Grain size effects on Ni/Al nanolaminate combustion


Reactions in Ni/Al nanolaminates exhibit high combustion temperatures and wave speeds that are customizable through changes to nanostructure. Nanolaminates fabricated via vapor deposition exhibit columnar grains with average diameters on the order of the individual layer thickness; yet, their role on nanolaminate combustion has not been previously investigated. The current work uses molecular dynamics simulations to elucidate the effect of grain size on reaction rates and combustion temperatures in Ni/Al nanolaminates. Decreasing grain size is shown to increase reaction rates as well as increase peak temperatures consistent with the excess enthalpy of smaller grain sizes. Additionally, grain boundaries provide heterogenous nucleation sites for the diffusion-restricting B2–NiAl phase. Focusing on Ni diffusion into liquid Al, an effective diffusion coefficient is computed as a function of grain size, which may be used in thermodynamic models for this stage of the reaction.

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The authors gratefully acknowledge support from the U.S. Department of Defense Science, Mathematics & Research for Transformation (SMART) Scholarship and the Munitions Directorate of the Air Force Research Laboratory. (Distribution A. Approved for public release; distribution unlimited. 96TW-2018-0407.)

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Correspondence to Douglas E. Spearot.

Appendix A. peak temperature increase calculation

Appendix A. peak temperature increase calculation

The peak temperature increase in Figs. 1 and 3 can be estimated by assuming any increase in initial enthalpy, Hinit, due to increasing GB density will be converted to additional heat, Q, since the system is not performing any external work. The heat capacity equation provides a conversion between additional heat and a change in system temperature,

$$Q = m \cdot {C_{\rm{p}}} \cdot {\rm{\Delta}}T\quad ,$$

where m represents the mass, Cp, the specific heat capacity, and ΔT, the temperature change of the system. If Eq. (A.1) is rearranged and terms for both the Ni and Al layers are separated,

$${\rm{\Delta}}T = {Q \over {{{\left({m \cdot {C_{\rm{p}}}} \right)}_{{\rm{Ni}}}} + {{\left({m \cdot {C_{\rm{p}}}} \right)}_{{\rm{Al}}}}}}\quad ,$$

which provides an estimate for the temperature increase of any single model. The relative temperature increase of the 1.5·Λ nanocrystalline (nc) and the single grain (sg) models, for example, can then be compared by subtracting the ΔT of one model from the other and substituting Hinit at 925 K for Q,

$$\matrix{{{\rm{\Delta}}{T_{{\rm{nc}}}} - {\rm{\Delta}}{T_{{\rm{sg}}}}} \hfill & = \hfill & {{{\left[{{{{H_{{\rm{init}}}}} \over {{{\left({m \cdot {C_{\rm{p}}}} \right)}_{{\rm{Ni}}}} + {{\left({m \cdot {C_{\rm{p}}}} \right)}_{{\rm{Al}}}}}}} \right]}_{{\rm{nc}}}}} \hfill \cr {} \hfill & {} \hfill & {- {{\left[{{{{H_{{\rm{init}}}}} \over {{{\left({m \cdot {C_{\rm{p}}}} \right)}_{{\rm{Ni}}}} + {{\left({m \cdot {C_{\rm{p}}}} \right)}_{{\rm{Al}}}}}}} \right]}_{{\rm{sg}}}}\quad .} \hfill \cr} $$

This calculation results in a peak temperature increase of 26 K, which compares well with the measured result of 32 K in Fig. 1.

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Witbeck, B., Spearot, D.E. Grain size effects on Ni/Al nanolaminate combustion. Journal of Materials Research 34, 2229–2238 (2019).

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