the effect of boron and carbon additions on the ni-19si-3nb-based

Dependence of Strain Rate and Environment on the Mechanical Properties of the

Ni-19Si-3Nb-1Cr-0.2B Intermetallic Alloy at High Temperature

C. C. Fu, C. M. Chung, L. J. Chang, C. F. Chang, and J. S. C. Jang

Department of Materials & Engineering, I-Shou University, Kaohsiung, Taiwan, R.O.C.

ABSTRACT

The dependence of strain rate and environment on the high temperature mechanical

behavior for the Ni-18Si-2Nb-1Cr-0.2B alloy was investigated by atmosphere-controlled

tensile test in various conditions (with different strain rate at different temperature under

vacuum, air, or water vapor atmosphere). The results revealed that the addition of boron and

chromium result in improving the elongation as well as ultimate tensile strength (UTS) of the

Ni-19Si-3Nb based alloys over a wide range of temperature under air and water vapor

atmosphere. The UTS and elongation can reach to 1270 MPa and 14%, respectively at 873K

in each kind of atmosphere. On contrary, the alloy without boron addition only presents

ductile mechanical behavior in vacuum. This is evident that boron and Cr elements present

positive effect on suppressing the environmental embrittlement in air and water vapor

atmosphere over a wide range of temperatures (room temperature to 1073 K) for the

Ni-19Si-3Nb base alloy. In addition, both of ultimate tensile strength and elongation present

quite insensitive on the strain rate when test at the temperature below 973 K. However, the

ultimate tensile strength exhibits very dependent on the strain rate when test in air at the

temperature above 973 K, decreasing the ultimate tensile strength with decreasing strain rate.

Keywords: environmental embrittlement, intermetallics, mechanical behavior, strain rate

dependence, and nickel silicide

Corresponding author: J. S. C. Jang, e-mail: [email protected]

Jason S. C. Jang et al. 1

INTRODUCTION

Because Ni3Si intermetallics exhibits excellent corrosion resistance over a wide range

and an increasing yield strength with temperature [1~3], therefore the Ni3Si-based alloy has

been considered to be a candidate as high-temperature structural material (especially chemical

parts). However, binary Ni3Si intermetallic compound is analogous to Ni3Al [4] suffering

from environmental embrittlement at room temperature, which is exacerbated by the presence

of hydrogen. The room temperature ductility of the Ni3Si-based alloys was proposed can be

improved by means of increasing the nickel content and adding titanium element [5-7],

adding niobium element [8-10]. In addition, to further improve the mechanical and related

properties of the Ni3Si based alloy, microalloying with small amount of interstitial elements

such as boron, carbon, and beryllium has been successfully applied to improve the

room-temperature ductility for these L12 structured alloys [11-17]. At the same time, these

Ni3Si-based alloys also reported to suffer from environmental embrittlement at elevated

temperatures [6,11-13,18-23]. However, the result of our previous study revealed that the high

temperature ductility of Ni-19Si-3Nb-0.2B alloy can be significantly increased by adding 1

at% chromium element [24]. Therefore, the present study is developed to further understand

the dependence of strain rate on the mechanical behavior of the Ni-19Si-3Nb-1Cr-0.2B alloy

over a range of temperatures in different environments including air, vacuum and pure water

vapor.

EXPERIMENTAL

The Ni3Si based alloys, with compositions of Ni–19%Si–3%Nb (in at%, corresponding

to Ni-9.9%Si-5.17%Nb in wt%) and Ni–19%Si–3%Nb-1%Cr-0.2%B (in at%, corresponding

to Ni-9.9%Si-5.17%Nb-1.1%Cr-0.04%B in wt%), were prepared by arc melting the

appropriate amounts of carbonyl nickel, electronic grade silicon, 99.9% pure niobium, and


99.9% pure silicon carbide before drop casting into a cold copper mold. The composition-loss

ratio of elements was less than 0.02 wt%, as determined by glowing discharge spectrum

analysis; hence, the weight loss of the alloy during melting can be neglected. These ingots

were annealed at 1080℃ for 24 hours and aged at 700℃ for 10 hours in a vacuum of 5 x

10-5 torr, and then machined into tensile specimens with a gauge section of 3 mm in width, 2

mm in thickness and 20 mm in length by electro-discharge machining. To prevent specimen

surface damage, the specimen was carefully polished to reach a roughness of 25 μm. Tensile

testing was performed at various temperatures ranging from room temperature to 800℃ at

different constant crosshead speeds with initial strain rates from 8 x 10-5 to 2 x 10-2 s-1. All of

these tests were designed to be conducted in different environments including air (with a

humidity level of 60% RH), vacuum (with an environmental pressure less than 2 x 10-4 torr),

and water vapor (with the water content about 850 ppm). For tests performed in water vapor, a

vacuum (less than 2 x 10-4 torr) was achieved first in the test chamber prior to backfilling with

distilled water.

