Mechanistic studies in combustion synthesis of NiAl–TiB₂ composites: Effects of gravity

Abstract

Combustion synthesis (CS) of NiAl-based materials reinforced by TiB 2 particles was investigated under both terrestrial and microgravity conditions. The synthesized metal matrix composites (MMC) are characterized by very fine (<1 m) reinforced particulates, which have strong bonding along their entire surface with matrix (NiAl) and are distributed uniformly in it. It was found that microgravity leads to a decrease in the average TiB 2 particle size, while higher volume fraction of NiAl component in the material leads to the formation of coarser reinforced particulates. The mechanism of structure formation of different MMCs during CS was identified by using the quenching technique. For example, it was shown that TiB 2 grains appear due to crystallization from the complex (Ni-Al-Ti-B) liquid solution formed in the combustion front. An overall decrease of microstructural transformation rates was observed under microgravity.

Low‐gravity combustion synthesis: Theoretical analysis of experimental evidences

“…It has been shown that products with finer and more uniform microstructure are typically obtained under low-gravity conditions with respect to terrestrial ones. 10,11,21,24 The main experimental results related to the synthesis of the composites TiB 2 -xTiAl and TiB 2 -xTiAl 3 , under both terrestrial and low-gravity (10 Ϫ2 g where g is the terrestrial gravity acceleration) conditions are first reported in this work. Experiments under low-gravity conditions have been performed during the 32 nd ESA Parabolic Flights Campaign (Bordeaux, March 2002).…”

“…Specifically, as compared to the corresponding behavior on the ground, combustion wave typically travels slower under reduced gravity. 8,10,11,13,14,21,24 Moreover, under terrestrial conditions, bottom ignition of the sample results in a highercombustion velocity with respect to top ignition, 8,10,14,24 so that the following inequality holds:…”

“…8,24 Furthermore, final products synthesized in low-gravity environments, generally present finer and more uniform microstructure than those obtained under normal gravity conditions. 10,11,21,24 Indeed, theoretical explanations of relatively lower combustion front velocity under reduced gravity conditions have not been provided yet in the literature, at least to our best knowledge. Justifications of negligible gravity effect on combustion temperature have also not been given.…”

“…On the other hand, analysis regarding the effect of gravity on product microstructure are reported in the literature. 9,10,14,21,24 However, in our opinion, when dimensional analysis is performed for describing heat and mass-transfer phenomena involved in the combustion process, the choice of the characteristic length is not appropriate.…”

Historical perspective and contribution of US researchers into the field of self-propagating high-temperature synthesis (SHS)/combustion synthesis (CS): Personal reflections

“…His initial research interest was focused on mathematical modeling of combustion fronts. However, very quickly his research evolved toward experimental investigation of reaction kinetics of heterogeneous reactions as well as understanding of system heterogeneity and melting effects on propagation of combustion fronts in the condensed phase [163][164][165][166][167][168][169][170][171][172][173][174][175][176][177]. Professor Varma invited a few Russian scientists, including Drs.…”