Download ligas inoxidáveis com efeito memória de forma submetidas a

NiTi ALLOYS WITH SHAPE MEMORY EFFECT DEFORMED BY ECAE
F. L. C. Lucas(a), V. Guido(a), K. A. Käfer(b), H. H. Bernardi(b,*), J. Otubo(b).
de Engenharia, Arquitetura e Urbanismo – FEAU – Universidade do
Vale do Paraíba, 12.244-000, São José dos Campos, SP, Brasil.
(b)Departamento de Materiais e Processos – Divisão de Engenharia Mecânica –
Instituto Tecnológico de Aeronáutica, 12.228-900, São José dos Campos, SP, Brasil.
*[email protected]
(a)Faculdade
ABSTRACT
The present paper studies the ECAE processed NiTi Shape Memory Alloys (SMA)
with two composition: Ti-55.27wt.%Ni and Ti-56.00wt.%Ni. The samples were
deformed by 1 ECAE pass using a die with an intersection angle of 120°. After
processing, Ti-55.27wt.%Ni was annealed at 300oC, 400oC and 500oC for 1 h to
evaluate the microstructural changes. Microstructural characterization was performed
by scanning electron microscopy (SEM) equipped with an energy dispersive
spectrometer (EDS) device. Martensitic transformations temperatures were analyzed
by differential scanning calorimetry (DSC).
Keywords: NiTi alloys, Shape Memory Effect, ECAE.
1. INTRODUCTION
The Shape Memory Alloys (SMA) are materials that have the capacity to
recover its original shape based on reverse martensitic transformations, i.e., when
the alloys are deformed in the martensitic state and subsequently heated, they
recover its original shape and size through transformation of the martensite to
austenite phase
[1].
In the NiTi alloys, the shape memory effect is associated to
martensitic transformation from B19’ (martensitic) to B2 (austenite).
In recent years, various factors that affects the structure including Ni content,
thermo-mechanical treatment and addition of alloying elements are being
investigated aiming to improve the shape memory effect (SME)
[1].
For NiTi alloys,
the microstructural changes, such as grain refinement, interferes in martensitic
transformation and consequently the SME. One alternative to refine the
microstructure is by severe plastic deformation, using the equal channel angular
extrusion (ECAE) technique
[2].
During ECAE, the billet is pressed through a die
consisting of two channel of same cross section intersecting each other at an angle
. Using ECAE process
[3-5],
some studies have been done in NiTi SMA with respect
to microstructural changes that influence the direct martensitic transformation from
B2  R phase promoted by cold working associated to annealing. The present paper
shows the results of NiTi SMA deformed by 1 ECAE pass in two different
compositions and correlates the microstructure to the hardness and to the martensitic
transformations temperatures.
2. EXPERIMENTAL
Two NiTi alloys, Ti-55.27wt.%Ni (hereafter called Ti55Ni) and Ti-56.00wt.%Ni
(hereafter called Ti56Ni), used in this work were produced by vacuum induction
melting (VIM) and their chemical compositions are shown in Table 1. These materials
were hot swaged to 10 mm in diameter rod, solution treated (ST) at 850oC for 1 h and
then water quenched. Subsequently, they were machined to 7x7x60 mm billets and
then ECAE processed at 250oC at an extrusion speed of 10 mm/min. The ECAE die
angles was  = 120o and  = 0o.
The deformed samples of the Ti55Ni alloy were annealed at 300oC, 400oC and
500oC for 1 h and air cooled. The microstructural characterization was performed
using a JEOL scanning electron microscope (SEM) operating at 22 kV equipped with
an energy dispersive spectrometer (EDS) system. The thermal transformation
behavior during heating and cooling was investigated using a differential scanning
calorimeter NETZSCH DSC404C Pegasus, with liquid nitrogen cooling system.
Vickers hardness test was performed using a Future-Tech FM-7000 microindenter
with a load of 200g/15s. The micrograph analyses and hardness measurements was
performed in the extrusion plane of specimens, in other words, Transversal DirectionNormal Direction (TD-ND).
Table 1: Chemical composition (wt.%) of the NiTi alloys with shape memory effect.