The microstructure for each alloy and the fractured surface after testing were examined

using a Hitachi scanning electron microscope (SEM), model S-2700, operating at 15 kV with

energy dispersive spectrometry capability. In addition, samples for observation of the polished

surface were metallographically prepared and etched with Marble's etching solution (HCl(aq)

50 ml + CuSO4(s) 10 grams + H2O(l) 50 ml).

RESULTS and DISCUSSION

Microstructures

The SEM image in Figure 1 shows the heat-treated microstructure of

Ni-19Si-3Nb-1Cr-0.2B alloy, which consists of the dendritic β phase (L12 Ni3Si), the α (fcc

solid solution)- β eutectic, and many equiaxed precipitates and the round-shaped Ni5Si2 phase


existing in the β matrix. Those plate-like precipitates change to small equiaxed precipitates

for the Ni-19Si-3Nb-1Cr-0.2B alloy, as shown in Figure 1(c).

Figure 2 shows the TEM image of the Ni-19Si-3Nb-1Cr-0.2B alloy in this study. The

image of this alloy includes β phase (L12 Ni3Si) and the cubic Nb3Ni2Si phase. The calculated

lattice parameters, a0, of the β phase for this alloy is 0.3527 nm almost the same as that

without Cr addition, which indicates that the chromium element does not affect the lattice

structure of the β phase.

Effects of temperature on the tensile properties at constant strain rate

The yield stress (defined at 0.2%-offset strain) of the Ni-19Si-3Nb base alloys with and

without boron and chromium additions as a function of testing temperature are shown as

Figure 3. The yield stress of the Ni-19Si-3Nb-1Cr-0.2B alloy is higher than the Ni-19Si-3Nb

base alloy. Both curves show that the yield stress increases with temperature starting from

room temperature at first, then reaches a maximum at 873K and finally decreases rapidly as

further increase in temperature. In addition, the yield stress was enhanced by the addition of

boron and chromium over a wide range of temperature.

The tensile elongation of the Ni-19Si-3Nb alloy and the Ni-19Si-3Nb-1Cr-0.2B alloy as

a function of temperature under different environments with a strain rate of 2 x 10-4 s-1 is

shown in Figure 4(a). When tested in vacuum, both Ni-19Si-3Nb alloy and

Ni-19Si-3Nb-0.15B-0.1C alloy exhibited identical high elongation values (i.e. ε > 12%) at

room temperature. When tensile tested in air and water vapor atmosphere, the tensile

elongation of the unalloyed Ni-19Si-3Nb exhibited relatively low values from room

temperature (~ 5 %) to 1073K (< 2%). With additions of boron and chromium, the tensile

elongation of Ni-19Si-3Nb-1Cr-0.2B alloy was significantly improved over a wide range of

temperature both in vacuum, air and 850 ppm water vapor atmospheres, the tensile elongation

only decreases slightly along with temperature up to 973K, and then increases steeply with


temperature up to 1073K. This result is analogous to those reported by Jang et al [24]; the

addition of chromium has a positive effect on suppressing the environment embrittlement at

high temperature for the Ni-19Si-3Nb-0.15B based alloy.

The ultimate tensile strength (UTS) of the Ni-19Si-3Nb alloy and Ni-19Si-3Nb-1Cr-0.2B

alloy as a function of temperature is shown in Figure 4(b). Both alloys attain high values of

UTS (i.e. > 1200 MPa) at room temperature when tested in vacuum. In addition, these two

alloys sustain high value of tensile strength in vacuum with temperature up to 873 K, and then

decreases with temperature from 873 K to 1073 K. When tested in air and 850 ppm water

vapor, the Ni-19Si-3Nb-1Cr-0.2B alloy presents higher UTS values than the Ni-19Si-3Nb

alloy over the entire temperature range. The result of UTS appears to be well correlated with

those results of tensile elongation.

