Ta system multilayers

Accepted Manuscript Mechanical properties study of W/TiN/Ta system multilayers Ying Zhang, Gaoyong Xu, Yan Wang, Cheng Zhang, Shuaijie Feng, Jinping Suo PII:

S0925-8388(17)32480-5

DOI:

10.1016/j.jallcom.2017.07.115

Reference:

JALCOM 42531

To appear in:

Journal of Alloys and Compounds

Received Date: 2 May 2017 Revised Date:

0925-8388 0925-8388

Accepted Date: 9 July 2017

Please cite this article as: Y. Zhang, G. Xu, Y. Wang, C. Zhang, S. Feng, J. Suo, Mechanical properties study of W/TiN/Ta system multilayers, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2017.07.115. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Mechanical properties study of W/TiN/Ta system multilayers Ying Zhanga, Gaoyong Xua, Yan Wanga, Cheng Zhanga, Shuaijie Fenga, Jinping Suoa*

State Key Laboratory of Mould Technology, Institute of Materials Science and

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a

Engineering, Huazhong University of Science and Technology, Wuhan 430074,

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China

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Key words: Layered toughening; W/TiN/Ta multilayers; mechanical properties; DBTT

*Corresponding author. Tel.: +86 027 87558055; fax: +86 027 87558055

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E-mail address: [email protected]

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ACCEPTED MANUSCRIPT Abstract

In the present work, W/TiN/Ta system multilayers are developed and compared with W/Ta multilayers through three point bending and tensile tests. W/TiN/Ta,

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especially W/TiN(Ti)/Ta multilayers have much higher strength than that of W/Ta multilayers due to the intrinsic high strength of TiN and the increased layered interfaces, while with less ductility. W/TiN/Ta and W/TiN(Ti)/Ta with tTa/W=1 also exhibit multiple cracks propagation characteristic as W/Ta, while the fracture

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characteristics are similar to that of W/Ta multilayers with smaller tTa/W due to lower interface strength. The plastic deformation and bridging effect of Ta layer are smaller;

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the interface cracking is much more intense; and more micro-cracks generated under load in these two multilayers. Their DBTT by tensile test is all about 300

, the

ductility of W/Ta is better than W/TiN/Ta and W/TiN(Ti)/Ta above 300

, and

reaches 37% at 400 400-700

, while the ductility of W/Ta decreases dramatically during

due to oxidation. At the whole temperature range, the ductility of , which is

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W/TiN/Ta and W/TiN(Ti)/Ta is close, and keeps highest during 400-600

about 12%. The interface of W/TiN/Ta system multilayers can be protected from

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serious oxidation by TiN coating at high temperature.

1. Introduction

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Tungsten is the most promising plasma facing material in nuclear reactor due to

its high melting point, high thermal conductivity, high sputtering threshold, low tritium retention and low corrosion rate, while the high brittleness restricted its application [1, 2]. Various methods such as alloying-strengthening [1], grain refining [3], particle dispersion strengthening [4], thermo-mechanical treatment [5] and fiber toughening [6], layered toughening [7] have been undertaken to improve its ductility. In our previous work, W/Ta multilayers were developed for plasma facing materials application. The effect of Ta/W thickness ratio on toughening effect of W/Ta multilayers was investigated and the toughening mechanisms were presented [8]. 2

ACCEPTED MANUSCRIPT Although the W/Ta multilayers have high strength and toughness, for the sake of long-term high temperature service, the interface of W layer and Ta layer needs to be treated to avoid inter-diffusion and solution between W and Ta, and the generation of brittle products. Thus on the basis of W/Ta multilayers, W/TiN/Ta multilayers have

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been developed with a TiN layer at the interface of W and Ta. TiN layer can avoid the solution of W, Ta and the generation of brittle products. On the other hand, the TiN layer at interface has the potential to achieve tritium permeation resistance in the inner of composite material.

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As many studies reported, the function of low-energy high beam of hydrogen isotopes can lead to the retention of hydrogen in tungsten and generate bubbles in

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tungsten, which will deteriorate the thermal and mechanical properties of tungsten [9, 10]. And the effect of neutron irradiation on deuterium tritium retention was more intense, which can generate irradiation defects in the bulk material of tungsten based PFMs and lead to the increased deuterium tritium retention [11, 12]. TiN coating is a common used and easily prepared tritium permeation resistance coating on the surface

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of structural material [13]. It has good tritium permeation resistance of which the tritium permeation resistance factor can be as high as 10000. Tong Zhou of our team has measured the tritium permeation resistances of TiN coating with various crystal

