I am a sucker for anything that has to do with space and the cosmos. From trying to understand black holes to the possibility of terraforming Venus is very fascinating to me. So I recently ran across a Kurzgesagt YouTube video (my social media of choice) that talked about building a sustainable colony on the moon [1]. Once I found this video I watched it multiple times, because 1) I enjoy the animation style of Kurzgesagt and 2) the Kurzgesagt team often make videos that allow me to think and imagine how my research can be used for futuristic applications.

This weeks blog will dive into a proceeding of the Space and Nuclear Conference titled: “Niobium-Base Alloys for Space and Nuclear Applications” [2]. This paper ties into my deep interest in space and my research focus materials manufacturing and nondestructive evaluation (NDE). The authors of the paper focus on two categories of niobium alloys: 1) low strength, high ductility and 2) moderate strength, moderate ductility for structural space nuclear reactor applications. Niobium-based alloys are the material of choice for space nuclear reactors because they have excellent high temperature mechanical properties with an emphasis placed on ductility (the material’s ability to be permanently deformed without complete fracture) [3]. There are a few commercially available niobium alloys such as C-103 and Nb-1Zr. The name designations are not important for this post but the second alloy Nb-1Zr means that it is a Niobium (Nb) balanced alloy with a minimum of 1 weight percent Zirconium (Zr). These two alloys being commercially available is key to making components for space nuclear reactors or any other application that require their unique properties. For the purpose of this post I will focus on the commercially available grades.

In these next couple of paragraphs I will give a run down of the highlights of the paper and at the end wrap everything up with my conclusions. The physical properties of niobium alloys are the chemical composition that make them up and give them their material properties that are desirable for specific engineering design. For example, all of the niobium alloys that were discussed in this paper contain zirconium, which is extremely useful in protecting the niobium alloy from alkali metal corrosion [2]. Alkali metal such as lithium, potassium, and cesium have been identified as key elements for heat transfer and working fluids in space nuclear reactors [4] and they can lead to corrosion if exposed onto other materials due to alkali metals being highly reactive. In addition, the mechanism by which the niobium is strengthened may also vary based on the composition.

The mechanical properties describes a material’s attributes when a force is applied to it. In the case for nuclear reactors, this includes unirradiated (not exposed to radiation) tensile yield strength, ultimate tensile strength, and elongation [2]. Furthermore, the mechanical properties can be linked to the alloying elements that make up the material. For example, tungsten (W) and vanadium (V) when added to the niobium based alloy can help strengthen the material. However, in the case for tungsten, it can decrease the materials ability to be fabricated. Also, different alloying elements can change the strengthening mechanism of the alloy (e.g., dispersion only versus solid solution and dispersion). Dispersion strengthening means that a second material phase can precipitate from the main phase to help reinforce the material microstructure [5]. Solid solution strengthening occurs when an element can either substitute or insert itself interstitially into the microstructure matrix. Whether an element is a substitutional or interstitial element depends on its size. On average, dispersion and solid solution strengthened niobium alloys have a higher strengths than dispersion only strengthened niobium alloys. But, dispersion strengthened alloys retain more strength at higher temperatures when compared to dispersion and solid solution strengthened niobium alloys.

Creep properties are another important property for nuclear applications. Creep is a measure of how a material component deforms when exposed to a mechanical force for a prolong period of time [6]. Creep tests can be performed at high temperatures, in irradiated conditions, and with mechanical loads that may be experience when a material is in service. Creep properties give a measure of how an alloy will perform when exposed anticipated working conditions for long durations. Niobium based alloys have been tested and their measured creep property data is mixed based on the alloy. In reference to the commercial grade niobium alloys, the Nb-1Zr have poor creep data at high temperatures at low mechanical loads. Alloying additions such as tungsten, tantalum, zirconium and carbon may increase the creep resistance at higher temperature. But adding these elements will need to be optimized to ensure that fabrication is also obtainable to make complex nuclear reactor components. Alloy C-103 has a limited amount of creep data at the moment and the data measured was not favorable for this alloy as some of its phases have been observed to be unstable at higher temperatures. When compared to Nb-1Zr, C-103 creep properties are pretty much the same, poor at elevated temperatures when compared to more experimental niobium alloy grades.

The ability for the niobium based alloy to be fabricated and welded is important. Especially, since any structure built on the moon will have to be put together piece by piece to cut down on launch cost. Gas tungsten arc and electron beam welding are prime candidates for structural assembly [2]. Welding processes and personnel will have to maintain cleanliness for welded components and control the welding environment to produce sound welds. Post weld heat treatment can be used to increase the ductility after fabrication for engineering design purposes.

In conclusion, commercially available niobium based alloys have some short comings to overcome to be fully endorsed for structural lunar nuclear reactor applications. There is still quite a bit of experimentation to conduct to understand their mechanical properties, creep attributes, and how they respond to fabrication. As far as the commercially available niobium grades, Nb-1Zr and C-103, there will need to be more alloying optimization of these alloys to withstand the thermal condition demand of nuclear applications. Niobium based alloys should see an increase in research because of other applications they can be used for in future engineering designs such as hypersonic flight. For example, alloy C-103 has been developed into powder form for additive manufacturing (AM) or 3D printing of aerospace engine components.

One important concept of manufacture was not talked about in this proceeding was NDE, which is a cornerstone of manufacturing and fabrication if consistent, high-integrity components are desired. NDE experiments can be carried out in parallel with the mechanical tests to understand how defects form in niobium alloys based on the welding process. NDE process optimization can be carried out to ensure majority of the defects that may affect the component’s performance are identified and remediated from the component before service.

I hope you found this post helpful. Thank you for your time!

–DB PhD

References:

  1. Kurzgesagt – In a Nutshell. (2018). How We Could Build a Moon Base TODAY – Space Colonization 1. [Video]. YouTube. https://www.youtube.com/watch?v=NtQkz0aRDe8
  2. Leonard, K. J., Duty, C. E., Zinkle, S. J., Luther, R. F., Gold, R. E., & Buckman, R. W. (2005). Niobium-base alloys for space nuclear applications. In Proc. Space Nuclear Conf., American Nuclear Society, San Diego, CA (pp. 286-293).
  3. Philips, N. R., Carl, M., & Cunningham, N. J. (2020). New opportunities in refractory alloys. Metallurgical and Materials Transactions A, 51(7), 3299-3310.
  4. W. O. Harms & A. P. Litman (1968) Compatibility of Materials with Alkali Metals for Space Nuclear Power Systems, Nuclear Applications, 5:3, 156-172, DOI: 10.13182/NT68-A28045
  5. Kou, S. (2003). Welding metallurgy. New Jersey, USA, 431(446), 223-225.
  6. Dieter, G. E., & Bacon, D. (1976). Mechanical metallurgy (Vol. 3). New York: McGraw-hill.