Dynamic assessment of direct-current mobility in field-assisted sintered oxide dispersion-strengthened V-4Cr-4Ti alloys
© The Author(s). 2017
Received: 3 June 2017
Accepted: 9 July 2017
Published: 24 July 2017
Vanadium alloy is one of the potential candidate material for structural applications in a commercial fusion reactor. Extended survival of a structural material has a direct consequence on the net energy produced in a fusion reaction, it is important to develop ultra-functional materials with tailored microstructures, to meet the harsh fusion environments. Microstructure of material, indeed depend upon the thermodynamics and kinetics of material processing.
Aiming to meet the harsh fusion conditions, we have developed oxide dispersion strengthened V-4Cr-4Ti alloys by high energy ball milling and field assisted sintering technique. Possible microstructural, morphological aftermaths observed in ball milled yttria dispersed V-4Cr-4Ti powders is explored.
Results and conclusion
Electron microscopy and laser particle analysis acknowledge that yttria addition aids powder agglomeration during ball milling. Ball milled powder was then consolidated (to a relative density of ~100%) using field assisted sintering technique, under optimal sintering conditions. Densification profile has implied that heterogeneous powder characteristic (apparent particle size and shape of powder) tends to impede the direct-current conductivity across the powder particle during various stages of field assisted sintering. In order to understand the kinetics of the field assisted sintering process on the starting powders, a new method was developed to compute the activation energy required for the direct-current conductivity across the individual powder particles. Relatively higher activation energy (for direct-current conductivity) is required for sintering yttria dispersed V-4Cr-4Ti powder than its V-4Cr-4Ti counterpart.
KeywordsVanadium Yttria Milling Sintering Densification Activation energy
Paybacks of V–4Cr–4Ti alloy as a candidate material in fusion power generation has been extensively discussed elsewhere (Zinkle et al. 2008; Smith et al. 1996; Johnson and Smith 1998; Smith et al. 1995; Muroga et al. 2002). V–4Cr–4Ti alloys have been relentlessly proving themselves as a potential alternative to basic steels and oxide dispersion-strengthened steels owing to their superior performance as a structural material like high temperature mechanical properties, irradiation resistance at elevated temperatures (>400 °C), low activation under fast neutron flux, and high ductility (Chung et al. 1996; Rice and Zinkle 1998; Gelles 1998). Even though the microstructural stability of V–4Cr–4Ti alloys at elevated temperature (>650 °C) is superior to ferritic/martensitic steels, the anticipated power conversion efficiencies expected out of plasma-facing components have increased tremendously (Loomis et al. 1994). This intrigued fusion scientists to look for an ultra-functional V–4Cr–4Ti alloy to withstand harsh conditions inside the fusion reactor, consequently enhancing the power generation efficiency of the same. One such work was attempted in the year 1996, by dispersing ultrafine yttria particles in V–4Cr–4Ti alloy (Yamagata et al. 1996). Shibayama and his co-workers developed the oxide dispersion strengthened (ODS) vanadium alloy by a novel powder metallurgy route, namely mechanical-alloying (MA) (Suryanarayana and Norton 1999; Alamo et al. 1992; Miller et al. 2005; Miller et al. 2003; Plant and Steel 1995). Earlier reports have consolidated ODS vanadium alloys using the hot isostatic pressing (HIP) technique, while a recent report suggests that field-assisted sintering (FAST) or spark plasma sintering (SPS) technique offers a variety of microstructural merits that can enhance the high temperature mechanical properties of V–4Cr–4Ti alloys (Krishnan and Sinnaeruvadi 2016). Vital merits offered by the FAST process include self-cleaning of the oxide layer in powder particles before sinter-bonding, retaining the nanostructure, and consolidating the powder particle to full density within a short span of time at lower temperature regimes (Mamedov 2000; Aalund 2008; Yucheng and Zhengyi 2002; Anselmi-tamburini et al. 2005; Chen et al. 2005). But no single literature is available to explain the densification mechanism behind sintering (FAST) V–4Cr–4Ti and yttria-dispersed V–4Cr–4Ti powders. Once the densification mechanism and its associated kinetics behind FAST-processing of the V–4Cr–4Ti powders are understood, tailoring the material properties via FAST process to match fusion-relevant conditions become conceivable (Tong et al. 2009; Thornton 1999; Thornton et al. 2004; Iimura et al. 2009; Yu et al. 1997).
In the present investigation, variations in current consumption while sintering V–4Cr–4Ti and yttria-dispersed V–4Cr–4Ti powders are discussed briefly. Further, the DC (direct-current) conductivity profile while sintering powder was obtained in order to understand the effect of yttria addition on the microstructural and morphological characteristics of ball-milled powders. Finally, the activation energy (E a) required for the sintering current to traverse through the individual powders was deduced.
Ball-milling conditions employed for synthesizing V–4Cr–4Ti and yttria-dispersed V–4Cr–4Ti powders
Type of ball mill
High-energy planetary ball mill
Milling vessel and media
Tungsten carbide (WC)
High purity argon (99.99% pure)
Powder morphology, apparent particle size, and chemical composition of ball-milled V–4Cr–4Ti and yttria-dispersed V–4Cr–4Ti powders were studied with the help of scanning electron microscopy images accompanied with energy-dispersive absorptive X-ray spectroscopy (SEM-EDAX). The milled powders were dispersed in ethanol and ultrasonicated for 120 s. The sonicated powders were transferred onto a glass plate and dried in air; subsequently, the powder was transferred onto a carbon tape placed over an aluminum stub and maintained under vacuum 24 h prior to electron microscopy-imaging. From the electron microscopy images, quantitative estimates of apparent particle size (μm) were calculated using ImageJ (version 1.50b) software. Further, to confirm the apparent particle size and its distribution, laser particle size analysis was carried out using Mastersizer 3000 laser diffraction particle size analyzer (Malvern Instruments, UK).
Results and discussion
Comparison of particle size outcomes obtained from the SEM and laser particle analysis
d SEM (μm)
d LPA (μm)
Unmilled V–4Cr–4Ti powder
40 ± 15
54 ± 9
8 ± 2
12 ± 2.4
14 ± 4
15 ± 1.4
19 ± 7
21 ± 2.4
24 ± 8
27 ± 8.2
V–4Cr–4Ti and yttria-dispersed V–4Cr–4Ti powders have been synthesized by ball-milling and consolidated using FAST process. Yttria addition has principally suppressed the surfactant’s role as a process control agent during milling, resulting in powder agglomeration and the subsequent increase in the apparent particle size distribution. Electron microscopy imaging and laser particle size analysis of ball-milled powders do endorse the increase in the apparent particle size with varying yttria content in V–4Cr–4Ti powder. The steady increment in the apparent particle size distribution of ball-milled powders (with varying yttria content) has attributed an exponential increase in the current consumption during the FAST process. Hence, the magnitude of sintering current consumption (for sintering the discussed powders) during the FAST process can be seen as a function of yttria content in V–4Cr–4Ti powders. For the given sintering condition, FAST-processing of V–4Cr–4Ti powders consume a relatively low order of current when compared with its yttria-dispersed V–4Cr–4Ti counterpart. Activation energy values computed from the DC current conductivity profile also confirms the fact that yttria addition (0.3 to 0.9 at.%) steadily increases the insulation resistance for DC current conductivity during FAST process.
Novel method developed for computing activation energy during sintering
Effect of yttria-doping (in V–4Cr–4Ti) on DC conductivity during sintering
Yttria addition in V–4Cr–4Ti powder leads to powder agglomeration
Powder characteristics affect dynamic current mobility during sintering
The authors declare that they have no competing interests.
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