The role of two-stage phase formation for the solid-state runaway reaction in Al/Ni reactive multilayers

Abstract

While extensively studied for heating rates below 1.7 K/s and above 1000 K/s, the solid-state phase transformations in Al/Ni reactive multilayers have not been examined at intermediate heating rates between 100 K/s and 1000 K/s. Combined nanocalorimetry and time-resolved synchrotron x-ray diffraction studies are utilized to address this range of heating rates for multilayers with an overall composition of 10 at. % Ni and a bilayer thickness of 220 nm. It was found that a two-stage phase formation of Al 3 Ni proceeds up to a heating rate of 1000 K/s. The two growth stages occur in the solid-state and are kinetically separated. The activation energy of the first growth stage is determined to be 137 kJ/mol, which agrees well with the literature data at low heating rates. At 1000 K/s, a transition to a runaway reaction is observed. Unusual for metallic multilayers, the reaction proceeds completely in the solid-state which is also known as "solid flame." Using nanocalorimetry, a critical input power density for ignition of 5.8 Â 10 4 W/cm 3 was determined. The rapid succession of the two Al 3 Ni formation stages was identified as the underlying mechanism for the self-sustaining reaction.

Two-Layer Heat-Resistant Protective Coatings for Turbine Engine Blades

“…In the case of the protective coating made with the APS + SLURRY method, it can be stated that too much activity of the Al source in the SLURRY aluminizing process leads to a significant increase in the brittleness of the coating at a temperature below 750 • C [12,22], which is visible during its operation due to numerous cracks. Oxidized zones were found in the coatings, as well as areas of corrosion on the border with the transition (diffusion) zone.…”

“…This significantly increases the cost of operating the aircraft and reduces the safety of flights. The analyzes carried out by the authors of this article show that the structure and thermophysical properties of the protective coating must be adapted to the given alloy from which the blade is made [9,[21][22][23]. Therefore, efforts should be made to achieve the best possible convergence of the structure and thermophysical properties of the protective coating and the base material of the blade.…”

“…Due to the presence of a significant amount of Cr in it, it protects the base material of the blade against oxidation and high-temperature corrosion. The advantages of the NiCoCrAlY layer are that it is possible to shape the content of aluminum and chromium and other elements in it and that its properties do not depend on the chemical composition of the blade material [22][23][24][25][26]. This layer is characterized by a low transition temperature from the brittle state to the plastic state, amounting to 473-573 K. The second layer, the outer one, was a diffusion thermal layer of the NiAl type with low thermal conductivity.…”

Heterogeneous Microstructures Tuned in a High Throughput Architecture

“…Currently the state of the art method to measure the thermal properties of small samples at fast heating rates is nanocalorimetry [2]. The morphological and phase changes occuring during these rapid temperature pulses have been of scientific interest for some time, some examples of which are studying self propagating reactions [3][4][5][6], or silicides for use in the microelectronics industry [7], and have stimulated work into improvements in methodology [8,9] in situ electron microscopy [10], time resolved X-ray Diffraction (XRD) [11], and mass spectrometry [12] in order to better characterize phase transitions. The general setup, consisting of a platinum heating strip on which the sample is deposited, allows heating rates beyond the million K/s range.…”