PrecipitationShape memory Alloy of Second Phases in Aged Ni Rich

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Precipitation of second phases in aged Ni richNiTiRe shape memory alloy

N. El-Bagoury*1,2

This study investigated the effect of aging on the structure and precipitation of second phases of

Ni52Ti47?7Re0?3 shape memory alloys. The alloy was solutionised at 1000uC for 24 h before aging

at various temperatures ranging from 300 to 600uC for 3 h. The matrix phase in both solutionised

and aged specimens was martensite. Ti2Ni phase was also present in the microstructure of both

solutionised and aged specimens and its volume fraction decreased as the aging temperature

increased. Ni4Ti3 phase began in appearance by increasing aging temperature to 400uC. Ni4Ti3

precipitates had lenticular and non-geometry shapes. Aging at 600uC led to precipitation of Ni3Ti

phase in the microstructure. This precipitated phase formed in white blocky shapes. Ti/Ni ratio

increased and/or Ni content decreased in the matrix with increasing in aging temperature.

Keywords: Ni rich NiTiRe shape memory alloys, Aging, Martensite, Second phase precipitate, Ti/Ni ratio

Introduction

Ti50Ni to Ti55Ni (at-%) can be termed as the pioneer

of shape memory alloys (SMA) and a key system for

studying phase transformations and precipitate evolution

in shape memory alloys. Shape memory alloys are

martensitic metals that remember the original shape of

their parent modification under specific conditions of

temperature and mechanical loading/unloading. The

thermodynamics of the parent bcc-structured, ordered

austenitic B2 phase and the monoclinic martensitic B199

phase are well understood. In order to improve shape

memory and mechanical properties, SMA is usually aged

at temperatures where precipitations of second phases

from the thermodynamically stable TiNi3 phase. In

particular, Ti3Ni4plays an important role for martensite

formation. The martensite start temperature, Ms, is

strongly influenced by changes of plastic deformation

limits associated with precipitation hardening and the

change of the matrix composition due to precipitation.18

For instance, an increase in the martensite start

temperature of approximately 30uC was reported due to

aging of Ti50?7Ni2 (at-%). Recently, the fatigue failureof TiNi SMA was related to the occurrence of TiNi3.

9

Control of transformation temperatures of shape

memory alloys has been an important research subject

to enhance the reliability and applicability of these

functional materials.1014 Factors influencing the Ms

temperature are believed to be the elastic properties of

the parent austenite crystal and certain microstructural

features such as precipitates.11,15,16

There are many factors that have great influence onthe transformation behaviours, such as, the compositionof alloys, aging treatment with precipitates, mechanical

treatment (cold working), addition of a third element,point defects, dislocations and degree of order.17 Amongthese factors, aging treatment is the most simple and

effective way, and is widely used to treat the TiNi basedalloys.18,19,2022,2325

It is known that precipitates will form from the

supersaturated sample during aging; and the existenceof precipitates will greatly affect the transformationbehavior.19,21,26

The precipitation process in nickel rich NiTi shapememory alloys is significantly influences both thestructural and functional properties of the alloy. It isknown that the precipitation sequence is as follows: botob1zNi4Ti3to b2zNi3Ti2to b3zNi3Ti wherebo,b1,b2 and b3 indicate changes in the matrix composition.The Ni4Ti3 particles precipitating in early stages ofageing have a rhombohedral structure and are uniformlydistributed in the matrix.

Until now, various abnormal transformation beha-

viours after aging have been found and their originshave been discussed.1012 As a result, the equilibriumrelationship between TiNi matrix and Ni4Ti3 precipi-tates is still subject to uncertainty.

In this present study, microstructure and precipitationof second phases such as Ti2Ni, Ni4Ti3 and Ni3Ti ofsolutionised and aged NiTiRe shape memory alloy willbe investigated.

Materials and proceduresPolycrystalline Ni51Ti48?7Re0?3 shape memory alloy wasmade by melting pure elements (more than 99?99%purity) in an induction vacuum furnace. The alloy was

melted four times to ensure homogeneity and cast intoan investment casting ceramic mould. This mould waspreheated to 1000uC before pouring process.