Strain rate dependence on the tensile properties of Cr and B doped alloy

Ultimate tensile strength as a function of temperature for the Ni-19Si-3Nb-1Cr-0.2B

alloy tested in different atmosphere with different strain rate is shown in Figure 5. Both of

ultimate tensile strength and elongation curves exhibits a trend of sustaining relatively high

values along the testing temperature up to 873 K, then drop significantly with temperature

from 873 K to 1073 K. In addition, both of ultimate tensile strength and elongation present

quite insensitive on the strain rate when test at the temperature below 973 K, as shown in

Figure 5(a) and (b). However, the ultimate tensile strength exhibits very dependent on the

strain rate only when test in air at the temperature of 1073 K, decreasing the ultimate tensile

strength with decreasing strain rate, as shown in Figure 5(c).

Strain rate dependence on fracture behavior of Cr and B doped alloy

The variation of fractographic patterns observed on the Ni-19Si-3Nb-1Cr-0.2B alloy

fractured in vacuum and in air with different strain rate (8 x10-5 and 2 x 10-2 s-1) at 973 K are


shown in Figure 6. Both fracture surfaces of Ni-19Si-3Nb-1Cr-0.2B alloy at different strain

rate exhibit the similar image, the intergranular cracking mixed with partial dimpled fracture.

These results appear to be well correlated with those results of tensile elongation tested at

973K. When increasing the temperature to 1073 K, both fracture surfaces tested at 1073K in

vacuum or in air atmosphere with different strain rate (8 x10-5 and 2 x 10-2 s-1) presents a

typical ductile fracture surface, fully fine dimpled fracture pattern, as shown in Figure 7. This

is an evidence to explain the dramatically increasing in tensile elongation at 1073 K for the

Ni-19Si-3Nb-1Cr-0.2B alloy.

The environmental effect at low temperatures was suggested to be associated with

hydrogen-induced embrittlement [25-27]. While at high temperature, the environmental effect

is related with oxygen-induced embrittlement [8,28]. Therefore, the effect of boron and

chromium addition on the suppression of environmental embrittlement could be due to the

interaction of hydrogen-boron and hydrogen-chromium, and the interaction between

oxygen-boron and oxygen-chromium in the corresponding temperature regime.

The undoped Ni-19Si-3Nb alloy, analogous to a number of L12-type compounds [29-32],

actually suffered from environmental (i.e. air or 850 ppm water vapor) embrittlement. It is

recognized that environmental embrittlement in materials resulting in the fracture of materials

involves kinetic process, such as decomposition, permeation and condensation of hydrogen

species and bond breaking processes [33-35]. However, the results observed in the boron and

chromium doped Ni-19Si-3Nb alloy clearly presented that the suppression of environmental

embrittlement in air and water vapor atmosphere over a wide temperature regime (room

temperature to 1073 K), as shown in Figure 4(a). The similar beneficial effect of chromium

has been also reported for Ni3Al(B,Cr)-based alloy[36] and Ni3Si based alloys [20]. Li and

Liu[36] proposed that the addition of chromium produces a kinetic effect on increasing the

activation energy for oxygen penetrating into Ni3Al. In addition, the increase in elongation at

high temperature was proposed to be attributed to the dynamic recrystallization that takes


place preferentially around the dispersion phase or grain boundaries [10,24,37]. Those new

grains nucleated at high temperature reduce the stress concentration at or near grain

boundaries (or dispersion phase) and so as to enhance the ductility.

CONCLUSION

The tensile properties of Ni-19Si-3Nb alloy and Ni-19Si-3Nb-1Cr-0.2B alloy were

evaluated over a wide range of temperatures, different testing environments, and different

strain rate. The results of this study are summarized below:

(1) The addition of B and Cr results in improving the tensile elongation as well as ultimate

tensile strength of the Ni-19Si-3Nb based alloy over a wide range of temperature (room

temperature to 1073 K) under air and water vapor atmosphere. The UTS and elongation

can reach to 1270 MPa and 14%, respectively at 873K in each kind of atmosphere. On

contrary, the alloy without boron addition only exhibits ductile mechanical behavior in

vacuum. This is evident that boron and Cr elements have positive effect on suppressing

the environmental embrittlement in air and water vapor atmosphere over a wide range of

temperatures (room temperature to 1073 K) for the Ni-19Si-3Nb base alloy.

(2) Both of ultimate tensile strength and elongation present quite insensitive on the strain rate

when test at the temperature below 973 K. However, the ultimate tensile strength exhibits

very dependent on the strain rate when test in air at the temperature above 973 K,

decreasing the ultimate tensile strength with decreasing strain rate.

