Porous nickel–titanium alloy prepared by gel-casting

Bo-Hua Duan*, Hai-Xia Hong, De-Zhi Wang,

Hui-Jiang Liu, Xiao-Jia Dong, Dan-Dan Liang

Received: 25 November 2012 / Revised: 19 April 2013 / Accepted: 15 June 2013

� The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2013

Abstract To explore the preparation of porous nickel–

titanium alloy with excellent properties, larger size and

complex shape, the premixed powder of Ni and Ti with

atomic ratio of 1:1 was shaped by gel-casting. The effects of

solids loading and the content of dispersant on flow ability of

nickel–titanium slurry and the mechanical properties of

nickel–titanium sintered body were studied. The drying

models under different solids loading were also discussed.

The results show that the viscosity of slurries significantly

increases with an increase in solids loading. After a proper

process of drying, the green body with complex shape is

obtained. The sintered body with porosity rate reaching up to

49.5 % and compression strength reaching to 364.74 MPa

could meet the basic demands of implant materials.

Keywords Gel-casting; Porous nickel–titanium alloy;

Solids loading; Drying

1 Introduction

The porous nickel–titanium (NiTi) shape memory alloy

(SMA) shows a good application prospect in the field of

medical implants because of its excellent biocompatibility,

superelasticity and adjustable mechanical property [1, 2].

Moreover, it has a special shape memory effect. The point of its

thermoelastic martensitic transformation is between -50 and

40 �C which is lower than most of other SMAs. There were

many preparation methods of this SMA, such as mechanical

alloying acrylamide (AM), spark plasma sintering, self-prop-

agating high temperature and powder injection moulding

[3–5]. The NiTi SMAs were successfully used in the field of

medicine [6, 7]. However, some methods to prepare porous

NiTi SMA may have the disadvantages of high production cost

and uncontrollable process. Especially these methods could

not form parts with larger size and complex shape, so it is

contributed to promote the development and application of

porosity NiTi SMA to exploit new preparation technique.

Gel-casting, a new near-net-shape ceramic processing tech-

nique, was invented by Omatate and Janney [8] of Oak Ridge

National Laboratory in early 1990s. This new powder metallurgy

of wet forming process had the advantages of manageable pro-

cess, low cost and broad applicable scope, especially for forming

the parts with larger size and complex shape [9, 10]. It becomes a

mature technique to form ceramic powders after nearly 20 years

of development. A series of ceramics (e.g. Al2O3, ZrO2 and

Si3N4) with excellent properties were prepared successfully by

gel-casting [11, 12]. Recently, researchers applied the gel-casting

to form metal powders. A series of metal parts (e.g. copper, steel

and titanium) were formed [13–16]. In this article, gel-casting

process was developed as a new moulding process in the field of

NiTi powder metallurgy to manufacture metal parts with better

properties and complex shape.

2 Experimental

2.1 Raw materials

The Ni powders of 99.5 % in purity, with the mean grain

size of 28.41 lm, and the Ti powders of 99.1 % in purity,

B.-H. Duan*, H.-X. Hong, D.-Z. Wang, H.-J. Liu, X.-J. Dong,

D.-D. Liang

School of Materials Science and Engineering, Central South

University, Changsha 410083, China

e-mail: duan-bh@csu.edu.cn

B.-H. Duan, H.-X. Hong

State Key Laboratory of Powder Metallurgy, Central South

University, Changsha 410083, China

123

Rare Met. RARE METALSDOI 10.1007/s12598-013-0195-x www.editorialmanager.com/rmet

with the mean grain size of 14.58 lm were used in this

work. Their morphologies are shown in Fig. 1. Table 1

shows the chemical reagent system based AM on organic

monomer.

2.2 Experimental process

The flow chart of Ni–Ti powders gel-casting process is

shown in Fig. 2. Firstly, the AM and N,N0-methylene-bis-

acrylamide (MBAM) were mixed at a proper ratio, and then

were dissolved in the deionized water to prepare a trans-

parent premixed solution. Secondly, the dispersant and two

kinds of powders with the atomic ratio of 1:1 were added into

the premixed solution to get a preliminary slurry. A certain

content of defoamer was added into the slurry. The slurry was

ball-milled in Ar atmosphere for a certain time in a plastic

tank using zirconia balls as the milling media to get homo-

geneous slurry. Thirdly, an appropriate content of APS and

TEMED was added into the slurry. Then the slurry was

poured into the mould after a sufficient stirring, then the

mould was put into the calorstat with the temperature of

80 �C for a few minutes. The slurry in the mould was poly-

merised into a certain shape. After demoulding, the wet

green body was dried in the vacuum drying oven for several

hours at a proper temperature to make sure that most of the

water in the body was eliminated. Finally, the dried green

body was moved into the vacuum sintering furnace for fur-

ther thermal degreasing and sintering. After a proper sin-

tering, the porous NiTi sintered body was prepared.