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orientations [14], and the TiN coating on one side and two sides of Ta slice by electrochemical hydrogen permeation test (Preparation and Research of Structure and

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Function Integrated Layered Tungsten Matrix Composites, a thesis submitted to Huazhong University of Science and Technology for the degree of master of engineering 2016). All the TiN coatings were deposited on Ta substrate by arc ion plating. When the hydrogen charging electric current density was 10mA/cm2, the steady-state hydrogen permeation current density of TiN coating with (200) preferred orientation on Ta substrate was 9.4uA/cm2. The steady-state hydrogen permeation current density of TiN coating with (111) preferred orientation was 4uA/cm2, which reflected that the hydrogen permeation resistance was better than that of (200). While the steady-state hydrogen permeation current density of the two-side coated TiN 3

ACCEPTED MANUSCRIPT coating with (111) preferred orientation on Ta substrate was as low as 0.053mA/cm2, which was much smaller than the corresponding one-side coated TiN coating. We can see that, the hydrogen permeation resistance didn’t just increase proportionally with increased TiN layer, instead, the hydrogen permeation resistance of two layered TiN

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was about 75 times higher than that of one layered TiN coating. The responsible mechanisms of the effect of preferred orientation and number of layers of TiN coating on hydrogen permeation resistance were given in his paper and not presented here.

So, for W/TiN/Ta multilayers, not only the compositing of high ductile Ta layer

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and high strength W can increase the strength and toughness of layered composite, but also TiN in the inner of layered composite has the possibility to achieve in-depth

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deuterium tritium permeation resistance. Due to the limitation of electrochemical hydrogen permeation test, the samples were eroded by electronic solution before H permeated out when H permeated through W/TiN/Ta composite with much better hydrogen permeation resistance than TiN/Ta layers. So gas deuterium permeation test must be carried out to assess the deuterium permeation resistance of W/TiN/Ta

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composite, which will be carried out in Japan in the future. In the present work, three point bending test and tensile test have been used to access the mechanical property of W/TiN/Ta multilayers. For comparison, W/Ta multilayers and W/TiN(Ti)/Ta

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multilayers with Ti layer on the surface of TiN coating will also be measured.

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2. Experimental procedure

Ta foil and W foil with thickness of 100 um were bought from BAOJI TST

NONFERROUS METAL CO., LTD. Ta was treated by cold rolling and vacuum annealing at 1000

for 1h and furnace cooled to room temperature. W was treated

by warm rolling and vacuum annealing at 900

for 1 h and furnace cooled. TiN

coating and Ti coating with thickness of 1um were both deposited by arc ion plating in Shenzhen allmerit Technology Co., Ltd. For W/TiN/Ta, TiN was deposited on two sides of Ta substrate; for W/TiN(Ti)/Ta, Ti was further deposited on two sides of TiN surface. 4

ACCEPTED MANUSCRIPT W/Ta, W/TiN/Ta and W/TiN(Ti)/Ta multilayers were prepared by alternately stacking W layer with Ta, Ta/TiN or Ta/TiN(Ti) respectively and sintered by spark plasma sintering with SINTER LAND LABOX-1575. The sintering was conducted at 1700

for 5min under 95 MPa. Three point bending tests were conducted using a

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Zwick/Roell Z020 universal testing machine with a cross head velocity of 0.3mm/min at room temperature. The sample size was about 18 (l) x 2.5 (w) x 2 (h) mm, the testing span was 14 mm. Tensile tests were conducted at room temperature, 200 300

, 400

, 500

, 600

, 700

,

in the atmosphere. Sketch of samples was shown

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in Fig.1. Cracks distributions on the side face of three point bending samples and fracture surface of bending and tensile samples were both observed by FEI Quanta

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200 Scanning Electron Microscope (ESEM).

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Fig.1 Schematic diagram of tensile sample and the thickness is 1.4mm

3. Results and discussion

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3.1 Three point bending test

The stress–strain curves of three kinds of multilayers by three-point bending test

are presented in Fig.2. The bending strength is calculated using the equation:σ =3PL/(2tw2), and the strain is determined from the expression:ε=100·6wδ/L2 [15]. P is the load, L is the span, t is the specimen thickness, w is the specimen width, and δis the deflection. The strength of W/TiN/Ta is 1200MPa, which is about two times of that of W/Ta multilayers. The high strength is due to the intrinsic high strength of TiN coating, and the increased layered interfaces which affects the strength by dislocation pile up effect. Accordingly, the ductile of multilayers decreases due to the 5