1Chemistry Department, Faculty of Science, TAIF University, PO Box 888,El-Haweyah, El-Taif, Saudi Arabia2

Casting Technology Lab., Manufacturing Technology Dept., CMRDI, POBox 87, Helwan, Cairo, Egypt

*Corresponding author, email [email protected]

W. S. Maney & Son Ltd. 2014Received 30 May 2014; accepted 5 October 2014DOI 10.1179/1878641314Y.0000000033 Materials at High Temperatures 2014 VOL 00 0 NO 0 00 1


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Specimens from this alloy were solutionised at 1000uC

for 24 h followed by quenching in iced water. The aging

process was carried out at various temperatures ranging

from 300 to 600uC for 3 h then iced water quenching.

The microstructure of both solutionised and aged

specimens was investigated by Meiji optical microscope

fitted with digital camera as well as JEOL JSM5410

scanning electron microscope (SEM). The specimens

for microstructure examination were prepared by stan-

dard metallographic procedures according to standard

ASTM: E3-11 then etched in a solution of HNO3, HFand H2O in a ratio of 4 : 1 : 5, respectively to investi-

gate different types of precipitates. Another etching

solution of HNO3/HF/CH3COOH in a ratio of 4:4:2

was used to examine the martensite phase. The dif-

ferent phases existing in the structure were analysed

using energy dispersive X-ray spectrometry (EDS)

attached in the SEM operated at 20 kV. The phase

transformations of the solutionised and aged alloys

were measured by Netzsch CC 200 F1 differential

scanning calorimetry (DSC) with a cooling/heating

rate of 10uC min21 in the temperature range from 230

to 150uC. Moreover, X-ray diffraction (XRD) was

carried out to identify the existing different phases inthe structure by using Cu K

a radiation with a step

scanning in 2h range of 3080u.

1 Microstructure of solution treated and aged NiTiRe shape memory alloys: a solutionised at 1000C/24 h; b aged at

300C/3 h; caged at 400C/3 h; daged at 500C/3 h; eaged at 600C/3 h

El-Bagoury Precipitation of second phases

Materials at High Temperatures 2014 VOL 00 0 NO 00 02
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6 X-ray diffraction for solution treated and aged NiTiRe shape memory alloys

7 Ti2Niprecipitated phase in aged alloy at 300C for 3 h


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Results and discussion

Phase constituents and microstructure ofsolutionised and aged NiTiRe alloysThe microstructures of both solutionised and aged Nirich NiTiRe shape memory alloys are shown in Fig. 1.The parent phase in all of these microstructures isthe martensite phase (B19). This means that themartensitic phase transformation temperatures forall of these specimens are above the room tempera-ture. Additionally, precipitates of Ti2Ni second phase isfound in the microstructure of both solutionised andaged specimens.

Ti2Ni phase precipitated in a blocky agglomeratednon-geometric shape in the microstructure of Ni richNiTiRe shape memory alloys, as shown in Fig. 2.

Figure 3 shows the distribution and size of the Ti2Ni

precipitates in the microstructures of aged NiTiRealloys. Ti2Ni phase is well distributed in the micro-structure of the aged specimens.

The size of precipitated Ti2Ni phase are differingamong microstructures according to the conditions ofthe heat treatment processes, as shown in Fig. 3.Whereas the aging temperature increases the Ti2Niprecipitates dissolute in the NiTi matrix. Therefore, theNi/Ti ratio in the matrix decreases with increasing agingtemperature leading to elevates the martensitic transfor-mation temperature.

The size of the Ti2Ni precipitates in the micro-structure of NiTiRe specimen aged at 300uC (Fig. 3a),

decreases as the aging temperature elevates to 400uC asshown in Fig. 3b. As the aging temperature increases to500 and 600uC, in addition to the dissolution of Ti2Niphase, some other precipitates appeared in the micro-structure of these alloys in a small size as shown inFig. 3c and d.

From the microstructure shown in Fig. 3, the volumefraction was measured and the obtained values arerepresented in Fig. 4. It is obvious that the volume

fraction of Ti2Ni decreases by increasing the agingtemperature. This means that the volume fraction ofTi2Ni phase decreases as the aging temperature increase

as shown in Fig. 4.Figure 5a and b shows the SEM image of the NiTiRe

shape memory alloy aged at 300u

C for 3 h followed byquenching in iced water. It is obvious that the micro-structure of the aged alloy is dominantly constituted by

the martensite plates. It is also clear that the martensite

found in some areas in the microstructure consists of fine

plates structure and are uniform in size (Fig. 5a). In other

some areas the microstructure reveals a random distribu-

tion of martensite plates having fine structure (white

circles) and coarse structure (black circles) as shown in

Fig. 5b.