Sample
Ti
Ni
C
O
Ti55Ni
balance
55.27
0.060
0.0564
Ti56Ni
balance
56.00
0.036
0.0515
3. RESULTS AND DISCUSSION
The Table 2 shows the hardness values of Ti55Ni and Ti56Ni. Comparing
Ti55Ni to Ti56Ni in the ST state it is observed that the alloy with higher Ni content
has a higher hardness. For alloy Ti56Ni, in the ST state, the hardness of samples
was about 262 HV and after 1 pass the hardness increased to 298 HV. This
hardness increase is related to work hardening. On the other hand, it is seen that for
a Ti55Ni there is no hardness increase after 1 pass. Even the heat treatment at
300°C does not show hardness increase. The increase in hardness (262 HV) is
observed for sample treated at 400°C possibly due precipitation of Ti 3Ni4
[6].
The
softening is observed for sample annealed at 500°C with its hardness of 200HV.
Karaman et al.
[7]
observed that in Ti-49.8at.%Ni samples deformed by 1 ECAE pass
and then submitted to heat treatment up to 500oC, the recovery process occurred,
however, above this temperature the recrystallization process started. Therefore, it is
supposed that the hardness reduction in Ti55Ni alloy annealed at 500oC is
associated to the beginning of recrystallization process.
Table 2: Hardness value of solution treated (ST), deformed and annealing samples.
Sample
Ti55Ni (HV)
Ti56Ni (HV)
ST
221  26
262  12
1 ECAE
225  12
298  11
o
1 ECAE + 300 C/1h
---231  10
1 ECAE + 400oC/1h
---262  13
o
1 ECAE + 500 C/1h
---200  21
The Fig. 1 shows the microstructures of Ti55Ni in the ST state (Fig. 1a) and
deformed by 1 ECAE pass (Fig. 1b). In the ST condition, the microstructure is
essentially composed by martensite plates and small precipitates. After ECAE
process, it is observed changes in the microstructure such as finer martensite plates
induced by deformation and deformation inhomogeneities, as well as the presence of
precipitate. The Fig. 2a shows the microstructures of Ti56Ni sample in the ST state. It
can be observed a uniform microstructure consisting of coarse grained austenite.
After 1 pass, it is observed a refinement in the microstructure, although, exist the
presence of austenitic grains of around 30 m.
Analyzing the Fig. 1a and 2a, it is possible to confirm that in ST state the Ti55Ni
alloy is martensite phase (B19’) and Ti56Ni alloy is austenite phase (B2). The
microstructural analysis is in agreement to hardness values shown in Table 2, where
the martensite phase presents a lower hardness compared to austenite phase. In a
deformed state, it is observed that Ti56Ni alloy presented a more deformed
microstructure compared to Ti55Ni alloy after 1 ECAE pass (Fig. 1b and 2b). These
results can be confirmed by hardness measurements that are 225HV and 298HV to
Ti55Ni and Ti56Ni, respectively.
(a)
(b)
Fig. 1. SEM backscattered electron images (TD-ND) of Ti55Ni alloy: (a) ST and (b)
deformed by 1 ECAE pass. (SEM-SE).
(a)
(b)
Fig. 2. SEM backscattered electron images (TD-ND) of Ti56Ni alloy: (a) ST and (b)
deformed by 1 ECAE pass. (SEM-SE).
The Fig. 3 shows the microstructure of the Ti55Ni alloy annealed at 300oC and
500oC by 1 h. The annealing treatment performed at 400°C resulted in no significant
changes in the microstructure and it is not shown. The SEM investigation, show no
significant microstructural changes comparing deformed state (Fig. 1b) to annealed
state (Fig. 3a and 3b). It was observed two types of precipitates in the Ti55Ni alloy
(see arrows in the Fig. 3a and 3b). This precipitates are present in the ST, deformed
and annealed samples, and the increase in the heat treatment temperature, no
change was observed in terms of precipitate morphology. The EDS analysis (Fig. 4)
revealed that one of the precipitate type was enriched in Ti (Fig. 4a), while the other
(Fig. 4b) the Ti and Ni concentration are similar to the matrix. The precipitate shown
in Fig. 4a the Ti and Ni concentration were 97 wt.% and 3 wt.%, respectively. On the
other hand, in the precipitate type analyzed in Fig. 4b the Ti and Ni concentration are
53 wt.% and 47 wt.%. It is possible that the precipitates rich in Ti could be carbides
(TiC), because the Ti55Ni alloy has 0.06 wt.% C, however, the precipitate with Ti and
Ni near concentrations could be Ti4Ni2O, formed by oxygen contamination.
Precipitates containing carbon and oxygen modify the original matrix composition
and thus influence the martensitic transformation temperatures
[8].
TiC particles and
Ti4Ni2O complex-oxide decreases Ti contained in the matrix, therefore, decreasing
the peak martensitic transformation temperature (Mp) [8].
