Structure and phase composition of tungsten alloys modified by compression plasma flows and high-intense pulsed ion beam impacts

doi:10.1016/j.apsusc.2019.06.113

Applied Surface Science

Volume 491, 15 October 2019, Pages 43-52

https://doi.org/10.1016/j.apsusc.2019.06.113 Get rights and content

Highlights

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The tungsten surface was modified by compression plasma flows and pulsed ion beams.
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W-Ti alloys were formed by compression plasma flows impact.
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W alloying with Ti prevent from the carbide phase formation after pulsed ion beams treatment.

Abstract

The results of structure and phase composition change in tungsten after compression plasma flow and high-intensive pulsed ion beams impacts are discussed. The compression plasma flows with the absorbed energy in range 35–70 J/cm² were used for the sub-surface modification. The preliminary Ti coating deposition on the tungsten surface allowed us to form W-Ti alloys by plasma flows treatment. The layer of the W-Ti alloy consists of solid solutions W(Ti) and β-Ti(W) as well as nitride phase (Ti,W)N. The high-intensive pulsed ion beam impact provides the tungsten carbide W₂C formation in the sub-surface layer. When forming the W-Ti alloy the carbon ions implanted into the tungsten take part in the carbo-nitride (Ti,W)(N,C) formation without brittle W₂C phase.

Introduction

Tungsten is considered as a main plasma facing material for ITER and DEMO because of its low sputtering yield, high melting point and high thermal conductivity [[1], [2], [3]]. During the operation of a fusion reactor the tungsten-contained parts will be subjected to high thermal influences and mechanical loads during the plasma disruptions and the plasma direct interaction with the first-wall material. Besides, the tungsten will be irradiated with neutrons or different ions, like helium, deuterium, tritium, that are produced in the thermonuclear reactions. A lot of radiation effects like blistering, flecking or bubbles formation take place during the irradiation [[4], [5], [6]]. As tungsten possesses a low temperature of brittle-to-ductile transition, the surface will be cracked and eroded that will promote to the plasma contamination with the eroded products. The tungsten atoms transport to the plasma and retard the reaction [7]. So, it is necessary to minimize the high-Z elements ejection into the plasma for the reaction stabilization. The formation of the tungsten-based alloys with additional elements can be considered as a promising approach for this purpose. The addition of other elements into the tungsten matrix changes the elastic and ductile properties influencing on the crack formation and surface erosion processes. However, to produce a tungsten-based alloy by any tradition methods connected to heating, melting and casting is a rather difficult problem because of the high melting point of tungsten. The powder technologies are unsuitable for the first-wall materials production.

In the present work the problem of tungsten-based alloys synthesis is solved by mean of high-energy compression plasma flow application. Such plasma flows with a high directly speed, high energy and pulse duration about 100 μs are generated by quasi-stationary plasma accelerators. Directed dense plasma flows with small divergence and high energy (up to several tens of J/cm²) allows one to modify a layer with a thickness of tens of micrometers in an extremely short time 100 μs. The previous results showed the possibility to use the compression plasma flow (CPF) impact for alloying of the sub-surface layers of materials with other metals deposited as a thin coating on the surface [[8], [9], [10]]. The energy of the plasma flow is enough for melting of both the coating and a part of the substrate. The melted state of two (or more) metals is mixed in hydrodynamic mode and fixed after solidification.

The main purpose of the work is to produce the tungsten-titanium alloy by CPF impact on the bi-layered Ti/W system. Ti was used as an alloying material because of decreasing in Young's modulus in W-Ti alloys comparable to that of pure tungsten. Moreover, the ductility of the W phase is reported to be improved through Ti alloying and metallic bonding [11]. As the main aim of producing the tungsten-based alloy is to improve its mechanical properties under the ion irradiation, the obtained alloys were testing on the structure and phase composition changes after high-intense pulsed ion beams influences. This type of the irradiation combines the ion irradiation with high thermal and mechanical loads that brings it closer to the conditions in the fusion reactors.

Section snippets

Experimental

The plates of pure tungsten with dimensions of 10 × 10 mm and thickness 2 mm were used as experimental samples. After the ultrasonic cleaning in the acid media the tungsten plates were subjected to the compression plasma flows (CPF) impact in the magnetoplasma compressor of compact geometry. The compression plasma flow is produced by a special device presented in the Fig. 1. The experiments were performed in a “residual gas” mode in which a pre-evacuated vacuum chamber was filled with a working

CPF treatment of pure tungsten

When interacting with the tungsten surface, a part of the plasma flow energy transfers to heat and provides increase in the substrate temperature. It is heat and cooling of the tungsten surface layer that results in structure changes after plasma flow influence. The classical heat transfer equation was solved for to describe the surface temperature evolution during the plasma impact: $\frac{\partial T (x, t)}{\partial t} = \frac{cρ}{κ} \frac{\partial^{2} T (x, t)}{\partial x^{2}},$ where c is the specific heat capacity of tungsten (1.40·10³ J/kg·K), ρ is the density of

Conclusions

The influence of compression plasma flows with pulse duration of 100 μs and absorbed energy density higher than 60 J/cm² melts the tungsten surface layer and provides the grain structure modification. Due to direct solidification of the melted layer the globular grains changing to columnar grains forms. The compression plasma flows impact to the “Ti coating – W substrate” produces the W-Ti alloy with composition depending on the absorbed energy density higher than 35 J/cm². The W-Ti alloy with

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Full length articleStructure and phase composition of tungsten alloys modified by compression plasma flows and high-intense pulsed ion beam impacts

Highlights

Abstract

Introduction

Section snippets

Experimental

CPF treatment of pure tungsten

Conclusions

Fusion Eng. Des.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

Vacuum

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Vacuum

Full length article
Structure and phase composition of tungsten alloys modified by compression plasma flows and high-intense pulsed ion beam impacts