X-ray diffraction for solution treated and agedNiTiRe alloysThere are five XRD patterns; the first one related to the

solution treated alloy and the other four patters represent

the aged Ni51Ti48?7Re0?3 alloys as shown in Fig. 6. The

parent phase, which has the main diffraction peak (11), in

all solution treated and aged alloysis the martensite phase,

denoted as M. This means that the martensitic phase

transformation temperatures for these specimens are

above room temperature. In addition to the martensitephase, the microstructure of the solution treated specimen

contains a precipitated Ti2Ni phase only. The latter

precipitated phase is a common denominator in all

microstructures of the investigated specimens with threes

peaks (110), (440) and (123) in all patterns.

The microstructure of the specimen aged at 300uC for

3 h has the same phase constitution as the solution

treated alloy, where they have identical X-ray pattern, as

shown in Fig. 6.

The XRD pattern of aged specimen at 400uC has a

new small peak for Ni4Ti3 phase, (112), as shown

in Fig. 6. By elevating aging temperature to 500 and

600u

C, the Ni3Ti4 phase peak get more obvious thanthat in XRD pattern of aged specimen at 400uC. The

presence of Ni3Ti phase in the microstructure of aged

8 Lenticular coherent (Ni4Ti3) precipitation in alloy aged

at 600C

9 Bulky and non-geometry precipitates of Ni4Ti3 phase in

aaged alloy at 500C and baged alloy at 600C


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specimen at 600uC was confirmed by a new peak, (203),appeared in its XRD pattern, as shown in Fig. 6.

Second phases precipitated in NiTiRe matrix

According to the heat treatment conditions, there arevarious second phases precipitated in the microstructureof NiTiRe shape memory alloys. These precipitatedsecond phases could be the equilibrium Ti2Ni and Ni3Ti

phases or metastable Ni4Ti3 phase.

In addition to the XRD pattern shown in Fig. 6, theprecipitation of Ti2Ni phase was confirmed using SEM

and EDX analysis as shown in Fig. 7. This precipitatedphase was found in the microstructures of all investi-

gated specimens but with various volume fractions, asshown in Fig. 4.

Surprisingly the precipitate has two phases as a

function of temperature, and one phase transforms tothe other martensitically by changing temperature. The

higher temperature phase has a tetragonal structure, thelattice parameters beinga50?3095 nm andc51?3585 nm(at 100uC), while the low temperature phase has an

orthorhombic structure, the lattice parameters being

a50?4398 nm, b50?4370 nm and c51?3544 nm (at

20uC). See Ref. 27 for more details.

The precipitation of Ni4Ti3 phase was observed onlyin the microstructures of the specimens aged at 400, 500

and 600uC. The microstructure of both solutionised andaged specimen at 300uC was free of any precipitations of

Ni4Ti3 phase.

Figure 8 shows the precipitates of Ti2Ni and Ni4Ti3phases in the aged alloy at 600uC. Ni4Ti3 phase

precipitated in a lenticular coherent shape as shown inFig. 8. The Ni4Ti3 precipitates have a rhombohedralatomic structure with a50?670 nm, a5113?9u.28 The

composition of this precipitate was first determined by

EDS to be Ti11Ni14by Nishida and Honma,29 and later

10 Ni3Ti precipitates in aged NiTiRe alloy at 600C for 3 h

11 Spectrum and EDS analysis of colonies found in microstructure of aged alloy at 600C




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it was taken to be Ti3Ni4by taking account the accuracyof EDS.

In addition to lenticular shape, Ni4Ti3 phase pre-cipitated in bulky non-geometric shapes as shown inFig. 8. This shape of Ni4Ti3 precipitates was found in

the NiTiRe alloy aged at 500uC, Fig. 9a and in alloyaged at 600uC (Fig. 9b).

Sitepu et al. reported that precipitation of Ni4Ti3particles occurred in a matrix of Ni rich NiTi SMA ofnominal composition Ni50?7Ti (at-%), when it wassolution annealed at 850uC for 15 min followed by waterquenching and aging at 400uC for 20 h.30

Pelton et al.31 reported dissolution of Ni4Ti3 between500 and 600uC in Ti50?8Ni (at-%). Moreover the solvustemperature of Ni4Ti3 phase is 560uC as estimated byYufeng Zheng et al.32 However in this study, Ni4Ti3precipitates are still appeared in the microstructure ofaged specimen at 600uC for 3 h followed by iced water

quenching.The equilibrium Ni3Ti phase precipitated only in the

microstructure of aged specimens at 600uC as shown in

Fig. 10. This phase precipitated in the microstructure asbulky white shape. The TiNi3 phase has the hexagonal

DO24 type ordered structure. The lattice constants area50?5101 nm, c50?8306 nm and c/a51?6284.33

Moreover, in Fig. 11, and according to EDS analysisthere are some colonies in the microstructure classifiedas a matrix phase.