ACCEPTED MANUSCRIPT existence of TiN, but is still acceptable which exhibits layered fracture characteristic. For W/TiN(Ti)/Ta multilayers, the Ti coating on TiN surface further increases the strength of layered composite, which reaches 1350MPa. The Ti layer has little effect on the ductility of W/TiN/Ta multilayers seen from the stress-strain curves. Fig.3

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shows the crack distributions of three point bending samples of three multilayers. The thickness ratio of Ta/W (tTa/W) is 1.0 in all multilayers. In our previous study, we have investigated the crack distributions of W/Ta multilayers with various thickness ratios [8]. And we concluded that, when tTa/W <0.62, the W/Ta multilayers exhibited 0.62, W/Ta multilayers exhibited

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macroscopic crack propagation, and when tTa/W

multiple cracks propagation. Formation of multiple cracks distributed damage and

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remarkably enhanced the toughness. Similar to W/Ta multilayers, W/TiN/Ta and W/TiN(Ti)/Ta with tTa/W=1 also exhibit multiple cracks propagation characteristic, while with some differences. The differences mainly exist in the following three aspects. (1) The plastic deformation and bridging effect of Ta layer which are typical characteristics of W/Ta are not obvious in W/TiN/Ta and W/TiN(Ti)/Ta, so the

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overall plastic deformation of these multilayers is small. (2) The interface cracking is much more intense than W/Ta, there exists several long interface cracks, while in W/Ta multilayers, only short interface cracks generated in the fracture area due to

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bridging of Ta and the connection of cracks in different W layers. (3) Micro-cracks in these two multilayers, especially in W/TiN(Ti)/Ta multilayers are much more than

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that in W/Ta. These phenomena all indicate that W/TiN/Ta and W/TiN(Ti)/Ta are more harder and brittle than W/Ta. The introduction of TiN layer and TiN(Ti) with Ti transition layer make the interface of W/TiN/Ta layered system more weaker than W/Ta system, thus the tendency of interface cracking increases under load. And the brittle W layer directly adjoins to the brittle TiN layer, while not to the ductile Ta layer (the structure was

W/TiN/Ta/TiN/W

), thus the release of stress in W layer

and at interface by plastic deformation of Ta has been weakened. The stress in W layer and at interface leads to a lot of micro-cracks in W layer and the formation of interface cracking. 6

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Fig.2 Stress-strain curves of three point bending of W based layered composites

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Fig.3 Cracks distribution of (a) W/Ta, (b) W/TiN/Ta and (c) W/TiN(Ti)/Ta multilayers by three point bending test

The fracture surface morphologies of W/Ta, W/TiN/Ta and W/TiN(Ti)/Ta are

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shown in fig.4-6 respectively. The fracture of W layers in three multilayers is all cleavage fracture. As we have researched in our previous study, for W/Ta multilayers,

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the cracks propagated in Ta layers followed by plastic deformation of Ta till fracture, leaving a lot of dimples as shown in fig.4 (b) and fig.4 (c) [8]. The fracture characteristic of W/TiN/Ta and W/TiN(Ti)/Ta are similar, while compared with W/Ta multilayers, the plastic deformation of Ta is smaller, and the interface cracking are more serious due to low interface strength. In some cases like fig.5 (b) and fig.6 (b), for W/TiN/Ta and W/TiN(Ti)/Ta, similar to W/Ta multilayers, cracking propagation happens in Ta layer followed by plastic fracture of Ta, but the dimples is relatively shallow. In more cases, bidirectional crack propagation in Ta like fig.5 (c) and fig.6 (c) and unidirectional crack propagation in Ta which can be largely found in fig.5 (a) and 7

ACCEPTED MANUSCRIPT fig.6 (a) happen till the fracture of Ta layer. The three situations that unidirectional crack propagation in Ta till fracture, bidirectional crack propagation in Ta till fracture, and firstly crack propagation followed by plastic fracture of Ta have been illustrated in our previous work [8]. When tTa/W is small, i.e. Ta layer is relatively thin,

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bidirectional crack propagation is more likely to happen in Ta layer; as tTa/W increases, the tendency of unidirectional crack propagation increases; as tTa/W continuously increases, like 1:1, firstly unidirectional crack propagation followed by plastic fracture of Ta will happen. While for W/TiN/Ta and W/TiN(Ti)/Ta multilayers, although tTa/W

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=1:1, these two multilayers exhibit similar fracture characteristic as W/Ta multilayers with smaller tTa/W than 1:1 due to lower interface strength. These indicate that, except

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mechanism of multilayers.

