Impurity stabilization of nanocrystalline grains in pulsed laser deposited tantalum
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Khalid Hattar Department of Radiation Solid Interactions, Sandia National Laboratories, Albuquerque, NM 87185
Jason R. Trelewicza) Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794 (Received 6 September 2016; accepted 8 February 2017)
Thermal stability of pulsed laser deposited (PLD) nanocrystalline tantalum was explored through in situ transmission electron microscopy (TEM) annealing over the temperature range of 800–1200 °C. The evolution of the nanostructure was characterized using grain size distributions collectively with electron diffraction analysis and electron energy loss spectroscopy (EELS). Grain growth dynamics were further explored through molecular dynamics (MD) simulations of columnar tantalum nanostructures. The as-deposited grain size of 32 nm increased by only 18% at 1200 °C, i.e., 40% the melting point of tantalum, conflicting with the MD simulations that demonstrated extensive grain coalescence above 1000 °C. Furthermore, the grain size remained stable through the reversible a-to-b phase transition near 800 °C, which is often accompanied by grain growth in nanostructured tantalum. The EELS analysis confirmed the presence of oxygen impurities in the as-deposited films, indicating that impurity stabilization of grain boundaries was responsible for the exceptional thermal stability of PLD nanocrystalline tantalum.
I. INTRODUCTION
At the core of designing advanced materials for harsh environments involving high temperatures, extreme stresses, and aggressive irradiation conditions lies unprecedented thermal stability.1–3 Nanocrystalline metals have gained a significant interest for extreme environment applications due to their advantageous mechanical properties despite their propensity to undergo grain growth even at moderate homologous temperatures.4,5 For example, heating nanocrystalline Ni with an average grain size of 20 nm to 40% of its melting point produced an order of magnitude increase in its grain size after only 30 s.6 Alloying has been pursued for suppressing nanostructural instabilities via solute stabilization of grain boundaries.7,8 Thermodynamic models argue that the segregated solute effectively reduces or eliminates the energetic penalty of grain boundaries,8–11 whereas kinetic treatments focus on the role of solute drag in limiting grain boundary mobility.12,13 Despite the mechanistic underpinnings, nanocrystalline alloys have demonstrated enhanced thermal stability relative to their single component counterparts.14
Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.68
Refractory metals have gained attention to enhance the operating temperatures of nanocrystalline materials due to their high melting points.1 Tantalum in particular offers a good combination of microstructural stability and mechanical properties 15,16 and consequently has been implemented in a myriad of applications spanning electronics, ballistics, and biomedical devices. 17–19 Re
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