(490d) Kinetics of High Temperature Reaction in Ni-Al System: Influence of Mechanical Activation

High temperature (>1000 K) reaction kinetics in the stoichiometric (1:1 by molar ratio) Al-Ni system was investigated by using the, so-called, electrothermal analysis (ETA) method. ETA is the only technique that allows studying kinetics of a heterogeneous gasless reaction at temperatures above the melting points of the precursors. Special attention was focused on methodological aspects of the ETA method.

Two different reaction systems were studied: (i) initial Al/Ni clad particles; (ii) the same powders but after 15 min of high energy ball milling (HEBM). HEBM of powders is widely used for preparation of amorphous and other advanced alloys. This approach can be applied for both the equilibrium phases as well as to mixtures with large negative heats of mixing, including highly reactive systems. A similar approach but with shorter processing time is used to enhance the powder mixture reactivity. This termed arresting ball milling or mechanical activation process, leads to a decrease of self-ignition temperature in the reactive mixture or to an increase of the reaction front propagation velocity, as well as allows a self-propagating mode for the compositions that do not burn without the mechanical treatment. Such mechanical treatment changes the main route of chemical interaction. Without ball milling, thermal explosion (TE) occurs at temperatures above eutectics (912 K) in the investigated system; thus the chemical route primarily involves solid-liquid reactions. After mechanical treatment the TE initiates at much lower temperatures and solid state reactions are predominantly responsible for the process.

Analysis of the obtained results leads to the conclusion that such mechanical treatment decreases the apparent activation energies of the reaction in the Ni-Al system, from 47 ± 7 kcal/mol for the initial powder to 25 ± 3 kcal/mol after high energy ball milling. Comparison of these data with those reported previously was also made.