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the thickness ratio, interface strength can also have an important effect on fracture

Fig.4 Fracture surface morphologies of W/Ta multilayers by three point bending test

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(a) macroscopic fracture morphology, (b-c) amplified fracture morphologies

Fig.5 Fracture surface morphologies of W/TiN/Ta multilayers by three point bending test (a) macroscopic fracture morphology, (b-c) amplified fracture morphologies

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Fig.6 Fracture surface morphologies of W/TiN(Ti)/Ta multilayers by three point bending test (a) macroscopic fracture morphology, (b-c) amplified fracture morphologies

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3.2 Tensile test

The tensile curves of three multilayers under test temperature from 25

to 700

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is shown in fig.7-9 respectively, and their corresponding maximum strength, ductility, integration calculated by origin software are shown in table 1-3 respectively. All the

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tensile tests were done in the atmosphere.

Fig.7 Tensile curves of W/Ta multilayers at various test temperature

Table 1 Maximum strength, ductility and integration of W/Ta multilayers at

Temperature/

various test temperature 25

200

300

400

500

600

700

323

334

376

417

403

363

285

4

5

10

37

29

25

22.5

Maximum strength /MPa Ductility /%

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650

2701

12509

8916

5925

5268

MPa*% For W/Ta multilayers, the DBTT is about 300 excellent ductility at 400

. The multilayers exhibit

which is about 37%, the strength also reaches maximum

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as 417MPa. The strength and ductility gradually decrease with temperature increasing, but their ductility is still good which are all above 20%. The integrations of multilayers at various temperatures reflect that the toughness is the highest at 400 then at 500

and 600

in turn. Usually, for bulk materials, the strength decreases

strength and ductility all increase before 400

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and ductility increases with temperature increasing. For W/Ta multilayers, the . The increased strength maybe due to

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the increased interface strength as a part of defects can be healed at the heating and heat preservation processes. Above 400

, the strength and ductility all decrease due

to oxidation at high test temperature. If the tensile tests are done under vacuum or protective atmosphere, the ductility of multilayers is suspected to increase with

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temperature increasing.

Fig.8 Tensile curves of W/TiN/Ta multilayers at various test temperature

Table 2 Maximum strength, ductility and integration of W/TiN/Ta multilayers at various test temperature Temperature/

25

200

300

400

500

600

700

521

612

565

548

503

472

391

Maximum strength /MPa 10

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4

5

7

11

11

11.5

8

912

1232

2447

4111

3665

3855

1952

Integration /

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MPa*%

Fig.9 Tensile curves of W/TiN(Ti)/Ta multilayers at various test temperature Table 3 Maximum strength, ductility and integration of W/TiN(Ti)/Ta multilayers at various test temperature 25

Maximum strength /MPa

300

400

500

600

700

551

635

614

587

579

540

445

4.5

4.5

8

12

14

12.5

10.5

1584

2988

5177

5921

4184

3197

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Ductility /%

200

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Temperature/

Integration /

995

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MPa*%

For W/TiN/Ta and W/TiN(Ti)/Ta multilayers their DBTT are also at 300

strength reach maximum at 200 during 300

-600

. Their

and decrease slightly with temperature increasing

. Their ductility changes little during 400

-600

, and the

strength and ductility of W/TiN(Ti)/Ta are both higher than those of W/TiN/Ta. At 700

, the strength and ductility of these two multilayers all decrease further, while

still acceptable.

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Fig.10 Tensile fracture surfaces of three multilayers, (a), (c), (e) is the fracture surface of W/Ta, W/TiN/Ta, W/TiN(Ti)/Ta respectively at 200

; (b), (d), (f) is the

fracture surface of them respectively at 400 The fracture surface features of three multilayers at 200

, 400

and 700

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shown in fig.10-12 respectively. From the tensile curves we can see that, at 200

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the multilayers are hard and brittle; at 400

is , all

, the ductility of three multilayers are all

near maximum and their strength are also high; at 700

, the strength and ductility of

three multilayers all deteriorate seriously. As shown in fig.10 (a), (c), (e), Ta layer in three multilayers all exhibit some ductility at 200

while not much as having no

much dimples. And W layer in three multilayers all exhibit cleavage fracture characteristics with flat surfaces. Fig.10 (b), (d), (f) is their fracture surface at 400 respectively. We can see that, a lot of dimples have appeared in Ta layers which reflect intense ductile fracture characteristic. And the ductility of W layers in three multilayers are also improved with fluctuate fracture surfaces. 12

