Mechanism of the Intermediary Phase Formation in Ti-20 wt. % Al Mixture during Pressureless Reactive Sintering.

Andrea Školáková,Pavel Salvetr,Davy Deduytsche,Jindřich Leitner,Pavel Novák

doi:10.3390/ma12132171

Abstract

This work aims to describe the mechanism of intermediary phases formation in TiAl20 (wt. %) alloy composition during reactive sintering. The reaction between titanium and aluminum powders was studied by in situ diffraction and the results were confirmed by annealing at various temperatures. It was found that the Ti2Al5 phase formed preferentially and its formation was detected at 400 °C. So far, this phase has never been found in this alloy composition during reactive sintering processes. Subsequently, the Ti2Al5 phase reacted with the titanium, and the formation of the major phase, Ti3Al, was accompanied by the minor phase, TiAl. Equations of the proposed reactions are presented in this paper and their thermodynamic and kinetic feasibility are supported by Gibbs energies of reaction and reaction enthalpies.

Highlights

A Ti-Al system consists of five important phases, including a Ti3 Al compound with a hexagonal close-packed superlattice, an equiatomic TiAl compound with a tetragonal structure, and aluminum-rich intermetallic compounds, namely TiAl2, Ti2 Al5, and TiAl3, with a tetragonal structure
The extreme reactivity of molten titanium usually causes contamination of the obtained products [6]. These compounds are produced by a melt-metallurgy process comprising vacuum induction melting (VIM), vacuum arc remelting (VAR), centrifugal casting, conventional melting, and hot isostatic pressing (HIP) [1]
We have studied intermetallics for several years and we have experienced that reaction kinetics mainly depend on particle size and compaction pressure

Summary

Introduction

A Ti-Al system consists of five important phases, including a Ti3 Al compound with a hexagonal close-packed superlattice (space group P63/mmc), an equiatomic TiAl compound with a tetragonal structure (space group P4/mmm), and aluminum-rich intermetallic compounds, namely TiAl2 (space group I41/amd), Ti2 Al5 (space group P4/mmm), and TiAl3 (space group I4/mmm), with a tetragonal structure. Titanium aluminides belong to the group of innovative materials that gradually replace nickel-based superalloys in highly demanding applications [1] They possess a great combination of stable mechanical properties at high temperatures (500–900 ◦ C), low density, and good oxidation resistance. For this reason, they are suitable candidates as structural materials for the aerospace and automotive industries. The current research is mainly focused on the development of alloys with microstructures containing TiAl and Ti3 Al phases, which should ensure great creep resistance [1,2,3,4,5] Despite this advantage, the application range of titanium aluminides is still limited because they suffer from room-temperature brittleness and have poor melt-metallurgic properties [5]. One of the currently studied processes of these compounds is Self-propagating

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