(3) The fact of increasing in elongation at high temperature (1073K) is suggested to be

attributed to the dynamic recrystallization that occurs preferentially around the dispersion

phase or grain boundaries and so as to enhance the ductility by reducing the stress

concentration at or near grain boundaries.


ACKNOWLEDGEMENT

The authors would like to gratefully acknowledge the sponsorship from the National

Science Council of ROC under the project NSC93-2745-E-214-001.

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Figure captions


(a)

(b)

Fig. 1. The SEM images of the Ni-18Si-2Nb-1Cr-0.2B alloy; (a) as-cast and (b) after heat

treatment

(a)


Matrix (Ni3Si) Precipitate (Nb3Ni2Si)

500nm

(a)

(b) (c)

Fig.2 TEM image of Ni-18Si-2Nb-1Cr-0.2B alloy (a) bright field, (b) SAD with zone

axis (011) of matrix and (c) SAD with zone axis (011) of precipitate.


200 400 600 800 1000 12000

300

600

900

1200

Yie

ld st

ress

(MPa

)

Temperature ( K )

Ni-18Si-2Nb-1Cr-0.2B Ni-19Si-3Nb

Fig. 3 Yield stress as a function of temperature for the Ni-19Si-3Nb alloy (with mark of □)

and Ni-19Si-3Nb-1Cr-0.2B alloy (with mark of ■) tested in vacuum with the strain rate

of 2 x 10-4 s-1.


200 400 600 800 1000 12000

4

8

12

16

20

24

(a)

Elo

ngat

ion

(%)

Temperature ( K )

B,Cr doped, Air B,Cr doped, water vapor B,Cr doped, vaccum undoped, Air undoped, water vapor undoped, vaccum

200 400 600 800 1000 12000

300

600

900

1200

1500

(b)

UTS

(MPa

)

Temperature (K)

B,Cr doped, Air B,Cr doped, water vapor B,Cr doped, vaccum undoped, Air undoped, water vapor undoped, vaccum

Fig. 4. (a)Elongation and (b)Ultimate tensile strength and as a function of temperature for the Ni-19Si-3Nb alloy (with mark of □, ○,and△) and Ni-19Si-3Nb-1Cr-0.2B alloy (with mark of ■, ●,and ▲) tested in different atmosphere with the strain rate of 2 x 10-4 s-1.


0

200

400

600

800

1000

1200

1400

1600

1800

2x10-22x10-32x10-4

UT

S (M

Pa)

Strain rate ( s-1)

UTS in vaccum UTS in air

elongation in vaccum elongation in air

8x10-5 0

5

10

15

20

25

30

(a)

Tested at 873K

Elo

ngat

ion

(%)

0

200

400

600

800

1000

1200

1400

1600

1800

Strain rate ( s-1)

UT

S (M

Pa)

2x10-22x10-32x10-48x10-5



0

5

10

15

20

25

30

(b)

Tested at 973K

Elo

ngat

ion

(%)

0

200

400

600

800

1000

1200

1400

1600

1800

Strain rate ( s-1)

UT

S (M

Pa)

2x10-22x10-32x10-48x10-5



0

5

10

15

20

25

30

(c)

Tested at 1073K

Elo

ngat

ion

(%)

Fig. 5. Ultimate tensile strength and elongation as a function of strain rate for the

Ni-19Si-3Nb-1Cr-0.2B alloy tested in air and vacuum at (a)873 K, (b)973 K, and (c) 1073 K, respectively.


(a) 973K vacuum, 8 x 10-5 s-1 (b) 973K vacuum, 2 x 10-2 s-1

(c) 973K air, 8 x 10-5 s-1 (d) 973K air, 2 x 10-2 s-1

Fig. 6. SEM images of fracture surface for Ni-19Si-3Nb-1Cr-0.2B alloy tested in vacuum

at 973 K with different strain rate



(a) 1073K vacuum, 8 x 10-5 s-1

(d) 1073K air, 8 x 10-5 s-1 (c) 1073K air, 8 x 10-5 s-1

(b) 1073K vacuum, 2 x 10-2 s-1

Fig. 7. SEM images of the fracture surfaces for Ni-19Si-3Nb-1Cr-0.2B alloy tested at 1073

K with different strain rate

the effect of boron and carbon additions on the ni-19si-3nb-based

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