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Fig.11 Tensile fracture surfaces of three multilayers at 700

and their

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corresponding surface scanning analyses , (a), (c), (e) is the fracture surface of W/Ta, W/TiN/Ta, W/TiN(Ti)/Ta respectively at 700

; (b), (d), (f) are their corresponding

surface scanning analyses

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Fig.11 (a), (c), (e) shows the fracture surface of W/Ta, W/TiN/Ta and W/TiN(Ti)/Ta multilayers at 700

respectively and fig.11 (b), (d), (f) is the

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corresponding fracture surface scanning analysis. Their fracture surfaces are seriously oxidized, O atom is nearly full of the whole scanning surface, while the O at interface of W/TiN/Ta and W/TiN(Ti)/Ta is much less than that of W/Ta which indicates the oxidation is slighter. Oxidation causes the property deterioration of W/Ta multilayers. The oxidation condition at 400

has also been analyzed as shown in fig.12. We

can see that the O content of the three multilayers on the fracture surfaces are both low which indicates that these multilayers have not been oxidized. Above 400 oxidation has happened and become serious at 700

, which makes the strength and

ductility of these three multilayers seriously deteriorated. 13

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Fig.12 Tensile fracture surfaces of three multilayers at 400

and their

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corresponding surface scanning analyses , (a), (c), (e) is the fracture surface of W/Ta, W/TiN/Ta, W/TiN(Ti)/Ta respectively at 400

; (b), (d), (f) are their corresponding

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surface scanning analyses

Fig13 Strength, ductility and integration comparisons of three multilayers at various test temperature 14

ACCEPTED MANUSCRIPT Fig.13 is comparison and summarize of the maximum strength, ductility and integration of three multilayers at various test temperature. The tensile strength of W/Ta, W/TiN/Ta, W/TiN(Ti)/Ta increases successively which is in accordance with the three point bending test results. The introduction of TiN with high strength and the

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increased layered interfaces contributed to higher strength of W/TiN/Ta than that of W/Ta. For W/TiN(Ti)/Ta, more layered interfaces by the introduction of Ti transition layer makes the strength of composite increase further. The ductility of three ; and the ductility of W/Ta is better than W/TiN/Ta

and W/TiN(Ti)/Ta above 300

. At the whole temperature range, the ductility of

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multilayers is close below 300

W/TiN/Ta and W/TiN(Ti)/Ta is close and keep highest during 400-600

, which is

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about 12%. While the ductility of W/Ta decreases dramatically during 400-700

.

This is because that the oxidation degree at interface of W/Ta is more serious than the other two composites of which the interface is protected by TiN coating. Fig.13 (c) is the integration of three multilayers which reflects the toughness. For W/Ta, the maximum toughness is at 400

4. Conclusion

with W/TiN(Ti)/Ta a little higher than W/TiN/Ta due to higher strength.

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at 400-600

; for W/TiN/Ta and W/TiN(Ti)/Ta, toughness is close

In the present work, W/TiN/Ta system multilayers are developed and compared

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with W/Ta multilayers through three point bending and tensile tests. W/TiN/Ta, especially W/TiN(Ti)/Ta multilayers have much higher strength than W/Ta multilayers

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due to the intrinsic high strength of TiN and the increased layered interfaces, and their ductility decreases accordingly. The interface property has a great effect on the fracture characteristics of multilayers. Hard TiN coating which separated the hard W and soft Ta layer makes the multilayers more hard-brittle. So the plastic deformation and bridging effect of Ta layer are smaller; the interface cracking and micro-cracks are much more in these two multilayers under load. But their DBTT by tensile test is all about 300

. Although the ductility of W/TiN/Ta and W/TiN(Ti)/Ta W/Ta is

smaller than W/Ta above 300

, the ductility of W/TiN/Ta and W/TiN(Ti)/Ta is more

stable at the whole temperature range and is still as high as about 12% during 15

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. The interface of W/TiN/Ta system multilayers can be protected from

serious oxidation by TiN coating at high temperature. Acknowledgements This work has been supported by Natural Science Foundation of China Program

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(51571095). We really appreciate the spark plasma sintering, the mechanical properties tests and SEM characterizations provided by State Key Laboratory of Materials Processing and Die&Mould Technology of HUST and Analytical and

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Testing Center of HUST respectively.

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ACCEPTED MANUSCRIPT Highlights: (1) W/TiN/Ta system multilayers have high strength and ductility (2) Comparisons were made between W/TiN/Ta and W/Ta (3) Layered interface has a great effect on mechanical property

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(4) DBTT of W/Ta and W/TiN/Ta, W/TiN(Ti)/Ta is about 300℃