(a)
(b)
Fig. 3. SEM backscattered electron images (TD-ND) of Ti55Ni alloy deformed by 1
ECAE pass and annealed in (a) 300oC/1h and 500oC/1h. (MEV-SE).
Fig. 4. EDS analysis (TD-ND) of Ti55Ni alloy deformed by 1 ECAE pass.
The Fig. 5 shows the heat flow vs. temperature curves for ST and deformed
samples of the Ti55Ni and Ti56Ni alloys. During the heating and cooling cycle of the
ST samples, in both alloys, only one endothermic and one exothermic peak were
observed. These peaks are related to reverse and direct B19’  B2 martensitic
transformation. The martensite start (Ms) and austenite start (As) temperatures were
29oC and 44oC, respectively, in the ST state to Ti55Ni alloy. In the Ti56Ni alloy, the
ST condition presents Ms and As temperatures at -31oC and -17oC, respectively,
therefore it austenitic at room temperature.
These results corroborate with the microstructure analyses (Fig. 1a and 2a). For
Ti55Ni alloy in the ST state, the DSC measurements show that martensitic
transformation (Mp) occurs at 24oC (Mp) and the microstructure at room temperature
is composed by martensite phase (B19’). For Ti56Ni alloy, in the same condition, the
DSC analyses shows that occurs a martensitic transformation (Mp) at -36oC, resulting
in a B2 microstructure at room temperature justifying the higher hardness value of
Ti56Ni compared to Ti55Ni that is martensitic at same temperature. Only B2B19’
transformation occurred for ST samples for both alloys.
For Ti55Ni alloy deformed by 1 ECAE pass (Fig. 5a), it was observed a
decrease in the reverse martensitic transformation temperature (Ap) and the
appearance of two small exothermic peaks during the cooling. The Mp values was
slightly decreased comparing ST condition to 1 ECAE pass sample (M pST = 24oC and
Mp1ECAE = 20oC and 6oC). On the other hand, Ti56Ni alloy (Fig. 5b) showed a small
endothermic peak at -24oC (Ap), however, no exothermic peak was observed in the
cooling step for this alloy. Recently, Fan et al.
[3]
found the same characteristic for Ti-
50.9at.%Ni deformed with 2 ECAE passes, in which no transformation peak (B2 
B19’) was observed in cooling step. Li et al.
[5]
attributed the absence of cooling peak
for Ti-50.3at.%Ni deformed with 2 ECAE passes to transformation temperatures
below the analyzed range or no transformation at all due to high degree of
deformation.
The Ti55Ni alloy deformed by 1 ECAE pass was thermally treated to observe
the annealing effect on martensite transformation temperature as shown in Fig. 6 for
different conditions. It was observed that the annealing treatments resulted in a
decrease in reverse and direct martensitic transformations temperatures compared to
the ST condition. After the annealing treatment performed at 300°C, the sample
maintains the characteristics of deformed state, presenting two exothermic peaks on
cooling. However, increasing the annealing temperature to 500°C, promotes an
increment in phase transformation temperatures, approaching to ST condition.
Studies in NiTi alloys deformed by ECAE
[3, 5, 9],
observed the R phase formation
after annealing, associated to Ti3Ni4 precipitation. The appearance of two peaks in
the deformed and annealed sample at 300oC for 1 h for Ti55Ni could be associated
to B2RB19’ transformation associated to cold work followed by annealing step at
300°C/1h promoting the precipitation of Ti3Ni4
[10, 11].
Although not shown, it was
observed the same behavior for sample ECAE deformed and annealed at 400°C/1h,
that is, the appearance of two peak on cooling, the first related to B2  R and the
second to R  B19’.
0,6
0,6
Solution treatment
1 ECAE
EXO
0,5
0,4
0,4
0,3
0,3
DSC/(mW/mg)
DSC/(mW/mg)
0,5
0,2
cooling
0,1
0,0
-0,1
heating
-0,2
0,2
cooling
0,1
0,0
-0,1
heating
-0,2
-0,3
-40
-0,3
(a)
Ti55Ni
-0,4
-60
Solution treatment
1 ECAE
EXO
-20
0
20
40
60
80
100
(b)
Ti56Ni
-0,4
-80
120
-60
-40
-20
0
20
40
60
Temperature (oC)
Temperature (oC)
Fig. 5. Differential scanning calorimetry for (a) Ti55Ni and (b) Ti56Ni alloys.