The apparent viscosity of the slurry was measured by a

rotational viscometer with the model of NDJ-79. The

density of sintered body was measured using Archimedes’

principle. The compression strength of the sintered body

was measured on AG-10TA materials testing machine. The

size of sample was U10 mm 9 10 mm. Microstructures of

the samples were observed by scanning electric microscope

Fig. 1 SEM images of Ni and Ti powder: a Ni and b Ti

Table 1 Chemical reagent system based acrylamide on organic monomer

Solvent Organic

monomer

Cross-linker Dispersant Defoamer Initiator Catalyst

Deionized

water

Acrylamide

(AM)

N,N0-methylene-bis-

acrylamide (MBAM)

Ammonium

citrate

N-octanol Ammonium

persulphate (APS)

Tetramethylethylenediamine

(TEMED)

Fig. 2 Flow chart of NiTi gel-casting process

B.-H. Duan et al.

123 Rare Met.

(SEM). The phase analysis of sintered body were got

through the X-ray diffractometer (XRD).

3 Results and discussion

3.1 Rheological properties of slurry

The key step for gel-casting is to prepare the stable slurry

with lower viscosity. The slurry with a lower viscosity can be

poured into mould successfully without additional force. The

viscosity of slurry is closely related to the solids loading in

the slurry. Figure 3 shows the effect of solids loading on

apparent viscosity of slurry under different shear rates. It can

be seen that the apparent viscosity increases along with the

increase of solids loading. The apparent viscosity increases

from 30 to 340 mPa�s with the increase of solids load-

ing(38 %–50 %) under the shear rate of 344 s-1. The higher

the solids loading is, the less the dispersion of powder is in

the slurry. The solid particles move closer together to form

stronger acting force which would result in the reuniting of

powders in the slurry, then the apparent viscosity increases. It

can also be found from Fig. 3 that the apparent viscosity

decreases nearly along with the increase of shear rates under

a certain solids loading. Part of powders are separated from

the slurry under the action of the shear stress in a higher shear

rate, then the solids loading decreases relatively, so the vis-

cosity decreases.

In addition, to make the slurry more stable and homo-

geneous and improve the rheological properties of slurry,

usually, an appropriate dispersant is added into the slurry.

Figure 4 shows the effect of the content of ammonium

citrate on the apparent viscosity of NiTi slurry with solids

loading of 42 % under two shear rates. It can be found from

Fig. 4 that the viscosity of slurry decreases along with the

increase of dispersant with content of under 2 wt%. At this

stage, the ammonium citrate is dissolved into the slurry to

be absorbed on the surface of the powders. The powders in

the slurry would form a repulsive force which would

improve rheological properties, so the viscosity of slurry

decreases. However, the absorption capacity of powders

absorbing ammonium citrate would reaches a saturation

point with the increase of content of ammonium citrate.

And the concentration of free ion would increase along

with the further increase of ammonium citrate that would

result in the increase of viscosity. The content of 2 % is

chosed as the optimal content in this work.

3.2 Drying and degreasing of NiTi green body

There is a part of water existing in the NiTi wet green

body, so the drying is an essential step in the gel-casting

process. Most of the free water would remove from the

green body during the drying process. Figure 5 shows the

drying curve of NiTi wet green body under different solids

loading at 100 �C. It can be seen that the drying process is

divided mainly into three steps. In the first stage, the water

existing in the surface and endosexine of the green body is

Fig. 3 Effect of solids loading on viscosity of NiTi slurry under

different shear rates

Fig. 4 Effect of content of ammonium citrate (based on the weight of

the powder) on viscosity of 42 vol % NiTi slurry under two shear

rates

Fig. 5 Drying curves of NiTi wet green body under different solids

loading at 100 �C

Porous nickel–titanium alloy prepared by gel-casting

123Rare Met.

volatilised out. The drying rate appears a rapid increasing

tendency. In the second stage, the water in the inner of

green body is removed through pores existing in the green

body. The drying rate presents a steady growing tendency.

In the following stage, the residual water in the green body

is removed. The drying rate shows a slow increasing trend.

