Effect of Alloying Content on the Defect Structure Formation and Evolution in the Ta‐W system
Authors: Ipatova Iuliia , Jimenez‐Melero Enrique 
Published: December 2016

I INTRODUCTION

Nowadays tantalum coating of tungsten targets used in spallation sources (e.g. ISIS, LANCE) is considered as a promising route to improve the target integrity, neutron production, operational reliability, and hopefully reduce the need of handling radioactive materials at the end of the target lifetime [1]. Tantalum offers attractive corrosion and mechanical resistance properties, and its neutronic performance is relatively similar to that of tungsten. However, the difference in the thermal expansion coefficient of tantalum and tungsten may lead to significant geometry variations between the coating and the substrate that could cause the degradation and failure of the Ta/W component at elevated temperatures. This suggests that doping Ta with controlled amounts of W (up to 10wt.%) would improve the thermal stability of the compound target.

Supplementary to this another question which is apparently open now is that fatigue failure of tantalum cladding will be the limiting factor of target lifetime. Tensile pre‐stress and radiation embrittlement can make the fatigue situation worse and irradiation creep and stress relaxation may reduce the average stress. And in this case the addition of tungsten to tantalum can potentially increase the yield strength and the rate of work hardening of the final material.

II EXPERIMENTAL

A Non‐radioactive materials

In this work pure tantalum and tungsten samples, together with selected binary alloy compositions (Ta2.5wt.%W; Ta5wt.%W and Ta10wt.%W) have been analyzed before study of the proton irradiation. Four alloys were annealed at temperatures close to the onset of recrystallization. Changes in a grain size and grains orientation have been observed together with the presence and nature of nano‐scale defects such as dislocation lines, nets and tangles Tungsten has been reported to influence the dislocation density and dynamics in non‐irradiated Ta‐W alloys during mechanical testing [3]. However, W contents about 10wt.% would induce the formation of a secondary brittle phase in the structure, according to the Ta‐W phase diagram.

Using analytical electron microscopy techniques different structure of dislocations has been detected with increasing concentration of tungsten in tantalum in non‐irradiated samples. In alloys with highest tungsten concentration of 10% dislocations form nets of intersected lines with increasing its number density (Fig.1) which directly affects modifications in hardening behaviour of these kinds of materials. Detailed examination of thin foils indicates that dislocations presented in this alloy are screw dislocations with ½ a [111] type Burgers vector.

B Proton irradiation of Tantalum‐Tungsten alloys

Two proton irradiation experiments have been implemented using a 5MV Tandem Pelletron ion accelerator (NEC model 15SDH‐4) and high current TORVIS source in order to go up to 3 MeV 1H+ at 350C and up to 1.6 dpa level in Dalton Cumbrian Facility (University of Manchester) for the aim of investigating tungsten doping impact on the irradiation‐induced defects in microstructure and changes in mechanical behaviour such as radiation‐induced embrittlement in Ta‐W alloys.

Pure Tantalum and two Tantalum‐Tungsten alloys with different tungsten concentration have been loaded together on a specially designed target stage, pumped with high level of vacuum (7.44 *10‐7Torr) and equipped with ceramic heater and cooling system allowing not overheating the stage.

III STUDY OF IRRADIATED SAMPLES

A Bragg peak position. Hardness test

Hardness of bulk UHP Ta, Ta5wt.%W and Ta10wt.%W irradiated with single proton beam of 3 MeV during 36 hours at 350oC, as described above is assessed by nanoindentation.

Nanoindentation tests (Fig.2) have been carried out from the cross‐section area of each sample exposed under irradiation in order to observe changes in hardness depending on different dpa level and defect density. This experiment also has been performed in order to compare the data with SRIM calculations of Bragg peak position (Fig.3). As it is showed on the graph above Bragg peak position for all alloys is varied between 25‐40 μm which corresponds to the SRIM calculations.

B Scanning electron imaging

The determination of the Bragg peak can be straight forward in Scanning electron microscope as different contrast may arise with back scattered electrons detector (BSE) since the irradiation damage mutes more the crystal structure. In the Fig. 4 presented below, the BSE detector revealed (at high voltage of 30 kV) a line of brighter contrast, perfectly parallel to the irradiated surface and all along the sample. It crosses several grains of different orientation without interruption, as clearly showed by the different BSE contrast. Also nano indenters after hardness test are showed in order to prove matches of hardness data. It should be noticed that the higher contrast line crosses the point with the highest hardness.

C Current work on irradiated materials

Irradiation‐induced dislocation loops formation are reported to occur in pure tantalum at a damage dose of ≤ 0.3 dpa at a relatively high temperatures of 700°C [2]. However, radiation‐induced hardening in pure Ta as well as in Ta‐W alloys seems to take place already at a dose of ≤ 0.3 dpa and temperatures up to 350°C, based on mechanical testing data of irradiated samples [4‐6]. Nevertheless, the correlation of the hardening phenomenon with the characteristics of the irradiated structures and mechanism of dislocation loops formation still remains unknown. Irradiated sample preparation of irradiated Ta and Ta‐W alloys is carrying out for advanced Transmission microscopy analysis in order to observe irradiation‐induced defects such as vacancy clusters and dislocation loops and influence of dose on the nature and forming of these defects. This detailed study constitutes the stepping stone in understanding the effect of the alloying content as well as radiation dose on the defect formation and dynamics of these materials under mechanical deformation and irradiation conditions.