0,6
0,5
Solution treated
1 ECAE
1 ECAE 300
1 ECAE 500
EXO
0,4
DSC/(mW/mg)
0,3
cooling
0,2
0,1
0,0
-0,1
-0,2
heating
-0,3
-0,4
-0,5
Ti55Ni
-0,6
-60
-40
-20
0
20
40
60
80
100
120
Temperature (oC)
Fig. 6. DSC measurements for Ti55Ni deformed by 1 ECAE pass and annealed.
4. CONCLUSION
NiTi alloys were deformed by 1 ECAE pass. In a ST the Ti55Ni is martensitic
and Ti56Ni is austenitic at room temperature as shown by microstructure, DSC
measurement and hardness values. The Ti56Ni presented a hardness increase after
1 ECAE pass denoting the work hardening but no hardness variation was observed
for Ti55%Ni for the same condition, this could be attributed to the austenitic state to
the first and martensitic state for the second. The annealing at 300°C for Ti55Ni after
1 pass presented no variation in hardness but it was observed the two step
martensitic transformation related to B2RB19’. The hardness increase for Ti55Ni
after annealing at 400°C is attributed to the precipitation of metastable Ti3Ni4
precipitation. The lowest value of hardness for Ti55Ni annealed at 500°C is attributed
to the beginning of recrystallization and no two peak transformation was observed.
5. ACKNOWLEDGEMENTS
Authors are thankful to Fapesp, CNPq Universal, CNPq Casadinho (UFCG/ITA),
Finep/ProInfra, Villares Metals SA, Multialloy Metais Especiais Ltda, AEB, IEAv and
ABTLus/LNNano.
6. REFERENCES
1. OTSUKA, K.; WAYMAN C. M. Shape Memory Materials. Cambridge University
Press: Cambridge, 1998.
2. SEGAL, V. M. Equal channel angular extrusion: from macromechanics to structure
formation. Materials Science and Engineering A, v.271, n.1-2, p. 322-333, 1999.
3. FAN, Z.; XIE, C. Phase transformation behaviors of Ti-50.9 at.% Ni alloy after
equal channel angular extrusion. Materials Letters, v.62, n.6-7, p. 800-803, 2008.
4. PUSHIN, V. G.; STOLYAROV, V. V.; VALIEV, R. Z. Features of structure and
phase transformations in shape memory TiNi-based alloys after severe plastic
deformation. Annales de Chimie Science des Matériaux, v.27, n.3, p. 77-88, 2002.
5. LI, Z.; CHENG, X.; SHANGGUAN, Q. Effects of heat treatment and ECAE process
on transformation behaviors of TiNi shape memory alloy. Materials Letters, v.59,
n.6, p. 705-709, 2005.
6. NISHIDA, M.; WAYMAN, C. M.; HONMA, T. Precipitation processes in nearequiatomic TiNi shape memory alloys. Metalurgical Transactions A, v.17, p. 15051515, 1986.
7. KARAMAN, I.; ERSIN KARACA, H.; MAIER, H. J.; LUO, Z. P. The effect of severe
marforming on shape memory characteristics of a Ti-Rich NiTi alloy processed using
equal channel angular extrusion. Metalurgical and Materials Transactions A, v.34,
p. 2527-2539, 2003.
8. OTUBO, J.; RIGO O. D.; COELHO, A. A.; NETO, C. M.; MEI, P. R. The influence
of carbon and oxygen content on the martensitic transformation temperatures and
enthalpies of NiTi shape memory alloy. Materials Science and Engineering A,
v.481-482, p. 639-642, 2008.
9. ZHANG, X.; XIA, B.; SON, J.; CHEN, B.; TIAN, X.; HAO, Y.; XIE, C. Effects of
equal channel angular extrusion and aging treatment on R phase transformation
behaviors and Ti3Ni4 precipitates of Ni-rich TiNi alloys. Journal of Alloys and
Compounds, v.509, n.21, p. 6296-6301, 2011.
10. MORAWIEC, H.; STRÓŻ, D.; GORYCZKA, T.; CHROBAK, D. Two-stage
martensitic transformation in a deformed and annealed NiTi alloy. Scripta Materialia,
v.35, n.4, p. 485-490, 1996.
11. KABAYAMA, L. K.; RIGO, O. D.; OTUBO, J. Influence of thermomechanical
processing on the martensitic transformation temperatures of NiTi SMA wire.
Materials Science Forum, v.643, p. 43-48, 2010.