It can also be found obviously from Fig. 5 that mass loss

and drying rate increase with the decrease of solids load-

ing. The water content is higher in a lower solids loading,

then the moisture evaporation would increases with the

decrease of solids loading in the same amount of time, so

the mass loss and drying rate increase. And the green

bodies were dried for about 10 h at 100 �C to make sure

that most of water in the green bodies was removed in this

work. As shown in Fig. 6, the NiTi green body with

complex shape was prepared by gel-casting.

Similarly, some organics including the polyacrylamide

exist in the NiTi green body after the polymerisation and

drying. And the organics should be removed in the

degreasing process. In order to discuss the changes in green

body during the degreasing process, the NiTi green body

was taken into the vaccum oven for degreasing. The basic

degreasing process with heating rate of 3 �C�min-1 at

600 �C for 4 h in vacuum atmosphere was adopted in this

work. The organics are mainly removed during the

degreasing process. And the two kinds of powders are

bound together with each other only through mutual

accumulation in the NiTi green body. As can be seen from

Fig. 7b, two kinds of powders distribute evenly in the green

body after degreasing. Figure 7a shows the fracture mor-

phology of NiTi green body after the drying process.

Compared with Fig. 7b, it can be obviously found that

there are organics distributed evenly in the green body.

Besides, the organics existing in space between the pow-

ders can strengthen the bonding of powders. And the green

body after drying had a higher strength just because of the

organics existing in the green body.

3.3 Properties of porous NiTi sintered body

The dried green bodies were moved into the vacuum sin-

tering furnace for subsequent sintering. The theoretical

melting point of NiTi alloy is about 1,300 �C. Generally,

the alloying temperature is about 0.8 times of theoretical

melting point, so the sintering temperature of NiTi alloy

was set as 1,050 �C. And it was sintered for 4 h in this

work. Figure 8 shows the effect of solids loading on

compression strength and porosity rate of NiTi sintered

body. As can be seen from Fig. 8, the porosity rate shows a

decreasing trend with the increase of solids loading,

reduces from 49.50 % to 34.57 %. With the increase of

solids loading, the space between two powders in NiTi

slurry decreases relatively. The size of pore created by the

volatilising of organics in the body would be further

reduced due to the mutual diffusion between metal parti-

cles in the subsequent sintering. At the same time, the

number of pores decreases, so is the porosity rate. How-

ever, its compression strength increases from 47.43 to

364.74 MPa along with the increase of solids loading. WithFig. 6 NiTi green body with complex shape prepared by gel-casting

Fig. 7 SEM images of fracture morphology of NiTi green body: a before degreasing and b after degreasing

B.-H. Duan et al.

123 Rare Met.

the increase of solids loading, the porosity rate reduces.

The densification extent of sintered body intensifies, and

the compression strength increases.

Figure 9 shows the fracture morphology of NiTi sin-

tered body. It can be found from Fig. 9 that pores are

distributed evenly in the sintered body, but the size of pores

largely differs, which might be caused by nonuniform

diffusion of two powders during the alloying process.

Figure 10 shows the XRD pattern of NiTi sintered at

1,050 �C. It can be seen that Ni3Ti, Ni4Ti3 and NiTi appear

in the sintered sample. The two impurity phases of Ni3Ti

and Ni4Ti3 might be caused by the uneven mixing of metal

powders. The sedimentation of powders during the pre-

paring process of NiTi slurry and different diffusion rates

of metal atoms during the alloying process would result in

that the atomic ratio of Ni and Ti cannot achieve 1:1.

4 Conclusion

Apparent viscosity of NiTi slurry increases from 30 to

340 mPa�s-1 with the increase of solids loading (38 %–

50 %) under a certain shear rate, but decreases with the

increase of shear rates under a certain solids loading.

Adding an appropriate dispersant can reduce the apparent

viscosity obviously. The optimal addition content of dis-

persant is about 2 wt%. The drying rate and mass loss of

NiTi green bodies increase with the decrease of solids

loading. Most of water in the NiTi wet green body is

eliminated after drying at 100 �C for 10 h. And the

organics in the green body are almost removed after

degreasing at 600 �C for 4 h. The NiTi green body with

complex shape is prepared by gel-casting. The sintered

body with the main phase of NiTi was prepared after sin-

tering at 1,050 �C for 4 h. Its compression strength

increases from 47.42 to 364.74 MPa. And its porosity rate

decreases from 49.5 % to 34.57 % with the increases of

solids loading (38 %–48 %).

Acknowledgments This project was financially supported by the

National Natural Science Foundation of China (No. 51274246) and

the Project Supported by State Key Laboratory of Powder Metallurgy

of China (No. 26358766).

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