation strain

than R → B19’ and B19’ → B2 transformations.

5. Acknowledgements

The authors would like to acknowledge the financial

support from the CNPq, from CASADINHO project no

620091/2008-8, from Projeto Universal project no

471831/2009-3 and the concession of scholarship to

George Carlos S. Anselmo.

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