From the above results, it can be concluded that theoptimum aging treatment conditions are ranging from

450 to 550uC for 3 h. Where the preferred Ni4Ti3phasethat elevating martensite transformation temperatureabove the room temperature,33 is precipitated in themicrostructure of Ni rich NiTiRe shape memory alloyafter aging at 450uC for 3 h. However, the precipitation

of undesirable Ni3Ti phase that affects the fatigue lifenegatively33 in the microstructure of Ni rich NiTiRe

shape memory alloy starts with aging at 600uC for 3 h.

There are some other strange precipitates found in the

microstructures of the investigated alloy specimen. Forinstance in the aged alloy at 400uC, the microstructurecontains some precipitates other than the recognised

12 Ti3Ni2 phase found in microstructure of aged alloy at 400 C


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ones such as Ti2Ni, Ni3Ti2, Ni4Ti3and Ni3Ti that appear

in the binary NiTi phase diagram.33

According to the EDS analysis and atomic percen-

tages, it was found precipitates of Ti3Ni2and Ti4Ni3, as

shown in Figs. 12 and 13 respectively. The precipitation

of these phases and its conditions need more work in the

near future to be investigated.

Table 1 summarises the precipitated phases found

in different microstructures of solutionised and aged

NiTiRe shape memory alloy specimen.

The precipitation of the second phase formed at

different aging temperatures alter the Ti/Ni ratio or Ni

concentration in B19 matrix as shown in Fig. 14. As the

aging temperature increases the Ti/Ni ratio in B19

increases due to the dissolution of the second phase

Ti2Ni in the B19 matrix. In the same direction, the

precipitation of both Ni4Ti3 and NiTi3phases at higher

temperature of aging process, 400 to 600uC, supports thedecreasing of Ni concentration or increasing Ti/Ni ratio

in the B19 matrix, as shown in Fig. 14.

As a result of the precipitation of Ni4Ti3phase in the

microstructure of aged alloys at 400, 500 and 600uC

in addition to the precipitation of Ni3Ti phase in the

13 Ti4Ni3 phase found in microstructure of aged alloy at 400 C

14 Ti/Ni ratio in matrix of NiTiRe alloy versus aging tem-

perature

Table 1 Existence of precipitated phases in investigatedalloys

Phase B19 Ti2Ni Ni4Ti3 Ni3Ti Ti4Ni3 Ti3Ni2

Sol. treated alloy ! ! 6 6 6 6Aged at 300uC ! ! 6 6 6 6

Aged at 400u

C ! ! ! 6 ! !Aged at 500uC ! ! ! 6 6 6Aged at 600uC ! ! ! ! 6 6




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microstructure of aged alloy at 600uC, the Ti/Ni ratioincreases while Ni content in the matrix decreases.34

Therefore, this changing in the chemical composition ofthe matrix may affect the martensite transformation

temperature, which increases as the ageing temperatureincreases.35

Conclusions

1. Martensite and Ti2Ni phases are the commondenominator in the microstructure of all investigatedalloy specimens. Ti2Ni phase dissolves in the matrix asthe aging temperature increase.

2. Ni4Ti3phase starts in formation slightly in lenticularas well as non-geometry shapes in the microstructure ofNi52Ti47?7Re0?3alloys with aging at 400uC and obviouslyat 500 and 600uC for 3 h.

3. The precipitates of Ni3Ti phase appeared as whiteblocky shapes in the microstructure of Ni52Ti47?7Re0?3alloy after aging at 600uC for 3 h.

4. The optimum aging temperature for Ni52Ti47?7Re0?3shape memory alloy is above 450uC to precipitate the

preferred Ti4Ni3 phase and below 550uC to avoid theundesired Ni3Ti phase.

5. Ti/Ni ratio and/or Ti content in the matrix ofNi52Ti47?7Re0?3 alloy increases as the aging temperatureincrease.

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PrecipitationShape memory Alloy of Second Phases in Aged Ni Rich

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